Abstract
Abstract: According to the characteristics of sewage dispersion with small flow and high concentration, fluctuation and rainfall impact, the process combining multi-level drop aeration-type ditch and ecological pond was designed to dispose the distributed wastewater of the rural areas in China. The combination process contains five parts: water inlet tank, constant flow pump, multi-level drop aeration-type ditch, ecological pond and water storage tank. Among them, multi-level drop aeration-type ditch includes five sections: advection I, drop I, advection II, drop II and advection III. Each section consists of water inlet area (A), sewage disposal area with gravel and vermiculite laid at the bottom (B) and water outlet area (C). Ditch based on drop aeration is conducive to the growth and metabolism of biofilm in vermiculite. Most of the pollutants can be removed into easily biodegradable organism or small molecular substances, achieving the purpose of purifying water quality. The 50 mm soil rich in microorganisms was laid in the bottom of the ecological pond, and the biozone, which was 50 mm wide and 400 mm long, was fixed on the surface of the pond with the iron shelf for 4 lines and each line has 5 colomns. And they were immersed in the sewage water. One hand, the treatment effect of ecological pond could strengthen the ditch system, and solve the problem of poor treatment of nitrogen and phosphorus; on the other hand, it would take in nitrogen and phosphorus of surface runoff in rainy days, increase the residential time of the rain and strengthen the treatment effect. Meanwhile, the hanging biozone would improve the ecological pond load and purify pollutants efficiently. At last, the hydraulic load, operation mode, water fluctuation, rainfall impact factors and parameters were studied. The results showed that: 1) The average removal rate of pollutant decreased when the hydraulic loading was increasing from 35 to 55 cm/d. The removal rates of chemical oxygen demand (COD), NH4+-N, total nitrogen (TN) and total phosphorous (TP) of the combined process were 62.18%-98.84%, 69.49%-92.46%, 63.37%-88.29% and 63.93%-91.58%, respectively. 2) The treatment effect of intermittent operation was better than the continuous operation. When the combination process was operated intermittently, the average removal rate of COD and NH4+-N was 95.02% and 77.04%, respectively, which was better than constantly running. The average removal rate of TN was 49.10% with a slight decrease, and the average removal rate of TP was 82.61% which remained unchanged. 3) The removal rate of pollutants varied with water fluctuation. The maximum removal rate of COD and NH4+-N was decreased nearly by 20%, and TN was the largest, about 25%, while TP was the least, about 15%. 4) When rainfall was in large-scale, the removal rate of each pollutant in the combined process was declined from 53.95%, 52.44%, 49.92% and 71.64% to 40.94%, 43.75%, 34.93% and 53.84%, respectively. 5) Adopting intercepting measure and controlling first flush pollution was the key of controlling pollution in the combined process. The relationship between removal rate and interception ratio was linear. Taking the interception ratio of 5 in torrential rain, the water quality still reached Class I according to the standard of Discharge Standards of Pollutants for Municipal Wastewater Treatment Plants (GB 18918-2002). It is of great significance to remove rural wastewater during the rainfall.
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