Study On Purification Of Nitrogen And Phosphorus Nutrients From Seawater By Chlorella Salina And C.salina-Mercenaria Mercenaria Coupling

The impact of aquaculture industry development on the environment has become a common concern,among which the treatment of inorganic nitrogen and phosphorus is a technical difficulty in the treatment of mariculture wastewater.In this study,the absorption process and efficiency of nitrogen and phosphorus nutrients in seawater by Chlorella under different inoculation densities and different treatment times were studied based on the characteristics of efficient absorption of nitrogen and phosphorus nutrients by microalgae.In order to provide reference for the establishment and optimization of aquaculture seawater purification process,the purification effect of Chlorella and hard clams coupling treatment process on nitrogen and phosphorus nutrients in seawater was studied,and the changes of microbial community structure were also discussed,.1.Different inoculum densities（1×10⁶,2×10⁶,3×10⁶ cells/ml）were set up to compare the absorption rule of nitrate and phosphate and the growth status of Chlorella at different salinities（16 and 26）.The results showed that the biomass of algal dry weight was positively correlated with the initial algal density,and the maximum algal dry weight was reached at 3×10⁶ cells/ml.Multiple linear regression analysis showed that both the increase of salinity and algal inoculation density promoted the growth of Chlorella,and the influence weight of algal inoculation density was greater.Within the time range of 24-120 h,the higher Chlorella density（1×10⁶-3×10⁶ cells/ml）in the 16 S group,the better the NO₃^-removal effect was.Within the time range of 24-96 h,the higher Chlorella density（1×10⁶-3×10⁶ cells/ml）in the 26 S group,the better the NO₃^-removal effect.It showed that the higher the algal density,the better the NO₃^-removal effect of Chlorella in the early and middle period of the experiment（24-96 h）.In the later period of the experiment（120 and 144 h）,the NO₃^-removal rate of the higher algal density group（2×10⁶ and 3×10⁶ cells/ml）tended to be consistent due to the depletion of NO₃^-.By comparing the removal effect of PO₄^3-in different algal density groups in the same salinity group,it was found that the higher the density of Chlorella（1×10⁶-3×10⁶cells/ml）,the better the removal effect of PO₄^3-in the 24 h to 144 h time range.Correlation analysis results showed that the NO₃^-and PO₄^3-removal rates of Chlorella increased with the increase of the number of algae,and the two showed a significant positive correlation.Therefore,the increase of salinity and algal inoculation density promoted the removal of NO₃^-and PO₄^3-by promoting Chlorella growth.In the 16 S and 26 S groups,the NO₃^-removal rate of the lowest algal density group（1×10⁶ cells/ml）always decreased from 24 h to 144 h,and the optimal removal effect may not be reached until the end of the experiment.In groups 16 S and 26 S,the NO₃^-removal rates of groups with higher algal density（2×10⁶ and 3×10⁶ cells/ml）reached the highest at 120 h,and there was no significant difference compared with the removal rates at 144 h.PO₄^3-removal rates of 1×10⁶,2×10⁶,26 S-3×10⁶ cells/ml groups in 16 S and 26 S groups all reached the highest at 120 h,which had no significant difference compared with 144 h,and all reached the highest PO₄^3-removal rates in3×10⁶ group.The results indicated that there were differences in the time points for achieving the best NO₃^-removal effect in different algal density groups,while there was no difference in the time points for achieving the best PO₄^3-removal effect.The removal rates of NO₃^-and PO₄^3-in the two salinity groups reached the highest at 120 h,and the2×10⁶ and 3×10⁶ cells/ml groups achieved the best NO₃^-removal effect,and the 3×10⁶/ml group had the best PO₄^3-removal effect.2.Different inoculum densities（5×10⁵,1×10⁶,2×10⁶,3×10⁶ cells/ml）were set to compare the removal effect and growth status of Chlorella on different forms of nitrogen and phosphorus nutrients in seawater with different salinities（16 and 26）.The results showed that low inoculum density（5×10⁵ and 1×10⁶ cells/ml）was conducive to maintaining high specific growth rate and growth status of Chlorella,and increasing salinity（16-26）promoted the growth of Chlorella.In group 16 S,the removal rates of NH₄⁺,NO₃^-and TDN in groups with higher algal density（1×10⁶,2×10⁶,3×10⁶ cells/ml）were 85.31%-85.87%,60.85%-61.66%and 54.98%-57.64%,respectively.There was no significant difference in the same index between groups.In 26 S,the removal rates of NH₄⁺,NO₃^-and TDN in the high density algae group（1×10⁶,2×10⁶,3×10⁶ cells/ml）were 87.23%-88.16%,56.70%-57.79%and 53.31%-54.62%,respectively.There was no significant difference in the same index among the groups.In the same salinity（16 or26）,the increase of algae density（1×10⁶,2×10⁶,3×10⁶ cells/ml）had no effect on the removal of NH₄⁺,NO₃^-and TDN.The removal rates of PO₄^3-and TDP were97.72%-99.71%and 98.31%-99.53%in the 16 S high density algae group（2×10⁶ and3×10⁶ cells/ml）,respectively,and there was no significant difference in the same index between the groups.The removal rates of PO₄^3-and TDP were 98.90%-99.05%and96.52%-96.83%in the high-density algae group（2×10⁶ and 3×10⁶ cells/ml）for 26 S,respectively.There was no significant difference in the same index between the groups.The results showed that the increase of algae density（2×10⁶ and 3×10⁶ cells/ml）at the same salinity（16 or 26）had no effect on the removal of PO₄^3-and TDP.Therefore,suitable algal inoculation density should be selected to improve the effect of nitrogen and phosphorus removal and save cost.When the initial inoculation density was 1×10⁶cells/m L,the removal effect of nitrogen in seawater was the best,and the removal effect of phosphorus was better.Therefore,1×10⁶ cells/m L was used as the optimal density for chlorella inoculation.It was found that increasing salinity（16-26）had no significant effect on the removal of NO₃^-,TDN,PO₄^3-and TDP by Chlorella,while increasing salinity increased the removal effect of NH₄⁺by Chlorella.Therefore,C.salina can efficiently remove nitrogen and phosphorus nutrients from seawater with salinity of 16～26.The results showed that C.salina had obvious sequential characteristics in the uptake of the three forms of nitrogen.The first is the rapid absorption of NH₄⁺.When the concentration of NH₄⁺drops below 0.2 mg/L,Chlorella begins to absorb NO₃^-and finally NO₂^-.3.The C.salina and M.mercenaria was used to purify nitrogen and phosphorus in seawater with different salinities（16 and 26）.The experiment was divided into chlorella stage and mercenaria stage.The results showed that the growth of Chlorella in 26 S group was significantly better than that in 16 S group（P<0.05）.The removal rates of NO₃^-,TDN,PO₄^3-and TDP in group 26 S were significantly higher than those in group16 S（P<0.05）.In the hard clam stage,the clearance rate and water filtration rate of Chlorella in 26 S group were significantly higher than those in 16 S group（P<0.05）.The concentration of NH₄⁺,PO₄^3-,TDN and TDP decreased steadily during the algal filtration period,and the removal effect was the same as that of the algal blank group.However,the removal effect of NO₃^-in the clam group was not as good as that in the algal blank group,which may be because the filtration feeding of clams reduces the concentration of chlorella and affects the absorption of NO₃^-by algae.During the experiment,the concentration of DON in the water increased,which may be due to the organic matter produced during the growth of algae.High-throughput sequencing technology was used to analyze the effects of chlorella stage and clams stage on microbial community structure in seawater.Alpha diversity analysis showed that filter feeding of clams significantly reduced microbial community diversity（P<0.05）.The analysis of microbial community structure at phyla level showed that proteobacteria were the main phylum in the initial stage.In chlorella stage,the abundance of Bacteroidetes increased significantly.In the clam stage,Bacteroidetes decreased and Proteobacteria abundance increased.The analysis of microbial community structure at the genus level showed that NS3a＿marine＿group had the highest abundance at the Chlorella stage,and Altermonas had the highest abundance at the clams stage.Further analysis of the relative abundance ratio of the main bacterial groups showed that the presence of heterotrophic denitrifying bacteria such as Flavobacteriaceae in Chlorella stage may lead to the accumulation of nitrite,and Altermonas in clam stage promoted the degradation and transformation of DON in the tail water.PCo A analysis showed that both Chlorella and clams affected the overall structure of microbial community in the seawater.The clustering of chlorella at the stage was different from that at the initial stage,while that of clams stage was similar to that at the initial stage.