waste water treatment essay

Wastewater Treatment and Reuse: a Review of its Applications and Health Implications

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• Published: 10 May 2021
• Volume 232 , article number  208 , ( 2021 )

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• Kavindra Kumar Kesari   ORCID: orcid.org/0000-0003-3622-9555 1   na1 ,
• Ramendra Soni 2   na1 ,
• Qazi Mohammad Sajid Jamal 3 ,
• Pooja Tripathi 4 ,
• Jonathan A. Lal 2 ,
• Niraj Kumar Jha 5 ,
• Mohammed Haris Siddiqui 6 ,
• Pradeep Kumar 7 ,
• Vijay Tripathi 2 &
• Janne Ruokolainen 1  

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Water scarcity is one of the major problems in the world and millions of people have no access to freshwater. Untreated wastewater is widely used for agriculture in many countries. This is one of the world-leading serious environmental and public health concerns. Instead of using untreated wastewater, treated wastewater has been found more applicable and ecofriendly option. Moreover, environmental toxicity due to solid waste exposures is also one of the leading health concerns. Therefore, intending to combat the problems associated with the use of untreated wastewater, we propose in this review a multidisciplinary approach to handle wastewater as a potential resource for use in agriculture. We propose a model showing the efficient methods for wastewater treatment and the utilization of solid wastes in fertilizers. The study also points out the associated health concern for farmers, who are working in wastewater-irrigated fields along with the harmful effects of untreated wastewater. The consumption of crop irrigated by wastewater has leading health implications also discussed in this review paper. This review further reveals that our current understanding of the wastewater treatment and use in agriculture with addressing advancements in treatment methods has great future possibilities.

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1 Introduction

Rapidly depleting and elevating the level of freshwater demand, though wastewater reclamation or reuse is one of the most important necessities of the current scenario. Total water consumption worldwide for agriculture accounts 92% (Clemmens et al., 2008 ; Hoekstra & Mekonnen, 2012 ; Tanji & Kielen, 2002 ). Out of which about 70% of freshwater is used for irrigation (WRI, 2020 ), which comes from the rivers and underground water sources (Pedrero et al., 2010 ). The statistics shows serious concern for the countries facing water crisis. Shen et al. ( 2014 ) reported that 40% of the global population is situated in heavy water–stressed basins, which represents the water crisis for irrigation. Therefore, wastewater reuse in agriculture is an ideal resource to replace freshwater use in agriculture (Contreras et al., 2017 ). Treated wastewater is generally applied for non-potable purposes, like agriculture, land, irrigation, groundwater recharge, golf course irrigation, vehicle washing, toilet flushes, firefighting, and building construction activities. It can also be used for cooling purposes in thermal power plants (Katsoyiannis et al., 2017 ; Mohsen, 2004 ; Smith, 1995 ; Yang et al., 2017 ). At global level, treated wastewater irrigation supports agricultural yield and the livelihoods of millions of smallholder farmers (Sato et al., 2013 ). Global reuse of treated wastewater for agricultural purposes shows wide variability ranging from 1.5 to 6.6% (Sato et al., 2013 ; Ungureanu et al., 2018 ). More than 10% of the global population consumes agriculture-based products, which are cultivated by wastewater irrigation (WHO, 2006 ). Treated wastewater reuse has experienced very rapid growth and the volumes have been increased ~10 to 29% per year in Europe, the USA, China, and up to 41% in Australia (Aziz & Farissi, 2014 ). China stands out as the leading country in Asia for the reuse of wastewater with an estimated 1.3 M ha area including Vietnam, India, and Pakistan (Zhang & Shen, 2017 ). Presently, it has been estimated that, only 37.6% of the urban wastewater in India is getting treated (Singh et al., 2019 ). By utilizing 90% of reclaimed water, Israel is the largest user of treated wastewater for agriculture land irrigation (Angelakis & Snyder, 2015 ). The detail information related to the utilization of freshwater and treated wastewater is compiled in Table 1 .

Many low-income countries in Africa, Asia, and Latin America use untreated wastewater as a source of irrigation (Jiménez & Asano, 2008 ). On the other hand, middle-income countries, such as Tunisia, Jordan, and Saudi Arabia, use treated wastewater for irrigation (Al-Nakshabandi et al., 1997 ; Balkhair, 2016a ; Balkhair, 2016b ; Qadir et al., 2010 ; Sato et al., 2013 ).

Domestic water and treated wastewater contains various type of nutrients such as phosphorus, nitrogen, potassium, and sulfur, but the major amount of nitrogen and phosphorous available in wastewater can be easily accumulated by the plants, that’s why it is widely used for the irrigation (Drechsel et al., 2010 ; Duncan, 2009 ; Poustie et al., 2020 ; Sengupta et al., 2015 ). The rich availability of nutrients in reclaimed wastewater reduces the use of fertilizers, increases crop productivity, improves soil fertility, and at the same time, it may also decrease the cost of crop production (Chen et al., 2013 a; Jeong et al., 2016 ). The data of high nutritional values in treated wastewater is shown in Fig. 1 .

Nutrient concentrations (mg/L) of freshwater/wastewater (Yadav et al., 2002 )

Wastewater reuse for crop irrigation showed several health concerns (Ungureanu et al., 2020 ). Irrigation with the industrial wastewater either directly or mixing with domestic water showed higher risk (Chen et al., 2013). Risk factors are higher due to heavy metal and pathogens contamination because heavy metals are non-biodegradable and have a long biological half-life (Chaoua et al., 2019 ; WHO, 2006 ). It contains several toxic elements, i.e., Cu, Cr, Mn, Fe, Pb, Zn, and Ni (Mahfooz et al., 2020 ). These heavy metals accumulate in topsoil (at a depth of 20 cm) and sourcing through plant roots; they enter the human and animal body through leafy vegetables consumption and inhalation of contaminated soils (Mahmood et al., 2014 ). Therefore, health risk assessment of such wastewater irrigation is important especially in adults (Mehmood et al., 2019 ; Njuguna et al., 2019 ; Xiao et al., 2017 ). For this, an advanced wastewater treatment method should be applied before release of wastewater in the river, agriculture land, and soils. Therefore, this review also proposed an advance wastewater treatment model, which has been tasted partially at laboratory scale by Kesari and Behari ( 2008 ), Kesari et al. ( 2011a , b ), and Kumar et al. ( 2010 ).

For a decade, reuse of wastewater has also become one of the global health concerns linking to public health and the environment (Dang et al., 2019 ; Narain et al., 2020 ). The World Health Organization (WHO) drafted guidelines in 1973 to protect the public health by facilitating the conditions for the use of wastewater and excreta in agriculture and aquaculture (WHO, 1973 ). Later in 2005, the initial guidelines were drafted in the absence of epidemiological studies with minimal risk approach (Carr, 2005 ). Although, Adegoke et al. ( 2018 ) reviewed the epidemiological shreds of evidence and health risks associated with reuse of wastewater for irrigation. Wastewater or graywater reuse has adverse health risks associated with microbial hazards (i.e., infectious pathogens) and chemicals or pharmaceuticals exposures (Adegoke et al., 2016 ; Adegoke et al., 2017 ; Busgang et al., 2018 ; Marcussen et al., 2007 ; Panthi et al., 2019 ). Researchers have reported that the exposure to wastewater may cause infectious (helminth infection) diseases, which are linked to anemia and impaired physical and cognitive development (Amoah et al., 2018 ; Bos et al., 2010 ; Pham-Duc et al., 2014 ; WHO, 2006 ).

Owing to an increasing population and a growing imbalance in the demand and supply of water, the use of wastewater has been expected to increase in the coming years (World Bank, 2010 ). The use of treated wastewater in developed nations follows strict rules and regulations. However, the direct use of untreated wastewater without any sound regulatory policies is evident in developing nations, which leads to serious environmental and public health concerns (Dickin et al., 2016 ). Because of these issues, we present in this review, a brief discussion on the risk associated with the untreated wastewater exposures and advanced methods for its treatment, reuse possibilities of the treated wastewater in agriculture.

2 Environmental Toxicity of Untreated Wastewater

Treated wastewater carries larger applicability such as irrigation, groundwater recharge, toilet flushing, and firefighting. Municipal wastewater treatment plants (WWTPs) are the major collection point for the different toxic elements, pathogenic microorganisms, and heavy metals. It collects wastewater from divergent sources like household sewage, industrial, clinical or hospital wastewater, and urban runoff (Soni et al., 2020 ). Alghobar et al. ( 2014 ) reported that grass and crops irrigated with sewage and treated wastewater are rich in heavy metals in comparison with groundwater (GW) irrigation. Although, heavy metals classified as toxic elements and listed as cadmium, lead, mercury, copper, and iron. An exceeding dose or exposures of these heavy metals could be hazardous for health (Duan et al., 2017 ) and ecological risks (Tytła, 2019 ). The major sources of these heavy metals come from drinking water. This might be due to the release of wastewater into river or through soil contamination reaches to ground water. Table 2 presenting the permissible limits of heavy metals presented in drinking water and its impact on human health after an exceeding the amount in drinking water, along with the route of exposure of heavy metals to human body.

Direct release in river or reuse of wastewater for irrigation purposes may create short-term implications like heavy metal and microbial contamination and pathogenic interaction in soil and crops. It has also long-term influence like soil salinity, which grows with regular use of untreated wastewater (Smith, 1995 ). Improper use of wastewater for irrigation makes it unsafe and environment threatening. Irrigation with several different types of wastewater, i.e., industrial effluents, municipal and agricultural wastewaters, and sewage liquid sludge transfers the heavy metals to the soil, which leads to accumulation in crops due to improper practices. This has been identified as a significant route of heavy metals into aquatic resources (Agoro et al., 2020 ). Hussain et al. ( 2019 ) investigated the concentration of heavy metals (except for Cd) was higher in the soil irrigated with treated wastewater (large-scale sewage treatment plant) than the normal ground water, also reported by Khaskhoussy et al. ( 2015 ).

In other words, irrigation with wastewater mitigates the quality of crops and enhances health risks. Excess amount of copper causes anemia, liver and kidney damage, vomiting, headache, and nausea in children (Bent & Bohm, 1995 ; Madsen et al., 1990 ; Salem et al., 2000 ). A higher concentration of arsenic may lead to bone and kidney cancer (Jarup, 2003 ) and results in osteopenia or osteoporosis (Puzas et al., 2004 ). Cadmium gives rise to musculoskeletal diseases (Fukushima et al., 1970 ), whereas mercury directly affects the nervous system (Azevedo et al., 2014 ).

3 Spread of Antibiotic Resistance

Currently, antibiotics are highly used for human disease treatment; however, uses in poultries, animal husbandries, biochemical industries, and agriculture are common practices these days. Extensive use and/or misuse of antibiotics have given rise to multi-resistant bacteria, which carry multiple resistance genes (Icgen & Yilmaz, 2014 ; Lv et al., 2015 ; Tripathi & Tripathi, 2017 ; Xu et al., 2017 ). These multidrug-resistant bacteria discharged through the sewage network and get collected into the wastewater treatment plants. Therefore, it can be inferred that the WWTPs serve as the hotspot of antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs). Though, these antibiotic-resistant bacteria can be disseminated to the different bacterial species through the mobile genetic elements and horizontal gene transfer (Gupta et al., 2018 ). Previous studies indicated that certain pathogens might survive in wastewater, even during and after the treatment processes, including methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE) (Börjesson et al., 2009 ; Caplin et al., 2008 ). The use of treated wastewater in irrigation provides favorable conditions for the growth and persistence of total coliforms and fecal coliforms (Akponikpe et al., 2011 ; Sacks & Bernstein, 2011 ). Furthermore, few studies have also reported the presence of various bacterial pathogens, such as Clostridium , Salmonella , Streptococci , Viruses, Protozoa, and Helminths in crops irrigated with treated wastewater (Carey et al., 2004 ; Mañas et al., 2009 ; Samie et al., 2009 ). Goldstein ( 2013 ) investigated the survival of ARB in secondary treated wastewater and proved that it causes serious health risks to the individuals, who are exposed to reclaimed water. The U.S. Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) have already declared the ARBs as the imminent hazard to human health. According to the list published by WHO, regarding the development of new antimicrobial agents, the ESKAPE ( Enterococcus faecium , S. aureus , Klebsiella pneumoniae , Acinetobacter baumannii , Pseudomonas aeruginosa , and Enterobacter species) pathogens were designated to be “priority status” as their occurrence in the food chain is considered as the potential and major threat for the human health (Tacconelli et al., 2018 ).

These ESKAPE pathogens have acquired the multi drug resistance mechanisms against oxazolidinones, lipopeptides, macrolides, fluoroquinolones, tetracyclines, β-lactams, β-lactam–β-lactamase inhibitor combinations, and even those antibiotics that are considered as the last line of defense, including carbapenems and glycopeptides (Giddins et al., 2017 ; Herc et al., 2017 ; Iguchi et al., 2016 ; Naylor et al., 2018 ; Zaman et al., 2017 ), by the means of genetic mutation and mobile genetic elements. These cluster of ESKAPE pathogens are mainly responsible for lethal nosocomial infections (Founou et al., 2017 ; Santajit & Indrawattana, 2016 ).

Due to the wide application of antibiotics in animal husbandry and inefficient capability of wastewater treatment plants, the multidrug-resistant bacteria such as tetracyclines, sulfonamides, β-lactam, aminoglycoside, colistin, and vancomycin in major are disseminated in the receiving water bodies, which ultimately results in the accumulation of ARGs in the irrigated crops (He et al., 2020 ).

4 Toxic Contaminations in Wastewater Impacting Human Health

The release of untreated wastewater into the river may pose serious health implications (König et al., 2017 ; Odigie, 2014 ; Westcot, 1997 ). It has been already discussed about the household and municipal sewage which contains a major amount of organic materials and pathogenic microorganisms and these infectious microorganisms are capable of spreading various diseases like typhoid, dysentery, diarrhea, vomiting, and malabsorption (Jia & Zhang, 2020 ; Numberger et al., 2019 ; Soni et al., 2020 ). Additionally, pharmaceutical industries also play a key role in the regulation and discharge of biologically toxic agents. The untreated wastewater also contains a group of contaminants, which are toxic to humans. These toxic contaminations have been classified into two major groups: (i) chemical contamination and (ii) microbial contamination.

4.1 Chemical Contamination

Mostly, various types of chemical compounds released from industries, tanneries, workshops, irrigated lands, and household wastewaters are responsible for several diseases. These contaminants can be organic materials, hydrocarbons, volatile compounds, pesticides, and heavy metals. Exposure to such contaminants may cause infectious diseases like chronic dermatoses and skin cancer, lung infection, and eye irritation. Most of them are non-biodegradable and intractable. Therefore, they can persist in the water bodies for a very long period and could be easily accumulated in our food chain system. Several pharmaceutical personal care products (PPCPs) and surfactants are available that may contain toxic compounds like nonylphenol, estrone, estradiol, and ethinylestradiol. These compounds are endocrine-disrupting chemicals (Bolong et al., 2009 ), and the existence of these compounds in the human body even in the trace amounts can be highly hazardous. Also, the occurrence of perfluorinated compounds (PFCs) in wastewater, which is toxic in nature, has been significantly reported worldwide (Templeton et al., 2009 ). Furthermore, PFCs cause severe health menaces like pre-eclampsia, birth defects, reduced human fertility (Webster, 2010 ), immunotoxicity (Dewitt et al., 2012 ), neurotoxicity (Lee & Viberg, 2013 ), and carcinogenesis (Bonefeld-Jorgensen et al., 2011 ).

4.2 Microbial Contamination

Researchers have reported serious health risks associated with the microbial contaminants in untreated wastewater. The diverse group of microorganisms causes severe health implications like campylobacteriosis, diarrhea, encephalitis, typhoid, giardiasis, hepatitis A, poliomyelitis, salmonellosis, and gastroenteritis (ISDH, 2009 ; Okoh et al., 2010 ). Few bacterial species like P. aeruginosa , Salmonella typhimurium , Vibrio cholerae , G. intestinales , Legionella spp., E. coli , Shigella sonnei have been reported for the spreading of waterborne diseases, and acute illness in human being (Craun et al., 2006 ; Craun et al., 2010 ). These aforementioned microorganisms may release in the environment from municipal sewage water network, animal husbandries, or hospitals and enter the food chain via public water supply systems.

5 Wastewater Impact on Agriculture

The agriculture sector is well known for the largest user of water, accounting for nearly 70% of global water usage (Winpenny et al., 2010 ). The fact that an estimated 20 million hectares worldwide are irrigated with wastewater suggests a major source for irrigation (Ecosse, 2001 ). However, maximum wastewater that is used for irrigation is untreated (Jiménez & Asano, 2008 ; Scott et al., 2004 ). Mostly in developing countries, partially treated or untreated wastewater is used for irrigation purpose (Scott et al., 2009 ). Untreated wastewater often contains a large range of chemical contaminants from waste sites, chemical wastes from industrial discharges, heavy metals, fertilizers, textile, leather, paper, sewage waste, food processing waste, and pesticides. World Health Organization (WHO) has warned significant health implications due to the direct use of wastewater for irrigation purposes (WHO, 2006 ). These contaminants pose health risks to communities (farmers, agricultural workers, their families, and the consumers of wastewater-irrigated crops) living in the proximity of wastewater sources and areas irrigated with untreated wastewater (Qadir et al., 2010 ). Wastewater also contains a wide variety of organic compounds. Some of them are toxic or cancer-causing and have harmful effects on an embryo (Jarup, 2003 ; Shakir et al., 2016 ). The pathway of untreated wastewater used in irrigation and associated health effects are shown in Fig. 2 .

Exposure pathway representing serious health concerns from wastewater-irrigated crops

Alternatively, in developing countries, due to the limited availability of treatment facilities, untreated wastewater is discharged into the existing waterbodies (Qadir et al., 2010 ). The direct use of wastewater in agriculture or irrigation obstructs the growth of natural plants and grasses, which in turn causes the loss of biodiversity. Shuval et al. ( 1985 ) reported one of the earliest evidences connecting to agricultural wastewater reuse with the occurrence of diseases. Application of untreated wastewater in irrigation increases soil salinity, land sealing followed by sodium accumulation, which results in soil erosion. Increased soil salinity and sodium accumulation deteriorates the soil and decreases the soil permeability, which inhibits the nutrients intake of crops from the soil. These causes have been considered the long-term impact of wastewater reuse in agriculture (Halliwell et al., 2001 ). Moreover, wastewater contaminated soils are a major source of intestinal parasites (helminths—nematodes and tapeworms) that are transmitted through the fecal–oral route (Toze, 1997 ). Already known, the helminth infections are linked to blood deficiency and behavioral or cognitive development (Bos et al., 2010 ). One of the major sources of helminth infections around the world is the use of raw or partially treated sewage effluent and sludge for the irrigation of food crops (WHO, 1989 ). Wastewater-irrigated crops contain heavy metal contamination, which originates from mining, foundries, and metal-based industries (Fazeli et al., 1998 ). Exposure to heavy metals including arsenic, cadmium, lead, and mercury in wastewater-irrigated crops is a cause for various health problems. For example, the consumption of high amounts of cadmium causes osteoporosis in humans (Dickin et al., 2016 ). The uptake of heavy metals by the rice crop irrigated with untreated effluent from a paper mill has been reported to cause serious health concerns (Fazeli et al., 1998 ). Irrigating rice paddies with highly contaminated water containing heavy metals leads to the outbreak of Itai-itai disease in Japan (Jarup, 2003 ).

Owing to these widespread health risks, the WHO published the third edition of its guidelines for the safe use of wastewater in irrigating crops (WHO, 2006 ) and made recommendations for threshold contaminant levels in wastewater. The quality of wastewater for agricultural reuse have been classified based on the availability of nutrients, trace elements, microorganisms, and chemicals contamination levels. The level of contamination differs widely depending on the type of source, household sewage, pharmaceutical, chemical, paper, or textile industries effluents. The standard measures of water quality for irrigation are internationally reported (CCREM, 1987 ; FAO, 1985 ; FEPA, 1991 ; US EPA, 2004 , 2012 ; WHO, 2006 ), where the recommended levels of trace elements, metals, COD, BOD, nitrogen, and phosphorus are set at certain limits. Researchers reviewed the status of wastewater reuse for agriculture, based on its standards and guidelines for water quality (Angelakis et al., 1999 ; Brissaud, 2008 ; Kalavrouziotis et al., 2015 ). Based on these recommendations and guidelines, it is evident that greater awareness is required for the treatment of wastewater safely.

6 Wastewater Treatment Techniques

6.1 primary treatment.

This initial step is designed to remove gross, suspended and floating solids from raw wastewater. It includes screening to trap solid objects and sedimentation by gravity to remove suspended solids. This physical solid/liquid separation is a mechanical process, although chemicals can be used sometimes to accelerate the sedimentation process. This phase of the treatment reduces the BOD of the incoming wastewater by 20–30% and the total suspended solids by nearly 50–60%.

6.2 Secondary (Biological) Treatment

This stage helps eliminate the dissolved organic matter that escapes primary treatment. Microbes consume the organic matter as food, and converting it to carbondioxide, water, and energy for their own growth. Additional settling to remove more of the suspended solids then follows the biological process. Nearly 85% of the suspended solids and biological oxygen demand (BOD) can be removed with secondary treatment. This process also removes carbonaceous pollutants that settle down in the secondary settling tank, thus separating the biological sludge from the clear water. This sludge can be fed as a co-substrate with other wastes in a biogas plant to obtain biogas, a mixture of CH 4 and CO 2 . It generates heat and electricity for further energy distribution. The leftover, clear water is then processed for nitrification or denitrification for the removal of carbon and nitrogen. Furthermore, the water is passed through a sedimentation basin for treatment with chlorine. At this stage, the water may still contain several types of microbial, chemical, and metal contaminations. Therefore, to make the water reusable, e.g., for irrigation, it further needs to pass through filtration and then into a disinfection tank. Here, sodium hypochlorite is used to disinfect the wastewater. After this process, the treated water is considered safe to use for irrigation purposes. Solid wastes generated during primary and secondary treatment processes are processed further in the gravity-thickening tank under a continuous supply of air. The solid waste is then passed into a centrifuge dewatering tank and finally to a lime stabilization tank. Treated solid waste is obtained at this stage and it can be processed further for several uses such as landfilling, fertilizers and as a building.

Other than the activated sludge process of wastewater treatment, there are several other methods developed and being used in full-scale reactors such as ponds (aerobic, anaerobic, facultative, and maturation), trickling filters, anaerobic treatments like up-flow anaerobic sludge blanket (UASB) reactors, artificial wetlands, microbial fuel cells, and methanogenic reactors.

UASB reactors are being applied for wastewater treatment from a very long period. Behling et al. ( 1996 ) examined the performance of the UASB reactor without any external heat supply. In their study, the COD loading rate was maintained at 1.21 kg COD/m 3 /day, after 200 days of trial. They achieved an average of 85% of COD removal. Von-Sperling and Chernicharo ( 2005 ) presented a combined model consisted of an Up-flow Anaerobic Sludge Blanket-Activated Sludge reactor (UASB–AS system), using the low strength domestic wastewater with a BOD 5 amounting to 340 mg/l. Outcomes of their experiment have shown a 60% reduction in sludge construction and a 40% reduction in aeration energy consumption. In another experiment, Rizvi et al. ( 2015 ) seeded UASB reactor with cow manure dung to treat domestic wastewater; they observed 81%, 75%, and 76% reduction in COD, TSS, and total sulfate removal, respectively, in their results.

6.3 Tertiary or Advanced Treatment Processes

The tertiary treatment process is employed when specific constituents, substances, or contaminants cannot be completely removed after the secondary treatment process. The tertiary treatment processes, therefore, ensure that nearly 99% of all impurities are removed from wastewater. To make the treated water safe for drinking purposes, water is treated individually or in combination with advanced methods like the US (ultrasonication), UV (ultraviolet light treatment), and O 3 (exposure to ozone). This process helps to remove bacteria and heavy metal contaminations remaining in the treated water. For the purpose, the secondarily treated water is first made to undergo ultrasonication and it is subsequently exposed to UV light and passed through an ozone chamber for the complete removal of contaminations. The possible mechanisms by which cells are rendered inviable during the US include free-radical attack and physical disruption of cell membranes (Phull et al., 1997 ; Scherba et al., 1991 ). The combined treatment of US + UV + O 3 produces free radicals, which are attached to cell membranes of the biological contaminants. Once the cell membrane is sheared, chemical oxidants can enter the cell and attack internal structures. Thus, the US alone or in combination facilitates the deagglomeration of microorganisms and increases the efficiency of other chemical disinfectants (Hua & Thompson, 2000 ; Kesari et al., 2011a , b ; Petrier et al., 1992 ; Phull et al., 1997 ; Scherba et al., 1991 ). A combined treatment method was also considered by Pesoutova et al. ( 2011 ) and reported a very effective method for textile wastewater treatment. The effectiveness of ultrasound application as a pre-treatment step in combination with ultraviolet rays (Blume & Neis, 2004 ; Naddeo et al., 2009 ), or also compared it with various other combinations of both ultrasound and UV radiation with TiO 2 photocatalysis (Paleologou et al., 2007 ), and ozone (Jyoti & Pandit, 2004 ) to optimize wastewater disinfection process.

An important aspect of our wastewater treatment model (Fig. 3 ) is that at each step of the treatment process, we recommend the measurement of the quality of treated water. After ensuring that the proper purification standards are met, the treated water can be made available for irrigation, drinking or other domestic uses.

A wastewater treatment schematic highlighting the various methods that result in a progressively improved quality of the wastewater from the source to the intended use of the treated wastewater for irrigation purposes

6.4 Nanotechnology as Tertiary Treatment of Wastewater Converting Drinking Water Alike

Considering the emerging trends of nanotechnology, nanofillers can be used as a viable method for the tertiary treatment of wastewater. Due to the very small pore size, 1–5-nm nanofillers may eliminate the organic–inorganic pollutants, heavy metals, as well as pathogenic microorganisms and pharmaceutically active compounds (PhACs) (Mohammad et al., 2015 ; Vergili, 2013 ). Over the recent years, nanofillers have been largely accepted in the textile industry for the treatment of pulp bleaching pharmaceutical industry, dairy industry, microbial elimination, and removal of heavy metals from wastewater (Abdel-Fatah, 2018 ). Srivastava et al. ( 2004 ) synthesized very efficient and reusable water filters from carbon nanotubes, which exhibited effective elimination of bacterial pathogens ( E. coli and S. aureus ), and Poliovirus sabin-1 from wastewater.

Nanofiltration requires lower operating pressure and lesser energy consumption in comparison of RO and higher rejection of organic compounds compared to UF. Therefore, it can be applied as the tertiary treatment of wastewater (Abdel-Fatah, 2018 ). Apart from nanofilters, there are various kinds of nanoparticles like metal nanoparticles, metal oxide nanoparticles, carbon nanotubes, graphene nanosheets, and polymer-based nanosorbents, which may play a different role in wastewater treatment based on their properties. Kocabas et al. ( 2012 ) analyzed the potential of different metal oxide nanoparticles and observed that nanopowders of TiO 2 , FeO 3 , ZnO 2 , and NiO can exhibit the exceeding amount of removal of arsenate from wastewater. Cadmium contamination in wastewater, which poses a serious health risk, can be overcome by using ZnO nanoparticles (Kumar & Chawla, 2014 ). Latterly, Vélez et al. ( 2016 ) investigated that the 70% removal of mercury from wastewater through iron oxide nanoparticles successfully performed. Sheet et al. ( 2014 ) used graphite oxide nanoparticles for the removal of nickel from wastewater. An exceeding amount of copper causes liver cirrhosis, anemia, liver, and kidney damage, which can be removed by carbon nanotubes, pyromellitic acid dianhydride (PMDA) and phenyl aminomethyl trimethoxysilane (PAMTMS) (Liu et al., 2010 ).

Nanomaterials are efficiently being used for microbial purification from wastewater. Carbon nanotubes (CNTs) are broadly applied for the treatment of wastewater contaminated with E. coli , Salmonella , and a wide range of microorganisms (Akasaka & Watari, 2009 ). In addition, silver nanoparticles reveal very effective results against the microorganisms present in wastewater. Hence, it is extensively being used for microbial elimination from wastewater (Inoue et al., 2002 ). Moreover, CNTs exhibit high binding affinity to bacterial cells and possess magnetic properties (Pan & Xing, 2008 ). Melanta ( 2008 ) confirmed and recommended the applicability of CNTs for the removal of E. coli contamination from wastewater. Mostafaii et al. ( 2017 ) suggested that the ZnO nanoparticles could be the potential antibacterial agent for the removal of total coliform bacteria from municipal wastewater. Apart from the previously mentioned, applicability of the nanotechnology, the related drawbacks and challenges cannot be neglected. Most of the nanoengineered techniques are currently either in research scale or pilot scale performing well (Gehrke et al., 2015 ). Nevertheless, as discussed above, nanotechnology and nanomaterials exhibit exceptional properties for the removal of contaminants and purification of water. Therefore, it can be adapted as the prominent solution for the wastewater treatment (Zekić et al., 2018 ) and further use for drinking purposes.

6.5 Wastewater Treatment by Using Plant Species

Some of the naturally growing plants can be a potential source for wastewater treatment as they remove pollutants and contaminants by utilizing them as a nutrient source (Zimmels et al., 2004 ). Application of plant species in wastewater treatment may be cost-effective, energy-saving, and provides ease of operation. At the same time, it can be used as in situ, where the wastewater is being produced (Vogelmann et al., 2016 ). Nizam et al. ( 2020 ) analyzed the phytoremediation efficiency of five plant species ( Centella asiatica , Ipomoea aquatica , Salvinia molesta , Eichhornia crassipes , and Pistia stratiotes ) and achieved the drastic decrease in the amount of three pollutants viz. total suspended solids (TSS), ammoniacal nitrogen (NH 3 -N), and phosphate levels . All the five species found to be efficient removal of the level of 63.9-98% of NH 3 -N, TSS, and phosphate. Coleman et al. ( 2001 ) examined the physiological effects of domestic wastewater treatment by three common Appalachian plant species: common rush or soft rush ( Juncus effuses L.), gray club-rush ( Scirpus Validus L.), and broadleaf cattail or bulrush ( Typha latifolia L.). They observed in their experiments about 70% of reduction in total suspended solids (TSS) and biochemical oxygen demand (BOD), 50% to 60% of reduction in nitrogen, ammonia, and phosphate levels, and a significant reduction in feacal coliform populations. Whereas, Zamora et al. ( 2019 ) found the removal efficiency of chemical oxygen demand (COD), total solids suspended (TSS), nitrogen as ammonium (N-NH 4 ) and nitrate (N-NO 3 ), and phosphate (P-PO 4 ) up to 20–60% higher using the three ornamental species of plants viz. Canna indica , Cyperus papyrus , and Hedychium coronarium . The list of various plant species applied for the wastewater treatment is shown in Table 3 .

6.6 Wastewater Treatment by Using Microorganisms

There is a diverse group of bacteria like Pseudomonas fluorescens , Pseudomonas putida , and different Bacillus strains, which are capable to use in biological wastewater systems. These bacteria work in the cluster forms as a floc, biofilm, or granule during the wastewater treatment. Furthermore, after the recognition of bacterial exopolysaccharides (EPS) as an efficient adsorption material, it may be applied in a revolutionary manner for the heavy metal elimination (Gupta & Diwan, 2017 ). There are few examples of EPS, which are commercially available, i.e., alginate ( P. aeruginosa , Azotobacter vinelandii ), gellan (Sphingomonas paucimobilis ), hyaluronan ( . aeruginosa , Pasteurella multocida , Streptococci attenuated strains ), xanthan (Xanthomonas campestris ), and galactopol ( Pseudomonas oleovorans ) (Freitas et al., 2009 ; Freitas, Alves, & Reis, 2011a ; Freitas, Alves, Torres, et al., 2011b ). Similarly, Hesnawi et al. ( 2014 ) experimented biodegradation of municipal wastewater using local and commercial bacteria (Sludge Hammer), where they achieved a significant decrease in synthetic wastewater, i.e., 70%, 54%, 52%, 42% for the Sludge Hammer, B. subtilis , B. laterosponus , and P. aeruginosa , respectively. Therefore, based on the above studies, it can be concluded that bioaugmentation of wastewater treatment reactor with selective and mixed strains can ameliorate the treatment. During recent years, microalgae have attracted the attention of researchers as an alternative system, due to their applicability in wastewater treatment. Algae are the unicellular or multicellular photosynthetic microorganism that grows on water surfaces, salt water, or moist soil. They utilize the exceeding amount of nutrients like nitrogen, phosphorus, and carbon for their growth and metabolism process through their anaerobic system. This property of algae also inhibits eutrophication; that is to avoid over-deposit of nutrients in water bodies. During the nutrient digestion process, algae produce oxygen that is constructive for the heterotrophic aerobic bacteria, which may further be utilized to degrade the organic and inorganic pollutants. Kim et al. ( 2014 ) observed a total decrease in the levels of COD (86%), total nitrogen (93%), and total phosphorus (83%) after using algae in the municipal wastewater consortium. Nmaya et al. ( 2017 ) reported the heavy metal removal efficiency of microalga Scenedesmus sp. from contaminated river water in the Melaka River, Malaysia. They observed the effective removal of Zn (97-99%) on the 3 rd and 7 th day of the experiment. The categorized list of microorganisms used for wastewater treatment is presented in Table 4 .

7 The Computational Approach in Wastewater Treatment

7.1 bioinformatics and genome sequencing.

A computational approach is accessible in wastewater treatment. Several tools and techniques are in use such as, sequencing platforms (Hall, 2007 ; Marsh, 2007 ), metagenome sequencing strategies (Schloss & Handelsman, 2005 ; Schmeisser et al., 2007 ; Tringe et al., 2005 ), bioinformatics tools and techniques (Chen & Pachter, 2005 ; Foerstner et al., 2006 ; Raes et al., 2007 ), and the genome analysis of complex microbial communities (Fig. 4 ). Most of the biological database contains microorganisms and taxonomical information. Thus, these can provide extensive details and supports for further utilization in wastewater treatment–related research and development (Siezen & Galardini, 2008 ). Balcom et al. ( 2016 ) explored that the microbial population residing in the plant roots immersed in the wastewater of an ecological WWTP and showed the evidence of the capacity for micro-pollutant biodegradation using whole metagenome sequencing (WMS). Similarly, Kumar et al. ( 2016 ) revealed that bioremediation of highly polluted wastewater from textile dyes by two novel strains were found to highly decolorize Joyfix Red. They were identified as Lysinibacillus sphaericus (KF032717) and Aeromonas hydrophila (KF032718) through 16S rDNA analysis. More recently, Leddy et al. ( 2018 ) reported that research scientists are making strides to advance the safety and application of potable water reuse with metagenomics for water quality analysis. The application of the bio-computational approach has also been implemented in the advancements of wastewater treatment and disease detection.

A schematic showing the overall conceptual framework on which depicting the computational approach in wastewater treatment

7.2 Computational Fluid Dynamics in Wastewater Treatment

In recent years, computational fluid dynamics (CFD), a broadly used method, has been applied to biological wastewater treatment. It has exposed the inner flow state that is the hydraulic condition of a biological reactor (Peng et al., 2014 ). CFD is the application of powerful predictive modeling and simulation tools. It may calculate the multiple interactions between all the water quality and process design parameters. CFD modeling tools have already been widely used in other industries, but their application in the water industry is quite recent. CFD modeling has great applications in water and wastewater treatment, where it mechanically works by using hydrodynamic and mass transfer performance of single or two-phase flow reactors (Do-Quang et al., 1998 ). The level of CFD’s capability varies between different process units. It has a high frequency of application in the areas of final sedimentation, activated sludge basin modeling, disinfection, and greater needs in primary sedimentation and anaerobic digestion (Samstag et al., 2016 ). Now, researchers are enhancing the CFD modeling with a developed 3D model of the anoxic zone to evaluate further hydrodynamic performance (Elshaw et al., 2016 ). The overall conceptual framework and the applications of the computational approach in wastewater treatment are presented in Fig. 4 .

7.3 Computational Artificial Intelligence Approach in Wastewater Treatment

Several studies were obtained by researchers to implement computer-based artificial techniques, which provide fast and rapid automated monitoring of water quality tests such as BOD and COD. Recently, Nourani et al. ( 2018 ) explores the possibility of wastewater treatment plant by using three different kinds of artificial intelligence methods, i.e., feedforward neural network (FFNN), adaptive neuro-fuzzy inference system (ANFIS), and support vector machine (SVM). Several measurements were done in terms of effluent to tests BOD, COD, and total nitrogen in the Nicosia wastewater treatment plant (NWWTP) and reported high-performance efficiency of artificial intelligence (Nourani et al., 2018 ).

7.4 Remote sensing and Geographical Information System

Since the implementation of satellite technology, the initiation of new methods and tools became popular nowadays. The futuristic approach of remote sensing and GIS technology plays a crucial role in the identification and locating of the water polluted area through satellite imaginary and spatial data. GIS analysis may provide a quick and reasonable solution to develop atmospheric correction methods. Moreover, it provides a user-friendly environment, which may support complex spatial operations to get the best quality information on water quality parameters through remote sensing (Ramadas & Samantaray, 2018 ).

8 Applications of Treated Wastewater

8.1 scope in crop irrigation.

Several studies have assessed the impact of the reuse of recycled/treated wastewater in major sectors. These are agriculture, landscapes, public parks, golf course irrigation, cooling water for power plants and oil refineries, processing water for mills, plants, toilet flushing, dust control, construction activities, concrete mixing, and artificial lakes (Table 5 ). Although the treated wastewater after secondary treatment is adequate for reuse since the level of heavy metals in the effluent is similar to that in nature (Ayers & Westcot, 1985 ), experimental evidences have been found and evaluated the effects of irrigation with treated wastewater on soil fertility and chemical characteristics, where it has been concluded that secondary treated wastewater can improve soil fertility parameters (Mohammad & Mazahreh, 2003 ). The proposed model (Fig. 3 ) is tested partially previously at a laboratory scale by treating the wastewater (from sewage, sugar, and paper industry) in an ultrasonic bath (Kesari et al., 2011a , b ; Kesari & Behari, 2008 ; Kumar et al., 2010 ). Advancing it with ultraviolet and ozone treatment has modified this in the proposed model. A recent study shows that the treated water passed quality measures suited for crop irrigation (Bhatnagar et al., 2016 ). In Fig. 3 , a model is proposed including all three (UV, US, nanoparticle, and ozone) techniques, which have been tested individually as well as in combination (US and nanoparticle) (Kesari et al., 2011a , b ) to obtain the highest water quality standards acceptable for irrigation and even drinking purposes.

A wastewater-irrigated field is a major source of essential and non-essential metals contaminants such as lead, copper, zinc, boron, cobalt, chromium, arsenic, molybdenum, and manganese. While crops need some of these, the others are non-essential metals, toxic to plants, animals, and humans. Kanwar and Sandha ( 2000 ) reported that heavy metal concentrations in plants grown in wastewater-irrigated soils were significantly higher than in plants grown in the reference soil in their study. Yaqub et al. ( 2012 ) suggest that the use of US is very effective in removing heavy or toxic metals and organic pollutants from industrial wastewater. However, it has been also observed that the metals were removed efficiently, when UV light was combined with ozone (Samarghandi et al., 2007 ). Ozone exposure is a potent method for the removal of metal or toxic compounds from wastewater as also reported earlier (Park et al., 2008 ). Application of US, UV, and O 3 in combination lead to the formation of reactive oxygen species (ROS) that oxidize certain organics, metal ions and kill pathogens. In the process of advanced oxidizing process (AOP) primarily oxidants, electricity, light, catalysts etc. are implied to produce extremely reactive free radicals (such as OH) for the breakdown of organic matters (Oturan & Aaron, 2014 ). Among the other AOPs, ozone oxidization process is more promising and effective for the decomposition of complex organic contaminants (Xu et al., 2020 ). Ozone oxidizes the heavy metal to their higher oxidation state to form metallic oxides or hydroxides in which they generally form limited soluble oxides and gets precipitated, which are easy to be filtered by filtration process. Ozone oxidization found to be efficient for the removal of heavy metals like cadmium, chromium, cobalt, copper, lead, manganese, nickel, and zinc from the water source (Upadhyay & Srivastava, 2005 ). Ultrasonic-treated sludge leads to the disintegration of biological cells and kills bacteria in treated wastewater (Kesari, Kumar, et al., 2011a ; Kesari, Verma, & Behari, 2011b ). This has been found that combined treatment with ultrasound and nanoparticles is more effective (Kesari, Kumar, et al., 2011a ). Ultrasonication has the physical effects of cavitation inactivate and lyse bacteria (Broekman et al., 2010 ). The induced effect of US, US, or ozone may destroy the pathogens and especially during ultrasound irradiation including free-radical attack, hydroxyl radical attack, and physical disruption of cell membranes (Kesari, Kumar, et al., 2011a ; Phull et al., 1997 ; Scherba et al., 1991 ).

8.2 Energy and Economy Management

Municipal wastewater treatment plants play a major role in wastewater sanitation and public health protection. However, domestic wastewater has been considered as a resource or valuable products instead of waste, because it has been playing a significant role in the recovery of energy and resource for the plant-fertilizing nutrients like phosphorus and nitrogen. Use of domestic wastewater is widely accepted for the crop irrigation in agriculture and industrial consumption to avoid the water crisis. It has also been found as a source of energy through the anaerobic conversion of the organic content of wastewater into methane gas. However, most of the wastewater treatment plants are using traditional technology, as anaerobic sludge digestion to treat wastewater, which results in more consumption of energy. Therefore, through these conventional technologies, only a fraction of the energy of wastewater has been captured. In order to solve these issues, the next generation of municipal wastewater treatment plants is approaching total retrieval of the energy potential of water and nutrients, mostly nitrogen and phosphorus. These plants also play an important role in the removal and recovery of emerging pollutants and valuable products of different nature like heavy and radioactive metals, fertilizers hormones, and pharma compounds. Moreover, there are still few possibilities of improvement in wastewater treatment plants to retrieve and reuse of these compounds. There are several methods under development to convert the organic matter into bioenergy such as biohydrogen, biodiesel, bioethanol, and microbial fuel cell. These methods are capable to produce electricity from wastewater but still need an appropriate development. Energy development through wastewater is a great driver to regulate the wastewater energy because it produces 10 times more energy than chemical, thermal, and hydraulic forms. Vermicomposting can be utilized for stabilization of sludge from the wastewater treatment plant. Kesari and Jamal ( 2017 ) have reported the significant, economical, and ecofriendly role of the vermicomposting method for the conversion of solid waste materials into organic fertilizers as presented in Fig. 5 . Solid waste may come from several sources of municipal and industrial sludge, for example, textile industry, paper mill, sugarcane, pulp industry, dairy, and intensively housed livestock. These solid wastes or sewage sludges have been treated successfully by composting and/or vermicomposting (Contreras-Ramos et al., 2005 ; Elvira et al., 1998 ; Fraser-Quick, 2002 ; Ndegwa & Thompson, 2001 ; Sinha et al., 2010 ) Although collection of solid wastes materials from sewage or wastewater and further drying is one of the important concerns, processing of dried municipal sewage sludge (Contreras-Ramos et al., 2005 ) and management (Ayilara et al., 2020 ) for vermicomposting could be possible way of generating organic fertilizers for future research. Vermicomposting of household solid wastes, agriculture wastes, or pulp and sugarcane industry wastes shows greater potential as fertilizer for higher crop yielding (Bhatnagar et al., 2016 ; Kesari & Jamal, 2017 ). The higher amount of solid waste comes from agricultural land and instead of utilizing it, this biomass is processed by burning, which causes severe diseases (Kesari & Jamal, 2017 ). Figure 3 shows the proper utilization of solid waste after removal from wastewater; however, Fig. 5 showing greater possibility in fertilizer conversion which has also been discussed in detail elsewhere (Bhatnagar et al., 2016 ; Nagavallemma et al., 2006 )

Energy production through wastewater (reproduced from Bhatnagar et al., 2016 ; Kesari & Jamal, 2017 )

9 Conclusions and future perspectives

In this paper, we have reviewed environmental and public health issues associated with the use of untreated wastewater in agriculture. We have focused on the current state of affairs concerning the wastewater treatment model and computational approach. Given the dire need for holistic approaches for cultivation, we proposed the ideas to tackle the issues related to wastewater treatment and the reuse potential of the treated water. Water resources are under threat because of the growing population. Increasing generation of wastewater (municipal, industrial, and agricultural) in developing countries especially in India and other Asian countries has the potential to serve as an alternative of freshwater resources for reuse in rice agriculture, provide appropriate treatment, and distribution measures are adopted. Wastewater treatment is one of the big challenges for many countries because increasing levels of undesired or unknown pollutants are very harmful to health as well as environment. Therefore, this review explores the ideas based on current and future research. Wastewater treatment includes very traditional methods by following primary, secondary, and tertiary treatment procedures, but the implementation of advanced techniques is always giving us a big possibility of good water quality. In this paper, we have proposed combined methods for the wastewater treatment, where the concept of the proposed model works on the various types of wastewater effluents. The proposed model not only useful for wastewater treatment but also for the utilization of solid wastes as fertilizer. An appropriate method for the treatment of wastewater and further utilization for drinking water is the main futuristic outcome. It is also highly recommendable to follow the standard methods and available guidelines provided WHO. In this paper, the proposed role of the computational model, i.e., artificial intelligence, fluid dynamics, and GIS, in wastewater treatment could be useful in future studies. In this review, health concerns associated with wastewater irrigation for farmers and irrigated crops consumers have been discussed.

The crisis of freshwater is one of the growing concerns in the twenty-first century. Globaly, about 330 km 3 of municipal wastewater is generated annually (Hernández-Sancho et al., 2015 ). This data provides a better understanding of why the reuse of treated wastewater is important to solve the issues of the water crisis. The use of treated wastewater (industrial or municipal wastewater or Seawater) for irrigation has a better future for the fulfillment of water demand. Currently, in developing countries, farmers are using wastewater directly for irrigation, which may cause several health issues for both farmers and consumers (crops or vegetables). Therefore, it is very imperative to implement standard and advanced methods for wastewater treatment. A local assessment of the environmental and health impacts of wastewater irrigation is required because most of the developed and developing countries are not using the proper guidelines. Therefore, it is highly required to establish concrete policies and practices to encourage safe water reuse to take advantage of all its potential benefits in agriculture and for farmers.

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Acknowledgements

All the authors are highly grateful to the authority of the respective departments and institutions for their support in doing this research. The author VT would like to thank Science & Engineering Research Board, New Delhi, India (Grant #ECR/2017/001809). The Author RS is thankful to the University Grants Commission for the National Fellowship (201819-NFO-2018-19-OBC-UTT-78476).

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Kavindra Kumar Kesari and Ramendra Soni contributed equally to this work.

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Department of Applied Physics, Aalto University, Espoo, Finland

Kavindra Kumar Kesari & Janne Ruokolainen

Department of Molecular and Cellular Engineering, Sam Higginbottom University of Agriculture, Technology and Sciences, Naini, Allahabad, India

Ramendra Soni, Jonathan A. Lal & Vijay Tripathi

Department of Health Informatics, College of Public Health and Health Informatics, Qassim University, Al Bukayriyah, Saudi Arabia

Qazi Mohammad Sajid Jamal

Department of Computational Biology and Bioinformatics, Sam Higginbottom University of Agriculture, Technology and Sciences, Naini, Allahabad, India

Pooja Tripathi

Department of Biotechnology, School of Engineering & Technology, Sharda University, Greater Noida, UP, India

Niraj Kumar Jha

Department of Bioengineering, Faculty of Engineering, Integral University, Lucknow, India

Mohammed Haris Siddiqui

Department of Forestry, NERIST, Nirjuli, Arunachal Pradesh, India

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Wastewater treatment involves several steps. First, solids are removed. Then, bacteria break down harmful substances. Finally, the water is disinfected.

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Sewage and wastewater treatment is a crucial aspect of environmental conservation. This process involves the removal of contaminants from wastewater, primarily from household sewage, to produce an effluent that can be safely returned to the environment or reused.

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500 Words Essay on Sewage and Wastewater Treatment

Sewage and wastewater treatment are integral components of modern urban infrastructure, playing a crucial role in maintaining public health and environmental sustainability. They involve a series of physical, biological, and chemical processes designed to remove contaminants from wastewater, making it suitable for discharge back into the environment or for reuse.

The Importance of Sewage and Wastewater Treatment

The treatment of sewage and wastewater is not merely a matter of public health, but also one of environmental responsibility. Untreated wastewater carries a host of pathogens and pollutants that can cause disease outbreaks, degrade ecosystems, and contaminate water sources. Proper treatment reduces these risks, ensuring water quality and conserving precious resources.

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Wastewater treatment generally involves three stages: primary, secondary, and tertiary. The primary stage is a physical process, where large solids are separated from the wastewater through sedimentation. The secondary stage is a biological process, where bacteria and other microorganisms decompose the organic matter. The tertiary stage is a chemical process, where any remaining pollutants are removed to meet the required standards for discharge or reuse.

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In response to these challenges, innovative solutions are being developed. For example, the use of membrane bioreactors, which combine the secondary and tertiary stages into one process, can significantly increase treatment efficiency. Similarly, the adoption of anaerobic digestion can transform sludge into biogas, a renewable energy source, thereby reducing the environmental footprint of the treatment process.

Sewage and wastewater treatment stand at the intersection of public health, environmental conservation, and sustainable development. As the pressures on our water resources continue to grow, so too does the importance of these processes. By embracing new technologies and approaches, we can ensure that our treatment systems are not only effective but also sustainable, contributing to a healthier and greener future.

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Home — Essay Samples — Science — Agriculture — Steps of the Wastewater Treatment Process

Steps of The Wastewater Treatment Process

• Categories: Agriculture Water Pollution Water Quality

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Introduction, the steps of the wastewater treatment process, the environmental and health significance, the role of technological advancements.

• Preliminary Treatment : The first step involves the removal of large objects and debris, such as sticks, leaves, and plastics, through screens or gratings. This helps protect downstream equipment from damage and ensures that smaller particles can be effectively removed in subsequent treatment stages.
• Primary Treatment : In this phase, the wastewater is allowed to settle in large tanks, allowing solids to settle at the bottom and form sludge. This sludge is then removed and further treated or disposed of. Although primary treatment removes a significant portion of suspended solids, it is not effective in eliminating dissolved contaminants.
• Secondary Treatment : Secondary treatment is designed to remove dissolved and suspended biological matter, such as organic materials and bacteria. It relies on microorganisms to break down these pollutants into less harmful substances. Common methods include activated sludge processes and trickling filters, which provide a habitat for beneficial bacteria to thrive and purify the water.
• Tertiary Treatment : Also known as advanced treatment, this stage goes beyond secondary treatment to further polish the water quality. It involves additional processes like filtration, chemical treatment, and disinfection to remove remaining contaminants, including nutrients (e.g., nitrogen and phosphorus), pathogens, and trace chemicals. The treated water is now safe for release into the environment or for reuse.
• Metcalf & Eddy, Inc., & Tchobanoglous, G. (2002). Wastewater Engineering: Treatment and Reuse. McGraw-Hill Education.
• Cheremisinoff, N. P. (2019). Handbook of Water and Wastewater Treatment Technologies. Butterworth-Heinemann.
• Tchobanoglous, G., Burton, F. L., & Stensel, H. D. (2002). Wastewater Engineering: Treatment and Reuse (Metropolitan Los Angeles Edition). McGraw-Hill.
• Mara, D., & Horan, N. (2003). Handbook of Water and Wastewater Microbiology. Elsevier.
• Khan, S. R., Tawabini, B. S., & Al-Zahrani, M. A. (2019). Advanced Technologies in Water and Wastewater Management. Springer.
• US Environmental Protection Agency. (n.d.). Wastewater Technology Fact Sheet: Membrane Bioreactors.
• World Health Organization. (2011). Guidelines for Drinking-water Quality. World Health Organization.

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• Published: 22 February 2024

Fundamentals and applications in water treatment

Nature Water volume  2 ,  page 101 ( 2024 ) Cite this article

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For papers regarding water and wastewater treatment, we are interested in both conceptual advance and potential for practical use.

There should be some kind of limit to the number of times a journal writes about itself. It certainly may seem an overkill to publish an introspective Editorial just one issue after we reflected on what we achieved in our first year of existence 1 . However, it was precisely during the work of reflection in preparation for the January issue Editorial that we realized the need to dwell on the way we consider fundamental and applied research work — admittedly we also received some comments from our readership almost at the same time. Although such views could be applied to all of the topics we cover, they are easily exemplified by the areas of water and wastewater treatment, and for the sake of this Editorial we focus on those.

Already while planning the launch of Nature Water , it was clear to us that a journal on water and society should publish research that provides practical and sustainable solutions. We felt that it was particularly important to insist on this point because the tradition of journals in the Nature Portfolio has been for a long time to look for conceptual advance or mechanistic understanding — though the trend towards technology has changed in the last few years. In our view, if a manuscript demonstrates the potential real-world applicability of a concept developed previously, the potential impact could be enough, at least from an editorial perspective, to be considered for publication. Perhaps the best example of this line of reasoning is the paper by Song et al. that reported the demonstration of water capture in a very dry location with a device conceived previously 2 (Fig. 1 ). In terms of reviews, the Review Article by Dang et al. in this issue beautifully illustrates the concept of starting from the working principle of a technology to explore its real-life application even to an industrial level.

Adapted from ref. 2 , Springer Nature Ltd.

It is undeniable that most of the papers we have published so far have a strong applied slant. However, we want to clarify that we are still interested in fundamental insight that would have relevance for water treatment down the line. In our pre-launch collection we included papers like the one by Song et al. 3 that explored the mechanisms of water permeability in artificial aquaporins. In a News & Views in June 2023 4 we highlighted a paper studying the fundamental aspects of water transport in reverse osmosis membranes 5 . Among the papers we published, a clear example is the paper by Hu et al. 6 , which explored the effect of lattice strain and element speciation on the chemical microenvironment of Fe 0 particles with a view to optimize the reduction properties of the material. Although Fe 0 nanomaterials are considered for environmental remediation, the specific study investigates the change of their chemical properties by introducing a chemistry-related approach, and from an editorial perspective the findings were interesting enough in themselves to be considered without a practical demonstration. Another example is the review by Mitch et al. 7 (Fig. 2 ). Disinfection byproducts (DBPs) are clearly connected to water treatment. But the Review Article itself explores primarily the chemical properties of a special category of DBPs and the associated organic matter precursors.

Reproduced from ref. 7 , Springer Nature Ltd.

To state it plainly, we would not consider research about the chemical or physical properties of water with no connection to water treatment. Those results are of course of value, but in our view are better suited in venues focussing on chemistry or physics. However, if there is a connection with water treatment, our criteria for consideration remains solely the perceived significance and potential impact of the results, whether they improve our fundamental understanding or show promise for applications.

Nat. Water 2 , 1 (2024).

Song, W., Zheng, Z., Alawadhi, A. H. & Yaghi, O. M. Nat. Water 1 , 626–634 (2023).

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Song, W. et al. Nat. Nanotechnol. 15 , 73–79 (2020).

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Lueptow, R. M. Nat. Water 1 , 492–493 (2023).

Wang, L. et al. Sci. Adv. 9 , eadf8488 (2023).

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Mitch, W. A., Richardson, S. D., Zhang, X. & Gonsior, M. Nat. Water 1 , 336–347 (2023).

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Effectiveness of wastewater treatment systems in removing microbial agents: a systematic review

• Zahra Aghalari 1 ,
• Hans-Uwe Dahms 2 , 3 , 4 ,
• Mika Sillanpää 5 ,
• Juan Eduardo Sosa-Hernandez 6 &
• Roberto Parra-Saldívar 6  

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Due to unrestricted entry of wastewater into the environment and the transportation of microbial contaminants to humans and organisms, environmental protection requires the use of appropriate purification systems with high removal efficiency for microbial agents are needed. The purpose of this study was to determine the efficacy of current wastewater treatment systems in removing microbes and their contaminants.

A systematic review was conducted for all articles published in 5 Iranian environmental health journals in 11 years. The data were collected according to the inclusion and exclusion criteria and by searching the relevant keywords in the articles published during the years (2008–2018), with emphasis on the efficacy of wastewater treatment systems in removing microbial agents. Qualitative data were collected using a preferred reporting items for systematic reviews and meta-analyzes (PRISMA) standard checklist. After confirming the quality of the articles, information such as the name of the first author and the year of publication of the research, the type of study, the number of samples, the type of purification, the type of microbial agents and the rate of removal of microbial agents were entered into the checklist. Also the removal rates of the microbial agents mentioned in the studies were compared with united states environmental protection agency (US-EPA) standards.

In this study, 1468 articles retrieved from 118 issues of 5 environmental health journals were reviewed. After reviewing the quality of the articles in accordance with the research objectives, 14 articles were included in the study that were published between 2010 and 2018. In most studies, two main indicators Total coliforms and Fecal coliforms in wastewater were investigated. Removing fungi and viral contamination from wastewater was not found in any of the 14 studies. Different systems (activated sludge, stabilization ponds, wetlands, and low and medium pressure UV disinfection systems were used to remove microbial agents in these studies. Most articles used active sludge systems to remove Total coliforms and Fecal coliforms , which in some cases were not within the US-EPA standard. The removal of Cysts and Parasitic eggs was only reporte from stabilization pond systems (SPS) where removal efficiency was found in accordance with US-EPA standards.

Conclusions

Different types of activated sludge systems have higher efficacy to remove microbial agents and are more effective than other mentioned systems in removing the main indicators of sewage contamination including Total coliforms and Fecal coliforms . However, inappropriate operation, maintenance and inadequate handling of activated sludge can also reduce its efficiency and reduce the removal of microbial agents, which was reported in some studies. Therefore, it is recommended to conduct research on how to improve the operation, maintenance, and proper management of activated sludge systems to transfer knowledge to users of sludge systems and prevent further health issues related to microbial agents.

Introduction

Due to hazardous impacts of municipal, industrial and hospital wastewater on water, soil, air and agricultural products, wastewater treatment and the proper disposal of the sludge produced are indispensable from an environmental safety point of view [ 1 , 2 ]. Economically, effective wastewater treatment has important effects on saving water, and preventing unnecessary water losses [ 3 ]. In arid and semiarid countries such as Iran, the water demand has increased and annual rainfall is low also in regions of North Africa, Southern Europe, and in large countries such as Australia and the United States. Consequently, reuse of sewage is the most sustainable and long-term solution to the problem of water scarcity [ 4 , 5 ]. In the next 30 years, the world’s population will increase by more than double. Due to population growth, the amount of water available in 1960 was reduced to 3300 cubic meters and in 1995 it was reduced to 1250 cubic meters. This trend is projected to decrease to 650 cubic meters worldwide by 2025 [ 6 ]. Due to this water shortage crisis, water from wastewater treatment need to be reused increasingly in the near future [ 6 ]. Wastewater reuse requires treatment and application of appropriate wastewater treatment systems [ 7 ]. In recent years, increased research has been done on wastewater treatment using simple, low-cost, easy-to-use methods in developing countries [ 8 , 9 ]. Systems and processes such as activated sludge, aerated lagoons, stabilization ponds, natural and synthetic wetlands, trickling filters, rotating biological contactors (RBCs) have been used for wastewater treatment and removal of physical, chemical and biological contaminants [ 10 , 11 ]. Among different contaminants of wastewater, microbial agents becoming increasingly important and their removal efficiency should be reported in different wastewater treatment systems [ 12 , 13 ]. Biological contaminants in wastewater are different types of bacteria ( Fecal coliforms and Escherichia coli , Salmonella , Shigella , Vibrio cholerae ), diverse Parasite cysts and eggs , viruses and fungi. All of them can be hazardous to environmental and human health depending on the type and amount [ 14 , 15 ]. For example, bacteria in wastewater cause cholera, typhoid fever, and tuberculosis, viruses can cause hepatitis, and protozoa can cause dysentery [ 16 , 17 ]. Many microbial agents attached to suspended solids in wastewater if inadequately treated and wastewater discharge into the environment, such as river water, green space, and crops, put humans and aquatic organisms at risk [ 18 , 19 ]. Therefore, utilization of appropriate wastewater treatment systems tailored to a variety of microbial agents is essential to achieve as complete as possible elimination of biological agents. For example, in the study of Sharafi et al., (2015) with the aim of determining the removal efficiency of parasites from wastewater using a wetland system, the removal rates of protozoan cysts and Parasite eggs were 99.7 and 100%, respectively [ 20 ]. Okoh, et.al. (2010) reported that activated sludge processes, oxidation pools, activated carbon filtration, lime and chlorination coagulation eliminated removed 50–90% of wastewater viruses [ 21 ]. Wastewater from wastewater treatment plants, is used in Iran without restrictions and controls like in many other countries. Therefore, it is necessary to employ proper sewage treatment systems, before water can be publicly used such as for irrigation. This study is focusing on the efficacy of different wastewater treatment systems in removing microbial agents.

Study protocol

This systematic review study was carried out to determine the efficacy of wastewater treatment systems in the removal of microbial agents (bacteria, parasites, viruses, and fungi) by searching all articles published in 5 Iranian Journals of Environmental Health. The data were collected by referring to the specialized site of each journal, from the beginning of 2008 to the latest issue of 2018. Reviewed journals included; Iranian Journal of Health and Environment (IJHE), Journal of Environmental Health Engineering (JEHE), Journal of Research in Environmental Health (JREH), and two English-language journals, Environmental Health Engineering and Management Journal (EHEMJ), Journal of Environmental Health Science and Engineering (JEHSE).

Search strategy

Inquired information was collected by searching for keywords on the sites of Iranian specialty health journal. Key words included; ‘waste water’ OR ‘waste-water’ OR ‘wastewater treatment’ OR ‘effluent’ OR ‘sewage’ OR ‘sewage treatment’ OR ‘sewage disposal’ OR ‘wastewater disposal’ AND ‘treat’ OR ‘remove’ AND ‘microb’ AND ‘pathogen’ AND ‘bacteria’ AND ‘virus’ AND ‘parasite’ AND ‘FCs’ OR ‘Fecal coliforms ’ AND ‘Iran’.

A manual search was performed by checking all published articles. This way, the abstracts of all published articles were reviewed over the period of 11 years between 2008 and 2018.

Inclusion criteria

Inclusion criteria for this study included the year of publication, type of wastewater samples (municipal wastewater, domestic wastewater, hospital wastewater), number of samples (more than 5 wastewater samples), treatment procedures (different types), state the required and mention the type of purification (type of treatment, type of microbial agents, amount or percentage of microbial agents removed).

Exclusion criteria

Exclusion criteria for this study were: lack of access to the full article, inappropriate subject matter, inadequacy of the method of treatment and purification, lack of expression of the type of microbial agents removed, review studies, and letters to the editor.

Quality assessment articles

This study is based on standard checklist PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-analyzes). The US-based National Institute of Health Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies [ 22 ] for qualitative studies was used. This checklist is made based on the following criteria: Yes, No, cannot determine, Not applicable, and Not reported. It has eliminated the scoring problems. The checklist included 14 questions that were used for research purposes, samples, inclusion and exclusion criteria, findings, results and publication period of each of the 14 articles (Table  1 ).

Extract information from articles

In order to extract information, all articles were evaluated independently by two reviewers based on inclusion and exclusion criteria. Both reviewers eventually summarized the information and in cases where the information was inconsistent a third reviewer’s comments was used. The information extracted from the articles was included in the researcher’s checklist for qualitative approval and used in other prior author studies of this paper [ 23 , 24 , 25 ]. The checklist included the name of the first author, the year of publication of the research, the type of study, the number of samples, the type of purification, the type of microbial agents and the rate of microbial removal. Additionally, the removal rates of the microbial agents mentioned in the studies were compared with US-EPA standards [ 26 , 27 ] (Table  2 ).

Search results

In this study, 1468 articles related to 118 issues of 5 environmental health journals were reviewed. In the first phase of the search process, 216 articles on wastewater treatment were identified. Then, 196 inappropriate and irrelevant articles were excluded for the purpose of the study. Finally, after reviewing the information and quality of the articles, 14 articles were eligible for systematic review (Fig.  1 ).

Flowchart describing the study design

Descriptive results of articles

Of the 14 articles reviewed, the largest number of articles (9 articles; 64.2%) were published between 2014 and 2018. Most of the experiments were carried out on wastewater samples in Tehran (28.58%). In total, studies were conducted in 10 cities of Iran (Fig.  2 ).

Cities selected for wastewater sampling in 14 articles

Concerning the type of microbial agents, it was found that a total of 14 articles have eliminated types of bacteria and parasites from municipal, hospital and industrial wastewater (Fig.  3 ). In 11 articles, two main microbial indices ( Total coliforms and Fecal coliforms ) were used as bioindicators to evaluate the efficacy of the wastewater treatment systems (Fig. 3 ).

Types of microbial agents removed in wastewater based on the articles

Quality assessment of articles

The qualitative results of the articles showed that most of the studies were of good quality but in many articles the method of determination of sample size (Q5) was not specified. In the articles, participation rate of eligible persons, inclusion and exclusion criteria, exposure (s) were evaluated more than once, and blinding of participant exposure status was not relevant and not applicable (Q10, Q4, Q3 and Q12) (Table  3 ).

Article features

Articles on the efficacy of a variety of purification systems for the removal of microbial agents were published between 2010 and 2018. All studies don in the laboratory. The largest sample size was related to Derayat et al., 2011 [ 30 ] in Kermanshah with 120 wastewater samples. Wastewater studies were carried out in different cities of North, East, West and Central Iran. Most studies have investigated bacterial factors in wastewater and the efficacy of removing fungi and viral contamination in wastewater was not found in any study (Table  4 ). In most articles, the type of sewage treatment system was activated sludge. For example were the removal rates of microbial agents in wastewater investigated in the study by Derayat et al., 2011 [ 30 ], Baghapour et al., 2013 [ 31 ] and Nahavandi et al., 2015 [ 37 ] on Conventional Activated Sludge, Ghoreishi et al., 2016 [ 38 ] on extended aeration activated sludge (Table 4 ).

Evaluation of the removal of microbial agents in accordance with US-EPA standards showed that in some articles the removal of Total coliforms and Fecal coliforms was not within acceptable ranges. For example, in the study of Ghoreishi et al., 2016 [ 38 ], although several different systems were used to remove Total coliforms, eimination efficiency never reached US-EPA standards. Moreover, the activated sludge process did not have the efficiency to remove Parasitic eggs as reported in the study by Nahavandi et al., 2015 [ 37 ] (Table 4 ).

Examination of microbial removal rates in the study of Ghoreishi et al., 2016 [ 38 ] that none of the Total Coliforms removal was US-EPA standard although both extended aeration activated sludge and conventional activated sludge systems were used to remove Total coliforms . The US-EPA standard for Total coliforms removal is 1000 MPN/100 mL, and wastewater showing this amount of Total coliforms is capable of being discharged into the receiving waters [ 26 , 27 ]. A study by Paiva et al., 2015 on domestic wastewater in tropical Brazil also showed that removal of Total coliforms through the use of activated sludge was not a desirable remediation method [ 42 ]. The reason for the poor performance of activated sludge to remove Total coliforms can be attributed to factors such as management problems and operation of the activated sludge system, which results in the production of bulk waste and sludge. This problem is one of the most important disadvantages of activated sludge systems and should be addressed once a month by experienced staff and monitoring experts to correct it. Overall, different activated sludge systems are the best choice for this type of wastewater due to the amount of municipal wastewater pollutants because of high purification efficiency to reduce biochemical oxygen demand (BOD 5 ) [ 43 , 44 ].

Removal of Cysts and Parasitic eggs in the study of Derayat et al., (2011), which used stabilization pond systems, was reported as being in accordance with US-EPA standards [ 30 ]. A study by Amahmid et al. (2002) aimed at the treatment of municipal wastewater with a stabilized pond system in Morocco showing that Cyst and Parasitic egg removal efficiency was 100% and that the pond system showed a proper performance [ 45 ]. A large number of stabilized pond systems were been constructed and used in countries such as the United States, New Zealand, India, Pakistan, Jordan and Thailand [ 3 ]. In Iran, a number of these systems were constructed for the treatment of wastewater in Arak, Gilan West and Isfahan [ 46 ]. Stabilization ponds have a high acceptability due to their simplicity of operation, and lack of mechanical and electrical equipment compared to other sewage treatment systems, their high efficiency in removing pathogenic organisms [ 47 ]. A major drawback for stabilization ponds is the need for extensive land, the low quality of effluents due to the presence of algae, and odor production that limits the use of this type of treatment system near habitated areas. To improve the quality of resulting effluents, chemical compounds need to be consolidated, such as by coagulation and the application of microstrainers, stabilization ponds and rock filters [ 47 , 48 ].

As for wetlands by Karimi et al. (2014) on Fecal coliforms , Escherichia coli and Fecal streptococci show that wetlands did not perform well to remove microbial agents (removal rate for Fecal coliforms 1.13 × 1014 MPN/100 mL and Escherichia coli 5.03 × 1012 MPN/100 mL) [ 34 ]. In a study by Decamp et al. (2000), the mean removal of Escherichia coli through the wetland was 41 to 72% at the in situ scale and 96.6 to 98.9% at the experimental scale [ 49 ]. In the study of Evanson et al. (2006), Fecal coliforms removal rate was 82.7 to 95.99% [ 50 ]. Removal of Total coliforms and Fecal coliforms in the wetlands is done by various biological factors such as nematodes, protozoa, bacterial activity, bacteriophage production, chemical factors, oxidation reactions, bacterial uptake and toxicity [ 51 ] and the interference in each of these (microbial communities) will affect the rate of removal of Total coliforms and other microbial agents. Removal of pathogens such as Escherichia coli and Cryptosporidium was also performed in wetlands but is often not in compliance with environmental standards [ 52 ]. In addition, although wetlands are economical and widely used in wastewater treatment systems because of easy to operate, maintain, and operate at a low price [ 53 , 54 , 55 ], but they don’t seem to be a good option for removing all of the microbial agents.

In a study by Hashemi, et.al. (2010) on UV disinfection system included low pressure (LP) and UV disinfection system including medium pressure (MP) to remove Total coliforms , Fecal coliforms and Fecal streptococci. All investigated microbial agents were completely eliminated [ 28 ]. However, it was reported that the direct disinfection of secondary effluents with LP and MP systems and even their integration due to high concentrations of suspended solids was not possible. Therefore, disinfection of wastewater with UV irradiation requires higher effluent quality through improved system utilization or application of an advanced treatment plant prior to disinfection [ 28 ]. In 1988, about 300 and in 2004 about 4300 sewage treatment plants in the United States, (that are more than 20% of filtration plants) used a UV system for wastewater disinfection. The number of wastewater treatment plants having UV systems has increased in the US, Europe and East Asia. This trend is expected to expand further in the coming decades. Although the use of UV radiation for wastewater disinfection has many potential advantages, it also has disadvantages in terms of cost, lamp deposition, and the possible reactivation of targeted pathogenic microorganisms after treatment [ 56 ]. Wastewater treatment professionals should therefore be aware of new replacement processes and perform pilot scale assessments prior to changing treatment processes.

One of the strengths of this study is addressing the efficacy of wastewater treatment systems by comparing the removal efficiency of various microbial agents that have received little attention as yet. In most studies, only one type of system for removing different physical, chemical and microbial contaminants in a single type of wastewater was investigated and it was not possible to compare the removal efficiency of microbial agents. One of the limitations of this study was the lack of reviewing published articles on wastewater treatment systems in other than the 5 Iranian journals. This limitation, however, might be negligible because the research on wastewater treatment was done by environmental health professionals. Therefore, most studies in this area are published in specialized environmental health journals.

Different types of activated sludge systems have better efficacy to remove microbial agents and are more effective than other systems in removing the main indicators of sewage contamination including Total coliforms and Fecal coliforms . However, inappropriate operation, maintenance and inadequate handling of activated sludge can also reduce the efficiency of microbial agent removal, which has been reported in some studies. Therefore, it is recommended to conduct research on how to increase the operation, maintenance and proper management of activated sludge systems and provide the results to utility personnel responsible to work with this system in order to correct the system quality output in a timely manner. In future research, it is recommended that employed treatment methods integrate two or more purification systems, which then could more effectively remove microbial agents. Additionally, the reports of removal efficiency should include each of the indicated microbes so that health and environmental professionals can make better decisions about using the systems or prevent future eventualities.

Availability of data and materials

Not applicable.

Abbreviations

Anaerobic baffled reactor

Biochemical Oxygen Demand

Environmental Health Engineering and Management Journal

Fluidized Bed Reactor

Iranian Journal of Health and Environment

Journal of Environmental Health Engineering

Journal of Environmental Health Science and Engineering

Journal of Research in Environmental Health

Low pressure

Medium pressure

Most Probable Number

Preferred Reporting Items for Systematic Reviews and Meta-analyzes

Rotating Biological Contactors

Stabilization Pond Systems

United States Environmental Protection Agency

Ultraviolet

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Acknowledgements

Since this research is part of a research project approved at Gonabad University of Medical Sciences, it is hereby sponsored by Gonabad University of Medical Sciences Research and Technology, which supported the research (Project No. T/4/95) and the Code of Ethics. (IR.GMU.REC.1396.110), is appreciated.

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This research was funded by the Deputy of Research and Technology of Gonabad University of Medical Sciences. The funders did not have any role in the design of the study and collection, analysis, and interpretation of data and in writing the manuscript.

A grant from MOST to Tan Han Shih (Hans-Uwe Dahms) is gratefully acknowledged (MOST 107–2621-M-037-001 and MOST 108–2621-M-037-001 to T.H. Shih). A NSYSU/KMU collaboration is acknowledged (108-PO25).

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Zahra Aghalari

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ZA conceived the study, made final decisions on the inclusion of journal articles and extracted data from them, and wrote and revised the manuscript. HUD, MS, JESH and RPS wrote and revised the manuscript. All authors read and approved the final manuscript.

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Aghalari, Z., Dahms, HU., Sillanpää, M. et al. Effectiveness of wastewater treatment systems in removing microbial agents: a systematic review. Global Health 16 , 13 (2020). https://doi.org/10.1186/s12992-020-0546-y

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The Anti-Abortion Endgame That Erin Hawley Admitted to the Supreme Court

Somewhat lost in the debate around abortion pills and oral arguments that took place at the Supreme Court in FDA v. Alliance for Hippocratic Medicine on Tuesday was one deeply uncomfortable truth: The very notion of what it means to practice emergency medicine is in dispute, with anti-abortion doctors insisting upon a right to refuse treatment for any patient who doesn’t meet their test of moral purity. Indeed, the right asserted is that in the absence of certainty about which patients are morally pure, the doctors want to deny medication to all patients, nationwide.

In public, the plaintiffs in this case—a group of doctors and dentists seeking to ban medication abortion—have long claimed they object to ending “unborn life” by finishing an “incomplete or failed” abortion at the hospital. But in court, they went much further. Their lawyer, Erin Hawley, admitted at oral argument that her clients don’t merely oppose terminating a pregnancy—they are pursuing the right to turn away a patient whose pregnancy has already been terminated . Indeed, they appear to want to deny even emergency care to patients whose fetus is no longer “alive,” on the grounds that the patient used an abortion drug earlier in the process. And they aim to deploy this broad fear of “complicity” against the FDA, to demand a nationwide prohibition on the abortion pill to ensure that they need never again see (and be forced to turn away) patients who’ve previously taken it. This is not a theory of being “complicit” in ending life. It is a theory that doctors can pick and choose their patients based on the “moral distress” they might feel in helping them.

It should come as no surprise that the same judge who tried to ban mifepristone in this case, Matthew Kacsmaryk, has also attempted to legalize anti-LGBTQ+ discrimination in health care nationwide. This is the ballgame: weaponize subjective religious beliefs against secular society to degrade the quality of care for everyone. If you can’t persuade Americans to adopt hardcore evangelical views, exploit the legal system to coerce them into it anyway.

Alliance for Hippocratic Medicine is at once embarrassingly frivolous and existentially important. Don’t let the jokes about how silly the Comstock Act seems , or how speculative the theory of standing is, get in the way of taking a serious look at the claims on offer. The plaintiffs say they are terrified that one day, a patient may walk into their emergency room suffering complications from a medication abortion prescribed by some other doctor. This patient may need their assistance completing the abortion or simply recovering from the complete abortion, which these plaintiffs deem “complicity” in sin. And they say the solution is either a total, nationwide ban on mifepristone, the first drug in the medication abortion sequence, or a draconian (and medically unnecessary) set of restrictions that would place mifepristone out of reach for many patients. (The U.S. Court of Appeals for the 5 th Circuit ruled to reinstate those restrictions at their behest.)

It is a twisted line of logic, one that should never have reached the Supreme Court in the first place. But it is also a product of the court’s past indulgence of outlandish claims about moral “complicity.” As was made plain in the oral arguments and briefing, activist doctors are no longer satisfied with personal conscience exemptions already granted under state and federal law; they now insist that nobody, anywhere, should have access to the abortion pill, in order to ensure that they themselves won’t have to treat patients who took one. At a minimum, they say, they should be able to radically roll back access to the pill in all 50 states to reduce the odds that one of these handful of objectors might someday encounter a patient who took it. This extremist argument lays bare the transformation of the idea of “complicity” from a shield for religious dissenters to a sword for ideologues desperate to seize control over other people’s lives and bodies.

At oral arguments, several justices pressed Hawley, who argued on behalf of Alliance for Hippocratic Medicine, with an obvious retort: Why can’t her clients simply refuse to treat these hypothetical someday patients on the grounds that they cannot help end the “life” of a fetus or embryo? After all, federal law guarantees doctors the right not to have to provide an abortion if doing so is “contrary to his religious beliefs or moral convictions.” Justices Amy Coney Barrett and Brett Kavanaugh secured assurances from Solicitor General Elizabeth Prelogar, early in the arguments, that under no circumstances could the government force any health care provider to ever participate in an abortion in violation of their conscience. Justice Elena Kagan asked Prelogar: “Suppose somebody has bled significantly, needs a transfusion, or, you know, any of a number of other things that might happen.” Would the plaintiffs object to treating them? Prelogar said the record was unclear.

Hawley, who is married to far-right Republican Sen. Josh Hawley, then approached the lectern and cleared up any confusion: Yes, she insisted, treating a patient who has undergone a medication abortion violates the conscience of the plaintiff physicians even if there is no “live” fetus or embryo to terminate anymore. “Completing an elective abortion means removing an embryo fetus, whether or not they’re alive, as well as placental tissue,” Hawley told Kagan. So the plaintiffs don’t object just to taking a “life.” They also object to the mere act of removing leftover tissue, even from the placenta.

Of course, these doctors must remove “dead” fetal tissue and placentas all the time—from patients who experienced a spontaneous miscarriage. By their own admission, the plaintiffs regularly help women complete miscarriages through surgery or medication. Those women they will gladly treat. Other women, though—the ones who induced their own miscarriage via medication—are too sinful to touch. Before the plaintiffs can administer even lifesaving emergency treatment, they need to know the circumstances of this pregnancy loss: Spontaneous miscarriages are OK; medication abortions are not.

Justice Ketanji Brown Jackson, too, zeroed in on this admission. She told Hawley that she had thought the objection was to “participating in a procedure that is ending the life [of the fetus].” Hawley told her no: Any participation in an abortion, even through the indirect treatment of a patient without a “live” fetus, violated the doctors’ conscience. So, wait. What about “handing them a water bottle?” Jackson asked. Hawley dodged the question, declining to say whether helping a patient hydrate would constitute impermissible complicity in sin.

All this is reminiscent of Little Sisters of the Poor , a case about a Catholic charitable group that was afforded an exemption from the Affordable Care Act’s contraception mandate. The Little Sisters were asked to check a box signaling to the government that they could not comply with the mandate, at which point the government would step in to cover their employees. But the Little Sisters refused, viewing this action—the checking of a box to opt out of coverage—as “complicity” in abortion because it would in turn trigger government payment for contraception (which they viewed as abortifacients). The Supreme Court and the Trump administration ultimately indulged the Little Sisters’ claim .

Here, we have emergency room physicians asserting that they will not participate in lifesaving medical intervention unless they approve of the reason for the pregnancy loss. Presumably, if the pregnant patient is an unwed mother, or a gay or transgender person, the doctor would be similarly complicit in sin and decline service. Seen through this lens, since one can never know which sins one is enabling in the ER, each and every day, a narrow conscience exemption becomes a sweeping guarantee that absolutely nobody in the country can ever have access to basic health care, let alone miscarriage management. (Of course, these plaintiffs might focus only on one set of “sins” they see as relevant.) In a country effectively governed by Kacsmaryk and his plaintiff friends, a gay person suffering a stroke could be turned away from any hospital because of his sexual orientation, all to spare a doctor from a glancing encounter with prior sin. As Tobias Barrington Wolff, a professor of law at the University of Pennsylvania Law School, put it to us in an email, this unbounded view of complicity “is part of enacting the social death of people and practices you abhor, which in turn can contribute to the material death of people and practices you abhor.”

One of the most exhausting lessons of post- Roe America is that being “pro-life” definitively means privileging the life of the presumptively sin-free unborn—or even their “dead” remains—over the life of the sin-racked adults who carry them. This is why women are left to go septic or to hemorrhage in hospital parking lots; it is why C-sections are performed in nonviable pregnancies, at high risk to mothers; it’s why the women who sued in Texas to secure exceptions to that state’s abortion ban are condemned by the state as sinners and whores . And it’s why—in the eyes of the Alliance for Hippocratic Medicine — it is a greater hardship for a physician to “waste precious moments scrubbing in, scrubbing out” of emergency surgery, as Hawley put it, so long as they don’t believe that the emergency warrants their professional services, than it is for a pregnant person, anywhere in the country, including in states that permit abortion, to be forced to give birth.

At oral argument, Hawley explained that her clients have “structured [their] medical practice to bring life into the world. When they are called from their labor and delivery floor down to the operating room to treat a woman suffering from abortion drug harm, that is diametrically opposed to why they entered the medical profession. It comes along with emotional harm.” The emotional harm alleged here is that unless these doctors approve of the specific circumstances of the ER visit, they violate not only their own medical preference but also their religious convictions. But they will never truly know enough about the sins of their patients to be able to shield themselves against being a link in a chain of subjective lifelong sin. And to be a doctor, especially an emergency physician, should be to understand that your patients’ private choices and spiritual life are not really open to your pervasive and vigilant medical veto. This deep-rooted suspicion of patients deemed insufficiently pure for lifesaving treatment didn’t begin with the availability of medication abortion. It will assuredly not end there.