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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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