Enrichment And Immobilization Of Ammonia-oxidizing And Nitrifying Bacteria Community

Ammonia-contaminated wastewater discharge is known as a main reason of eutrophication.To remove the ammonia from wastewater by specific facilities in wastewater treatment plants(WWTPs)is useful to control this potential problem.The contaminant acceptance threshold of ammonia in the related standard is more stringent since the increasingly serious tendency of this problem.Compared with full nitrification,nitrite is an economically favourable solution for the removal of nitrogen,contributing to the development of wastewater treatment.However,it is difficult to achieve partial nitrification because it is difficult to control the ammonia oxidation process stopping at nitrite.Low nitrification rate in general and more serious in winter is also the current problem of sewage treatment.The nitrifying bacteria concentration is one of the main factors of the reaction rate variations,which reflects the effect of wastewater treatment.Increasing the nitrifying bacteria concentration is a feasible way to improve the ammonia oxidation rate for ammonia-contaminated sewage and wastewater treatment in real projects.In addition,a bacterial community has more advantages due to its biodiversity compared to a pure strain of bacteria.However,how to obtain a high concentration of target bacteria community is the key to realize the improvement of ammonia oxidation rate.The biological autotrophic nitrification process is performed by ammonia-oxidizing bacteria(AOB)and nitrite-oxidizing bacteria(NOB).On the basis of summarizing and analyzing the similarities and differences of physiological characteristics between AOB and NOB,a continuous ammonia feeding mode for intermittent operation was proposed;the calculation formulas and the method of data presentation were also established in this study.This is a novel method which is different from the traditional wastewater treatment method including continuous flow and intermittent operation,resulting in the NH4~+-N and NO2~--N concentrations can be controlled by the relationship of the ammonia feed stream supply rate and ammonia oxidation rate,with the pH and temperature were controlled synthetically,so as to achieve the expected free ammonia(FA)and free nitrous acid(FNA)levels.Therefore,this method is of important theoretical and practical significance.Then the feasibility of this method used in the enrichment of AOB community was further investigated using the activated sludge taken from the A~2/O process in a wastewater treatment plant(WWTP)as the seed sludge.The results showed that the AOB community can be enriched by a continuous ammonia feeding mode for intermittent operation,resulting in the maximum ammonia oxidation rate reached a high level,over 110 mg/(L·h),and the nitrite accumulation ratio was approximately85%after approximately 10 days.The AOB community became the dominant bacteria in the cultivated sludge.To validate the repeatability of this novel method,reducing the reaction temperature from around 30°C to 25°C,and regulating the relationship of the ammonia feed stream supply rate and ammonia oxidation rate to control the expected FA and FNA levels.AOB community can be enriched after more than 20 days,using the activated sludge taken from the A~2/O process in a wastewater treatment plant(WWTP)as the seed sludge,and a relatively high DO level improves the ammonia oxidation rate.With the dissolved oxygen(DO)level of 0.7~1.5 mg/L,the maximum ammonia oxidation rate was over 170 mg/(L·h)and nitrite accumulation ratio over80%.High-throughput sequencing showed that Nitrosomonas belonging to AOB,were highly enriched,having increased from 0.24%to 53.03%.Whereas the levels of Nitrospira affiliated with NOB decreased from 2.70%to a very low level of 0.01%.The method of the continuous ammonia feeding mode for intermittent operation was proven to be feasible and repeatable.Further study was focused on the feasibility of the enrichment of nitrifying bacteria by an alternate mode of intermittent operation mode in the daytime and a continuous ammonia feeding mode for intermittent operation during the night.The results indicated that the nitrifying bacteria community can be enriched by this alternate operation mode,resulting in a maximum ammonia oxidation rate reaching approximately 110 mg/(L·h)and a nitrite oxidation rate of approximately 50 mg/(L·h),with the temperature and DO level of around23°C and 1.0~1.5 mg/L,respectively.High-throughput sequencing demonstrated that Nitrosomonas and Nitrospira belonging to AOB and NOB,respectively,were predominant in the activated sludge after more than 20 days at a ratio extending from2.94%to 60.66%,indicating that the nitrifying bacteria community growth can be achieved by this alternate mode.The similarity of the enrichment of ammonia-oxidizing bacteria and nitrifying bacteria is the regulation of the relationship of ammonia feed stream supply rate and ammonia oxidation rate.However,the difference between them is FA and FNA should be regulated properly to inhibit NOB but not AOB during the process of AOB community enrichment.FA and FNA should also be regulated properly to prevent the AOB and NOB inhibition during the process of nitrifying community enrichment,which can be realized by the regulation of ammonia feed stream supply rate,ammonia oxidation rate and nitrite accumulation ratio.In addition,the ratio of MLVSS/MLSS and SVI were reduced during the cultivation process,which indicated that the inorganic substance in the system increased,and the settleability of activated sludge improved significantly as well as the sludge properties changed through the transformation of floc sludge to granular sludge.Based on the enrichment of bacterial community,the effects of different levels of FA by the variation of pH on the ammonia oxidation rate,nitrite accumulation and nitrate production were evaluated.The effects of nitrifying bacteria concentrations on the efficiency of ammonia oxidation were also assessed.The results showed that a suitable pH level is crucial to increase the ammonia oxidation rate with a certain initial ammonia concentration and a relatively high nitrifying bacteria concentration is the basic for the substantial improvement of the ammonia oxidation performance.The enriched ammonia-oxidizing and nitrifying bacteria community can be used for cell immobilization,and the ammonia oxidation rates were 41.35 mg/(L·h)and46.64 mg/(L·h),respectively.The bacteria concentration can be improved by adding the biological active fillers into the reactors,so as to improve and maintain ammonia oxidation rate.In addition,the technology of bioaugmentation was discussed in this study.The highly enriched ammonia-oxidizing or nitrifying bacteria community can be used to improve the ammonia oxidation rate,resulting in the shorter time of ammonia-contaminated wastewater treatment.Besides,the enriched bacteria community can be used to enhance the discharge standard of nitrogen,so as to provide theoretical basis for the effective control of eutrophication from the source.