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 thermodynamics. The adhesion of hydrophilic cells appeared to be enhanced at a low liquid surface tension (γ(LV)), while the adhesion of hydrophobic cells was favored at a high γ(LV). Growth in protein-rich growth media resulted in low granular biomass yields; addition of polycations, such as poly-L-lysine and chitosan, increased the γ(LV) and the granular biomass yield. On the basis of the results of activity tests and microbial counts with wash-out cells, we identified two types of structured granules that were related to the influence of γ(LV). In one type of granules, hydrophilic acidogens surrounded a more hydrophobic methanogenic association. These granules were selected at a low γ(LV) provided that carbohydrates were available as substrates. The other type of granules was selected at a high γ(LV) hydrophobic cells (i.e., methanogens) were predominant throughout these granules. The granules which had acidogens as solid-phase emulsifiers around a methanogenic association appeared to allow more stable reactor performance. Decreasing the γ(LV) in the reactor by adding trace amounts of a surfactant also increased reactor stability.