Full-scale operational experience and laboratory studies revealed that thermophilic aerobically digested sludge generally had poorer dewatering properties than mesophilic aerobically digested sludge. Several earlier studies reported that cations appeared to affect the dewatering properties
of wastewater activated sludge and mesophilic aerobically digested sludge. This paper reported on a laboratory study that investigated how the thermophilic and mesophilic digestion and the shear that was applied to the sludge during digestion, affected the distribution of cations among the
solid and liquid phases of the digested sludge, also assessed if the poor dewaterability of thermophilically digested sludge correlated to the distribution of cations in sludge. Bench-scale digesters, using waste activated sludge as the feed, were batch-operated at 60°C and 20°C, respectively.
Measured cations included Na+, K+, Ca2+, Mg2+, and Fe. This study found that monovalent cations (Na+ and K+) in the feed sludge were mainly present in the filtrate of the sludge. Digestion, either
thermophilically or mesophilically, resulted in some increase in the concentration of monovalent ions in the filtrate, likely due to the break down of sludge floc. Multivalent cations (Ca2+, Mg2+, Fe) in the feed sludge were mainly associated with the solids
matrix of the sludge. High mechanical shear that was applied to the sludge during digestion, coupled with high digestion temperature, were effective in freeing these multivalent cations from the solids matrix. It appears that the deterioration in dewaterability of thermophilically digested
sludge was unlikely related to changes in distribution of cations in sludge. It was more likely that high mechanical shear applied to the sludge resulted in the disintegration of sludge floc (evidenced by the release of cations into the filtrate), created fine particles, and blocked water
passage channels in sludge, thereby, negatively affecting the dewaterability of digested sludge.
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