Study On Rural Domestic Wastewater Treatment By Modified Subsurface Soil Infiltration System

Recently, water pollution and water shortages in rural areas of China are very serious. However, most of the rural areas lack wastewater collection pipes and purification facilities, so domestic wastewater discharge directly without treating, which pollutes the rivers and lakes and other water bodies and even threats the safety of drinking water quality in village. Therefore, rural domestic wastewater must be treated. Subsurface soil infiltration system, as a typical decentralized small sewage treatment facility, is one of the best means of treating rural domestic wastewater. But traditional Subsurface soil infiltration system can’t form a good aerobic and anaerobic environment at the same time for microbial nitrification and denitrification due to structural and operational reasons, so purification effect of the total nitrogen is not good. Therefore, in order to improve the total nitrogen removal effect, contrast experiments are adopted to study the impact of fillers combinations and operating conditions on pollutants removal rate, and a proposal of modifying the structure and optimizing the operating process is proposed finally; Meanwhile, the microbial amount, activity and metabolic diversity are measured,and the correlations with water quality indicators are also analyzed,which provides a theoretical support to nitrogen removal effect improvement.The results obtained through the research are as follow:(1)The ways of matrix combination can affect the purification effect of the system. The system with complete mixing mode and with the volume ratio 8:3:3 of zeolite, activated carbon and ceramic clay have a better purification effect than the system with layered filling mode and with volume ratio 3:2:2 respectively. The purification effect of each system is:System4> System3 (or System2)> System2 (or System3)> Systeml. The removal rates of CODCr, NH4+-N and TN are 79.5%,88.7% and 87.7%,which are 7.0% 4.5% and 3.7% higher than that of systeml respectively.(2) Treatment effect of the system is increased with the decrease of hydraulic loading rate and the increasing of dry-wet ratio. There is no difference of treatment effect under the hydraulic loading rate of 0.1～0.3 m3/(m2·d),but 15%～20% decline of CODCr, NH4+-N and TN removal rates is founded under 0.4m3/(m2·d) than that under 0.1m3/(m2·d). Contrasted with continuous operation, intermittent operation has a better removal effect,and the removal rate of CODCr and TN increase 10%～20% under the dry-wet ratio of 3:1,but there is little influence on NH4+-N. So 0.1m3/(m2·d) and dry-wet ratio of 3:1 are the best operating condition, however, taking both purification efficiency and treatment capability into consideration, hydraulic loading rate of 0.1m3/(m2·d) and dry-wet ratio of 2:1 are recommended.(3) Microbial amount and activity have a very significant positive correlation with purification effect, that is to say, when microbial amount and activity improve, the removal rate of pollutants improves simultaneously. But microbial metabolic activity and diversity are not relevant with pollutants removal, so they will not affect the removal rate of all pollutants in the system.