Study On Process Optimization And Practice Application Of Solid-liquid Separation Tower Coupled Vertical Subsurface Flow Constructed Wetland In Treating Tailwater Of Pond Juan Yang System

Intensive pond aquaculture inevitably leads to pollutants accumulation in the aquaculture pond.Moreover,the discharge of untreated aquaculture tailwater could cause a series of ecological and environmental problems,such as eutrophication of aquatic systems.In comparison,the pond Juan Yang mode has several practical advantages,including ecologically friendly,resource conservation,and better economic effectiveness,which is of great significance to the healthy and green development of freshwater aquaculture in China.As one of the critical steps of Juan yang processes,the development of efficient and low-cost tailwater treatment technology is very important for the sustainable development of the pond culture model.Vertical subsurface constructed wetland has the advantages of high efficiency,low cost and easy management,which is very suitable for the treating tailwater of Juan yang system.This study systematically analyzed the characteristics of pollutant content in pond Juan Yang tailwater,investigated the effect of solid-liquid separation tower-vertical subsurface constructed wetland technology on pond Juan Yang tail water treatment.Based on this technology,the optimized intermittent aeration strategy was successfully applied to the treatment of pond tail water.The main research conclusions are as follows:（1）The concentrations of total nitrogen（TN）and total phosphorus（TP）in the Juan Yang tail water of the ponds were 12.65～15.12 mg/L and 4.74～6.89 mg/L,respectively.The concentrations of ammonium nitrogen（NH₄⁺-N）and nitrous nitrogen（NO₂^--N）were 3.91～5.03 mg/L and 0.26～0.41 mg/L.Its content is significantly higher than that of traditional pond aquaculture tail water.respectively,indicating that the captive system and the sewage device can effectively collect the nitrogen produced in the captive process phosphorus and other pollutants.（2）The sedimentation of the solid-liquid separation tower for 1 h can realize the removal of nitrogen and phosphorus in the tail water of Juan Yang ponds to a large extent.After 1 h of sedimentation treatment,the average removal rates of TN and TP in the pond tail water were 35.05%and 62.85%,respectively.After sedimentation,the supernatant of tail water is treated by continuous aeration constructed wetland,the removal rate of NH₄⁺-N and NO₂^--N can reach 87.91%and 92.57%,respectively,but due to the lack of carbon source,the denitrification process is inhibited,and the removal rate of TN Only 38.68%.Using volcanic rock as the substrate of continuous aeration constructed wetland,the removal rate of TP can reach 55.41%.（3）In order to improve the TN removal efficiency of constructed wetlands,the effect of dissolved oxygen（DO）concentration on the denitrification effect and denitrification pathways of intermittent aerated constructed wetlands under different influent carbon-nitrogen ratios（COD/N）was explored through a small-scale mechanism study.The results show that increasing the DO concentration during aeration can effectively improve the NH₄⁺-N removal efficiency of constructed wetlands,but the optimal DO supply for TN removal should be adjusted according to the influent COD/N.In addition,the nitrogen removal mechanism of the system was analyzed from the perspective of microorganisms,and it was found that under various DO concentrations and influent COD/N conditions,simultaneous nitrification and denitrification（SND）were the main nitrogen removal pathways in intermittent aeration constructed wetlands.When COD/N is high,simultaneous short-range nitrification and denitrification（NH₄⁺→NO₂^-→N₂）may occur,and SND efficiency is easily damaged by high DO concentration when COD/N is low.（4）Applying the optimized intermittent aeration strategy to the actual pond Juan Yang tail water treatment,the TN removal rate of the constructed wetland can be increased from 38.68%to 49.47%.Among them,after adjusting the aeration mode and DO concentration of A1,the SND efficiency of A1 was enhanced,and the TN removal rate of A1 could reach 27.79%.After the pond Juan Yang tail water is treated by the solid-liquid separation tower-intermittent aeration constructed wetland process,the average concentrations of NH₄⁺-N,NO₂^--N and soluble phosphate(PO₄^3--P)in the reused tailwater are 0.45 mg/L,respectively,0.02 mg/L and 0.48 mg/L,the removal rates of TN and TP can reach 68.04%and 81.49%,which can basically realize the recycling or discharge of pond Juan Yang tailwater.