During the thermophilic AD of WAS, converting sludge particles to the supernatant is the first step and also the limiting step, those soluble matters would be finally utilized by anaerobic microbes, accompanied with the sludge reduction (Silvestre et al., 2014). The composition of organic matters in the supernatant before and after the supplementation of FeCl3 with different dosing time was investigated in Fig. 4. In the initial stage, acetate, butyrate and unknown organic matters (such as ethanol, amino acids and long chain fatty SB203580 (LCFA) etc.) were the main component in SCOD (910 ± 20 mg/L), which contributed to about 22%, 41% and 26%, respectively. The SCOD and its component changed significantly after 12 days’ digestion with a time interval of 6 days in terms of R6 (dosing time at the 144th hour after the experimental startup). Compared with R1, an enhanced SCOD was achieved in R4 (22,280 ± 400 mg/L), while that in R2, R3, R5 and R6 reduced by 7%, 3%, 12% and 10%, respectively. Particularly, acetate in all the treatment groups with FeCl3 additive was lower than that in R1 (7020 ± 310 mg/L), indicating that the supplementation of FeCl3 could accelerate the decomposition of acetate. At the end of the digestion (43 days), the main component was the unknown organic matters, which accounted for 76% in R4. The acetate in R1 also kept in a high level (4110 ± 260 mg/L, P(0.05) = 2.45 × 10−13 < 0.05) in comparison with 1100 ± 110 mg/L (R2), 1110 ± 140 mg/L (R3), 1550 ± 160 mg/L (R4), 2480 ± 190 mg/L (R5) and 1050 ± 120 mg/L (R6), respectively, which confirmed that acetic acid was the main inhibitor and could be disinhibited by the supplementation of FeCl3. Acetate, propionate, butyrate and valerate were summed as VFAs, and the contribution proportion of VFAs in SCOD was shown in Table 3. With the time extension, the proportion of the FeCl3 added groups showed a declined trend, while that in R1 maintained in a high level (from 62.78% to 38.47% in R1) and inhibited the biogas production. In contrast, R4 achieved the lowest proportion (19.44%) at the end of digestion, confirming that the effective disinhibition of excessive VFAs could be obtained with the dosing time at 72nd hour.