Sewage treatment

  (Redirected from Sewage treatment plants)

Sewage treatment (or domestic wastewater treatment, municipal wastewater treatment) is a type of wastewater treatment which aims to remove contaminants from sewage to produce an effluent that is suitable for discharge to the surrounding environment or an intended reuse application, thereby preventing water pollution from raw sewage discharges.[2] Sewage contains wastewater from households and businesses and possibly pre-treated industrial wastewater. There are a high number of sewage treatment processes to choose from. These can range from decentralized systems (including on-site treatment systems) to large centralized systems involving a network of pipes and pump stations (called sewerage) which convey the sewage to a treatment plant. For cities that have a combined sewer, the sewers will also carry urban runoff (stormwater) to the sewage treatment plant. Sewage treatment often involves two main stages, called primary and secondary treatment, while advanced treatment also incorporates a tertiary treatment stage with polishing processes and nutrient removal. Secondary treatment can reduce organic matter (measured as biological oxygen demand) from sewage,  using aerobic or anaerobic biological processes

Aerial photo of Kuryanovo activated sludge sewage treatment plant in Moscow, Russia.
Constructed wetlands for sewage treatment near Gdansk, Poland
Waste stabilization ponds at a sewage treatment plant in the South of France.
UASB for domestic wastewater treatment in Bucaramanga, Colombia
Sewage treatment plants (STPs) come in many different sizes and process configurations. Clockwise from top left: Aerial photo of Kuryanovo activated sludge STP in Moscow, Russia; Constructed wetlands STP near Gdansk, Poland; Waste stabilization ponds STP in the South of France; Upflow anaerobic sludge blanket STP in Bucaramanga, Colombia
Sewage treatment
SynonymWastewater treatment plant (WWTP), water reclamation plant
Position in sanitation chainTreatment
Application levelCity, neighborhood[1]
Management levelPublic
InputsSewage, could also be just blackwater (waste), greywater[1]
OutputsEffluent, sewage sludge, possibly biogas (for some types)[1]
TypesList of wastewater treatment technologies
Environmental concernsWater pollution , Environmental health, Public health, sewage sludge disposal issues

A large number of sewage treatment technologies have been developed, mostly using biological treatment processes. Engineers and decision makers need to take into account technical and economical criteria, as well as quantitative and qualitative aspects of each alternative when choosing a suitable technology.[3]: 215  Often, the main criteria for selection are: desired effluent quality, expected construction and operating costs, availability of land, energy requirements and sustainability aspects. In developing countries and in rural areas with low population densities, sewage is often treated by various on-site sanitation systems and not conveyed in sewers. These systems include septic tanks connected to drain fields, on-site sewage systems (OSS), vermifilter systems and many more. On the other hand, advanced and relatively expensive sewage treatment plants in cities that can afford them may include tertiary treatment with disinfection and possibly even a fourth treatment stage to remove micropollutants.

At the global level, an estimated 52% of sewage is treated.[4] However, sewage treatment rates are highly unequal for different countries around the world. For example, while high-income countries treat approximately 74% of their sewage, developing countries treat an average of just 4.2%.[4]

The treatment of sewage is part of the field of sanitation. Sanitation also includes the management of human waste and solid waste as well as stormwater (drainage) management.[5] The term "sewage treatment plant" is often used interchangeably with the term "wastewater treatment plant".[6][3]


Activated sludge sewage treatment plant in Massachusetts, US

The term "sewage treatment plant" (STP) (or "sewage treatment works" in some countries) is nowadays often replaced with the term wastewater treatment plant (WWTP).[6][7] Strictly speaking, the latter is a broader term that can also refer to industrial wastewater.

Purposes and overviewEdit

The overall aim of treating sewage is to produce an effluent that can be discharged to the environment while causing as little water pollution as possible, or to produce an effluent that can be reused in a useful manner.[8] This is achieved by removing contaminants from the sewage. It is a form of waste management.

With regards to biological treatment of sewage, the treatment objectives can include various degrees of the following: transform dissolved and particulate biodegradable components (especially organic matter) into acceptable end products, transform and remove nutrients (nitrogen and phosphorus), remove or inactivate pathogenic organisms, and remove specific trace organic constituents (micropollutants).[6]: 548 

Some types of sewage treatment produce sewage sludge which can be treated before safe disposal or reuse. Under certain circumstances, the treated sewage sludge might be termed "biosolids" and can be used as a fertilizer.

Sewage characteristicsEdit

Raw sewage arriving at a sewage treatment plant in Syria (note that protective gloves should be worn when sampling sewage).

Sewage (or domestic sewage, domestic wastewater, municipal wastewater) is a type of wastewater that is produced by a community of people. It is typically transported through a sewer system.[9]: 175  Sewage consists of wastewater discharged from residences and from commercial, institutional and public facilities that exist in the locality.[10]: 10  Sub-types of sewage are greywater (from sinks, bathtubs, showers, dishwashers, and clothes washers) and blackwater (the water used to flush toilets, combined with the human waste that it flushes away). Sewage also contains soaps and detergents. Food waste may be present from dishwashing, and food quantities may be increased where garbage disposal units are used. In regions where toilet paper is used rather than bidets, that paper is also added to the sewage. Sewage contains macro-pollutants and micro-pollutants, and may also incorporate some municipal solid waste and pollutants from industrial wastewater.

Sewage usually travels from a building's plumbing either into a sewer, which will carry it elsewhere, or into an onsite sewage facility. Collection of sewage of several households together usually takes places in either sanitary sewers or combined sewers. The former is designed to exclude stormwater flows whereas the latter is designed to also take stormwater. The production of sewage generally corresponds to the water consumption. A range of factors influence water consumption and hence the sewage flowrates per person. These include: Water availability (the opposite of water scarcity), water supply options, climate (warmer climates may lead to greater water consumption), community size, economic level of the community, level of industrialization, metering of household consumption, water cost and water pressure.[10]: 20 

The main parameters in sewage that are measured to assess the sewage strength or quality as well as treatment options include: solids, indicators of organic matter, nitrogen, phosphorus, and indicators of fecal contamination.[10]: 33  These can be considered to be the main macro-pollutants in sewage. Sewage contains pathogens which stem from fecal matter. The following four types of pathogens are found in sewage: pathogenic bacteria, viruses, protozoa (in the form of cysts or oocysts) and helminths (in the form of eggs).[11][12] In order to quantify the organic matter, indirect methods are commonly used: mainly the Biochemical Oxygen Demand (BOD) and the Chemical Oxygen Demand (COD).[10]: 36 


Sewerage (or sewage system) is the infrastructure that conveys sewage or surface runoff (stormwater, meltwater, rainwater) using sewers. It encompasses components such as receiving drains, manholes, pumping stations, storm overflows, and screening chambers of the combined sewer or sanitary sewer. Sewerage ends at the entry to a sewage treatment plant or at the point of discharge into the environment. It is the system of pipes, chambers, manholes, etc. that conveys the sewage or storm water.

In many cities, sewage (or municipal wastewater) is carried together with stormwater, in a combined sewer system, to a sewage treatment plant. In some urban areas, sewage is carried separately in sanitary sewers and runoff from streets is carried in storm drains. Access to these systems, for maintenance purposes, is typically through a manhole. During high precipitation periods a sewer system may experience a combined sewer overflow event or a sanitary sewer overflow event, which forces untreated sewage to flow directly to receiving waters. This can pose a serious threat to public health and the surrounding environment.

The system of sewers is called sewerage or sewerage system in British English and sewage system in American English.[13]

Types of treatment processesEdit

Sewage can be treated close to where the sewage is created, which may be called a "decentralized" system or even an "on-site" system (on-site sewage facility, septic tanks, etc.). Alternatively, sewage can be collected and transported by a network of pipes and pump stations to a municipal treatment plant. This is called a "centralized" system (see also sewerage and pipes and infrastructure).

A large number of sewage treatment technologies have been developed, mostly using biological treatment processes (see list of wastewater treatment technologies). Very broadly, they can be grouped into high tech (high cost) versus low tech (low cost) options, although some technologies might fall into either category. Other grouping classifications are "intensive" or "mechanized" systems (more compact, and frequently employing high tech options) versus "extensive" or "natural" or "nature-based" systems (usually using natural treatment processes and occupying larger areas) systems. This classification may be sometimes oversimplified, because a treatment plant may involve a combination of processes, and the interpretation of the concepts of high tech and low tech, intensive and extensive, mechanized and natural processes may vary from place to place.

Low tech, extensive or nature-based processesEdit

Constructed wetland (vertical flow) at Center for Research and Training in Sanitation, Belo Horizonte, Brazil
Trickling filter sewage treatment plant at Onça Treatment Plant, Belo Horizonte, Brazil.

Examples for more low-tech, "natural", often less expensive sewage treatment systems are shown below. They often use little or no energy. Some of these systems do not provide a high level of treatment, or only treat part of the sewage (for example only the toilet wastewater), or they only provide pre-treatment, like septic tanks. On the other hand, some systems are capable of providing a good performance, satisfactory for several applications. Many of these systems are based on natural treatment processes, requiring large areas, while others are more compact. In most cases, they are used in rural areas or in small to medium-sized communities. For example, waste stabilization ponds are a low cost treatment option with practically no energy requirements but they require a lot of land.[3]: 236  Due to their technical simplicity, most of the savings (compared with high tech systems) are in terms of operation and maintenance costs.[3]: 220–243 

Examples for systems that can provide full or partial treatment for toilet wastewater only:

High tech, intensive or mechanized processesEdit

Aeration tank of activated sludge sewage treatment plant (fine-bubble diffusers) near Adelaide, Australia

Examples for more high-tech, intensive or "mechanized", often relatively expensive sewage treatment systems are listed below. Some of them are energy intensive as well. Many of them provide a very high level of treatment. For example, broadly speaking, the activated sludge process achieves a high effluent quality but is relatively expensive and energy intensive.[3]: 239 

Disposal or treatment optionsEdit

There are other process options which may be classified as disposal options, although they can also be understood as basic treatment options. These include: Application of sludge, irrigation, soak pit, leach field, fish pond, floating plant pond, water disposal/groundwater recharge, surface disposal and storage.[14]: 138 

Application of sewage to land can be considered as a form of final disposal or of treatment, or both.[3]: 189  It leads to groundwater recharge and/or to evapotranspiration. Land application include slow-rate systems, rapid infiltration, subsurface infiltration, overland flow. It is done by flooding, furrows, sprinkler and dripping. It is a treatment/disposal system that requires a large amount of land per person.

Design aspectsEdit

Upflow anaerobic sludge blanket (UASB) reactor in Brazil (picture from a small-sized treatment plant), Center for Research and Training in Sanitation, Belo Horizonte, Brazil

Process selectionEdit

When choosing a suitable sewage treatment process, decision makers need to take into account technical and economical criteria, as well as quantitative and qualitative aspects of each alternative.[3]: 215  Therefore, each analysis is site-specific. A life cycle assessment (LCA) can be used, and criteria or weightings can be attributed to the various aspects. The final decision may have a degree of subjectivity.[3]: 216  A range of publications exist to help with technology selection.[3]: 221 [14][15][16]

In industrialized countries, the critical items in process selection are, in decreasing order of importance: efficiency, reliability, sludge disposal aspects and land requirements. In developing countries, the main critical items might be different and revolve more around construction costs, sustainability, simplicity and operational costs.[3]: 218 

Choosing the most suitable treatment process is complicated and requires expert inputs, often in the form of feasibility studies. This is because the main important factors to be considered when evaluating and selecting sewage treatment processes are numerous: process applicability, applicable flow, acceptable flow variation, influent characteristics, inhibiting or refractory compounds, climatic aspects, process kinetics and reactor hydraulics, performance, treatment residuals, sludge processing, environmental constraints, chemical product requirements, energy requirements, requirements of other resources, personnel requirements, operating and maintenance requirements, ancillary processes, reliability, complexity, compatibility, area availability.[3]: 219 

With regards to environmental impacts the following aspects are included in the selection process: Odors, vector attraction, sludge transportation, sanitary risks, air contamination, soil and subsoil contamination, surface water pollution or groundwater contamination, devaluation of nearby areas, inconvenience to the nearby population.[3]: 220 

Odor controlEdit

Odors emitted by sewage treatment are typically an indication of an anaerobic or "septic" condition.[17] Early stages of processing will tend to produce foul-smelling gases, with hydrogen sulfide being most common in generating complaints. Large process plants in urban areas will often treat the odors with carbon reactors, a contact media with bio-slimes, small doses of chlorine, or circulating fluids to biologically capture and metabolize the noxious gases.[18] Other methods of odor control exist, including addition of iron salts, hydrogen peroxide, calcium nitrate, etc. to manage hydrogen sulfide levels.[19]

Energy requirementsEdit

The energy requirements vary with type of treatment process as well as sewage strength. For example, constructed wetlands and stabilization ponds have low energy requirements, associated mainly with the occasional presence of pumps and other equipment. On the other hand, the activated sludge process includes an aeration step, which is highly energy consuming.[20] Sewage treatment plants that produce biogas in their sewage sludge treatment process with anaerobic digestion can produce enough energy to meet most of the energy needs of the sewage treatment plant itself.[6]: 1505  For activated sludge treatment plants in the United States, around 30 percent of the annual operating costs is usually required for energy.[6]: 1703  Most of this electricity is used for aeration, pumping systems and equipment for the dewatering and drying of sewage sludge. Advanced sewage treatment plants, e.g. for nutrient removal, require more energy than plants that only achieve primary or secondary treatment.[6]: 1704 

Small rural plants using trickling filters may operate with no net energy requirements, the whole process being driven by gravitational flow, including tipping bucket flow distribution and the desludging of settlement tanks to drying beds. This is usually only practical in hilly terrain and in areas where the treatment plant is relatively remote from housing because of the difficulty in managing odors.[21] [22]

Co-treatment of industrial effluentEdit

In highly regulated developed countries, industrial wastewater usually receives at least pretreatment if not full treatment at the factories themselves to reduce the pollutant load, before discharge to the sewer. The pretreatment has the following aims: to remove constituents that may pose risks to the sewerage system and its workers; prevent toxic or inhibitory compounds to the microorganisms in the biological stage in the municipal treatment plant; hinder beneficial use of the produced sewage sludge; or that will still be present in the final effluent from the treatment plant.[3]: 59  Some industrial wastewater may contain pollutants which cannot be removed by sewage treatment plants. Also, variable flow of industrial waste associated with production cycles may upset the population dynamics of biological treatment units.

Design aspects of secondary treatment processesEdit

A poorly maintained anaerobic treatment pond in Kariba, Zimbabwe (sludge needs to be removed)

Non-sewered areasEdit

Urban residents in many parts of the world rely on on-site sanitation systems without sewers, such as septic tanks and pit latrines, and fecal sludge management in these cities is an enormous challenge.[23]

For sewage treatment the use of septic tanks and other On-Site Sewage Facilities (OSSF) is widespread in some rural areas, for example serving up to 20 percent of the homes in the U.S.[24]

Available process stepsEdit

Sewage treatment often involves two main stages, called primary and secondary treatment, while advanced treatment also incorporates a tertiary treatment stage with polishing processes.[25] Different types of sewage treatment may utilize some or all of the process steps listed below.

Preliminary treatmentEdit

Preliminary treatment (sometimes called pretreatment) removes coarse materials that can be easily collected from the raw sewage before they damage or clog the pumps and sewage lines of primary treatment clarifiers.


Preliminary treatment arrangement at small and medium-sized sewage treatment plants: Manually-cleaned screens and grit chamber (Jales Treatment Plant, São Paulo, Brazil)

The influent in sewage water passes through a bar screen to remove all large objects like cans, rags, sticks, plastic packets, etc. carried in the sewage stream.[26] This is most commonly done with an automated mechanically raked bar screen in modern plants serving large populations, while in smaller or less modern plants, a manually cleaned screen may be used. The raking action of a mechanical bar screen is typically paced according to the accumulation on the bar screens and/or flow rate. The solids are collected and later disposed in a landfill, or incinerated. Bar screens or mesh screens of varying sizes may be used to optimize solids removal. If gross solids are not removed, they become entrained in pipes and moving parts of the treatment plant, and can cause substantial damage and inefficiency in the process.[27]: 9 

Grit removalEdit

Preliminary treatment: Horizontal flow grit chambers at a sewage treatment plant in Juiz de Fora, Minas Gerais, Brazil

Grit consists of sand, gravel, rocks, and other heavy materials. Preliminary treatment may include a sand or grit removal channel or chamber, where the velocity of the incoming sewage is reduced to allow the settlement of grit. Grit removal is necessary to (1) reduce formation of deposits in primary sedimentation tanks, aeration tanks, anaerobic digesters, pipes, channels, etc. (2) reduce the frequency of tank cleaning caused by excessive accumulation of grit; and (3) protect moving mechanical equipment from abrasion and accompanying abnormal wear. The removal of grit is essential for equipment with closely machined metal surfaces such as comminutors, fine screens, centrifuges, heat exchangers, and high pressure diaphragm pumps.

Grit chambers come in three types: horizontal grit chambers, aerated grit chambers, and vortex grit chambers. Vortex grit chambers include mechanically induced vortex, hydraulically induced vortex, and multi-tray vortex separators. Given that traditionally, grit removal systems have been designed to remove clean inorganic particles that are greater than 0.210 millimetres (0.0083 in), most of the finer grit passes through the grit removal flows under normal conditions. During periods of high flow deposited grit is resuspended and the quantity of grit reaching the treatment plant increases substantially. It is therefore important that the grit removal system not only operates efficiently during normal flow conditions but also under sustained peak flows when the greatest volume of grit reaches the plant.[6]

Flow equalizationEdit

Equalization basins can be used to achieve flow equalization, with the aim to reduce peak dry-weather flows or peak wet-weather flows in the case of combined sewer systems.[6]: 334  The benefits are performance improvements of the biological treatment processes, the secondary clarifiers and any effluent filtration equipment.[6]: 334  Disadvantages include the basins' capital cost and space requirements. Basins can also provide a place to temporarily hold, dilute and distribute batch discharges of toxic or high-strength wastewater which might otherwise inhibit biological secondary treatment (such was wastewater from portable toilets or fecal sludge that is brought to the sewage treatment plant in vacuum trucks). Flow equalization basins require variable discharge control, typically include provisions for bypass and cleaning, and may also include aerators and odor control.[28]

Fat and grease removalEdit

In some larger plants, fat and grease are removed by passing the sewage through a small tank where skimmers collect the fat floating on the surface. Air blowers in the base of the tank may also be used to help recover the fat as a froth. Many plants, however, use primary clarifiers with mechanical surface skimmers for fat and grease removal.

Primary treatmentEdit

Rectangular primary settling tanks at a sewage treatment plant in Oregon, USA

Primary treatment is the "removal of a portion of the suspended solids and organic matter from the sewage".[6]: 11 It consists of allowing sewage to pass slowly through a basin where heavy solids can settle to the bottom while oil, grease and lighter solids float to the surface and are skimmed off. These basis are called "primary sedimentation tanks" or "primary clarifiers" and typically have a hydraulic retention time (HRT) of 1.5 to 2.5 hours.[6]: 398  The settled and floating materials are removed and the remaining liquid may be discharged or subjected to secondary treatment. Primary settling tanks are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank where it is pumped to sludge treatment facilities.[27]: 9–11 

Sewage treatment plants that are connected to a combined sewer system sometimes have a bypass arrangement after the primary treatment unit. This means that during very heavy rainfall events, the secondary and tertiary treatment systems can be bypassed to protect them from hydraulic overloading, and the mixture of sewage and storm-water receives primary treatment only.[29]

Primary sedimentation tanks can be expected to remove 50-70% of the suspended solids and 25-40% of the biological oxygen demand (BOD).[6]: 396 

Secondary treatmentEdit

Simplified process flow diagram for a typical large-scale treatment plant using the activated sludge process.

The main processes involved in secondary sewage treatment are designed to remove as much of the solid material as possible.[25] They use biological processes to digest and remove the remaining soluble material, especially the organic fraction. This can be done with either suspended-growth or biofilm processes. The microorganisms that feed on the organic matter present in the sewage grow and multiply, constituting the biological solids, or biomass. These grow and group together in the form of flocs or biofilms and, in some specific processes, as granules. In several treatment processes, the biological floc or biofilm and remaining fine solids can then be settled as a sludge, leaving a liquid substantially free of solids, and with a greatly reduced concentration of pollutants.[25]

Secondary treatment can reduce organic matter (measured as biological oxygen demand) from sewage,  using aerobic or anaerobic processes. The organisms involved in these processes are sensitive to the presence of toxic materials, although these are not expected to be present at high concentrations in typical municipal sewage.

Secondary treatment is the removal of biodegradable organic matter (in solution or suspension) from sewage or similar kinds of wastewater.[30]: 11  The aim is to achieve a certain degree of effluent quality in a sewage treatment plant suitable for the intended disposal or reuse option. A "primary treatment" step often precedes secondary treatment, whereby physical phase separation is used to remove settleable solids. During secondary treatment, biological processes are used to remove dissolved and suspended organic matter measured as biochemical oxygen demand (BOD). These processes are performed by microorganisms in a managed aerobic or anaerobic process depending on the treatment technology. Bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, and organic short-chain carbon molecules from human waste, food waste, soaps and detergent) while reproducing to form cells of biological solids. Secondary treatment is widely used in sewage treatment and is also applicable to many agricultural and industrial wastewaters.

Tertiary treatmentEdit

Overall setup for a micro filtration system

Advanced sewage treatment generally involves three main stages, called primary, secondary and tertiary treatment but may also include intermediate stages and final polishing processes. The purpose of tertiary treatment (also called "advanced treatment") is to provide a final treatment stage to further improve the effluent quality before it is discharged to the receiving water body or reused. More than one tertiary treatment process may be used at any treatment plant. If disinfection is practiced, it is always the final process. It is also called "effluent polishing". Tertiary treatment may include biological nutrient removal (alternatively, this can be classified as secondary treatment), disinfection and removal of micropollutants, such as environmental persistent pharmaceutical pollutants.

Tertiary treatment is sometimes defined as anything more than primary and secondary treatment in order to allow discharge into a highly sensitive or fragile ecosystem such as estuaries, low-flow rivers or coral reefs.[31] Treated water is sometimes disinfected chemically or physically (for example, by lagoons and microfiltration) prior to discharge into a stream, river, bay, lagoon or wetland, or it can be used for the irrigation of a golf course, greenway or park. If it is sufficiently clean, it can also be used for groundwater recharge or agricultural purposes.

Sand filtration removes much of the residual suspended matter.[27]: 22–23  Filtration over activated carbon, also called carbon adsorption, removes residual toxins.[27]: 19  Micro filtration or synthetic membranes are also used, for example in membrane bioreactors which also remove pathogens.[6]: 854 

Settlement and further biological improvement of treated sewage may be achieved through storage in large man-made ponds or lagoons. These lagoons are highly aerobic, and colonization by native macrophytes, especially reeds, is often encouraged.


Disinfection of treated sewage may be attempted to kill pathogens (disease-causing microorganisms) prior to disposal, and is increasingly effective after more elements of the foregoing treatment sequence have been completed.[32]: 359  The purpose of disinfection in the treatment of sewage is to substantially reduce the number of pathogens in the water to be discharged back into the environment or to be reused. The effectiveness of disinfection depends on the quality of the water being treated (e.g. turbidity, pH, etc.), the type of disinfection being used, the disinfectant dosage (concentration and time), and other environmental variables. Water with high turbidity will be treated less successfully, since solid matter can shield organisms, especially from ultraviolet light or if contact times are low. Generally, short contact times, low doses and high flows all militate against effective disinfection. Common methods of disinfection include ozone, chlorine, ultraviolet light, or sodium hypochlorite.[27]: 16  Monochloramine, which is used for drinking water, is not used in the treatment of sewage because of its persistence.

Chlorination remains the most common form of treated sewage disinfection in many countries due to its low cost and long-term history of effectiveness. One disadvantage is that chlorination of residual organic material can generate chlorinated-organic compounds that may be carcinogenic or harmful to the environment. Residual chlorine or chloramines may also be capable of chlorinating organic material in the natural aquatic environment. Further, because residual chlorine is toxic to aquatic species, the treated effluent must also be chemically dechlorinated, adding to the complexity and cost of treatment.

Ultraviolet (UV) light can be used instead of chlorine, iodine, or other chemicals. Because no chemicals are used, the treated water has no adverse effect on organisms that later consume it, as may be the case with other methods. UV radiation causes damage to the genetic structure of bacteria, viruses, and other pathogens, making them incapable of reproduction. The key disadvantages of UV disinfection are the need for frequent lamp maintenance and replacement and the need for a highly treated effluent to ensure that the target microorganisms are not shielded from the UV radiation (i.e., any solids present in the treated effluent may protect microorganisms from the UV light). In many countries, UV light is becoming the most common means of disinfection because of the concerns about the impacts of chlorine in chlorinating residual organics in the treated sewage and in chlorinating organics in the receiving water.

Ozone (O3) is generated by passing oxygen (O2) through a high voltage potential resulting in a third oxygen atom becoming attached and forming O3. Ozone is very unstable and reactive and oxidizes most organic material it comes in contact with, thereby destroying many pathogenic microorganisms. Ozone is considered to be safer than chlorine because, unlike chlorine which has to be stored on site (highly poisonous in the event of an accidental release), ozone is generated on-site as needed from the oxygen in the ambient air. Ozonation also produces fewer disinfection by-products than chlorination. A disadvantage of ozone disinfection is the high cost of the ozone generation equipment and the requirements for special operators. Ozone sewage treatment requires the use of an ozone generator, which decontaminates the water as ozone bubbles percolate through the tank.

Membranes can also be effective disinfectants, because they act as barriers, avoiding the passage of the microorganisms. As a result, the final effluent may be devoid of pathogenic organisms, depending on the type of membrane used. This principle is applied in membrane bioreactors.

Biological nutrient removalEdit

Nitrification process tank at an activated sludge plant in the United States.

Sewage may contain high levels of the nutrients nitrogen and phosphorus. Typical values for nutrient loads per person and nutrient concentrations in raw sewage in developing countries have been published as follows: 8 g/person/d for total nitrogen (45 mg/L), 4.5 g/person/d for ammonia-N (25 mg/L) and 1.0 g/person/d for total phosphorus (7 mg/L).[3]: 57  The typical ranges for these values are: 6-10 g/person/d for total nitrogen (35-60 mg/L), 3.5-6 g/person/d for ammonia-N (20-35 mg/L) and 0.7-2.5 g/person/d for total phosphorus (4-15 mg/L).[3]: 57 

Excessive release to the environment can lead to nutrient pollution, which can manifest itself in eutrophication. This process can lead to algal blooms, a rapid growth, and later decay, in the population of algae. In addition to causing deoxygenation, some algal species produce toxins that contaminate drinking water supplies.

Ammonia nitrogen, in the form of free ammonia (NH3) is toxic to fish. Ammonia nitrogen, when converted to nitrite and further to nitrate in a water body, in the process of nitrification, is associated with the consumption of dissolved oxygen. Nitrite and nitrate may also have public health significance if concentrations are high in drinking water, because of a disease called metahemoglobinemia.[3]: 42 

Phosphorus removal is important as phosphorus is a limiting nutrient for algae growth in many fresh water systems. Therefore, an excess of phosphorus can lead to eutrophication. It is also particularly important for water reuse systems where high phosphorus concentrations may lead to fouling of downstream equipment such as reverse osmosis.

A range of treatment processes are available to remove nitrogen and phosphorus. Biological nutrient removal (BNR) is regarded by some as a type of secondary treatment process,[6] and by others as a tertiary (or "advanced") treatment process.

Nitrogen removalEdit

Constructed wetlands (vertical flow) for sewage treatment near Shanghai, China.

Nitrogen is removed through the biological oxidation of nitrogen from ammonia to nitrate (nitrification), followed by denitrification, the reduction of nitrate to nitrogen gas. Nitrogen gas is released to the atmosphere and thus removed from the water.

Nitrification itself is a two-step aerobic process, each step facilitated by a different type of bacteria. The oxidation of ammonia (NH4+) to nitrite (NO2) is most often facilitated by bacteria such as Nitrosomonas spp. ("nitroso" referring to the formation of a nitroso functional group). Nitrite oxidation to nitrate (NO3), though traditionally believed to be facilitated by Nitrobacter spp. (nitro referring the formation of a nitro functional group), is now known to be facilitated in the environment predominantly by Nitrospira spp.

Denitrification requires anoxic conditions to encourage the appropriate biological communities to form. "Anoxic conditions" refers to a situation where oxygen is absent but nitrate is present. Denitrification is facilitated by a wide diversity of bacteria. The activated sludge process, sand filters, waste stabilization ponds, constructed wetlands and other processes can all be used to reduce nitrogen.[27]: 17–18  Since denitrification is the reduction of nitrate to dinitrogen (molecular nitrogen) gas, an electron donor is needed. This can be, depending on the wastewater, organic matter (from the sewage itself), sulfide, or an added donor like methanol. The sludge in the anoxic tanks (denitrification tanks) must be mixed well (mixture of recirculated mixed liquor, return activated sludge, and raw influent) e.g. by using submersible mixers in order to achieve the desired denitrification.

Over time, different treatment configurations for activated sludge processes have evolved to achieve high levels of nitrogen removal. An initial scheme, the Ludzack–Ettinger Process, placed an anoxic treatment zone before the aeration tank and clarifier, using the return activated sludge (RAS) from the clarifier as a nitrate source. The sewage (either raw or as effluent from primary clarification) serves as the electron source for the facultative bacteria to metabolize carbon, using the inorganic nitrate as a source of oxygen instead of dissolved molecular oxygen. This denitrification scheme was naturally limited to the amount of soluble nitrate present in the RAS. Nitrate reduction was limited because RAS rate is limited by the performance of the clarifier.[6]: 616 

The "Modified Ludzak–Ettinger Process" (MLE) is an improvement on the original concept, for it recycles mixed liquor from the discharge end of the aeration tank to the head of the anoxic tank to provide a consistent source of soluble nitrate for the facultative bacteria.[6]: 616  In this instance, raw sewage continues to provide the electron source, and sub-surface mixing maintains the bacteria in contact with both electron source and soluble nitrate in the absence of dissolved oxygen.

There are other process configurations, including anoxic tanks before and after the aeration tanks, such as variations of the Bardenpho process.[33]: 160 

Phosphorus removalEdit

Studies of United States sewage in the late 1960s estimated mean per capita contributions of 500 grams (18 oz) in urine and feces, 1,000 grams (35 oz) in synthetic detergents, and lesser variable amounts used as corrosion and scale control chemicals in water supplies.[34] Source control via alternative detergent formulations has subsequently reduced the largest contribution, but naturally the phosphorus content of urine and feces remained unchanged.

Phosphorus can be removed biologically in a process called enhanced biological phosphorus removal. In this process, specific bacteria, called polyphosphate-accumulating organisms (PAOs), are selectively enriched and accumulate large quantities of phosphorus within their cells (up to 20 percent of their mass).[33]: 148–155 

Phosphorus removal can also be achieved by chemical precipitation, usually with salts of iron (e.g. ferric chloride) or aluminum (e.g. alum), or lime.[27]: 18  This may lead to a higher sludge production as hydroxides precipitate and the added chemicals can be expensive. Chemical phosphorus removal requires significantly smaller equipment footprint than biological removal, is easier to operate and is often more reliable than biological phosphorus removal. Another method for phosphorus removal is to use granular laterite or zeolite.[35][36]

Some systems use both biological phosphorus removal and chemical phosphorus removal. The chemical phosphorus removal in those systems may be used as a backup system, for use when the biological phosphorus removal is not removing enough phosphorus, or may be used continuously. In either case, using both biological and chemical phosphorus removal has the advantage of not increasing sludge production as much as chemical phosphorus removal on its own, with the disadvantage of the increased initial cost associated with installing two different systems.

Once removed, phosphorus, in the form of a phosphate-rich sewage sludge, may be sent to landfill or used as fertilizer in admixture with other digested sewage sludges. In the latter case, the treated sewage sludge is also sometimes referred to as biosolids.

Fourth treatment stageEdit

Micropollutants such as pharmaceuticals, ingredients of household chemicals, chemicals used in small businesses or industries, environmental persistent pharmaceutical pollutants (EPPP) or pesticides may not be eliminated in the commonly used sewage treatment processes (primary, secondary and tertiary treatment) and therefore lead to water pollution.[37] Although concentrations of those substances and their decomposition products are quite low, there is still a chance of harming aquatic organisms. For pharmaceuticals, the following substances have been identified as "toxicologically relevant": substances with endocrine disrupting effects, genotoxic substances and substances that enhance the development of bacterial resistances.[38] They mainly belong to the group of EPPP.

Techniques for elimination of micropollutants via a fourth treatment stage during sewage treatment are implemented in Germany, Switzerland, Sweden[citation needed] and the Netherlands and tests are ongoing in several other countries.[39] Such process steps mainly consist of activated carbon filters that adsorb the micropollutants. The combination of advanced oxidation with ozone followed by granular activated carbon (GAC) has been suggested as a cost-effective treatment combination for pharmaceutical residues. For a full reduction of microplasts the combination of ultrafiltration followed by GAC has been suggested. Also the use of enzymes such as laccase secreted by fungi is under investigation.[40] [41] Microbial biofuel cells are investigated for their property to treat organic matter in sewage.[42]

To reduce pharmaceuticals in water bodies, "source control" measures are also under investigation, such as innovations in drug development or more responsible handling of drugs.[38][43] In the US, the National Take Back Initiative is a voluntary program with the general public, encouraging people to return excess or expired drugs, and avoid flushing them to the sewage system.[44]

Sludge treatment and disposalEdit

Mechanical dewatering of sewage sludge with a centrifuge at a large sewage treatment plant (Arrudas Treatment Plant, Belo Horizonte, Brazil)

Sewage sludge treatment describes the processes used to manage and dispose of sewage sludge produced during sewage treatment. Sludge treatment is focused on reducing sludge weight and volume to reduce transportation and disposal costs, and on reducing potential health risks of disposal options. Water removal is the primary means of weight and volume reduction, while pathogen destruction is frequently accomplished through heating during thermophilic digestion, composting, or incineration. The choice of a sludge treatment method depends on the volume of sludge generated, and comparison of treatment costs required for available disposal options. Air-drying and composting may be attractive to rural communities, while limited land availability may make aerobic digestion and mechanical dewatering preferable for cities, and economies of scale may encourage energy recovery alternatives in metropolitan areas.

Sludge is mostly water with some amounts of solid material removed from liquid sewage. Primary sludge includes settleable solids removed during primary treatment in primary clarifiers. Secondary sludge is sludge separated in secondary clarifiers that are used in secondary treatment bioreactors or processes using inorganic oxidizing agents. In intensive sewage treatment processes, the sludge produced needs to be removed from the liquid line on a continuous basis because the volumes of the tanks in the liquid line have insufficient volume to store sludge.[45] This is done in order to keep the treatment processes compact and in balance (production of sludge approximately equal to the removal of sludge). The sludge removed from the liquid line goes to the sludge treatment line. Aerobic processes (such as the activated sludge process) tend to produce more sludge compared with anaerobic processes. On the other hand, in extensive (natural) treatment processes, such as ponds and constructed wetlands, the produced sludge remains accumulated in the treatment units (liquid line) and is only removed after several years of operation.[46]

Sludge treatment options depend on the amount of solids generated and other site-specific conditions. Composting is most often applied to small-scale plants with aerobic digestion for mid-sized operations, and anaerobic digestion for the larger-scale operations. The sludge is sometimes passed through a so-called pre-thickener which de-waters the sludge. Types of pre-thickeners include centrifugal sludge thickeners,[47] rotary drum sludge thickeners and belt filter presses.[48] Dewatered sludge may be incinerated or transported offsite for disposal in a landfill or use as an agricultural soil amendment.[49]

Environmental impactsEdit

Sewage treatment plants can have significant effects on the biotic status of receiving waters and can cause some water pollution, especially if the treatment process used is only basic. For example, for sewage treatment plants without nutrient removal, eutrophication of receiving water bodies can be a problem.

Water pollution (or aquatic pollution) is the contamination of water bodies, usually as a result of human activities, in such a manner that negatively affects its legitimate uses.[50]: 6  Water pollution reduces the ability of the body of water to provide the ecosystem services that it would otherwise provide. Water bodies include for example lakes, rivers, oceans, aquifers, reservoirs and groundwater. Water pollution results when contaminants are introduced into these water bodies. For example, releasing inadequately treated wastewater into natural waters can lead to degradation of these aquatic ecosystems. All plants and organisms living in or being exposed to polluted water bodies can be impacted. The effects can damage individual species and impact the natural biological communities they are part of. Water pollution can also lead to water-borne diseases for people using polluted water for drinking, bathing, washing or irrigation.[51]
Treated effluent from sewage treatment plant in Děčín, Czech Republic is discharged to surface waters.


Sludge drying beds for sewage sludge treatment at a small treatment plant at the Center for Research and Training in Sanitation, Belo Horizonte, Brazil.


Increasingly, people use treated or even untreated sewage for irrigation to produce crops. Cities provide lucrative markets for fresh produce, so are attractive to farmers. Because agriculture has to compete for increasingly scarce water resources with industry and municipal users, there is often no alternative for farmers but to use water polluted with sewage directly to water their crops. There can be significant health hazards related to using water loaded with pathogens in this way. The World Health Organization developed guidelines for safe use of wastewater in 2006.[52] They advocate a ‘multiple-barrier’ approach to wastewater use, where farmers are encouraged to adopt various risk-reducing behaviors. These include ceasing irrigation a few days before harvesting to allow pathogens to die off in the sunlight, applying water carefully so it does not contaminate leaves likely to be eaten raw, cleaning vegetables with disinfectant or allowing fecal sludge used in farming to dry before being used as a human manure.[53]

Circular secondary sedimentation tank at activated sludge sewage treatment plant at Arrudas Treatment Plant, Belo Horizonte, Brazil.

Reclaimed waterEdit

Water reclamation (also called wastewater reuse, water reuse or water recycling) is the process of converting municipal wastewater (sewage) or industrial wastewater into water that can be reused for a variety of purposes. Types of reuse include: urban reuse, agricultural reuse (irrigation), environmental reuse, industrial reuse, planned potable reuse, de facto wastewater reuse (unplanned potable reuse). For example, reuse may include irrigation of gardens and agricultural fields or replenishing surface water and groundwater (i.e., groundwater recharge). Reused water may also be directed toward fulfilling certain needs in residences (e.g. toilet flushing), businesses, and industry, and could even be treated to reach drinking water standards. Treated municipal wastewater reuse for irrigation is a long-established practice, especially in arid countries. Reusing wastewater as part of sustainable water management allows water to remain as an alternative water source for human activities. This can reduce scarcity and alleviate pressures on groundwater and other natural water bodies.[54]

Global situationEdit

Share of domestic wastewater that is safely treated (in 2018)[55]

Before the 20th century in Europe, sewers usually discharged into a body of water such as a river, lake, or ocean. There was no treatment, so the breakdown of the human waste was left to the ecosystem. This could lead to satisfactory results if the assimilative capacity of the ecosystem is sufficient which is nowadays not often the case due to increasing population density.[3]: 78 

Today, the situation in urban areas of industrialized countries is usually that sewers route their contents to a sewage treatment plant rather than directly to a body of water. In many developing countries, however, the bulk of municipal and industrial wastewater is discharged to rivers and the ocean without any treatment or after preliminary treatment or primary treatment only. Doing so can lead to water pollution. Few reliable figures exist on the share of the wastewater collected in sewers that is being treated in the world. A global estimate by UNDP and UN-Habitat in 2010 was that 90% of all wastewater generated is released into the environment untreated.[56] A more recent study in 2021 estimated that globally, about 52% of sewage is treated.[57] However, sewage treatment rates are highly unequal for different countries around the world. For example, while high-income countries treat approximately 74% of their sewage, developing countries treat an average of just 4.2%.[57]

The Joint Monitoring Programme (JMP) for Water Supply and Sanitation by WHO and UNICEF report in 2021 that 82% of people with sewer connections are connected to sewage treatment plants providing at least secondary treatment.[58]: 55 However, this value varies widely between regions. For example in Europe, North America, Northern Africa and Western Asia, a total of 31 countries had universal (>99%) wastewater treatment. However, in Albania, Bermuda, North Macedonia and Serbia "less than 50% of sewered wastewater received secondary or better treatment" and in Algeria, Lebanon and Libya the value was less than 20% of sewered wastewater that wasbeing treated. The report also found that "globally, 594 million people have sewer connections that don’t receive sufficient treatment. Many more are connected to wastewater treatment plants that do not provide effective treatment or comply with effluent requirements.".[58]: 55 

Global targetsEdit

Sustainable Development Goal 6 has a Target 6.3 which is formulated as follows: "By 2030, improve water quality by reducing pollution, eliminating dumping and minimizing release of hazardous chemicals and materials, halving the proportion of untreated wastewater and substantially increasing recycling and safe reuse globally."[55] The corresponding Indicator 6.3.1 is the "proportion of wastewater safely treated".

Data in 2020 showed that there is still too much uncollected household wastewater: Only 66% of all household wastewater flows were collected at treatment facilities in 2020 (this is determined from data from 128 countries).[7]: 17  Based on data from 42 countries in 2015, the report stated that "32 per cent of all wastewater flows generated from point sources received at least some treatment".[7]: 17  For sewage that has indeed been collected at centralized sewage treatment plants, about 79% went on to be safely treated in 2020.[7]: 18 


The history of sewage treatment had the following developments: It began with land application (sewage farms) in the 1840s in England, followed by chemical treatment and sedimentation of sewage in tanks, then biological treatment the late 19th century, which led to the development of the activated sludge process starting in 1912.[59][60]

It was not until the late 19th century that it became possible to treat the sewage by biologically decomposing the organic components through the use of microorganisms and removing the pollutants. Land treatment was also steadily becoming less feasible, as cities grew and the volume of sewage produced could no longer be absorbed by the farmland on the outskirts.

Edward Frankland conducted experiments at the sewage farm in Croydon, England, during the 1870s and was able to demonstrate that filtration of sewage through porous gravel produced a nitrified effluent (the ammonia was converted into nitrate) and that the filter remained unclogged over long periods of time.[61] This established the then revolutionary possibility of biological treatment of sewage using a contact bed to oxidize the waste. This concept was taken up by the chief chemist for the London Metropolitan Board of Works, William Libdin, in 1887: all probability the true way of purifying sewage...will be first to separate the sludge, and then turn into neutral effluent... retain it for a sufficient period, during which time it should be fully aerated, and finally discharge it into the stream in a purified condition. This is indeed what is aimed at and imperfectly accomplished on a sewage farm.[62]
From 1885 to 1891 filters working on this principle were constructed throughout the UK and the idea was also taken up in the US at the Lawrence Experiment Station in Massachusetts, where Frankland's work was confirmed. In 1890 the LES developed a 'trickling filter' that gave a much more reliable performance.[63]


In most countries, sewage collection and treatment are subject to local and national regulations and standards.

By countryEdit



The Urban Waste Water Treatment Directive (full title "Council Directive 91/271/EEC of 21 May 1991 concerning urban waste-water treatment") is a European Union directive regarding urban wastewater collection, wastewater treatment and its discharge, as well as the treatment and discharge of "waste water from certain industrial sectors". It was adopted on 21 May 1991.[64] It aims "to protect the environment from the adverse effects of urban waste water discharges and discharges from certain industrial sectors" by mandating waste water collection and treatment in urban agglomerations with a population equivalent of over 2000, and more advanced treatment in places with a population equivalent above 10,000 in sensitive areas.[65]



In India, wastewater treatment regulations come under three central institutions: "The Ministry of Environment Forest and Climate Change (MoEF&CC), the Ministry of Housing and Urban Affairs (MoHUA), and the recently formed Ministry of Jal Shakti."[66] The various water and sanitation policies such as the "National Environment Policy 2006" and "National Sanitation Policy 2008" also lay down wastewater treatment regulations. State governments and local municipalities hold responsibility for the disposal of sewage and construction and maintenance of "sewerage infrastructure." Their efforts are supported by schemes offered by the Government of India, such as the National River Conservation Plan, Jawaharlal Nehru National Urban Renewal Mission, National Lake Conservation Plan. Through the Ministry of Environment and Forest, India's government also has set-up incentives that encourage industries to establish "common facilities" to undertake the treatment of wastewater.[67]


Currently, Japan's methods of wastewater treatment include rural community sewers, wastewater facilities, and on-site treatment systems such as the Johkasou system to treat domestic wastewater.[68] Larger wastewater facilities and sewer systems are generally used to treat wastewater in more urban areas with a larger population. Rural sewage systems are used to treat wastewater at smaller domestic wastewater treatment plants for a smaller population. Johkasou (jōkasō) systems are on-site wastewater treatment systems tanks. They are used to treat the wastewater of a single household or to treat the wastewater of a small number of buildings in a more decentralized manner than a sewer system.[69]



In Libya, municipal wastewater treatment is managed by the general company for water and wastewater in Libya, which falls within the competence of the Housing and Utilities Government Ministry. There are approximately 200 sewage treatment plants across the nation, but few plants are functioning. In fact, the 36 larger plants are in the major cities; however, only nine of them are operational, and the rest of them are under repair.[70]

The largest operating wastewater treatment plants are situated in Sirte, Tripoli, and Misurata, with a design capacity of 21,000, 110,000, and 24,000 m3/day, respectively. Moreover, a majority of the remaining wastewater facilities are small and medium-sized plants with a design capacity of approximately 370 to 6700 m3/day. Therefore, 145,800 m3/day or 11 percent of the wastewater is actually treated, and the remaining others are released into the ocean and artificial lagoons although they are untreated. In fact, nonoperational wastewater treatment plants in Tripoli lead to a spill of over 1,275, 000 cubic meters of unprocessed water into the ocean every day.[70]


United StatesEdit

The United States Environmental Protection Agency (EPA) and state environmental agencies set wastewater standards under the Clean Water Act.[71] Point sources must obtain surface water discharge permits through the National Pollutant Discharge Elimination System (NPDES). Point sources include industrial facilities, municipal governments (sewage treatment plants and storm sewer systems), other government facilities such as military bases, and some agricultural facilities, such as animal feedlots.[72] EPA sets basic national wastewater standards: The "Secondary Treatment Regulation" applies to municipal sewage treatment plants,[73] and the "Effluent guidelines" which are regulations for categories of industrial facilities.[74]

See alsoEdit


  1. ^ a b c "Sanitation Systems – Sanitation Technologies – Activated sludge". SSWM. 27 April 2018. Retrieved 31 October 2018.
  2. ^ Khopkar, S.M. (2004). Environmental Pollution Monitoring And Control. New Delhi: New Age International. p. 299. ISBN 978-81-224-1507-0.
  3. ^ a b c d e f g h i j k l m n o p q r Von Sperling, M. (2015). "Wastewater Characteristics, Treatment and Disposal". Water Intelligence Online. 6: 9781780402086. doi:10.2166/9781780402086. ISSN 1476-1777.
  4. ^ a b Jones, Edward R.; van Vliet, Michelle T. H.; Qadir, Manzoor; Bierkens, Marc F. P. (2021). "Country-level and gridded estimates of wastewater production, collection, treatment and reuse". Earth System Science Data. 13 (2): 237–254. Bibcode:2021ESSD...13..237J. doi:10.5194/essd-13-237-2021. ISSN 1866-3508.
  5. ^ "Sanitation". Health topics. World Health Organization. Retrieved 2020-02-23.
  6. ^ a b c d e f g h i j k l m n o p Metcalf & Eddy (2014). Wastewater engineering : treatment and resource recovery. George Tchobanoglous, H. David Stensel, Ryujiro Tsuchihashi, Franklin L. Burton, Mohammad Abu-Orf, Gregory Bowden (Fifth ed.). New York, NY. ISBN 978-0-07-340118-8. OCLC 858915999.
  7. ^ a b c d UN-Water, 2021: Summary Progress Update 2021 – SDG 6 – water and sanitation for all. Version: July 2021. Geneva, Switzerland
  8. ^ WWAP (United Nations World Water Assessment Programme) (2017). The United Nations World Water Development Report 2017. Wastewater: The Untapped Resource. Paris. ISBN 978-92-3-100201-4. Archived from the original on 8 April 2017.
  9. ^ Tilley, E., Ulrich, L., Lüthi, C., Reymond, Ph., Zurbrügg, C. (2014). Compendium of Sanitation Systems and Technologies – (2nd Revised ed.). Swiss Federal Institute of Aquatic Science and Technology (Eawag), Duebendorf, Switzerland. ISBN 978-3-906484-57-0. Archived from the original on 8 April 2016.CS1 maint: multiple names: authors list (link)
  10. ^ a b c d Von Sperling, M. (2015). "Wastewater Characteristics, Treatment and Disposal". Water Intelligence Online. 6: 9781780402086. doi:10.2166/9781780402086. ISSN 1476-1777.
  11. ^ World Health Organization (2006). Guidelines for the safe use of wastewater, excreta, and greywater. World Health Organization. p. 31. ISBN 9241546859. OCLC 71253096.
  12. ^ Andersson, K., Rosemarin, A., Lamizana, B., Kvarnström, E., McConville, J., Seidu, R., Dickin, S. and Trimmer, C. (2016). Sanitation, Wastewater Management and Sustainability: from Waste Disposal to Resource Recovery Archived 2017-06-01 at the Wayback Machine. Nairobi and Stockholm: United Nations Environment Programme and Stockholm Environment Institute. ISBN 978-92-807-3488-1, p. 56
  13. ^ "sewerage – definition of sewerage in English from the Oxford dictionary". Archived from the original on 2015-09-24. Retrieved 2015-09-04.
  14. ^ a b Tilley, E., Ulrich, L., Lüthi, C., Reymond, Ph., Zurbrügg, C. (2014). Compendium of Sanitation Systems and Technologies – (2nd Revised ed.). Swiss Federal Institute of Aquatic Science and Technology (Eawag), Duebendorf, Switzerland. ISBN 978-3-906484-57-0. Archived from the original on 8 April 2016.CS1 maint: multiple names: authors list (link)
  15. ^ Spuhler, Dorothee; Germann, Verena; Kassa, Kinfe; Ketema, Atekelt Abebe; Sherpa, Anjali Manandhar; Sherpa, Mingma Gyalzen; Maurer, Max; Lüthi, Christoph; Langergraber, Guenter (2020). "Developing sanitation planning options: A tool for systematic consideration of novel technologies and systems". Journal of Environmental Management. 271: 111004. doi:10.1016/j.jenvman.2020.111004. PMID 32778289. S2CID 221100596.
  16. ^ Spuhler, Dorothee; Scheidegger, Andreas; Maurer, Max (2020). "Comparative analysis of sanitation systems for resource recovery: Influence of configurations and single technology components". Water Research. 186: 116281. doi:10.1016/j.watres.2020.116281. PMID 32949886. S2CID 221806742.
  17. ^ Harshman, Vaughan; Barnette, Tony (2000-12-28). "Wastewater Odor Control: An Evaluation of Technologies". Water Engineering & Management. ISSN 0273-2238.
  18. ^ Walker, James D. and Welles Products Corporation (1976)."Tower for removing odors from gases." U.S. Patent No. 4421534.
  19. ^ Sercombe, Derek C. W. (April 1985). "The control of septicity and odours in sewerage systems and at sewage treatment works operated by Anglian Water Services Limited". Water Science & Techmology. 31 (7): 283–292. doi:10.2166/wst.1995.0244.
  20. ^ Hoffmann, H., Platzer, C., von Münch, E., Winker, M. (2011). Technology review of constructed wetlands – Subsurface flow constructed wetlands for greywater and domestic wastewater treatment. Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) GmbH, Eschborn, Germany, p. 11
  21. ^ Galvão, A; Matos, J; Rodrigues, J; Heath, P (1 December 2005). "Sustainable sewage solutions for small agglomerations". Water Science & Technology. 52 (12): 25–32. doi:10.2166/wst.2005.0420. Retrieved 27 March 2021.
  22. ^ "Wastewater Treatment Plant - Operator Certification Training - Module 20:Trickling Filter" (PDF). Pennsylvania Department of Environmental Protection. 2016. Retrieved 27 March 2021.
  23. ^ Chowdhry, S., Koné, D. (2012). Business Analysis of Fecal Sludge Management: Emptying and Transportation Services in Africa and Asia – Draft final report. Bill & Melinda Gates Foundation, Seattle, USA
  24. ^ U.S. Environmental Protection Agency, Washington, D.C. (2008). "Septic Systems Fact Sheet." Archived 12 April 2013 at the Wayback Machine EPA publication no. 832-F-08-057.
  25. ^ a b c Henze, M.; van Loosdrecht, M. C. M.; Ekama, G.A.; Brdjanovic, D. (2008). Biological Wastewater Treatment: Principles, Modelling and Design. IWA Publishing (Spanish and Arabic versions are available online for free). doi:10.2166/9781780401867. ISBN 978-1-78040-186-7. External link in |publisher= (help)
  26. ^ Water and Environmental Health at London and Loughborough (1999). "Waste water Treatment Options." Archived 2011-07-17 at the Wayback Machine Technical brief no. 64. London School of Hygiene & Tropical Medicine and Loughborough University.
  27. ^ a b c d e f g EPA. Washington, DC (2004). "Primer for Municipal Waste water Treatment Systems." Document no. EPA 832-R-04-001.
  28. ^ "Chapter 3. Flow Equalization". Process Design Manual for Upgrading Existing Wastewater Treatment Plants (Report). EPA. October 1971.
  29. ^ "How Wastewater Treatment Works...The Basics" (PDF). EPA. 1998. Retrieved 27 March 2021.
  30. ^ Wastewater engineering : treatment and reuse. George Tchobanoglous, Franklin L. Burton, H. David Stensel, Metcalf & Eddy (4th ed.). Boston: McGraw-Hill. 2003. ISBN 0-07-041878-0. OCLC 48053912.CS1 maint: others (link)
  31. ^ "Stage 3 - Tertiary treatment". Sydney Water. 2010. Retrieved 27 March 2021.
  32. ^ Metcalf & Eddy, Inc. (1972). Wastewater Engineering. New York: McGraw-Hill. ISBN 978-0-07-041675-8.
  33. ^ a b Von Sperling, M. (2015-12-30). "Activated Sludge and Aerobic Biofilm Reactors". Water Intelligence Online. 6: 9781780402123. doi:10.2166/9781780402123. ISSN 1476-1777.
  34. ^ Process Design Manual for Phosphorus Removal (Report). EPA. 1976. pp. 2–1. EPA 625/1-76-001a.
  35. ^ Wood, R. B.; McAtamney, C.F. (December 1996). "Constructed wetlands for waste water treatment: the use of laterite in the bed medium in phosphorus and heavy metal removal". Hydrobiologia. 340 (1–3): 323–331. doi:10.1007/BF00012776. S2CID 6182870.
  36. ^ Wang, Shaobin; Peng, Yuelian (2009-10-09). "Natural zeolites as effective adsorbents in water & wastewater treatment" (PDF). Chemical Engineering Journal. 156 (1): 11–24. doi:10.1016/j.cej.2009.10.029. Retrieved 2019-07-13.
  37. ^ UBA (Umweltbundesamt) (2014): Maßnahmen zur Verminderung des Eintrages von Mikroschadstoffen in die Gewässer. Texte 85/2014 (in German)
  38. ^ a b Walz, A., Götz, K. (2014): Arzneimittelwirkstoffe im Wasserkreislauf. ISOE-Materialien zur Sozialen Ökologie Nr. 36 (in German)
  39. ^ Borea, Laura; Ensano, Benny Marie B.; Hasan, Shadi Wajih; Balakrishnan, Malini; Belgiorno, Vincenzo; de Luna, Mark Daniel G.; Ballesteros, Florencio C.; Naddeo, Vincenzo (November 2019). "Are pharmaceuticals removal and membrane fouling in electromembrane bioreactor affected by current density?". Science of the Total Environment. 692: 732–740. Bibcode:2019ScTEn.692..732B. doi:10.1016/j.scitotenv.2019.07.149. PMID 31539981.
  40. ^ Margot, J.; et al. (2013). "Bacterial versus fungal laccase: potential for micropollutant degradation". AMB Express. 3 (1): 63. doi:10.1186/2191-0855-3-63. PMC 3819643. PMID 24152339.
  41. ^ Heyl, Stephanie (2014-10-13). "Crude mushroom solution to degrade micropollutants and increase the performance of biofuel cells". Bioeconomy BW. Stuttgart: Biopro Baden-Württemberg.
  42. ^ Logan, B.; Regan, J. (2006). "Microbial Fuel Cells—Challenges and Applications". Environmental Science & Technology. 40 (17): 5172–5180. Bibcode:2006EnST...40.5172L. doi:10.1021/es0627592.
  43. ^ Lienert, J.; Bürki, T.; Escher, B.I. (2007). "Reducing micropollutants with source control: Substance flow analysis of 212 pharmaceuticals in faeces and urine". Water Science & Technology. 56 (5): 87–96. doi:10.2166/wst.2007.560. PMID 17881841.
  44. ^ "National Prescription Drug Take Back Day". Washington, D.C.: U.S. Drug Enforcement Administration. Retrieved 2021-06-13.
  45. ^ Henze, M.; van Loosdrecht, M. C. M.; Ekama, G.A.; Brdjanovic, D. (2008). Biological Wastewater Treatment: Principles, Modelling and Design. IWA Publishing (Spanish and Arabic versions are available online for free). doi:10.2166/9781780401867. ISBN 978-1-78040-186-7. External link in |publisher= (help)
  46. ^ Von Sperling, M. (2015). "Wastewater Characteristics, Treatment and Disposal". Water Intelligence Online. 6: 9781780402086. doi:10.2166/9781780402086. ISSN 1476-1777.
  47. ^ "Centrifuge Thickening and Dewatering. Fact sheet". EPA. September 2000. EPA 832-F-00-053.
  48. ^ "Belt Filter Press. Fact sheet". Biosolids. EPA. September 2000. EPA 832-F-00-057.
  49. ^ Panagos, Panos; Ballabio, Cristiano; Lugato, Emanuele; Jones, Arwyn; Borrelli, Pasquale; Scarpa, Simone; Orgiazzi, Albert o; Montanarella, Luca (2018-07-09). "Potential Sources of Anthropogenic Copper Inputs to European Agricultural Soils". Sustainability. 10 (7): 2380. doi:10.3390/su10072380. ISSN 2071-1050.
  50. ^ Von Sperling, M. (2015). "Wastewater Characteristics, Treatment and Disposal". IWA Publishing. 6. doi:10.2166/9781780402086. ISBN 9781780402086.
  51. ^ "Water Pollution". Environmental Health Education Program. Cambridge, MA: Harvard T.H. Chan School of Public Health. July 23, 2013. Retrieved 2021-09-18.
  52. ^ WHO (2006). WHO Guidelines for the Safe Use of Wastewater, Excreta and Greywater – Volume IV: Excreta and greywater use in agriculture Archived 17 October 2014 at the Wayback Machine. World Health Organization (WHO), Geneva, Switzerland
  53. ^ Wastewater use in agriculture: Not only an issue where water is scarce! Archived 2014-04-09 at the Wayback Machine International Water Management Institute, 2010. Water Issue Brief 4
  54. ^ Andersson, K., Rosemarin, A., Lamizana, B., Kvarnström, E., McConville, J., Seidu, R., Dickin, S. and Trimmer, C. (2016). Sanitation, Wastewater Management and Sustainability: from Waste Disposal to Resource Recovery. Nairobi and Stockholm: United Nations Environment Programme and Stockholm Environment Institute. ISBN 978-92-807-3488-1
  55. ^ a b Ritchie, Roser, Mispy, Ortiz-Ospina (2018) "Measuring progress towards the Sustainable Development Goals." (SDG 6), website
  56. ^ Corcoran, E., C. Nellemann, E. Baker, R. Bos, D. Osborn, H. Savelli (eds) (2010). Sick water? : the central role of wastewater management in sustainable development : a rapid response assessment (PDF). Arendal, Norway: UNEP/GRID-Arendal. ISBN 978-82-7701-075-5.CS1 maint: multiple names: authors list (link) CS1 maint: extra text: authors list (link)
  57. ^ a b Jones, Edward R.; van Vliet, Michelle T. H.; Qadir, Manzoor; Bierkens, Marc F. P. (2021). "Country-level and gridded estimates of wastewater production, collection, treatment and reuse". Earth System Science Data. 13 (2): 237–254. Bibcode:2021ESSD...13..237J. doi:10.5194/essd-13-237-2021. ISSN 1866-3508.
  58. ^ a b WHO and UNICEF (2021) Progress on household drinking water, sanitation and hygiene 2000-2020: Five years into the SDGs. Geneva: World Health Organization (WHO) and the United Nations Children’s Fund (UNICEF), 2021. Licence: CC BY-NC-SA 3.0 IGO
  59. ^ P.F. Cooper. "Historical aspects of wastewater treatment" (PDF). Archived from the original (PDF) on 2011-05-11. Retrieved 2013-12-21.
  60. ^ Benidickson, Jamie (2011). The Culture of Flushing: A Social and Legal History of Sewage. UBC Press. ISBN 9780774841382. Archived from the original on 19 April 2021. Retrieved 2013-02-07.
  61. ^ Colin A. Russell (2003). Edward Frankland: Chemistry, Controversy and Conspiracy in Victorian England. Cambridge University Press. pp. 372–380. ISBN 978-0-521-54581-5.
  62. ^ Sharma, Sanjay Kumar; Sanghi, Rashmi (2012). Advances in Water Treatment and Pollution Prevention. Springer Science & Business Media. ISBN 978-94-007-4204-8.
  63. ^ "Epidemics, demonstration effects, and municipal investment in sanitation capital" (PDF). Archived from the original (PDF) on 2006-09-04.
  64. ^ "Council Directive 91/271/EEC of 21 May 1991 concerning urban waste-water treatment (91/271/EEC)". Retrieved 19 July 2009.
  65. ^ "Urban Waste Water Directive Overview". European Commission. Retrieved 19 July 2009.
  66. ^ Schellenberg, Tatjana; Subramanian, Vrishali; Ganeshan, Ganapathy; Tompkins, David; Pradeep, Rohini (2020). "Wastewater Discharge Standards in the Evolving Context of Urban Sustainability–The Case of India". Frontiers in Environmental Science. 8. doi:10.3389/fenvs.2020.00030. ISSN 2296-665X. S2CID 215790363.
  67. ^ Kaur, R; Wani, SP; Singh, AK. "Wastewater production, treatment and use in India" (PDF). AIS. Retrieved 2020-11-17.
  68. ^ Motoyuki Mizuochi: Small-Scale Domestic Wastewater Treatment Technology in Japan, and the Possibility of Technological Transfer, Asian Environment Research Group, National Institute for Environmental Studies, Japan, retrieved on January 6, 2011
  69. ^ "Japan Edducation Center of Environmental Sanitation". Retrieved 2021-04-23.
  70. ^ a b "Wastewater Treatment Plants in Libya: Challenges and Future Prospects". International Journal of Environmental Planning and Management.
  71. ^ United States. Federal Water Pollution Control Act Amendments of 1972. Pub.L. 92–500 Approved October 18, 1972. Amended by the Clean Water Act of 1977, Pub.L. 95–217, December 27, 1977; and the Water Quality Act of 1987, Pub.L. 100–4, February 4, 1987.
  72. ^ "National Pollutant Discharge Elimination System". EPA. 2020-02-21.
  73. ^ EPA. "Secondary Treatment Regulation." Code of Federal Regulations, 40 CFR Part 133.
  74. ^ "Industrial Effluent Guidelines". EPA. 2020-02-12.

External linksEdit