| With the development of social economy,there are more and more pollutants discharged from sewage and domestic sewage.Common pollutants include COD,NH4+-N and heavy metals etc,and heavy metals are the main factors causing threats to China’s ecological environment.In response to the problem of heavy metal pollution,China has formulated many relevant laws and regulations,but there are still problems such as corporate smuggling and excessive discharge,coupled with inevitable discharge of domestic sewage,agricultural pollution and road runoff etc,making surface water The phenomenon of heavy metals in resources is difficult to eradicate.Therefore,environmental workers have been looking for suitable economic and environmentally friendly heavy metal treatment processes.Constructed wetland is a complex ecosystem that uses the synergy of matrix,microbes and animal and plant communities to remove organic matter,nitrogen,phosphorus and heavy metals from sewage.Studies have shown that constructed wetlands can effectively treat heavy metal sewage,but in actual engineering,the processing performance of constructed wetlands will be affected by factors such as the size of hrt,the type of substrate,the type of wetland plants,and the concentration and type of influent pollutants.In view of the above problems,in this study,reeds and cattails were used as wetland plants,and gravel was used as a substrate to construct a horizontal subsurface flow constructed wetland to treat comprehensive wastewater containing COD,NH4+-N,Cd2+ and Zn2+.Research indicates:(1)When the HRT is 2 d,the NH4+-N in the wetland effluent reaches the discharge standard B standard and the TN and COD reaches the discharge standard A standard under different Cd2+and Zn2+concentrations.(2)When the HRT of sewage was 4 d,the removal rates of Cd2+in the constructed wetland were 4 mg/L,10 mg/L and 16 mg/L,respectively,which were 99.6%,99.3%,98.6%.The removal rates of Zn2+in wetland at concentrations of 12 mg/L,20 mg/L,and 28 mg/L were 99.1%,94.7%,and 91.9%.The effluent heavy metal concentration of Cd2+was 4 mg/L and Zn2+of 12 mg/L is close to or meets the discharge standard,and the artificial wetland removal effect is stronger than that of the unplanted plant.(3)The order of adsorption of heavy metals by matrix follows:Zn2+>Cd2+;in the experiment of constructed wetland,the removal of Cd2+in wetland is not only directly affected by Zn2+,but also affected by its own concentration.The removal of low concentration Zn2+is not affected by Cd2+.When the concentration of Zn2+is higher,the removal rate decreases with the increase of Cd2+ concentration;thus,the heavy metal in the wetland can be seen.The competitive adsorption relationship is far more complicated than the competitive adsorption relationship between heavy metals in a single matrix.(4)The results of continuous operation showed that the concentration of NH4+-N in the effluent gradually decreased and stabilized at 4.40 mg/L after 72 days of wetland operation,and the removal rate was stable at about 78.0%.The effluent NH4+-N was used throughout the operation period.The effluent TN concentration was finally stabilized at about 4.7 mg/L,and the removal rate was about 76.5%.The concentration of effluent TN in the whole operation period reached the emission level A standard.The concentration of the effluent COD concentration and removal rate are not affected,the removal rate is about 85.0%,the effluent meets the discharge standard level A standard;the operation reaches 54 d-82 d,the wetland effluent Cd is very stable,the maximum value is only 0.01 mg/L,the effluent meets the discharge standard;the maximum Zn concentration of effluent is 0.37 mg/L,and the effluent Zn2+concentration meets the discharge standard throughout the operation period.(5)The enrichment of heavy metals in Phragmites australis and Typha angustifolia was mainly concentrated in the roots.The enrichment ability of Typha angustifolia to Cd and Zn was significantly stronger than that of reeds.The enrichment ability of reed and cattail to Zn was significantly stronger than that of cadmium;Cd and Zn The content of depth increases gradually,the accumulation of Cd and Zn is mainly concentrated in the bottom layer.The main occurrence of Cd and Zn in the matrix of constructed wetland is the residual state.The residual state of Cd and Zn increases with the increase of matrix depth.In the trend,the Cd carbonate binding state in the matrix layer is larger than that of Zn.In the most active exchange state,the affinity sequence of heavy metals to the matrix is Cd>Zn,and the heavy metals Cd and Zn are bound by iron-manganese oxide.The ratio of occurrence is higher,but the ratio of Zn in the iron-manganese oxide bound state is higher,and the organic bound Cd and Zn tend to decrease with the increase of the matrix layer depth. |
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