Sewage treatment is the process of removing contaminants
from wastewater, including household sewage and runoff (effluents). 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). With suitable
technology, it is possible to re-use sewage effluent for drinking water,
although this is usually only done in places with limited water supplies, such
as Windhoek and Singapore.
History
The Great Stink of 1858 stimulated research into the problem
of sewage treatment. In this caricature in The Times, Michael Faraday reports
to Father Thames on the state of the river.
Basic sewer systems were used for waste removal in ancient
Mesopotamia, where vertical shafts carried the waste away into cesspools.
Similar systems existed in the Indus Valley civilization in modern day India
and in Ancient Crete and Greece. In the Middle Ages the sewer systems built by
the Romans fell into disuse and waste was collected into cesspools that were
periodically emptied by workers known as 'rakers' who would often sell it as
fertilizer to farmers outside the city.
Modern sewage systems were first built in the mid-nineteenth
century as a reaction to the exacerbation of sanitary conditions brought on by
heavy industrialization and urbanization. Due to the contaminated water supply,
cholera outbreaks occurred in 1832, 1849 and 1855 in London, killing tens of
thousands of people. This, combined with the Great Stink of 1858, when the
smell of untreated human waste in the River Thames became overpowering, and the
report into sanitation reform of the Royal Commissioner Edwin Chadwick, led to
the Metropolitan Commission of Sewers appointing Sir Joseph Bazalgette to
construct a vast underground sewage system for the safe removal of waste.
Contrary to Chadwick's recommendations, Bazalgette's system, and others later
built in Continental Europe, did not pump the sewage onto farm land for use as
fertilizer; it was simply piped to a natural waterway away from population
centres, and pumped back into the environment.
Early attempts
One of the first attempts at diverting sewage for use as a
fertilizer in the farm was made by the cotton mill owner James Smith in the
1840s. He experimented with a piped distribution system initially proposed by
James Vetch that collected sewage from his factory and pumped it into the
outlying farms, and his success was enthusiastically followed by Edwin Chadwick
and supported by organic chemist Justus von Liebig.
The idea was officially adopted by the Health of Towns
Commission, and various schemes (known as sewage farms) were trialled by
different municipalities over the next 50 years. At first, the heavier solids
were channelled into ditches on the side of the farm and were covered over when
full, but soon flat-bottomed tanks were employed as reservoirs for the sewage;
the earliest patent was taken out by William Higgs in 1846 for "tanks or
reservoirs in which the contents of sewers and drains from cities, towns and
villages are to be collected and the solid animal or vegetable matters therein
contained, solidified and dried..." Improvements to the design of the
tanks included the introduction of the horizontal-flow tank in the 1850s and
the radial-flow tank in 1905. These tanks had to be manually de-sludged
periodically, until the introduction of automatic mechanical de-sludgers in the
early 1900s.
The precursor to the modern septic tank was the cesspool in
which the water was sealed off to prevent contamination and the solid waste was
slowly liquified due to anaerobic action; it was invented by L.H Mouras in
France in the 1860s. Donald Cameron, as City Surveyor for Exeter patented an
improved version in 1895, which he called a 'septic tank'; septic having the
meaning of 'bacterial'. These are still in worldwide use, especially in rural
areas unconnected to large-scale sewage systems.
Chemical treatment
Sir Edward Frankland, a distinguished chemist, who
demonstrated the possibility of chemically treating sewage in the 1870s.
It was not until the late 19th century that it became
possible to treat the sewage by chemically breaking it down through the use of
microorganisms and removing the pollutants. Land treatment was also steadily
becoming less feasible, as cities grew and the volume of sewage produced could
no longer be absorbed by the farmland on the outskirts.
Sir Edward Frankland conducted experiments at the Sewage
Farm in Croydon, England, during the 1870s and was able to demonstrate that
filtration of sewage through porous gravel produced a nitrified effluent (the
ammonia was converted into nitrate) and that the filter remained unclogged over
long periods of time.This established the then revolutionary possibility of
biological treatment of sewage using a contact bed to oxidize the waste. This
concept was taken up by the chief chemist for the London Metropolitan Board of
Works, William Libdin, in 1887:
...in all
probability the true way of purifying sewage...will be first to separate the
sludge, and then turn into neutral effluent... retain it for a sufficient
period, during which time it should be fully aerated, and finally discharge it
into the stream in a purified condition. This is indeed what is aimed at and
imperfectly accomplished on a sewage farm.
From 1885 to 1891 filters working on this principle were
constructed throughout the UK and the idea was also taken up in the US at the
Lawrence Experiment Station in Massachusetts, where Frankland's work was
confirmed. In 1890 the LES developed a 'trickling filter' that gave a much more
reliable performance.
Contact beds were developed in Salford, Manchester and by
scientists working for the London City Council in the early 1890s. According to
Christopher Hamlin, this was part of a conceptual revolution that replaced the
philosophy that saw "sewage purification as the prevention of decomposition
with one that tried to facilitate the biological process that destroy sewage
naturally."
Contact beds were tanks containing the inert substance, such
as stones or slate, that maximized the surface area available for the microbial
growth to break down the sewage. The sewage was held in the tank until it was
fully decomposed and it was then filtered out into the ground. This method
quickly became widespread, especially in the UK, where it was used in
Leicester, Sheffield, Manchester and Leeds. The bacterial bed was
simultaneously developed by Joseph Corbett as Borough Engineer in Salford and
experiments in 1905 showed that his method was superior in that greater volumes
of sewage could be purified better for longer periods of time than could be achieved
by the contact bed.
The Royal Commission on Sewage Disposal published its eighth
report in 1912 that set what became the international standard for sewage
discharge into rivers; the '20:30 standard', which allowed 20 mg Biochemical
oxygen demand and 30 mg suspended solid per litre.
Activated sludge
The Davyhulme Sewage Works Laboratory, where the activated
sludge process was developed in the early 20th century.
The development of secondary treatments to sewage in the
early twentieth century led to arguably the single most significant improvement
in public health and the environment during the course of the century, the
invention of the 'activated sludge' process for the treatment of sewage.
In 1912, Dr. Gilbert Fowler, a scientist at the University
of Manchester, observed experiments being conducted at the Lawrence Experiment
Station at Massachusetts involving the aeration of sewage in a bottle that had
been coated with algae. Fowler's engineering colleagues, Edward Ardern and
William Lockett, who were conducting research for the Manchester Corporation
Rivers Department at Davyhulme Sewage Works, experimented on treating sewage in
a draw-and-fill reactor, which produced a highly treated effluent. They aerated
the waste-water continuously for about a month and were able to achieve a
complete nitrification of the sample material. Believing that the sludge had
been activated (in a similar manner to activated carbon) the process was named
activated sludge.
Their results were published in their seminal 1914 paper,
and the first full-scale continuous-flow system was installed at Worcester two
years later. In the aftermath of the First World War the new treatment method
spread rapidly, especially to the USA, Denmark, Germany and Canada. By the late
1930s, the activated sludge treatment was the predominant process used around
the world.
Origins of sewage
Sewage is generated by residential, institutional,
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 storm water are known as combined sewer systems. This
design was common when urban sewerage systems were first developed, in the late
19th and early 20th centuries.:119 Combined sewers require much larger and more
expensive treatment facilities than sanitary sewers. Heavy volumes of 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 (dilution by stormwater and/or groundwater) is
allowed into a sanitary sewer system. Communities that have urbanized in the
mid-20th century or later generally have built separate systems for sewage
(sanitary sewers) and stormwater, because precipitation causes widely varying
flows, reducing sewage treatment plant efficiency.
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).
Process overview
Sewage can be treated close to where the sewage is created,
a decentralized system (in septic tanks, biofilters or aerobic treatment
systems), or be collected and transported by a network of pipes and pump
stations to a municipal treatment plant, a centralized 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.
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.
Simplified process flow diagram for a typical large-scale
treatment plant
Process flow diagram for a typical treatment plant via
subsurface flow constructed wetlands (SFCW)
Pretreatment
Pretreatment removes all materials that can be easily
collected from the raw sewage before they damage or clog the pumps and sewage
lines of primary treatment clarifiers. Objects that are commonly removed during
pretreatment include trash, tree limbs, leaves, branches, and other large
objects.
The influent in sewage water passes through a bar screen to
remove all large objects like cans, rags, sticks, plastic packets etc. carried
in the sewage stream. 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.
Grit removal
Pretreatment 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.[21] Grit chambers come in 3 types: horizontal grit chambers, aerated
grit chambers and vortex grit chambers.
Flow equalization
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 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
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.Grease and oil from the
floating material can sometimes be recovered for saponification.
Secondary treatment
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.The fixed-film principle 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.
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.
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 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.
Activated sludge
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.
Biological floc, as mentioned above, is an ecosystem of
living biota that subsists on nutrients from the inflowing primary settling
tank (or clarifier) effluent. These mostly carbonaceous dissolved solids
undergo aeration to be broken down and biologically oxidized or converted to
carbon dioxide. Likewise, nitrogenous dissolved solids (amino acids, ammonia,
etc.) are also oxidized (=eaten) by the floc to nitrites, nitrates, and, in
some processes, to nitrogen gas through denitrification.
While denitrification is encouraged in some treatment
processes, in many suspended aeration plants denitrification will impair the
settling of the floc and lead to poor quality effluent.
In either case, the settled floc is both recycled to the
inflowing primary effluent to regrow, or is partially 'wasted' (or diverted) to
solids dewatering, or digesting, and then dewatering.
Interestingly, like most living creatures, activated sludge
biota can get sick. This many times takes the form of the floating brown foam,
Nocardia. While this so-called 'sewage fungus' (it isn't really a fungus) is
the best known, there are many different fungi and protists that can
overpopulate the floc and cause process upsets. Additionally, certain incoming
chemical species, such as a heavy pesticide, a heavy metal (e.g.: plating
company effluent) load, or extreme pH, can kill the biota of an activated
sludge reactor ecosystem. Such problems are tested for, and if caught in time,
can be neutralized.
A typical surface-aerated basin (using motor-driven floating
aerators)
Aerobic granular sludge
Activated sludge systems can be transformed into aerobic
granular sludge systems (aerobic granulation) which enhance the benefits of
activated sludge, like increased biomass retention due to high sludge
settlability.
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. 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
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 impede their use.
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 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 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
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
Secondary sedimentation tank at a rural treatment plant.
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.
Tertiary treatment
The purpose of tertiary treatment is to provide a final
treatment stage to further improve the effluent quality before it is discharged
to the receiving environment (sea, river, lake, wet lands, ground, etc.). More
than one tertiary treatment process may be used at any treatment plant. If
disinfection is practised, it is always the final process. It is also called
"effluent polishing."
Filtration
Sand filtration removes much of the residual suspended
matter.Filtration over activated carbon, also called carbon adsorption, removes
residual toxins.
Lagooning
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.
Nutrient removal
Wastewater may contain high levels of the nutrients nitrogen
and phosphorus. Excessive release to the environment can lead to a buildup 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 phosphorus.
Nitrogen removal
Nitrogen is removed through the biological oxidation of
nitrogen from ammonia to nitrate (nitrification), followed by denitrification,
the reduction of nitrate to nitrogen gas. Nitrogen gas is released to the
atmosphere and thus removed from the water.
Nitrification itself is a two-step aerobic process, each
step facilitated by a different type of bacteria. The oxidation of ammonia
(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. 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) pumps.
The bacteria Brocadia anammoxidans, is being researched for
its potential in sewage treatment. It can remove nitrogen from waste water. In
addition the bacteria can perform the anaerobic oxidation of ammonium and can
produce the rocket fuel hydrazine from waste water.
Phosphorus removal
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.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.[citation needed] 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 fertilizer.
Disinfection
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. 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 uses.
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 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 disinfection.
Ozone (O3) is generated by passing oxygen (O2) through a
high voltage potential resulting in a third oxygen atom becoming attached and
forming O3. Ozone is very unstable and reactive and oxidizes most organic
material it comes in contact with, thereby destroying many pathogenic
microorganisms. Ozone is considered to be safer than chlorine because, unlike
chlorine which has to be stored on site (highly poisonous in the event of an
accidental release), ozone is generated on-site as needed. 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.
Odor control
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 noxious 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 for reducing 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 cyclic activated sludge (CASSBR). 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 CASSBR 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.
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.
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 larger-scale operations.
The sludge is sometimes passed through a so-called
pre-thickener which de-waters the sludge. Types of pre-thickeners include
centrifugal sludge thickeners[39] rotary drum sludge thickeners and belt filter
presses.
Anaerobic digestion
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 biochemical oxygen demand (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 a 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 four 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.
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 ± 3 °C.
Aerobic digestion
Aerobic digestion (a subset of the activated sludge process)
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. However, recent technological advances
include non-electric aerated filter systems that use natural air currents for
the aeration instead of electrically operated machinery.
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.
Composting
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.
Incineration
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.
Sludge disposal
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 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 to 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 carried out 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. Because of 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 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
percent 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, 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 desalination plants.
Most of sub-Saharan Africa is without wastewater
treatment.[citation needed]
SUBSCRIBERS - (
LINKS) :FOLLOW / REF / 2 /
findleverage.blogspot.com
Krkz77@yahoo.com
+234-81-83195664
No comments:
Post a Comment