Study On Pyrite And Limestone Constructed Wetlands For Polluted River Water Treatment

In recent years, discharge of domestic wastewater and industrial wastewater increases with the rapid economic development in China. Many rivers have therefore been polluted, and lost their functions. It is imperious to search for a cost-effective technology to control polluted river water. As an effective substitute technology for the traditional wastewater treatment technology, constructed wetland with low cost, good purifying performance and fine ecological function is attracting more and more attentions. Limestone and pyrite-limestone subsurface horizontal constructed wetlands were constructed to treat polluted river water in this research. The aims were to know performances of the two wetlands on removal of common pollutants, especially the nitrogen and phosphorus, and analyze the actions of these minerals. The main contents of this study include:(1) constructing limestone and pyrite-limestone subsurface horizontal constructed wetlands, investigating the performance of limestone and pyrite-limestone subsurface horizontal constructed wetlands, discussing the effects of parameters of wetland on pollutants removal, and analysing the transfer and degradation laws of the pollutants; (2) studying the mechanism of phosphorous removal in wetlands. The main results are shown as follows:(1) Limestone and pyrite-limestone subsurface horizontal constructed wetlands have good performance on pollutant removal. The hydraulic retention time highly influenced the purification ability of constructed wetlands, and the optimal hydraulic retention time was 3d for constructed wetlands treating polluted river water. At 3d HRT, the average removal efficiency of COD. NH4+-N. TN and TP were 53.93%, 82.13%,66%,50.9%, and 51.66%,77.43%,72.06%,97.35% for limestone and pyrite-limestone constructed wetlands, respectively. The seasonal efficiencies of pollutant removals for both wetlands were in the order of winter< autumn< spring< summer. There were few differences in COD removal between limestone and pyrite-limestone wetlands, but the nitrogen and phosphorus removals of pyrite-limestone constructed wetland were higher than those of limestone constructed wetland. The phosphorus removal of pyrite-limestone wetland was very efficient and stable, not affected by temperature. Concentrations of Cu、Zn、Cd、Cr、Pb、Ni、As、Fe、 Mn and other heavy metals in effluent of the two constructed wetlands are up to standards of discharge standard of pollutants for municipal wastewater treatment plant, and the third grade of the Environmental quality standard for surface water(2) In summer and winter, the pollutants concentration changed with some regularity along the limestone and pyrite-limestone constructed wetlands. The pollutant concentrations decreased along horizontal direction of water flow, but did not along up-to-down direction. The concentrations of COD and NH4+-N in upper layer were lower than the middle in two wetlands, possibly because of higher DO in the upper layer. The concentrations of TP decreased along horizontal direction and vertical up to down direction. The decreasing rate of pollutants concentration along water flow in winter is much lower than in summer, and so did the removal efficiency.(3) Batch experiments were carried out to investigate the characteristics of phosphorus removal by limestone and pyrite. The effects of initial phosphate concentration, pH, temperature and interfering ions (NH4+, NO3- and SO42-) on the phosphorus removal by substrates were investigated. Freundich isotherm was observed to fit the phosphate adsorption on limestone. The Langmuir isotherm was more suitable to describe the phosphate adsorption on pyrite, the maximal adsorption capacity reached 11.01 mg/kg. The phosphorus removal efficiency of limestone increased with increasing temperature, and the phosphorus removal efficiency of pyrite decreased with increasing temperature. When pH< 6.5, the phosphorus removal efficiency of limestone increased slowly with increasing pH; when pH≥ 6.5, the phosphorus removal efficiency of limestone decreased rapidly with increasing pH. For pyrite, when pH< 8.5, the phosphorus removal efficiency increased with increasing pH, the phosphorus removal efficiency rapidly decreased at pH≥8.5. NH4+, NO3- and SO42- had no significant effects on phosphorus removal.(4) Kinetic data of limestone and pyrite fitted the pseudo-second-order kinetic model well. With the same reaction conditions, the phosphorus adsorption capacity of limestone is bigger than that of pyrite. Thermodynamics research indicated that the phosphorus adsorption on limestone was a spontaneous and endothermic process, and the phosphorus adsorption on pyrite was a spontaneous and exothermic process. The removed phosphorus by limestone mainly existed in Ga、Mg-P, and the removed phosphorus by pyrite mainly existed in Fe、Al-P. The phosphorus adsorption on limestone and pyrite was mainly physical adsorption, and accompanied by chemical adsorption.In conclusion, limestone and pyrite-limestone subsurface horizontal constructed wetlands could treat polluted river water. Consider the removal efficiency and the stability of effluent, it’s preferred to choose pyrite-limestone constructed wetland to treat polluted river water with low organic matter content and high nitrogen and phosphorus content. It’s of great significance in selection of pyrite as constructed wetland substrate to remove the nitrogen and phosphorus in polluted river, the results can provide a reference for the design and application of mineral constructed wetlands.