Sewage treatment (or domestic wastewater treatment, municipal wastewater treatment) is a type of
wastewater treatment which aims to remove
contaminants
Contamination is the presence of a constituent, impurity, or some other undesirable element that spoils, corrupts, infects, makes unfit, or makes inferior a material, physical body, natural environment, workplace, etc.
Types of contamination
...
from
sewage to produce an
effluent that is suitable for discharge to the surrounding environment or an intended reuse application, thereby preventing
water pollution
Water pollution (or aquatic pollution) is the contamination of water bodies, usually as a result of human activities, so that it negatively affects its uses. Water bodies include lakes, rivers, oceans, aquifers, reservoirs and groundwater. ...
from raw sewage discharges.
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
Biochemical oxygen demand (BOD) is the amount of dissolved oxygen (DO) needed (i.e. demanded) by aerobic biological organisms to break down organic material present in a given water sample at a certain temperature over a specific time period. T ...
) from sewage, using aerobic or anaerobic biological processes.
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.
Often, the main criteria for selection are: desired effluent quality, expected construction and operating costs, availability of land, energy requirements and
sustainability
Specific definitions of sustainability are difficult to agree on and have varied in the literature and over time. The concept of sustainability can be used to guide decisions at the global, national, and individual levels (e.g. sustainable livi ...
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.
However, sewage treatment rates are highly unequal for different countries around the world. For example, while
high-income countries
A high-income economy is defined by the World Bank as a nation with a gross national income per capita of US$12,696 or more in 2020, calculated using the Atlas method. While the term "high-income" is often used interchangeably with "First World" a ...
treat approximately 74% of their sewage, developing countries treat an average of just 4.2%.
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. The term "sewage treatment plant" is often used interchangeably with the term "wastewater treatment plant".
Terminology
The term "sewage treatment plant" (STP) (or "sewage treatment works" in some countries) is nowadays often replaced with the term
wastewater treatment plant (WWTP).
Strictly speaking, the latter is a broader term that can also refer to industrial wastewater.
The terms "water recycling center" or "water reclamation plants" are also in use.
Purposes and overview
The overall aim of treating sewage is to produce an
effluent that can be discharged to the environment while causing as little
water pollution
Water pollution (or aquatic pollution) is the contamination of water bodies, usually as a result of human activities, so that it negatively affects its uses. Water bodies include lakes, rivers, oceans, aquifers, reservoirs and groundwater. ...
as possible, or to produce an effluent that can be
reused
Reuse is the action or practice of using an item, whether for its original purpose (conventional reuse) or to fulfill a different function ( creative reuse or repurposing). It should be distinguished from recycling, which is the breaking down of ...
in a useful manner.
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).
Some types of sewage treatment produce
sewage sludge
Sewage sludge is the residual, semi-solid material that is produced as a by-product during sewage treatment of industrial or municipal wastewater. The term " septage" also refers to sludge from simple wastewater treatment but is connected to s ...
which can be
treated before safe disposal or reuse. Under certain circumstances, the treated sewage sludge might be termed "
biosolids
Biosolids are solid organic matter recovered from a sewage treatment process and used as fertilizer. In the past, it was common for farmers to use animal manure to improve their soil fertility. In the 1920s, the farming community began also to use ...
" and can be used as a
fertilizer.
Sewage characteristics
Collection
Types of treatment processes
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
Onsite sewage facilities (OSSF), also called septic systems, are wastewater systems designed to treat and dispose of effluent on the same property that produces the wastewater, in areas not served by public sewage infrastructure.
A septic tank a ...
,
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 processes
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.
Due to their technical simplicity, most of the savings (compared with high tech systems) are in terms of operation and maintenance costs.
*
Anaerobic digester types and
anaerobic digestion, for example:
**
Upflow anaerobic sludge blanket reactor
**
Septic tank
**
Imhoff tank
The Imhoff tank, named for German engineer Karl Imhoff (1876–1965), is a chamber suitable for the reception and processing of sewage. It may be used for the clarification of sewage by simple settling and sedimentation, along with anaerobic dig ...
*
Constructed wetland (see also
biofilters)
*
Decentralized wastewater system
*
Nature-based solutions
*
On-site sewage facility
Onsite sewage facilities (OSSF), also called septic systems, are wastewater systems designed to treat and dispose of effluent on the same property that produces the wastewater, in areas not served by public sewage infrastructure.
A septic tank a ...
*
Sand filter
Sand filters are used as a step in the water treatment process of water purification.
There are three main types; rapid (gravity) sand filters, upward flow sand filters and slow sand filters. All three methods are used extensively in the water i ...
*
Vermifilter
*
Waste stabilization pond with sub-types:
**
Facultative ponds
**
Anaerobic pond – facultative ponds systems
** Facultative
aerated lagoons
** Complete-mix aerated lagoon sedimentation pond systems
** High rate ponds
** Maturation ponds
Examples for systems that can provide full or partial treatment for toilet wastewater only:
*
Composting toilet (see also
dry toilets
A dry toilet (or non-flush toilet, no flush toilet or toilet without a flush) is a toilet which, unlike a flush toilet, does not use flush water. Dry toilets do not use water to move excreta along or block odors. They do not produce sewage, and ...
in general)
*
Urine-diverting dry toilet
A urine-diverting dry toilet (UDDT) is a type of dry toilet with urine diversion that can be used to provide safe, affordable sanitation in a variety of contexts worldwide. The separate collection of feces and urine without any flush water has ma ...
*
Vermifilter toilet
Vermifilter toilet, also known as a primary vermifilter, vermidigester toilet, tiger toilet or tiger worm toilet, is an on-site sanitation system in which human excreta are delivered from a toilet (usually by flushing) onto a medium containing a wo ...
High tech, intensive or mechanized processes
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.
*
Activated sludge systems
*
Aerobic granulation
The biological treatment of wastewater in the sewage treatment plant is often accomplished using conventional activated sludge systems. These systems generally require large surface areas for treatment and biomass separation units due to the genera ...
*
Aerobic treatment system
*
Enhanced biological phosphorus removal
*
Expanded granular sludge bed digestion
*
Extended aeration
*
Filtration
*
Membrane bioreactor
*
Moving bed biofilm reactor
*
Reverse osmosis
*
Rotating biological contactor
*
Sequencing batch reactor
*
Trickling filter
*
Ultrafiltration
*
Ultraviolet disinfection
Ultraviolet germicidal irradiation (UVGI) is a disinfection method that uses short- wavelength ultraviolet ( ultraviolet C or UV-C) light to kill or inactivate microorganisms by destroying nucleic acids and disrupting their DNA, leaving them ...
Disposal or treatment options
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
Groundwater recharge or deep drainage or deep percolation is a hydrologic process, where water moves downward from surface water to groundwater. Recharge is the primary method through which water enters an aquifer. This process usually occurs ...
, surface disposal and storage.
Application of sewage to land can be considered as a form of final disposal or of treatment, or both.
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 aspects
Population equivalent
The "per person organic matter load" is a parameter used in the design of sewage treatment plants. This concept is known as
population equivalent (PE). The base value used for PE can vary from one country to another. Commonly used definitions used worldwide are: 1 PE equates to 60 gram of BOD per person per day, and it also equals 200 liters of sewage per day.
This concept is also used as a comparison parameter to express the strength of
industrial wastewater compared to sewage.
Process selection
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.
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.
A range of publications exist to help with technology selection.
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
Specific definitions of sustainability are difficult to agree on and have varied in the literature and over time. The concept of sustainability can be used to guide decisions at the global, national, and individual levels (e.g. sustainable livi ...
, simplicity and operational costs.
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
Kinetics ( grc, κίνησις, , kinesis, ''movement'' or ''to move'') may refer to:
Science and medicine
* Kinetics (physics), the study of motion and its causes
** Rigid body kinetics, the study of the motion of rigid bodies
* Chemical kin ...
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.
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.
Odor control
Odors
An odor (American English) or odour (Commonwealth English; see spelling differences) is caused by one or more volatilized chemical compounds that are generally found in low concentrations that humans and animals can perceive via their sense ...
emitted by sewage treatment are typically an indication of an anaerobic or "septic" condition. 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. Other methods of odor control exist, including addition of iron salts,
hydrogen peroxide,
calcium nitrate, etc. to manage
hydrogen sulfide levels.
Energy requirements
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. Sewage treatment plants that produce biogas in their
sewage sludge treatment
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 red ...
process with
anaerobic digestion can produce enough energy to meet most of the energy needs of the sewage treatment plant itself.
For activated sludge treatment plants in the United States, around 30 percent of the annual operating costs is usually required for energy.
Most of this electricity is used for aeration, pumping systems and equipment for the dewatering and drying of
sewage sludge
Sewage sludge is the residual, semi-solid material that is produced as a by-product during sewage treatment of industrial or municipal wastewater. The term " septage" also refers to sludge from simple wastewater treatment but is connected to s ...
. Advanced sewage treatment plants, e.g. for nutrient removal, require more energy than plants that only achieve primary or secondary treatment.
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.
Co-treatment of industrial effluent
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.
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 processes
Non-sewered areas
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.
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.
Available process steps
Sewage treatment often involves two main stages, called primary and secondary treatment, while advanced treatment also incorporates a tertiary treatment stage with polishing processes.
Different types of sewage treatment may utilize some or all of the process steps listed below.
Preliminary treatment
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
clarifier
Clarifiers are settling tanks built with mechanical means for continuous removal of solids being deposited by sedimentation. A clarifier is generally used to remove solid particulates or suspended solids from liquid for clarification and/or thi ...
s.
Screening
The influent in sewage water passes through a
bar screen
A bar screen is a mechanical filter used to remove large objects, such as rags and plastics, from wastewater. It is part of the primary filtration flow and typically is the first, or preliminary, level of filtration, being installed at the influen ...
to remove all large objects like cans, rags, sticks, plastic packets, etc. carried in the sewage stream. 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.
[EPA. Washington, DC (2004)]
"Primer for Municipal Waste water Treatment Systems."
Document no. EPA 832-R-04-001.
Grit removal
Grit consists of
sand
Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class ...
,
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 , 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.
Flow equalization
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.
The benefits are performance improvements of the biological treatment processes, the secondary clarifiers and any effluent filtration equipment.
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
A portable or mobile toilet (colloquial terms: thunderbox, portaloo, porta-john or porta-potty) is any type of toilet that can be moved around, some by one person, some by mechanical equipment such as a truck and crane. Most types do not require ...
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.
Fat and grease removal
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 treatment
Primary treatment is the "removal of a portion of the
suspended solids
Suspended solids refers to small solid particles which remain in suspension in water as a colloid or due to motion of the water. Suspended solids can be removed by sedimentation if their size or density is comparatively large, or by filtration. ...
and
organic matter from the sewage".
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
clarifier
Clarifiers are settling tanks built with mechanical means for continuous removal of solids being deposited by sedimentation. A clarifier is generally used to remove solid particulates or suspended solids from liquid for clarification and/or thi ...
s" and typically have a hydraulic retention time (HRT) of 1.5 to 2.5 hours.
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.
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.
Primary sedimentation tanks can be expected to remove 50–70% of the suspended solids and 25–40% of the
biological oxygen demand
Biochemical oxygen demand (BOD) is the amount of dissolved oxygen (DO) needed (i.e. demanded) by aerobic biological organisms to break down organic material present in a given water sample at a certain temperature over a specific time period. T ...
(BOD).
Secondary treatment
The main processes involved in secondary sewage treatment are designed to remove as much of the solid material as possible.
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.
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.
Tertiary treatment
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
An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...
such as
estuaries, low-flow rivers or
coral reefs
A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups.
...
. Treated water is sometimes disinfected chemically or physically (for example, by lagoons and
microfiltration) prior to discharge into a
stream,
river
A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of ...
,
bay
A bay is a recessed, coastal body of water that directly connects to a larger main body of water, such as an ocean, a lake, or another bay. A large bay is usually called a gulf, sea, sound, or bight. A cove is a small, circular bay with a na ...
,
lagoon
A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into '' coastal lagoons'' (or ''barrier lagoons ...
or
wetland
A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free (Anoxic waters, anoxic) processes prevailing, especially in t ...
, 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
Groundwater recharge or deep drainage or deep percolation is a hydrologic process, where water moves downward from surface water to groundwater. Recharge is the primary method through which water enters an aquifer. This process usually occurs ...
or agricultural purposes.
Sand filtration removes much of the residual suspended matter.
Filtration over
activated carbon, also called ''carbon adsorption,'' removes residual
toxins.
Micro filtration or
synthetic membranes
An artificial membrane, or synthetic membrane, is a synthetically created membrane which is usually intended for separation purposes in laboratory or in industry. Synthetic membranes have been successfully used for small and large-scale industrial ...
are also used, for example in
membrane bioreactors which also remove pathogens.
Settlement and further biological improvement of treated sewage may be achieved through storage in large human-made ponds or lagoons. These lagoons are highly aerobic, and colonization by native
macrophytes, especially reeds, is often encouraged.
Disinfection
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.
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.
Monochloramine
Monochloramine, often called chloramine, is the chemical compound with the formula NH2Cl. Together with dichloramine (NHCl2) and nitrogen trichloride (NCl3), it is one of the three chloramines of ammonia. It is a colorless liquid at its melting p ...
, 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
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
tic structure of bacteria,
virus
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsk ...
es, and other
pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a g ...
s, 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.
As with UV treatment, heat
sterilization
Sterilization may refer to:
* Sterilization (microbiology), killing or inactivation of micro-organisms
* Soil steam sterilization, a farming technique that sterilizes soil with steam in open fields or greenhouses
* Sterilization (medicine) rende ...
also does not add chemicals to the water being treated. However, unlike UV, heat can penetrate liquids that are not transparent. Heat disinfection can also penetrate solid materials within wastewater, sterilizing their contents.
Thermal effluent decontamination systems provide low resource, low maintenance effluent decontamination once installed.
Ozone () is generated by passing
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
() through a high
voltage potential resulting in a third oxygen
atom becoming attached and forming . 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 removal
Sewage may contain high levels of the nutrients
nitrogen and
phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ea ...
. 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).
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).
Excessive release to the environment can lead to
nutrient pollution, which can manifest itself in
eutrophication
Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phyt ...
. 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 (NH
3) 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.
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
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: ...
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,
and by others as a tertiary (or "advanced") treatment process.
Nitrogen removal
Nitrogen is removed through the biological
oxidation of nitrogen from
ammonia to
nitrate (
nitrification
''Nitrification'' is the biological oxidation of ammonia to nitrite followed by the oxidation of the nitrite to nitrate occurring through separate organisms or direct ammonia oxidation to nitrate in comammox bacteria. The transformation of ...
), followed by
denitrification
Denitrification is a microbially facilitated process where nitrate (NO3−) is reduced and ultimately produces molecular nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products. Facultative anaerobic bacteria perform denit ...
, 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 (NH
4+) to nitrite (NO
2−) is most often facilitated by bacteria such as ''
Nitrosomonas'' spp. ("nitroso" referring to the formation of a
nitroso functional group). Nitrite oxidation to nitrate (NO
3−), 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
A constructed wetland is an artificial wetland to treat sewage, greywater, stormwater runoff or industrial wastewater. It may also be designed for land reclamation after mining, or as a mitigation step for natural areas lost to land development. ...
and other processes can all be used to reduce nitrogen.
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 mixer
A submersible mixer is a mechanical device that is used to mix sludge tanks and other liquid volumes. Submersible mixers are often used in sewage treatment plants to keep solids in suspension in the various process tanks and/or sludge holding tanks ...
s 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.
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.
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.
Phosphorus removal
Studies of United States sewage in the late 1960s estimated mean per capita contributions of in urine and feces, in synthetic detergents, and lesser variable amounts used as corrosion and scale control chemicals in water supplies. 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).
Phosphorus removal can also be achieved by chemical
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
, usually with
salts of
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
(e.g.
ferric chloride
Iron(III) chloride is the inorganic compound with the formula . Also called ferric chloride, it is a common compound of iron in the +3 oxidation state. The anhydrous compound is a crystalline solid with a melting point of 307.6 °C. The col ...
) or
aluminum (e.g.
alum), or lime.
This may lead to a higher sludge production as hydroxides precipitate and the added chemicals can be expensive.
Chemical phosphorus removal Chemical phosphorus removal is a wastewater treatment method, where phosphorus is removed using salts of aluminum (e.g. alum or polyaluminum chloride), iron (e.g. ferric chloride), or calcium (e.g. lime). Phosphate forms precipitates with the metal ...
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.
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
Sewage sludge is the residual, semi-solid material that is produced as a by-product during sewage treatment of industrial or municipal wastewater. The term " septage" also refers to sludge from simple wastewater treatment but is connected to s ...
, 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. 22% of the world's phosphorus needs could be satisfied by recycling residential wastewater.
Fourth treatment stage
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. 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.
[Walz, A., Götz, K. (2014)]
Arzneimittelwirkstoffe im Wasserkreislauf
ISOE-Materialien zur Sozialen Ökologie Nr. 36 (in German) 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 and the Netherlands and tests are ongoing in several other countries. 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
Laccases () are multicopper oxidases found in plants, fungi, and bacteria. Laccases oxidize a variety of phenolic substrates, performing one-electron oxidations, leading to crosslinking. For example, laccases play a role in the formation of l ...
secreted by fungi is under investigation. Microbial biofuel cells are investigated for their property to treat organic matter in sewage.
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.
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.
Sludge treatment and disposal
Environmental impacts
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
Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phyt ...
of receiving water bodies can be a problem.
Reuse
Irrigation
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
The World Health Organization (WHO) is a specialized agency of the United Nations responsible for international public health. The WHO Constitution states its main objective as "the attainment by all peoples of the highest possible level o ...
developed guidelines for safe use of wastewater in 2006.
[WHO (2006)]
WHO Guidelines for the Safe Use of Wastewater, Excreta and Greywater – Volume IV: Excreta and greywater use in agriculture
. World Health Organization (WHO), Geneva, Switzerland 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.
Reclaimed water
Global situation
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
An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...
. 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.
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
Water pollution (or aquatic pollution) is the contamination of water bodies, usually as a result of human activities, so that it negatively affects its uses. Water bodies include lakes, rivers, oceans, aquifers, reservoirs and groundwater. ...
. 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. A more recent study in 2021 estimated that globally, about 52% of sewage is treated.
However, sewage treatment rates are highly unequal for different countries around the world. For example, while
high-income countries
A high-income economy is defined by the World Bank as a nation with a gross national income per capita of US$12,696 or more in 2020, calculated using the Atlas method. While the term "high-income" is often used interchangeably with "First World" a ...
treat approximately 74% of their sewage,
developing countries treat an average of just 4.2%.
As of 2022, without sufficient treatment, more than 80% of all wastewater generated globally is released into the environment. High-income nations treat, on average, 70% of the wastewater they produce, according to UN Water.
Only 8% of wastewater produced in low-income nations receives any sort of treatment.
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.
[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 IGOHowever, 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.".
In October 2021,
British Members of Parliament voted to continue allowing untreated sewage from combined sewer overflows to be released into waterways.
Global targets
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."
The corresponding Indicator 6.3.1 is the "proportion of wastewater safely treated". It is anticipated that wastewater production would rise by 24% by 2030 and by 51% by 2050.
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).
[UN-Water, 2021]
Summary Progress Update 2021 – SDG 6 – water and sanitation for all
Version: July 2021. Geneva, Switzerland 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".
For sewage that has indeed been collected at centralized sewage treatment plants, about 79% went on to be safely treated in 2020.
History
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.
Regulations
In most countries, sewage collection and treatment are subject to local and national
regulations and standards.
By country
Overview
Europe
In the European Union, 0.8% of total energy consumption goes to wastewater treatment facilities.
The European Union needs to make extra investments of €90 billion in the water and waste sector to meet its 2030 climate and energy goals. An estimated 60–70% of wastewater's potential value is still untapped (that is, heat, energy, nutrients, minerals, metals, chemicals, etc.).
Asia
India
The '
Delhi Jal Board
Delhi Jal Board (DJB) is the government agency responsible for supply of potable water to the most of the National Capital Territory region of Delhi, India. Delhi Jal Board was constituted on 6 April 1998 through an Act of the Delhi Legislative As ...
' (DJB) is currently operating on the construction of the largest sewage treatment plant in India. I
will be operational by the end of 2022with an estimated capacity of 564 MLD. It is supposed to solve the existing situation wherein untreated sewage water is being discharged directly into the river ‘Yamuna’.
Japan
Africa
Libya
Americas
United States
See also
*
Decentralized wastewater system
*
List of largest wastewater treatment plants
*
List of water supply and sanitation by country
*
Nutrient Recovery and Reuse: producing agricultural nutrients from sewage
*
Organisms involved in water purification
*
Sanitary engineering
*
Waste disposal
References
External links
Water Environment Federation– Professional association focusing on municipal wastewater treatment
{{DEFAULTSORT:Sewage Treatment
Environmental engineering
Pollution control technologies
Sanitation
Treatment
Sewerage infrastructure
Water pollution