Experimental Study On Domestic Wastewater Treatment By Tidal-flow Constructed Wetlands And Greenhouse Gases Emission During The Treatment

The rapid development of China's economy has resulted in deteriorating water environment.Decentralized domestic wastewater is difficult to be collected by city sewer and has to be treated by decenterlized ways.Constructed wetlands?CWs?wastewater treatment technology shows great advantages in decentralized wastewater treatment.However,the conventional CWs have some disadvantages,such as low treatment capacity and large land requirement.In this study,a three-stage tidal flow constructed wetland?TFCW?was applied to achive high rate treatment of decentralized domestic sewage.The overall treatment performance and influencing factors were studied.In addition,greenhouse gases?GHGs?emissions of the TFCW were also investigated.Under a total cycle time of 8 h?contact/rest,4 h/4 h?and a hydraulic loading rate of 1.26 m3/?m2·d?,the three-stage TFCW achieved 85%,85%,39% and 32% removals for COD,NH₄⁺-N,TN and TP respectively.The system was very effective in the removal of COD and NH₄⁺-N,but showed limited TN and TP removals.The comtaminants' removal mainly occurred in the first stage,which accounted for 83.8%,75.9%,79.9% and 75.3% of the overall removal of COD,NH₄⁺-N,TN and TP respectively.Under a fixed cycle time of 8 h,the duration of the contact and rest phases had a significant impact on the treatment efficiency.With the prolongation of the contact time?2,4,6 h?,the COD removal in the first stage increased to 75.7% from the lowest 65.9%.However,the removal of NH₄⁺-N decreased to 31.4% from 68%,while the accumulation ratio of NO3--N decreased to 1.7% from 38.8%.The removal of TP increased to 29.8% from 11.5%.Under a fixed ratio of the contact/rest time?1:1?,the total cycle time affected significantly the treatment performance.When the cycle time was extended to 8 h from 6 h,the pollutant removal in first stage increased significantly.However,only slight enhancement was observed with further increase of the cycle time.The NH₄⁺-N removal even decreased when the cycle time was extended to 16 h.The best treatment performance was obtained at a cycle time of 12 h,with effluent NH₄⁺-N of 2⁴ mg N/L and effluent BOD5 <10 mg/L.The removal efficiencies were also affected by the change of water level.Among them,the removals of COD and TP were less affected,while the removals of NH₄⁺-N and TN were greatly affected.Under a cycle time of 12 h?contact/rest,6 h/6 h?,the removals of COD?⁷5%?and TP?³0%?were more or less constant at different water levels?15 cm above the media level;at the media level;20 cm below the media level?.High water level restricted the oxygen transfer,and the NH₄⁺-N removal decreased to 58.5% from 67.5%?at the media level?;while the restricted oxygen transfer favored denitrification and the TN removal increased to 42.5% from 35.1%.Low water level increased NH₄⁺-N removal to 71.8%,while TN removal decreased to 34.6% due to restricted denitrification.Low influent C/N ratio was a main reason for the poor TN removal in this study.However,the enhanced oxygen transfer and lack of anoxic condition is the intrinsic factor limiting TN removal in TFCWs.To enhance TN removal,methods of step-feeding,effluent recirculation and high influent inorganic carbon were explored in this study.The first two methods aimed at enhancing TN removal by optimizing the utilization of the influent organic carbon,while the third method aimed at stimulating short-cut nitrification and denitrification.Under a cycle time of 12 h?contact/rest,6 h/6 h?,step-feeding?59.8%?and recirculation?64.2%?enhanced the TN removal by 15% and 19% compared with the control.By adding inorganic carbon to the influent,the TN removal?59.7%?was enhanced by 21%,under a cycle time of 8 h?contact/rest,4 h/4 h?.The GHGs emission in TFCW displayed certain regularity,and was affected by the change of temperature and water level.N2O?50.4 mg CO₂ /?m2×h??was the main GHGs compared with CH4?24.8 mg CO₂ /?m2×h??in TFCW,which was attributed to the enhanced oxygen transfer in TFCWs.During the contact period,all the GHGs emission displayed a significant positive correlation to temperature?N2O: p<0.001;CH4: p<0.005?.During the rest period,N2 O emission was positively related to temperature,while the effect of temperature on CH4 emission was not significant.As to different water levels,high water level?15 cm above media?led to the highest GHGs flux?90.8 mg CO₂ /?m2×h??,while lowest GHGs flux?61.1 mg CO₂ /?m2×h??was recorded when the water level was at the media layer.In general,the GHGs emission?average 75 mg CO₂ /?m2×h??of TFCW was substantially lower than other types of CWs?surface flow,257 mg CO₂ /?m2×h?;horizontal subsurface flow,358 mg CO₂ /?m2×h?;vertical flow,162 mg CO₂ /?m2×h??.