The development of intact anaerobic granular sludge was studied in UASB reactors under varying conditions of CaCO3 precipitation and biomass yield. Varying precipitating quantities were obtained using different calcium concentrations in the influent and different biomass yields were obtained by feeding with completely or partly acidified substrate. Four reactors were operated at a constant organic loading rate of 13g COD l-1 d-1. Treatment of completely acidified waste water with high calcium concentrations (780-1560 mgCa2 l-1) resulted in the rapid formation of dense granules with a high ash content of which the specific methanogenic activity rapidly dropped and which easily agglomerated, leading to serious cementation of the sludge bed. At a lower influent calcium concentration (390 mgCa2 l-1), treatment of completely acidified waste water resulted in precipitation on reactor compartments, while precipitation within the aggregates did hardly take place. The presence of a fraction of non-acidified substrate retarded cementation of the sludge bed and resulted in development of sludge with a high ash content and a satisfactory quality. The biofilm model developed was shown to provide a clear, qualitative insight in the factors that affect the location of precipitation, and thereby the quality of high ash content sludge. Moreover, it explains the differences observed in the location of precipitation during the experiments. A low Ca/Na ratio, a small granule diameter and a high crystallization rate constant (=low CaCO3 growth inhibition) were found to stimulate precipitation in the bulk solution, and are considered to be favourable parameters for the development of sludge with a high ash content and a good quality.
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