Research On Drainage And Purification Capacity Of Ecological Canal Ponds In Irrigation District

Starting from soaking with the paddy field in the north,multiple regressions are required throughout the entire production cycle.Due to excessive use of fertilizers,the degree of regressing pollution in the paddy irrigation area has increased,and the non-point source pollution load in farmland has become the main contributor to water eutrophication.After precipitation or irrigation,nitrogen,phosphorus and other chemical fertilizers applied in paddy fields enter nearby water bodies through surface runoff,recession and underground infiltration,causing water pollution.Ecological canal pond utilize the self-organization and self-regulation capabilities of ecosystems to achieve pollutant reduction,which is an effective method for in-situ reduction of non-point source pollution in farmland and can fully utilize local ponds and biological natural resources.However,the control of non-point source pollution in farmland requires to definite the law of the water withdrawal amount,withdrawal time,and withdrawal load concentration.This paper took Wangwenzhuang Irrigation District in Tianjin as the research object,used drainage ditches as the monitoring unit to carry out monitoring of drainage and pollutant concentration,and used paddy fields to control drainage and rainfall runoff.By measuring the amount of water quantity and quality combined with rainfall observation,the temporal and spatial changing-laws of the amount of water quantity and quality were obtained,providing a basis for formulating a scheme for reducing return flow pollution in paddy fields.Reducing the concentration of pollutants in farmland drainage and achieving the“standard discharge”of farmland drainage are the requirements for ecological environmental protection.There are many studies on aquatic plant combined with ecological canal pond to handle polluted water bodies.However,aquatic plant are selected differently due to regional differences.There are fewer studies on aquatic plant that are suitable for the climate conditions of northern coastal areas and have the ornamental value to beautify river water bodies.Aquatic plant such as scallion validus,canna,iris,and lythrumsalicaria are considered to be the preferred aquatic plant because of their long growth cycle,high survival rate,salt tolerance,and high ornamental properties.This study selected four aquatic ornamental plants,including canna,scallions,chicoris,and iris,to study the purification effect of individual plants and different plant combinations respectively.According to the water quality and quantity of the drainage of paddy fields in the project area,the ecological canal pond was designed and optimized.According to the regional land use status,two kinds of ecological canal ponds were designed and compared,respectively,the shallow canal pond and deep canal pond.The main conclusions of this paper are as follows:（1）The total amount of paddy field drainage in irrigation area showed a decreasing trend,and the overall pollutant content of surface water was higher than the levelⅣrequirement.The pollutant concentration in three controlled drainage and one-rainstorm runoff drainage showed a decreasing trend.The initial control drainage volum was maximum,which was 100m3/mu.The second control drainage volum during the tillering stage drainage was equal,together with the rainstorm runoff,so the total of drainage volum of the second time was maximum,which was 152.3 m3/mu.The relationship of the controlled drainage volume of the research area plot showed a gradual decreasing trend.The drainage relationship and law during the rice planting process were significantly influenced by the crop growth cycle and weather,and the drainage amount will also changed with weather.（2）By calculating the water pollution load of the water withdrawal during different periods,it could be seen that the changing-law of the concentration of COD,ammonia nitrogen and total phosphorus showed a decreasing trend.Among them,the highest COD during the first water withdrawal was 55mg/L,the highest ammonia nitrogen was 3.4mg/L,and the highest total phosphorus was 0.5mg/L during the soaking period.The amount of COD,ammonia nitrogen and total phosphorus showed a downward trend,and the total amount of drainage pollutants was at a high point for the first time,with COD 3.56 kg/mu,ammonia nitrogen 0.06 kg/mu,and total phosphorus 0.06 kg/mu,respectively.（3）Among the aquatic plant preferrence,the aquatic plant with the most stable removal rate of TP and NH₄⁺-N in the paddy field water withdrawal by a single plant was scallion validus,and the difference was the smallest among the five treatments.The highest removal rate of TP with canna at the early stage was 75%,the highest NH₄⁺-N removal rate of scallion at the early stage was 56%,while the removal effect of NH₄⁺-N and TP was relatively poor.The removal ability of single plant for NH₄⁺-N and TP in water was in the order of scallion validus>canna>iris>lythrumsalicaria.（4）The combination of scallion validus and canna had the best removal effect on TP and NH₄⁺-N in the paddy field water withdrawal,with the smallest difference and the most stable in the five repetitions.The combination of canna and iris had the highest TP removal rate,with 77%.The combination of scallion validus and iris also achieved the higher removal rate,with 71%,however,the removal rate decreased with a large difference at the later stage,resulting in a decrease in removal efficiency.The removal ability of NH₄⁺-N and TP in water with plant combinations was in the order of scallion validus+canna>scallion validus+iris>canna+iris.Scallion validus and canna obtained the best TP and NH₄⁺-N absorption capacity.In aquatic plant,the removal capacity of NH₄⁺-N and TP in water with a single plant and three plant combinations was as follows:scallion validus+canna>scallion validus+iris>canna+iris>scallion validus>canna>iris>lythrumsalicaria.Among them,the combination of scallion validus and canna had the most stable and best adsorption effect on NH₄⁺-N and TP.（5）In the optimization design of ecological canal pond,through data comparison and analysis,the treatment efficiency of shallow ecological pond under the same area was three times that of deep ecological canal pond,making it the optimal solution.The shallow ecological pond combining biological filtration and plant absorption had a strong adaptability to the water quality and quantity change when water withdrawal into the paddy field.Taking the paddy field drainage ecological canal pond in Wangwenzhuang Irrigation District as an example,the total area was 786 000 m²,with the average water depth of the system of 0.5 m,the hydraulic retention time of about 16.5 h,and the corresponding surface hydraulic load of about 0.4 m³/（m²·d）.According to the distribution of paddy field in the irrigation area,paddy field per mu was equipped with about 30 m²ecological canal pond to treat itself water withdrawl and complete in-situ reduction.The shallow ecological canal pond performed well in the removal of COD,NH₄⁺-N and TP,and improved the purification efficiency of wastewater in the purification zone.It has positive significance in the sustainable development of paddy field water withdrawl reduction and efficient utilization of water resources in the Wangwenzhuang Irrigation District.