| Constructed rapid infiltration system which developed from rapid infiltration system, is one of the sewage land treatment technology and it is also the sustainable and ecological wastewater treatment technology.The terrain of the Three Gorges Reservoir Area is complex, and the climate is special too, the special purple soil of china is the primary soil of this area. The town of this area is small, widely distributed, with backward economy and poor management of infrastructure, which is in urgent need of sewage treatment processes with low cost, low energy consumption with less processes and easy to operate.Theoretical basis and technical support to expand the scope of CRI system will be provided owing to the research on applying CRI system to the small town of the Three Gorges Reservoir Area and exploring the construction and operation parameters of CRI system suitable to the soils and climatic conditions of the area.Based on the basic principles and requirements of RI system, the special purple soil and some other common media were applied to construct the CRI testers, and the experiments were conducted in the field. Parameters such as hydraulic loading, hydraulic loading cycle, flooding time and wet/dry ratio and other operating parameters were studied out through the parameter tests such as water and wastewater infiltration permeability, specific yield of soil etc. The domestic wastewater treatment performance of five CRI testers with different configurations were investigated under the condition of four hydraulic loading level of 0.08m3/ m2·d,0.10m3/ m2·d,0.12m3/ m2·d,0.14m3/ m2·d and three wet/dry ratio of 1d:4d,1d:3d,1d:2d. The study found:The prefered CRI tester is constructed with purple soil (70cm)/ceramic(20cm) /pebble(10cm). The preferred operating parameters of the tester are: hydraulic load being 0.1m3/m2·d and the wet/dry ratio being 1d:3d. The mean concentration of COD,NH4+-N,TN,TP of CRI tester effluent is 50.0mg/L,7.9mg/L,20.6mg/L,0.9mg/L and the mean removal rate is 84.2%,72.3%,57.7%,85.1% respectively. All the indexes have matched the B standard of"Pollutants Discharge standard of Urban Sewage treatment plant"except TN which is about 50% exceeding by the B standard.Hydraulic load, hydraulic loading cycle and the variation of wet/dry ratio have little effect on the removal of COD and TP, but have great impact on the removal of NH4+-N and TN. The removal rate of TN is negatively correlated with hydraulic loading, but positively correlated with wet/dry ratio and temperature in some degree. Alternation of flooding and dry is the main form of aeration, the best aeration combination of CRI system is the alternation of flooding and dry + plant canna.Aiming at the natural environments parameters such as 17℃~ 19℃of the temperature, 60~80% of the average relative humidity, about 30% of the sunshine rate and 1.12m/s of the average annual wind speed, the CRI system constructed mainly by purple soil is worked out compatible to the economical, technological and management level of small towns in the reservoir area. The construction form and operating parameters are the innovation and supplements of CRI system, also they are an an extension of the application scope of CRI system.The frequency of TN concentration in the effluent matching the B standard being about 50% becomes the primary cause when the improvement of hydraulic load or the performance of decontamination is concerned. For the reasons above, the main factors such as the ratio of purple soil and sand, thickness of the infiltration soil, the ratio of carbon and nitrogen of the sewage, and the operating parameters are examined carefully, for the purposes to study the method to improve nitrogen removal performance in two ways which are optimizing the CRI system structure and operation parameters. Studies have shown that:To the infiltration media mixed with purple soil and the sand, the less the ratio of soil and sand is, the more of NO3--N produced in the effluent, the more the ratio of soil and sand is, the less of NH4+-N,TN produced in the effluent. The CRI tester composed by the mixture of soil and sand in 4:1 ratio had relatively the best removal performance to NH4+-N and TN, the removal rate is 71.7% and 48.8% respectively. Integrated other factors such as hydraulic load, the medium with ratio of soil and sand in 4:1 is the more appropriate media choice.The primary mechanism of CRI system is metabolism of aerobic microorganisms. The oxidation of organic and kjeldahl nitrogen is occurred mostly within the 0.6 meter soil layer below the surface, and the removal process of TN is mainly occurred between the soil layer of 0.6~1.0m. The thickness of the soil cannot result in the better performance of N removal. The CRI tester constructed with 1.0 meter infiltration soil has relatively good removal effects of nitrogen.The ratio of C:N in the sewage can significantly effect the NO3--N,TN concentration of effluent, with the increased C:N, the NO3--N,TN concentration of effluent decreased markedly. It is a feasible measurement to improve TN removal performance to dose carbon into sewage (the maximum of COD is 400mg/L) and the best proportion of C:N in the sewage should be between 6.0 and 8.0.The less the wet/dry ratio is and so does the flooding, the better the NH4+-N removal performance is. The NH4+-N removal rate can achieve to 71.4% when wet/dry ratio is 1:9, the flooding time is 12 hours, and the hydraulic loading cycle is 120 hours. The less the wet/dry ratio is but the longer the flooding time is, the better the TN removal performance is. The TN removal rate can achieve to 59.1% when wet/dry ratio is 1:9, the flooding time is 48 hours, and the hydraulic loading cycle is 480 hours. Long flooding time is benefial to denitrification of CRI system.Studying further on the mechanism of nitrogen migration and transformation in the CRI system, It is found that NH4+-N is the key of nitrogen migration and transformation in the CRI system, so the study on the mechanism of nitrogen migration and transformation in the CRI tester is carried out at the condition that applied single nitrogen compound (NH4+-N and NO3--N) to one-dimensional soil column constructed by the mixture of purple soil and sand in the ratio of 3:1, and the hydraulic loading cycle is 4 days, the wet/dry ratio is 1d:3d.The experiments of NO3--N dosing into the CRI tester showed: adsorption of NO3--N in the soil column tester did not exist; there was denitrification process occured after flooding 5~9 hours, the space of denitrification is between 25cm~85cm as to the CRI soil column with the effective height of 1.0 meter.The experiments of NH4+-N dosing into the CRI tester showed: the nitrification and the adsorption processes begins when flooding; the adsorption is a simple and temporary processes, easy to desorption and desorption completely; the removal process of NH4+-N in the CRI system is a steady-state process,has nothing to do with the flooding time, but has negative linear correlation to the deepth of the soil column within a certain height.Based on the experiment results of single compound of NH4+-N and NO3--N dosing into the CRI system, referenced to the theory of solute migration and transportation through porous media and convection-dispersion equation, a one-dimensional equation of the vertical migration of NH4+-N was composed to simulate the process of NH4+-N dosing into the CRI system. In the simulation, the migration and transportation of NH4+-N in the CRI system is the results of water convection and hydrodynamic dispersion, and adsorption-desorption, nitrification and denitrification process. It is the first time that the water flow equation of sewage flow through the CRI soil column and the oxygen concentration as the constraints of nitrification, denitrification process were linked together. The expression of one-dimensional mathematical model of vertical migration of ammonia in the CRI system is:The parameters in the CRI model are determined as following: the retardarce coefficient is determined by static isothermal absorption experiment, the velocity of sewage travel through the soil column is determined by permeability test, the vertical dispersion coefficient is determined by test the electric conductivity of tracer in the dispersion experiment, the rate of nitrification and denitrification is determined by test of Baps technology. Finally, the migration and transformation mechanism of NH4+-N is analysised out by the test results of oxidization-Redox potentials along with the soil columns.The model is applied to simulate the single nitrogen compounds (NH4+-N) of the artificial sewage distributed to the surface of soil columns.The boundary and the original conditions were initialized, the parameters such as retardarce coefficient, the mean water velocity through the infiltration medium, vertical diffusion coefficient, nitrification and denitrification rates were substituted into the equation. The Matlab program was writed out based on the finite difference method. The simulated result and the parameters sensitivity analysis is worked out so as to understand the migration and transformation mechanism of NH4+-N in the CRI system.Comparison of simulation results and the measured data showed that simulation results could reflect the NH4+-N concentration when artificial nitrogen-containing sewage being distributed onto the surface of soil column at different periods. Both the trends and the value of concentration are in good agreement when NH4+-N concentration in soil solution in the CRI soil columns of different height concerned.The order of sensitive factors which affect the NH4+-N migration and transformation is: vertical dispersion coefficient D.≈retardarce coefficient Rd1 > pore velocity v = nitrification coefficient K1. The order which affect the NO3-N migration and transformation is: nitrification coefficient K1 > vertical dispersion coefficient D.> retardarce coefficient Rd2> denitrification coefficient K2 > pore velocity v, which is helpful on further research on the nitrogen removal performance in the CRI system.
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