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Activated sludge plants
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This is the most current field of application for FRINGS immersible aerators.
Biological waste water treatment in activated sludge plants uses microorganisms (bacteria) to decompose the organic waste water components. This is a normal biological process which also takes place in ponds, lakes or rivers. The difference is that activated sludge plants are teeming with millions of microorganisms. Artificial oxygen supply ensures the efficiency and activity of the bacteria. Activated sludge plants require constant waste water circulation to avoid biomass build-up at the bottom and subsequent oxygen starvation of the microorganisms.
An activated sludge plant always consists of an aeration basin with activated sludge and a basin for secondary clarification. Solids that are removed from the activated sludge process in the secondary clarification basin are pumped back into the aeration basin. This allows for a continuous biological cleaning process with high decomposition rates (up to 99%). Only a minor part of sludge is removed from the system (excess sludge).
Larger plants are equipped with a basin for preliminary treatment. This basin allows heavier particles to settle out while the organic materials remain suspended in the waste water and are carried on. Waste water treatment plants for small towns (up to 10,000 inhabitants) mostly do without preliminary treatment basins. In this case a scraper with a tine width between 10 and 20 or a sieve is used. If the waste water contains sand, a sand filter is also used.
Waste water load analysis is based on the biochemical oxygen demand (BOD5) or the chemical oxygen demand (COD). The BOD5 or the COD value is multiplied by the amount of waste water, obtaining the total BOD5 load or the total COD load. The size of the immersible aerator to be used depends on this load. For more information on the sizing of immersible aerators, please read the FRINGS information bulletin no. 402. This information is primarily for municipal waste water treatment. The sizing of an aerator for industrial waste water treatment is more difficult to calculate; therefore we recommended having this calculation done by a FRINGS application engineer. Fill out the FRINGS Waste water treatment questionnaire which is provided in the Download Area, and send it back to marketing@frings.com.
Generally, an activated sludge plant is considered as a low-load activated sludge system if the OC load (BOD5 load divided by usable basin volume) is < 1 kgBOD5 per m3 x d, whereas in a high load system the OC load is > 1 kgBOD5 per m3 x d. In industrial waste water treatment plants, a low-load stage is often combined with a high-load stage. Such a system is called a two-stage activated sludge plant. Depending on the materials contained in the waste water, chemical or physical processes are used as additional preliminary and tertiary stages.
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