| Subsurface soil infiltration systems are on-site domestic sewagetreatment systems can remove more than85%COD, more than50%of TN,more than90%of NH+4and more than95%of TP in domestic sewage.Invest and costs of the subsurface soil infiltration systems are low.However, soils with low permeability restrict the popular application ofthe systems. In order to reduce clogging, sand or sandy loams are usually asmain media to replace the soil in the systems, and invest and cost increaserapidly. After soil is clogging, drying process or changing soil was necessaryto deal with the systems and make them work correctly. Nevertheless, thesemethods had weak points including much more area, complicatedmanagement, etc.Generally, bio-clogging was the main reason for failure, EPS cloggedthe passageway of soil and the systems didn’t endure the designed flow rate.In order to find an effective, economical and friendly method to avoid clogging, polyurethane foam with high porosity works as media to improvethe permeability of soils. Polyurethane foam is a kind of plastic with lowdensity, high porosity, high water absorption, high elasticity and corrosionresistance. High porosity of polyurethane foam can improve the porosity andsaturated permeability of soils. Large specific surface of polyurethane foambe beneficial to microbe in soil matrix. Low density of polyurethane foamwill reduce transportation expense. Corrosion resistance of polyurethanefoam can make it exist in soils for a long time.The quality of outlet from soil infiltration systems was dominated byflow rate, pollution load, temperature, soil permeability, etc. Fortifying soilproperties by polyurethane foam can greatly reduce the quantity of sand andsandy loams in soil infiltration systems. Optimization process by soilcolumns was in laboratory. Sand and iron granule were added into the soil asbasic media in columns, then polyurethane foam at different quantity addedinto the basic media as different composite media. Four kinds of compositemedia were used in column experiments, and polyurethane foam in the fourkinds of composite media occupied0,3‰,6‰,8‰.Flow rate, pollution load, temperature were used to research the soils.Saturated permeability of soil columns decided the chance of clogging.Therefore, saturated permeability of columns was the key factor. The resultsindicated polyurethane foam could improve saturated permeability of soilevidently. The saturated permeability has a power function to the mass ofpolyurethane foam in columns. The soil columns with6‰polyurethane foam,59.6%soil,36.8%sand and3%steel slag had higher performance to treatdomestic sewage. During the two years, no clogging happened in columnsand the saturated permeability of the columns kept bigger than20m/d. Soil core analyses revealed high biogeochemical activity within theinfiltrative zone from0to10cm depth of treatment layer. All columnsexhibited comparable behavior and there were no significant differences inperformance attributable to infiltrative surface character or soil depth. It isspeculated that the comparable performance is due to a similar and sufficientdegree of soil clogging genesis coupled with bioprocesses that effectivelypurified the wastewater given the adequate retention times and highvolumetric utilization’s of soil infiltrative zone.Purification behaviors were influenced by soil permeability, organic load,and temperature. The quality of effluent from soil columns is in negativecorrelation with flow rate and pollution load, and in positive correction withtemperature. Therefore, it is important to design reasonable flow rate and setpre-treatment units was for soil infiltration systems.According to the optimized results of the soil columns, a full-scaleseptic-soil infiltration system was set up in a kindergarten of ChongmingCounty of Shanghai. Designed hydraulic loading of the soil infiltrationsystem was20cm/d, and the most adverse hydraulic loading30cm/d. CODin septic tank effluent was usually less than300mg/L, temperature wasbetween12~31℃. COD in outlet from the full-scale soil infiltration systemwas about22mg/L; more than90%of COD in septic tank effluent werepurified by soil infiltration. After polyurethane foam added to the situ soil,soil porosity was improved to53%from38%. Saturated permeabilityincreased evidently from0.05m/d to0.66m/d. Flow rate was14cm/d, on theaverage domestic wastewater COD removal rate was91%, average effluentCOD less than20mg/L, average TN in outlet was less than16mg/L，averageNH+4in outlet was less than2mg/L，average TP in outlet of was less than0.1 mg/L. No clogging happened in full-scale soil infiltration system. After morethan two years, the permeability of fortified soil decreased to43cm/d, whichwas much higher than5cm/d of sjtu soil.The results of full-scale experiment proved that polyurethane foam is aneffective material to improve soil porosity and saturated permeability.Saturated permeability of situ soil was5cm/d, after modified by thepolyurethane foam, the flow rate of soil infiltration increased to20cm/d, andoccupied less land area and cost less.Subsurface soil infiltration system treating wastewater mainly dependedon microbe in soil matrix.16S rDNA in soil columns provided direct insightinto microbial diversity, and pointed the main bacteria to treat domesticwastewater. The Proteobacteria and Bacteroidetes were most bacteria andtook up about75%of bacterial species in soil columns.Biological clogging of soil infiltration system is the main reason forfailure. When pollutants were purified by soil matrix in soil infiltrationsystems, a lot of microorganisms and EPS were raised and clogged the soilpores in soil vadose zone. In the end, the system cannot endure the hydraulicloading and was in failure. There were more Beta-Proteobacteria classes innormal soils and more Alpha-Proteobacteria classes in clogged soils.Based on soil column experiments and field experiments, dynamicmodel of soil infiltration systems was established and the dynamic model canpredict the state of the systems and provide some useful information formanagement. |
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