Mechanism Research On Cr(Ⅵ) Influence On Bio-nitrification And Nitrifying Bacteria Community

Many wastewater plants accept a large amount of industrial effluents, whichusually contain high levels of hazardous heavy metals and thus affect biologicalnitrification process seriously. One of urgent problems needing to be resolved forwastewater plants is how to deal with poison load on biological treatment system. Inthis work, the effects of Cr6+, a typical heavy metal in industrial wastewater, onmicrobial nitrification and nitrifying bacteria community were investigated throughstatic tests, shock load tests and sustained load tests in SBR. To explore theinhibition mechanisms of Cr6+on nitrification, nitrification ability, microbialnitrifying activity and the distribution of Cr6+in sludge solution were examined andmoreover the evolution of nitrifying bacteria was determined by using PCR-DGGEand FISH methods.In the static tests, Cr6+had inhibitory effects on the activities of heterotrophicbacteria, ammonia-oxidizing bacteria（AOB） and nitrite-oxidizing bacteria（NOB）.The inhibition order was AOB＞NOB＞heterotrophic bacteria. The inhibitioneffects of Cr6+on AOB were quickly observed in2h contact time. The absorptionrate of Cr6+onto sludge was very fast and achieved a high level of accumulation in2h. The distribution of Cr6+in the sludge solution indicates that Cr6+fast entered intocells of microorganisms in active sludge and reacted with molecular materials incells.The shock load tests indicate that Cr6+was absorbed quickly by bacteria andthen accumulated in bacteria cells. The activities of heterotrophic bacteria recoveredsignificantly in the natural recovery process, but AOB and NOB had no sign ofrecovery. DGGE and FISH proved that diversity of bacteria was decreased seriouslyunder the Cr6+inhibition. The flora number of AOB increased in a less degree in therecovery phase，but NOB did not change mainly.In the sustained load tests in SBR, heterotrophic bacteria was less inhibited byCr6+（≤10mg/L）and could quickly adapt to inhibition of Cr6+. Nitrifying bacteriawas inhibited to a great degree and the SOUR of AOB inhibition rate could reach toabove80%, so the SBR system had lost nitrification ability in theory. After a periodof natural recovery, heterotrophic bacteria could be fully recovered, and nitrifyingbacteria could only recovered to a limited extend gradually. The recovery rate orderwas heterotrophic bacteria＞AOB＞NOB. The analysis results of DGGE and FISH showed that under a long-term Cr6+inhibition, only a few bacteria had strongresistance to the toxicity of Cr6+. The recovery of flora number of AOB and NOBtook a long time, and the recovery degree order was AOB＞NOB.