Cultivation Of Polyurethane Autotrophic Nitrification Biofilm And Its Application In Water Purification Of Shrimp Culture

The increase of ammonia and nitrite content will lead to the deterioration of water quality in aquaculture,and then affect the growth of aquatic organisms even death.At present,biological methods are mostly used to remove ammonia and nitrite.In the process of treatment,ammonia oxidizing microorganisms(AOM),including ammonia oxidizing bacteria(AOB)and ammonia oxidizing archaea(AOA),transform ammonia into nitrite;nitrite oxidizing bacteria(NOB)transform ammonia converted nitrite into nitrate further.Ammonia and nitrite with high toxicity to aquatic organisms can be converted into nitrate with relatively low toxicity through this process,so it is widely used in aquaculture.Nitrifying microorganisms have the characteristics of autotrophy,generation time and attached growth,so it is necessary to provide suitable biofilm carrier for their growth.The ideal substrate should have the characteristics of large specific surface area,low price,easy adhesion of microorganisms and low density.At the same time,the proportion of ammonia oxidizing microorganisms and nitrite oxidizing bacteria,which attached to the carrier should be appropriate,otherwise the peak concentration of ammonia and nitrite will appear in the aquaculture water,which will cause harm to aquatic organisms.In this study,polyurethane was selected as the carrier of nitrifying microorganism.The cultivation process and appropriate preservation method of polyurethane biofilm under different conditions were determined by modification and control of physical and chemical factors.The application effect of polyurethane biofilm in Litopenaeus vannamei culture,the changes of biofilm activity and bacterial community structure were investigated.The following conclusions are obtained through the study:(1)The results showed that CS-PVA-GA modified polyurethane,dissolved oxygen 6-8mg/L and aeration ratio 2h/15min had the shortest establishment time of nitrification function compared with the control group by the cultivation process of autotrophic nitrifying biofilm was optimized with modifying polyurethane carrier and adjusting cultivation conditions.The process of self-cultivation nitrification biofilm hanging could be completed after 9 days,and the final ammoxidation rate and nitrite oxidation rates were 7.2mg.g^-1.h^-1,9.15mg.g^-1.h^-1,8.1mg.g^-1.h^-1;5.25mg.g^-1.h^-1,4.8mg.g^-1.h^-1,5.55mg.g^-1.h^-1.(2)The changes of ammonia oxidation rate and nitrite oxidation rate of polyurethane biofilm under normal temperature with normality and vacuum,normal refrigeration with normality and vacuum were investigated.The results showed that the ammonia oxidation activity was the highest under the condition of refrigeration with vacuum,and its ammonia oxidation rate was 36.93%of the initial value at the11th month,followed by efrigeration with normality;while the nitrite oxidation activity was the highest under the normal temperature with vacuum,and its nitrite oxidation rate was 13.03%of the initial value at the 12th month,followed by normal temperature with normality.(3)Autotrophic nitrifying biofilm polyurethane was applied to water purification of Litopenaeus vannamei culture system.The results showed that adding polyurethane biofilm could effectively remove NH₄⁺-N and NO₂^--N in culture water.After 60 days of culture,the concentrations of NH₄⁺-N and NO₂^--N in the experimental group showed a trend of low-level fluctuation-rising-falling-low-level fluctuation,but the concentration of NH₄⁺-N was always lower than 0.06 mg/L;the concentration of NO₂^--N was always lower than 0.5 mg/L in the stable period,while the concentrations of NH₄⁺-N and NO₂^--N in the control group showed an upward trend during the culture period and were always higher than those in the experimental grouput,they were within the acceptable range of shrimp farming;the content of NO₃^--N showed an upward trend,and the concentration of NO₃^--N(36.84±3.235mg/L)was higher than that of the control group(7.96±3.639mg/L).(4)The activity of polyurethane biofilm was determined after 7 days,20 days,40days,60 days and 90 days.The results showed that the ammonia oxidation rate decreased first and then increased gradually,while the nitrite oxidation rate decreased gradually.The ammonia oxidation rate and nitrite oxidation rate of polyurethane biofilm were 0.31 mg.g^-1.h^-1and 0.04 mg.g^-1.h^-1respectively at the end of culture cycle.(5)The number of ammonia oxidizing archaea,ammonia oxidizing bacteria and nitrite oxidizing bacteria in polyurethane biofilm at different culture stages was determined by real-time PCR.The results showed that the number of nitrifying microorganisms showed downward-upward-downward trend with the culture cycle.The number of ammonia oxidizing archaea was 1-4 orders of magnitude higher than that of AOB and NOB.(6)The results of high-throughput sequencing showed that the dominant phyla in the samples were Prteobacteria,Bacteroidota,Planctomycetota and Actinobacteriota;at the genus level,there were significant differences in the microbial communities of the five samples,and the most abundant genera were Nitrococcus,Pseudomonas,mariniradius and microscillaceae.