UNIVERSITY OF WEST HUNGARY
FACULTY OF AGRICULTURAL AND FOOD SCIENCES
WASTE WATER TREATMENT
IN THE UNITED KINGDOM
Balint Szule
PhD-student, NYME-MÉK, Mosonmagyaróvár
Correspondence:
Balint Szule
37 Creighton Road
London
N17 8JU
2014
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Abstract:
The waste water treatment in the UK has a long history. It was obvious to collect
information from the UK, especially from London the capital city of the United
Kingdom (and one of the biggest city in the world). More than 10 million people live
and work in London. Collecting and treating this amount of waste water needs special
expertise and system. Having knowledge of other treatment methods from other
countries, is important to improve the system in Hungary. In this research the UK’s and
Hungarian’s waste water treatment were compared.
Key words: waste water, treatment, environmental, methods
Introduction
Freshwater is a vital natural resource that will continue to be renewable as long as it is
well managed. Preventing pollution from domestic, industrial, and agro-industrial
activities is important to ensure the sustainability of the locale’s development. (NG Wun
Jern, 2006)
In recent years in the UK all sewage water needs to be treated, whether it comes from a
home or a factory. The treatment are 99% takes place in the country and this work meet
the standard of the EU legislation.
Treatment of wastewater is an essential process that prevents contamination and the
destruction of our waterways, drinking water resources and natural water resources.
Although untreated waste water is mostly water (generally less than 0,1% is solid
material), without treatment the waste water produced every day would cause significant
damage to the environment. The removal of these solids and disinfection of the water
before discharge is the basic concept of wastewater treatment. If wastewater was
discharged without treatment directly to a receiving water system, it would severely
damage the water quality and render it unsuitable for swimming, fishing and other
activities. (URL1) The impacts of untreated waste water range from, chronic ecosystem
damage due to oxygen depletion of receiving waters from the biodegradation of organic
matter; ecosystem damage of eutrophication of waters resulting from excessive input of
nutrients present in waste water; potential health risks from water-born pathogens from
discharges to waters used for recreational activities. Wastewater is a carrier of harmful
bacteria and microorganisms known as pathogens. Several pathogens include
Cryptosporidium, Salmonella, Typhoid, E-Coli, Hepatitis A & B and Giardiasis also
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known as “beaver fever”. Wastewater is also rich in nitrogen and phosphorus nutrients
which stimulate excessive aquatic growth, which in turn, can be detrimental to aquatic
life such as fish and waterfowl. Untreated waste water also contains sewage litter and
other sewage solids that can impact the environment, for example, through smothering
of river beds or posing a hazard through its ingestion by wildlife. (Waste water
treatment in the United Kingdom - 2012)
Discussion
Common regulation
In the United Kingdom the authority follows the European Union directive. The Council
Directive 91/271/EEC concerning urban waste-water treatment was adopted on 21 May
1991. Its objective is to protect the environment from the adverse effects of urban waste
water discharges and discharges from certain industrial sectors and concerns the
collection, treatment and discharge of:
Domestic waste water;
Mixture of waste water;
Waste water from certain industrial sectors;
Four main principles are laid down in the Directive:
1. Planning
2. Regulation
3. Monitoring
4. Information and reporting
Specifically the Directive requires:
The Collection and treatment of waste water in all agglomerations of> 2000
population equivalents;
Secondary treatment of all discharges from agglomerations of> 2000 p.e., and more
advanced treatment for agglomerations> 10 000 population equivalents in designated
sensitive areas and their catchments;
A requirement for pre-authorisation of all discharges of urban wastewater, of
discharges from the food-processing industry and of industrial discharges into urban
wastewater collection systems;
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Monitoring of the performance of treatment plants and receiving waters; and
Controls of sewage sludge disposal and re-use, and treated waste water re-use
whenever it is appropriate.
1. Planning
The planning aspects of the Directive require Member States to:
Designate sensitive areas (sensitive water bodies) in accordance with three specific
criteria, and to review their designation every four years;
Identify the relevant hydraulic catchment areas of the sensitive areas and ensure that
all discharges from agglomerations with more than 10 000 p.e. located within the
catchment shall have more stringent than secondary treatment;
Establish less sensitive areas if relevant;
Establish a technical and financial programme for the implementation of the Directive
for the construction of sewage collecting systems and wastewater treatment plants
addressing treatment objectives within the deadlines set up by the Directive (and the
Accession Treaties for new Member States).
2. Regulation
The regulation aspects of the Directive require Member States to:
Establish systems of prior regulation or authorisation for all discharges of urban
wastewater;
Establish systems of prior regulation or authorisation for discharges of industrial
wastewater into urban sewage collecting systems to ensure:
o Treatment plant operation and sludge treatment will not be impeded;
o No adverse effect on the environment (including receiving waters) will occur;
and
o The safe disposal of sewage sludge.
Establish systems of prior regulation and/or specific authorisation and permits for food
processing industries;
Ensure that all urban wastewater generated in agglomerations with more than 2000
p.e. are supplied with collecting systems, and that the capacity of these is such that all
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urban waste water is collected, taking account of normal local climatic conditions and
seasonal variations;
Ensure that national authorities take measures to limit pollution of receiving waters
from storm water overflows via collecting systems under unusual situations, such as
heavy rain;
Ensure that wastewater treatment is provided for all agglomerations at the level
specified by the Directive and within the required deadline:
o Secondary treatment is the basic level that should be provided, with more
stringent treatment being required in sensitive areas and their catchments;
o For certain discharges in coastal waters treatment may be less stringent (i.e.
primary treatment) under certain conditions and subject to the agreement of the
European Commission;
o For agglomerations with a population equivalent of less than 2000 but
equipped with a collecting system, appropriate treatment must be provided.
Ensure that technical requirements for the design, construction, operation and
maintenance of wastewater treatment plants treating urban wastewater are maintained
and that they ensure adequate capacity of the plant and treatment of urban wastewater
generated in agglomerations taking into account normal climatic conditions and
seasonal variations;
Ensure that the environment is protected from adverse effects of the discharge of
wastewater;
Ensure that the environmentally and technically sound reuse or disposal of sewage
sludge is subject to general rules, registration or authorisation and that the requirement
of specific inter-linked Directives for agricultural re-use (86/278/EEC), incineration
(89/429/EEC and 89/369/EEC), and landfill (99/31/EC) are respected. The disposal of
sewage sludge to surface waters is banned.
3. Monitoring
The monitoring aspects of the Directive require Member States to ensure that monitoring
programmes are in place and that the programmes correspond to the requirements laid down
in Annex I D of the Directive in terms of parameters monitored, analytical method and
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sampling frequency. Member States are required to ensure that both discharges from urban
wastewater treatment plants and receiving waters are monitored.
4. Information and reporting
The information and reporting provisions of the Directive require Member States to ensure
that the following are put in place:
Adequate mechanisms to allow the co-operation and exchange of information with
other Member States in cases where discharges of wastewater have a transboundary
effect on water quality of shared waters;
Adequate reporting procedures and databases to allow the provision of information to
the Commission on:
o Transposition of the Directive into national legislation, implementation
programmes and situation reports on the disposal and re-use of treated urban
wastewater and sewage sludge;
o Status of collecting systems, efficiency of treatment plants (i.e. treatment level
and monitoring results) and the quality of receiving waters; and
o Status of discharges from the food-processing industry to surface waters;
Access for the public to relevant information and the publication of status reports
every two years on the status of wastewater collection and treatment and disposal or
re-use of sludge. (URL2)
The common law in the European Union speaks three different type of waste water. These are
the urban, domestic and industrial waste water. Urban waste water means domestic waste
water or the mixture of domestic waste water with industrial waste water and/or run-off
rainwater; domestic waste water means waste water from residential settlements and services
which originates predominantly from the human metabolism and from household activities.
Industrial waste water intend any waste water which is discharged from premises used for
carrying on any trade or industry, other than domestic waste water and run-off rainwater
(Urban waste water directive /91/271/ECC).
Sewage treatment is the process of removing contaminants from wastewater, including
household sewage and run off (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
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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. (URL3)
Most wastewater treatment processes have three main disadvantage (URL4):
high energy requirements;
high operation and maintenance requirements, including production of large
volumes of sludge (solid waste material);
they are geared towards environmental prohibition rather than human health
protection.
History of wastewater treatment
Many ancient cities had drainage systems, but they were primarily intended to carry rainwater
away from roofs and pavements. A notable example is the drainage system of ancient Rome.
It included many surface conduits that were connected to a large vaulted channel called the
Cloaca Maxima (“Great Sewer”), which carried drainage water to the Tiber River. Built of
stone and on a grand scale, the Cloaca Maxima is one of the oldest existing monuments of
Roman engineering.
There was little progress in urban drainage or sewerage during the Middle Ages. Privy vaults
and cesspools were used, but most wastes were simply dumped into gutters to be flushed
through the drains by floods. Toilets (water closets) were installed in houses in the early 19th
century, but they were usually connected to cesspools, not to sewers. In densely populated
areas, local conditions soon became intolerable because the cesspools were seldom emptied
and frequently overflowed. The threat to public health became apparent. In England in the
middle of the 19th century, outbreaks of cholera were traced directly to well-water supplies
contaminated with human waste from privy vaults and cesspools. It soon became necessary
for all water closets in the larger towns to be connected directly to the storm sewers. This
transferred sewage from the ground near houses to nearby bodies of water. Thus, a new
problem emerged: surface water pollution.
It used to be said that “the solution to pollution is dilution.” When small amounts of sewage
are discharged into a flowing body of water, a natural process of stream self-purification
occurs. Densely populated communities generate such large quantities of sewage, however,
that dilution alone does not prevent pollution. This makes it necessary to treat or purify
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wastewater to some degree before disposal. As populations in towns and cities grew the rivers
could not absorb the pollution. They began to smell and became unable to support life. The
situation was particularly serious in London and culminated in the House of Commons having
to suspended as a result of the “Great Stink” from the Thames in 1859. This encouraged the
construction of a new sewer system in London that set the standard for the rest of the country.
The construction of centralized sewage treatment plants began in the late 19th
and early 20th
centuries, principally in the United Kingdom and the United States. Instead of discharging
sewage directly into a nearby body of water, it was first passed through a combination of
physical, biological, and chemical processes that removed some or most of the pollutants.
Also beginning in the 1900s, new sewage-collection systems were designed to separate storm
water from domestic wastewater, so that treatment plants did not become overloaded during
periods of wet weather.
After the middle of the 20th century, increasing public concern for environmental quality led
to broader and more stringent regulation of wastewater disposal practices. Higher levels of
treatment were required. For example, pretreatment of industrial wastewater, with the aim of
preventing toxic chemicals from interfering with the biological processes used at sewage
treatment plants, often became a necessity. In fact, wastewater treatment technology advanced
to the point where it became possible to remove virtually all pollutants from sewage. This was
so expensive, however, that such high levels of treatment were not usually justified.
Wastewater treatment plants became large, complex facilities that required considerable
amounts of energy for their operation. After the rise of oil prices in the 1970s, concern for
energy conservation became a more important factor in the design of new pollution control
systems. Consequently, land disposal and subsurface disposal of sewage began to receive
increased attention where feasible. Such “low-tech” pollution control methods not only might
help to conserve energy but also might serve to recycle nutrients and replenish groundwater
supplies. (URL5)
Treatment
Collection
Before waste water can be treated it needs to be collected. Every day in the UK over 624,200
kilometres of sewers collect over 11 billion litres of waste water from homes, municipal,
commercial and industrial premises and rainwater run-off from roads and other impermeable
surfaces.
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This is treated at about 9,000 sewage treatment works before the treated effluent is discharged
to inland waters, estuaries and the sea. (URL6)
The UK’s sewerage undertakers are responsible for building, maintaining and improving main
sewers, pumping stations, and waste water treatment facilities that service around 96% of the
UK population. The remaining 4% of the population, represented by the smallest of
communities and individual properties in rural areas remote from mains sewers, are generally
served by privately owned, small-package treatment plants catering for small groups of
houses, to septic tanks, cesspits and other in-situ treatment systems, generally serving
individual properties. There are three main types of collection system:
surface-water drainage that collects rainwater run-off from roads and urban areas and
discharge direct to local waters;
combined sewerage that collects rainwater run-off and waste water from domestic,
industrial, commercial and other premises; and
foul drainage that collects domestic waste water from premises (no rainwater is
collected).
Both surface water and foul drainage may eventually connect to combined sewerage where
there are no local environmental waters to which surface water drainage can discharge.
Combined sewerage systems are not uncommon in the UK and elsewhere in Europe. A basic
requirement of combined sewerage systems is that they need to cater for all normal local
climatic conditions. In other words, they need to be large enough to receive and effectively
manage storm water from peak seasonal wet weather. However, even when designed to deal
with such weather, there may be times when heavy continuous rainfall will temporarily
exceed the capacity of combined sewerage systems. To deal with such situations ‘combined
sewer overflows’ are designed and built as an integral part of combined sewerage systems.
The purpose of combined sewer overflows is to allow excess waste water to be discharged to
local waters to avoid sewers being overwhelmed and waste water ‘backing up’ along sewers
and flooding streets and properties, or overwhelming waste water treatment plants. The
backflow of waste water to properties and streets would present human health hazards and
flooded and overflowing treatment plants would disrupt treatment processes and have the
potential to cause more environmental damage than can be caused by discharges from
combined sewer overflows. (Waste Water Treatment in the UK, 2012)
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Treatment methods
Usually wastewater treatment will involve collecting the wastewater in a central, segregated
location (the Wastewater Treatment Plant) and subjecting the wastewater to various treatment
processes. Most often, since large volumes of wastewater are involved, treatment processes
are carried out on continuously flowing wastewaters (continuous flow or "open" systems)
rather than as "batch" or a series of periodic treatment processes in which treatment is carried
out on parcels or "batches" of wastewaters. While most wastewater treatment processes are
continuous flow, certain operations, such as vacuum filtration, involving as it does, storage of
sludge, the addition of chemicals, filtration and removal or disposal of the treated sludge, are
routinely handled as periodic batch operations. Wastewater treatment, however, can also be
organized or categorized by the nature of the treatment process operation being used; for
example, physical, chemical or biological. Examples of these treatment steps are shown
below. A complete treatment system may consist of the application of a number of physical,
chemical and biological processes to the wastewater.
Physical Chemical Biological
Sedimentation
(Clarification)
Chlorination Aerobic
Screening Ozonation Activated Sludge Treatment
Methods
Aeration Neutralization Trickling Filtration
Filtration Coagulation Oxidation Ponds
Flotation and Skimming Adsorption Lagoons
Degassification Ion Exchange Aerobic Digestion
Equalization Anaerobic
Lagoons
Septic Tanks
Table 1: Some Physical, Chemical and Biological Wastewater Treatment Methods (Source: URL7)
Conventional wastewater treatment consists of a combination of physical, chemical, and
biological processes and operations to remove solids, organic matter and, sometimes,
nutrients from wastewater. General terms used to describe different degrees of treatment, in
order of increasing treatment level, are preliminary, primary, secondary, and tertiary and/or
advanced wastewater treatment.
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Figure 3: Generalized flow diagram for
municipal wastewater treatment (Asano et
al. 1985)
Preliminary treatment
The objective of preliminary treatment is the removal of coarse solids and other large
materials often found in raw wastewater. Removal of these materials is necessary to enhance
the operation and maintenance of subsequent treatment units. Preliminary treatment
operations typically include coarse screening, grit removal and, in some cases, comminution
of large objects.
Figure 4: Simplified process flow diagram
for a typical large-scale treatment plant
(Source: URL8)
Screens
On entering a sewage treatment works,
dirty water passes through screens to
remove paper, wood and other large
articles that could damage machinery
or block pipe systems. Screens consist of vertical bars spaced close together or perforated
plates that are cleaned by rakes or water jets. The cleared material (known as screenings) is
washed and safely disposed of at a landfill site. It is important to cut the amount of screenings
which can block sewers before the treatment works with unpleasant results. Only toilet paper
should be flushed down the toilet. Water companies run ‘Bag It and Bin It’ campaign to
encourage the public not to flush cotton buds or plastic and sanitary items. In some European
countries the sewer pipes are so small that not even paper may be flushed.
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Grit Removal
Grit includes sand, gravel, cinder, or other heavy solid materials that are “heavier” (higher
specific gravity) than the organic biodegradable solids in the wastewater. Grit also includes
eggshells, bone chips, seeds, coffee grounds, and large organic particles, such as food waste.
Removal of grit prevents unnecessary abrasion and wear of mechanical equipment, grit
deposition in pipelines and channels, and accumulation of grit in anaerobic digesters and
aeration basins. Grit removal facilities typically precede primary clarification, and follow
screening and comminution. This prevents large solids from interfering with grit handling
equipment. In secondary treatment plants without primary clarification, grit removal should
precede aeration (Metcalf and Eddy, 1991)
In grit chambers, the velocity of the water through the chamber is maintained sufficiently
high, or air is used, so as to prevent the settling of most organic solids. Grit removal is not
included as a preliminary treatment step in most small wastewater treatment plants.
Comminutors are sometimes adopted to supplement coarse screening and serve to reduce the
size of large particles so that they will be removed in the form of a sludge in subsequent
treatment processes. Flow measurement devices, often standing-wave flumes, are always
included at the preliminary treatment stage.
Removal of oil and grease
At some treatment works this process is thought necessary to protect the downstream
processes. Materials such as oil and grease should not be poured down drains or discharged to
a sewer
Primary treatment
The objective of primary treatment is the removal of settleable organic and inorganic solids
by sedimentation, and the removal of materials that will float (scum) by skimming.
Approximately 25 to 50% of the incoming biochemical oxygen demand (BOD5), 50 to 70% of
the total suspended solids (SS), and 65% of the oil and grease are removed during primary
treatment. Some organic nitrogen, organic phosphorus, and heavy metals associated with
solids are also removed during primary sedimentation but colloidal and dissolved constituents
are not affected. The effluent from primary sedimentation units is referred to as primary
effluent.
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In many industrialized countries, primary treatment is the minimum level of preapplication
treatment required for wastewater irrigation. It may be considered sufficient treatment if the
wastewater is used to irrigate crops that are not consumed by humans or to irrigate orchards,
vineyards, and some processed food crops. However, to prevent potential nuisance conditions
in storage or flow-equalizing reservoirs, some form of secondary treatment is normally
required in these countries, even in the case of non-food crop irrigation. It may be possible to
use at least a portion of primary effluent for irrigation if off-line storage is provided.
Primary sedimentation tanks or clarifiers may be round or rectangular basins, typically 3 to 5
m deep, with hydraulic retention time between 2 and 3 hours. Settled solids (primary sludge)
are normally removed from the bottom of tanks by sludge rakes that scrape the sludge to a
central well from which it is pumped to sludge processing units. Scum is swept across the
tank surface by water jets or mechanical means from which it is also pumped to sludge
processing units.
In large sewage treatment plants, primary sludge is most commonly processed biologically by
anaerobic digestion. In the digestion process, anaerobic and facultative bacteria metabolize
the organic material in sludge, thereby reducing the volume requiring ultimate disposal,
making the sludge stable (nonputrescible) and improving its dewatering characteristics.
Digestion is carried out in covered tanks (anaerobic digesters), typically 7 to 14 m deep. The
residence time in a digester may vary from a minimum of about 10 days for high-rate
digesters (well-mixed and heated) to 60 days or more in standard-rate digesters. Gas
containing about 60 to 65% methane is produced during digestion and can be recovered as an
energy source. In small sewage treatment plants, sludge is processed in a variety of ways
including: aerobic digestion, storage in sludge lagoons, direct application to sludge drying
beds, in-process storage (as in stabilization ponds), and land application.
Secondary treatment
The settlement process is very effective in removing organic material, but if the settled
sewage were discharged to a watercourse, the dissolved organic matter in settled sewage
would still cause problems. The objective of secondary treatment is the further treatment of
the effluent from primary treatment to remove the residual organics and suspended solids. In
most cases, secondary treatment follows primary treatment and involves the removal of
biodegradable dissolved and colloidal organic matter using aerobic biological treatment
processes. Aerobic biological treatment is performed in the presence of oxygen by aerobic
microorganisms (principally bacteria) that metabolize the organic matter in the wastewater,
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thereby producing more microorganisms and inorganic end-products (principally CO2, NH3,
and H2O). Several aerobic biological processes are used for secondary treatment differing
primarily in the manner in which oxygen is supplied to the microorganisms and in the rate at
which organisms metabolize the organic matter.
High-rate biological processes are characterized by relatively small reactor volumes and high
concentrations of microorganisms compared with low rate processes. Consequently, the
growth rate of new organisms is much greater in high-rate systems because of the well
controlled environment. The microorganisms must be separated from the treated wastewater
by sedimentation to produce clarified secondary effluent. The sedimentation tanks used in
secondary treatment, often referred to as secondary clarifiers, operate in the same basic
manner as the primary clarifiers described previously. The biological solids removed during
secondary sedimentation, called secondary or biological sludge, are normally combined with
primary sludge for sludge processing.
Common high-rate processes include the activated sludge processes, trickling filters or
biofilters, oxidation ditches, and rotating biological contactors (RBC). A combination of two
of these processes in series (e.g., biofilter followed by activated sludge) is sometimes used to
treat municipal wastewater containing a high concentration of organic material from industrial
sources.
Activated Sludge
In the activated sludge process, the dispersed-growth reactor is an aeration tank or basin
containing a suspension of the wastewater and microorganisms, the mixed liquor. The
contents of the aeration tank are mixed vigorously by aeration devices which also supply
oxygen to the biological suspension. Aeration devices commonly used include submerged
diffusers that release compressed air and mechanical surface aerators that introduce air by
agitating the liquid surface. Hydraulic retention time in the aeration tanks usually ranges from
3 to 8 hours but can be higher with high BOD5 wastewaters. Following the aeration step, the
microorganisms are separated from the liquid by sedimentation and the clarified liquid is
secondary effluent. A portion of the biological sludge is recycled to the aeration basin to
maintain a high mixed-liquor suspended solids (MLSS) level. The remainder is removed from
the process and sent to sludge processing to maintain a relatively constant concentration of
microorganisms in the system. Several variations of the basic activated sludge process, such
as extended aeration and oxidation ditches, are in common use, but the principles are similar.
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An adaptation of the activated sludge process is often used to remove nitrogen and
phosphorus. Effluent from primary clarifiers flows to the biological reactor, which is
physically divided into five zones by baffles and weirs. In sequence these zones are:
anaerobic fermentation zone (characterized by very low dissolved oxygen levels and
the absence of nitrates);
anoxic zone (low dissolved oxygen levels but nitrates present);
aerobic zone (aerated);
secondary anoxic zone; and
final aeration zone.
The function of the first zone is to condition the group of bacteria responsible for phosphorus
removal by stressing them under low oxidation-reduction conditions, which results in a
release of phosphorus equilibrium in the cells of the bacteria. On subsequent exposure to an
adequate supply of oxygen and phosphorus in the aerated zones, these cells rapidly
accumulate phosphorus considerably in excess of their normal metabolic requirements.
Phosphorus is removed from the system with the waste activated sludge.
Most of the nitrogen in the influent is in the ammonia form, and this passes through the first
two zones virtually unaltered. In the third aerobic zone, the sludge age is such that almost
complete nitrification takes place, and the ammonia nitrogen is converted to nitrites and then
to nitrates. The nitrate-rich mixed liquor is then recycled from the aerobic zone back to the
first anoxic zone. Here denitrification occurs, where the recycled nitrates, in the absence of
dissolved oxygen, are reduced by facultative bacteria to nitrogen gas, using the influent
organic carbon compounds as hydrogen donors. The nitrogen gas merely escapes to
atmosphere. In the second anoxic zone, those nitrates which were not recycled are reduced by
the endogenous respiration of bacteria. In the final re-aeration zone, dissolved oxygen levels
are again raised to prevent further denitrification, which would impair settling in the
secondary clarifiers to which the mixed liquor then flows.
An experimentation programme on this plant demonstrated the importance of the addition of
volatile fatty acids to the anaerobic fermentation zone to achieve good phosphorus removal.
These essential short-chain organics (mainly acetates) are produced by the controlled
fermentation of primary sludge in a gravity thickener and are released into the thickener
supernatent, which can be fed to the head of the biological reactor. Without this supernatent
return flow, overall phosphorus removal quickly dropped to levels found in conventional
activated sludge plants.
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In many situations, where the risk of public exposure to the reclaimed water or residual
constituents is high, the intent of the treatment is to minimize the probability of human
exposure to enteric viruses and other pathogens. Effective disinfection of viruses is believed
to be inhibited by suspended and colloidal solids in the water, therefore these solids must be
removed by advanced treatment before the disinfection step.
Trickling Filters
A trickling filter or biofilter consists of a basin or tower filled with support media such as
stones, plastic shapes, or wooden slats. Wastewater is applied intermittently, or sometimes
continuously, over the media. Microorganisms become attached to the media and form a
biological layer or fixed film. Organic matter in the wastewater diffuses into the film, where it
is metabolized. Oxygen is normally supplied to the film by the natural flow of air either up or
down through the media, depending on the relative temperatures of the wastewater and
ambient air. Forced air can also be supplied by blowers but this is rarely necessary. The
thickness of the biofilm increases as new organisms grow. Periodically, portions of the film
'slough off the media. The sloughed material is separated from the liquid in a secondary
clarifier and discharged to sludge processing. Clarified liquid from the secondary clarifier is
the secondary effluent and a portion is often recycled to the biofilter to improve hydraulic
distribution of the wastewater over the filter.
Rotating Biological Contactors
Rotating biological contactors (RBCs) are fixed-film reactors similar to biofilters in that
organisms are attached to support media. In the case of the RBC, the support media are slowly
rotating discs that are partially submerged in flowing wastewater in the reactor. Oxygen is
supplied to the attached biofilm from the air when the film is out of the water and from the
liquid when submerged, since oxygen is transferred to the wastewater by surface turbulence
created by the discs' rotation. Sloughed pieces of biofilm are removed in the same manner
described for biofilters.
High-rate biological treatment processes, in combination with primary sedimentation,
typically remove 85 % of the BOD5 and SS originally present in the raw wastewater and some
of the heavy metals. Activated sludge generally produces an effluent of slightly higher
quality, in terms of these constituents, than biofilters or RBCs. When coupled with a
disinfection step, these processes can provide substantial but not complete removal of bacteria
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and virus. However, they remove very little phosphorus, nitrogen, non-biodegradable
organics, or dissolved minerals.
Tertiary and/or advanced treatment
Tertiary and/or advanced wastewater treatment is employed when specific wastewater
constituents which cannot be removed by secondary treatment must be removed. As shown in
Figure 3, individual treatment processes are necessary to remove nitrogen, phosphorus,
additional suspended solids, refractory organics, heavy metals and dissolved solids. Because
advanced treatment usually follows high-rate secondary treatment, it is sometimes referred to
as tertiary treatment. However, advanced treatment processes are sometimes combined with
primary or secondary treatment (e.g., chemical addition to primary clarifiers or aeration basins
to remove phosphorus) or used in place of secondary treatment (e.g., overland flow treatment
of primary effluent).
Disinfection
Disinfection normally involves the injection of a chlorine solution at the head end of a
chlorine contact basin. The chlorine dosage depends upon the strength of the wastewater and
other factors, but dosages of 5 to 15 mg/l are common.
Ozone and ultraviolet (uv) irradiation can also be used for disinfection but these methods of
disinfection are not in common use.
Figure 5: Ultraviolet (UV)
irradiation at South-Pest Wastewater
Treatment Plan, Hungary
Chlorine contact basins are
usually rectangular channels,
with baffles to prevent short-
circuiting, designed to provide a
contact time of about 30
minutes. However, to meet
advanced wastewater treatment
requirements, a chlorine contact time of as long as 120 minutes is sometimes required for
specific irrigation uses of reclaimed wastewater. The bactericidal effects of chlorine and other
disinfectants are dependent upon pH, contact time, organic content, and effluent temperature.
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Conclusion
The world’s water is limited and valuable. It is continually used and reused in the water cycle.
Every day in the UK about 624,200 kilometres of sewers collect over 11 billion litres of waste
water. This is treated at about 9,000 sewage treatment works before the treated effluent is
discharged to inland waters, estuaries and the sea.
Waste water is the mixture of domestic waste water from the kitchens, bathrooms and toilets,
the waste water from industries discharging to sewers and rainwater run-off from roads and
other impermeable surfaces such as roofs, pavements, and roads draining to sewers.
Without treatment the water from toilets, baths, sinks and washing machines from domestic
and residential premises, industrial waste water discharges to sewers and the rainwater
contaminated with metals, oils and other pollutants in rainwater run-off from urban areas
draining to sewers would have significant adverse impacts on the water environment.
Without suitable treatment, the waste water we produce every day would damage the water
environment and create public health problems. Untreated sewage contains organic matter,
bacteria and chemicals. The impacts of untreated waste water range from, chronic ecosystem
damage due to oxygen depletion of receiving waters from the biodegradation of organic
matter; ecosystem damage of eutrophication of waters resulting from excessive inputs of
nutrients present in waste water; potential health risks from water-borne pathogens from
discharges to waters used for recreational activities, such as swimming and canoeing.
Untreated waste water also contains sewage litter and other sewage solids that can impact the
environment, for example, trough the smothering of river beds or posing a hazard through its
ingestion by wildlife.
In a modern society wastewater is carried from houses in underground pipes or sewers. In
older parts of towns and cities sewers may also collect rainwater from roofs and roadways.
Sewage received at the treatment works is normally greyish in colour with a slight ‘fruity’
smell and an occasional tint of ‘bad eggs’. It is basically dirty water less than 0,1% being
waste that needs to be treated.
There are four main stages in wastewater treatment; preliminary, primary, secondary
(biological) and tertiary treatment. The number of stages applied depends on the quality of
discharge required to protect the environment.
I
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