| Activated sludge process is widely used in sewage treatment,during which a large amount of waste activated sludge(WAS)will inevitably be produced as by-products.Anaerobic fermentation(AF)is one of the most common and cost-effective methods for the stable treatment of WAS and the recovery of energy products such as biogas,hydrogen and short-chain fatty acids(SCFAs).By report,over 98%of microplastics(MPs)in wastewater are trapped in WAS.MPs pollution in WAS and its potential ecological health risks are urgent problems to be solved.Several studies have indicated that the addition of polyethylene(PE),polyethylene terephthalate(PET)as well as polyvinyl chloride(PVC)MPs in WAS may impose restrictions on value-added products production in AF by releasing toxic additives and/or inducing redox damage.MPs can be further categorized as nanoscale plastics(<1000 nm)and microscale plastics(1 μm-5 mm).Compared to MPs,nanoplastics(NPs)are likely to induce more serious environmental problems.Studies have shown that the presence of MPs in sludge may have a negative effect on SCFAs during AF.However,the effect of NPs on SCFAs during AF is still unclear.We conclude that NPs may have a more serious negative effect on sludge acidification than MPs.In order to overcome the inhibitory effect of NPs on sludge acidification,a low-cost in-situ treatment method of free nitrous acid(FNA)was used to assist calcium peroxide(CaO2)pretreatment to promote the process of sludge acidification.Polystyrene(PS)NPs(50 nm)decreased the methane production by 15.5%compared with the control group,while PS MPs(1 μm and 10 μm)had no significant effect on the methane production.The results of mechanism analysis showed that PS NPs significantly increased reactive oxygen species(ROS)during sludge anaerobic fermentation,and the cell viability decreased concurrently which was 85.95 ± 0.27%of that of the control group.Therefore,it can be concluded that the inhibitory effect of PS NPs on sludge methane production was resulted from the strong oxidative stress induced by NPs.PS NPs/MPs had no obvious effect on sludge solubilization process,but promoted sludge hydrolysis process.The degradation efficiency of bovine serum albumin(BSA)in PS-50 nm group was increased by 12.6%,and the removal efficiency of dexan was increased.Moreover,the activities of protease and cellulase were increased in the PS-50 nm as well.PS NPs/MPs inhibited acetate kinase(AK)activity and thus inhibited acidification.PS NPs/MPs promoted methane metabolism.In the PS-50 nm,PS-1 μm and PS-10 μm groups,the activity of coenzyme F420 was 145.0%,173.8%and 177.5%of the control group,respectively.Analysis of microbial communities and key genes showed that PS NPs reduced the relative abundance of Candidatus Methanofastidiosum and Methanobacterium,and the absolute abundance of mcrA gene encoding methylcoenzyme M reductase,which was 54.4%of that in the control group,while the relative abundance of Mariniphaga,Candidatus Microthrix,Brevefilum and Perlabentimonas related to sludge hydrolysis were increased.The co-treatment of FNA and CaO2 was more conducive to WAS decomposition and organic matter release compared with FNA and CaO2 pretreatments.Notably,the highest SCOD concentration of 3319.3 ± 197.1 mg/L was observed in the FNA+CaO2(0.15)group,which was 2.6 and 3.1 times higher than that in the CaO2(0.15)and FNA groups,respectively.In addition,the concentration of soluble protein and polysaccharide was also the highest in the FNA+CaO2(0.15)group,which was 699.0 and 916.4 mg COD/L,respectively.The synergism between FNA and CaO2 in the FNA+CaO2(0.15)group resulted in the enhancement of DMPO·-OH and DMPO·-O2-intensities compared to the corresponding intensities of the individual pretreatments.The intensified intensity of DMPO·-OH and DMPO·-O2-might account for the enhanced sludge decomposition and organics release observed in the FNA+CaO2(0.15),and the sufficient substrate promoted SCFAs production during sludge fermentation.Specifically,the highest concentration of total SCFAs found in the FNA+CaO2(0.15)was 3380.8 ± 33.8 mg COD/L.Compared with FNA and CaO2 pretreatments,FNA and CaO2 co-treatment increased the relative abundance ofMacellibacteroides,Petrimonas,and Sedimentibacter,and thus to increase the production of SCFAs.Metagenomic analysis showed that the relative abundance of genes encoding hexokinase(HK),6-phosphofructokinase 1(pfkA)and pyruvate kinase(pyk)increased,indicating that the glycolysis pathway was enhanced by FNA+CaO2 co-treatment.Compared with the control,the FNA+CaO2 co-treatment enhanced the phosphate acetyltransferase-acetate kinase pathway,and the genes involved in pyruvate metabolism(pta,E2.3.1.8,K15024 and ackA)were enriched in the FNA+CaO2 group.In addition,the relative abundance of key genes involved in amino acid metabolism increased in the FNA+CaO2 group compared to the control group.The methane production pathway based on CO2 reduction was inhibited in the FNA+CaO2 assay,and the relative abundance of fwdBCDEFG,ftr,mch,mtd,and mer decreased by 15.8-61.5%in the FNA+CaO2 assay as compared with the control.FNA+CaO2 cotreatment resulted in a decrease in the relative abundance of key genes involved in the common pathway of methane production by aceticlastic reaction and CO2 reduction.FNA+CaO2 co-treatment had negative effects on the biosynthesis of coenzymes M,B,F420 and methanofuran.This study comprehensively evaluated the toxicity of PS MPs with different particle sizes on anaerobic fermentation performance of sludge,and clarified the response mechanisms of microbial community structure and key genes to NPs/MPs.Furthermore,the FNA and CaO2 co-treatment of sludge was constructed to enhance the production of SCFAs,and the synergistic mechanism of FNA and CaO2 was revealed.The biosynthesis mechanism of SCFAs in the FNA+CaO2 co-treatment was analyzed.This paper provided theoretical and technical guidance for sludge reduction and energy resource recovery. |
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