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This work reports a new waste activated sludge (WAS) pretreatment method based on free ammonia (FA) for promoting the generation of short-chain fatty acids (SCFAs). Experimental results showed that pretreatment of WAS for 3 days with FA largely improved WAS disintegration, with the highest dissolution (soluble chemical oxygen demand (COD), 3400 ± 120 mg/L at initial FA level of 237.8 mg/L) being 4.5-fold that without FA pretreatment. The pretreatment method by FA facilitated the breakdown of extracellular polymeric substances and cell envelope of sludge cells and killed more live microbial cells, which thereby accelerated the dissolution of substances from WAS. It was also found that FA severely suppressed the SCFA consumption process, but the acetogenesis process was unaffected. Although FA also inhibited hydrolysis, acidogenesis, and homoacetogenesis to some extent, the inhibitions did not largely affect the biodegradation of the relevant substances at all the tested FA levels. Finally, using FA to pretreat WAS for SCFA enhancement was confirmed. When FA concentrations ranged from 53.5 to 176.5 mg/L, the maximum generation of SCFA was enhanced from 196.8 to 267.2 mg COD/g VSS, which was 2.3−3.2 times that of the blank. Further FA leveling (237.8 mg/L) caused a slight decline of maximum SCFA generation (226.9 mg COD/g VSS). The findings reported may instruct engineers to develop an economic and effective strategy to enhance SCFA production, which might support the operation of wastewater treatment plants in sustainable paradigms with low energy input in the future.

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The expanding use of chitosan in sewage and sludge treatment processes raises concerns about its potential environmental impacts. However, investigations of the impacts of chitosan on sewage sludge anaerobic digestion where chitosan is present at substantial levels are sparse. This study therefore aims to fill this knowledge gap through both long-term and batch tests. The results showed that 4 g/kg total suspended solid (TSS) chitosan had no acute effects on methane production, but chitosan at 8–32 g/kg TSS inhibited methane production by 7.2–30.3%. Mass balance and metabolism of organic analyses indicated that chitosan restrained the transfer of organic substrates from solid phase to liquid phase, macromolecules to micromolecules, and finally to methane. Further exploration revealed that chitosan suppressed the secretion of extracellular polymeric substances of anaerobes by occupying the connection sites of indigenous carbohydrates and increased the mass transfer resistance between anaerobes and substrates, which thereby lowered the metabolic activities of anaerobes. Although chitosan could be partly degraded by anaerobes, it is much more persistent to be degraded compared with indigenous organics in sludge. Microbial community and key enzyme encoding gene analyses further revealed that the inhibition of chitosan to CO2-dependent methanogenesis was much severer than that to acetate-dependent methanogenesis.

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