Farmland water purification in Yellow River irrigation area based on near-natural eco-circulation system

The Yellow River irrigation area in Ningxia is one of the important river sections affecting the water quality of the Yellow River. The main pollution source is the farmland backwater in the irrigation area, and the water quality is generally lightly polluted. In order to implement the comprehensive management of agricultural non-point source pollution, it is necessary to strictly control the discharge of farmland backwater into the Yellow River and strengthen the construction of ecological interception and purification facilities. This study investigated the effects of water quality biological purification technology and near-natural ecological circulation system technology on the water purification in the farmland. In 2022, in a pond at the end of the first drainage ditch of Zhongwei, the study comprehensively used the constructed wetland water quality biological purification technology and the near-natural ecological circulation system technology, selected suitable aquatic and terrestrial plants, and used eutrophication pollution factors as the core indicators. The near-natural eco-circulation system included sediment reuse, selection and growth control of aquatic organisms, fillers for purification zones, construction of bird shelters, and reuse systems for purified water irrigation. The system was constructed by the followed steps. First, after the Yellow River water was channeled through irrigation canals at all levels to farmlands, the return water flowed into constructed wetlands via drainage ditches at all levels. Through eco-biological purification processes involving aquatic plants, aquatic animals, and microorganisms, a small portion of the water meeting the standards was reused for irrigation, while most of it was discharged back into the Yellow River for recycling in the lower reaches. The sediment was comprehensively utilized as ecological products, such as improving sandy soil, increasing farmland fertility, and processing into organic fertilizers. Secondly, the constructed wetland itself, composed of aquatic and terrestrial systems, formed a wetland park and a large bird habitat system. It not only beautified the environment but also provided places for birds to inhabit, forage, breed as temporary shelters. It can also be used for human recreation and bird watching under low-disturbance conditions. In the food chain system, there was an aquatic food chain consisting of aquatic plants, herbivorous fish, shrimps, mollusks, crustaceans, and carnivorous fish. These, together with terrestrial plants, their fruits and seeds, and terrestrial insects, were involved in the food chain of birds (both herbivorous and carnivorous birds). A larger near-natural ecosystem was formed by wetlands, woodlands, farmlands, and animals and plants. The water quality pH value, chemical oxygen demand, total nitrogen, ammonia nitrogen and total phosphorus indicators were analyzed and evaluated before and in the two years after the implementation of the technology. Before the technique implement, the water standard from the farmland presented above Class V. After the water purification was applied, the average mass concentration reduction rates of chemical oxygen demand, total nitrogen, ammonia nitrogen and total phosphorus in the second year after the implementation were 41.97%, 33.27%, 56.96% and 72.22%, respectively, compared with those before the implementation. The comprehensive pollution index of water quality decreased from 0.72 to 0.45, and it was generally stable at Class IV water and above standards. A near-natural ecological circulation system suitable for the treatment of lightly polluted water in the Yellow River was constructed, which enriched biodiversity, and the agricultural water environment continued to improve, providing a low-interference and low-pollution living space for birds. This study provides a reference for the ecological purification treatment of farmland drainage.