Nutrient Removal In Hybrid Constructed Wetlands For Treating Ighly Polluted River Water

In recent years, the comprehensive treatment of WeiHe River has always been oneof the priorities in Shanxi Province. The present situation of the WeiHe River, can besummed up as: less water in upstream, water polluted highly in midstream, severedeposition of the channel in downstream. In order to apply the ecological idea ofwastewater treatment, build constructed wetland（CW） in the flood area of the ZaoHeRiver for the river sewage treatment according to the pollution and environmentcharacteristic of it will be the optimal choice for the regulation of Weihe River.Therefore, a CW of20mu as a demonstration project were constructed at the bank ofthe ZaoHe River which is the most polluted tributary of WeiHe River, providing theorybasis for using CW improvement the water quality of the WeiHe River.The demonstration project contains5groups of different hybrid CWs with acontinuous influent about350m3/d, planted reeds, cattail and ryegrass. After one year’sexperiment on the treatment efficiency of the system and the plant growthcharacteristics. Found out the influence factors on the removal of nitrogen andphosphorus in CWs and determined the best combination of different types of wetlands.The results show that:（1） For5groups of different CWs, the nitrogen and phosphorous removalefficiency of series3which is a combination of the vertical subsurface flow, horizontalsubsurface flow and surface flow CW was the best, the removal rates of TN, NH₃-N andTP was79.4±14.0%、79.2±13.7%、82.3±12.0%, respectively. The concentrations of theeffluent was9.7±3.3mg/L、5.7±2.6mg/L、0.65±0.42mg/L, respectively. （2） The influence of seasonal changes of temperature on the nitrogen removal inthe CWs was obviously, the removal rates of TN and NH3-N in summer were25%³5%higher than that in winter. The removal rate of TP was influenced by seasonaltemperature change lightly.（3） The concentration of the dissolved oxygen （DO）in the CWs was low in summerbut higher in winter. Low DO concentration in summer limited the treatment efficiencyof the wetlands, while higher DO concentration in winter inhibited the denitrificationresulted the accumulation of NO₂^-and NO₃^-in the effluent.（4）The removal efficiencies of nitrogen and phosphorus in Wetland varied underdifferent hydraulic retention time, when the hydraulic retention time was3d treatmenteffect was the best.（5） CWs had a strong ability to resist loads fluctuation, in the pollution loadsfluctuation range of ZaoHe River. The removal rates of TN and TP increased as the TNand TP load sincreased, subsurface wetland type with a better load resistance than thesurface flow wetland, the optimal load of TN and TP was35mg/L-40mg/L and3.9mg/L-4.4mg/L, respectively.（6） The removal efficiencies of nitrogen and phosphorus were the highest in thereed area followed by the mixed plant area and cattail area. The removal rates ofnitrogen and phosphorus decreased obviously after harvesting the plants, TN removalrate decreased by more than20%after the harvested, which for TP was5%.Plantshas arole in maintain the stability of wetland, the aboveground part plays a key role inmaintain the root microenvironment which is an important part of the wetland system.