Study On The Factors And Mechanism Of The Heavy Metal Treatment In Sludge With Constructed Wetland

With the increase of municipal sewage treatment,the sludge quantity also increases rapidly.The sludge contains high moisture content and difficult to dehydrate,the composition is complex and difficult to treat.And it contains many heavy metal is the most important factors to limit the utilization.It become an important and difficult problem in the field of environmental protection,has aroused great attention of the governments and scientific researcher.In recent years,constructed wetland treatment technology has been widely used in sludge treatment and disposal because of its low construction cost,simple operation and no secondary pollution.Unfortunately,the current research mainly concentrated on the constructed wetland treatment sludge dehydration and stabilization,few studies involving the removal of heavy metal,and fewer researchers pay attention to study on the factors and mechanism of the heavy metal treatment in sludge with constructed wetland.Based on this,this study employed constructed wetland to treat sludge heavy metal,the factors of chemical pretreatment methods,plants and matrixes on the effect of constructed wetland to remove heavy metal in sludge were investigated,and the related mechanism was analyzed.This study aims to provide data support for the treatment of sludge and its heavy metals in constructed wetlands,and provide references for the treatment and utilization of sludge.The main research contents and achievements are summarized as follows,(1)Citric acid(CA)and glutamic acid N,N-2 acetic acid(GLDA)were employed to pretreat sludge,examined the chemical pretreatment on the sludge heavy metal content and morphological change,and the pretreatment conditions were optimized also.Based on this,the effects of different pretreatment methods on the treatment of heavy metals in constructed wetland were analyzed.The results showed that the chemical pretreatment can promote the transfer of heavy metal from the solid phase to the liquid phase,and improve the heavy metal content of effective state,which is beneficial for the subsequent wetland treatment.Compared with the control,the removal rate of heavy metal Cd,Cu,Pb and Ni in the constructed wetland was increased by 60.19%,34.29%,37.22% and 25.51%,respectively.At the same time,in the CA and GLDA pretreatment system,the plant biomass at the end of experiment increased by 158.65% and 172.89% respectively,is more than 89.14% of the control,it is indicated that chemical pretreatment of sludge beneficial to wetland plants,thereby increasing removal rate of heavy metals.(2)Yellow flag,scirpus tabernaemontani and reed were employed,with no plants for comparison,studied the removal efficiency of different plants on the constructed wetland to treat sludge heavy metal.The results showed that the yellow flag,scirpus tabernaemontani and reed system average removal rate of heavy metal were 54.03%,66.61% and 47.63%,respectively,compared with the control increased by 34.06%,46.64% and 46.64%,respectively.At the same time,the biomass of yellow flag,scirpus tabernaemontani and reed increased by 89.14%,189.30% and 52.00% respectively.This showed that scirpus tabernaemontani is more suitable for constructed wetland treatment system,and the treatment effect of heavy metals is also the highest.The contribution of scirpus tabernaemontani on the removal of heavy metals in sludge is 30.13%,higher than that of yellow flag and reeds,21.78% and 22.23%.(3)Pebbles,ceramsite and porous concrete were employed as matrix to constructed wetlands.The results showed that the average removal rate of heavy metals were 54.03%,51.38% and 49.48% respectively.The main reason for the low removal rate of heavy metal in the system is the clay and concrete alkaline characteristics.At the same time,the pebbles system is more beneficial to plant growth,and its heavy metal removal rate higher.(4)The variance analysis and significance test were carried out to clarify the effects of chemical pretreatment,plants and matrixes on the heavy metal removal.The results showed that the effect of pretreatment on the removal of heavy metals was the most,and matrixes was the minimum.The wetland system had the best removal efficient on Ni,Pb was the worst,and the removal efficient size was the Ni>Cu>Cd>Pb.