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Newsletter and Technical Publications
Freshwater Management Series No. 10
Managing Urban Sewage
An Introductory Guide for Decision-makers
III. Sewage Facilities
A. Sewage Collection
There are two types of sewage collection systems - combined and separate. Combined sewers
carry away both rainwater and wastewater, while separate sewers take care of wastewater
and rainwater in separate pipes. Combined sewage collection systems are often used because
the cost of construction is less than that of a separate system. In a combined sewage collection
system, the rainwater mixed with wastewater is allowed to flow directly into rivers and
adjacent water bodies during wet weather conditions. In a separate sewage collection system,
rainwater mixing with wastewater is minimal, but the problem of pollutants on road surfaces
being carried into rivers and coastal waters still exists. The construction of a separate sewage
collection system is relatively expensive and more complex due to the need to install two
sets of pipes.
B. Pumping Stations
Wastewater collected by the sewers flows by gravity and is relayed to treatment plants by
pumping stations. In flat land, sewers tend to be laid deeper and pumping stations must be
used to lift the sewage closer to the surface where it can be treated in a sewage treatment
plant. Pumping stations often have both wastewater and rainwater pumping facilities. Where
this is the case, the rainwater is usually pumped into subsurface aquifers below sea level or
into the sea directly to prevent inundation of rivers and surface waters.
C. Wastewater Treatment Plants
The principal role of a wastewater treatment plant is to remove pollutants from wastewater
and to discharge the treated effluent into an adjacent waterbody such as a river or the sea.
Treatment plants can be divided into discrete unit processes as outlined below.
- Grit Chamber - Within a wastewater treatment plant, raw wastewater first enters a
grit chamber. As the wastewater flows gently through the chamber, solids (such as
sand, grit and gravel) settle to the bottom, and are removed by buckets, while large
suspended matter is removed by screens. The wastewater is then pumped into a primary
sedimentation tank.
- Primary Sedimentation Tank - As the wastewater flows slowly in a primary sedimentation
tank for two to three hours, organic solids gradually settle to the bottom. This mass of
solids is called raw sludge, and is sent to a sludge treatment facility for further treatment.
To make the most of available land, double-decker sedimentation tanks are used at
some wastewater treatment plants.
- Aeration Tank - The major role of the aeration tank is to remove the soluble organic
material that escaped treatment in the primary sedimentation tank and to provide further
removal of suspended solids. In order to ensure the sufficient and rapid decomposition
of organic material, it is necessary to promote the growth of microorganisms capable
of absorbing these soluble organic materials. In the activated sludge process, the
aeration tank mixes and agitates wastewater and activated sludge. During the 6 to 8-
hour aeration period, the microorganisms absorb the organic matter as nutrients, and
they grow as a result. This decomposes the organic matter into inorganic substances
such as water and carbonic acid gas. The suspended solids adhere to microorganisms
and then form clots that can be easily removed as sediment.
- Activated sludge contains a large quantity of microorganisms and is based on the
same principle as nature's process of purifying water. The activated sludge process
involves putting activated sludge into wastewater and decomposing organic matter
into such inorganic substances as water and carbon dioxide through the metabolic
activity of the microorganisms.
- Secondary Sedimentation Tank - While the mixture from the aeration tank flows slowly
in a secondary sedimentation tank, it is separated into solids (activated sludge) and an
aqueous portion (or supernatant). Part of the activated sludge is returned to the aeration
tank, and the rest is treated in a sludge treatment facility. The secondary effluent is
usually discharged into the receiving environment after chlorination. Following
advanced treatment, part of the sewage treatment plant effluent is often used for
miscellaneous purposes in the plant and as water for toilets in buildings. In addition,
it can be used to augment the flow of smaller streams. In some cities such a Tokyo,
double-decker secondary sedimentation tanks are used to make more effective use of
available land.
- Sludge Treatment - The raw sludge from the primary sedimentation tank and the excess
sludge are pumped to thickening tanks. In the thickening tank, the volume of the
sludge is reduced to about one-quarter of the volume of the raw sludge. The thickened
sludge is then mechanically dehydrated. The sludge is sometimes sent to a digestion
tank after being thickened. There are different types of sludge drying (or dehydrating)
machines, including vacuum, centrifugal, filter press, and belt press. Dehydrated sludge
is often burned and becomes ash. The ash generated by incinerating sludge is usually
about 1% of the original sludge volume.
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