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Treatment stages
Primary
treatment
Primary treatment is to reduce oils, grease, fats, sand, grit, and
coarse (settleable) solids. This step is done entirely with
machinery, hence the name mechanical treatment.
Influx (influent) and removal of large objects
In the mechanical treatment, the influx (influent) of sewage water
is strained to remove all large objects that are deposited in the
sewer system, such as rags, sticks, condoms, sanitary towels
(sanitary napkins) or tampons, cans, fruit, etc. This is most
commonly done using a manual or automated mechanically raked screen.
This type of waste is removed because it can damage the sensitive
equipment in the sewage treatment plant.
Sand and grit removal
This stage (also known as screening or maceration) typically
includes a sand or grit channel where the velocity of the incoming
wastewater is carefully controlled to allow sand grit and stones to
settle but still maintain the majority of the organic material
within the flow. This equipment is called a detritor or sand
catcher. Sand grit and stones need to be removed early in the
process to avoid damage to pumps and other equipment in the
remaining treatment stages. Sometimes there is a sand washer (grit
classifier) followed by a conveyor that transports the sand to a
container for disposal. The contents from the sand catcher may be
fed into the incinerator in a sludge processing plant but in many
cases the sand and grit is sent to a land-fill.
Screening and Maceration
The grit free liquid is then passed through fixed or rotating
screens to remove floating and larger material such as rags and
smaller particulates such as peas and corn. Screenings are collected
and may be returned to the sludge treatment plant or may be disposed
of off site by landfilling or incineration. Maceration, in which
solids are cut into small particles through the use of rotating
knife edges mounted on a revolving cylinder, is used in plants that
are able to process this particulate waste. Macerators are, however,
more expensive to maintain and are less reliable than physical
screens.
Sedimentation
Many plants have a sedimentation stage where the sewage is allowed
to pass through large circular or rectangular tanks. These tanks are
commonly called primary clarifiers or primary sedimentation tanks.
The tanks are large enough that faecal solids can settle and
floating material such as grease and plastics can rise to the
surface and be skimmed off. The main purpose of the primary stage is
to produce a generally homogeneous liquid capable of being treated
biologically and a sludge that can be separately treated or
processed. Primary settlement tanks are usually equipped with
mechanically driven scrapers that continually drive the collected
sludge towards a hopper in the base of the tank from where it can be
pumped to further sludge treatment stages.
Secondary treatment
Secondary treatment is designed to substantially degrade the
biological content of the sewage such as are derived from human
waste, food waste, soaps and detergent. The majority of municipal
and industrial plants treat the settled sewage liquor using aerobic
biological processes. For this to be effective, the biota require
both oxygen and a substrate on which to live. There are number of
ways in which this is done. In all these methods, 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 particles. Secondary
treatment systems are classified as fixed film or suspended growth.
In fixed film systems - such as rock filters - the biomass grows on
media and the sewage passes over its surface. In suspended growth
systems - such as activated sludge - the biomass is well mixed with
the sewage. Typically, suspended growth systems require smaller
footprints than fixed film systems for an equivalent capacity ;
however, fixed film systems are more able to cope with shocks in
biological loading and provide higher removal rates for BOD and
suspended solids than suspended growth systems.
Roughing filters
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. They are
typically tall, circular filters filled with open synthetic filter
media to which sewage is applied at a relatively high rate. The
design of the filters allows high hydraulic loading and a high
flow-through of air. On larger installations, air is forced through
the media using blowers. The resultant liquor is usually within the
normal range for conventional treatment processes.
Activated sludge
Activated sludge plants use a variety of mechanisms and processes to
use dissolved oxygen to promote the growth of biological floc that
substantially removes organic material. It also traps particulate
material and can, under ideal conditions, convert ammonia to nitrite
and nitrate and ultimately to nitrogen gas.
Filter Beds (Oxidising beds)
In older plants and plants receiving more variable loads, trickling
filter beds are used where the settled sewage liquor is spread onto
the surface of a deep bed made up of coke (carbonised coal),
limestone chips or specially fabricated plastic media. Such media
must have high surface areas to support the biofilms that form. The
liquor is distributed through perforated rotating arms radiating
from a central pivot. The distributed liquor trickles through this
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 medias' surfaces and eat or otherwise reduce the organic
content. This biofilm is often fed on by insects and worms.
Rotating plates and spirals
In some smaller plants slowly revolving plates or spirals are used
which are partially submerged in the liquor. A biotic floc is
created which provides the required substrate.
Moving Bed Biological Reactor
Moving Bed Biological Reactor (MBBR) involve the addition of inert
media into existing activated sludge basins to provide active sites
for biomass attachment. This conversion results in a strictly
attached growth system. Advantages of attached growth systems
include 1) maintain a high density of biomass population 2) increase
the efficiency of the system without the need for increasing the
mixed liquor suspended solids (MLSS) concentration and 3) eliminate
the cost of operating the return activated sludge (RAS) line.
Biological Aerated Filters
Biological Aerated (or Anoxic) Filter (BAF) combines 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.
Membrane Biological Reactors
Membrane Biological Reactors (MBR) includes a semi-permeable
membrane barrier system either submerged or in conjunction with an
activated sludge process. This technology guarantees removal of all
suspended and some dissolved pollutants. The limitation of MBR
systems is directly proportional to nutrient reduction efficiency of
the activated sludge process. The cost of building and operating a
MBR is usually higher than conventional wastewater treatment.
Secondary sedimentation
The final step in the secondary treatment stage is to settle out the
biological floc or filter material and produce sewage water
containing very low levels of organic material and suspended matter.
Tertiary treatment
Tertiary treatment provides a final stage to raise the effluent
quality to the standard required before it is discharged to the
receiving environment (sea, river, lake, ground, etc.) More than one
tertiary treatment process may be used at any treatment plant. If
disinfection is practiced, it is always the final process. It is
also called Effluent polishing.
Filtration
Sand filtration removes much of the residual suspended matter.
Filtration over activated carbon removes residual toxins.
Lagooning
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.
Constructed wetlands
Constructed wetlands include engineered reedbeds and a range of
similar methodologies, all of which provide a high degree of aerobic
biological improvement and can often be used instead of secondary
treatment for small communities, also see phytoremediation.
One example is a small reedbed used to clean the drainage from the
elephants' enclosure at Chester Zoo in England.
Nutrient removal
Wastewater may also contain high levels of nutrients (nitrogen and
phosphorus) that in certain forms may be toxic to fish and
invertebrates at very low concentrations (e.g. ammonia) or that can
create nuisance conditions in the receiving environment (e.g. weed
or algal growth). Weeds and algae may seem to be an aesthetic issue,
but algae can produce toxins, and their death and consumption by
bacteria (decay) can deplete oxygen in the water and suffocate fish
and other aquatic life. Where receiving rivers discharge to lakes or
shallow seas, the added nutrients can cause severe eutrophication
losing many sensitive clean water fish. The removal of nitrogen
and/or phosphorus from wastewater can be achieved either
biologically or by chemical precipitation.
Nitrogen removal is effected through the biological oxidation of
nitrogen from ammonia to nitrate (nitrification involving nitrifying
bacteria such as Nitrobacter and Nitrosomonas), and then by
reduction from nitrate to nitrogen gas (denitrification), which is
released to the atmosphere. These conversions require carefully
controlled conditions to encourage the appropriate biological
communities to form. Sand filters, lagooning and reed beds can all
be used to reduce nitrogen. Sometimes the conversion of toxic
ammonia to nitrate alone is referred to as tertiary treatment.
Phosphorus removal can be effected biologically in a process called
enhanced biological phosphorus removal. In this process specific
bacteria, called Polyphosphate accumulating Organisms, are
selectively enriched and accumulate large quantities of phosphorus
within their cells. When the biomass enriched in these bacteria is
separated from the treated water, the bacterial biosolids have a
high fertilizer value. Phosphorus removal can also be achieved,
usually by chemical precipitation with salts of iron (e.g. ferric
chloride) or aluminum (e.g. alum). The resulting chemical sludge,
however, is difficult to dispose of, and the use of chemicals in the
treatment process is expensive. Although this makes operation
difficult and often messy, chemical phosphorous removal requires
significantly smaller equipment footprint than biological removal
and is easier to operate.
Disinfection
The purpose of disinfection in the treatment of wastewater is to
substantially reduce the number of living organisms in the water to
be discharged back into the environment. The effectiveness of
disinfection depends on the quality of the water being treated
(e.g., turbidity, pH, etc.), the type of disinfection being used,
the disinfectant dosage (concentration and time), and other
environmental variables. Turbid 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, or UV light. Chloramine, which is used for drinking
water, is not used in waste water treatment because of its
persistence.
Chlorination remains the most common form of wastewater 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 is becoming the most common means of
disinfection in the UK because of the concerns about the impacts of
chlorine in chlorinating residual organics in the wastewater and in
chlorinating organics in the receiving water. UV radiation is used
to damage 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).
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
disease-causing microorganisms. Ozone is considered to be safer than
chlorine because, unlike chlorine which has to be stored on site
(highly poisonous in the event of an accidental release), ozone is
generated onsite as needed. Ozonation also produces fewer
disinfection by-products than chlorination. A disadvantage of ozone
disinfection is the high cost of the ozone generation equipment and
the requirements for highly skilled operators.
Package
plants and batch reactors
In
order to use less space, treat difficult waste, deal with
intermittent flow or achieve higher environmental standards, a
number of designs of hybrid treatment plants have been produced.
Such plants often combine all or at least two stages of the three
main treatment stages into one combined stage. In the UK, where a
large number of sewage treatment plants serve small populations,
package plants are a viable alternative to building discrete
structures for each process stage.
For example, one process which combines secondary treatment and
settlement is the Sequential Batch Reactor (SBR). Typically,
activated sludge is mixed with raw incoming sewage and mixed and
aerated. The resultant mixture is then allowed to settle producing a
high quality effluent. The settled sludge is run off and re-aerated
before a proportion is returned to the head of the works. SBR plants
are now being deployed in many parts of the world including North
Liberty, Iowa, and Llanasa, North Wales.
The disadvantage of such processes is that precise control of
timing, mixing and aeration is required. This precision is usually
achieved by computer controls linked to many sensors in the plant.
Such a complex, fragile system is unsuited to places where such
controls may be unreliable, or poorly maintained, or where the power
supply may be intermittent.
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 floculate for a relatively long
time.
Sludge treatment and disposal
See also Sewage sludge treatment 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.
The choice of a wastewater solid treatment method depends on the
amount of solids generated and other site-specific conditions.
However, in general, composting is most often applied to
smaller-scale applications followed by aerobic digestion and then
lastly anaerobic digestion for the larger-scale municipal
applications.
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, thus smaller tanks, thermophilic digestion
is more expensive in terms of energy consumption for heating the
sludge.
One major feature of anaerobic digestion is the production of
biogas, which can be used in generators for electricity production
and/or in boilers for heating purposes.
Aerobic digestion
Aerobic digestion is a bacterial process occurring in the presence
of oxygen. Under aerobic conditions, bacteria rapidly consume
organic matter and convert it into carbon dioxide. The operating
costs are characteristically much greater for aerobic digestion
because of energy costs for aeration needed to add oxygen to the
process.
Composting
Composting is also an aerobic process that involves mixing the
wastewater solids 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.
Thermal depolymerization
Thermal depolymerization uses hydrous pyrolysis to convert reduced
complex organics to oil.
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
requirements for disposal of biosolids.
Treatment
in the receiving environment
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,
exposure to ultraviolet radiation, etc. 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 certain
contaminants in wastewater, including hormones (from animal
husbandry and residue from human birth control pills) 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. A significant threat in the coming
decades will be the increasing uncontrolled discharges of wastewater
within rapidly developing countries.
Copyrights:
Wikipedia information about Sewage Treatment
This article is licensed
under the
GNU Free Documentation License. It uses material from the
Wikipedia article "Sewage
treatment". |
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Contents
Treatment
Stages
-
Primary
-
Secondary
-
Tertiary
Package
plants and batch reactors
Sludge treatment and disposal
Treatment
in the receiving environment
Sewage Treatment Plant
used to create the Wonga Wetlands
Primary sedimentation
tank at a rural treatment plant
Trickling filter bed
using plastic media
Secondary Sedimentation
tank at a rural treatment plant
The outlet of a
wastewater treating plant flows into a small river |
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