Engineering factors affecting the granulation of sludge in the UASB reactor

The influence of inoculum sludge involving different microbial populations, different modes of stream flow and different flow rates in a bioreactor on the granulation of sludge was studied. In the granulation process there were four periods: formation of microbial floccus, formation of subcore, growth of subcore, and maturity of granules. The production of microfloccus could be attributed to acid-forming bacteria. The hydrodynamic momentum transfer caused by moving phases and the hydraulic shear effect are the crucial factors affecting the formation of the subcore. For this a new concept-the lowest limited flow rate-is proposed, i.e., the lowest flow rate to form an expanded sludge bed. A sufficient feed rate (higher than the lowest flow rate), an appropriate sludge load, and an equal distribution of feed, as well as a suitable alkalinity in the medium are the essential factors for the scaling-up and process-control of the UASB reactor.