Factors influencing dewaterability of thermophilic aerobically digested biosolids

Thermophilic aerobic digestion is a high temperature (50–65°C) sludge treatment process that produces Class A biosolids. Full-scale experience revealed that dewatering thermophilically digested sludge required large amounts of polymers for conditioning, resulting in a high chemical cost for sludge dewatering. This research investigated how feed sludge composition, digestion temperature, digestion time, and mixing-induced shear affect the dewatering properties of thermophilic aerobically digested sludge. The characteristics and the dewatering mechanism of thermophilically digested sludge were also studied. The experimental work was carried out at laboratory scale, using batch operated aerobic digesters. Dewaterability was measured as specific capillary suction time (SCST).; This research found that feed sludge composition was an important factor affecting dewaterability of the digested sludge. A higher proportion of the secondary sludge in the feed resulted in a higher SCST in the digested sludge. Digestion temperature had a significant effect on dewaterability of the digested sludge. When the secondary sludge was digested at 55°C or higher temperatures, digestion resulted in immediate and substantial increases in SCST of the digested sludge; continued digestion resulted in a reduction in SCST. When the same type of sludge was digested at 40°C or 50°C, digestion also resulted in a significant increase in SCST, but the rate of increase in SCST was lower; continued digestion at these two temperatures did not result in much change in SCST. Thermophilic digestion did not result in much change in floc charge, but an immediate reduction in floc size of the digested sludge.; Digestion at all temperatures resulted in reductions in total and volatile solids, changes in pH and conductivity, changes in concentrations of ammonia and phosphate of the digested sludge. However, the deterioration in dewaterability of the digested sludge did not correlate to changes in the preceding parameters; instead, SCST correlated to concentrations of soluble extracellular proteins and polysaccharides in the digested sludge. Regardless of digestion temperatures, the mixing-induced shear had a significant effect on dewaterability; such an effect was due to the reduction in floc charge and size. The results from this study indicated that the effect of thermophilic digestion on dewaterability was physical-chemical, not microbiological, in nature. The substances affecting the dewaterability were mainly associated with the liquid phase of the thermophilically digested sludge. The soluble extracellular proteins had small sizes, with 86% of such materials less than 7,000 Daltons. Protease treatment confirmed that extracellular proteins played a role in affecting the dewaterability of thermophilically digested sludge.