The physico-chemical characteristics of thermophilic aerobic sludges

An examination of the rheological properties of two aerobically digested sludges, one from an autothermal thermophilic digester (ATAD), the other from a thermophilic digester (TAD), showed that both sludges exhibited yield stresses which were an order of magnitude lower than activated sludges at similar solids' concentrations. However, their filtration characteristics, as measured by the capillary suction time method, were appreciably poorer than those of thickened activated sludge. Particle size analysis and microscopic examinations showed that both the thermophilic sludges contained a predominance of very small particles which would cause blinding of the filters. The surface charge carried by the thermophilic sludges was lower than that associated with activated sludge samples. These results are comparable with earlier work which showed that rheological properties were associated with surface charge. However, the bound water content of the thermophilic sludges did not appear to be associated with the yield stress. Live/dead staining showed that in both samples there were free-swimming viable bacteria. (C) 2001 Society of Chemical Industry.