Research On Puriifcation Effect Of Subsurface Wastewater Infiltration Systems For Distributed Rural Sewage Treatment

One key objective of “The Twelfth-Five Year” environmental planning of China is tostrengthen rural drinking water source protection and comprehensive control of waterpollution． The methods of investigation， monitoring and indoor simulation were used todiscuss the purification effect of subsurface wastewater infiltration systems (SWIS) forrural sewage treatment under the conditions of the rural sewage dispersion， lack ofcollection facilities， water pollution treatment method and application technology andother practical problems．According to the removal characteristic of organic pollutants， three different kinds ofSWIS were established to study the removal efficiency of the distributed rural sewageunder different structure conditions． To establish the subsurface wastewater infiltrationsystems， two types of matrixes were studied． One of the matrixes was composed of flyash (5%) and soil (95%)， which was called FAS． The other was composed ofdecomposed cow dung (5%) and soil (95%)， which was called DCDS． To study thepurification effect， the SWIS were run at different hydraulic loads and distribution modeunder the optimal structure conditions．Among the three different structure filling layered systems， the one with most DCDSshowed the highest removal efficiency of total nitrogen (TN)， and the others showedinsignificant distinction． The average removal efficiency of chemical oxygen demand(COD)，total phosphorus (TP) and ammonia nitrogen (NH3-N) was89%，86%and86%，respectively． The average effluent concentration of TP and NH4-N was0．1mg/L and1．0mg/L， respectively．The removal efficiency of TN was increased with the increasing filling height ofDCDS． Compared to the removal efficiency of TN in the systems without DSCD (25%)，the removal efficiency in the systems filling half of the height and one fourth of the heightwas increased by72%and31%， respectively．According to the different seasonal discharge in the rural family， the SWIS were runat three different hydraulic loads of6．5cm/d，13cm/d and20cm/d， and which werecalled low， medium and high loads， respectively． The experimental results showed thatthe fluctuated COD removal efficiency was mainly due to the influent COD concentrationranging from45mg/L to360mg/L， while the effluent COD concentration was keptconstant． The stable removal efficiency of TP was obtained in both low and medium hydraulic loads， and the average removal efficiency was higher than98%． The resultsalso showed that if the SWIS were run successively under high hydraulic load and highorganic load conditions， the removal efficiency was decreased． However， the removalefficiency could be returned to the normal level if the organic load decreased． Among allthe three hydraulic loads， high removal efficiency of NH3-N was obtained due tonitrification could be easily occurred． However， the removal efficiency of TN wasaffected by the higher hydraulic load． The effluent concentration of TN had the sametrend with the influent TN concentration， and the average removal efficiency was88%，which illustrated that SWIS could run well at a high hydraulic load．The SWIS were run under two different inlet arrangement conditions， which wascalled once inlet and many times inlet，respectively．The results showed that，comparedto the system operated under once inlet conditions， the effluent concentration of thecontaminants in the system operated at many times inlet was much lower． The inletarrangement had a little influence on the removal of COD and TP， while it had asignificant influence on NH3-N and TN removal． The effluent NH3-N concentration atonce inlet was two times higher than that of many times inlet， and thus the removalefficiency of TN was affected by the inlet arrangement conditions．According to the experimental results， SWIS with DSCD filling half of the heightwas selected to treat the distributed rural sewage． The results showed that no matter howthe contaminant concentration changed， the system could keep high removalefficiency． The effluent concentration could achieve different criteria specified in SurfaceWater Quality Standard at the initial stage． The effluent COD concentration could meetwith the III criteria specified in Surface Water Quality Standard at the fourth week， whilethe effluent TP concentration could meet with the II criteria specified in Surface WaterQuality Standard at the beginning． The effluent NH3-N concentration could meet with IIcriteria from the III criteria at the beginning． The effluent TN could meet with III criteriafrom the IV criteria at the beginning． While the effluent NO3-N concentration could meetwith the criteria I specified in Groundwater Quality Standard， far less than the limitedvalues (≤10mg/L) specified in Surface Water Source of Drinking Water SupplementProject Standard．Apparently， the optimized SWIS had an obvious advantage for the distributed ruralsewage treatment，such as simple configuration and management，no sewage remaining，and high removal efficiency． So， SWIS had an abroad application prospect for thetreatment of sewage in the rural area．...