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Biodrying of dewatered sludge (DS) mixed with air-dried sludge (AS, Trial A), spent coffee ground (SCG, Trial B) and sawdust (SD, Trial C) was investigated by analyzing general physico-chemical parameters, organic matters degradation and extracellular enzyme activities. Results showed that SCG accelerated biodrying in terms of the highest water removal (80.50%) in the shortest period (8 days) in Trial B. Compared to Trial B, Trial A showed a shorter high-temperature period, and Trial C reached a lower temperature peak and biodrying period was long. During the high temperature period in Trial B, hemicellulose, lipids and proteins successively served as main sunbstrates for producing metabolic heat, and their degradation accounted for 28.9%, 34.7% and 10.9% of the consumed VS, respectively. Mean while, proteins and cellulose, representing 65.6% and 32.1% of the consumed VS were the main energy sources in Trials A and C, respectively. During high-temperature period in Trial B, SCG accelerated the conversion of VS to dissolved organic matters (DOM), and DOM to humic- and fulvic-like substances. The foremost induced enzyme was mannanase with activity of 3103 μg min−1 g−1 TS (total solids) on d 2, followed by protease, xylanase and cellulase with activities of 183, 1356 and 648 μg min−1 g−1 TS on d 6 in Trial B. Trial C showed evolution of xylanase and cellulase while Trial A slowly induced protease. These findings indicated that bulking agents with different compositions influenced the DS biodrying by stimulating different extracellular enzymes and thereby affecting the sequence and extent of organic matters degradation. |
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