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Factors influencing dewaterability of thermophilic aerobically digested biosolids Zhou, Jianpeng

Abstract

This dissertation reports on research findings from a study investigating factors that influence the dewaterability and other characteristics of thermophilic aerobically digested biosolids. Thermophilic aerobic digestion is a high temperature (50-65°C) sludge treatment process that produces Class A biosolids. Experience from operating full-scale facilities has revealed that dewatering thermophilically digested sludge requires substantially higher dosages of polymers for conditioning, compared to dewatering of mesophilically digested sludge. The objectives of this research program were to investigate how feed sludge composition, digestion temperature, digestion time, and mixing induced shear affect the dewatering properties of digested sludge. The characteristics of thermophilically digested sludge and mechanisms related to dewatering 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 is an important factor affecting dewaterability (measured as SCST) of the digested sludge. Regardless of how the sludge was digested, the measured SCST exponentially correlated to the weight proportion of the secondary sludge contained in the feed. A higher proportion of secondary sludge in the feed resulted in a higher SCST in the sludge. Dewaterability did not correlate to pH, volatile solids, concentrations of ammonia and phosphate in digested sludge, but correlated to concentrations of soluble extracellular proteins and polysaccharides. Digestion temperature had a significant effect on dewaterability (measured as SCST) of the digested sludge. When the sludge, containing 100% secondary sludge, was digested at 55°C or higher temperatures, digestion resulted in immediate and significant increases in SCST. When the same type of sludge was digested at 40 or 50°C, digestion also resulted in a significant increase in SCST, but the rate of increase in SCST was lower than the rate when the sludge was digested at 55°C or higher. Following the initial surge in SCST, continued digestion at 55°C or higher temperatures for more than 1 d resulted in a reduction in SCST; while continued digestion at 40°C or 50°C did not result in much change in SCST. Mesophilic digestion resulted in a progressive increase in SCST over the entire duration of 10-12 d digestion. Thermophilic digestion did not result in much change in floe charge, but did cause an immediate reduction in floe size. Digestion at all temperatures resulted in a reduction in solids, changes in pH, conductivity, concentrations of ammonia and phosphate, and phosphorus distribution among the solid and liquid phases of the sludge. However, the deterioration in dewaterability of digested sludge did not correlate to changes in these parameters; instead, it correlated to concentrations of soluble extracellular proteins and polysaccharides in the digested sludge. The substances affecting dewaterability of thermophilically digested sludge was mainly associated with the liquid phase of the digested sludge. The soluble extracellular proteins had small sizes, with 86% of such materials less than 7,000 Daltons. These proteins could not be effectively stained by Coomassie Brilliant Blue dye, and were not affected by boiling treatment. Protease treatment confirmed that protein played a role in affecting dewaterability. The deterioration in dewaterability, due to thermophilic digestion, was a physical-chemical phenomenon, not a microbiological phenomenon, although the substances resulting in deterioration of dewaterability (such as extracellular proteins) were originated from bacterial cells. Mechanical shear applied to the digested sludge had a significant effect on dewaterability, regardless of digestion temperatures. The effect of mechanical shear was due to the reductions in floe charge and size. Digestion and mixing induced shear resulted in changes in distribution of cations among the solid and liquid phases of the digested sludge. However, dewaterability was not associated with changes in ratios of monovalent to divalent cations. Thermophilic digestion and mechanical shear resulted in a reduction in the initial yield and adhesion coefficients of digested sludge. Digested sludge showed non-Newtonian characteristics, in particular, the shear-thinning property. For thermophilically digested sludge, an initial polymer demand needed to be satisfied first, before the SCST could be substantially reduced by polymer conditioning.

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