Contact angle measurement and cell hydrophobicity of granular sludge from upflow anaerobic sludge bed reactors

The contact angle, which is generally used to evaluate the hydrophobicities of pure bacterial strains and solid surfaces, was used to study mixed cell cultures of bacteria involved in anaerobic digestion. Previously published data and data from this study showed that most acidogens are hydrophilic (contact angle, <45(deg)) but most of the acetogens and methanogens isolated from granular sludge are hydrophobic (contact angle, >45(deg)). The hydrophobicities of mixtures of hydrophilic and hydrophobic cells were found to be linearly correlated with the cell mixing ratio. The hydrophobicities of cells present in effluents from upflow anaerobic sludge bed reactors which were treating different types of substrates were different depending on the reactor conditions. When the reactor liquid had a high surface tension, cells sloughing off from sludge granules, as well as cells present on the outer surfaces of the granules, were hydrophobic. Short-term batch enrichment cultures revealed that proteins selected for highly hydrophilic cells. Long-term in-reactor enrichment cultures revealed that sugars selected for hydrophilic acidogens on the surfaces of the granules, while fatty acids tended to enrich for hydrophobic methanogens. When linear alkylbenzenesulfonate was added, the cells on the surfaces of granules became more hydrophilic. Control tests performed with pure cultures revealed that there was no change in the surface properties due to linear alkylbenzenesulfonate; hence, the changes in the wash-out observed probably reflect changes in the species composition of the microbial association. A surface layer with moderate hydrophobicity, a middle layer with extremely high hydrophobicity, and a core with high hydrophobicity could be distinguished in the grey granules which we studied.