| With the increasing demand for seafood,the aquaculture industry is rapidly expanding.A large amount of untreated marine aquaculture wastewater is directly discharged into the aquatic environment,containing nitrogen,phosphorus,and COD,which may lead to eutrophication of the water body and ultimately lead to water quality deterioration.At the same time,marine aquaculture wastewater also contains a large amount of unused antibiotics,posing a threat to the ecological environment and human health.In order to achieve the requirement of effective removal of pollutants in aquaculture wastewater before discharge and achieve the recycling of aquaculture wastewater,this article adopts a symbiotic system of bacteria and algae for treatment.Firstly,the gradient increases the salinity in wastewater,domesticates salt tolerant activated sludge,and provides a source of bacteria for the symbiotic system of bacteria and algae;Secondly,explore the impact of inoculation concentration of Chlorella on the treatment efficiency of marine aquaculture wastewater,and select the optimal inoculation concentration;Next,change the inoculation ratio of bacteria and algae,determine the optimal inoculation ratio of bacteria and algae by comparing the removal efficiency of pollutants,and construct a complete and efficient symbiotic system of bacteria and algae;Finally,the effect of lighting conditions on the treatment of wastewater by the symbiotic system of bacteria and algae was studied,and control experiments were conducted with light intensity and light to dark ratio as single variables to determine the optimal lighting conditions for the symbiotic system of bacteria and algae.Following conclusions are drawn:(1)After 90 d of salinity incubation,the normal freshwater activated sludge could be domesticated into salt-tolerant activated sludge.The removal rates of COD,ammonia nitrogen and phosphorus could reach 97.6%,86.45% and 30.78%,respectively.The increase in salinity had no effect on the removal of COD and denitrification of the system,but was detrimental to the growth of phosphorus-polymerizing bacteria in the system.(2)The optimum inoculation concentration of marine Chlorella was 6×106 cells/m L,with the removal rates of 70.0%,70.1%,95.3% and 52.3% for COD,ammonia nitrogen,phosphorus and SMX,respectively.Too high inoculation concentration would cause self-shading phenomenon of microalgae,increase the competition among microalgae,lead to the death of algal cells and reduce the removal efficiency of nutrients and SMX.Too low inoculation concentration would extend the experimental period,which is not conducive to the rapid construction of the bacterial-algal symbiosis system.(3)The symbiotic system of marine chlorella and activated sludge can effectively remove pollutants from mariculture wastewater.With the optimum inoculation ratio of 3(Chlorella maritima: activated sludge = 120 mg/L: 360 mg/L),the removal rates of COD,ammonia nitrogen,phosphorus and sulfamethoxazole could reach 93.2%,91.4%,93.6% and 93.1%,respectively.The higher bacterial-algal activity and EPS secretion made it easier to form bacterial-algal colloids in the anaerobic,anoxic and aerobic zones in order from the inside to the outside,which realized an efficient denitrification symbiotic system with the simultaneous presence of nitrifying bacteria,denitrifying bacteria and anaerobic ammonia oxidizing bacteria.Biodegradation is the main mechanism of SMX degradation in the symbiotic system,and the proteins and polysaccharides in EPS support the transport and diffusion of SMX,which is the follow-up force for further SMX degradation.A suitable ratio of bacteria to algae(inoculum ratio of 3)is beneficial for screening the system with higher abundance of functional bacteria for nitrogen and phosphorus removal and increasing the abundance of SMX degrading bacteria norak＿f＿Microscillaceae.(4)The optimal light intensity was determined to be 4500 lux,and the removal rates of COD,ammonia nitrogen,phosphorus and SMX were 94.9%,93.7%%,94.3% and 94.7%,respectively.Each pollutant was removed mainly during the light period,and the removal rate was lower during the dark period.Too low light intensity reduced the photosynthetic rate of Chlorella and too high light intensity limited the dark reaction phase in the Calvin Cycle,both of which reduced the activity of both and affected the EPS secretion.Light intensity caused significant differences in the microbial communities of the symbiotic system.Excessive light intensity reduces the abundance and diversity of microbial communities,with a gradual shift of bacteria in the system to the more light-tolerant Ruegeria and Stappia,decreasing the abundance of the denitrifying bacterium Marinicella and the sulfamethoxazole-degrading bacterium norank＿f＿Microscillaceae.However,it would increase the abundance of Planctomycetes and Cyanobacterias,which inhibit the genus of algal blooms.(5)The optimum light-to-dark ratio was determined to be 10:14,with removal rates of95.4%,95.4%,94.9% and 95.7% for COD,ammonia nitrogen,phosphorus and SMX,respectively.The light-to-dark ratio would limit the effective time of pollutant removal by the bacterial-algal symbiotic system.Too low light-to-dark ratios could delay the growth of Chlorella and too high light-to-dark ratios could lead to damage of Chlorella due to extended light exposure,both of which would inhibit the interaction between Chlorella and bacteria,reduce the activity of both and decrease EPS secretion.The light-dark ratio could affect the abundance and diversity of microorganisms and cause changes in the microbial community structure.Under suitable light conditions,it will increase the abundance of nitrifying or denitrifying bacteria such as unclassified＿f＿Rhodobacteraceae,Aestuariicoccus,Ruegeria,Defluviimonas and Denitromonas in the symbiotic system and promote the nitrification denitrification process of the system,which improve the denitrification efficiency.In addition,the abundance of the genera Chloroflexi and sulfamethoxazole-degrading bacteria norank＿f＿Microscillacea,which have good degradation ability for antibiotics,would also be increased. |
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