A Study On The Mechanism Of Constructed Rapid Infiltration System And The Application For Rural Domestic Sewage Treatment

Rural domestic sewage has already been one of the main pollution sources of rural areas in our country. In order to find the appropriate process and technology for treatment of rural domestic sewage in our country, this dissertation is concentrated on investigating the rural domestic sewage quantity and quality characteristics; proving the feasibility of the rural domestic sewage treatment with constructed rapid infiltration system(CRI); analyzing the mechanism of the organic degradation, nitrogen and phosphorus removal; proposing the optimal inside structure and operation parameters of the CRI system; discussing actual operating effect of typical engineering example, by means of field survey, laboratory experimental and typical engineering example study, etc.Rural domestic sewage is consisted of kitchen, laundry, live stock breeding wastewater (accounted for 80-85%) and the other kinds of wastewater (accounted for 15-20%); Rural domestic sewage generation ratio is 55-69%, the maximum hourly variation coefficient is 2.27-6.14; The pollutant concentration of rural domestic sewage is higher than that of the urban domestic sewage, but the pollutant production per capita is less.The process and technology for treatment of rural domestic sewage in our country demands the features of flexibility; low investment, energy consumption and operating cost; convenient maintenance and management, etc. CRI system, which meets all the requirements, is suitable for the treatment of rural domestic sewage.The COD removal rate of the simulated soil column effluent is over 85%, mainly due to the aerobic biodegradation reaction happening in the 0-900mm section of the simulated soil column; The COD removal rate of the improved simulated soil column effluent is over 90%, mainly due to the facultative anaerobic and aerobic biodegradation reaction happening in the 0-1800mm section of the improved simulated soil column, which is 10% more than that of the simulated soil column effluent.The ammonia nitrogen removal rate of the simulated soil column effluent is over 85%, mainly due to the nitration reaction happening in the aerobic(0-900mm) and facultative anaerobic(900-1200mm) sections of the simulated soil column; The nitrate nitrogen concentration reaches maximum, 14.08-15.06mg/l, at the 900mm distance to infiltration media surface; In the facultative anaerobic(900-1200mm) and anaerobic (1200-1500mm) sections of the simulated soil column, which is not submerged by the sewage, the residence time (0.4d) is short, C/N ratio is small, dissolved oxygen concentration can not meet the demand of denitrification reaction, then the anti-nitration reaction must not be thorough ,only a small amount of the nitrate nitrogen is transformed to N₂. So the descent range of nitrate nitrogen concentration is 10% of its maximum. These are the reasons that the ammonia nitrogen removal rate of the simulated soil column effluent is over 85%, but the total nitrogen removal rate is only 28.35-29.78%.The improved simulated soil column is modified from the simulated soil column by the means of adding the overflow tank and step feed-water. According to the comparative test results of different feed-water means, the intake combination 0mm : 900mm is better than 0mm : 700mm, the water intake ratio 2: 1 is better than 1: 1. The removal rate of ammonia and total nitrogen reaches maximum, at the 1800mm distance to infiltration media surface of the improved simulated soil column, the maximum of ammonia nitrogen removal rate is 97.97-98.77%, the maximum of total nitrogen removal rate is 58.55-61.92%; In the anaerobic(1800-2200mm) section of the improved simulated soil column, it causes the ammonia and total nitrogen concentration increase 5-8%,that the endogenous denitrification and ammoniation reactions happen.when the hydraulic loading is 1.0m/d, in order to meet I-class A criteria specified in "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB18918-2002)", the optimal parameters of the CRI system are as followed: the optimal water intake mode is step feed-water, with the intake combination 0mm : 900mm and the water intake ratio 2:1; the length of the optimal aerobic section in the drying periods is 700mm; the length of the optimal facultative aerobic section in the drying periods is 700mm; the length of the optimal anaerobic section in the drying periods is 400mm; the length of the optimal submerged section in the drying periods is 1100mm; the optimal effluent discharging mode is overflow through the pipe which is 1100mm above the bottom of the simulated soil column.The total phosphorus removal rate of the simulated soil column and the improved simulated soil column is 40-45%, the total phosphorus concentration of the effluent is over 2.0mg/l; According to the adsorption test results, the sponge iron has the best adsorption efficiency among the five kinds of special medium tested. According to the phosphorous removal efficiency contrast test among different infiltration medias, the total phosphorus removal rate of the tested column increases about 60%.The Sewage Treatment Project of some unit in chengdu military region and the second ditch of Fenghuan-river have good COD, SS, ammonia nitrogen removal effect, the COD, SS, ammonia nitrogen concentration of the effluent also can meet the design standard, but the total nitrogen and phosphorus removal rate can not. The former has advantages in many aspects, such as better treatment effect, less occupied area, less investment cost, less operation cost. It proves that the CRI system with the suitable hydraulic load is superior to " CRI+ Constructed Wetland " technology.