SELECTOR VOLUME IN PULP AND PAPER ACTIVATED-SLUDGE WASTE-WATER TREATMENT PLANTS - BENCH-SCALE AND PILOT-PLANT STUDIES

Laboratory activated sludge reactors were operated on effluent wastewa ter from a kraft mill. Two of the three reactors were initially preced ed with a ''normal'' size aerated selector, 2% of the total aerated vo lume, with unsatisfactory sludge volume index development. When the se lectors were replaced by larger ones, 13% of the aerated volume, the s ludge volume index could be kept below 50 ml/g for the selector proces ses while continuing to be higher and more unstable in the reference p rocess. A pilot plant, operated in situ on the same wastewater, showed a comparable improvement in sludge volume index when its selector, 7% of the total volume, was replaced by one that constituted 13% of the total volume, corresponding to a selector load of 3 g BOD/g VSS d. A ccording to studies of the COD balance around one bench scale selector , the COD removal mechanism in the selector was respiration/assimilati on rather than uptake/storage.