A digital computer simulation of an automatically controlled complete mix activated sludge wastewater treatment plant using food : mass ratio control



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With the activated sludge wastewater treatment process being as complicated as it is, a concerted effort is needed to develop an automatic control system which will respond effectively to variations in influent flow and concentration. Such a control system has been devised and evaluated in the research study reported herein. The primary control objective established for this study was to maintain a near constant food:mass ratio using return sludge flow as the control variable. Hydraulic retention time was allowed to vary from three to fifteen hours while the solids retention time was held constant at a value of eight days. A quasi food:mass ratio set point of 0.35 Kg/day/Kg was intended for steady state operation. Results of the study indicate that the previously mentioned control strategy allowed the food:mass ratio to vary from 0.1 to 0.7 Kg/day/Kg when the influent, flow and concentration were varied sinusoidally from plus to minus fifty percent of the steady state values. Effluent quality remained essentially constant; however, the final settling tank model indicated that the limiting solids flux was exceeded for periods up- to eight hours. This wouTd contribute to a degraded effluent should the condition last long enough for the sludge blanket to reach the effluent weir. A tighter control of the food:mass ratio was attempted by varying the reactor volume so as to maintain a constant hydraulic retention time of six hours. This control system modification reduced the food:mass ratio excursions significantly; however, no improvement was obtained in effluent quality. Finally, the performance of food:mass ratio control was compared with that of a more conventional control strategy—portional return sludge flow control. For the influent variations applied, the conventional control strategy gave as good a performance as foodfmass ratio control. Future studies should incorporate a much more detailed settler model which describes the effects of food:mass ratio changes on settling characteristics of the biological floc.