Effects Of Fa And Cr⁶⁺ On Bio-nitrification Inhibition And Recovery Processes And Community Structure

Many municipal sewage treatment plants receive a large amount of industrial effluents with many hazardous pollutants. These hazardous pollutants could cause a negative effect on nitrogen removal performance of the biological treatment system. When the concentrations of some pollutants are suddenly rising, nitrification efficiency will be drastically reduced. It is difficult for nitrifying bacteria to recover their activities from the inhibitory effects of the pollutants in a short term and as a result the nitrogen concentration in the effluent will not meet the discharge standard. Thus, it is necessary to study the impacts of poison pollutants shock and continuous loadings on the nitrification and the following recovery process. In this study, synthetic domestic wastewaters with high levels of free ammonia（FA） and Cr⁶⁺ were used to study the effects of FA and Cr⁶⁺ on bio-nitrification process, respectively. Moreover, to investigate the mechanisms of the nitrification inhibition and recovery from a microbial ecology perspective, fluorescence in situ hybridization（FISH）, polymerase chain reaction-denaturing gradient gel electrophoresis（PCR-DGGE） and real-time quantitative（q-PCR） technique were used to analyze the changes of nitrifying bacterial quantity and community structure during the nitrification inhibition and recovery processes.During the shock loadings of high levels of ammonia（150-2000 mg/L）, activity and quantity of ammonia-oxidizing bacteria（AOB） and nitrite-oxidizing bacteria（NOB） increased at first and then decreased when FA concentration loadings increased. The FA critical inhibitory concentrations for AOB and NOB activities were 8.11 mg/L and 6.60 mg/L, respectively. The results of sustained loading experiments showed that alkalinity affected the activity of AOB and NOB, and low alkalinity level inhibited the activity and quantity of AOB, while it improved the activity and quantity of NOB. The addition of alkalinity increased the activity and quantity of AOB, while it inhibited the activity and quantity of NOB. With a high alkalinity, high FA concentration seriously inhibited the activity of AOB and NOB, and consequently resulted in severe reduction in the quantity of AOB and NOB. The FA inhibitory concentrations for AOB and NOB were 23.06-43.18 mg/L and 6.07-11.99 mg/L, respectively.The results of Cr⁶⁺ shock and sustained loading experiments showed that nitrifying bacteria had a high sensitivity to Cr⁶⁺. The more Cr⁶⁺ was added, the faster the inhibition occurred. And the more significant the inhibitory degree was, the longer the recovery time was required. During the recovery process, the recovery rates of AOB activity were faster than that of NOB. Activated sludge had a strong uptake and absorption ability for chromium, which could mainly accumulate in the interior of activated sludge and the surface of the floc. Nitrification inhibition was up to the interior chromium of activated sludge, which was found to be hardly released from the cells in the recovery process. DGGE analysis indicated that the loading of Cr⁶⁺ changed the diversity and community structure of nitrifying bacteria. During the inhibition process, the diversity of nitrifying bacteria increased, population distribution became disperse, and the quantity of predominant population decreased. In the recovery process, nitrifying bacterial diversity reduced, and some more tolerant nitrifying bacteria（Nitrosospira） gradually became the predominant flora, which made the nitrification performance be recovered.Activated sludge obtained a stronger tolerance to Cr⁶⁺ after acclimation with Cr⁶⁺ gradient continuous and gradient intermittent loading methods, but 12 mg/L Cr⁶⁺ still caused AOB complete inactivation. DGGE, cloning and sequencing results implied that the diversity of AOB population increased at first and then decreased. AOB predominant population shifted gradually from Nitrosomonas to Nitrosospira, indicating that Nitrosospira has a higher tolerance to Cr⁶⁺. q-PCR results showed that low concentrations of Cr⁶⁺（1-2 mg/L） caused amo A gene over-expression, while adding a higher concentration of Cr⁶⁺ inhibited the expression of amo A gene. Compared with Cr⁶⁺ gradient intermittent dosing acclimation method, it is more beneficial for Cr⁶⁺ gradient continuous dosing acclimation method to improve the tolerance of AOB to Cr⁶⁺, while the additional concentration of Cr⁶⁺ should not exceed 10 mg/L during Cr⁶⁺ gradient continuous dosing acclimation process.