VERIFICATION AND APPLICATIONS OF A MATHEMATICAL-MODEL FOR AEROBIC BULKING

The AEROFIL model is verified by experiments in a full-scale and a pilot-scale plant and several model applications are demonstrated. It is shown that long hydraulic retention times in primary settling tanks tend to favour aerobic bulking. Even small amounts of obligate aerobic filamentous microorganisms in the influent, eroded from the slime layers in sewers, may provoke serious problems with the settling properties of the activated sludge. The design of aerobic selectors with 10 kg BOD5/m(3) d, as recommended by the ATV Task Group, generally leads to optimal selector volumes. The positive effect of compartmentalized aerobic selectors is demonstrated by the AEROFIL model. If anoxic selectors are designed with 2.5 kg BOD5/m(3) d, as recommended by the ATV Task Group, the growth of obligate aerobic filamentous microorganisms is successfully suppressed. Extended anoxic sludge blankets in secondary clarifiers hinder the obligate aerobic filamentous microorganisms in their competition against facultative aerobic floc-formers.