Đang tải... (xem toàn văn)
In order to promote the application of constructed wetland technology for wastewater treatment and for preventing the Lake Dianchi Valley of China from further eutrophication, field studies was carried out, which included the small-scale, pilot-scale experiments, and finally demonstration treatment systems. In this study, the performance of a combined free water, subsurface constructed wetlands and ecological pond system was designed and adopted to treat one village’s sewage. The monitor results of 430 days showed that the mean removal rates of total nitrogen, ammonia-nitrogen, total phosphorus and COD were 89.9%, 85.1%, 85.1% and 80.6%, respectively. The water quality could achieve the design requirement. The performance of wetland system in the storm was also investigated. The system could resist the impulse load of stormwater and had a good capacity to reduce the non-point source pollutants brought by the stormwater runoff.
Journal of Water and Environment Technology, Vol.2, No.2, 2004 - 49 - PERFORMANCE OF A COMBINED CONSTRUCTED WETLAND SYSTEM FOR TREATING VILLAGE SEWAGE IN LAKE DIANCHI VALLEY Chao-Xiang Liu* , **, Hu Hong-Ying*, Huang Xia*, Shi Hang-chang*, Qian Yi * and Koichi Fujie*** * Environment Simulation and Pollution Control State Key Joint Laboratory , Department of Environmental Science and Engineering, Tsinghua University, Beijing, 100084, China ** Institute for Recycling & Environmental Control System, Fukuoka University, 10 Koyocho, Wakamatsu-ku, Kitakyushu 808-0002, Japan ***Department of Ecological Engineering, Toyohashi University of Technology, Toyohashi, Aichi 441-8580, Japan ABSTRACT In order to promote the application of constructed wetland technology for wastewater treatment and for preventing the Lake Dianchi Valley of China from further eutrophication, field studies was carried out, which included the small-scale, pilot-scale experiments, and finally demonstration treatment systems. In this study, the performance of a combined free water, subsurface constructed wetlands and ecological pond system was designed and adopted to treat one village’s sewage. The monitor results of 430 days showed that the mean removal rates of total nitrogen, ammonia-nitrogen, total phosphorus and COD were 89.9%, 85.1%, 85.1% and 80.6%, respectively. The water quality could achieve the design requirement. The performance of wetland system in the storm was also investigated. The system could resist the impulse load of stormwater and had a good capacity to reduce the non-point source pollutants brought by the stormwater runoff. Keywords: Constructed wetlands; Village sewage; Non-point source pollution, Nitrogen and phosphorus INTRODUCTION Dianchi, the largest lake in southwest of China, has been polluted severely in recently years due to excessive discharge of nitrogen and phosphorus nutrients with population growth and economic development. Total nitrogen (T-N) and total phosphorous (T-P) concentrations of water in Dianchi lake were about 2.10 mg/L and 0.33 mg/L respectively in 2000 (Liu, 2001), which shows it was in serious eutrophication status. Non-point source (NPS) pollution discharged from decentralized rural areas around Dianchi is one of the main reasons that cause Dianchi’s eutrophication, which includes village sewage, agriculture wastewater, storm water runoff and agricultural solid waste, and accounts for well over half of all impairments to its water quality now. So it has important significance to treat the large amounts of nutrient from agriculture and village effectively with practicable technology in order to control the eutrophication of Lake Dianchi. According to field investigation, the population is mainly concentrated in the numerous decentralized small villages in Lake Dianchi valley, in which the discharge system is too simple that the sewage flows into the lake freely without any treatment because of undeveloped economy level there. Constructed wetland (CW) is regarded as a simple Journal of Water and Environment Technology, Vol.2, No.2, 2004 - 50 - cost-effective ecological technology for wastewater treatment in small communities or decentralized villages (USEPA, 1988; Gao, 1991; Cameron et al., 2003). To this general condition, a demonstration treatment system, in which the constructed wetland was as core technology, combined with the ecological and scenical design, was adopted to treat a typical village sewage. There is the long rain season in local place always accompanied with strong storms at the beginning. The stormwater runoff not only brings the large amounts of NPS pollutant, but also has an effect on the sewage treatment system due to its strong impulse and washing role. Therefore, this study was conducted with the following objective: (1) the long term performance of demonstration system treatment of village sewage. (2) the performance of demonstration systems when suffering the impulse load of stormwater and its effect on the reducing non-point source pollutants containing in the stormwater runoff. It has important significance to accumulate the design and maintaining experience for the promotion of constructed wetland technology in the Lake Dianchi valley. MATERIALS AND METHODS Constructed wetland system design The population serviced by the constructed wetland system was 1820 people. The design sewage flux was 80m 3 /d. According to parameters obtained form the pilot-scale experiment results (Liu, 2003), the system was designed as following process (seen in Fig.1.). Sewer System Sweage Effluent Free-water surface CW Sedimentation & distribution tank Ecological pond Subsurface CW Fig.1 The flow chart of constructed wetland system First, Sewage was distributed evenly to the free-water surface constructed wetland through sedimentation&distribution tank, then entered into the subsurface constructed wetland for secondary treatment, finally entered into a ecological pond which set up behind constructed wetlands in order to make full use of the land advantage to strengthen ecological and scenical effect of ecological system. The area was 2000 m 2 and hydraulic loading rate was 4cm/d in the free-surface constructed wetland, in which the 20-30cm depth of free water was kept above the bottom and the Phragmites communis was dominant macrophyte. The area was 300m 2 and hydraulic loading rate was 30cm/d in the subsurface constructed wetland, which was filled into some media with good hydraulic and adsorption property. The area was 1400 m 2 and water depth was 0.6m in the ecological pond. The overlook of completed constructed wetland system was shown in Fig. 2. Journal of Water and Environment Technology, Vol.2, No.2, 2004 - 51 - Fig. 2 The overlook of constructed wetland system Operation conditions The demonstration system was built from February, 2003, and began to run at June, 2003 when the macrophyte in the wetland grew mature. The experiment lasted about 430 days until September, 2003. The weather was warm during the experiment period. The temperature was varied from 8℃to 25℃ and the mean temperature was about 15℃. The water quality of village sewage was shown in Table 1. The influent flux was in a wide fluctuation from 20 to 60m 3 /d, which behaved as that the flux was small and the pollutants concentration was high because of high evaporation (from November to the April of next year) in dry season, and the flux was big and the pollutants concentration was low in the rain season (from May to October). The data from the local weather station showed that the average annual rainfall and evaporation was 802mm and 2093mm, respectively. Table 1 Water quality of village sewage TN NH 4 + -N TP COD pH 5-85 2-60 1-13 100-700 7.0-8.0 Note：Unit is mg/L except pH Sample and analyze methods The performance of the system was investigated by monitoring the influent and effluent quality 4-7 times every month. The parameters analyzed, using procedures outlined in the Standard Methods (Chinese EPA, 1997) were: COD, NH 4 + -N, total-N (TN) and total-P (TP). To the storm monitoring, the sample of influent and effluent was taken at the start, middle and end of storm. The flux was measured by downflow weir or float method. RESULTS AND DISCUSSION Removal performance of constructed wetland system for village sewage The COD Profiles of influent and effluent, as well as corresponding removal rate are shown in Fig. 3. It was observed that the influent COD was as high as 600 mg/L from the Journal of Water and Environment Technology, Vol.2, No.2, 2004 - 52 - December to the April of next year, which was caused by the high evaporation during the period of dry season. Although the influent COD was varied violently from 100 to 700mg/L, the effluent COD of system was lower than 52 mg/L and the COD removal rate could achieve above 80%. 0 200 400 600 800 1000 2002/6/1 2002/9/1 2002/12/2 2003/3/4 2003/6/4 2003/9/4 Date COD （ mg/L ） 0 20 40 60 80 100 Removal rate （ % ） Infulent Effulent Removal rate Fig.3 COD removal performance 0 20 40 60 80 100 2002/6/1 2002/9/1 2002/12/2 2003/3/4 2003/6/4 2003/9/4 Date TN （ mg/L ） 0 20 40 60 80 100 Removal rate （ % ） Infulent Effulent Removal rate Fig.4 TN removal performance Journal of Water and Environment Technology, Vol.2, No.2, 2004 - 53 - 0 20 40 60 80 100 2002/6/1 2002/9/1 2002/12/2 2003/3/4 2003/6/4 2003/9/4 Date NH 4 + -N （ mg/L ） 0 20 40 60 80 100 Removal rate （ % ） Infulent Effulent Removal rate Fig.5 NH 4 + -N removal performance 0 5 10 15 2002/6/1 2002/9/1 2002/12/2 2003/3/4 2003/6/4 2003/9/4 Date TP （ mg/L ） 0 20 40 60 80 100 Removal rate （ % ） Influent Effluent Removal rate Fig.6 TP removal performance The TN and ammonia profiles over the operation period are shown in Fig. 4 and 5. The influent TN was varied from 5 to 85 mg/L, the effluent COD of system was not stable until one month later, then the removal rate was increased and could achieved 89.9% with an effluent TN less than 3 mg/L. The influent NH 4 + -N was varied from 2 to 60 mg/L, the effluent NH 4 + -N was lower than 2 mg/L and the removal rates was 85.1%. It could be concluded that high nitrogen removal efficiency was maintained in the constructed wetland system. The profiles of TP removal performance over the operation period are shown in Fig. 6. The influent TP was varied from 1 to 14 mg/L, the effluent TP was lower than 0.6 mg/L and removal rate could achieved 85.1%. Table 2 shows average removal performance of the constructed wetland system over 430 days operation. The mean removal rates of total nitrogen, ammonia-nitrogen, total phosphorus and COD in the demonstration system were 89.9%, 85.1%, 85.1% and 80.6%, respectively. The effluent quality was good enough to achieved the design requirement. Journal of Water and Environment Technology, Vol.2, No.2, 2004 - 54 - Table 2 Removal performance of constructed wetland system treatment of village COD TN NH 4 + -N TP Average influent concentration（mg/L） 364.6 24.6 19.0 5.33 Average effluent concentration（mg/L） 52.2 2.3 1.4 0.55 Average removal rate (%) 80.6 89.9 85.1 85.1 Note: the number of available sample is 80 Removal performance of constructed wetland system for storm runoff It entered into the rain season after June. A storm fell from 21:00 at June 24, 2003 and lasted about 15 hours with a 120mm rainfall. The storm flux in the constructed wetland system is shown in Fig.7. 0 200 400 600 800 0 5 10 15 20 Time(h) Storm flux(m 3 /h) Fig. 7 The storm flux in the constructed wetland system The mass removal pollutants can be roughly calculated through integrating the flux and time curve. The result is shown in Table 3. The total load of stormwater received by system was 4408 m 3 . The removal amounts of COD, TN, NH 4 + -N and TP were 273Kg, 14.1Kg, 6.7Kg and 7.3Kg, and corresponding mass removal rates were 74.6%, 63.1%, 62.7% and 59.5%, respectively. Moreover, the system could discharge flood and operation well in the storm. It could be concluded that the system could resist the impulse load of stormwater and had a good capacity to reduce the large amounts of pollutants brought by the stormwater runoff in a short time. Table 3 Removal performance of the constructed wetland system in the storm Strom (V, m 3 ) COD（Kg） TN（Kg） NH 4 + -N（Kg） TP（Kg） Influent 432 22.4 11.2 9.8 Effluent 4408 159 8.4 4.6 2.5 Mass Removal / 273 14.1 6.7 7.3 Removal rate / 63.1 62.7 59.5 74.6 Journal of Water and Environment Technology, Vol.2, No.2, 2004 - 55 - The above results showed that the constructed wetland system not only reduced the pollutants of village sewage, but also could effectively reduced the large amounts of non-point source pollutants brought by the rainfall, which caused by the rain washing the village grounds and fields. On the other hand, the rainfall would renew the ecological system and promote the metabolism of organism. CONCLUSIONS Based on the results of this study, the following conclusions can be made: (1) The monitor results of 430 days on constructed wetland system showed that the average removal rates of total nitrogen, ammonia-nitrogen total phosphorus and COD were 89.9%, 85.1%, 85.1% and 80.6%, respectively. The water quality achieved the design requirement. (2) The systems could resist the impulse load of stormwater and had a good capacity to reduce the non-point source pollutants brought by the stormwater runoff. ACKNOWLEDGEMENT This Project was financially supported by Chinese Science and Technology Ministry. The author thanks the Environmental Protection Institute of Kunming and local office for their helps in the fields experiment. The author should also thank many people who joined in the project for their hard work. REFERENCES Cameron, K., Madramootoo, C., Crolla, A., Kinsley, C. (2003) Pollutant removal from municipal sewage lagoon effluents with a free-surface wetland., Water Res, Vol.37, 2803-2812. Methods for water and wastewater analysis. (1997). 3 rd edn, China Environmental Protection Agency. Gao Z.M., Li X.F. (1991). Handbook for soil treatment of domestic wastewater. Chinese standard Publisher, pp. 251 -277. Liu, C.X., Hu, H.-Y., Huang, X., Shi, H.C, Qian, Y. and K. Fujie. (2003). Pilot constructed wetland systems for treating rural domestic wastewater in Lake Dianchi Valley., Proceedings of Asian waterqual’2003, IWA Asia-Pacific Regional conference, Abstract on pp. 70, Paper on CD-ROM, Bangkok. Liu, C.X., Hu, H.-Y., Huang, X., Shi, H.C, Qian, Y. (2003). Design of aquatic ecosystem for treatment of rural swage in Lake Dianchi area., China water & wastewater. Vol.19, No.2, 93-94. Liu L. P. (2001). Discussion on the effects and experience in eutrophication control in Dianchi Lake., Chongqing Environmental Science, Vol.23, No.5, 24~26. Journal of Water and Environment Technology, Vol.2, No.2, 2004 - 56 - Design manual: constructed wetlands and aquatic plant systems for municipal wastewater treatment. (1988). U.S. Environmental Protection Agency)., Cincinnati, USA. . Technology, Vol .2, No .2, 20 04 - 53 - 0 20 40 60 80 100 20 02/ 6/1 20 02/ 9/1 20 02/ 12/ 2 20 03/3/4 20 03/6/4 20 03/9/4 Date NH 4 + -N （ mg/L ） 0 20 40 60 80 100. lower than 52 mg/L and the COD removal rate could achieve above 80%. 0 20 0 400 600 800 1000 20 02/ 6/1 20 02/ 9/1 20 02/ 12/ 2 20 03/3/4 20 03/6/4 20 03/9/4 Date