CONTROL STRATEGIES FOR ACTIVATED-SLUDGE TREATMENT PLANTS

Control strategies for dissolved oxygen, sludge withdrawal and nutrien t addition are proposed for activated sludge treatment of bisulphite c hemimechanical pulp mill effluents. The benefits of the control method s were investigated with a mill-site pilot plane facility. Simple tune d proportional-integral-derivative control was found adequate for diss olved oxygen control in the aeration basins. About 20% saving in air c onsumption can be achieved by this technique since the dissolved oxyge n concentration can be maintained at a lower level than that for manua l operation, without the danger of oxygen depletion in shock load situ ations. A hierarchical strategy for sludge withdrawal was found to be very effective. Mixed liquor Suspended solids concentration was the up per level control parameter and sludge retention time was used for low er level control. This strategy reduced the standard deviation of both mixed liquor suspended solids concentration (from 139 to 69 mg/L) and sludge retention time (from 0.94 to 0.42 days). Conductivity was foun d to correlate well with effluent chemical oxygen demand and can be us ed as a tool for feed forward control of nutrient addition.