Overview of the sewage treatment process
April 2012
Valves, Actuators & Pump Control
Initially, incoming domestic sewage is discharged into the pre-settling basin where it is mechanically cleaned. Then the pre-purified sewage is conveyed to the activating basin, where, in the first, the nitrification phase, oxygen is added in the form of compressed air to support the growth of oxygen-consuming bacteria to form what is known as ‘activated sludge’.
In the second, the denitrification phase, no oxygen is added subjecting the previously strongly multiplying bacteria to oxygen deficiency. They now have to utilise other oxygen compounds, thus the chemical compounds of the nitrogen oxides are cracked and rendered harmless.
In the last phase the sewage is placed into the final sedimentation tank where the remaining matters are precipitated by adding ferric chloride. In this process the heavier substances settle and the lighter substances float to the top and are skimmed off. Then the clarified water can be passed into rivers. During regeneration, compressed air is added to the settling basin beneath the surface. The intermediate and end valves used are metal butterfly valves. In addition to manually operated types, automatic valves with motorised actuators are used. Sizes range from DN 80 to 1000 mm.
Plant technology
There are several large air pumps in a compressor house from where the air, compressed to 4 to 6 bar, is brought together in a collecting main and transported to settling basin through a piping system. At the border of a basin the collecting main opens into a manifold from where it discharges into the sewage along the basin, under the surface the compressed air is distributed via nozzles. Each pipe section and the individual feeding conducts are closed by butterfly valves. The compressor pipes and main distributors are equipped with motorised butterfly valves, the individual distribution pipes with manually operated butterfly valves and the sewage inlet and outlet is controlled by butterfly valves installed under water. Their gear is flooded and the motorised actuator is mounted outside the basin.
The valve solution
Gemü wafer-type butterfly valves in cast iron/epoxy coated (400 µm) with stainless steel discs and nitrile packing ring (NBR/Perbunan) in sizes DN 80 to 100 mm. The butterfly valves up to DN 125 are manually operated by means of a lever, they have a LUG design, and those larger than DN 400 are in double flange design. The flange class is PN 10 (DIN).
For more information contact Philipp Tholen, Gemü Valves Africa, +27 (0)11 462 7795, [email protected], www.gemue.co.za
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