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 hydrophobic ities of pure bacterial strains and solid surfaces, was used to study mixed cell cultures of bacteria involved in anaerobic digestion. Previ ously published data and data from this study showed that most acidoge ns are hydrophilic (contact angle, < 45 degrees) but most of the aceto gens and methanogens isolated from granular sludge are hydrophobic (co ntact angle, > 45 degrees). The hydrophobicities of mixtures of hydrop hilic and hydrophobic cells were found to be linearly correlated with the cell mixing ratio, The hydrophobicities of cells present in efflue nts from upflow anaerobic sludge bed reactors which were treating diff erent types of substrates were different depending on the reactor cond itions. When the reactor liquid had a high surface tension, cells slou ghing off from sludge granules, as well as cells present on the outer surfaces of the granules, were hydrophobic. Short-term batch enrichmen t cultures revealed that proteins selected for highly hydrophilic cell s. Long-term in-reactor enrichment cultures revealed that sugars selec ted for hydrophilic acidogens on the surfaces of the granules, while f atty acids tended to enrich for hydrophobic methanogens. When linear a lkylbenzenesulfonate was added, the cells on the surfaces of granules became more hydrophilic. Control tests performed with pure cultures re vealed that there was no change in the surface properties due to linea r alkylbenzenesulfonate; hence, the changes in the wash-out observed p robably reflect changes in the species composition of the microbial as sociation. A surface layer with moderate hydrophobicity, a middle laye r with extremely high hydrophobicity, and a core with high hydrophobic ity could be distinguished in the grey granules which we studied.