Floc or Granule? Evidence of Granulation in a Continuous Flow System

Floc structure can have a significant impact on the successful operation of the activated sludge process. In addition to facilitating solids separation in clarifiers, large flocs have also been reported to provide an environment for simultaneous nitrification and denitrification (SND). Very large, dense, and well defined flocs, called granules, can be formed in aerobic sequencing batch reactors (SBRs). However, granular sludge is not believed to be achievable in continuous flow systems. This paper reports on the performance of a full scale, 720,000 m3/day (189 mgd) facility that exhibits several properties of granular sludge in a flow through system. Field testing related to settling characteristics, particle size distribution, and microbial ecology indicate that this flow through activated sludge system has several similarities to an aerobic granular sludge system. Modeling is currently being completed to examine potential mechanisms for the selection of the potential granules.