Anaerobic digestion (AD) is a sustainable pathway towards recovering chemical energy from excess sludge, and humic substances (HSs) contained in sludge can inhibit energy (methane/CH ₄ ) conversion efficiency. This study aims to investigate the impact of humic acids (HA) on the various processes in a batch anaerobic digestion process. For this purpose, “clean” sludge was cultivated in a laboratory to avoid HSs presence. The cultivated sludge was used in a series of batch experiments, with humic acids added at different levels. A complete AD test, as well as three sub-phase tests (hydrolytic phase; acidogenic phase; methanogenic phase) was performed and analyzed with and without HA dosing. In the single-phase AD system, dosing with HA inhibited the methanogenic efficiency by 35.1% at HA:VSS = 15%. However, the effects of HA on the three sub-phases revealed something very different. HA inhibited hydrolytic efficiency by 38.2%, promoted acidogenic efficiency by 101.5%, and finally inhibited methanogenic efficiency by 52.2%. The combined efficiency of the three sub-phases without HA dosing is calculated at 15.7%; and with HA dosing (HA:VSS = 15%) at 10.2%. Overall, the combined inhibition efficiency of the three sub-phases is equal to 35.0%, which is almost identical (35.1%) to the result observed in the single-phase AD process. The possible mechanisms behind the phenomena were analyzed and summarized in the context.

Carbon-neutral operation of wastewater treatment plants (WWTPs) requires enhancing anaerobic digestion (AD) of excess sludge for a higher energy conversion efficiency. Among others, iron has been identified to function on enhancing methane production in AD. As an industrial residual, waste iron scraps (WISs) have been reported as potentially enhancing CH₄ production in AD. With this study, the mechanisms of AD enhanced by WISs are analysed in a two-phase process: acidogenic phase (AP) and methanogenic phase (MP). Semi-continuous tests substantially excluded ORP reduction and hydrogen-evolution corrosion induced by WISs in enhancing CH₄ production, although WISs (10 g Fe/L) could indeed increase CH₄ production by 10.1% and 21.4% when added in AP and MP respectively. Detection on both FISH and enzymatic activities of involved microorganisms revealed that the stimulating effects of WISs on anaerobes (both catabolism and anabolism) could play an important (96.3%) role in enhancing CH₄ production, which would facilitate hydrolysis of refractory organics and improvement of electron transport rate (ETR).

Methane (CH₄) recovery from excess sludge plays an important role in wastewater treatment aimed at achieving carbon neutrality. Besides pretreatment for excess sludge, exogenous CO₂and even H₂have already been tested in enhancing CH₄production from anaerobic digestion (AD). However, exogenous H₂injected into AD might simultaneously influence both upstream acidogenesis and downstream methanogenesis. In this study, a series of batch tests were conducted to ascertain the influences of hydrogen partial pressure (P_H2) on these two processes. Interestingly, the batch tests demonstrated that injecting exogenous H₂into AD at the beginning of each batch cycle can both stimulate acidogenesis (organic conversion rate) and enhance methanogenesis (both CH₄and biogas productions) at an appropriate P_H2(=0.33 bar) and an optimal revolution (R_s = 200 rpm) of a shaker.