Synergistic effects of the chitosan addition and polysaccharides-EPS on the formation of anaerobic granules.

Concomitant early granulation with chitosan addition under a syntroph-specific substrate and enhancement of extracellular polymeric substances (EPS) production were aimed at to build anaerobic granules with high syntrophic activities in a short period. Two laboratory-scale upflow anaerobic sludge blanket reactors were operated as control (R1) and chitosan addition (R2) reactors during early granulation (phase 1). Chitosan decreased the negativity of microbial surface charges (zeta potential) to -10.5 mV on day 58 which led to increases in average diameter sizes, nuclei and granule ratio of approximately 115 µm, 55.1% and 8.2%, respectively. While zeta potential in R1 slightly changed, this resulted in less microbial aggregation. Although microbial aggregation in R2 was rapidly triggered by chitosan addition during phase 1, its structure was clumpy with rough surface due to lack of EPS. Substrate switching to glucose increased polysaccharides-EPS during phase 2 which was synergistically improved on the structural characteristics of microbial aggregate in R2, that is, more spherical and compact, with a smoother surface. Rapid-growth microorganism was also boosted, which then dominated the outer layer of the aggregate. The Archaea clumps were observed at a deeper layer and were surrounded by Eubacteria, presumably acetogens, indicating a syntrophic relationship due to substrate association between these microbial groups.