Analysis On Water Quality Of Runoff From Roof In Wuhan

Wuhan is abundant in rainfall, with lots of rivers and lakes in it, but the distribution of water resources is unbalance and the quantity of rain throughout seasons is uneven. With the development of urbanization, the impervious surface in Wuhan increases, so dose the volume of its runoff. Now the surface of roof becomes one important component of the urban impervious areas. However, as the surface of roof is the carrier of the atmospheric dry and wet sedimentation, along with the corrosion of roofing materials, the roof runoff takes lots of pollutants, which makes it contaminative.Compared to runoff formed through other confluent mediums, the quality of roof runoff is much better, and its collection is easier. Reasonable reuse of the roof runoff is advantageous to economize the high-quality resources of water, reduce the city's non-point pollution load and relieve the pressure of its drainage system in flood time. Roof runoff after collection can be used as landscap water, municipal water, cooling water and recharging groundwater.According to the main types of roof materials and the geographic position of Wuhan City, we chose three roofs with different materials: asphalt,tile and cement in Wuhan university of technology as the sampling points for rainfall runoff. Analysis on the quality of rainwater runoff from the three points during precipitation were done for 14 times in 2008. The main results of this study are shown as followed:Water quality of roof runoff is complex and of strong randomicity. And its pollution level differs greatly with the roof materials: the concentrations of contaminates in the runoff from asphalt roof and cement roof are obviously higher than in that from tile roof. Organic contaminants are the main pollutants in the runoff.Although the concentration of the pollutants changes along with the course of runoff process, the general tendency is that the runoff is more seriously polluted in the initial stage of runoff; the concentration of pollutants in it can reach very high level. As the precipitation time goes by, the concentration of the pollutants drop gradually until it is stabile. The quality of rainwater from different sampling points changes a lot at a time during precipitation. The quality before it is more seriously polluted than that after it. First flush effects are found to be obvious. The concentration of contaminates in various roof runoff are much higher in the early 16-26 minutes when the precipitation is frequent, and that is the initial stage of runoff. When rain interval is longer, the initial stage of runoff will also last longer.At most time, the average values of all the monitoring items for the runoff samples are higher than those required in miscellaneous domestic water quality standard, for example, the average concentrations of COD in samples from asphalt roof,tile roof and cement roof are 286.97 mg/L, 233.24 mg/L and 86.36mg/L, and the turbidity is 54.15NTIU,20.27NTU and 36NTU separately. Sometime the quality of roof runoff is worse than that of group V in environmental quality standard of surface water; but sometimes it can reach the quality standard of miscellaneous domestic water and reuse of reclaimed water, and it can even reach the requirement of group III or group IV in environmental quality standard of surface water.We can see precipitation condition such as the properties of runoff surface, the frequency, intensity, volume and duration of rainfall; season, temperature and atmospheric deposition have great influence on the quality of roof runoff, especially the quality of initial rainfall in urban area of Wuhan City. All these factors make the variation regularity of runoff quality complex.