Studies On The Stability Of Constructed Wetland In Treating Wastewater In Subtropical Zone

Constructed wetlands (CWs) are engineered systems that widely used in wastewater treatment.They are complex systems in terms of biology, hydraulics and water chemistry, and treatment efficiency was affected by many factors, such as hydraulic loading rate, retention time, water column depth, influent nutrient concentration, vegetation, water chemistry and temperature. An integrated-vertical flow constructed wetland was built in Hangzhou Botanical Garden, Zhejiang Provinec, China, to purify light-eutrophic water from Jade fish-seeing pond, and other three CWs of the same structure were built to purify high-eutrophic wastewater. In this paper, in order to evaluate the stability of the function of the CW in subtropical zone, we studied annual and seasonal variations of the removal efficiency for water pollutions, and also the influence of other factors such as typoon events and the riatio of influent carbon concentration to nitrogen concentration.Integrated vertical flow constructed wetland had good purification performance and long term performance of the system was stable. Most indexes of its effluent reached or near the first class standard of Chinese surface water, such as NH₄-N, NO₃-N, TP, COD, BOD, etc. Annual variation showed no degeneration of purifying efficiency except BOD₅ in treating light-eutrophic water. The removal rate (RR) of BOD₅ gradually decreased since 2005 (before 2005 RR>60%, after 2005 RR<30%), it might related to typhoon events. RR for TP (85%) was highest, for BOD₅ (67%), COD (45%) and NH₄-N (59%) was a liitle lower, and RR for TN (24%) and NO₃-N (16%) was lowest.When the intergrated-flow constructed wetland was used to treat light-eutrophic wastewater, seasonal variations of RR for BOD₅ and COD was found, but no seasonal differentce in removal efficiency of nitrogen and phosphorus. RR for BOD₅ was lowest in summer (31%) and for COD was lowest in autumn (15%).Plants in CW system play important roles in the accumulation of organic carbon, bio-available nitrogen and bio-available phosphorus in matrix. The average accumulated velocity for bio-available nitrogen in planted area (15.73 g y^-1 kg^-1 dry soil) was significantly higher than in unplanted area (12.50 g y^-1 kg^-1 dry soil) (P<0.05). However, average accumulated velocity for bio-available phosphorus had no significant difference between planted (3.62 g y^-1 kg^-1 dry soil) and unplanted area (4.92 g y^-1 kg^-1 dry soil), and for organic carbon also showed the same average accumulated velocity, 0.19 g y^-1 kg^-1 dry soil in planted area and 0.18 g y^-1 kg^-1 in unplanted area, respectively.Seasonal variations indicated that accumulation for organic carbon in unplanted area was much higher in summer (0.73 g/kg dry soil) than in winter (0.49 g/kg dry soil), for bio-available nitrogen was higher in winter (53.57 g/kg dry soil) than in summer (49.99 g/kg dry soil). However, there were no significant differences among four seasons in planted area in accumulation of organic carbon and bio-available nitrogen. The accumulation for organic carbon in summer and winter was 0.78 g/kg dry soil and 0.82 g/kg dry siol, respectively. And for bio-available nitrogen was 62.93 g/kg dry soil and 64.36 g/kg dry soil, respectively. It indicated that plants in CW systems played siginificant roles in keeping stability of the removal efficiency.Water quality in the ornamental fishpond (OFP) changed little whlie CW stopped operating in winter compared to the period while kept on operating. Only TN concentration in OFP kept on a high level (Class V ) , other indexes reached or near class I standard of Chinese surface water. Consequently, CW can stop operating in winter when used to treat light- eutrophic wastewater in subtropical region.Furthermore, we taken typhoon events into consideration, results showed that removal efficiency of NH₄-N and BOD₅ decreased significantly. RR for NH₄-N decreased from 64% to 15%, and for BOD₅ decreased form 48% to 8%. Influent NH₄-N concentration varied little (increased from 0.08mg/1 to 0.11mg/1), but effluent concentrations increased significantly from 0.02mg/l to 0.09mg/l. However, NH₄-N concentration in OFP and effluent still reached class I standard of Chinese surface water (<0.15mg/l) (GB3838-2002). BOD₅ concentration both of the influent and effluent increased strongly, from 4.89mg/l to 12.83mg/l in influent, and 3.39mg/l to 12.00mg/l in effluent. pH value and COND both in influent and effluent were also significantly decreased.Effluent concentrations of the nutrients increased when influent concentrations rised. And massremoval rate (MRR) showed a positive relationship with mass loading rate (MLR), which indicatedthat increase MLR can improve treatment efficiecy. The carbon supplied in CW of HangzhouBontanical Garden was inadequate (BOD₅3-N<4), while in other three CWs thattreat high-eutrophic wastewater influent carbon source was enough (BOD₅/TN>3, COD/NO₃-NM).The whole study analyzed the factors that influence the stability of treatment efficiency of theconstructed wetland, and provided reference for plant collocating model. This study established thetheoretical basis for the improvement in treatment efficiecy in constructed wetlands, and alsoprovided reference for treating environment with constructed wetland.