Effects Of Different Pretreatment Methods On Anaerobic Digestion Of Sludge Containing Microplastics

There is growing concern about the toxic effects of microplastics and their chemical additives.Most additives of microplastics are not chemically but physically bound to plastics,thus,when microplastics are exposed to external stress,the additives can be easily released into the environment.The leachate including plastic debris or the additives,especially with the influence of aging processes,could pose an ecotoxicological risk in freshwater or seawater.More than 90% of the microplastics in wastewater treatment plants(WWTPs)accumulate in waste activated sludge(WAS)due to their hydrophobicity.Recent studies have shown that microplastics accumulated in WAS have toxic effects on the anaerobic digestion of sludge.Anaerobic digestion is a multi-stage process,including solubilization,hydrolysis,acidification,acetogenic and methanogenic stages,where hydrolysis is considered as the rate-limiting step of WAS anaerobic digestion.Therefore,a series of physical,chemical and biological pretreatment techniques are applied to improve sludge hydrolysis process,such as ultrasonic,alkaline,Fenton and thermal pretreatment.In view of the increasing concentration of microplastics in the environment and the prevalence of WAS pretreatment application,this paper investigated the effects of different pretreatment techniques on the anaerobic digestion of sludge containing polystyrene microplastics(PS-MPs).Firstly,the effects of physical(ultrasonic and thermal pretreatment)and chemical pretreatment(alkaline and Fenton pretreatment)on the anaerobic digestion performance of WAS containing PS-MPs were investigated.Thermal pretreatment and two chemical pretreatment methods significantly improved sludge disintegration and the solution of organic matters.The methane yields of thermal and chemical pretreated groups increased by 17.5 ～ 20.4%.Although the anaerobic digestion efficiency of the alkaline and Fenton pretreated groups was relatively high,the concentrations of sodium dodecyl sulfate(SDS)leached from PS-MPs in their reactors were 1.7 ～ 2.6 times higher than those of the physical pretreated groups.In addition,there was no significant difference in reactive oxygen species(ROS)levels among the four pretreated groups(p > 0.05),which all showed a slight increase compared with the control group.In terms of the effects of different pretreatment methods on the microbial communities in the long-term anaerobic digestion reactors,the thermal pretreated group had the highest proportion of methanogens(Halobacterota and Euryarchaeota)among all groups,accounting for 13.2%.From the perspective of anaerobic digestion performance and changes in leaching behavior of microplastics,thermal pretreatment with high improvement on methane production and simultaneously a weak effect on the leaching of PS-MPs additive,is regarded as a more suitable method for improving the anaerobic digestion performance of WAS containing PS-MPs.Based on the above results,the effects of low-temperature thermal pretreatments on anaerobic digestion of WAS containing PS-MPs were further explored.Four low-temperature thermal treatment methods(50°C,60°C,70°C,80°C)were applied for comparative experiments.Pretreatments of 60°C ～ 80°C significantly promoted sludge disintegration and the increase of soluble microbial products,which improved the biodegradability of the sludge,while the pretreatment of 50°C did not show an advantage in this process.All four temperatures showed a remarkable increase in SCOD(soluble chemical oxygen demand),and the pretreatment groups of 60°C ～ 80°C led to a significant increase in soluble protein and polysaccharide.The pretreatment group of 80°C had a greater increase in VFAs compared with other pretreatments.The results of cumulative methane production from different low-temperature thermal pretreatment methods were the same as above.The cumulative methane yields of pretreated groups of 60°C ～ 80°C increased by 18.5 ～ 21.8%,while that of50°C showed a relatively weak improvement.The leaching concentration of SDS was positively correlated with pretreatment temperature.The higher the temperature of pretreatment,the greater the leaching concentration of SDS.Therefore,to comprehensively evaluate the effects of different low-temperature thermal pretreatments on anaerobic digestion performance and leaching behavior of microplastics,the pretreatment at the temperature range of 60°C ～ 70°C is more environmentally friendly for anaerobic digestion of sludge containing PS-MPs to alleviate the environmental impact of leaching additives from microplastics.The above research provided new ideas and directions for the selection of pretreatments for anaerobic digestion of sludge containing microplastics by comparing and analyzing the effects of different pretreatment methods.