Study On Pollutant Removal Mechanism Of Stromwater Runoff Through Urban Greenbelt

As one of the most important components of city ecosystem, the city greenbelt has great effect on water cycle and pollution control, which is benificial to maintain and improve city ecosystem. It is a natural and effective way to control runoff pollutant with low cost and long-term ecological effects through greenbelt. Based on the characters and needs of central district of Shanghai, it is urgent to study the effect of greenbelt on pollution control It also has important instructional meaning to save the city water resources and protect the city water environment. The author studies the pollutant removal mechanism of stromwater runoff through artificial greenbelt and rainfall device and gets the following conclusions:1. The city greenbelt has good effects on pollutant control. The designed repetition periods of rainfall are one year, three years and every five years. During rainfall lasting two hours, the removal efficiencies of COD, NH4⁺-N, NO₃ ^--N, organic nitrogen, TN and TP are 28.5%～50.9%, 31.7%～51.3%, 18.8%～41.9%, 19.7%～32.1%, 18.0%～30.2% and 26.1%～41.1% respectively. The removal efficiencies change with different intensities and decrease rapidly in the process of rainfall.2. The city greenbelt has great effect on total quantity control. With the rainfall intensities of 36 mm·h^-1, 50 mm·h^-1, 57 mm·h^-1, the removal efficiency of organic matter, nitrogen and phosphorus are 47.6%～61.3%, 48.3%～64.4% and 41.3%～50.9% respectively.3. As an important component of the greenbelt, plants play an important role in removal of runoff pollutants. Compared with bare soil, the removal efficiencies of COD, NH₄⁺-N and TP are increased by 0.8%～7.7%, 1.3%～6.2% and 1.9%～9.2% respectively.4. During seven-day lasting rainfall, the first day removal efficiency of COD, NH₄⁺-N are 40.0%～50.0%, 67.1%～75.2%, higher than the other days. And the removal efficiency of pollutants has great relevance with depth of soil, especial for TN and TP.5. Actual city greenbelt system also has great effect on pollutant removal. The average removal efficiencies of COD, NH₄⁺-N and TP are 44.4%, 56.6% and 42.3% respectively. The function of the greenbelt on removing the pollutants in rainwater differs under different conditions, such as different rainfall intensities, initial stage and later stage in a same rainfall. The pollutants removal efficiencies in moderate rain are better than that in light rainfall and downfall, also in initial stage than that in latter's. (5) Most pollutants are cut down by adsorption and interception of soil and the plant root system during rainfall. Half an hour after the rain begins, pollutants are mainly adsorbed by topsoil. During the rain break, the pollutant adsorbed by greenbelt was decomposed by soil microorganisms, which mainly take place on the second to the eighth day after the rain. The organic matters are decomposed by bacteria, and Nitrogens are decomposed by nitrification and denitrification, which occurs on the fifth to the eighth day after rain. And 14～17 days later, the contents of organic mater, TN and TP reduce to the levels before rain basically.