Activated sludge tanks

The primary difference between the PRF and the SBR during react is that the equivalent to true plug flow conditions can be established in an SBA but cannot be achieved in a single activated sludge tank because of the dispersion resulting from the aeration system. However, a cascade of CMFRs consisting three or four tanks in series can be considered as a suitable approximation (Wilderer et al., 2001). 

The activated sludge tanks (2) are fed by the acidic tank (4) and the alkaline tank (5). In order to replace evaporation losses and also to prevent salt formation in the sembbing fluid, a small quantity of fresh water runs continuously into the tank (7). Small quantities of fluid are continuously removed from the two sembbing stages and then clarified in sedimentation tanks (8). The resultant activated sludge is recirculated to the sembber. 

Bacterial decomposition of dissolved organic matter takes place in trickling filter tanks and activated sludge tanks. 

Figure 14.24. Combination of processes to eliminate phosphate in waste treatment. The numbers in the figure give typical concentrations of P (mg P per liter). The arrows after the primary settling tank and after the final settling tank indicate the addition of phosphate precipitants and flocculants. Fe(III) or Al(III) can be used for the precipitation of phosphate in the activated sludge tank and as a destabilizer in the final sand filter. (From Boiler, EAWAG, 1993.)...

Biological reactors — which are designed to replace conventional bio-active beds and activated sludge tanks may be classified as being borderline between UF and membrane reactors. The processes in question will be analyzed later. 

The model used for the simulation is the Matlab implementation of BSMl [3], BSMl was developed for unbiased comparison of different control laws. The WWTP is modelled with five activated sludge tanks in series using the Activated Sludge Model 1 (ASMl) [5] and a secondary clarifier modelled according to the method of Takacs et al. [6] (see Fig. 1). All parameters are set as defined in the BSMl protocol. 

In this study, the NDNs control law is used [2], which allows the exact control of the quantity of oxygen necessary for optimal performances of the activated sludge for both reactions of nitrification and denitrification. This control law is based on two PI controllers in cascade. The first one controls the concentration of nitrate (Sno hi g.m" ) by manipulating the set-point of ammonia (S h hi g m ) while the second controller acts on the level of ammonia by manipulating the oxygen transfer coefficient (KLa in s ). In real WWTPs, the oxygen transfer coefficient is controlled with air flow rate but this is not included in our model. The measurements are made in the last activated sludge tank and the same KLa is applied to the three last tanks. 

Pure culture work" is being carried on in connection with the development of the activated sludge process. Organisms which ars found to be especially rapid nitrifiers are chosen and added in large amounts to the activated sludge tanks. 

The activated sludge process uses an enriched bacteria suspension to which normal or 02-enriched air is added. Surface aerators blow the oxygen required for microbiological decomposition into the water of the activated sludge tank. A downstream settling tank is used to separate and prethicken the bacteria suspension. The following criteria are essential for smooth operation of the facility... 

Sour effluents need to be neutralized under all circumstances before they are allowed to enter the activated sludge tank, whereas an alkaline waste water inrush rarely disturbs the system because the steady production of CO2 as a consequence of microbic activity tends to neutralize the system automatically. Toxic substartces have to be eliminated by a specific pretreatment. Flocculation and adsorption processes have been found suitable for this purpose. Under special circumstances, the respective effluent stream may even have to be branched off for separate treatment. 

A 8500 activated sludge tank with ten 70/50 kW surface aerators. 

Biochemical oxygen demand (BOD) volumetric loading can be increased due to a high concentration of activated sludge [mixed liquor suspended solids (MLSS)] being possible. This enables miniaturization of activated sludge tank, reduction of treatment time, and the like. 

Excess sludge can be directly removed from the activated sludge tank and subjected to dehydration treatment. 

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