Analysis Of Nitrogen And Phosphorus Removal Effect Of Optimized Microalgae In The Tail Water Of Intensive Culture

At present,the aquaculture in China is developing rapidly in the direction of high density and intensification.At the same time,the associated treatment of aquaculture tail water has been paid more and more attention.The cost of the traditional tail water treatment method is high,but the biological treatment method is economical and effective.Zero-water exchange bioflocs-based culture is a new intensive culture model in the aquaculture industry,which can effectively control harmful nitrogen such as ammonia nitrogen（NH₄⁺-N）and nitrite nitrogen（NO₂^--N）in the aquaculture water environment at a very low level,but there is a prominent problem of excessively high concentrations of nitrate nitrogen（NO₃^--N）and phosphate(PO₄^3--P)during the process.Based on the selection principles of being common,benefit,efficiency in absorbing nitrogen and phosphorus,being easy to collect and having economic value in aquaculture,we selected Cyanophyta Spirulina platensis,Diatoms Chaetoceros muelleri,Thalassiosira sp.and Chlorophyta Dunaliella sp.for the research.Their growth status and removal efficiency of nitrogen and phosphorus,in the tail water with high-concentration nitrate nitrogen and phosphate,were compared and evaluated.Oscillatoria sp.was used as the reference strain of Spirulina platensis during the research.The content and main results of this research are as follows:1)Two water environments,i.e.,intensive culture tail water and conventional laboratory microalgae culture medium,were set up in the experiment.By comparing the growth status of microalgae in the tail water with high concentrations of nitrate nitrogen and phosphate,the survival potentials and nitrogen and phosphorus removal effects of microalgae were verified.The results showed that,compared with the other three microalgae species,Spirulina platensis had the best removal effect on nitrogen and phosphorus in the intensive culture tail water,and the maximum removal rates of NO₃^--N,total inorganic nitrogen（TIN）and PO₄^3--P were 79.60%,46.06%and 98.55%,respectively.The corresponding concentration values decreased from 130.04 mg·L^-1,130.85 mg·L^-1and 10.23 mg·L^-1to 26.53 mg·L^-1,70.58 mg·L^-1and 0.15 mg·L^-1,respectively,and the absolute values of the removals were 103.51 mg·L^-1,60.27 mg·L^-1and 10.08 mg·L^-1,respectively.The rates and effects of Chaetoceros muelleri,Thalassiosira sp.and Dunaliella sp.removing nitrogen and phosphorus in tail water of intensive culture were slower or weaker than that of Spirulina platensis.The concentrations of NO₃^--N and TIN in the intensive culture tail water and the laboratory microalgae culture medium treated by Chaetoceros muelleri and Dunaliella sp.showed downward trends,but it may take a longer time for them to decrease to the lower level.Chaetoceros muelleri had a removal effect of more than 98%on PO₄^3--P in the tail water.Chaetoceros muelleri and Thalassiosira sp.had very good purification effects on the water with low concentrations of nitrogen and phosphorus.2)The linear relationship between microalgae cell number,dry weight and the contents of nitrogen,phosphorus and carbon was fitted,and the potentials of microalgae to absorb nitrogen and phosphorus in the tail water were calculated.The establishment of fitting equation provided a mathematical model for comparing the nitrogen and phosphorus absorption capacities of different microalgae.The results showed that the contents of nitrogen,phosphorus and carbon per unit cell number and per unit dry weight of Spirulina platensis were much higher than that of Oscillatoria sp.and Chaetoceros muelleri,and it indicated that Spirulina platensis needed to absorb more nutrients in the water environment to support its own growth and proliferation.It also verified the nitrogen and phosphorus absorption ability of Spirulina platensis from the dimension of material cycle.In theory,if Spirulina platensis was inoculated into the zero-water exchange bioflocs-based culture pond with volume of 1000 m³with the initial concentration of 10³cells·m L^-1,when the concentration of Spirulina platensis increased to 10⁶cells·m L^-1,it was calculated that31.42 kg of nitrogen and 4.97 kg of phosphorus could be absorbed to support its own growth and proliferation.3)The effect of ecologically synergistic purification of culture tail water by microalgae and bacteria together was evaluated.Spirulina platensis selected in the previous research was separately treated with Bacillus subtilis YT7210 and Rhodococcus rubrum HDRR2Y to co-process the intensive culture tail water.It was found that the microalgae and bacteria had obviously synergistic effect on nitrogen and phosphorus removals in the tail water environment.Among them,the NH₄⁺-N and PO₄^3--P removals were faster,and the removal rate of 100%were reached on the 6th day of the experiment.The NO₃^--N and TIN removals were shown,and the trend was generally decreasing.The NH₄⁺-N and PO₄^3--P removals for group added with bacteria were less effective than that for the group added with bacteria and microalgae.