The Effect Of Seeding Biofloc And Addition Of Nitrite And Betaine On Establishment Of Nitrification In MBBR

How to accelerate the nitrification of biofilters in recirculating aquaculture systems has been a key research issue in the industry,especially in the seawater environment where the growth of nitrifying bacteria is inhibited.In order to optimize the culture of nitrifying bacteria in reactors,this paper uses a moving bed biofilm reactor as a research vehicle to investigate the effects of inoculating bioflocs,changing the preexisting additional nitrogen source and adding betaine on the establishment of nitrification and microbial communities in freshwater and seawater reactors during the cultivation of nitrifying bacteria.1.Effects of inoculated biofloc on the establishment of nitrification in a moving bed biofilm reactor（MBBR）In this study,the performance of the establishment of nitrification according to three different strategies:conventional method（Control group A）,inoculation with biofloc recovered from a tilapia biofloc culture system（Group B）,and addition with extra nitrite（Group C）in the Moving bed biofilm reactor（MBBR）was compared.Among them,the biofloc-inoculated group considerably accelerated the nitrification process in the MBBR（38 d）,which is roughly 18 d faster than the control group（A）（56 d）and 21 d faster than group C（59 d）.Less ammonia（8 mg/L NH₄⁺-N,10 mg/L in other groups）and external nitrite（2 mg/L NO₂^--N）in the influent caused effluent ammonia to drop more slowly（5 d slower than the control group,8 d slower than the B group）,which is detrimental to the nitrification process’development.Notably,the influent’s hydraulic retention time（HRT）was reduced from 12 h to 6 h following the successful establishment of nitrification.During the adaptation to reduced HRT,the MBBR inoculated with biofloc experienced short-term changes in the water quality index of the effluent water,whereas the other groups did not.The biofilm seeded with biofloc had the highest mean gray value ratio（1.42）of live/dead cell fluorescence,which grew better and could cover the entire groove under multiple microscope observations.However,the other groups did not demonstrate a similar trend.In summary,the research found that seeding biofloc use as nitrification bioaugmentation into the MBBR of the recirculating aquaculture system（RAS）to greatly speed up the nitrification process.2.Effects of using only nitrite as an additional nitrogen source in the early stage on the establishment of nitrification in MBBRIn this study,the effects of conventional incubation（group A）,pre-staging with high concentrations of nitrite instead of ammonium as an additional nitrogen source（group B）and low concentrations of nitrite as an additional nitrogen source（group C）were compared on the establishment of nitrification.The experiment was divided into three phases:start-up,intensification and stable operation.In the first phase,group C with low nitrite concentration（2 mg NO₂^--N/L）had the fastest start-up in the first phase（27 d）,which was 3 d faster（30 d）than group C with high nitrite concentration（5 mg NO₂^--N/L）and the control group A.In the second and third stages,group C had a more stable nitrification performance,but all groups could efficiently remove ammonia nitrogen from the water in the later stages of MBBR.Under the observation of laser confocal microscopy,there was no difference in the live death of biofilm cells among the three culture strategies.The microbial diversity of the three strategies was compared,and high concentration of nitrite as an additional nitrogen source in the early stages affected the microbial diversity of the biofilm,but the abundance of Nitrosomonas,a bacterium associated with ammonia oxidation,was increased.Low concentration of nitrite as an additional nitrogen source had no significant effect on microbial diversity.In conclusion,pre-addition of nitrite instead of ammonium as an additional nitrogen source could be a potential strategy for Nitrosomonas culture.3.Contrasting effects of betaine or nitrite addition on the establishment of nitrification in seawater MBBRIn this study,the effects of three different strategies on the establishment of nitrification in a seawater moving bed biofilm reactor were compared between conventional culture（group A）,the addition of a certain amount of betaine（group B）and the addition of nitrite only as an additional nitrogen source upfront（group C）.The experiments were divided into three stages of nitrifying bacteria cultivation at salinities of 10‰,20‰and 30‰.Group A with conventional culture was able to establish nitrification rapidly（30 d）at 10‰salinity,25 d faster than group B（55 d）with betaine addition and 22 d faster than group C（52 d）with nitrite as an additional nitrogen source in the early stage.Groups B and C were able to adapt quickly to a salinity of 30‰,and the establishment of nitrification at the target salinity of 30‰was completed in group C at 69 d,6 d faster than in groups B and C.Nitrosomonas spp.and Nitrospira spp.were the major AOB and nitrite-oxidizing bacteria（Nitrite-oxidizing bacteria,NOB）in all groups.oxidizing bacteria（NOB）,it was the addition of betaine（group B）and nitrite（group C）that significantly increased the abundance of AOB,and group C also significantly increased the abundance of NOB.In conclusion,in contrast to the conventional culture strategy,the addition of betaine had no significant promoting effect on the establishment of nitrification.The previous incubation strategy with nitrate as the nitrogen source could accelerate the establishment of nitrification in the 30‰seawater moving bed biofilm reactor and significantly increase the abundance of nitrifying bacteria.