| At present,rural domestic sewage has attracted wide attention due to its arbitrary discharge,wide spread,difficult collection and difficult management.The pollution of water environment in rural areas is becoming more and more serious.The rural economy is weak and the geographical environment is complex,which restricts the development,popularization and application of rural domestic sewage treatment technology.So it is imminent to study the rural sewage treatment technology which is economical,practical,cheap and easy to operate.In this paper,HABR is used as pretreatment.The wetland system of IVCW-HF was used for advanced treatment of rural domestic sewage.The purpose is to verify the feasibility of combined constructed wetland system for treating rural domestic sewage in Northwest China.In this experiment,waste bricks and shale ceramsite as the matrix,common plants（reed and cattail）in Northwest C hina as the main plants.The artificial wetland system under outdoor natural conditions was set up.First,the single stage horizontal flow constructed wetland was used to treat rural domestic sewage.Secondly,the effects of the compound vertical flow（IVCW）-horizontal flow（HF）artificial wetland system on the treatment of pollutants under the conditions of starting,stable operation,plant harvest and seasonal change were studied.The main influencing factors（hydraulic load,wetland system change and pollution load）of wetland system in deep treatment of rural domestic sewage were analyzed.Finally,the following conclusions are drawn:（1）The hydraulic load of the single level horizontal flow constructed wetland system was0.15 m3/（m2·d）,and the average removal rates of COD,NH4+-N,TN,TP in the wetland system were 52.18%,42.64%,31.97%,39.58%respectively.The effect of single stage horizontal flow constructed wetland for removing pollutants from rural domestic sewage is relatively poor.The removal of organic matter is slightly better and the removal efficiency of nitrogen and phosphorus is not satisfactory.It is verified that horizontal flow constructed wetland is suitable for multi-level or combined with other constructed wetlands to bring into full play its advantages.（2）In the starting stage of the experiment,with the recovery of reeds and cattail in the wetland system and the gradual formation of the biofilm,the oxygen transport capacity of plant roots was enhanced,and the amount of water in the matrix aerobic,facultative oxygen and anaerobic microorganism increased gradually,pH remained unchanged at 7.017.89.The COD removal rate of IVCW-HF constructed wetland system could reach 77.40%on average from 45.83%to later.The removal of NH4+-N is relatively stable,with an average of 93.77%.The removal rate of TN increased from 50.96%to 76.71%.The removal rate of TP increased from 46.58%to 86.83%.At this stage,the average height of Phragmites communis and Typha angustifolia increased from 38cm and 32cm to 85cm and 80cm respectively,and the growth of IVCW plants was better than that of HF.（3）In the stable operation stage of the experimental system,the removal efficiency of COD,TN and TP decreased with the increase of hydraulic loading,especially on COD and TN removal.Under the low hydraulic load of 0.15 m3/（m2·d）,the comprehensive treatment effect of each pollutant is the best.The lowest concentration of COD,NH4+-N,TN,TP in the effluent of the IVCW-HF constructed wetland system can reach 18.95mg/L,3.10mg/L,7.07mg/L,0.57mg/L,and the average removal rate can reach 61.27%,77.84%,69.11%,88.81%.（4）Under the four hydraulic loads of 0.15,0.25,0.35,0.45 m3/（m2·d）,the system COD,NH4+-N,TN,TP along the downward flow-upflow-horizontal flow concentration gradient first rapidly decreased and then slowed down.Under the influence of dissolved oxygen and nutrient demand,the removal efficiency of downflow to pollutants is better than that of upstream and horizontal flow.The removal rate of nitrogen and phosphorus on the surface of the wetland system is higher than that of the middle and lower layers.（5）The influent pollution concentration has an obvious effect on the removal efficiency of pollutants in IVCW-HF constructed wetland system.The removal rates of COD,NH4+-N,TN,TP decreased with the increase of influent concentration.The experimental study on the highest water influent concentration has a certain removal rate,indicating that the IVCW-HF system has a strong resistance to pollutants and can carry the higher concentration of the influent pollutants to full play its pollution removal capacity.（6）After the plant harvest stage,the hydraulic load is 0.15 m3/（m2·d）and IVCW-HF system purification effect analysis:the effect on NH4+-N is more stable,the removal rate of COD,TN,TP is reduced,and the low concentration COD adaptation is strong.So the appropriate harvest time should be chosen to minimize the effect of the purification on the purification effect of the wetland system.（7）When the season changes,the effect of sewage purification in the wetland system is compared:summer>autumn,spring>winter,and in July,the removal efficiency of pollutant removal is best,and the removal rate of COD,NH4+-N,TN,TP can reach 90.51%,96.30%,81.34%,91.73%respectively.The lowest removal rates were 31.63%,28.82%,25.27%,27.91%in winter.The comparison of microbial species and quantity in the whole wetland system:bacteria>actinomycetes>fungi.The number of different microbes in wetland types:downflow>upward flow>horizontal flow,vertical flow>horizontal flow.Seasonal change of microorganism quantity comparison:summer>autumn>spring>winter,that the highest in summer in July and the lower in winter.It shows that the effect of different types of wetland purification sewage is closely related to the species and quantity of microorganisms.（8）The effect of single level horizontal flow and composite vertical flow（downflow+upflow）-horizontal flow constructed wetland system in deep treatment of rural domestic sewage was compared,and the combined vertica l flow and horizontal flow constructed wetland system had better treatment effect.The IVCW-HF artificial wetland system can effectively reduce the load of influent pollutants in the four stages of starting,stable operation,plant harvesting and seasonal change.It is suitable for the deep treatment of rural domestic sewage.The efficiency of the HF section can be improved by about 20%on the basis of the IVCW section.The results show that the selection of composite vertical flow and horizontal flow constructed wetland system combined with the advantages and disadvantages of different types of constructed wetland can effectively reduce the concentration of pollutants in rural domestic sewage. |
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