Study On Using Algal-bacterial Biofilm For Aquaculture Wastewater Treating And Its Potential For Feed Utilization

Aquaculture wastewater discharge is large,and the dissolved oxygen is generally maintained at 4～8 mg/L.The treatment by traditional biological denitrification technology is quite costly and easy to lead to nitrate accumulation.The aerobic denitrification technology can realize denitrification under aerobic conditions,so it is gradually becoming a hot spot for research and application of new denitrification technology for aquaculture wastewater.Microalgae show great potential in the resource utilization of pollutants in wastewater.In this study,aerobic denitrifying bacteria enriched by acclimation of activated sludge formed biofilms coupled with chlorella,thus constructing an algal-bacteria symbiotic biofilm system to treat aquaculture wastewater.Its process parameters and operation effect were investigated,and the biofilm composition and its extracellular polymeric substances（EPS）properties were studied,the long-term stability of the system was investigated,and the nitrogen and phosphorus elements removal pathways and microbial community changes analyzed.Finally,the potential for feeding utilization of algal-bacterial biofilm was evaluated in order to provide some theoretical and technical references for the efficient treatment of aquaculture wastewater and the recovery of carbon,nitrogen and phosphorus elements.The main conclusions are as follows:（1）The activated sludge was domesticated with nitrate as the only nitrogen source and kept high DO（5～6 mg/L）.The results showed that when the domestication experiment lasted for 24 days,the effluent TN and PO₄^3--P average removal rates stabilized at 89.09%and 54.51%,respectively,at which time the aerobic denitrification sludge was domesticated successfully.When the original sludge and aerobic denitrification sludge are mixed in the ratio of 1:4,the system has the best nitrogen removal effect.The relative abundance of common aerobic denitrifying bacteria such as Flavobacteria,Rhodococcus,Paracoccus and Pseudomonas which were almost 0 in the original sludge increased to 5.92%,2.87%,1.36%and 1.06%,becoming the dominant bacteria genus in the domesticated sludge.（2）The characteristics of Chlorella biofilm,activated sludge biofilm and algal bacteria biofilm and the removal effect of N and P in aquaculture wastewater were compared and analyzed.The results show that the algal-bacterial symbiotic biofilm system has the best removal effect on N and P element in aquaculture wastewater.The effluent concentrations of TN and PO₄^3--P were 0.43～0.66 mg/L and 0.024～0.036 mg/L,respectively.Environment scanning electron microscopy photograph showed that microalgae and bacteria are closely bonded on the algal biofilm.The elemental analysis showed the relative contents of C,H,N and S were 35.93%,6.4%,6.38%and 0.09%,respectively,which were higher than other two groups,and the results of Fourier transform infrared analysis also showed that the absorption intensity protein,polysaccharide and lipid characteristic peaks were all the highest in algal-bacterial symbiotic biofilm,which indicated that algal-bacterial symbiotic system enhanced and improved the assimilation of C,N and S elements in water by algal-bacterial symbiotic biofilm and the relative contents of nutrients such as protein and polysaccharide in biofilm.EPS characteristic analysis results showed that algal-bacterial symbiotic system enhanced the activity and catabolism of microorganisms.（3）The algal-bacterial symbiotic sequencing batch biofilm system（SBBR）was constructed and its operating parameters were optimized,investigating the treatment effect of aquaculture wastewater through long-term operation of the system,studying the nitrogen and phosphorus removal pathways by the system nitrogen balance calculation and continuous fractional determination of phosphorus components in the biofilm,and explore changes of biofilm microbial communities by high-throughput sequencing.The results showed that the optimum conditions are light intensity at 4000 lux,temperature at 25℃,aerobic reaction time at 11h.After long-term operation for 40 days,Chlorella microalgae grows well in the biofilm,and its relative content was 0.86 g/g VSS.The average removal rates of TN,PO₄^3--P and COD in this system were stable at 97.48%,98.66%and 92.22%,respectively.About 49.33%of N elements were removed by microbial assimilation and50.67%were removed by denitrification in the simulated wastewater.Biological assimilation and absorption were the main removal pathway of P element,chemisorption and precipitation were the secondary removal pathway.Common aerobic denitrifying bacteria which belongs to Flavobacteria（15.13%）and Paranococcus（11.18%）are the dominant bacteria in this system.Aquimonas,Nodosilinea PCC-7104 as common bacterium in Phycosphere and Azoarcus were also dominant bacteria in this system.（4）The content of protein,amino acid,mineral elements and other nutrient components of algal-bacteria biofilm were determined to explore the fodder potential of algal bacteria biofilm from the perspective of nutrient components,At the same time,zebrafish were fed with algal-bacterial biomass instead of fish meal to evaluate the feasibility of algal biofilm feeding.Finally,the economic and environmental benefits of algal-bacteria biofilm feeding utilization were estimated.The results showed that algal-bacteria biofilm was rich in various nutrients and mineral elements,and the relative contents of oil,polysaccharide,protein,carotenoid and chlorophyll were 14.96%,6.71%,29.23%,0.19%and 0.62%,respectively.Compared with the reference protein,its essential amino acid index（EAAI）values are all greater than 0.95.Zebrafish grew well in the experiment which indicated that algal-bacterial symbiotic biofilm could replace part of aquatic feed.Treating 1 ton of aquaculture wastewater by algal-bacterial biofilm can harvest 158.2 g biomass bait,saving 0.514～1.19￥ of aquaculture cost while fixing about 208.4 g CO2,assimilating about 10.09 g nitrogen and 1.376 g phosphorus which can bring environmental benefits.Purifying aquaculture wastewater by algal-bacterial symbiotic biofilm can degraded pollutants while harvesting high nutritional value biomass bait which has good economic and environmental benefits.