Preliminary Study On Distribution Characteristics And Removal Mechanisms Of Microplastics In Sludge Treatment Units

Microplastics are plastic fragments or particles with a particle size of<5 mm.Environmental microplastic pollution has aroused widespread global concern,and has been listed by the United Nations Environment Programme and the Ocean Conference as one of the major environmental issues that need to be addressed at the same time,and has also been included in China’s"New Pollutant Management Action Program"in2022 to promote governance.Wastewater treatment plants are an important"source"and"sink"of environmental microplastics,and microplastics are precipitated into activated sludge during the wastewater treatment process,but effective information on the changes of microplastic distribution characteristics during sludge treatment and disposal is very limited,which amplifies the environmental risk of microplastics in sludge.Based on the establishment of a rapid separation,extraction and identification of microplastics from sludge samples,the microplastic distribution characteristics of different activated sludge treatment units in a typical wastewater treatment plant in Beijing were explored,and the mechanism of microplastic removal by thermal hydrolysis treatment of activated sludge was further studied.The main research results are as follows:（1）In this study,the digestion effect of sludge organic matter by four digestion schemes:30%H₂O₂,10%KOH,Fenton reagent,and 30%H₂O₂+Fenton reagent was compared.The 30%H₂O₂+Fenton reagent showed the highest efficiency of 92.07±7.19%for the digestion of organic matter in the sludge samples and was essentially without destruction to morphology and quality of microplastics,which is recommended as a separation and extraction solution for the extraction of microplastics from sludge samples.Based on these,an efficient method was established:30%H₂O₂+Fenton reagent digestion─potassium formate solution（1.5 g/cm³）flotation─Cellulose enzyme digestion─absolute ethanol concentration─Fourier transform infrared microscopy spectroscopy（μ-FTIR）/Laser direct infrared spectroscopy（LDIR）detection.（2）The average abundance of microplastics in the activated sludge of a typical urban wastewater treatment plant in Beijing was investigated to be 1013.33±460.99（particles/g d.w.）and 2640.20±2126.09（particles/g d.w.）,respectively.The abundance of microplastics in activated sludge in summer was 2.6 times higher than that in activated sludge in winter,but the abundance of microplastics in activated sludge in both seasons was highest in small size microplastics,with the average percentage of20-100μm microplastics higher than 85%,while 500-5000μm microplastics were lower than 4%.In terms of microplastics material,PVC,PP and Acrylates are the three types of polymers with the highest content,accounting for 41.49～43.56%,21.54～25.57%and7.73～8.89%respectively.In terms of microplastics shape,the highest average percentage of fragmented microplastics（>55%）and the lowest percentage of granular microplastics（<5%）.（3）The activated sludge centrifugal pre-dewatering,thermal hydrolysis and anaerobic digestion treatment processes all efficiently reduced microplastic abundance in activated sludge by 54-75%,10%and 13-16%,respectively.Compared to the mixed activated sludge without any treatment,the combined treatment process of activated sludge pre-dewatering,thermal hydrolysis and anaerobic digestion significantly reduced the abundance of activated sludge microplastics（p<0.05）by 61-78%,with the centrifugal activated sludge pre-dewatering process playing an important role.In terms of microplastic particle size,both the single treatment process of activated sludge pre-dewatering,thermal hydrolysis and anaerobic digestion and the combined process of pre-dewatering,thermal hydrolysis and anaerobic digestion significantly reduced the abundance of 20-100μm microplastics,with the reduction ratios ranging from 49-75%,1-9%,7-13%and 56-79%,respectively.In terms of microplastic material,the single treatment process of activated sludge pre-dewatering and thermal hydrolysis could significantly reduce the abundance of PVC microplastics,while the combined process of pre-dewatering,thermal hydrolysis and anaerobic digestion could significantly reduce the abundance of PVC microplastics（p<0.05）,with a reduction ratio of 77-98%.（4）The existing activated sludge thermal hydrolysis process（170°C,0.5h）can obviously change the apparent morphological characteristics of microplastics,such as discoloration,surface roughness and pores visible to the naked eye,and generate smaller particle size microplastics.Meanwhile,the high energy breakage of methylene（-CH₂）and the generation of oxygen-containing functional groups such as carbonyl groups（C=O）in the long chains of microplastic polymer molecules occurred after thermal hydrolysis treatment of activated sludge,implying significant changes in the molecular structure of the polymer.It can be seen that thermal hydrolysis treatment can promote the aging and degradation of microplastics in activated sludge,and the aging and degradation of microplastics is facilitated with the increase of thermal hydrolysis temperature and residence time.