Effects Of Different Microplastics On The Anaerobic Digestion Performance Of Straw Sludge And Changes In Their Own Degradation Characteristics

Anaerobic digestion is a reasonable method to treat sludge,but the sludge often contains large amounts of microplastics（MPs）,the presence of which inevitably affects anaerobic digestion,and the transport and degradation characteristics of MPs after anaerobic digestion have also received attention from scholars.In this study,we investigated the effects of different MPs on anaerobic digestion and their degradation in anaerobic digestion systems based on the enrichment characteristics of MPs in sludge and the degradation characteristics of MPs under anaerobic digestion conditions in order to provide a theoretical basis for better resource utilization of sludge and degradation of MPs.In this study,sludge and straw were used as substrates to analyze the effect of adding different types（Polyvinyl chloride（PVC）,Polybutylene adipate terephthalate（PBAT）,Polylactic acid（PLA））,different particle sizes（250μm,550μm,1000μm）,and different aged treatments（Non-aged（NA）,Fenton（FT）,and Ultraviolet（UV））on the biogas production characteristics,process parameter characteristics and microbial characteristics of anaerobic digestion.The dynamic changes of the parameters of the anaerobic digestion system under different MPs treatments were investigated to clarify the relationship between them and the factors of the anaerobic digestion system;meanwhile,the degradation of different MPs in the anaerobic digestion system was clarified through the determination and analysis of the residual MPs in the digester after the digestion.The main conclusions of the study are as follows:（1）All three MPs had an effect on the biogas production and biogas production efficiency of anaerobic digestion of straw sludge,showing an overall inhibition of PVC and PBAT and promotion of PLA compared to CK₁.Among them,PVC₅₅₀,PBAT₁₀₀₀,and PLA₅₅₀had the highest total biogas production with growth rates of-18.13%,16.55%,and19.44%,respectively.Both the modified Gompertz model and the first-order hydrolysis equation model were able to better simulate the cumulative biogas production from anaerobic digestion under the influence of MPs types and particle size,and thus the maximum biogas production potential(M_max),maximum biogas production rate(R_max),lag time（λ）,and hydrolysis rate（k）were predicted.The effects of MPs type,MPs particle size and their interactions on M_max,R_max,λ,and k were found to be significant（P<0.05）by multi-factor analysis of variance.All three MPs increased the p H,Ammonia nitrogen（AN）,and Volatile fatty acids（VFA）concentrations in the pre-digestion system,but this phenomenon disappeared in the later stages.（2）Both aging of FT and UV modified the effects of MPs on anaerobic digestion biogas production and biogas production efficiency,and they behaved differently in different MPs types.Both PVC-FT and PVC-UV increased anaerobic digestion biogas production relative to PVC-NA,with growth rates of 33.55%and 26.65%,respectively;PBAT-FT and PBAT-UV increased and decreased anaerobic digestion biogas production relative to PBAT-NA,with growth rates of 4.15%and-15.51%,respectively;PLA-FT and PLA-UV decreased and increased anaerobic digestion biogas production relative to PLA-NA,with growth rates of-24.43%and 6.33%,respectively.Both the modified Gompertz model and the first-order hydrolysis equation model could better simulate the cumulative biogas production from anaerobic digestion under the influence of MPs type and aging treatment and then predict M_max,R_max,λ,and k.The effects of MPs type,MPs aging treatment,and their interactions on M_max,R_max,λ,and k were found to be significant by multifactorial multi-factor analysis of variance（P<0.05）.In addition,FT and UV aging weakened the VFA accumulation phenomenon caused by PVC in the prephase,but their effects on p H and AN concentration were not significant.（3）The surfaces of PLA and PBAT became rough and cracks appeared after anaerobic digestion,while the surfaces of PVC were relatively smooth.In addition,the abundance of the characteristic functional groups on the surfaces of all three MPs decreased after anaerobic digestion.The three MPs showed different degradation rates after anaerobic digestion,ranging from 0.70%～1.92%for PVC,2.39%～4.43%for PBAT,and 5.38%～8.90%for PLA,with an overall performance of PVC<PBAT<PLA.Where the highest degradation rates of each MPs were treated with PVC₂₅₀,PBAT₂₅₀,and PLA₅₅₀,respectively.After the FT and UV aging of the three MPs,it could be observed that the surface became rough and fine lines appeared.The surface of the aged MPs became rougher and had a large number of holes and cracks after anaerobic digestion.In addition,the abundance of characteristic functional groups on the surfaces of the three MPs decreased after FT and UV aging.Interestingly,the abundance of characteristic functional groups on the surface of aged MPs increased dramatically after anaerobic digestion and all introduced─OH,which is important evidence of the aging degradation of MPs.The degradation rates of all three aged MPs after anaerobic digestion were higher than those of the original MPs,ranging from0.72%～3.87%for PVC,2.95%～5.97%for PBAT,and 7.92%～14.51%for PLA.（4）In the middle stage of anaerobic digestion,the OTUs of unique bacteria were greater than CK for PVC and PBAT and the opposite for PLA,suggesting that PVC and PBAT may have reduced the enrichment of dominant bacteria by the anaerobic digestion system.The OTUs of unique archaea were greater than CK for both MPs addition groups.suggesting that MPs may have reduced the enrichment of dominant archaea by the anaerobic digestion system.In the late stage of anaerobic digestion,the unique bacterial OTUs of PVC and PBAT were smaller than CK,and the opposite was true for PLA,indicating that PVC and PBAT may exhibit toxic effects on some bacteria in the late stage.The unique archaeal OTUs of the MPs addition group were all smaller than CK,indicating that all three MPs had toxic effects on archaea in the late stage.In addition,all three MPs had insignificant effects on the homogeneity and diversity of bacterial and archaeal communities.Redundancy analysis showed that Proteobacteria and Firmicutes were positively correlated with changes in p H and AN and negatively correlated with changes in VFA;the opposite was true for Bacteroidetes.Methanosarcina was positively correlated with VFA changes but not with p H and AN changes,while Methanobacterium was positively correlated with p H and AN changes but negatively correlated with VFA changes.