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Graphical abstract Highlights • TCC enhanced hydrogen production from waste activated sludge. • TCC could be degraded during anaerobic fermentation system. • TCC promoted solubilization and acidogenesis. • TCC inhibited methanogenesis and homoacetogenesis. Abstract This work aims to investigate whether and how TCC affects hydrogen production using both experimental and model approaches. Experimental results showed that the exposure of TCC not only enhanced the hydrogen production yield but also promoted the hydrogen yield potential and hydrogen production rate. The maximum hydrogen production yield and hydrogen production rate increased from 10.1 ± 0.2 to 14.2 ± 0.2 mL/g VSS and 0.09 to 0.13 mL/g VSS·h, respectively, when TCC level increased from 0 to 1403 ± 150 mg/kg TSS. Mechanism exploration showed that the presence of TCC significantly promoted the release of substances and observably facilitated the acidification process but seriously inhibited the methonogenesis and homoacetogenesis processes. Further investigations with enzyme analysis revealed that TCC importantly increased the activities of acetate kinase and [FeFe] hydrogenase but seriously inhibited the activities of carbon monoxide dehydrogenase and Coenzyme F420. [ABSTRACT FROM AUTHOR]

Graphic abstract Highlights • FA aids ultrasound pretreatment enhanced the yield of SCFA. • FA aids ultrasound pretreatment accelerated sludge solubilization. • FA aids ultrasound pretreatment enhanced the release of biodegradable organics. • FA aids ultrasound pretreatment severely inhibited methanogenesis process. Abstract Ultrasound pretreatment was proved to be effective in accelerating waste activated sludge (WAS) disintegration and promoting anaerobic fermentative short-chain fatty acids (SCFA) production. However, due to the high activities of SCFA consumers, SCFA yield is limited. Herein we reported an efficient strategy, i.e., using free ammonia (FA) to aid ultrasound pretreatment, to enhance SCFA accumulation. Experimental results showed that the greatest SCFA yield of 316.7 ± 14.9 mg COD/g VSS was obtained at 2 w/mL ultrasound pretreatment for 15 min followed by initial 60 mg/L FA pretreatment for 2 d, which was 2.2 times (1.7 times) of that from sole ultrasound (FA) pretreatment. The mechanism study revealed that this combination method accelerated sludge solubilization, enhanced the release of biodegradable organics, and caused severe inhibitions to SCFA consumption. Moreover, this method has inhibitory effects on specific activities of hydrolytic microorganisms, SCFA-producing bacteria and methanogens, but the inhibitions to methanogens were severe. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • CaO 2 + ultrasonic synergistically improved the intensity of free radicals. • CaO 2 + ultrasonic facilitated solubilization and release of biodegradable organics. • CaO 2 + ultrasonic improved the hydrolysis rate and biochemical methane potential. • CaO 2 + ultrasonic enhanced the removal of refractory organic pollutants. Abstract This study reported a novel and high-efficient pretreatment method for anaerobic digestion, i.e., combining calcium peroxide (CaO 2) with ultrasonic (US), by which not only the methane production was remarkably improved but also the removal of refractory organic contaminants was enhanced. Experimental results showed the optimum condition for methane production was achieved at 0.1 g CaO 2 /g VSS combined with US (1 W/ml, 10 min). Under this condition, the maximal methane yield of 211.90 ± 2.6 L CH 4 /kg VSS was obtained after 36 d of anaerobic digestion, which was respectively 1.36-fold, 1.19-fold and 1.26-fold of that from the control, solo US (1 W/ml, 10 min) and solo CaO 2 (0.1 g/g VSS). Mechanism investigations revealed that CaO 2 + US not only improved the disintegration of waste activated sludge (WAS) but also increased the proportion of biodegradable organic matters. Moreover, the total frequency of recalcitrant contaminants contained in WAS decreased significantly when CaO 2 + US was applied. [ABSTRACT FROM AUTHOR]

This study reported a new, renewable and high-efficient strategy for anaerobic fermentation, i.e., using free ammonia (FA) to pretreat waste activated sludge (WAS) for 1 d and then combining with rhamnolipid (RL), by which the short-chain fatty acids (SCFA) production was remarkably improved. Experimental results showed the maximal SCFA production of 324.7 ± 13.9 mg COD/g VSS was achieved at 62.6 mg FA/L pretreatment combined with 0.04 g RL/g TSS, which was respectively 5.95-fold, 1.63-fold and 1.41-fold of that from control, FA pretreatment and RL pretreatment. Mechanism investigations revealed that FA + RL enhanced sludge solubilization and hydrolysis, providing more organics for subsequent SCFA production. It was also found that the combined method inhibited acidogenesis and methanogenesis, but the inhibition to methanogenesis was much severer than that to acidogenesis. Finally, the feasibility of NH 4 + -N and PO 3− 4 -P, released in fermentation liquor, being recovered as magnesium ammonium phosphate (MAP) was confirmed. [ABSTRACT FROM AUTHOR]

• Impact of pyrochar (PC) and hydrochar (HC) on the anaeribic digestion of WAS was compared. • The highest accumulative methane yield was observed at group HC180. • Pyrochar showed a slightly negative effect on methane production. • Specific DIET-based syntrophic microbes were enriched with HCs addition. This study compared the effects of sewage sludge-derived pyrochar (PC300, PC500, and PC700) and hydrochar (HC180, HC240, and HC300) on mesophilic anaerobic digestion of waste activated sludge (WAS). It was demonstrated that hydrochar can better promote the methane production compared with pyrochar. The highest accumulative methane yield of 132.04 ± 4.41 mL/g VS added was obtained with HC180 addition. In contrast, the PC500 and PC700 showed a slightly negative effect on methane production. Sludge-derived HC not only accelerated the solubilization and hydrolysis of sludge floc, but also improved the production of acetic acid and propionate, further resulting in improved methane production. Simultaneously, the syntrophic microbes facilitating direct interspecies electron transfer (DIET) such as Syntrophomonas, Peptococcaceae, Methanosaeta and Methanobacterium bred rapidly with the addition of HCs. These results indicated that the hydrochar is more ideal as the accelerant to promote the methane production from mesophilic anaerobic digestion of WAS than the pyrochar. [ABSTRACT FROM AUTHOR]

• Clarithromycin promoted volatile fatty acids production from sludge fermentation. • Clarithromycin was partly degraded during the anaerobic fermentation process. • Clarithromycin enhanced sludge solubilization process. • Clarithromycin inhibited all the other process of anaerobic fermentation. This study investigated the effect of Clarithromycin (CLA) on volatile fatty acids (VFAs) production during waste activated sludge (WAS) anaerobic fermentation for the first time. Experimental results showed that when CLA concentration in WAS increased from 0 to 1000 mg/kg TSS, the maximum yield of VFAs increased from 27.7 to 35.7 mg COD/g VSS (without pH pretreatment) and from 59.3 to 65.6 mg COD/g VSS (initial pH 9 pretreatment), respectively. Mechanism exploration revealed that CLA facilitated the disruption of extracellular polymeric substances, thus promoting WAS solubilization. CLA inhibited all the other anaerobic fermentation processes. However, its inhibition to acetogenesis and methanogenesis was severer than that to hydrolysis and acidogenesis, resulting in the decrease in VFAs consumption. Microbial analysis showed that CLA slightly increased the abundance of microorganisms responsible for hydrolysis and acidogenesis whereas decreased the proportion of VFAs-consuming microorganisms. [ABSTRACT FROM AUTHOR]

• Ultrasonic aids FNA pretreatment enhanced the yield of hydrogen. • Ultrasonic aids FNA pretreatment accelerated sludge solubilization. • Ultrasonic aids FNA pretreatment enhanced the release of biodegradable organics. • Ultrasonic aids FNA pretreatment severely inhibited hydrogen consumers. This study presents a novel and effective method, i.e., adding nitrite into acidic fermentations after ultrasonic (US) pretreatment to form free nitrous acid (FNA), to further enhance hydrogen yield. Experimental results showed that when 180 mg/L nitrite was added into the US (2 W/mL, 15 min) pretreated waste activated sludge (WAS), the maximal hydrogen yield of 24.81 ± 1.24 mL/g VSS (volatile suspended solids) was obtained under acidic fermentation (1.0 mg/L FNA was initially formed under this condition), which was 2.21-folds (or 1.36-folds) of that from US pretreatment (or FNA treatment) alone. This combination approach caused a positive synergy on sludge disintegration and enhanced the transformation of the released organics from non-biodegradable to biodegradable. Further study showed that the inhibiting effect of this combination method on hydrogen consuming microorganism was severer. Considering its pollution free, this combination strategy is an attractive technology for hydrogen recovery from WAS. [ABSTRACT FROM AUTHOR]