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The role of total solids (TS) content on sludge mesophilic anaerobic digestion (AD) was investigated in batch reactors. A range of TS below 10% was evaluated with two replicates. The volumetric biogas production rate (VBPR) increased with increasing initial TS. On day 5, VBPR increased from 133.2 ± 11.2 mL L −1 d −1 at TS 2% to 1111.7 ± 58.1 mL L −1 d −1 at TS 10%. The ultimate biogas yield ( P ) firstly increased and then decreased with the increasing TS. The greatest P was 246.8 mL g −1 -VS added at TS 8%. The corresponding free ammonia nitrogen (FAN) and total ammonia nitrogen (TAN) were 84.0 ± 16.1 and 1163.0 ± 108.6 mg L −1 , respectively. Normalized capillary suction time (NCST) and solid content of thickened sludge obtained by centrifuging sludge increased with increasing TS. On day 49, NCST were 1.2 ± 0.2 (TS = 2%) and 13.2 ± 1.1 (TS = 10%) s g −1 -TS. Meanwhile, solid content of thickened sludge was 7.5 ± 0.5 (TS = 2%) and 15.4 ± 0.0% (TS = 10%). In summary, high VBPR and dewatering effects were obtained after TS content optimization. [ABSTRACT FROM AUTHOR]