Process Design Of The Constructed Wetland For Purification Of Farmland Drainage

Farmland drainage is one of the main output way for agriculturalnon-point source pollution. It is of high quantity, and is a stable pollutionsource. Large amounts of farmland drainage without any purificationdirectly discharge into receiving waters, It not only wastes waterresources, but also makes receiving waters pollution. Constructed wetlandis a new ecological method to purify wastewater, and is of significantenvironmental benefits. The northeast farmland drainage was chosen tostudy. Based on the characteristics of water quality and water quantity offarmland drainage, combined with the effluent quality requirements, ahybrid subsurface flow constructed wetland was designed. Devices in thelaboratory were built. Through filtering design parameters, the processand parameters of constructed wetland sewage treatment were designed,and a layout of the structures was drawn. The main conclusions wereshown as follows:1. From types of constructed wetlands: The part constructed wetlandwas composed of aerobic and anaerobic subsurface-flow constructedwetlands. The subsurface flow pattern of aerobic subsurface-flowconstructed wetland is divided into vertical flow and horizontal flow. Anaerobic constructed wetland can raise water temperatures of3℃,which is beneficial to removal of contaminants. The wetland area can bereused. Aerobic constructed wetland takes full advantage of the benefitsof vertical and horizontal subsurface flows that is better than a single flowpattern of constructed wetlands.2. From substrates in the constructed wetlands: Constructed wetlandbeds are filled with several materials: soil, zeolite, vermiculite, slag andgravel. Grain size, porosity and thickness of materials are designed.3. From plants in the constructed wetland: Aerobic subsurface flowconstructed wetland planted Phragmites. It is typical of wetland plants,strong vitality, and high using value. A design parameter for Phragmitesis planting density.4. From parameters of constructed wetlands: Design parameters arecalculated by model and formula. Then the optimization of hydraulicloading is worked with the artificial neural network that is trained bydates of domestic and international subsurface constructed wetlands. Theresult is two hydraulic loadings:0.03~0.037m3/(m2·d) and0.078~0.083m3/(m2·d). Finally, validation of the constructed wetlandsand determining0.078~0.083m3/(m2·d) as a hydraulic parameter valueengineered is by experiment.5. From the main structures: The constructed wetland process includes:grille, regulating reservoir and constructed wetland. The constructed wetland is integrated anaerobic subsurface flow with aerobic subsurfaceflow that is composed of horizontal subsurface flow and vertical flow.6. From engineered: The constructed wetland system is more suitablefor Northeast farmland drainage featured by low water temperature andobviously varies water quantity. Each part was engineered, which mainlyincludes: selecting and calculating design parameters, mapping a relevantprocess elevation plan. It aims to provide a technical guidance andreference for drainage purification projects.