| Runoff directly discharging from separate storm sewer system can cause serious pollution of receiving water, whose quality were found even worse than combined sewer overflow by many studies. Urban runoff purification treatment before emissions is of great significance. Suspended solid (SS) is one of the main pollutants of storm, which is the carrier of many other pollutants (such as heavy metals, organic materials, PAHS and so on) in addition to its polluting. So the removal of SS can eliminate a lot of other pollutants at the same time, improving water quality significantly. On the basis of summarizing various existing SS removal measures, this paper is intended to develope pollution control technology and equipment using principle of precipitation and filtration to enhance SS removal efficiency at the end of drainage system and achieve purification of runoff discharging at a low cost. The main contents are as follows:(1) Sewer sediments which deposited during dry-weather or low flow rate would resuspend during rain events, making up a significant source of TSS of wet weather flow. This paper investigated size distributions, the concentrations of volatile organic solid (VS) and four heavy metal (Pb, Cu, Zn, Cr) of sewer sediments collected from seven categories of catchments of Jiaxing, China. Moreover, the samples were graded into five different settling velocity classes, sediment characteristics of each class were assessed. The result shows that sewer sediment characteristics of each sampling point vary from each other in accordance with local surroundings (such as human activities, traffic and so on). Heavy metal loads of CSS, RA, RDA and IA are relatively high, showing pollution risks to receiving waters. Sediment grain size alone can not entirely measure its settling velocity, it’s necessary to investigate pollution characteristics of particles with different settling velocities for estimating the removal rate of SS-associated pollutants when removing SS by precipitation. In general, particles with lower settling velocities are higher in pollution concentrations, when it comes to total pollution load, the contribution of particles with higher settling velocities becomes bigger than when only considering concentrations.(2) Basing on the design of plated sedimentation tank and restriction factors SS precipitation removal efficiency found in the experiment, a series of SS precipitation devices were designed. Model sands which are "settling velocity similar" to real sediments were used as experimental subject to explore SS efficiency (η) and head loss (Ah) for each device at different flow conditions. The result shows that these SS precipitation devices can improve η by different extents at different flow conditions respectively, while the increase of△h is very limited, and the structure of these devices is simple, so they can be used in standard sumps at upstream as well as end of drainage system. Of all the devices, SS removal effect of c-2and e-3is best and most stable.(3) In order to overcome shortcomings of traditional sand filter, SS up-flow filtration device which using foam particles as filter material was designed. Model sands and real pipe sediments were used as experimental subject to explore SS efficiency (η) and head loss (△h) for a series of filter units with different filter material particle sizes and filter layer thickness at different flow conditions. The result shows that the increment in η of filter units compared to the original sump is higher than that of precipitation devices, the SS removal efficiency can increased by more than20%at all test flow conditions. While the additional Ah is also bigger, so the SS up-flow filtration device is only suitable for using at the end of drainage systems. Because only95%of the filter layer height is filled with foam particles and ρss> ρwater> ρfilter materia, only by shaking the filter units lightly, almost all of the SS particles inside it would fall off and filter material would be cleaned conveniently. |
PDF Full Text Request