Study On Wastewater Treatment By The Double-Layer-Soil Land Treatment System Of Sandysoil And Clay

In this research, the soil layer is constructed based on actual working conditions. The land treatment system uses sandy soil and clay as filling materials. Its operational characteristics, including best hydraulic load and water quality of effluents in different depth are studied through experiments. The reaction mechanism of the system is studied and the kinetic models of the system are built. The results provide theoretical and technical supports for decentralized domestic sewage (especially rural domestic sewage) treatment. And by studying the treatment effects of surface plant and comparing them with other plants in earlier studies, this research provides reliable reference for surface plant selection in land treatment system.In studying the effects of hydraulic load on effluent water quality, it shows that under the influent condition of 0.10 m3/m2.d, the system has better treatment effects on COD and TP with the removal rate of 87.22% and 83.78% respectively. And under the influent condition of 0.08 m3/m2.d, the system has better treatment effects on TN and NH4+-N with the removal rate of 51.63% and 75.87% respectively. As the hydraulic load of the influent increases, its effect on COD removal is more evident than other pollutions.In studying water qualities of the effluents from different depth of the soil, it shows that TP was mainly removed by the clay layer while other pollutions are removed mainly in the sandy soil layer. The removal of TP is related to properties of the soil such as heaviness. While the removals of COD, TN and NH4+-N are mainly achieved by oxidation, nitration and denitrification reactions of the microorganisms within the system. The sandy soil has better reoxygenation ability as it is on the upper layer. This factor combined with the oxygen transference of the plants may contribute to the better removal efficiencies of COD, TN and NH4+-N. The final effluent's COD and NH4+-N concentration can meet the "A-level,1st grade" standard of "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant" (GB18918-2002), while its TN and TP concentration can meet the B-level in the same grade.In studying DO concentration of the system, it shows that effluent's DO has no apparent connection to hydraulic load. DO decreases as the depth increases. DO decreases more rapidly in sandy layers than clay layers. The change of DO level helps explain the system's reaction mechanism. Also, by studying the effects of surface plant, it is seen that chlorophytum comosum (var.marginatum) shows certain removal ability on TN and TP and little effect on COD. By further comparing its effects with reineckia carnea in earlier stydies, chlorophytum comosum shows better removal ability on N and P. This research studies a double-layer-soil land treatment system by increasing its hydraulic load. The effluent can meet relative standards. However, the hydraulic load has not been elevated largely enough to solve the problem of great area demand of land treatment system. There also lay questions about how to increase water treatment effect and to increase the reoxygenation ability of lower soil layer. We can see from the results that double -layer-soil land treatment system has certain pollution removal ability and can be easily managed which indicate its potential in future application.