Recovery Of Nitrogen And Phosphorus From Aquaculture Wastewater By Saline Farmland Drainage Combined With Modified Biochar

With the rapid development of aquaculture in Xinjiang,the emission of aquaculture waste is huge,and excessive nitrogen and phosphorus are brought into the external environment,which increases the environmental burden of the Tarim River Basin.As a mature and efficient nitrogen and phosphorus recovery method,MAP precipitation method is widely used to treat nitrogen and phosphorus in aquaculture wastewater because of its good treatment effect,rapid reaction,simple operation and economic benefits.However,the scarcity of conventional magnesium source required for map reaction is an important factor restricting the large-scale use of struvite precipitation method.Therefore,it is urgent to find an alternative magnesium source that can reduce the cost of guano precipitation.In this paper,saline farmland drainage in southern Xinjiang was used as an alternative magnesium source,and combined with modified fecal biochar to simultaneously recover nitrogen and phosphorus from aquaculture wastewater.The optimum reaction conditions for the recovery of nitrogen and phosphorus from aquaculture wastewater by high salt drainage were discussed,and the influence of interfering ions on the recovery efficiency of nitrogen and phosphorus was analyzed;In order to further improve the recovery efficiency of nitrogen and phosphorus,four kinds of magnesium modified biochar were prepared:G-PBC,XS-PBC,HS-PBC and Y-PBC.Combined with the nitrogen and phosphorus adsorption properties of various modified biochar,dry modified biochar（G-PBC）was selected as the raw material of coupling reaction,and combined with salinized farmland drainage to simultaneously recover nitrogen and phosphorus from aquaculture wastewater,and the best process conditions were found.The main results are as follows:（1）The recovery experiment of nitrogen and phosphorus from nitrogen and phosphorus wastewater was carried out by using farmland high salt drainage as magnesium source.The nitrogen and phosphorus recovery efficiency of high salt drainage and conventional magnesium source was compared,the main factors affecting its recovery were discussed,and the optimal reaction conditions for nitrogen and phosphorus recovery from high salt drainage were obtained through orthogonal test.The comparative analysis of nitrogen and phosphorus recovery with conventional MgCl₂ magnesium source shows that when p H=10,the efficiency of nitrogen and phosphorus recovery from high salt drainage is the highest,which is slightly higher than the reaction p H required by Mg Cl₂ magnesium source to achieve the maximum recovery;The effects of p H（8,9,10,11）,Mg:P molar ratio（1.0,1.5,2.0,2.5）and N:P molar ratio（1.0,1.5,2.0,2.5）on phosphorus recovery from high salt drainage magnesium source were investigated by L₃⁴orthogonal test.SPSS analysis results showed that p H and Mg:P molar ratio had significant effects on nitrogen and phosphorus recovery.The optimum reaction conditions for ammonia nitrogen recovery from high salt drainage were p H=9,n（Mg）:n（P）=2.5,n（N）:n（P）=4;The optimum reaction conditions of phosphorus are p H=10,n（Mg）:n（P）=2.5,n（N）:n（P）=4;Ions in the reaction system of high salt drained magnesium source have a restrictive effect,and K⁺,Cl^-,SO₄^2-can inhibit the recovery of ammonia nitrogen.HCO₃^-and SO₄^2-ions inhibited the recovery of total phosphorus,while Ca²⁺ions promoted the recovery of phosphorus;XRD and SEM-EDS analysis show that the product of nitrogen and phosphorus recovery from high salt drainage is mainly guanite,mixed with impurities such as apatite and clay minerals.（2）Since the concentration of magnesium ion in farmland drainage can only recover about 30%of nitrogen and phosphorus in simulated wastewater,in order to reduce the dosage of saline alkali water and improve the recovery rate of nitrogen and phosphorus,four kinds of biochar（G-pbc,XS-pbc,HS-pbc and Y-pbc）were prepared.Through isothermal adsorption experiment and microstructure characterization,G-pbc was selected as the best modified biochar for subsequent nitrogen and phosphorus recovery.The nitrogen and phosphorus recovery effect of farmland drainage coupling modified biochar with magnesium ion concentration of 1.0 g·L^-1 was further studied,and the reaction conditions and influencing factors were discussed.The results showed that the nitrogen and phosphorus content and p H in the coupling system changed greatly within 20 minutes before the reaction,and the recovery rate of nitrogen and phosphorus was high.After 20 minutes,the nitrogen and phosphorus content increased first and then decreased,and finally stabilized until the end of the reaction.The recovery rate of nitrogen and phosphorus can reach 80%with the addition of 0.5%modified biochar;DOM fixed part of ammonia nitrogen in the reaction system by electrostatic adsorption;The recovery effect of total phosphorus was reduced by complexation.The optimum conditions for nitrogen and phosphorus recovery from farmland drainage coupled modified biochar are as follows:p H value of 10,reaction time of 20 min and 0.5%G-pbc addition.Under the modified conditions,the recovery rate of nitrogen and phosphorus can reach more than 80%.（3）By establishing the reaction system of salinized farmland drainage combined with modified biochar to recover nitrogen and phosphorus from aquaculture wastewater,the whole process of nitrogen and phosphorus recovery was evaluated by using the life cycle assessment（LCA）system,focusing on the analysis of characteristic indicators in each stage.It was concluded that the total impact of each unit on the environment was the coupling reaction unit>biochar preparation unit>salinized farmland drainage unit.Data sensitivity analysis was used to analyze the components of the coupling reaction unit which caused heavy environmental burden.The analysis results showed that the environmental pressure caused by aquaculture wastewater accounted for 50%of the coupling reaction unit.According to the LCA evaluation results,the original reaction system was optimized to reduce the environmental pressure caused by the coupling reaction.In conclusion,it is feasible to use high salt drainage coupled modified biochar to recover nitrogen and phosphorus from aquaculture wastewater,and the recovered biochar and guano stone mixture can be used in cotton field soil.This study combines the treatment of salinization with the utilization of aquaculture waste resources,and studies the process system of the organic combination of nitrogen and phosphorus resources recycling and environmental pollution treatment.The research results are of great significance to improve the current aquaculture wastewater treatment technology.