Study On Nitrogen And Phosphorus Removal Characteristics Of Shallow Constructed Wetlands With Lawn Plants Treating Rural Wastewater

Constructed wetlands are favored in rural domestic sewage treatment projects due to its simple management,low investment and operation cost in realizing the function of denitrification and phosphorus removal.However it has been found for a long time that the constructed wetland system will inevitably have problems such as filling blockage,stopping growth of aquatic plants in winter and decreasing purification capacity during the long-term operation.Lawn plants have the advantages of short growth cycle,many species to choose from,and dense roots.By interplanting lawn plants in cold and warm seasons,wetland plants can be quickly changed into stubble,and the processing capacity in winter can be further improved.Based on this,four cold season turfgrass?Lolium perenne L,Agrostis palustris Huds,Poa pratensis L,Festuca arundinacea Sehreb?and two warm season turfgrass?Zoysia japonica L,Cynodon dactylon L.?were selected as artificial wetland plants in the experiment.The constructed wetland was constructed with loess,river sand,vermiculite,ceramsite and aerated concrete as the substrate,and the substrate layer was 0.1m thick.The water quality indicators of effluent were continuously monitored to explore the nitrogen and phosphorus removal effect of the tail water from rural domestic sewage treated by biological treatment.The purpose of this experiment is to provide theoretical and practical significance for the promotion of extremely shallow turf constructed wetland in the vast rural areas.The specific conclusions are as follows:?1?In autumn and winter,Lolium perenne,Agrostis palustris Huds,Poa pratensis L,Festuca arundinacea Sehreb were seeded in the substrate layer of the wetland,and the most appropriate hydraulic load was selected as 0.027m³/(m²·d^-1),The wetland was operated from September to the following June.Lolium perenne L and Festuca arundinacea Sehreb were evergreen all the year round and grew well.The Agrostis palustris Huds and Poa pratensis L began to turn yellow in the summer of the following year.The average removal rates of TP,TN and NH₄⁺-N in the constructed wetland with Lolium perenne were 83.27%,61.37%and54.45%.The average removal rate of TP,TN and NH₄⁺-N in the constructed wetland with Festuca arundinacea Sehreb reached 81.34%,58.48%and 52.38%.The concentration of TN in the effluent from the extremely shallow subsurface flow constructed wetlands with planting Agrostis palustris Huds and Poa pratensis both reached the level A of"Standards for pollutant discharge from urban sewage treatment plants"?GB 18918-2002?.Compared with the control group without plants,the effluent from the constructed wetland had significantly lower water quality indexes.?2?In spring and summer,the wetland substrate is seeded with Zoysia japonica L,Cynodon dactylon L,and the stolons could breed and quickly became flat around 20d.The wetland ran from May to December.Zoysia japonica L and Cynodon dactylon L grew well from May to early November,as the temperature droped in late November,some of the leaves turned yellow.The average removal rates of TP,TN and NH₄⁺-N in the constructed wetland with Zoysia japonica L were 85.26%,55.28%and 58.22%.The average removal rates of TP,TN and NH₄⁺-N in the constructed wetland with Cynodon dactylon L reached 84.31%,52.12%and 56.76%.The effluent TP,TN,and NH₄⁺-N of extremely shallow subsurface constructed wetlands planted with warm-season turfgrass plants all met the first-grade A of"Standards for pollutant discharge from urban sewage treatment plants"?GB 18918-2002?.?3?The absorption characteristics of nitrogen and phosphorus of the four cold season turfgrass were different:Lolium perenne was suitable for the repair of high NH₄⁺-N,PO₄^3--P concentration of tail water,especially the absorption of PO₄^3--P had advantages compared with the traditional wetland plants.Poa pratensis and Festuca arundinacea Sehreb were suitable for repairing tail water with low PO₄^3--P and high NH₄⁺-N concentration.The effect of Agrostis palustris Huds on nitrogen and phosphorus absorption was not good.?4?The adsorption capacity of the five substrates of loess,river sand,vermiculite,aerated concrete and ceramsite to PO₄^3--P ranked as:aerated concrete>vermiculite>ceramsite>loess,and the equilibrium adsorption capacity of the five substrates to NH₄⁺-N ranked as:vermiculite>aerated concrete>ceramsite>loess>river sand.?5?The optimal mass ratio of loess,river sand,vermiculite and aerated concrete is:4:2:1:1.?6?The results of the study on the migration and transformation of nitrogen and phosphorus in wetland showed that the removal of phosphorus in wetland system mainly depends on the enrichment of substrate,and the amount of phosphorus removal accounts for54.04%⁷3.39%of the phosphorus load in the water.Phosphorus removal by plant absorption accounted for 2.88%¹0.34%.The removal of nitrogen in wetland system mainly depended on the action of microorganisms,which accounts for 23.28%³7.63%of the nitrogen load of water.Secondly,the enrichment of substrate,which removal amount accounted for20.96%²6.11%of the nitrogen load of water intake.While plant absorption only accounted for a very small part,the proportion was 0.51%³.81%.