GRANULATION AND SLUDGE BED STABILITY IN UPFLOW ANAEROBIC SLUDGE BED REACTORS IN RELATION TO SURFACE THERMODYNAMICS

Adhesion of bacteria involved in anaerobic consortia was investigated in upflow anaerobic sludge bed reactors and was related to surface the rmodynamics. The adhesion of hydrophilic cells appeared to be enhanced at a low liquid surface tension (gamma(LV)), while the adhesion of hy drophobic cells was favored at a high gamma(LV). Growth in protein-ric h growth media resulted in low granular biomass yields; addition of po lycations, such as poly-L-lysine and chitosan, increased the gamma(LV) and the granular biomass yield. On the basis of the results of activi ty tests and microbial counts with wash-out cells, we identified two t ypes of structured granules that were related to the influence of gamm a(LV). In one type of granules, hydrophilic acidogens surrounded a mor e hydrophobic methanogenic association. These granules were selected a t a low gamma(LV) provided that carbohydrates were available as substr ates. The other type of granules was selected at a high gamma(LV); hyd rophobic cells (i.e., methanogens) were predominant throughout these g ranules. The granules which had acidogens as solid-phase emulsifiers a round a methanogenic association appeared to allow more stable reactor performance. Decreasing the gamma(LV) in the reactor by adding trace amounts of a surfactant also increased reactor stability.