WATER TREATMENT
BASIC KNOWLEDGE
THE WASTEWATER TREATMENT PLANT In a wastewater treatment plant, domestic wastewater is treated to enable it to be discharged back into a watercourse. The wastewater produced by private households is polluted largely by dissolved biodegradable substances. A wastewater treatment plant is essentially divided into the following sections: Mechanical treatment Biological treatment Sludge treatment Depending on the properties of the wastewater and the treated water quality requirements, further steps may be necessary, such as removal of phosphates.
Mechanical treatment
Biological treatment
In the first stage, suspended solids are mechanically removed from the wastewater. Initially, course materials such as pieces of wood, plastic bags and fabric are filtered out using a bar screen. Then the water flows into a grit chamber. In this sedimentation tank, mineral solids such as sand and gravel are separated by sedimentation. Organic solids have a much lower settling velocity than sand and, consequently, a low velocity sedimentation step is required to separate them. This process stage is termed primary clarification and the solids which separate at this stage are termed primary sludge.
Mechanical treatment is followed by biological treatment of the wastewater. The principle of biological treatment is the fact that microorganisms use the organic matter as a source of nutrition. In this way, they degrade the organic matter and remove it from the wastewater. The most frequently used method is the activated sludge process. In this process, organic matter is degraded by aerobic micro-organisms. In order to provide them with the necessary oxygen, the wastewater is aerated in the aeration tank. The products of the aerobic metabolism are biomass, water and carbon dioxide. The growing microorganisms form flocs – the so called “activated sludge” – which are continuously removed from the aeration tank together with the wastewater.
Primary components of a wastewater treatment plant
Aeration tank
The activated sludge is separated, by means of sedimentation in the secondary clarifier. The treated water contains only small amounts of organic matter, and can be safely discharged into watercourse (receiving water). More biomass is removed from the aeration tank than is produced in the same period of time. In order to balance out this loss of biomass in the aeration tank, part of the sludge separated in the secondary clarifier is returned to the aeration tank as “return sludge”. Sludge treatment
How a wastewater treatment plant works:
The portion of the activated sludge which is not returned is termed “surplus sludge”. Together with the primary sludge from the primary clarification, it forms the sewage sludge. Sewage sludge is a waste product, and is treated by further processes.
Secondary clarifier
Mechanical treatment 1 bar screen, 2 grit chamber, 3 primary clarifier, 4 primary sludge Biological treatment 5 aeration tank, 6 secondary clarifier, 7 return sludge, 8 surplus sludge A wastewater, B compressed air, C receiving water, D sewage sludge
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