ACTIVATED-SLUDGE SYSTEM-DESIGN FOR SPECIES SELECTION - ANALYSIS OF A DETAILED MULTISPECIES MODEL

We analyze a detailed activated sludge model to determine the boundari es for proliferation of flee formers during aerobic or anoxic selectio n and with soluble or solid substrates. The analysis of competition wi th several filamentous species shows that the outcome is similar to th at of competition with a single species. For the two-species competiti on we present a general procedure for determining the boundaries of de sired operation and compute these boundaries for a slightly modified v ersion of the AEROFIL model (Kappeler and Gujer, 1994a, b Water Resour ces 28, 303-322). To achieve high conversions in aerobic reactors a su bstrate concentration gradient should be established implying that the recycle ratio and the sludge-age should be sufficiently small while t he dissolved oxygen concentration should be sufficiently large. The an alysis of two mixed aerobic reactors suggests that the selector volume should be comparable or larger than that of the reactor. Analysis of an anoxic selector coupled with an aerobic reactor showed that good se lection can be obtained with small selectors and a sufficiently large recycle ratio. A control mode based on switching between these two reg imes has been suggested. Sustaining the growth of flee formers on a so lid-substrate feed that undergoes hydrolysis is a difficult task if th e kinetic parameters of the hydrolyzed and soluble substrate are simil ar. This subject requires further experimental and modeling efforts to account for better selection that has been observed with solid substr ates. (C) 1997 Elsevier Science Ltd.