In an activated sludge treatment system, an acclimatised, mixed, biological growth of microorganisms (activated sludge) is brought into contact with organic material in the wastewater in the presence of excess dissolved oxygen and nutrients (nitrogen and phosphorus) (e.g. Metcalf & Eddy, 2002). The microorganisms convert the soluble organic compounds to carbon dioxide and cellular material, i.e. new biomass. Oxygen is obtained from applied air which also maintains adequate mixing. The bioreactor effluent is settled to separate biological sludge and a portion of the sludge is recycled; the excess is sent for further treatment such as dewatering. Activated sludge systems utilised in the food processing industry are the extended aeration types: that is, they combine long aeration times with low applied organic loadings. Typical food/microorganisms (F/M) ratios are lower than 0.1 kg COD/kg mixed liquor volatile suspended solids (MLVSS) per day while sludge age (solid retention time, SRT) should be maintained at values greater than 20 days. This practice allows for a relatively low production of waste activated sludge, typically <0.3 kg VS per kilogram of COD removed. This value is however much higher than that obtained in anaerobic processes. The detention times are typically 1-2 days. The concentration of suspended solids is maintained at moderate levels, generally some 3-6 g/l. It is usually necessary to provide a primary treatment and flow equalization prior to the activated sludge process, to ensure optimum operation. Removal of BOD5 and suspended solids in the range of 95-98% can be achieved. One problem related to extended aeration systems can be the solid/liquid separation in the final clarifier; indeed these activated sludges are generally characterised by bad sedimentation tendencies. As this problem can drastically affect the process performances, membrane biore-actors have recently been introduced (Stephenson et al., 2000). Required oxygen is generally in the range 1-1.5 kg per kilogram of treated BOD5 for a number of food processing wastewaters (i.e. dairy, fruit and vegetable) while seafood wastes appear to require higher oxygen availability to stabilise these waste types (up to 2-3 kg per kilogram of treated BOD; Carawan et al., 1979). As required oxygen can be very high, the activated sludge process is sometimes applied after an anaerobic process for the removal of excess organic compounds. Moreover, the activated sludge process enables the possibility of removing nutrients (nitrogen and phosphorous) when operated as a pre-denitrification or a BNR process (Mauret et al., 2001). Temperature (for winter operations) can have a significant influence on the waste removal performance of the extended aeration system since pin-point floc can develop and loss of biological activity will decrease the performance efficiency of this system under cold-weather operating conditions.
This process, in all its different configurations, is widely applied in food wastewater treatment. Table 21.6 shows typical yields observed for this process when treating different kinds of wastewaters.
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