Sewage treatment is the process of removing contaminants from
waste water and household sewage, both runoffs (effluents), domestic, commercial
and institutional. It includes physical, chemical and biological processes to
remove physical chemical and biological contaminants. Its objective is to
produce an environmentally safe fluid waste stream (or treated effluent) and a
solid waste (or treated sludge) suitable for disposal or reuse (usually as farm
fertilizer). Using advanced technology, it is now possible to re-use sewage
effluent for drinking water, although Singapore is the only country to
implement such technology on a production scale in its production of new water.
ORIGINS OF SEWAGE
Sewage is generated by residential institutional,
commercial and industrial establishments. It includes household waste liquid
from toilets, bath, 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 grey water and
black water is becoming more common in the developed world, with grey water
being permitted to be used for watering plant or recycled for flushing toilet.
Sewage may include storm water runoff. Sewage systems capable of handling storm
water are known as combined sewer systems.
As rainfall travels over roofs and the
ground, it may pick up various contaminants including soil particles and other
sediments, heavy metals, organic compounds, animal waste, and oil and grease. Some jurisdictions require storm water to
receive some level of treatment before being discharged directly into water
ways. Examples of treatment processes used for storm water include retention
basin, wetlands, buried vaults with various kinds of media filters, and vortex
separator. Sewage can be treated close to where it is created, a decentralized
system (in septic tanks, biofilter or aerobic treatment systems), or be
collected and transported by a network of pipes and pump stations to a municipal
treatment plant, a centralised system, sewage collection and treatment is
typically subjected to local, state and federal regulations and standards.
Industrial sources of sewage often require specialized treatment processes.
Sewage treatment involves the
removal of solids by physical screening or sedimentation. The removal of
soluble and fine suspended organic pollutants by biological oxidation and
adsorption processes. Both forms of treatment produce sludge as by–product and
these have to be treated and used or disposed off in an economical and
environmentally acceptable ways.
The following
describes a typical sewage treatment sequence in practice, there are many
process variations employed according to locality and the standard of effluent
required.
PRELIMINARY TREATMENT
Screening:
Large solids (plastic, rag, toilet paper residues) are removed first by
mechanical screens. Traditionally, screening was used to remove only large
material (> 25 – 30mm) in order to protect down stream operations. Nowadays,
much finer screens (6mm. mesh) are commonly employed to remove small inert
solids. The material retained (“screenings”) is usually washed to remove faecal
matter and then compressed for disposal to land fill or to an incinerator.
GRIT REMOVAL
At the next preliminary stage, fine
mineral matter (grit and sand), originating mainly from road run off, is
allowed to deposit in long channels or circular traps. The retained solids are
removed and usually sent to landfill for disposal.
PRIMARY TREATMENT
Primary Sedimentation: The sewage passes into large sedimentation tanks to provide
a quiescent settlement period of about 8 hours. Most of the solids settle to
the bottom of the tanks and form a watery sludge, known as a “primary sludge”,
which is removed for separate treatment. The sewage remaining after settlement
has taken place is known as “settled sewage”.
Secondary (Biological) Treatment;
Settled sewage than flows to an aerobic biological treatment stage where it
comes into contact with micro-organisms which remove and oxidize most of the
remaining organic pollutants.
At smaller works, the biological
stage often takes the form of a packed bed of graded mineral media through
which the sewage trickles and on the surfaces of which the micro-organism grow.
At most larger works, the sewage is mixed for several hours with an aerated
suspension of flies of micro-organisms (known as the activated sludge process).
As well as removing most of the polluting organic matter, modern biological
treatment can where necessary remove much of the nitrogen and phosphorous in
the sewage, thus reducing the nutrient load on the receiving water.
FINAL SETTLEMENT
Following secondary (biological) treatment, the flow passes to final
settlement tanks where most of the biological solids are deposited as sludge
(secondary sludge) while the clarified effluent passes to the outfall pipe for
discharge to a water course. In the case of the activated sludge process, some
of the secondary sludge is returned to the aeration tanks for further contact
with the sewage. The secondary sludge from biological treatment also requires
separate treatment and disposal and may be combined with the primary sludge for
this purpose.
TERTIARY TREATMENT
In circumstances where the highest
quality of effluent is required, a third (tertiary) stage of treatment can be
used to remove most of the remaining suspended organic matter from the effluent
before it is discharged to a water course. Tertiary treatment is effected by
sand filters, mechanical filtration or by passing the effluent through a constructed
wetland such as a reed bed or grass plot.
SLUDGE TREATMENT
All methods of sewage treatment
generate organic sludge (or “biosolids”) as by-product and these must be
managed separately from the liquid sewage. Raw (untreated) sludge has a very high
oxygen demand and must not be allowed to enter the water environment. Sludge
also contains pathogenic organisms.
There is, therefore, a need to deal with them in a way that permits
their ultimate disposal in an environmentally acceptable and sustainable
manner. The sludge “route” selected for a given sewage treatment works will
depend on several factors including it’s location, the availability of suitable
farm land, the characteristics of the sludge and the over all cost.
Sludge produced by sewage treatment are organic in nature and contain
useful amount of plant nutrient such as nitrogen, phosphorous and essential
trace elements. Therefore, the first objective should be to utilize the sludge
as a fertilizer or soil conditioner on agricultural land. In their initial form,
most raw (untreated) sludge have a high water content (96 - 99%), are putrescent
and have an offensive odour. They will
also contain a variety of human and animal pathogen, derived from the contributing
population. Various forms of treatment may be used to achieve volume reduction
by removing some of the water content. Odour and pathogen reduction is achieved
by stabilization and disinfection processes. In recent years, the control of
odour emissions to the atmosphere has become an important requirement of sludge
treatment.
The following outlines the more common type of treatment employed, of
which various combinations are used according to the end product required.
PRIMARY CONSOLIDATION
As
a first stage of treatment, sludge is passed or subject to centrifugation to
reduce it’s water content and volume by up to 50%. The separated liquor is
returned to the sewage flow for treatment and the consolidated sludge passes
forward for further processing.
ANAEROBIC DIGESTION
In Anaerobic digestion (AD) consolidated liquid sludge B retained in an airtight tank (digester) and maintained at 350c for 12-20 day . Under the
anaerobic conditions in the tank various pathogen break down about half of the sludge organic matter
and convert it into a gas containing about 65% methane. The gas is used to heat
the digester and, in some cases, also to fuel gas engines to generate electricity.
The sludge resulting from anaerobic digestion is much less offense in odor than
the untreated raw sludge and, with certain restriction, is generally suitable
for use in agriculture in liquid or solid form. Further consolidation of sludge
after digestion, to reduce its volume, is a common practice.
MECHANICAL DEWATERING
Either untreated or digested sludge may be converted from a liquid to a
sludge “cake” by treating it first with conditioning chemical which releases
much of the water initially bound to the organic matter. Much of the free water
is then removed from the sludge in a filter press, a belt filter or a
centrifuge. The resultant sludge cake will have only 20% of the volume and
weight of the original sludge,
thereby reducing subsequent handling and transport costs. The conversion of
sludge to a solid form is essential prior to its disposal to landfill.
INCINERATION
This involves the
burning of the sludge at 850-900oc to destroy its organic content
and to leave a smaller residue of mineral ash for final disposal, usually to
landfill. Incineration is only suitable for large sewage works and is used when
the option of agricultural use of the sludge is not practicable. The process is
carried out under closely controlled conditions and is subject to strict
environment regulation to ensure that the ambient air quality is not
compromised by the combustion gases.
Thermal Drying
Some sewage works in the UK employ thermal drying systems to convert
the sludge to pelletised or granular form comprising about 90% solids. The
heating involved also destroys pathogens. Thermally-dried sludges are used in
agriculture or for amenity uses (for example, golf course, parks and other
amenity areas)
Pasteurization (disinfection)
To destroy all pathogens in liquid
sludge, it is heated to about 700c for at least 30minutes after
which it is cooled and subjected to anaerobic digestion. This combination of
pasteurization and digestion produces an “enhanced treated” product (4) which
enables it to be used more widely for various agricultural purposes.
Lime Stabilization
At some smaller works, lime is added to liquid sludge
to raise its pH to above 12.0 for several hours. The high alkalinity improves
its odour and eliminates pathogens.
Screening:
Sieve: The influent sewage water passes through a bar
screen to remove all large objects like cans, rags sticks, plastics packets
etc. carried in the sewage stream. This is most commonly done with an automated
mechanically raked bar screen in modern plants serving large populations,
whilst in smaller or less modern plants; a manually cleaned screen may be used.
The raking action of a mechanical bar screen is typically paced according to
the accumulation on the bar screens and/or flow rate. The solids are collected
and later disposed in a landfill or incinerated. Bar screen 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.
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Lansing M. Prescott, John P. Harley, Donal A. Klein,
(1999). General Microbiology 4th edition page 868-873.
Nduka Okafor, (1987). Industrial microbiology
University of ife press Ltd, Ile - Ife, Nigeria 1st edition page
398-399.
EPA, (2007). “Membrane Bioreactors” wastewater
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Khopkar, S.M., (2004). Environmental pollution
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Smart N. Uchegbu, (2002). Environmental Management and
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