Study On The Occurrence And Migration Of Microplastics In A Wastewater Treatment Plant

Microplastics have been found all over the world,even in the Antarctic.Microplastic pollution has become one of the global pollution problems and has attracted wide attention in recent years.As a new type of pollutant,microplastics have the characteristics of high toxicity and difficult degradation,as well as the characteristics of adsorption of persistent organic pollutants and heavy metals.Microplastics exist widely in the ecosystem and are easy to be ingested by organisms,thus harming human health through the food chain.Discharge from wastewater treatment plants is one of the important ways for microplastics to enter the natural environment.The study on microplastic pollution in wastewater treatment plants,especially the study on the occurrence and migration of microplastics in the current wastewater treatment processes,is of great significance for the control of microplastic pollution.The underground wastewater treatment plant has the advantages of small area,high effluent quality and small impact on the surrounding environment,which fully embodies the concept of green environmental protection.However,there is still a gap in the study of microplastic pollution in the underground wastewater treatment plant.This study aimed to study microplastics in an underground wastewater treatment plant in Beijing.The study mainly analyzed the overall occurrence of microplastics in this underground wastewater treatment plant and the removal efficiencies of microplastics in different treatment processes.According to the microplastic pollution in surrounding water environment,the influence of wastewater treatment plant discharge on the microplastic pollution to natural water was discussed.The main conclusions are as follows:(1)In an underground wastewater treatment plant in Beijing,the microplastic concentration in influent was 16.0 n/L(n is the number of microplastics),its concentration in effluent was 1.3 n/L,and the removal efficiency was 91.7%.Four shapes including fiber,film,debris and particle were detected in the wastewater treatment plant.The color of microplastics mainly presented transparent and blue.The size range of microplastics detected was between 0.04-4.16 mm,and the proportion of microplastics with small size(?0.5 mm)was the highest.Eleven polymer types of microplastics were found,and the top three abundant types were polypropylene(36.6%),polyester(22.2%)and polyethylene(19.8%).The microplastic concentration in the final sludge discharge of the wastewater treatment plant was 7.0×10~3 n/kg dry sludge.In sludge,there were three shapes of microplastics including fiber,film and debris.The color also mainly presented transparent.Six different polymer types of microplastics were found in sludge,and the most abundant type was polyethylene.(2)The removal efficiencies of microplastics in the primary treatment,the secondary treatment,and the advanced treatment were 27.1%,60.0%,and 74.1%,respectively.The coarse screen,membrane screen,advection sedimentation tank(secondary sedimentation tank),high efficiency sedimentation tank,and ultrafiltration played important roles in removing microplastics,among which the secondary sedimentation tank had the highest removal efficiency of 86.0%.However,the microplastic concentrations increased in the fine screen and multi level A/O process In the wastewater treatment plant,microplastics did not flow in one direction from the inlet to the outlet.The microplastics deposited in the advection sedimentation tank could return to the multi level A/O process with the sludge backflow,resulting in the accumulation of microplastics in the multi level A/O process.On the whole,the microplastics in wastewater treatment plant showed a trend of migration from wastewater to sludge.(3)The wastewater treatment plant discharges 100,000 tons water per day into urban river,and 1.3×10~8 microplastics(mainly composed of polyester fiber)will be discharged into water environment every day.The daily sludge discharge capacity is 200 tons(water content is 80%).With sludge transportation,2.8×10~8 microplastics will be discharged,which is twice as much as wastewater discharge.(4)The microplastic concentrations in the surrounding water environment of this underground wastewater treatment plant ranged from 1.7-3.3 n/L.The microplastic concentration in the upstream and downstream of the outlet did not increase significantly,indicating that no significant increase of microplastic concentration was caused by the wastewater treatment plant.However,the discharge from the wastewater treatment plant increased the types of microplastic pollution(polyester fiber)in the urban river,indicating that the complexity of microplastic pollution has been increased.In addition,the background value of microplastic concentration measured in the upstream of the river is slightly higher than that in some downstream samples,indicating that there are other sources of microplastic pollution in the river water.