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.
Tetrabromobisphenol A (TBBPA) has extensive applications in various fields; its release into ecosystems and the potential toxic effects on organisms are becoming major concerns. Here, we investigated the effects of TBBPA on anaerobic digestion, whose process is closely related to the carbon cycles under anaerobic conditions. The results revealed that TBBPA exhibited dose-dependent hormesis-like effects on methane production from glucose, i.e., the presence of 0.1 mg/L TBBPA increased the methane production rate by 8.79%, but 1.0−4.0 mg/L TBBPA caused 3.45−28.98% of decrement. We found that TBBPA was bound by the tyrosine-like proteins of the extracellular polymeric substances of anaerobes and induced the increase of reactive oxygen species, whose slight accumulation stimulated the metabolism activities but high accumulation increased the apoptosis of anaerobes. Owing to the differences between individual anaerobes in tolerance, TBBPA at 0.1 mg/L stimulated the acidogenesis and hydrogenotrophic methanogenesis, whereas higher levels (i.e., 1.0−4.0 mg/L) severely restrained all of the processes of acidogenesis, acetogenesis, and methanogenesis. Along with the accumulation of bisphenol A (BPA) produced from TBBPA by Longilinea sp. and Pseudomonas sp., the methanogenic pathway was partly shifted from acetate-dependent to hydrogen-dependent direction, and the activities of carbon monoxide dehydrogenase and acetyl-CoA decarbonylase/synthase were inhibited, while acetate kinase and F420 were hormetically affected. These findings elucidated the mechanism of anaerobic syntrophic consortium responses to TBBPA, supplementing the potential environmental risks of brominated flame retardants.
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