Inhibition Of Toxic Substances On Nitrification And Its Eliminating Methods

In reaction to the serious inhibition of nitrification during the startup of a full-scale A/O biological treatment facility for acrylonitrile wastewater,the inhibitory effects of toxic substances on nitrification and the inhibition characteristics were the studied,taking cyanide and thiourea as examples.An eliminating method of nitrification inhibition was proposed and was applied to the full-scale biological treatment facility for acrylonitrile wastewater to investigate its application in the practical engineering.The nitrification processes inhibited by four typical pollutants(acrylonitrile,acrylic acid,acetonitrile and cyanide)in acrylonitrile wastewater were investigated.By comparison,the inhibitory effect of cyanide on the nitrification was the strongest,with a 50%inhibitory concentration of 0.218 mg cyanide-gVSS-1 in a municipal activated sludge system.However,the performance of nitrification was recoverable when cyanide was degraded.To overcome cyanide inhibition,cyanide degradation ability was enhanced by cultivating cyanide-degrading bacteria.Four strains of cyanide-degrading bacteria which were able to grow with cyanide as carbon source and nitrogen source were isolated from the activated sludge after cultivation.They were shaped like short rods under SEM and were identified to be Brevundimonas sp.,Rhizobium sp.,Dietzia sp.and Microbacterium sp.,respectively.Thiourea also showed inhibitory effect on nitrification,and the inhibition was also recoverable.The 50%inhibitory concentration of thiourea on nitrification was determined to be 0.088 mg thiourea·gVSS-1 in a municipal activated sludge system.To eliminate the inhibition,two methods were proposed:H2O2 pretreatment before biological treatment and cultivating thiourea-degrading bacteria through biological methods.However,thiourea inhibition was eliminated in different mechanisms that the former converted thiourea to other lower toxic or non-toxic substances using H2O2 oxidation pretreatment,while the latter enhanced the thiourea degradation rate by cultivating thiourea-degrading bacteria.Compared with the former,the latter was with low cost,non-toxic by-products,and was resistant to the impact load of thiourea,which was benefit for the stable operation of the biological treatment.Four strains of thiourea-degrading bacteria which were able to grow with thiourea as carbon source and nitrogen source were isolated from the cultivated activated sludge,and were all identified to be Pseudomonas sp.with short rod shapes under SEM.The results obtained in the above researches on the inhibition characteristics of cyanide and thiourea on nitrification and their eliminating methods showed that,the inhibitory effects of toxic substances on nitrification were disappeared after the toxic substances were degraded.Enhancing the degradation abilities of toxic substances by cultivating toxic substances-degrading bacteria in the way of stepwise increasing the influent loads was able to successfully eliminate the inhibition on nitrification.To investigate the application of this eliminating method in the practical engineering,the eliminating method of enhancing the degradation abilities of toxic substances was applied into the full-scale acrylonitrile wastewater biological treatment facility.The inflow rate and influent concentration were increased stepwise during the second startup.The system was successfully started after 75 days,and has been operated stably for more than 3 years by now.During the latest days,the influent concentrations of COD,NH3-N,TKN and TCN were 831-2164,188-516,306-542 and 1.17-9.57 mg·L-1 and the effluent concentrations were 257±30.9,3.30±1.10,31.6±4.49 and 0.40±0.10 mg·L-1(n=100),respectively.The aerobic activated sludge in the full-scale biological treatment facility was collected for the biological phase analysis.The bacteria were mainly short rod-shaped,which were observed by SEM,and contained many types of cyanide-degrading bacteria.The method of enhancing the degradation abilities of toxic substances to eliminate the inhibition of toxic substances on nitrification was proposed in this paper.This method is simple and effective,with little additional costs,and has been successfully applied into the full-scale acrylonitrile wastewater biological treatment facility.The results obtained in this paper have guiding significance for the full-scale biological treatment processes of acrylonitrile wastewater and could also be referential for the field operation of the biological treatment plants for other toxic wastewater.