The settled AS is returned to the head of the aeration tank (RAS) to re-seed the new sewage (or industrial wastewater) entering the tank and to ensure the desired MLSS concentration in the aeration tank. At the effluent of the aeration tank, mixed liquor is discharged into settling tanks and the supernatant (treated waste water) is run off to be discharged to a natural water or undergo further treatment before discharge. Other factors are: Influent load (BOD5, COD, Nitrogen.) in relation to the AS solids present in the aeration tank (Food:Microorganism Ratio, F:M Ratio), oxygen supply, temperature, etc. the hydraulic residence time (HRT) in the aeration tank, which is defined by aeration tank volume divided by the flow rate. The removal efficiency is controlled by different boundary conditions, e.g. With all activated sludge plants, the concentration of biodegradable components present in the influent is reduced due to biological (and sometimes chemical) processes in the aeration tank. Typically, dry solids concentrations of mixed liquor (MLSS) range from 3 to 6 g/L. The mixture of raw sewage (or industrial wastewater) and biological mass is commonly known as Mixed Liquor. Atmospheric air or in rare cases pure oxygen is introduced to a mixture of primary treated or screened sewage (or industrial wastewater) combined with organisms to develop a biological floc ("Activated Sludge" AS). There is a large varity of design, however, in principle all AS consist of three main components: an aeration tank, which serves as bio reactor a settling tank ("final clarifier") for seperation of AS solids and treated waste water a return activated sludge (RAS) equipment to transfer settled AS from the clarifier to the influent of the aeration tank (Fig. Activated sludge (AS) is a process dealing with the treatment of sewage and industrial wastewaters and developed around 1912-1914.
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