Secondary clarifier
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Secondary clarifiers are usually the last technical element in wastewater treatment plants and thus have a central role in the cleaning performance of wastewater treatment plants . Secondary clarifiers are part of the activated sludge process .
tasks
- Optimal separation of the solids (sludge) from the purified water, low concentrations in the drain ( solids ).
- Separation of the activated sludge-water mixture.
- Thickening of the sludge to ensure a highly concentrated return sludge and excess sludge .
- Storage function to prevent the biomass from being washed out of the system, especially with increased hydraulic loading.
Designs
Secondary clarifiers differ in principle in addition to their geometric shape:
- Round basin or
- Rectangular pool,
also with regard to the direction of flow:
- flows through vertically or
- flows through horizontally.
There are other distinguishing features with regard to the design of the inlets and outlets:
- Circular pools can be flown through from the inside to the outside or from the outside to the inside.
- In the case of rectangular pools, a distinction is made between longitudinal and cross-flow pools.
See also: Dortmundbrunnen , Emscherbrunnen .
Working principle
Secondary clarifiers are based on separation (→ see settling tanks ). The wastewater reaches the secondary clarifier from the aeration tank or trickling filter. The sludge contained in the water settles and is collected and thickened in the lower area.
The secondary clarifier can be divided into 4 zones:
- the clear water zone
- the separation zone
- the storage zone
- the thickening or clearing zone.
The sludge from the thickening zone is withdrawn and returned to the primary treatment as return sludge (not to be confused with excess sludge that comes from the activation process). Usual soil sludge concentrations can be in the range of 10 to 20 g DS / liter (DS = dry matter ). This corresponds to a solids content of 1 to 2 percent. The clarified water is drawn off from the clear water zone using various techniques.
During the final clarification, various problems can arise due to poorly settable sludge. An indication of this is the sludge index
See: bulking sludge , floating sludge .
Downstream systems
To improve the drainage quality or to reduce the drainage concentrations, further systems can be connected downstream such as
- Membrane filtrations
- In the case of post-precipitation to eliminate phosphorus, another basin or a filter system (usually sand filtration) must be available to separate the flakes
- Fining ponds
- Fixed bed reactors
- Ozonation
- Activated carbon filtration
- Flocculation flotation / microflotation
- UV disinfection
Etc.
literature
- Wilhelm Hosang, Wolfgang Bischof: Sewage technology. 11th, revised and expanded edition. Teubner, Stuttgart 1998, ISBN 3-519-15247-9 .