Sewage treatment 1Sewage treatment
The objective of sewage treatment is to produce a disposable effluent
without causing harm to the surrounding environment, and also
Sewage treatment is the process of removing
contaminants from wastewater and household sewage,
both runoff (effluents) and domestic. It includes
physical, chemical, and biological processes to remove
physical, chemical and biological contaminants. Its
objective is to produce an environmentally-safe fluid
waste stream (or treated effluent) and a solid waste (or
treated sludge) suitable for disposal or reuse (usually as
farm fertilizer). Using advanced technology it is now
possible to re-use sewage effluent for drinking water,
although Singapore is the only country to implement
such technology on a production scale in its production
Origins of sewage
Sewage is generated by residential, institutional, and commercial and industrial establishments. It includes household
waste liquid from toilets, baths, showers, kitchens, sinks and so forth that is disposed of via sewers. In many areas,
sewage also includes liquid waste from industry and commerce. The separation and draining of household waste into
greywater and blackwater is becoming more common in the developed world, with greywater being permitted to be
used for watering plants or recycled for flushing toilets.
Sewage may include stormwater runoff, sewerage systems capable of handling stormwater are known as combined
systems. Combined sewer systems are usually avoided now because precipitation causes widely varying flows
reducing sewage treatment plant efficiency. Combined sewers require much larger and more expensive treatment
facilities than sanitary sewers. Heavy storm runoff may overwhelm the sewage treatment system, causing a spill or
overflow. Sanitary sewers are typically much smaller than combined sewers, and they are not designed to transport
stormwater. Backups of raw sewage can occur if excessive infiltration/inflow is allowed into a sanitary sewer
Modern sewered developments tend to be provided with separate storm drain systems for rainwater. As rainfall
travels over roofs and the ground, it may pick up various contaminants including soil particles and other sediment,
heavy metals, organic compounds, animal waste, and oil and grease. (See urban runoff.) Some jurisdictions require
stormwater to receive some level of treatment before being discharged directly into waterways. Examples of
treatment processes used for stormwater include retention basins, wetlands, buried vaults with various kinds of
media filters, and vortex separators (to remove coarse solids).
Sewage can be treated close to where it is created, a decentralised system (in septic tanks, biofilters or aerobic
treatment systems), or be collected and transported via a network of pipes and pump stations to a municipal
treatment plant, a centralised system (see sewerage and pipes and infrastructure). Sewage collection and treatment is
typically subject to local, state and federal regulations and standards. Industrial sources of sewage often require
specialized treatment processes (see Industrial wastewater treatment).
Sewage treatment generally involves three stages, called primary, secondary and tertiary treatment.
Sewage treatment 2
• Primary treatment consists of temporarily holding the sewage in a quiescent basin where heavy solids can settle
to the bottom while oil, grease and lighter solids float to the surface. The settled and floating materials are
removed and the remaining liquid may be discharged or subjected to secondary treatment.
• Secondary treatment removes dissolved and suspended biological matter. Secondary treatment is typically
performed by indigenous, water-borne micro-organisms in a managed habitat. Secondary treatment may require a
separation process to remove the micro-organisms from the treated water prior to discharge or tertiary treatment.
• Tertiary treatment is sometimes defined as anything more than primary and secondary treatment in order to allow
rejection into a highly sensitive or fragile ecosystem (estuaries, low-flow rivers, coral reefs,...). 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, green way or park.
If it is sufficiently clean, it can also be used for groundwater recharge or agricultural purposes.
Process Flow Diagram for a typical large-scale treatment plant
Process Flow Diagram for a typical treatment plant via Subsurface Flow Constructed
Pre-treatment removes materials that can be easily collected from the raw sewage before they damage or clog the
pumps and sewage lines of primary treatment clarifiers (trash, tree limbs, leaves, branches etc.).
The influent sewage water passes through a bar screen 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, whilst 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
Sewage treatment 3
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
Pre-treatment may include a sand or grit channel or chamber, where the velocity of the incoming sewage is adjusted
to allow the settlement of sand, grit, stones, and broken glass. These particles are removed because they may damage
pumps and other equipment. For small sanitary sewer systems, the grit chambers may not be necessary, but grit
removal is desirable at larger plants. Grit chambers come in 3 types: horizontal grit chambers, aerated grit
chambers and vortex grit chambers.
Clarifiers and mechanized secondary treatment are more efficient under uniform flow conditions. Equalization
basins may be used for temporary storage of diurnal or wet-weather flow peaks. Basins provide a place to
temporarily hold incoming sewage during plant maintenance and a means of diluting and distributing batch
discharges of toxic or high-strength waste which might otherwise inhibit biological secondary treatment (including
portable toilet waste, vehicle holding tanks, and septic tank pumpers). Flow equalization basins require variable
discharge control, typically include provisions for bypass and cleaning, and may also include aerators. Cleaning may
be easier if the basin is downstream of screening and grit removal.
Fat and grease removal
In some larger plants, fat and grease is 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.
In the primary sedimentation stage, sewage flows through large tanks, commonly called "pre-settling basins",
"primary sedimentation tanks" or "primary clarifiers". The tanks are used to settle sludge while grease and oils rise
to the surface and are skimmed off. 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.:9-11 Grease and oil from the floating material can sometimes be recovered for saponification.
The dimensions of the tank should be designed to effect removal of a high percentage of the floatables and sludge. A
typical sedimentation tank may remove from 50 to 70 percent of suspended solids, and from 30 to 35 percent of
biochemical oxygen demand (BOD) from the sewage.
Secondary treatment is designed to substantially degrade the biological content of the sewage which are derived
from human waste, food waste, soaps and detergent. The majority of municipal plants treat the settled sewage liquor
using aerobic biological processes. To be effective, the biota require both oxygen and food to live. The bacteria and
protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, organic short-chain carbon
molecules, etc.) and bind much of the less soluble fractions into floc. Secondary treatment systems are classified as
fixed-film or suspended-growth systems.
• Fixed-film or attached growth systems include trickling filters, biotowers, and rotating biological contactors,
where the biomass grows on media and the sewage passes over its surface.:11-13 The fixed-film principal has
further developed into Moving Bed Biofilm Reactors (MBBR ), and Integrated Fixed-Film Activated Sludge
(IFAS ) processes. An MBBR system typically requires smaller footprint than suspended-growth systems.
Sewage treatment 4
(Black & Veatch )
• Suspended-growth systems include activated sludge, where the biomass is mixed with the sewage and can be
operated in a smaller space than trickling filters that treat the same amount of water. However, fixed-film systems
are more able to cope with drastic changes in the amount of biological material and can provide higher removal
rates for organic material and suspended solids than suspended growth systems.:11-13
Roughing filters are intended to treat particularly strong or variable organic loads, typically industrial, to allow them
to then be treated by conventional secondary treatment processes. Characteristics include filters filled with media to
which wastewater is applied. They are designed to allow high hydraulic loading and a high level of aeration. On
larger installations, air is forced through the media using blowers. The resultant wastewater is usually within the
normal range for conventional treatment processes.
A generalized, schematic diagram of an activated sludge process.
A filter removes a small percentage of the
suspended organic matter, while the
majority of the organic matter undergoes a
change of character, only due to the
biological oxidation and nitrification taking
place in the filter. With this aerobic
oxidation and nitrification, the organic
solids are converted into coagulated
suspended mass, which is heavier and
bulkier, and can settle to the bottom of a
tank. The effluent of the filter is therefore
passed through a sedimentation tank, called
a secondary clarifier, secondary settling tank
or humus tank.
In general, activated sludge plants encompass a variety of mechanisms and processes that use dissolved oxygen to
promote the growth of biological floc that substantially removes organic material.:12-13
The process traps particulate material and can, under ideal conditions, convert ammonia to nitrite and nitrate
ultimately to nitrogen gas. (See also denitrification).
Sewage treatment 5
A Typical Surface-Aerated Basin (using motor-driven floating aerators)
Surface-aerated basins (Lagoons)
Many small municipal sewage systems
in the United States (1 million gal./day
or less) use aerated lagoons.
Most biological oxidation processes for
treating industrial wastewaters have in
common the use of oxygen (or air) and
microbial action. Surface-aerated basins
achieve 80 to 90 percent removal of
BOD with retention times of 1 to 10
days. The basins may range in depth
from 1.5 to 5.0 metres and use
motor-driven aerators floating on the
surface of the wastewater.
In an aerated basin system, the aerators
provide two functions: they transfer air into the basins required by the biological oxidation reactions, and they
provide the mixing required for dispersing the air and for contacting the reactants (that is, oxygen, wastewater and
microbes). Typically, the floating surface aerators are rated to deliver the amount of air equivalent to 1.8 to 2.7 kg
O2/kW·h. However, they do not provide as good mixing as is normally achieved in activated sludge systems and
therefore aerated basins do not achieve the same performance level as activated sludge units.
Biological oxidation processes are sensitive to temperature and, between 0 °C and 40 °C, the rate of biological
reactions increase with temperature. Most surface aerated vessels operate at between 4 °C and 32 °C.
Filter beds (oxidizing beds)
In older plants and those receiving variable loadings, trickling filter beds are used where the settled sewage liquor is
spread onto the surface of a bed made up of coke (carbonized coal), limestone chips or specially fabricated plastic
media. Such media must have large surface areas to support the biofilms that form. The liquor is typically distributed
through perforated spray arms. The distributed liquor trickles through the bed and is collected in drains at the base.
These drains also provide a source of air which percolates up through the bed, keeping it aerobic. Biological films of
bacteria, protozoa and fungi form on the media’s surfaces and eat or otherwise reduce the organic content.:12 This
biofilm is often grazed by insect larvae, snails, and worms which help maintain an optimal thickness. Overloading of
beds increases the thickness of the film leading to clogging of the filter media and ponding on the surface. Recent
advances in media and process micro-biology design overcome many issues with trickling filter designs.
Constructed wetlands (can either be surface flow or subsurface flow, horizontal or vertical flow), include engineered
reedbeds and belong to the family of phytorestoration and ecotechnologies; they provide a high degree of biological
improvement and depending on design, act as a primary, secondary and sometimes tertiary treatment, also see
phytoremediation. One example is a small reedbed used to clean the drainage from the elephants' enclosure at
Chester Zoo in England; numerous CWs are used to recycle the water of the city of Honfleur in France and
numerous other towns in Europe, the US, Asia and Australia. They are known to be highly productive systems as
they copy natural wetlands, called the "kidneys of the earth" for their fundamental recycling capacity of the
hydrological cycle in the biosphere. Robust and reliable, their treatment capacities improve as time go by, at the
opposite of conventional treatment plants whose machinery age with time. They are being increasingly used,
although adequate and experienced design are more fundamental than for other systems and space limitation may
Sewage treatment 6
impede their use.
A new process called soil bio-technology (SBT) developed at IIT Bombay has shown tremendous improvements in
process efficiency enabling total water reuse, due to extremely low operating power requirements of less than 50
joules per kg of treated water. Typically SBT systems can achieve chemical oxygen demand (COD) levels less
than 10 mg/L from sewage input of COD 400 mg/L. SBT plants exhibit high reductions in COD values and
bacterial counts as a result of the very high microbial densities available in the media. Unlike conventional treatment
plants, SBT plants produce insignificant amounts of sludge, precluding the need for sludge disposal areas that are
required by other technologies.
In the Indian context, conventional sewage treatment plants fall into systemic disrepair due to 1) high operating
costs, 2) equipment corrosion due to methanogenesis and hydrogen sulphide, 3) non-reusability of treated water due
to high COD (>30 mg/L) and high fecal coliform (>3000 NFU) counts, 4) lack of skilled operating personnel and 5)
equipment replacement issues. Examples of such systemic failures has been documented by Sankat Mochan
Foundation at the Ganges basin after a massive cleanup effort by the Indian government in 1986 by setting up
sewage treatment plants under the Ganga Action Plan failed to improve river water quality.
Biological aerated filters
Biological Aerated (or Anoxic) Filter (BAF) or Biofilters combine filtration with biological carbon reduction,
nitrification or denitrification. BAF usually includes a reactor filled with a filter media. The media is either in
suspension or supported by a gravel layer at the foot of the filter. The dual purpose of this media is to support highly
active biomass that is attached to it and to filter suspended solids. Carbon reduction and ammonia conversion occurs
in aerobic mode and sometime achieved in a single reactor while nitrate conversion occurs in anoxic mode. BAF is
operated either in upflow or downflow configuration depending on design specified by manufacturer.
Schematic diagram of a typical rotating biological contactor (RBC). The treated effluent
clarifier/settler is not included in the diagram.
Rotating biological contactors
Rotating biological contactors (RBCs)
are mechanical secondary treatment
systems, which are robust and capable
of withstanding surges in organic load.
RBCs were first installed in Germany
in 1960 and have since been developed
and refined into a reliable operating
unit. The rotating disks support the
growth of bacteria and
micro-organisms present in the
sewage, which break down and
stabilize organic pollutants. To be
successful, micro-organisms need both
oxygen to live and food to grow. Oxygen is obtained from the atmosphere as the disks rotate. As the
micro-organisms grow, they build up on the media until they are sloughed off due to shear forces provided by the
rotating discs in the sewage. Effluent from the RBC is then passed through final clarifiers where the
micro-organisms in suspension settle as a sludge. The sludge is withdrawn from the clarifier for further treatment.
A functionally similar biological filtering system has become popular as part of home aquarium filtration and
purification. The aquarium water is drawn up out of the tank and then cascaded over a freely spinning corrugated
fiber-mesh wheel before passing through a media filter and back into the aquarium. The spinning mesh wheel
Sewage treatment 7
develops a biofilm coating of microorganisms that feed on the suspended wastes in the aquarium water and are also
exposed to the atmosphere as the wheel rotates. This is especially good at removing waste urea and ammonia
urinated into the aquarium water by the fish and other animals.
Membrane bioreactors (MBR) combine activated sludge treatment with a membrane liquid-solid separation process.
The membrane component uses low pressure microfiltration or ultrafiltration membranes and eliminates the need for
clarification and tertiary filtration. The membranes are typically immersed in the aeration tank; however, some
applications utilize a separate membrane tank. One of the key benefits of an MBR system is that it effectively
overcomes the limitations associated with poor settling of sludge in conventional activated sludge (CAS) processes.
The technology permits bioreactor operation with considerably higher mixed liquor suspended solids (MLSS)
concentration than CAS systems, which are limited by sludge settling. The process is typically operated at MLSS in
the range of 8,000–12,000 mg/L, while CAS are operated in the range of 2,000–3,000 mg/L. The elevated biomass
concentration in the MBR process allows for very effective removal of both soluble and particulate biodegradable
materials at higher loading rates. Thus increased sludge retention times, usually exceeding 15 days, ensure complete
nitrification even in extremely cold weather.
The cost of building and operating an MBR is often higher than conventional methods of sewage treatment.
Membrane filters can be blinded with grease or abraded by suspended grit and lack a clarifier's flexibility to pass
peak flows. The technology has become increasingly popular for reliably pretreated waste streams and has gained
wider acceptance where infiltration and inflow have been controlled, however, and the life-cycle costs have been
steadily decreasing. The small footprint of MBR systems, and the high quality effluent produced, make them
particularly useful for water reuse applications.
Secondary Sedimentation tank at a rural
The final step in the secondary treatment stage is to settle out the biological
floc or filter material through a secondary clarifier and to produce sewage
water containing low levels of organic material and suspended matter.
The purpose of tertiary treatment is to provide a final treatment stage to raise
the effluent quality before it is discharged to the receiving environment (sea,
river, lake, ground, etc.). 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."
Sewage treatment 8
Sand filtration removes much of the residual suspended matter.:22-23 Filtration over activated carbon, also called
carbon adsorption, removes residual toxins.:19
A sewage treatment plant and lagoon in Everett,
Washington, United States.
Lagooning provides settlement and further biological improvement
through storage in large man-made ponds or lagoons. These lagoons
are highly aerobic and colonization by native macrophytes, especially
reeds, is often encouraged. Small filter feeding invertebrates such as
Daphnia and species of Rotifera greatly assist in treatment by
removing fine particulates.
Wastewater may contain high levels of the nutrients nitrogen and
phosphorus. Excessive release to the environment can lead to a build
up of nutrients, called eutrophication, which can in turn encourage the
overgrowth of weeds, algae, and cyanobacteria (blue-green algae). This may cause an algal bloom, a rapid growth in
the population of algae. The algae numbers are unsustainable and eventually most of them die. The decomposition of
the algae by bacteria uses up so much of the oxygen in the water that most or all of the animals die, which creates
more organic matter for the bacteria to decompose. In addition to causing deoxygenation, some algal species produce
toxins that contaminate drinking water supplies. Different treatment processes are required to remove nitrogen and
The removal of nitrogen is effected 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 (NH3) to nitrite (NO2
−) is most often facilitated by 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 almost exclusively by Nitrospira spp.
Denitrification requires anoxic conditions to encourage the appropriate biological communities to form. It is
facilitated by a wide diversity of bacteria. Sand filters, lagooning and reed beds can all be used to reduce nitrogen,
but the activated sludge process (if designed well) can do the job the most easily.:17-18 Since denitrification is the
reduction of nitrate to dinitrogen gas, an electron donor is needed. This can be, depending on the wastewater, organic
matter (from faeces), 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 [RAS], and raw influent) e.g. by
using submersible mixers in order to achieve the desired denitrification.
Sometimes the conversion of toxic ammonia to nitrate alone is referred to as tertiary treatment.
Many sewage treatment plants use centrifugal pumps to transfer the nitrified mixed liquor from the aeration zone to
the anoxic zone for denitrification. These pumps are often referred to as Internal Mixed Liquor Recycle(IMLR)
Sewage treatment 9
Each person excretes between 200 and 1000 grams of phosphorus annually. Studies of United States sewage in the
late 1960s estimated mean per capita contributions of 500 grams in urine and feces, 1000 grams 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 the content of
urine and feces will remain unchanged. Phosphorus removal is important as it is a limiting nutrient for algae growth
in many fresh water systems. (For a description of the negative effects of algae, see Nutrient removal). It is also
particularly important for water reuse systems where high phosphorus concentrations may lead to fouling of
downstream equipment such as reverse osmosis.
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). When the biomass
enriched in these bacteria is separated from the treated water, these biosolids have a high fertilizer value.
Phosphorus removal can also be achieved by chemical precipitation, usually with salts of iron (e.g. ferric chloride),
aluminum (e.g. alum), or lime.:18 This may lead to excessive sludge production as hydroxides precipitates 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.
Once removed, phosphorus, in the form of a phosphate-rich sludge, may be stored in a land fill or resold for use in
The purpose of disinfection in the treatment of waste water is to substantially reduce the number of microorganisms
in the water to be discharged back into the environment for the later use of drinking, bathing, irrigation, etc. The
effectiveness of disinfection depends on the quality of the water being treated (e.g., cloudiness, pH, etc.), the type of
disinfection being used, the disinfectant dosage (concentration and time), and other environmental variables. Cloudy
water 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.:16
Chloramine, which is used for drinking water, is not used in the treatment of waste water because of its persistence.
After multiple steps of disinfection, the treated water is ready to be released back into the water cycle by means of
the nearest body of water or agriculture. Afterwards, the water can be transferred to reserves for everyday human
Chlorination remains the most common form of waste water disinfection in North America 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 the
United Kingdom, UV light is becoming the most common means of disinfection because of the concerns about the
Sewage treatment 10
impacts of chlorine in chlorinating residual organics in the wastewater and in chlorinating organics in the receiving
water. Some sewage treatment systems in Canada and the US also use UV light for their effluent 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 onsite as needed. 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
Odors 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 obnoxious
gases. Other methods of odor control exist, including addition of iron salts, hydrogen peroxide, calcium nitrate,
etc. to manage hydrogen sulfide levels. High-density solids pumps are suitable to reduce odors by conveying sludge
through hermetic closed pipework.
Package plants and batch reactors
To use less space, treat difficult waste and intermittent flows, a number of designs of hybrid treatment plants have
been produced. Such plants often combine at least two stages of the three main treatment stages into one combined
stage. In the UK, where a large number of wastewater treatment plants serve small populations, package plants are a
viable alternative to building a large structure for each process stage. In the US, package plants are typically used in
rural areas, highway rest stops and trailer parks.
One type of system that combines secondary treatment and settlement is the sequencing batch reactor (SBR).
Typically, activated sludge is mixed with raw incoming sewage, and then mixed and aerated. The settled sludge is
run off and re-aerated before a proportion is returned to the headworks. SBR plants are now being deployed in
many parts of the world.
The disadvantage of the SBR process is that it requires a precise control of timing, mixing and aeration. This
precision is typically achieved with computer controls linked to sensors. Such a complex, fragile system is unsuited
to places where controls may be unreliable, poorly maintained, or where the power supply may be intermittent.
Extended aeration package plants use separate basins for aeration and settling, and are somewhat larger than SBR
plants with reduced timing sensitivity.
Package plants may be referred to as high charged or low charged. This refers to the way the biological load is
processed. In high charged systems, the biological stage is presented with a high organic load and the combined floc
and organic material is then oxygenated for a few hours before being charged again with a new load. In the low
charged system the biological stage contains a low organic load and is combined with flocculate for longer times.
Sewage treatment 11
Sludge treatment and disposal
The sludges accumulated in a wastewater treatment process must be treated and disposed of in a safe and effective
manner. The purpose of digestion is to reduce the amount of organic matter and the number of disease-causing
microorganisms present in the solids. The most common treatment options include anaerobic digestion, aerobic
digestion, and composting. Incineration is also used albeit to a much lesser degree.:19-21
The equipment used in treatment plants are the following: first, the sludge is passed trough a so-called pre-thickener
or main sludge thickener This dewaters the sludge. Types of pre-thickeners include: centrifugal sludge
thickeners, rotary drum sludge thickeners, and belt filter presses. After this, the actual digestion is done in a
tank, and after this, the remaining solid is moved off.
Sludge treatment depends 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
Anaerobic digestion is a bacterial process that is carried out in the absence of oxygen. The process can either be
thermophilic digestion, in which sludge is fermented in tanks at a temperature of 55°C, or mesophilic, at a
temperature of around 36°C. Though allowing shorter retention time (and thus smaller tanks), thermophilic digestion
is more expensive in terms of energy consumption for heating the sludge.
Anaerobic digestion is the most common (mesophilic) treatment of domestic sewage in septic tanks, which normally
retain the sewage from one day to two days, reducing the BOD by about 35 to 40 percent. This reduction can be
increased with a combination of anaerobic and aerobic treatment by installing Aerobic Treatment Units (ATUs) in
the septic tank.
Mesophilic Anaerobic Digestion (MAD) is also the most common method for treating sludge produced at Sewage
Treatment Plants. The sludge is fed into large tanks and held for a minimum of 12 days to allow the digestion
process to perform the 4 stages necessary to digest the sludge. These are Hydrolysis, acidogenesis, Acetogenesis and
Methanogenesis. In this process the complex proteins and sugars are broken down to form more simple compounds
such as water, carbon dioxide and methane.</ref>f>http:/ / www. esru. strath. ac. uk/ EandE/ Web_sites/ 03-04/
biomass/ background%20info8. html</ref>
One major feature of anaerobic digestion is the production of biogas (with the most useful component being
methane), which can be used in generators for electricity production and/or in boilers for heating purposes. Many
larger sites utilize the biogas for combined heat and power, using the cooling water from the generators to maintain
the temperature of the digestion plant at the required 35 degrees +/- 3
Aerobic digestion is a bacterial process occurring in the presence of oxygen. Under aerobic conditions, bacteria
rapidly consume organic matter and convert it into carbon dioxide. The operating costs used to be characteristically
much greater for aerobic digestion because of the energy used by the blowers, pumps and motors needed to add
oxygen to the process.
Aerobic digestion can also be achieved by using diffuser systems or jet aerators to oxidize the sludge. Fine bubble
diffusers are typically the more cost-efficient diffusion method, however, plugging is typically a problem due to
sediment settling into the smaller air holes. Coarse bubble diffusers are more commonly used in activated sludge
tanks (generally a side process in waste water management) or in the flocculation stages. A key component for
selecting diffuser type is to ensure it will produce the required oxygen transfer rate.
Sewage treatment 12
Composting is also an aerobic process that involves mixing the sludge with sources of carbon such as sawdust, straw
or wood chips. In the presence of oxygen, bacteria digest both the wastewater solids and the added carbon source
and, in doing so, produce a large amount of heat.:20
Incineration of sludge is less common because of air emissions concerns and the supplemental fuel (typically natural
gases or fuel oil) required to burn the low calorific value sludge and vaporize residual water. Stepped multiple hearth
incinerators with high residence time and fluidized bed incinerators are the most common systems used to combust
wastewater sludge. Co-firing in municipal waste-to-energy plants is occasionally done, this option being less
expensive assuming the facilities already exist for solid waste and there is no need for auxiliary fuel.:20-21
When a liquid sludge is produced, further treatment may be required to make it suitable for final disposal. Typically,
sludges are thickened (dewatered) to reduce the volumes transported off-site for disposal. There is no process which
completely eliminates the need to dispose of biosolids. There is, however, an additional step some cities are taking to
superheat sludge and convert it into small pelletized granules that are high in nitrogen and other organic materials. In
New York City, for example, several sewage treatment plants have dewatering facilities that use large centrifuges
along with the addition of chemicals such as polymer to further remove liquid from the sludge. The removed fluid,
called centrate, is typically reintroduced into the wastewater process. The product which is left is called "cake" and
that is picked up by companies which turn it into fertilizer pellets. This product is then sold to local farmers and turf
farms as a soil amendment or fertilizer, reducing the amount of space required to dispose of sludge in landfills. Much
sludge originating from commercial or industrial areas is contaminated with toxic materials that are released into the
sewers from the industrial processes. Elevated concentrations of such materials may make the sludge unsuitable
for agricultural use and it may then have to be incinerated or disposed of to landfill.
Treatment in the receiving environment
The outlet of the Karlsruhe sewage treatment
plant flows into the Alb.
Many processes in a wastewater treatment plant are designed to mimic
the natural treatment processes that occur in the environment, whether
that environment is a natural water body or the ground. If not
overloaded, bacteria in the environment will consume organic
contaminants, although this will reduce the levels of oxygen in the
water and may significantly change the overall ecology of the
receiving water. Native bacterial populations feed on the organic
contaminants, and the numbers of disease-causing microorganisms are
reduced by natural environmental conditions such as predation or
exposure to ultraviolet radiation. Consequently, in cases where the
receiving environment provides a high level of dilution, a high degree
of wastewater treatment may not be required. However, recent
evidence has demonstrated that very low levels of specific contaminants in wastewater, including hormones (from
animal husbandry and residue from human hormonal contraception methods) and synthetic materials such as
phthalates that mimic hormones in their action, can have an unpredictable adverse impact on the natural biota and
potentially on humans if the water is re-used for drinking water. In the US and EU, uncontrolled discharges of
wastewater to the environment are not permitted under law, and strict water quality requirements are to be met, as
Sewage treatment 13
clean drinking water is essential. (For requirements in the US, see Clean Water Act.) A significant threat in the
coming decades will be the increasing uncontrolled discharges of wastewater within rapidly developing countries.
Effects on Biology
Sewage treatment plants can have multiple effects on nutrient levels in the water that the treated sewage flows into.
These effects on nutrients can have large effects on the biological life in the water in contact with the effluent.
Stabilization ponds (or treatment ponds) can include any of the following:
• Oxidation ponds, which are aerobic bodies of water usually 1–2 meters in depth that receive effluent from
sedimentation tanks or other forms of primary treatment.
• Dominated by algae
• Polishing ponds are similar to oxidation ponds but receive effluent from an oxidation pond or from a plant with an
extended mechanical treatment.
• Dominated by zooplankton
• Facultative lagoons, raw sewage lagoons, or sewage lagoons are ponds where sewage is added with no primary
treatment other than coarse screening. These ponds provide effective treatment when the surface remains aerobic;
although anaerobic conditions may develop near the layer of settled sludge on the bottom of the pond.
• Anaerobic lagoons are heavily loaded ponds.
• Dominated by bacteria
• Sludge lagoons are aerobic ponds, usually 2–5 meters in depth, that receive anaerobically digested primary
sludge, or activated secondary sludge under water.
• Upper layers are dominated by algae 
Phosphorus limitation is a possible result from sewage treatment and results in flagellate-dominated plankton,
particularly in summer and fall.
At the same time a different study found high nutrient concentrations linked to sewage effluents. High nutrient
concentration leads to high chlorophyll a concentrations, which is a proxy for primary production in marine
environments. High primary production means high phytoplankton populations and most likely high zooplankton
populations because zooplankton feed on phytoplankton. However, effluent released into marine systems also leads
to greater population instability.
A study done in Britain found that the quality of effluent affected the planktonic life in the water in direct contact
with the wastewater effluent. Turbid, low-quality effluents either did not contain ciliated protozoa or contained only
a few species in small numbers. On the other hand, high-quality effluents contained a wide variety of ciliated
protozoa in large numbers. Due to these findings, it seems unlikely that any particular component of the industrial
effluent has, by itself, any harmful effects on the protozoan populations of activated sludge plants.
The planktonic trends of high populations close to input of treated sewage is contrasted by the bacterial trend. In a
study of Aeromonas spp. in increasing distance from a wastewater source, greater change in seasonal cycles was
found the furthest from the effluent. This trend is so strong that the furthest location studied actually had an inversion
of the Aeromonas spp. cycle in comparison to that of fecal coliforms. Since there is a main pattern in the cycles that
occurred simultaneously at all stations it indicates seasonal factors (temperature, solar radiation, phytoplankton)
control of the bacterial population. The effluent dominant species changes from Aeromonas caviae in winter to
Aeromonas sobria in the spring and fall while the inflow dominant species is Aeromonas caviae, which is constant
throughout the seasons.
Sewage treatment 14
Sewage treatment in developing countries
Few reliable figures exist on the share of the wastewater collected in sewers that is being treated in the world. In
many developing countries the bulk of domestic and industrial wastewater is discharged without any treatment or
after primary treatment only. In Latin America about 15% of collected wastewater passes through treatment plants
(with varying levels of actual treatment). In Venezuela, a below average country in South America with respect to
wastewater treatment, 97 percent of the country’s sewage is discharged raw into the environment. In a relatively
developed Middle Eastern country such as Iran, the majority of Tehran's population has totally untreated sewage
injected to the city’s groundwater. However now the construction of major parts of the sewage system, collection
and treatment, in Tehran is almost complete, and under development, due to be fully completed by the end of 2012.
In Isfahan, Iran's third largest city, sewage treatment was started more than 100 years ago.
In Israel, about 50 percent of agricultural water usage (total use was 1 billion cubic metres in 2008) is provided
through reclaimed sewer water. Future plans call for increased use of treated sewer water as well as more
Most of sub-Saharan Africa is without wastewater treatment.
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Sewage treatment 16
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Article Sources and Contributors 17
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Aaavinash, Abdallahdjabi, Abhishank.jajur, AdultSwim, Afluent Rider, Aitias, Alansohn, Aldie, Ale jrb, Alf ea, [email protected], Allstarecho, AlphaEta, Amalthea, Andrewpmk, Andy Dingley,
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Image Sources, Licenses and Contributors
File:Sewer Plant.jpg Source: http://en.wikipedia.org/w/index.php?...ewer_Plant.jpg License: Creative Commons Attribution 3.0 Contributors: Rjgalindo
File:ESQUEMPEQUE-EN.jpg Source: http://en.wikipedia.org/w/index.php?...EMPEQUE-EN.jpg License: Creative Commons Attribution-Sharealike 2.5 Contributors: Leonard
File:SchemConstructedWetlandSewage.jpg Source: http://en.wikipedia.org/w/index.php?...landSewage.jpg License: Public Domain Contributors: Yayasan
IDEP Foundation and Wastewater Gardens
File:Activated Sludge 1.png Source: http://en.wikipedia.org/w/index.php?...d_Sludge_1.png License: Public Domain Contributors: Original uploader was Mbeychok at
File:Surface-Aerated Basin.png Source: http://en.wikipedia.org/w/index.php?...ated_Basin.png License: Public Domain Contributors: Mbeychok
File:Rotating Biological Contactor.png Source: http://en.wikipedia.org/w/index.php?..._Contactor.png License: Public Domain Contributors: Mbeychok
File:Secondary sedimentation tank 1 w.JPG Source: http://en.wikipedia.org/w/index.php?...n_tank_1_w.JPG License: GNU Free Documentation License
Contributors: Mailer diablo, Million Moments, Velella, Vortexrealm
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File:MiRO3.jpg Source: http://en.wikipedia.org/w/index.php?...File:MiRO3.jpg License: Creative Commons Attribution 2.0 Contributors: User:Ikar.us
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