a case study of integrated wastewater treatment

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• Published: 26 September 2022

Pathways to a net-zero-carbon water sector through energy-extracting wastewater technologies

• Aishwarya Rani 1 ,
• Seth W. Snyder   ORCID: orcid.org/0000-0001-6232-1668 2 ,
• Hyunook Kim 3 ,
• Zhongfang Lei 4 &
• Shu-Yuan Pan   ORCID: orcid.org/0000-0003-2082-4077 1  

npj Clean Water volume  5 , Article number:  49 ( 2022 ) Cite this article

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• Climate sciences
• Pollution remediation

The energy-consuming and carbon-intensive wastewater treatment plants could become significant energy producers and recycled organic and metallic material generators, thereby contributing to broad sustainable development goals, the circular economy, and the water-energy-sanitation-food-carbon nexus. This review provides an overview of the waste(water)-based energy-extracting technologies, their engineering performance, techno-economic feasibility, and environmental benefits. Here, we propose four crucial strategies to achieve net-zero carbon along with energy sufficiency in the water sector, including (1) improvement in process energy efficiency; (2) maximizing on-site renewable capacities and biogas upgrading; (3) harvesting energy from treated effluent; (4) a new paradigm for decentralized water-energy supply units.

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Introduction

By 2030, 40% of the world’s population will experience water scarcity, creating stresses exacerbated by climate change 1 . The interrelationship between the water and energy sectors has been coined the “Water-Energy Nexus”. Energy use in the water sector largely depends on fossil sources, increasing carbon dioxide (CO 2 ) emissions. Specifically, the water sector accounts for 4% of total energy consumption, with highly energy-dependent wastewater treatment plants (WWTPs) accounting for 25% of the total energy use 2 . Globally, almost 400 billion m 3 of wastewater is produced annually, and it is expected to increase by 25 and 50% by 2030 and 2050, respectively. Due to finance and resource limitations, 80% of all wastewater is discharged untreated, creating a serious sanitation crisis for 4.5 billion people and impacting the environment and the biome 3 .

The “Water-Energy-Sanitation” crisis is evident considering ~800 million people live without clean water, 2.5 billion do not have adequate sanitation 4 , and 1.1 billion people have no access to electricity 5 . Energy, water, and sanitation are inextricably linked to agriculture, food security, health, gender, and education; thus, they are essential to achieving many sustainable development goals (SDGs) and environmental justice. Another concern is wastewater treatment processes that are carbon-intensive; for example, WWTPs in the USA generated 20 million metric tons (MMT) CO 2 -eq in 2017 6 . The energy needed for a typical domestic WWTP employing aerobic activated sludge processes and anaerobic digestion (AD) is 0.6 kWh per m 3 of wastewater treated, about half of which is used for providing electrical energy to sustain the aeration basins 7 . Biogas, a renewable methane source produced from AD, can be utilized for combined heat and power (CHP), decreasing energy consumption and CO 2 emissions. The US Environmental Protection Agency (USEPA) 8 noted that 25–50% of a WWTP’s energy needs could be met by biogas, even with conventional methods involving aerobic treatment.

In fact, wastewater contains approximately five times more embedded energy than is required for its treatment 9 . The American Biogas Council reports ~80% of the latent energy in wastewater is thermal, ~20% is chemical, and <1% of the potential exists in hydraulic generation 9 . While the thermal energy load is significant, it is low-grade heat and typically only useful for district heating. In terms of valuable resources, 16.6 MMT of nitrogen is embedded in wastewater produced worldwide annually, with 3 MMT of phosphorus and 6.3 MMT of potassium 10 . In the World Energy Outlook, the International Energy Agency states 3 , “If waste from all those who lack access to safely managed sanitation in rural areas today was captured and digested, the biogas potential could be roughly 20–50 billion m 3 ; this could be enough energy to provide clean cooking fuel to 60–180 million households.” The statement demonstrates the integration of water, energy, sanitation, and carbon into a single thread. Climate change increases global water demand and access uncertainty while aggravating regional water security. Therefore, it is essential to consider wastewater (sewage) treatment, water reuse, and resource recovery in an integrated manner to maximize benefits in managing the water-energy nexus 11 in sanitation. Broad areas to consider include “energy and water efficiency” as well as “energy and nutrient resource recovery”. To provide broad access to water, energy, and sanitation, system designs should consider the circular economy as the core backbone of the infrastructure.

Most developed nations have strict effluent discharge standards that regulate WWTPs. The regulatory approach focuses on minimizing the environment’s detrimental impact while decreasing the health risks associated with untreated wastewater. Less developed economies suffer from the costs of treating wastewater, resulting in the release of untreated wastewater. As the WWTPs evolve into water resource recovery facilities, the revenue from recovered energy, nutrients, materials, and even water could support the broader deployment of sanitation, decreasing the discharge of untreated effluent 12 , 13 . Ultimately, advanced water resource recovery facilities could decrease demand for freshwater, even if the reclaimed water is not used as a potable source.

Recently, solar energy has also gained attention for wastewater treatment. Usually, external energy is required to overcome the thermodynamical barriers to electromethanogenesis. However, solar light-driven electro-driving power could accelerate the conversion of waste organics to bioenergy. Wang et al. 14 found that the natural intermittent solar-powered mode was more beneficial for microorganisms involved in electron transfer and energy recovery than the manual sharp on-off mode and thus could be a promising perspective of solar-power-driven microbial biotechnology to boost bioenergy recovery from wastewater. For solar thermal sterilization coupled with bubble technologies, for example, bubble nucleation and cavitation, the interfacial properties of bubbles could improve the efficiency of biological wastewater treatment while also inactivating pathogens and mitigating biofouling 15 , 16 . Moreover, interfacial solar vapor generation is considered an efficient, sustainable, and low-cost method for producing clean water from solar desalination and wastewater treatment 17 .

WWTPs could become net energy producers with advanced design and possibly considered carbon-negative facilities. Numerous R&D programs have been conducted to make energy-dependent wastewater treatment technologies into energy-efficient or energy positive. Sewage can be the heart of the water-energy-sanitation-food-carbon nexus in a circular economy. However, to the best of our knowledge, little-to-no research on a systematic overview of energy-extracting technologies deployed in WWTPs has been conducted to build a pathway towards a net-zero water(waste) sector. In this study, we first review technologies developed for recovering energy from wastewater, including anaerobic bioreactors, salinity gradient energy (SGE) recovery processes, and fuel cells. Then, we summarize advances in existing technologies to reduce their energy footprint. We also evaluate different energy-extracting technologies from the aspects of engineering, economics, and environmental performance. Last, we suggest strategies for the role that WWTPs could play in achieving associated SDGs and net-zero carbon schemes.

Energy-extracting wastewater technologies

Typical wastewater treatment processes include screening, grit removal, primary settling tank, aeration (or activated sludge), secondary settling tanks, filtration, disinfection, and sludge treatment. These unit processes are usually energy-dependent and carbon-intensive. Several energy-extracting technologies could be adapted to existing WWTPs, such as anaerobic digestors or membrane reactors, salinity gradient or osmotic energy recovery processes, as well as fuel cells to realize net-zero carbon. In this section, we review and discuss the available energy-extracting technologies.

Anaerobic bioreactors

For decades, almost as an afterthought, AD has been used in WWTPs to stabilize sludge with biogas production (Fig. 1a ). Table 1 compiles a sample of AD-based case studies of self-sufficient and energy-positive WWTPs. The net positive energy is often achieved by co-digesting sludge with high organic content substrates, including agricultural and food waste, fats, oil, and grease 18 . For example, the WWTP in Bern (Switzerland) uses co-digestion of sludge with green and food waste to produce twice as much energy as the facility consumes 19 . Hybrid or combined systems are also used to increase the efficiency of the AD process, such as integrating bipolar membrane electrodialysis to recover ions and solid oxide fuel cells to produce electricity, which has the potential to achieve 55% of the maximum net energy efficiency 20 . Also, the CHP systems with AD facilitate the onsite conversion of produced biogas into power, and thermal energy could support operations across the facility or provide power to the grid 21 .

a Anaerobic digestion and b Anaerobic membrane bioreactor. Credit: www.flaticon.com for the cliparts.

The anaerobic membrane bioreactor (AnMBR) is an anaerobic bioreactor coupled with a membrane unit (Fig. 1b ), offering advantages such as improved effluent quality, low sludge production, compact size, and high biogas production (i.e., indirect energy generation). Anaerobic dynamic membrane bioreactors have proven to be an attractive option because they can be operated at an ambient temperature and can produce net energy of 0.05–0.06 kWh m −3   22 . The operation of AnMBR in phases improves the biogas yield and is thus reliable for energy-neutral or positive treatment for low-strength wastewater. Kong et al. 23 demonstrated a 5 m 3 large-scale submerged AnMBR with a biogas yield of 0.09–0.10 L per liter of raw wastewater. Another demonstration study indicated a 20 L submerged AnMBR installed in Sen-En WWTP (Tagajo city, Japan) can generate net electrical energy of 1.82–2.27 kWh d −1   24 .

In summary, anaerobic bioreactors have several inherent merits over other treatment processes, leading to their rapid universal adoption. AD-related processes employ a circular economy model by introducing wastewater into the supply chains, covering the energy demand of the wastewater utilities and communities, and promoting resource efficiency. It is a widely practiced technique that efficiently promotes renewable bioenergy and supports the bioeconomy. However, most of the current anaerobic bioreactors still need advancements in their designs and operating conditions to valorize the AD technological solutions.

Salinity gradient energy (SGE) recovery processes

SGE (so-called osmotic power, or blue energy) is the energy created from the difference in salt concentration between two fluids 25 . It is the thermodynamic reverse of using energy to desalinate saline water. Theoretically, around 2.24–2.25 MW of work 26 , 27 could be extracted from per m 3 of river water that flows into the ocean. Similarly, approximately 18 GW of salinity gradient power could be harvested while discarding treated wastewater into the oceans 28 . The chemical concentration gradient available in the aforementioned cases is a Gibbs-free energy that could be utilized for producing energy. This initiated series of attempts to develop technologies, such as pressure retarded osmosis (PRO), reverse electrodialysis (RED), and single-pore osmotic generators (OPGs). They are being explored as a renewable hybrid process for recovering energy from highly saline brine (e.g., effluent from desalination or salt mining) and treated wastewater effluents 29 . Table 2 summarizes several recent SGE studies that explored the power generation potential of wastewater or brine from desalination or salt mining 29 .

As shown in Fig. 2a , first conceived by Professor Sidney Loeb in 1974, PRO is a forward osmosis-based process utilizing Gibb’s free energy of mixing and is similar to hydropower technology 30 . The first standalone PRO-based energy power plant with a capacity of 10 kW was constructed by Statkraft in Oslo fjord, Tofte, Norway, in 2009; however, it was terminated in 2012 due to economic feasibility constraints 31 . After that, Toray industries developed the largest hybrid PRO powerplant with a capacity of 5 kW m −2   32 . Kyowakiden Industry Corporation Limited and Statkraft lead the PRO system development in Japan and Norway, respectively. A demonstration (Fig. 2b ) conducted by Kyowakiden Industry Corporation Limited used a mixed discharge system of treated sewage from Wajiro Wastewater Treatment Center and brine from Fukuoka Seawater Desalination Center Japan to feed into a hybrid PRO system and harvest 10 W m −2 of power. This demonstration showed decreased environmental stress, a gray water footprint, and an energy footprint later proved in the “Megaton Water System” national project organized by Toray Kurihara Fellow. Kyowakiden Industry Corporation Limited Japan has also registered a patent for this PRO system (PCT/JP2014/051873) in Israel, Saudi Arabia, Australia, the United States, and Japan 33 . Apart from these pilot or large-scale plants, lab-scale R&D advances PROs hybrids, or similar technologies continue to be developed. For example, Hon et al. 34 and Jiao et al. 35 introduced a novel hybrid forward osmosis-electrokinetic system similar to PRO (Fig. 2c ). The FO submodule ensures continuous fluid transportation in the entire system, and the electrokinetic submodule is responsible for electricity generation 35 . The other PRO hybrid process includes investigations of SWRO-PRO 36 (seawater reverse osmosis-pressure retarded osmosis), MD-PRO (Membrane distillation-pressure retarded osmosis) 37 , and a combined SWRO-MD-PRO 36 .

a Schematic diagram of a pressure retarded osmosis plant run on river water vs. seawater [adapted from ref. 28 ], b A demonstration conducted by Kyowakiden Industry Co., Ltd., used a mixed discharge system of treated sewage from Wajiro Wastewater Treatment Center and brine from Fukuoka Seawater Desalination Center, Japan, to feed into pressure retarded osmosis system and harvest power of 10 W m −2 [adapted from ref. 32 ], and c forward osmosis-pressure retarded osmosis hybrid system to mitigate membrane fouling for sustainable salinity gradient energy [adapted from ref. 167 ]. Credit: www.flaticon.com for the cliparts.

RED is an energy-free electrochemical technology or salt battery 38 originating from the natural, spontaneous, and irreversible mixing of river/fresh water and ocean/saline water (refer to Fig. 3 ). In 2005, REDStack with a European Salt Company Frisia jointly started a RED pilot project of 5 kW capacity 32 . The European REApower project’s framework extended a RED experiment utilizing less saline wastewater to the pilot project 39 . Showcasing the scale-up RED units, Tedesco et al. 40 installed three RED units with a total power capacity of 1 kW in Sicily, Italy; and on a laboratory scale, Nam et al. 41 reported the largest RED unit with 1000 cell pairs and a power density of 0.76 W m −2 . RED was extended under the European Seventh Framework Program to innovate membrane-free CapMix technology. The seawater and freshwater are alternately fed into a chamber comprising electrodes; that are charged when exposed to seawater and discharged when introduced to freshwater; this charging and discharging continues in cycles. Ongoing research aims to inherit the energy by mixing treated wastewater and brine from the desalination plant 42 .

a Vertical representation of a reverse electrodialysis mechanism [adapted from ref. 28 ] and b Horizontal representation of a reverse electrodialysis mechanism.

The ion-exchange membranes are the core of membrane-based energy-generation technologies; however, they have distinct physical limitations. Pore size is comparable to the ionic species’ size, and it is also thick (micro to millimeters), inhibiting mass transport and resulting in high membrane resistance, limiting power densities to <2.2 W m −2   43 . Therefore, transmembrane ion transport encounters steric hindrance and high electrical resistance, leading to low throughput of ions. Nanofluidic OPGs are technological breakthroughs because they can overcome challenges with pore size on a nanometer scale. For instance, Gao et al. 44 reported an ionic diode membrane-scale nanofluidic device comprising heterojunctions between cathode-microporous alumina and anode-mesoporous carbon for harvesting SGE up to a power density of 3.46 W m −2 .

Similarly, Hwang et al. 45 designed a mesoporous silica-based nanofluidic SGE harvesting system employing three monovalent electrolytes viz KCl, NaCl, and LiCl with power densities of 3.90, 2.39, and 1.29 W m −2 , respectively. Ji et al. 46 developed a 2D-material-based nanofluidic RED utilizing graphene oxide membrane pairs and ended up having 54% greater power density than commercial ion-exchange membranes. Several demerits of nanopores, such as electrical inhibition, high resistance, small switching currents leading to lower power generation, and weak signals causing difficulty in differentiating from background noise, made the researchers switch to the other emerging osmotic single-pore platforms. These brought the OPGs to an attractive high-power density of 26 pW (2.6 kW m −2 reported by Guo et al. 47 ) and 225 pW (10 3  kW m −2 by Feng et al. 48 ) under optimized conditions, compared to the conventional membrane-based energy harvesting processes.

Yeh et al. 49 reported a power density of 5.85 kW m −2 using a single alumina nanopore at pH 3.5 under a 1000-fold concentration ratio. Single-pore platforms overcome the issue of low power density and constant entering resistance in nanofluidic OPGs. In the direction of power density, Gao et al. 50 addressed the giant gap between the single-pore demonstration and the membrane-scale application originating from the different ion transport properties in a porous membrane, based on a reservoir-interface-nanopore resistance paradigm. Importantly the study highlighted by suppressing the reservoir and interfacial resistances, kW m –2 to MW m –2 power density could be achieved with multi-pore membranes, approaching the level of a single-pore system. Cao et al. 51 also developed integrated nanofluidic REDs, showing a lower fluidic resistance and a higher ionic flux. Power levels of 45, 29, and 17 pW were generated with the cation-selective negatively charged nanopores for KCl, NaCl, and LiCl, respectively. While with the anion-selective positively charged nanopores, a power of 22, 14, and 6 pW was produced.

In summary, SGE is considered a breakthrough in renewable energy and has the potential to produce energy at a scale comparable to intermittent solar and wind energy. While the deployment of salinity gradients for energy recovery is nascent, potential sites for productive energy generation are broadly distributed, especially in highly-populated coastal regions. Single-pore OPGs have shown extremely high-power density. More power can be harnessed in the future with advancements in the existing semipermeable and ion-exchange membranes, significantly reducing the required capital investment and thus supporting their economic viability. In addition, the SGE techniques’ hybrid scheme can complement the desalination and local (waste)water management strategies in dry urban coastal areas.

Fuel cells have gained popularity for generating electricity, hydrogen, and valuable chemicals. Fuel cells are functionally the reverse of an electrolysis cell. They are a device that can transform chemical energy into electrical energy. It normally consists of an anode and a cathode, which are connected through an external circuit and a chemical fuel. There is strong interest in hydrogen fuel cells for clean trucks 52 . The carbon content in wastewater is a viable feedstock for fuel cells. Figure 4 shows the typical diagrammatic representations of fuel cells, including microbial, enzymatic, and photocatalytic systems. Tables 3 , 4 also compile some recent studies utilizing wastewater for bioenergy or biohydrogen generation using MFC and AnMBR, as well as MEC, respectively.

a Microbial fuel cell, b Microbial electrolysis cell, c Plants microbial fuel cell or microbial solar cell, d Microbial desalination cell, e Enzymatic fuel cell, and f Photocatalytic fuel cell.

Microbial systems include microbial fuel cells (MFC, as shown in Fig. 4a ), microbial desalination cells (MDCs, in Fig. 4b ), and microbial electrolysis cells (MECs, in Fig. 4c ). MFC, introduced in 1911 by Potter 53 , is a type of microbial system that obtains electrical energy from the chemical energy of organics by utilizing the exoelectrogenic bio-agents activities of bacteria (e.g., Clostridium cellulolyticum , G. sulfurreducens, Enterobacter cloacae , and Clostridium butyricum ), as well as fungi (e.g., Aspergillus awamori and Phanerochaete chrysosporium ) 54 . In fact, an individual MFC unit offers relatively low energy density. In practice, a few studies 55 , 56 suggested the miniaturization in physical stacking and electrical connections of multiple MFCs to scale up the power output (see Table 3 ). A power density of ~0.15 W m −2 was achieved with swine wastewater 57 . Rabaey et al. 58 reported 4.31 W m −2 power density using closely spaced graphite blocks with a large ion-exchange membrane surface and a ferricyanide catholyte. A parallel configuration of five MFC units with a tubular approach produced 175.7 W m −2   59 .

As an estimate, ten times the current produced energy can be generated by converting a mere 1% solar energy 60 . Solar energy-based photosynthetic MFCs were first reported in 1980 61 and are gaining popularity. They consist of microbes with certain specialized light-harvesting complexes where photosystem I and photosystem II function as the photosynthetic units. Two kinds of photosynthetic MFC were reported, including sub-cellular photo MFCs utilizing anoxygenic photosynthetic entities attached to the electron acceptor and whole-cell photo MFCs using whole-cell autotrophic microbes. Moreover, biohydrogen can be further converted into bioelectricity to avoid any handling or recovery problems and can also be produced by photo MFCs. Pillot et al. 62 and Rashid et al. 63 highlighted a synergistic relationship and a better green electrical output on mixing photosynthetic autotrophic microbes with heterotrophic bacteria.

MDCs are a cheaper alternative that could support existing desalination technologies (reverse osmosis, electrodialysis, and capacitive deionization), which can desalinate and treat (waste)water, as well as generate electricity using bacteria (exoelectrogens). It has three chambers, i.e., anode, desalination, and cathode chambers, and is being practiced at a lab scale. For instance, a hydraulic-coupled MDC generated 0.86 W m −2 power density 64 ; a recirculation MDC 65 reported ~0.93 W m −2 , and a novel two-chamber MDC 66 produced 2.0 W m −2 . Besides electricity generation, MDCs have more benefits to offer, namely, >90% nitrate removal, >80% heavy metal and calcium carbonate removal, and 80% ammonia removal, along with the production of H 2 , acid, and base chemicals 65 , 67 .

The MEC is capable of removing organics while simultaneously producing H 2 (see Table 4 ). Microbes are utilized as biocatalysts to reduce the activation overpotential of a certain redox process, enhancing the voltage efficiency and production rate. According to Heidrich 68 , an onsite 0.12 m 3 MEC created 0.015 m 3 -H 2 m −3 d −1 (a purity of 100 ± 6.4%) with a Coulombic efficiency of 55% and retrieved ~70% of the electric power input. MECs combining new technologies with traditional ones may be used to overcome the thermodynamic limits, as well as material prices, methanogens, substrate concentration, and other issues the MEC faces on its own. When the MEC is integrated with other fermentation systems, 96% of the H 2 was recovered at a production rate of 2.11 m 3 H 2 m −3 d −1 , resulting in electrical energy productivity of 287% 69 , 70 . An integrated MEC-AD generated biogas of good quality, with CH 4 content coming to 86 ± 6%. For metropolitan wastewater, the benchtop investigation produced standardized net energy of 25.96 kWh m −3 d −1   71 . The net energy of a 1000-L pilot-scale framework with a cathodic surface area of 18.1 m 2  m −3 was determined to be 2.11 kWh m −3 d −1 for vineyard wastewater 72 . The H 2 produced by MECs is green hydrogen and the US Department of Energy’s Energy Earthshots Initiative are aiming to reduce the cost of clean (green) hydrogen by 80% to US$1 per 1 kg in 1 decade (“111”). As with salinity gradient technologies, microbial systems are nascent but have the potential for broad deployment. In addition, they provide additional benefits in (waste)water treatment.

An enzymatic fuel cell (EFC, as presented in Fig. 4e ) is a renewable and environmentally friendly energy source, first demonstrated by Yahiro et al. 73 . It employs enzymes as a catalyst that transform the released chemical energy from the enzymatic oxidation of fuels such as hydrogen, alcohols, and sugars, with oxygen as an oxidant, to electrical energy by the movement of electrons released from the chemical reaction. The wastewater-powered EFCs utilizing the hydrogen recovered from MEC could reduce ~9.3 kg CO 2 -eq emissions per kg of H 2 produced from the conventional steam methane reforming (SMR) technology to produce H 2 74 . A thermocatalytic method involving splitting methane from the rising natural gases and extracting viable H 2 can act as a wastewater H 2 feedstock 74 for EFCs. The optimized EFC was tested in the municipal wastewater of Zonguldak city in Turkey, generating 4.6 mW m −2   75 .

A photocatalytic fuel cell (PFC, a synergistic integration between photocatalysis and fuel cell, as presented in Fig. 4f ) can degrade organic contaminants present in sewage and recover the chemical energy. The main components of a PFC include a light source, cathode, photoanode, and organic wastewater as the chemical fuel. Hybridization of PFCs with nanobubbles is one of the techniques that may promise a synergistic effect in treating various types of wastewater 76 . A burr-like Ag-TiO 2 coated photoanode-based brewery effluent flow-PFC ran at 2.75 A m −2 over a 6-h period, generating a minimum voltage and power density of 0.65 V and 1.85 W m −2 , respectively 77 . Xu et al. 78 designed an integrated PFC-electro-Fenton process with WO 3 /W photoanode and Fe@Fe 2 O 3 /carbon felt cathode with maximum power output and current density of 3.4 W m −2 and 5.9 A m −2 , respectively. Liao et al. 79 reported a high-power density of 42.6 W m −2 and a current density of 112 A m −2 employing a visible-light responsive photoanode and air-breathing cathode.

The fuel cells function like batteries and offer several advantages over fossil fuel-based technologies. The microbial and enzymatic systems are bioelectrochemical-based solutions; therefore, the microbes and the enzymes play a vital role in the related treatments. The R&D goals to address the key challenges of fuel cells are (i) to minimize the cost, (ii) to increase the performance and durability, and (iii) to advance the designs of membranes and modules. Despite the challenges and shortcomings, it is well-known the energy systems discussed above play a vital role in the future energy mix and decarbonized energy systems, as they are sustainably sound in terms of energy conversion, production, storage, controlling pollution, and thus greenhouse gas emissions. To promote these wastewater-based renewable energy-generation technologies, several innovative ongoing projects to make WWTPs energy surplus are POWERSTEP 80 , Enerwater 80 , R3Water 80 , BioBZ 81 , DEMOSOFC 82 (installation of three fuel cell modules for co-production of 175 kW electric power) and SMART-Plant 80 .

Technology evaluation

This section surveys the state of technology to evaluate the opportunities and gaps in developing and deploying systems while maximizing wastewater treatment benefits. We compare the engineering, economics, and environmental performance of the aforementioned technologies.

Engineering performance

Technology readiness level (TRL, on a scale from 1 to 9: 1 being the lowest and 9 being the highest) is an indicator of the maturity of a particular technology, reflecting the implementation of an actual system in an operational environment. For the anaerobic bioreactors, AD is considered the most mature and widely practiced technology with TRL 9. AD is deployed over a range of small, isolated facilities to the world’s largest WWTPs. The AnMBRs are still at TRL 4 and could rise to TRL 6 by 2027 83 . For the SGE recovery technologies, PRO has reached TRL 7 because the power density of the PRO membrane is 6-fold higher than the commercial requirement of 5 W m −2 under lab conditions and twofold higher when tested with the waste streams 84 . The PRO membrane has demonstrated stable power generation under extensive pilot tests. RED has achieved TRL 7 because its prototype is fully demonstrated in an operational environment 85 . The MFCs and EFCs are at a general TRL of 3–4 because there is a need for increased electrical outputs by establishing pathways contributing to the transfer of extracellular electrons; cost-effective cathodes; feasibility establishment in real conditions; optimized designs for efficient performance; and extension to other complex substrates with specific bacteria for scale-up applications. However, it is expected to reach TRL 6 by 2035 86 . The MECs and MDCs are at TRL 3. The current challenges include cheaper cathodes, platinum substitution; catalyst reduction; lack of comprehensive understanding of unique electron transfer mechanisms in the complex matrix of electrodes, bacterial cells, or other microbes; and insufficient long-term experiments in real conditions. Similar to MFCs, the TRL level of MECs and MDCs will increase to 4 by 2027 and reach 6 by 2035 87 . The TRL of PFC varies as per its application: for water splitting and hydrogen production, the TRL is 2–3; for water treatment, the TRL is 3–4; however, it is a mature technology for air purification and self-cleaning 88 . The TRLs of standalone technologies are low; however, the integrated or hybrid technologies could have higher TRLs as they overcome the demerits and perform better with higher yields and efficiency. The technologies are marked on the TRL scale in Fig. 5 .

Inset is the TRL scale from 1 to 9. CFB represents common flow battery, and EF represents electro-Fenton [Data were collected from refs. 44 , 48 , 49 , 78 , 89 , 90 , 91 , 92 , 93 , 94 , 95 , 96 , 97 , 98 , 99 , 100 , 101 , 102 , 103 , 104 , 105 , 106 , 107 ].

Figure 5 compiles the power density and associated current density of energy-extracting wastewater technologies from the literature 44 , 48 , 49 , 78 , 89 , 90 , 91 , 92 , 93 , 94 , 95 , 96 , 97 , 98 , 99 , 100 , 101 , 102 , 103 , 104 , 105 , 106 , 107 . The single-pore OPGs have secured the highest coordinates on the power and current densities, illustrating their abundant potential. The hybrid schemes have generated power and current with higher densities than their standalone prototypes 78 , 89 , 107 . The power density of PRO is much higher than the RED, as the PRO can take advantage of the increased salinity gradient due to its logarithmic dependence on the solution concentration, unlike RED 108 . Therefore, the membrane permeability–selectivity at a small efficiency cost can improve the power density of PRO. The power and current densities of fuel cells are very low when implemented on a large scale. Fuel cells require more research in the direction of harvesting greater power and current density. However, the combination and integration could boost the power density to a significant extent. The anaerobic digestors and AnMBRs are biogas producers and indirect sources of electricity. The electricity generation depends on the installed CHP units or other energy recovery techniques. This area needs the attention of researchers to improve the current, power, and energy densities of the energy-generating technologies for scale-up and deployment.

Techno-economic analysis (TEA)

Green technologies for WWTPs are analyzed by carbon neutrality, energy, and economic analysis. TEA is a method to analyze economic performance, and it is important to evaluate the sustainability-based potential for deployment. Apart from the efficiency and technical assessment, economic assessment is necessary to understand the potential for the technology to meet market demand. TEA combines process modeling and engineering design with economic evaluation and helps to assess the economic viability of the technology or system. It is important in identifying the future potential of energy-extracting technologies. For example, the TEA of a Kraft pulp mill using AD to produce renewable diesel revealed production costs (€0.47–0.82 per liter of diesel equivalent) similar to conventional diesel in Sweden (€0.68 per liter of diesel) 109 . Stoll et al. 110 used TEA to compare different anodes (hard felt, carbon foam, and standard graphite brush) for a unit m 3 pilot-scale MFC. The TEA of energy generated by RED is reported by Turek et al. 111 , which determined a high specific cost equal to US$ 6.79 per kWh against a total investment cost of US$ 100 m −2 for the installed membrane. For the TEA of PRO, Benjamin et al. 112 used the Tampa Bay Seawater Desalination Plant as a case study and calculated a potential savings of 9% if PRO was installed. The electricity generation by OPG (US$ 0.04 kW h −1 ) is not competitive with the wholesale conventional US grid electricity reported in the TEA of OPG by Hickenbottom et al. 113 . For PRO to be commercially viable, the target power density is 5 W per m 2   114 . However, for techno-economic feasibility, Chung et al. 115 developed a model to calculate the minimum required net power density to achieve the target levelized cost of electricity. The minimum required net power density for the PRO system in achieving a levelized cost of electricity of US$ 0.074 per kWh (the capacity-weighted average levelized cost of electricity of solar photovoltaic in the USA) was found to be 56.4 W per m 2 . Hybrid technologies could perform better in acquiring required power densities. Trapero et al. 116 performed an economic assessment on three different scenarios, optimistic, pessimistic, and most likely scenarios, based on the maximum power density of a solo MFC implementation for wastewater treatment and compared with conventional activated sludge process; MFC was found to be a beneficial technology in all three scenarios.

Environmental benefits

Life-cycle assessment (LCA) methodology to evaluate the carbon emissions and environmental impacts of WWTP products’ and processes’ environmental aspects are described in ISO 14040:2006 standard. Rebello et al. 117 proposed a guideline framework suitable for urban WWTPs based on a review of over 111 LCA studies of WWTPs. Mueller et al. 118 compared RED to existing renewable energy technologies with a functional unit of 1 MWh of net electricity production, and under baseline assumptions, the impacts are an average of 50% higher for natural water compared to the concentrated brine. The study highlighted RED’s superior performance to other renewable energy technologies; however, several key environmental impacts were found in RED, including carcinogenic activity and eutrophication due to membranes. Foley et al. 119 conducted an LCA to compare the environmental impact of energy-producing wastewater treatment processes such as AD, MFC, and MEC. The study found significant environmental benefits of MFC compared with AD and MEC. In an LCA of AnMBR, two process subcomponents, including sludge management as well as sulfide and phosphorus removal, were indicated as environmental hotspots affecting the environment and costs 120 . In an LCA of sewage sludge, Lanko et al. 121 analyzed the Mesophilic, Thermophilic, and Temperature-Phased AD and reported the best performance of temperature-phased AD.

Table 5 presents the advantages, disadvantages, and future outlooks of the treatment techniques discussed in this article. To begin with, for the full-scale deployment of PRO, the effects of constant pressure operation, draw solution dilution, and feed solution concentration in membrane modules are crucial factors for overall performance 122 . The realistic membranes are neither perfectly selective nor achieve perfect hydrodynamics; and realistic modules also have limited area causing an incomplete mixing process. Therefore, more novel specific module designs, along with the hybrids such as FO-RO-PRO 123 are required to target the water-energy sustainability. Similarly, in RED, the ion-exchange membrane, as well as the stack/module designs, are crucial for highly efficient large-scale treatment plants 124 . The SGE has reached advanced levels on lab-scale R&D; however, for large-scale deployment, the designs are yet to be optimized for desired water-energy solutions. Despite numerous benefits, the fuel cells such as MFC, EFC, Plant MFC, and PFC are struggling for commercialization because their architecture, cost, and durability are yet to be optimized 125 . Most energy-generation technologies are in their infancy and require more advancement to overcome their drawbacks or limitations and improve their TRLs. The less efficient and expensive semipermeable membranes, friction losses in plant streams, and low efficiency of rotating components in the case of PRO; the expensive catalysts in biofuel cells; expensive electrodes in RED; slow and intrinsic electron transfer in EFCs must be replaced prior to scaling up these technologies to a full-scale plant. The thinner the membranes, the higher the power density; atomically thin membranes have gained the limelight because they maintain their structural integrity while providing minimal resistance to the ions. Extensive usage of membranes in membrane-based energy-extracting technologies has been contributing to membranes’ evolution.

Strategies On transforming Wwtp to net-zero Co 2

More countries are coming on board and joining the race towards net-zero. By 2021, Global Water Intelligence traced 65 water utilities and WWTPs with commitments toward the net-zero or climate neutrality targets; 26 utilities out of those have joined the United Nations Framework Convention on Climate Change’s Race To Zero global campaign to show their commitment and performance globally (Fig. 6 ) 126 . According to the USEPA, there are opportunities for energy efficiency, renewable energy, and water efficiency at each stage of the water use cycle 127 . The USEPA proposed a seven-step process in Ensuring a Sustainable Future: An Energy Management Guidebook for Water and Wastewater Utilities 128 based on the circular evolving Plan-Do-Check-Act management systems approach described in ENERGY STAR ® Guidelines for Energy Management 129 . To achieve net-zero carbon emissions by 2050, society needs a coordinated investment in research, sharing of best practices, and joint deployment. Net-zero water sectors can also contribute to several SDGs and life quality for all people.

a Summary of year-based commitments. b Net-zero carbon emission utilities, c Carbon-neutral utilities, and d Net-zero emission utilities [Adapted from ref. 168 ].

This section proposes four strategies based on the existing energy-positive WWTPs execution plans to become net energy producers and achieve a net-zero carbon emission sector, including (1) improvement in process energy efficiency, (2) maximization of on-site renewable capacities and biogas upgrading, (3) harvesting of energy from treated effluent, and (4) a new paradigm for decentralized water-energy supply facilities. These strategies aim to provide clean water to all of society while minimizing carbon emissions and environmental impacts, thereby effectively managing the implementation costs. These strategies could also serve as the basis for broader action plans for researchers, technology developers, service providers, operators, regulators, and municipalities.

Improvement in process energy efficiency

The most common energy-consuming processes at WWTPs are aeration systems and mechanical pumping. The first accounts for 45 to 75% of the WWTP’s energy expenditure, and the latter accounts for 18.9% 130 . The energy consumption of pumps can be improved by designing proper pumps in the right position. First, a broad investigation is required in the design stage to minimize the requirement to lift wastewater and consider the flooding flow mode 131 . Second, appropriate pumps considering the combination of sewage lifting amount and changing characteristics are needed to meet the high operating efficiency range and water level 132 . Panepinto et al. 133 identified several opportunities for energy saving, such as ~25% energy saving by optimizing the primary settling efficiency with coagulants, 20–36% by aerating oxidation tanks equipped with automatized controlled dissolved oxygen and sludge retention time, and 64% by optimizing dissolved air flotation into solids thickening. A partial and intermittent operation of blowers instead of 24/7 operation would guarantee an unchanged effect with reduced energy consumption. Turning off mixers when aerators operate can save 90% of the energy 130 .

Following the USEPA’s plan 128 , several (waste)water utilities have reduced their energy consumption and CO 2 emissions by improving their process energy efficiency. The Green Bay, Wisconsin Metropolitan Sewerage District serving 217,000 residents, saved 2,144,000 MWh yr −1 and 1480 metric tons of CO 2 -eq by installing new energy-efficient blowers. The East Bay Municipal Utility District uses the microturbine CHP units and water distribution via downhill pipes to consume 82% less energy than the California average in delivering each million gallons of drinking water. Millbrae, California, generating 1.7 million kWh yr −1 electricity, is 80% self-sufficient energy by utilizing inedible kitchen grease diverted from the city’s WWTP. Also, the sewage source heat pump can exchange heat between the sewage and the heat pump, and the internal heat pump is driven by electric power for heating or cooling purposes. Generally, the heating/cooling coefficient of the sewage source heat pump is 5.0–6.0 134 , which is much higher than the conventional air-source heat pumps. Also, it decreases the CO 2 and SO 2 emissions by 68% 135 and 75% 136 , respectively. Kollman et al. 137 reported a sewage source heat pump supplied by electricity from renewable resources only as the most sustainable option for producing the heat demand of 9057 MWh th yr −1 with an ecological footprint reduction of almost 99%. Awe et al. 138 mentioned, “variable frequency drives (VFDs) can be used to vary the speed of the pump to match the flow conditions and affinity laws for centrifugal pumps suggest that even a small reduction in motor speed can reduce pump energy by as much as 50%.”

Moreover, smartness and intelligence innovations are essential in the path toward sustainable circularity in the water sector. Smart systems such as sensors, tailored treatment systems, adaptive outputs, industrial resilience, continuous process improvement and learning, and reimagining water resource recovery facilities can reduce energy consumption and water loss while improving process energy efficiency. This could be the most important management step towards circularity and sustainability. It is estimated the energy requirements of sewage pumping are 69 kWh per population equivalent 139 per year, or exceeding double the average energy consumption for treating wastewater to a good quality 140 . Thus, intelligent wastewater pumping systems, real-time decision support systems, and adaptive mixers can cut 50% of the energy-related emissions; smart mixing and aeration systems can reduce the N 2 O emissions. Melbourne’s Main Outfall Sewer was recognized with a Gold Award for Sustainability in Design. Vacuum sewer systems are 24% cheaper 141 and can lower the overall energy by 30–35% for their operation compared to the conventional gravity sewers 140 . In other words, smart and automated technological modifications/add-ons can improve the process energy efficiency and reduce the energy requirement of the WWTPs, contributing to their goal of self-sufficiency.

Maximizing on-site renewable capacities and biogas upgrading

The on-site integration of renewables such as solar or wind energy, AD equipped with CHP facilities, or installation of energy extracting technologies will contribute to the self-sufficiency of WWTPs. The on-site installation of standalone or hybrid renewable energy generators with energy storage could provide the decarbonized power source for wastewater treatment systems (e.g., distillation, photocatalytic oxidation, direct heat, desalination, and UV disinfection) as well as pumping. Solar energy can be applied in the WWTPs, including (1) the solar thermal to increase the reaction temperature and improve treatment efficiency, (2) the sludge can be dewatered utilizing the solar thermal energy, and (3) it can be employed for desalination (reverse osmosis or electrodialysis) or evaporation purposes. Geothermal or industrial waste heat could also be used as a thermal source. Photovoltaic power generation electrolysis could remove and recover pollutants from wastewater and provide electricity for other unit operations. Yiannopoulos et al. 142 used anaerobic biofilter reactors to increase the sewage treatment temperature to 35  o C by solar heating. Ren et al. 143 proposed several improvements to anaerobic biological treatment utilizing solar thermal systems. Similarly, wind or other clean-electricity generators could drive treatment systems, pumps, or disinfection systems. Wind turbines start pumping at speeds between 2.5 and 3.5 m s −1 and could provide mechanical energy at lower speeds than electrical wind turbines (minimum average speed of 5–6 m s −1 ). Emerging hybrid systems (photovoltaics, wind turbines, backup generator, and battery storage) are gaining popularity for remote or islanded applications.

In addition, biogas (e.g., from the AD of sludge) upgrading should be deployed in WWTPs to produce renewable natural gas to replace fossil fuel sources 144 , 145 . Natural gas has broad applications in society and mature infrastructure, and hydrogen infrastructure and utilization are less mature. Therefore, deploying renewable natural gas from biogas is easier than hydrogen. In 2017, Europe ranked first, having 17,783 installed biogas-based power generation facilities with 18.4 billion Nm 3 ; and 340 biomethane plants out of 540 were fed into the grid 146 , 147 . Biogas can be upgraded to a sustainable fuel (i.e., biomethane) by the mature technologies of chemical or water scrubbing and pressure swing adsorption (market share of 25, 34, and 20% in Europe) 80 ; later, it can be fed into gas grids, CHPs, and vehicles.

Moreover, upgrading biogas to biomethane increases the calorific value facilitating utilization pathways, such as domestic stoves, boilers, internal combustion engines, Stirling engines, gas turbines/microturbines, and natural gas grid injections, vehicles’ fuel, and fuel cells. However, upgrade challenges include avoiding methane release, minimizing water leakage from the water scrubber, high energy/chemical usage, and operating cost, leaving the research scope for cost-effective and chemical-saving microbial/electrochemical or biological methods. Some new upgrading techniques reported recently are ecological lung, in-situ methane enrichment in AD, and cryogenic upgrading 148 . Besides methane, biogas comprises 25–50 vol% of CO 2 produced by the AD process, and it is estimated that ~32.2 Gt of CO 2 is emitted annually by AD around the world. The bioenergy-derived CO 2 can be further collected and utilized to realize a negative carbon scheme 149 . For instance, numerous approaches limit atmospheric release by injection for enhanced oil recovery or production of suits of chemicals such as Fischer-Tropsch liquids, polymers, alcohols, polyols, succinic acid, and syngas/hydrogen 80 . The WWTPs require innovative and integrated resource- and energy-efficient treatment technologies and enhanced carbon capture from a circular economy perspective.

Harvesting energy from treated effluent for coastal nations

Most coastal nations rely on desalination to meet some of their water demands, and desalination plants are intensive energy users, the production of which typically requires burning fossil fuels in large power plants. Heihsel et al. 150 developed a tailor-made multi-regional input-output model to examine the greenhouse gases for 2005–2015 from seawater desalination in Australia, using conventional energies. The electricity component contributed 69% during the zenith of the construction phase and 96% during the operating phase to the entire emissions of 1193 kt CO 2 -eq. Liu et al. 151 calculated the carbon emissions for three desalination plants used in the United Arab Emirates to produce one m 3 of clean water from seawater: ~13.7 t-CO 2 d −1 for multi-stage flash, ~0.72 t-CO 2 d −1 for multiple effect distillation, and ~1.46 t-CO 2 d −1 for reverse osmosis. Thus, the energy-efficient or energy-plus carbon-free SGE techniques can be integrated or hybridized for an effective energy-generating advanced treatment for brine effluent and treated wastewater effluent in the coastal nations 152 , 153 , 154 . PROs, REDs, and OPGs utilize the WWTPs’ treated effluent with freshwater/seawater, creating convenient conditions for blue energy production; some of the exemplary studies are listed in Table 2 . Ye et al. 155 proposed an electrochemistry-based charge-free mixing entropy battery (no membranes) which maintained 97% efficiency in capturing SGE for over 180 cycles. Fotiadou and Papagiannopoulos-Miaoulis 156 identified how blue energy introduction could function as a driving force for the Mediterranean Sea’s conceptualization as ‘marine space,’ acknowledged in the preamble of Directive 2014/89/EU of the European Parliament and the Council in 2014 for establishing a framework of Maritime Spatial Planning. The European Union launched the COASTENERGY project (2019–2021) 157 to adopt a participatory approach for gathering and involving Quadruple Helix actors in a multi-level network to develop a common roadmap and deploy coastal blue energy systems in pilot areas. Single-pore osmotic platforms, nanofluidic OPGs integrated with RED or PRO, can be the solutions for the urban water-energy nexus. As shown in Fig. 5 , the highest current and voltage density can be achieved by single-pore OPGs platforms. Blue energy can also restore unique marine ecological systems in coastal nations because it utilizes the highly concentrated brine from desalination plants (for example, creating a fish migration stream from salt to fresh water and vice-versa; SDG 6.6) 158 .

New paradigm for decentralized water-energy supply units

Decentralized water-energy supply facilities are standalone/hybridized facilities or can be integrated with centralized WWTPs, installed at/near the source of wastewater generation, and designed based on site-specific conditions. These decentralized WWTPs are smart and cost-effective alternatives because they can avoid large initial investments, operation, and maintenance costs, increase the potential for wastewater reuse, create jobs, promote business, use land and energy wisely, put less pressure on the natural water budget, consume less fertilizer, recover more nutrients and energy, and preserve green space. In fact, water providers and consumers play a vital role in achieving net-zero emissions in the water sector. For example, consumers in the United Kingdom’s housing sector saved 1.33 Mt CO 2 -eq yr −1 by reducing water consumption by 5–6% from 2017 to 2019 159 .

The energy and capital invested in sewage collection and transportation can be significantly decreased in decentralized systems. Applying natural treatment technologies increases up to 33% in decentralized facilities, implying simple operations with lower costs 160 can be implemented in the middle- and lower-income countries. The DEWATS initiative 161 aims for decentralized treatment and sanitation in developing countries, usually without technical energy inputs, thus providing more reliable operation and fewer effluent quality fluctuations. Tervahauta et al. 141 compared the decentralized and standardized collection systems and found the highest energy consumption of 914 MJ per capita per year within a centralized system; source separation into the urine, black- and gray water decreased the overall energy consumption to 208 MJ per capita per year for gravity-based systems and 190 MJ per capita per year for vacuum-based systems. Decentralization with a proper source separation stage is a long-term winning strategy for improving the energy efficiency of the urban water cycle; it offers closed loops of resource uses, which is in line with the circular economy principles. Kalehbasti et al. 162 tested a novel method to design and optimize the hourly demand and supply of integrated energy and water system in an urban district for environmental and economic sustainability. The model is tested on a sample neighborhood from San Francisco in California, with 21 building prototypes, 32 CHP engines, 16 chillers, and 3 wastewater treatment systems (one centralized and two decentralized membrane-based systems). The results indicated the normalized life-cycle cost, social cost of carbon, annual energy demand, and annual wastewater production of the integrated designs of the decentralized water-energy systems were 20, 75, 8, and 20%, respectively, lower than those of the centralized WWTPs.

Developed countries with high carbon footprints have already started progressing towards a low-carbon economy. For example, >50% of new capacity added to the grid in the US is carbon-free and is expected to shift to 90% carbon-free energy by 2035 163 , 164 . However, switching to a completely decarbonized energy system is still a big challenge for developing countries (economies in transition, least developed countries, and highly indebted poor countries), where water reuse is generally practiced out of necessity to avoid the carbon-intensive pathways while growing their economies, providing access to energy, and much more. This is strongly supported by a hypothesis of an inverted U-shaped relationship between economic output per capita and some measures of environmental quality known as the Environmental Kuznets Curve, “As GDP per capita rises, so does environmental degradation. However, beyond a certain point, increases in GDP per capita lead to reductions in environmental damage” 165 .

Decentralization can be one of the cost-effective solutions toward the net-zero goal and is also recommended for water reuse and improved energy efficiency by the USEPA 166 . There is a need to recognize the benefits of water reuse along with its safety; it can be accomplished by better regulation and the provision of incentives. Therefore, it is clear that with enough political will and the creation of adequate incentives for businesses and policymakers alike, sustainable and productive sanitation can be a major contributing factor to the achievement of greener economies, fostering job creation and poverty reduction along with the entire sanitation wastewater treatment and reuse chain. The link between water and city planning is clearly a major issue for the entire world, including the developing nations growing at an unprecedented rate and older or lower growth nation-states. Water cannot be the sole driver in urban planning, but it needs to be a large part of the equation to create sustainable cities. Often innovation takes place without the other two supporting legs, i.e., the need or future requirement that it might meet, and the capital - both human (knowledge and expertise) and financial. There is a mounting consensus that we should not be looking backward for solutions but be more innovative in delivering the outcomes required from our water-energy systems. Last, collaboration, coordination, and investment should be placed to develop and deploy an integrated suite of wastewater technologies to secure a water future for broad segments of society.

Data availability

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Acknowledgements

Sincere appreciation goes to the National Science and Technology Council (NSTC) of Taiwan (ROC) under Grant Number MOST 111–2636-M-002–026 for financial support. National Taiwan University also supports this study under Project Number 110-B-CD-5602–24910. The US Department of Energy supports S.W.S. through contract DE-AC07–05ID14517 (Idaho National Laboratory). H.K. was financially supported by the green venture research and development program (S3051540) funded by the Ministry of SMEs and Start-Ups (MSS, Korea).

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Rani, A., Snyder, S.W., Kim, H. et al. Pathways to a net-zero-carbon water sector through energy-extracting wastewater technologies. npj Clean Water 5 , 49 (2022). https://doi.org/10.1038/s41545-022-00197-8

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DOI : https://doi.org/10.1038/s41545-022-00197-8

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Integration of renewable energy in wastewater treatment during COVID-19 pandemic: Challenges, opportunities, and progressive research trends

Sasan zahmatkesh.

a Department of Chemical Engineering, University of Science and Technology of Mazandaran, P.O. Box 48518-78195, Behshahr, Iran

Kassian T.T. Amesho

b Institute of Environmental Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan

c The International University of Management, Centre for Environmental Studies, Main Campus, Dorado Park Ext 1, Windhoek, Namibia

Mika Sillanpaa

d Faculty of Science and Technology, School of Applied Physics, University Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia

e International Research Centre of Nanotechnology for Himalayan Sustainability (IRCNHS), Shoolini University, Solan 173212, Himachal Pradesh, India

f Department of Chemical Engineering, School of Mining, Metallurgy and Chemical Engineering, University of Johannesburg, P. O. Box 17011, Doornfontein 2028, South Africa

Chongqing Wang

g School of Chemical Engineering, Zhengzhou University, Zhengzhou 450001, China

SARS-CoV-2 has aroused drastic effects on the global economy and public health. In response to this, personal protective equipment, hand hygiene, and social distancing have been considered the most important ways to prevent the direct spread of the virus. SARS-CoV-2 would be possible survive in wastewater for a few days, leading to secondary transmission via contact with water and wastewater. Thus, the most economical and practical approaches for decentralized wastewater treatment are renewable energies such as the solar energy disinfestation process. However, as freshwater requirements increase and fossil fuels become unsustainable, renewable energy becomes more attractive for desalination applications. Solar photovoltaic, membrane-based, and electricity desalination technologies are becoming increasingly popular due to their lower energy requirements. Several aquatic environments could be benefitted from solar energy wastewater disinfection. Besides, utilizing solar energy during the day can inactivate SARS-CoV-2 to nearly 90%. However, conventional membrane-based desalination practices have also been integrated, including reverse osmosis (RO) and electrodialysis (ED). Several exciting membrane processes have been developed recently, including membrane distillation (MD), pressure-reduced osmosis (PRO), and reverse electrodialysis (RED). Such operations can produce clean and sustainable electricity from brine and impaired water, generally considered hazardous to the environment. As a result, neither PRO nor RED can produce electricity without mixing a high salinity solution (such as seawater or brine and wastewater, respectively) with a low salinity solution. Herein, we critically review the progress in applying renewable energy such as solar energy and geothermal energy for generating electricity from wastewater treatment and uniquely discuss the effects of these two types of renewable energy on SARS-CoV-2 in air and wastewater treatment. We also highlight the significant process made on the membrane processes utilizing renewable energy and research gaps from the standpoint of producing clean and sustainable energy. The significant points of this review are: (1) among various types of renewable energy, solar energy and geothermal energy have been predominantly studied for wastewater treatment, (2) effects of these two types of renewable energy on SARS-CoV-2 in air and wastewater treatment are critically analyzed, and (3) the knowledge gaps and anticipated future research outlook have been consequently proposed thereof.

1. Introduction

Several contaminants are common in wastewater systems, including urine and faeces ( Table 1 ) ( Foladori et al. 2020 ). Various pathways can lead to SARS-COV-2 RNA ( Figs. 1 and ​ and2 ) 2 ) getting into wastewater systems, emphasizing the virus's potential transmission path ( Ahmed et al., 2021 ). For example, hospitals and isolation centres can discharge wastewater containing SARS-COV-2 ( Zhang et al., 2020 ). A recent study reported that approximately 67% of stool samples from infected people were tested positive for SARS-COV-2 RNA. Also noted is that SARS-COV-2 RNA may still be detected in stool even after the respiratory infection has resolved; respiratory samples are negative ( Jones et al., 2020 ). Moreover, according to the Luo study, the gastrointestinal tract can replicate viruses ( Luo et al., 2021 ). As a result, contaminated wastewater may contain significant infectious viruses ( Crank et al., 2022 ). In addition, in low-income countries, wastewater can spread SARS-COV-2 if discharged directly into surface waters without proper treatment. Due to this, groundwater resources are not safe because they could also be contaminated with viruses from recharged groundwater ( Peccia et al., 2020 , Randazzo et al., 2020 ).

The detection of infectious SARS-CoV-2 in human feces and diarrhoea.

Network visualization of terms associated with wastewater treatment and SARS-CoV-2.

(a) Key mutation in the spike protein (b) Mechanism of coronavirus.

Increasing population and industrial development drive global energy demand to an all-time high. Over the past two generations, there have been significant increases in population, especially in developing countries ( Flow chart 1 ). A significant challenge of the 21st century is avoiding energy crises ( Brosemer et al., 2020 , Rosa et al., 2021 ). Due to a growing population, energy demand is increasing rapidly. In order to establish themselves in the world, different countries have different strategies, plans, policies, and control measures. The global population is growing, and resources are depleted ( Qazi et al., 2019 ). Therefore, consideration of energy sources plays a crucial role in satisfying the world's needs and population ( Shahsavari et al., 2018 ). Many factors affect the energy level available to people, including a country's development profile ( Neofytou et al., 2020 ), economic status ( Mofijur et al., 2021 ), and technological advancements within the nation ( Sütterlin et al., 2017 ). Effects on the ecosystem have been substantial due to the emission of various gases caused by the burning of fossil fuels, which are readily available and are frequently used to satisfy the world's energy demands ( Yoshida et al., 2019 ).

With the increasing demand for water, sustainable energy demand is essential to the sewage treatment process.

It is clear from the geographical location of ancient civilizations and cities that freshwater sources have been critical to the growth of civilization since ancient times ( Manuel et al., 2018 ). Water consumption has increased massively in the past few decades due mainly to improved living standards, increased population, and a highly industrialized economy ( Xie et al., 2018 ). Thus, the lack of fresh water has become a severe concern for many countries around the world. Also, there is a strong correlation between the amount and the development of civilization in water-stressed areas ( Kummu et al., 2011 ). According to UN-Water, more than half of the world's population will have no access to clean drinking water by 2025 (Almost one out of every ten people live without access to essential drinking water, and approximately 771 million people are without safe drinking water). In spite of the fact that water covers 75% of the earth's surface, the majority of the water contained in reservoirs cannot be used directly ( Tortajada et al., 2018 ). Most fresh water in the world comes from conventional sources, such as rivers ( Jackson et al., 2013 ), lakes ( Sinang et al., 2015 ), and groundwater ( Gude 2018 ). Therefore, drinking water, however, is primarily found in groundwater. Furthermore, freshwater resources in different parts of the world are not distributed proportionally to the population and water usage. There is no doubt that these statistics support the idea of obtaining portable water from non-conventional sources, such as the sea, rivers, and lakes located in water-stressed areas ( Rozemeijer et al., 2021 ).

Currently, replacing fossil fuels with sustainable energy is high on the international political agenda for climate protection due to the energy turn ( Kåberger 2018 ). This issue is addressed by several European initiatives and strategies that will help shape the energy industry's future ( Jensen et al., 2018 ). A temperature rises of 2°C or less is the goal of the 2020 Sustainable Development Goals. There now are three additional targets added to the 2030 climate & energy framework: a minimum 40% reduction in greenhouse gas emissions and 27% renewable energy usage by 2030 and improvement in energy efficiency by 27% ( Knopf et al., 2015 ). Research into low-carbon technologies and their development is established in the Strategic Energy Technology Plan. Globally, Sustainable Development Goals focus, among other things, on ensuring everyone has access to energy that is affordable, reliable, sustainable, and modern ( Fig. 3 ) ( Amesho 2019 ). Since it is regarded as a resource accessible to every household without interruption, wastewater attracts attention in this context ( Kollmann et al., 2017 ).

An increase in the use of sustainable energy in wastewater treatment around the world.

Nevertheless, Solar energy ( Pandey et al., 2021a , b ), wind energy ( Shoaib et al., 2019 ), hydropower ( Kougias et al., 2019 ), and geothermal ( Bayer et al., 2019 ) are all eco-friendly energy sources ( Fig. 4 ). For many reasons, solar energy may prove to be the best choice for the future: The first demonstration of solar energy's abundance is that the sun emits about 3.8 1023 kW of solar energy daily, out of which the earth intercepts about 1.8 1014 kW. Besides light and heat, solar energy is received by the Earth in different forms. The bulk of this energy is absorbed, scattered, and reflected by clouds as it travels. Several studies revealed that solar energy could fulfill most of the world's energy needs because it is abundant in nature and free to use. As a second reason for its promise, it is a source of energy that can be used indefinitely with stable and progressively higher output efficiencies than any alternative energy source ( Fig. 5 ) ( Elsheikh et al., 2019 ).

Network visualization of terms associated with renewable energy.

Electricity produced from renewable sources worldwide by 2020. Renewable energy sources include hydropower, solar energy, wind energy, biomass. Other sources include geothermal, wave, tidal, and waste.

However, wastewater treatment plants (WWTPs) are primarily designed to remove undissolved and dissolved matters from wastewater (cooking fats, oils, road grit, and nutrients) ( Tian et al., 2021 ). In this way, WWTPs play a crucial role in the control of water pollution as well as sanitary engineering. Increasingly, wastewater professionals are becoming interested in the additional energy generation potential of WWTPs beyond that of on-site digester gas combustion or cogeneration ( Capodaglio et al., 2020 ). The concept of wastewater as an energy source must be reconsidered, such as utilizing digested sewage sludge for incineration and electricity generation can provide a significant amount of energy recovery ( Wang et al., 2021 ). Furthermore, on-site energy generation at WWTPs could allow nutrient recycling from wastewater and reuse of treated wastewater for irrigation and industrial processes ( Marangon et al., 2020 ). Thus, wastewater-related research focuses on wastewater treatment plants operating as control components in energy distribution systems, wastewater treatment plants utilized as energy storage systems, and metallurgical phosphorus recycling to turn wastewater sludge into energy, fertilizer, and iron ( Cudjoe et al., 2020 ).

Although recent research initiatives and publications related to WWTPs have made significant contributions in energy generation from wastewater, the research could focus more on electrical optimization and self-sufficiency ( Yan et al., 2020 )). The issue of thermal energy seems to be playing only a minor role ( Yang et al., 2017 ). It is estimated that Austrian WWTPs using anaerobic digestion may reach high levels of electric self-sufficiency with optimal wastewater treatment and cogeneration ( Gandiglio et al., 2017 ). On the other hand, WWTPs using anaerobic digestion have lower chances of achieving thermal self-sufficiency than biogas combustion and heat recovery ( Duarte et al., 2018 ). Technologically, there are three primary approaches for generating or recovering heat at wastewater treatment plants: (a) combustion of digester gas via cogeneration, application of in-sewer heat exchangers ( Nourin et al., 2021 ), (b) external heat pumps for wastewater heat recovery ( Reiners et al., 2021 ), and (c) use of solar thermal generation ( Verma et al., 2019 ). In addition to sewage sludge incineration, heat can also be generated ( Tarpani et al., 2018 ). However, in recent decades, impressive technological advancements have led to the introduction of new chemicals, materials, and processes involving a variety of complexities, leading to increased releases of pollutants into the environment, resulting in a requirement for the efficient removal of these pollutants ( Levine et al., 2004 ). Besides, numerous studies have been conducted on various aspects of wastewater treatment technologies, concluding that various techniques have been developed to remove pollutants as well as treat them ( Sonune et al., 2004 ). Sedimentation ( Song et al., 2000 ), flotation ( Rubio et al., 2002 ), filtration ( Hube et al., 2020 ), coagulation ( Lee et al., 2012 ), and flocculation ( Lee et al., 2014 ) are conventional methods of removing solid particles from wastewater. Advanced oxidation processes (( Miklos et al., 2018 )), adsorption ( Zahmatkesh et al., 2020 ), and membrane processes ( Asif et al., 2021 ) are more appropriate for removing organic and inorganic compounds from biological treatment than conventional treatment. Despite the effectiveness of water treatment methods used today, they have some drawbacks, such as those that require high levels of energy, cause fouling, and generate many byproducts.

Furthermore, using electrochemical methods in water treatment has advanced to new levels. Electrochemical treatment is characterized by a high removal efficiency, clean energy conversion, pollution avoidance because no emissions are generated, and straightforward operation ( Guo et al., 2022 ). Several electrochemical processes are discussed, including electro-coagulation ( Kumari et al., 2021 ), electro-flotation ( Akarsu et al., 2021 ), electro-oxidation ( Guo et al., 2022 ), electro-disinfection ( Rivas et al., 2019 ), and electro-reduction ( Koparal et al., 2002 ). The electro-coagulation process occurs when metal ions are released into a solution by an anode made of aluminum or iron. According to much research, the use of the alkaline solution in the removal of chemical oxygen demand (COD) is preferable to a neutral or weak acidic solution in aluminum anodes ( Wang et al., 2013 , Liao et al., 2014 ). In addition to its advantages, electrocoagulation generates reactive coagulant in-situ and features compact equipment setup ( Butler et al., 2011 ). There are many ways to use electroflotation, such as recovering fine mineral particles, de-inking recycled paper, and separating oil from water ( Wang et al., 2009 ). Anodic oxidation can be classified into direct and indirect ( Panizza et al., 2006 ). The hydroxyl radicals (·OH) or active oxygen (MOx+1) absorbed by the anode surface directly oxidize pollutants in the direct electrooxidation ( Linares-Hernández et al., 2010 ). The process of indirect anodic oxidation commonly uses chlorine, hypochlorite, Fenton's reagent, peroxodisulphate, and ozone as oxidizing agents ( Martinez-Huitle et al., 2006 ). Electro-reduction is also categorized into direct and indirect reduction, as with electrooxidation ( Yao et al., 2019 ). Nitrogen gas is formed from reacting electrons from the cathode surface with nitrate compound, while hydroxyl ions are formed as a byproduct. It is known as a direct reduction process. Electro-reduction is commonly used to treat dye vat wastewater ( Roessler et al., 2003 , Yao et al., 2019 ).

By converting membranes into a renewable energy source, desalination can be made more environmentally friendly and sustainable. Membrane operations focus primarily on reverse osmosis (RO) and electrodialysis (ED) as renewable desalination methods. Despite this, some exciting membrane processes emerge with a potentially significant impact on desalination. In addition to membrane distillation (MD) and forward osmosis (FO), there are new processes called renewable energy desalination (RED) and Pressure Retarded Osmosis (PRO). Using these methods can harness/ produce renewable energy and solve issues associated with conventional desalination ( Ali et al., 2018 ).

The purpose of this article is to review the aforementioned promising approach of using renewable energy to generate electricity in wastewater treatment as well as the critical technology during SARS-CoV-2. Furthermore, describe the effect of solar and geothermal energy on wastewater treatment during SARS-CoV-2. finally, challenges to and future needs in the use of solar energy for treating wastewater containing SARS-CoV-2

1.1. Routes of SARS-CoV-2 RNA in the aquatic systems

Several channels for transmission of SARS-CoV-1 are identified in apartment buildings with wastewater plumbing systems. SARS-CoV-2 can be transmitted through aerosols or drops of water, much like the SARS-CoV-1 virus ( Gormley et al., 2017 ). It has been reported that the SARS-CoV-1 and SARS-CoV-2 viruses have similar stability in aerosols and on surfaces ( Leung et al., 2020 ). It is possible to remain infectious for several days on surfaces as well as in aerosols if the inoculum is shed ( Van Doremalen et al., 2020 ). Similarly, Ong et al., (2020) investigated SARS-CoV-2 survival in air, surfaces, and personal protective equipment of healthcare workers and disease carriers ( Ong et al., 2020 ). SARS-CoV-2 can be transmitted by stool samples obtained from air outlet fans, door handles, sinks, and toilet bowls, which proves that SARS-CoV-2 can be spread through stools. The following percentage of samples that tested positive for SARS-CoV-2 were collected by Hu et al., (2020) from 23 high-touch surfaces of a quarantine room: 70% (in the bedroom), > 50% (in the bathroom), > 33% (in the corridor) ( Hu et al., 2020 ). In addition, the toilet bowl and the sewer inlet were among the areas with the highest levels of viral contamination in the room. As a result of this transmission pathway, the sanitary plumbing (or wastewater) system may be responsible for contaminating the surrounding environment and spreading the COVID-19 virus to the nearby cities. Accordingly, Gormley et al., (2020) provided recommendations recently to minimize the transmission of pollutants through the wastewater plumbing system. Fig. 2 summarizes some valuable suggestions to avoid the risk of spreading the pathogen through the wastewater plumbing system in the buildings ( Gormley et al., 2020 ).

Diverse ways are available for SARS-CoV-2 RNA to spread through aquatic environments ( Fig. 6 ), which may lead to the transmission of COVID-19 ( Adelodun et al., 2020 , Carducci et al., 2020 ) These routes consist of wastewater from hospitals, isolation wards, and quarantine stations ( Wang et al., 2020 ). It has been confirmed that the discharged contamination is transmitted via contamination of water bodies ( Prüss-Ustün et al., 2019 ). Hence, water sources can become polluted in several different types of ways. Surface waters (streams and lakes) where wastewater is frequently discharged directly without appropriate treatment could be a prospective vehicle for SARS-CoV-2 to spread via waterways to different parts of communities that depend on these water sources for low-income nations daily requirements.

Sources and routes of SARS-CoV-2 in aquatic systems ( Adelodun et al., 2020 ).

Furthermore, groundwater sources could also become contaminated with disease-causing viruses during the replenishment process because they could be replenished with contaminated water. Hospital waste can also cause disease transmission if disposed of into aquatic bodies without appropriate treatment ( Adelodun et al., 2020 ) underlined a number of studies in which peppermint virus and surface and groundwater have been reported to contain other human enteric viruses-protecting routes through the aquatic system to prevent SARS-CoV-2 and other pathogens from unintentionally reaching these water sources. There is a higher risk of spreading these viruses and pathogens in areas where the water supply is inadequate. Workers involved in wastewater treatment are also at risk of infection ( Silva et al., 2020 ). Nevertheless, the half-life of SARS-CoV-2 in wastewater has been reported to be highly dependent on temperature ( Hart et al., 2020 ), UV ozone ( Yao et al., 2020 ), and chlorine-based disinfectants ( Zhang et al., 2020 ). The half-life of SARSCoV2 in hospital effluents has been estimated to range from 4.8 to 7.2 h at 20°C, and the nucleotide sequence agreement and spike glycoprotein similarity between SARS-CoV-2 variations is 99.9% ( Hart et al., 2020 ).

2. Renewable energy

Since 2000, wastewater treatment systems powered by renewable energy have drawn significant attention ( Fig. 7 ) ( Yang et al., 2021 ). Due to their ability to reduce carbon dioxide emissions, these systems can also provide sanitation and reuse of water in remote and isolated areas. In addition, renewable energy (solar, geothermal, wind, and tidal/wave) has become increasingly affordable in recent decades ( Fig. 8 ), which has led to its widespread use and application in wastewater treatment facilities ( Kollmann et al., 2017 ). In the event that energy grids are down after a natural disaster, a wastewater treatment system powered by renewable energy might be the best option ( Ali et al., 2020 ). However, water, resources, and potential energy can all be recovered by membrane processes in wastewater treatment. There are several different types of membrane processes depending on how they are driven ( Zhang et al., 2019 ): First, the filtration process can be pressure-based (microfiltration, ultrafiltration, nanofiltration, and reverse osmosis). Secondly, electricity is required for electrodialysis, and other electro-membrane processes and a gradient of concentration (forward osmosis) is essential. Finally, using membrane distillation and membrane evaporation to produce a thermal gradient.

Rate of growth of renewable energy in wastewater treatment worldwide.

There is a wide observation that renewable energy sources are widely used today ( Ghandriz et al., 2021 ).

For wastewater treatment and reuse, renewable energy can be utilized in various ways, such as electricity generation, heat generation (thermal gradient), wind flow (for evaporation), and concentration gradient generation (( Charcosset 2009 )).

2.1. Using renewable energy to generate electricity in wastewater treatment

Renewable Energy Sources are commonly recognized as beneficial in increasing the sustainability of energy use at WWTPs and reducing the cost of energy supply ( Fig. 9 ). Using on-site renewable energy sources, such as wind, solar, water, and waste generated on-site, can effectively reduce the energy supply's economic costs ( Mook et al., 2014 ). Eco-friendly technologies like bioenergy must be added to WWTPs in order to improve environmental efficiency.

Potential utilization of Renewable Energy for wastewater treatment and desalination purposes.

Solar photovoltaic (PV) systems have gained popularity due to converting sunlight into electric currents without implying any environmental harm ( Makrides et al., 2010 ). This source has several uses, including the water pump ( Meah et al., 2008 ), lamp ( Enaganti et al., 2020 ), chargers of batteries ( Masoum et al., 2004 ), and supply of electric utility grids ( Bhandari et al., 2014 ). Since the mid-1990s, the production of PV modules has risen dramatically at an astounding rate, indicating PV systems' great potential for the present and future ( Parida et al., 2011 ). Previous tests have demonstrated that the PV system is capable of producing power regardless of the weather. In partial cloudy conditions, PV is capable of even generating 80% of their potential energy ( Pali and Vadhera, 2020 ), while in hazy or humid conditions, they can generate 50%; and in heavily overcast conditions, they can still generate 30% ( Ishii et al., 2013 ).

Typically, two types of PV systems exist stand-alone systems and are connected to the grid. Stand-alone PV systems require batteries to operate, and such systems can be installed in remote areas. In contrast, grid-connected PV systems use the local power grid and produce solar electricity distributed through an independent power provider. For public sector installations of PV systems, a variety of financial incentives are available in an effort to promote the use of renewable energy sources and boost environmental awareness ( Singh 2013 , Sobri et al., 2018 ).

A few researchers have examined PV systems as power sources in electrochemical systems ( Table 2 ). Some have used textile wastewater as an electrolyte solution to reduce organic compounds (such as BOD, COD and so on) because organic compounds are incapable of breaking down during biological treatment. Textile wastewater contains azo dyes and their intermediates, which can be carcinogenic, toxic, and mutagenic. Hybrid PV is an electrochemical process that is able to remove organic compounds while also generating hydrogen gas at the same time ( Ganiyu et al., 2019 , Strazzabosco et al., 2019 ).

Contaminants removed by electro-coagulation and electro-flotation during wastewater treatment.

PV systems typically include PV modules, batteries, a controller, and an inverter. It is converted into electric energy (direct current) upon absorbing solar radiation on the PV module ( Table 3 ). After the electric energy has been produced, the power is sent to the batteries through a regulator. In the case of batteries, the regulator prevents them from being overcharged or discharged excessively. Backup electricity is generated using stored solar energy at night and during periods of low solar radiation ( Sahu et al., 2016 ). During low solar radiation periods, ( Dominguez‐Ramos et al., 2010 ) reported that a PV setup without batteries is inefficient to treat water, due to the inadequate backup power supply. Furthermore, the treatment procedure can only be performed during the day without a battery. Among other things, an inverter converts direct current (DC) into alternating current (AC) for devices that need AC power.

Applications of various technologies for Renewable Energy in WWTPs.

2.2. Membrane processes utilizing renewable energy

It is practical to use renewable energies for membrane processes either directly (for example, to draw energy from the wind to evaporate membranes) or indirectly (for instance, to drive reverse osmosis using the electricity generated by the wind turbine). In general, the direct use of renewable energy is more energy-efficient because energy conversion involves energy loss. However, the process and application restrict the necessary energy form ( Charcosset 2009 ). Typically, membrane processes are driven by electricity in order to treat wastewater. Membrane bioreactors (MBRs) are increasingly popular used renewable energy-driven membrane systems. Since numerous examples of photovoltaic and wind-powered MBRs have been installed worldwide, there have been no technology barriers to their implementation ( Zhang et al., 2019 ). Atlantic City was the first to build a WWTP powered by solar and wind, including GE Zenon's membrane bioreactor ( Sutherland 2007 ). Microfiltration (MF) or ultrafiltration (UF) are typically applied in MBR technology, with pore sizes ranging from 0.4 to 0.02 μm (( Li et al., 2017 )). Despite the fact that the application does not provide any information on the scale of energy consumption related to municipal MBRs, which Krzeminski and colleagues report in their study as 0.5-0.7 kWh/m 3 ( Krzeminski et al., 2012 ). Therefore, one solar panel should produce approximately 0.7 kWh/m 2 of electricity to treat one cubic meter of wastewater by MBR during the day in light of the PV pane's electricity generation intensity of 0.7 kWh/m 2 ( Björklund et al., 2001 ). Due to its ability to filter via ultrafiltration membranes, MBR effluent is capable of filtering particles, suspended solids, bacteria, and dissolved compounds; therefore, it can meet the standards for direct discharge or surface water recharge ( Wei et al., 2014 ). Nanofiltration or reverse osmosis (RO) is one way to remove organic compounds and salts from water (i.e., to produce industrial water or indirect potable water). It typically takes 0.77 kWh of energy to refine one cubic foot of wastewater using NF or low-pressure RO ( Obotey Ezugbe et al., 2020 ).

UF, NF, and RO membrane processes (driven by pressure) are not the only way to reclaim domestic wastewater ( Table 4 ). In Forward osmosis (FO), a dense membrane produces gradients of concentration to drive the membrane, ensuring that almost all wastewater pollutants are retained. The desalination concentration or thermal evaporation method can be applied to recover the diluted draw solution used in the FO process. For instance, the thermal evaporation method (membrane distillation or MD) uses solar energy for power, while desalination-concentration (RO or electrodialysis or ED) uses electricity from photovoltaics (PVs) or a wind turbine ( Qtaishat et al., 2013 , Obotey Ezugbe et al., 2020 ).

Evaluation of each membrane used in wastewater treatment.

Recent years have seen a focus on resource recovery from wastewater. A bio-electrochemical cell can generate electricity using methane from the anaerobic degradation of nutrients (P, N, K). Membranes can recover nutrient elements. Recent studies have focused on phosphate recovery. Zhang et al.2009 have recovered phosphate using an electrodialysis stack explicitly designed for phosphate recovery (namely "electrodialysis," SED).

Due to the ability of ion exchange membranes to separate nutrients using electricity, they can be used to drive this process for resource recovery using renewable energy ( Liu et al., 2017 ). As with a conventional fuel cell, microbial fuel cells (MFC) rely on an ion-exchange membrane ( Asensio et al., 2018 ). In a microbial fuel cell, mixed and pure cultures of cellulose-degrading bacteria generate power from cellulose. The process is beneficial in two ways: organic pollutants are degraded, and bioelectricity is produced; simultaneously, using electrochemistry, wastewater is treated for removing and recovering nutrients ions. ( Zhang et al., 2014 ) developed a bio-electrochemical system to recover phosphate. Bio-electrochemical systems consist of electrodes and ion-exchange membranes that remove phosphate and ammonia using an electric field generated by bioanodes and hollow cathodes. Bioelectrochemical systems have been investigated for wastewater resource recovery ( Bajracharya et al., 2016 ). It has shown that the system involves both electricity generation and separation, which is complex and causes the investigation to become increasingly complicated due to the interdependence of these two processes. On the other hand, in comparison with chemical catalysis, BES is capable of producing high-value chemicals at a lower cost using an inexpensive catalyst (protein catalysis).

Membrane filtration systems such as MF and UF, which are used in advanced wastewater treatment, have the potential to block the transmission of SARS-CoV-2 effectively. Furthermore, a modular membrane system structure can help eliminate SARS-CoV-2 from effluent using current WWTPs. The effectiveness of MF > 50 nm and UF 2–50 nm membranes at removing SARS-CoV-2 depends on the distribution of pore diameters in relation to the target virus. Thus, UF membranes can effectively remove SARS-CoV-2 with a diameter of 10 to 100 nm under certain conditions ( Kitajima et al., 2020 ). In addition, SARS-CoVs can be eliminated based on membrane surface characteristics based on electrostatic and hydrophobic interactions. An MBR can use ultrafiltration to enhance viral removal (not just for SARS-CoV) and steric removal, adsorption, and inactivation during biological treatment. Due to this, MBRs have shown to be more effective at removing enteric viruses (removing up to 6.8 logs) than conventional WWTPs (removing up to 3.6 logs). SARS-CoVs could also be entirely removed by high-pressure membrane systems using tighter and denser membranes (pore sizes <2 nm) such as NF and RO ( Lv et al., 2006 ).

2.3. Solar energy

There are many renewable energy sources worldwide, but solar energy is the most abundant. Some studies indicate that solar power probably can meet the entire world's energy needs from just 1% of the arid and semi-arid areas. According to reported data, many the Middle East and North Africa areas receive solar insolation of 5-7 kW h per solar day ( Fig. 10 ). They are generally characterized by abundant brackish or seawater but lack sufficient freshwater, so solar energy is ideal for desalination ( Yang et al., 2018 ). Desalination of saline water can be conducted directly by using solar energy or converting it into electricity. The first type mainly includes solar stills, humidification devices with dehumidification, solar chimneys, etc., while the second type primarily uses photoelectric modules ( Kasaeian et al., 2019 ). Solar power is converted into electricity with photovoltaic (PV) cells ( Singh 2013 ). In order to improve the efficiency of these cells, solar energy can be concentrated primarily. Although PV cells are attractive in cost-effectiveness, overall lifecycle, and energy storage, they are not yet practicable. It is possible to use PV thermal to produce thermal energy and electric power simultaneously. Solar energy is used nowadays to generate steam to run desalination units manually. Renewable desalination has gained high popularity because of solar energy's abundant availability and the fact that it can be converted to either electric or thermal power ( Table 5 ) ( Wang, 2010 ).

Solar power generation (worldwide), 2020. The amount of electricity generated by solar is measured as terawatt-hours (TWh) per year. Source: Our World in Data based on BP Statistical Review of World Energy & Ember, BP Statistical Review of World Energy: https://www.bp.com/en/global/corporate/energy-economics/statistical-review-of-world-energy.html– Ember: https://ember-climate.org/data/ .

Pros and cons of solar and geothermal energy.

2.4. Solar energy in wastewater treatment

As one of the renewable energy sources, solar energy is most widely used in order to reduce sludge water content. Despite the low costs and little maintenance, open solar drying beds were still used ( Zhang et al., 2018 ). Currently, greenhouse dryers are being used since they are more energy-efficient and aid in reducing pathogens and dry matter content. In greenhouse dryers, the air around the sludge or the bottom of the greenhouse can be heated to increase drying efficiency ( Abdel-Ghany et al., 2011 , Prakash et al., 2014 ). Supplemental heat can be generated through solar energy, such as through hot water pipes inside the greenhouse, a solar water heater, or a heat pump that uses treated wastewater as a heat source. In addition to integrating solar energy with other energy sources, there are also proposals to improve thermal drying. In order to dry sludge and conduct anaerobic digestion, biogas is generated by anaerobic digestion of sludge coupled with a parabolic trough collector fueled by biogas. CHP has provided enough electric energy for WWTP to meet its demand ( Bennamoun 2012 , Shao et al., 2015 ).

The demand for freshwater has risen exponentially in recent years. Globally, freshwater is of great importance. However, there are many ways the environment and humans misuse water, including wasting it on agriculture. In addition, manufacturing has become increasingly dependent on water in recent years. As a result, water is becoming scarce. In particular, this technique has been developed to treat non-biodegradable or regenerative wastewaters that cannot be successfully treated using the industry's conventional biological or medicinal treatment processes. A heterogeneous photocatalytic method was used to oxidize untreated effluent from the pharmaceutical industry to improve the efficiency of impurity oxidation and preserve economics ( Pandey et al., 2021a , b ). Typically, raw materials used for treating wastewater include ozone, chlorine, hydrogen peroxide, and ferrous iron (Fenton's reagent) and hydrogen peroxide ( Kannan et al., 2016 ).

Chemical pollutants such as persistent organic pollutants are found in industries, household wastewater effluents, and water leachate from wastewater ( Loganathan et al., 2020 ). These filters need to be replaced to ensure that our water sources remain clean. More and more techniques were implemented throughout the year and used to kill the toxins. Researchers have studied photocatalytic detoxification as an alternative method for treating polluted water since 1976. Based on quantum mechanics, photocatalysts have a well-defined energy level structure ( Tsoutsos et al., 2005 ).

Wastewater from industrial brines can now be treated with a solar-powered system. Wastewater from industrial brines can now be treated with a solar-powered system. An integrated system for treating industrial brine wastewater using solar energy has been developed, producing valuable benefits for the community. Water treatment techniques based on membranes and evaporation were integrated into the developed system. Various methods of evaporation, such as falling film and forced convection, were employed. Thermodynamically, this system is analyzed under varied operating conditions using energy and exergy approaches ( Sansaniwal 2019 , Li et al., 2021 ).

Solar system generation could be applied to future desalination ( Sharon et al., 2015 ), sterilization ( Li et al., 2018 ), and chemical purification ( Yang et al., 2018 ). Since photon control and thermal insulation have developed accelerating, solar stems are being manufactured by minimizing radiation, convection, and convective losses while improving light transmission. As a result of studying the standard transpiration mechanism in plants, researchers have proposed creating a 3D artificial transpiration system with all components, which minimized heat loss and relied predominantly on angular light absorption, producing maximum solar system performance under a single sun. As well as generating purified water from polluted heavy metal ions, the artificial transpiration process results in a low carbon footprint, recycling of heavy metals, and recycled heavy metals. Various technical solutions based on solar energy were presented, focusing on recent developments in energy recovery technology ( Marcelino et al., 2015 , Borges et al., 2016 , Pandey et al., 2021a , b ).

2.5. Effect of solar energy on SARS-CoV-2 in air and wastewater treatment

Using solar energy for air purification is becoming more common in recent years since it provides excellent UV and other thermal radiation sources that kill microorganisms and control pollution. There are several publications describing photocatalyst solar systems for air and water purification. A solar-energy-based photocatalyst degradation of formaldehyde (HCHO) was investigated by ( Litter et al., 2020 ). A TiO 2 sol-gel film covered the inner surface of the borosilicate glass reactor. TiO 2 was used as a light source, which provided a 5-Log reduction in HCHO in about 60 min when using solar UV-A with 0.6 mW/cm 2 . According to ( Ma et al., 2019 ) air purification is required in houses, cars, offices, and other occupied spaces. HCHO is a major carcinogenic component of indoor air pollution, primarily composed of volatile organic compounds (VOCs). When the appropriate temperature is reached, HCHO can be oxidized to CO2 and H 2 O by TCR using transition metal oxides and noble metals found in many building materials. In order to use solar energy as a heating source, Sun et al. (2020) investigated the TC oxidation reaction using MnOx-CeO 2 . An experiment was conducted to study the effects of initial HCHO concentration, indoor air temperatures, solar irradiation, and ambient temperatures on the oxidation process. According to the study, the initial HCHO concentration was vital compared to solar irradiation or ambient temperature. Using solar energy made it possible to significantly reduce oxidation times, significantly reducing the threshold limit for the Chinese Indoor Air Quality (IAQ) standards ( Wang et al., 2004 ).

Disinfestation methods based on solar energy lower the risk of transmission from high concentration sites, particularly in tropical areas with an abundance of solar energy. These methods are the most economical and practical for hospitals, isolation wards, and medical centers. However, solar energy wastewater disinfection in several aquatic environments is feasible and widely applicable. Water disinfection by solar energy is a sustainable approach that is widely endorsed for disinfecting water. In addition, there are primarily three factors that determine the impact of solar radiation: solar intensity, physical and chemical properties of wastewater, and type of virus. Solar radiation is higher than 2000 kWh/m 2 /y per year, making solar energy an available energy source. Several mechanisms can be used to disinfect wastewater. For instance, the direct mechanism involves photon absorption directly by viruses or endogenous components like proteins, nucleic acids, and other biomolecules. Thus, one advantage of UV treatment for water treatment is that it is an effective disinfectant, as it is able to kill most waterborne pathogens, as well as a few pollutants that are relatively resistant to the treatment. Under UV light, phosphodiester bonds and links with molecules are broken, which leads to a breakdown of the viral genome and proteins. Accordingly, the viral particles lose their ability to infect and replicate.

Furthermore, UV light is one of the most effective methods for disinfecting biologically contaminated water. Compared to other methods like chlorination, it does not generate harmful by-products and controls the growth of microorganisms. According to their wavelengths, UV types include UVA (315-400 nm), UVB (280-315 nm), and UVC (100-280 nm), abbreviated as UVA, UVB, and UVC (UVA photons are low-energy, while UVC shows that photons are powerful enough to damage the DNA of pathogens). However, Inactivating viruses occur through UV-B (280–315 nm) radiation; and hence, they absorb the UV-B radiation. Viruses cannot be effectively disinfected by solar radiation because most UVB wavelengths cannot reach the earth's surface; it may exacerbate the problem when UV intensity decreases. Under 550 W/m 2 at 45°C in the solar water disinfection simulation, coxsackievirus was wholly inactivated after two hours ( Heaselgrave et al., 2011 ), but it could not be entirely inactivated by exposing it to 75 W/m 2 for 6 h at a maximum of 34°C ( Alotaibi et al., 2011 ). In a recent study, ( Nicastro et al., 2020 ) investigated how UV-B in sunlight inactivates viruses in various populated cities worldwide. The result showed that the COVID-19 virus inactivated in the summer comparatively more rapidly, indicating that solar radiation has a vital role in its occurrence and spread. Finally, according to several reports, more than 90% of COVID-19 viruses in world cities were inactivated by exposure to mid-day solar radiation after 11-34 min ( Fig. 11 ).

SARS-CoV-2 can be removed effectively from wastewater via solar energy2.6 Geothermal energy.

An example of geothermal energy is the provision of steam and hot water from the Earth and then used to generate electricity. Geothermal energy is suitable for many applications due to the wide range of Earth's temperatures ( Fig. 12 ). In order to use geothermal energy ( Table 5 ), either vaporize liquids with lower boiling points so that steam can be manufactured or, based on the quality of the geothermal energy, heat a turbine directly. Geothermal energy technology has been proven for electricity production, although it is not widely used. Currently, geothermal energy serves more than 60 million people in 24 countries worldwide, providing approximately 10,000 megawatts of energy ( Asif et al., 2007 ). A significant advantage of geothermal energy is that it can be used directly for desalination, even though its net installed capacity is less than wind power. A geothermal well can be used for desalination if the depth exceeds 100 meters ( Goosen et al., 2010 ). Geothermal energy is an excellent energy source for areas with excellent resources for water. A high-pressure geothermal source can be used straight up for mechanically driven desalination, but high-temperature geothermal fluid can generate electricity to power RO or ED plants. Water's thermal desalination can be performed using geothermal heat from a low-temperature energy source. In comparison with other renewable energies, geothermal energy provides uninterrupted thermal power.

Installed geothermal energy capacity (worldwide), 2020. The cumulative installed capacity of geothermal energy, is measured in megawatts. Source: Statistical Review of World Energy - BP (2021), OurWorldInData.org/renewable-energy • CC BY, Link: https://www.bp.com/en/global/corporate/energy-economics/statistical-review-of-world-energy.html .

In comparison to other renewable energy sources, geothermal energy is unlike most other RES. It is not affected by intermittency; it is a suitable source of electricity due to its reliability and continued availability. Furthermore, the process is relatively standard on islands, usually located at the juncture of two plates or originate from volcanic activity, making them very promising. During the geothermal process, heat is extracted from porous or fissured rocks within Earth's crust which contain hot water or steam. Applications depend on the temperature of geo-fluids. For electricity generation, high-temperature fluids are used, such as conventional (150-350°C) or binary (90-200°C) power plants, while low- and medium-temperature fluids are used for industrial, agricultural, district heating, or space heating. In the future, binary geothermal power plants will likely become more popular due to the abundance of low-temperature geothermal energy sources and geo pressured ( Di Fraia et al., 2019 , Di Fraia et al., 2020 ).

2.7. Effect of geothermal energy on SARS-CoV-2 in wastewater treatment

Thermo-catalytic Inactivation (TCI) has been proposed to disinfect water against bacteria and viruses. When transition metal oxides such as MnO2, CuO, and TiO2 are used as thermal catalyst in Thermo-catalytic Reactors (TCR), a temperature range of 40-80°C is required ( Chen et al., 2018 ). Even at room temperature, disinfection can be accomplished using noble metals such as Pt, Pd, Au, and Ag for TCR. As a result, higher temperatures adversely affect SARS-CoV-2 and its conservative surrogates ( Guo et al., 2021 ). Much research showed that the temperature range for SARS-CoV-2 disinfection is the same as that suggested for TCR reactions for water purification. Researchers pursuing thermal inactivation to combat SARS-CoV-2 are in the early stages of this research ( Harussani et al., 2021 ). Under various environmental conditions, the TCI of a surrogate for human norovirus, a virus, takes 40 days to remain on diaper or feces for viral survival. Increase the temperature from 18°C to 30°C without any catalyst, and the inactivation time can be shortened to even one hour at 56°C, reducing the time from 40 days to 24 days. In addition, the virus appears to be susceptible to freezing and thawing but remains stable at room temperature ( Parsa et al., 2021 ).

3. Key challenge and future research needs in the use of solar energy to treat

All types of waste management are energy-intensive, which is challenging to achieve during the global energy crisis. Plentiful solar energy is proactively used to dispose of solid and liquid waste ( Pandey et al., 2021a , b ). Technologies such as solar pathogenic organic destruction, solar photocatalytic degradation, solar thermal desalination, and distillation are used to treat liquid waste ( Ugwuishiwu et al., 2016 ). Despite the benefits of integers in each method, specific problems need to be addressed. Fig. 13 illustrates the significant challenges associated with solar wastewater treatment.

• • Discarding waste after wastewater treatment has a negative impact on the environment. As a result, recycling technology must be used to treat waste residues, which increases overall cost-effectiveness.
• • Large-scale treatment facilities do not yet exist because of high capital costs. This is very relevant to the study for further analysis of the model before building any wastewater treatment plant.
• • The downside of solar energy is that wastewater is treated only during the day, while wastewater treatment plants are ineffective at night. It is more expensive to install energy storage systems in wastewater treatment plants.
• • Wastewater treatment plants are required to comply with Environmental Impact Assessment (EIA) regulations and therefore vary by location. As a result, firms lack skilled workers to boost productivity ( Sansaniwal, 2019 ).
• • Due to the lack of industrialization of laboratory research, scientific innovation in solar wastewater treatment is lagging.
• • Solar desalination is a reliable way to disinfect brackish water into drinking water. Even though the system is straightforward, the solar evaporation process must solve various problems, such as lacking a cost-effectively operational feasible application component.
• • Removing heavy metals from liquid waste involves further technological development to improve efficiency.
• • For wastewater treatment plants that use solar energy, water must also be treated for biological decontamination and to remove offensive odors.
• • As SARS-CoV-2 is very sensitive to temperature, it can survive within its environment for up to two days at low temperatures (4°C), room temperatures (20–25°C), and hot temperatures (33–37°C), but not at 56°C or 70°C where it lasts for less than an hour (30 minutes and 5 minutes, respectively) at those temperatures.
• • Due to the lower rate of solar UV in cold seasons (specifical winter), solar stills are not suitable for cold seasons (specifical winter). A sufficient amount of UV is essential to prevent pathogens from spreading by vapor and prevent pathogens from growing on solar stills.

Solar energy uses for wastewater treatment: critical challenges ( Pandey et al., 2021a , b ).

Solar energy could be used for wastewater treatment to enhance treatment significantly and solve water scarcity if researchers and scientists pay close attention to the above issues.

4. Conclusions

According to reports in several parts of the world, it is highly likely that novel Coronavirus SARS-CoV-2 will appear in water and wastewater. In water, coronaviruses are inactivated by the temperature and are sensitive to moisture. SARS-CoV-2 is most commonly transmitted through symptomatic people's stools to water, sewage, and wastewater. A current drinking water treatment process inactivates and destroys SARS-CoV-2 in water efficiently and effectively. SARS-CoV-2 can be detected using frequent sewage and wastewater monitoring, thus mitigating the threat of pathogen transmission and adverse health effects.

Furthermore, as part of the ongoing fight against COVID-19, this review article discusses the physical inactivation mechanisms that can be used to inactivate the SARS-CoV-2. Detecting viruses in the water is alarming for scientists because the virus could spread faster through the water, worsening the country's crisis during monsoon season. It also includes information on physical inactivation methods such as UV-inactivation and thermal-inactivation, which are used to eliminate pathogens and VOCs in air and water. As a method of disinfecting air and water, solar energy, an excellent source of UV and heat, is shown in COVID-19. This review article further aims to answer specific questions to understand better the application of renewable energy such as solar and geothermal energy in wastewater treatment. An important question to be resolved is the scientific details, feasibility, and methods studied so far for the mechanisms used to break down pollutants in wastewater while minimizing the spread of SARS-CoV-2 using solar and geothermal energy. Critical analysis of critical issues such as pollutants present in industrial and domestic wastewater, wastewater treatment methods, and environmental benefits of wastewater treatment.

Consent to publish

This version has been approved by all other coauthors.

Declaration of Competing Interest

The authors declare that they have no conflict of interest.

Acknowledgements

This work did not receive any financial support.

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Case study on water quality index for wastewater treatment in mumbai

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Preeti Shrivastava , Meenal Mategaonkar; Case study on water quality index for wastewater treatment in mumbai. Water Practice and Technology 2024; wpt2024081. doi: https://doi.org/10.2166/wpt.2024.081

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The traditional wastewater treatment process is laborious and demanding in terms of time, energy, and space. With rapid advancements, there's a quest for more efficient techniques that can achieve comparable treatment outcomes while minimizing these demands. This study proposes a solution by employing ozone as an Advanced Oxidation Process (AOP) to expedite the treatment while enhancing the water quality. The Water Quality Index (WQI) offers a consolidated representation of overall water quality by considering multiple quality parameters. Although typically used for assessing surface water quality, this study utilizes WQI to gauge the enhancement in parameters post-treatment. The research investigates the impact of varied ozone doses on raw sewage samples collected from four different sites in Mumbai, focusing on parameters such as dissolved oxygen, turbidity, hardness, chemical oxygen demand (COD), and WQI. The results demonstrate a notable percentage enhancement in WQI, ranging from 30 to 60% across various sites within a short 25-minute timeframe, attributed to the application of ozone.

Domestic wastewater treatment.

Time sensitive.

Efficient technique.

Advanced oxidation.

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