Research On The Improvement Of Nitrogen Removal By Two-stage Feeding In SWI System

In recent years, demand of fresh water and amount of domestic sewage has been rising with the rapid development of economy in rural areas. But most of domestic sewage has been discharged into lakes and rivers without treatment, causing serious pollution of water environment. The subsurface wasterwater filtration system(SWI) is advantageous to rural domestic sewage treatment, because of its efficient pollutants removal, low investment, easy operation and management, and trivial environmental impact etc. Subsurface infiltration system is effective in removing COD, TP and NH3-N.But the removal effect of N in subsurface filtration system is not as satisfactory due to lack of carbon source in the denitrification process.In the research, two-stage feeding approach was applied in the constructed SWI system to improve the nitrogen removal efficiency. There are 3 distribution pipes at the depth 20cm(1#), 45cm(2#) and 70cm(3#) in the experimental setup. The influence of shunt ratios and depth of the second feeding pipe were studied. The results showed that the removal efficiencies of COD, TP and NH3-N were 94.18%, 92.87% and 98.35% in the traditional SWI system, in which wastewater was distributed into the system only through the distribution pipe 1#. However the removal efficiency of TN was only 37.33%. The modified SWI system has two distribution pipes. When the two distribution pipes are 1# and 2#, the optimal shunt ratio was 2:1 and the removal efficiency of TN was 43.26%. and while the two distribution pipes are 1# and 3#, the optimal shunt ratio was 2:1 too, and the removal efficiency of TN was 60.42%, 23.09% higher than that of the traditional SWI system. Hence, distributed by pipe 1# and 3# with shunt ratio 2:1 was the optimal condition of two-stage feeding SWI system. And the average concentrations of COD, TP NH3-N and TN in effluent were 28.29mg/L、0.82mg/L、3.57mg/L and 16.38mg/L, respectively, which all reached the first level B of the Discharge standard of pollutants for municipal sewage treatment plant(GB18918-2002).Under the optimal operation condition, the amount of effluent balanced with the amount of influent in a complete intermittent cycle, and the effluent amount of drying period accounted for 40% of the whole. The average effluent concentrations of COD、TP、TN and NH3-N during an intermittent cycle were 38.32mg/L、0.79mg/L、16.73mg/L and 2.71mg/L, which also met the first level B of the Discharge standard of pollutants for municipal sewage treatment plant(GB18918-2002).Removal pathways of N were influenced by the depth of second distribution pipe. In the condition that the wastewater was distributed by pipe 1# and 2#, ammonification of organic nitrogen, adsorption of ammonia, and certain degree of nitrification were observed in the soil environment from the depth of 1# distribute pipe to the depth of 20 cm. From depth of 20 cm to 40 cm, nitrification is the main mechanism, while the area between the depth of 40 cm and 80 cm is the main zone of denitrification.Comparatively, when the wastewater was distributed by pipe 1# and 3#, ammonification of organic nitrogen, adsorption of ammonia, and certain degreed nitrification occurred in the area between the depth of 1# distribute pipe to the depth of 20 cm. And from the depth of 20 cm to 40 cm, nitrification is also the main mechanism. Certain degreed denitrification occurred between the depth of 40 cm and the depth of 60 cm. Under the condition with shunt ratio of 3:1, main areas of denitrification spreads from the depth of 80 cm to the outlet. When shunt ratio is changed to 1:1, one of the main zones for denitrification moves to the depth of 60 cm to the outlet, besides the 40cm-60 cm area. And with shunt ratio of 2:1,areas within the depth of 40 cm to 60 cm and 60 cm to 80 cm are the main zones for denitrification.