| Landfill leachate is a highly concentrated organic wastewater produced in the sanitary landfill process,which is complex in composition and environmentally hazardous,requiring treatment using a series of combined processes to satisfy effluent standards.Monitoring of leachate and combined process treated tailwater commonly uses physicochemical monitoring,but this monitoring does not reflect the biological and ecological toxicity of leachate.Caenorhabditis elegans is a free-living bacterial-feeding nematode,which is widely used for biotoxicity testing of contaminants.Meanwhile,a multitude of freeliving nematode species exist in various soil types,which play an important role in soil material cycling and have potential advantages in the ecological evaluation of contaminants.Thus,in this study,the biological toxicity and ecotoxicity of landfill leachate and tailwater of different treatment processes in Shanghai Laogang Landfill were evaluated by using C.elegans and free-living nematode communities,respectively.The feasibility of nematodes as indicators of landfill leachate toxicity evaluation was also investigated to provide scientific basis for landfill leachate toxicity reduction treatment process and tailwater disposal.The main results of the study are as follows:1.Biotoxicity of landfill leachate tailwater from different processes was assessed by indicators of survival,growth,motility and reproduction of the model species C.elegans,and the results showed that:1)the nematode mortality rate at 24 h of tailwater exposure by raw leachate(RAW),in situ microbial ponds(RM)and anaerobic bioreactor(A)was more than 75%,and the nematode mortality rate at 24 h of aerobic bioreactor(O)and membrane bioreactor(MBR)was significantly higher than that of mineralized waste bed(ARB),nanofiltration and reverse osmosis technology(NF+RO)and control(CK);2)the growth in body length of nematodes exposed to MBR and ARB treatments was significantly lower than that of both NF+RO and control within 48 h.It is evident that MBR and ARB treatments can inhibit nematode growth;3)the nematode body bending frequency and head oscillation frequency responded differently to different process tailwaters,and MBR tailwater treatment resulted in nematode sterility,but the number of eggs and egg hatching rate of nematodes in ARB and NF+RO treatments were not significantly different from the control.The above results suggest that the mortality of C.elegans in combination with physiological indicators can be a good indicator of the biotoxicity of the landfill leachate treatment process tailwater.2.The ecotoxicity of leachate was assessed by constructing a multi-trophic level nematode community microcosm tank and it was found that: 1)compared to the control,nematode density,genus abundance,Simpson’s index,and Shannon’s diversity index decreased significantly at each exposure time after raw landfill leachate(RAW)exposure,followed by anaerobic bioreactor(A)and membrane bioreactor(MBR),while nanofiltration and reverse osmosis technologies(NF+RO)did not differ from the control,indicating that the effects of different processes of waste leachate treatment on nematode diversity were in the following order: RAW > A = MBR > NF+RO = CK;2)the differences in nematode community structure between the tailwaters of different treatment processes increased with increasing exposure time,which shows that the ecotoxicity of landfill leachate to soil biological communities is time-series and persistent;3)at the functional group level,non-selective sediment-feeding nematodes(1B),which feed on decaying organic matter detritus,and algae-eating scavenger nematodes(2A),which are the dominant trophic taxa in the nematode community,show a significant decrease in abundance when affected by leachate,and are leachate-sensitive trophic taxa that can be used as indicator trophic taxa for monitoring leachate toxicity;4)RAW leachate exposure significantly increased the maturity index(MI),enrichment index(EI)and structural index(SI)of the nematode community and decreased the metabolic footprint(NMF),which shows that leachate addition can provide nutrients to the soil,but still has a negative impact on the ecological function of soil organisms.The above results indicate that nematode communities can be comprehensively assessed for the ecotoxicity of leachate and different process tailwaters at several levels,including diversity,food web status,and function.3.Through the combined analysis of microecological tank soil physicochemical factors,microorganisms and nematode communities,the study explored the intrinsic mechanisms affecting nematode communities after the addition of raw leachate and each treatment tailwater,and found that:1)leachate addition significantly changed the soil physicochemical properties,with raw leachate addition causing a significant increase in the ammonia nitrogen content of the soil in the microcosm tank and a significant increase in soil conductivity from RAW,A and MBR tailwater;2)leachate addition significantly changed the microbial community,and the diversity of the microbial community gradually increased with the extension of the treatment process(from RAW-A-MB-NFRO),with the dominant bacterial phylum Bacteroidete having a high tolerance to the raw leachate and rare bacterial taxa being sensitive to leachate toxicity;3)mental test showed significant effects of soil ammonia nitrogen,p H and conductivity on microorganisms,and redundancy analysis(RDA)showed that soil ammonia nitrogen and conductivity were the main factors affecting nematodes,and microorganisms can significantly affect nematode communities;4)piecewise SEM showed that leachate addition directly increased soil contamination levels,which indirectly led to a decrease in nematode densities and altered nematode feeding taxa composition.Since soil ecological functions were directly and positively affected by nematode feeding taxa composition,the level of soil ecological functions also decreased,revealing a potential pathway by which landfill leachate addition inhibits soil ecological functions. |
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