The Study On Effects Of Artificial Short Circuiting Path On Suspended Solid In Constructed Wetland

Constructed wetland is a sewage treatment technology that has been grown up since1970s. It has the advantages of simple process equipment, low cost and energy consumption,easy to construct, operate and maintain. The applying fields of CW are broader than thetraditional approaches, which mainly include agricultural sewage, urban stormwater runoff orsewage, eutrophication of the lake and some industrial wastewater. In the process of abovesewage treatment, organic matter, nutrients and heavy metals in the sewage mostly exist insuspended state. Therefore, the high efficient precipitation and purification of suspended solid(SS) had drawn more and more attention and had become one of the key points that must beconsidered in the design process of constructed wetland.This thesis mainly targets surface constructed wetland (SCW). Firstly, we verified thefeasibility of quasi-3D model in SCW by numerical calculation. Secondly, the movements ofSS in the constructed wetland were simulated under different shapes of artificial shortcircuiting path with HLE stably generated.In the case of model verification, a series of flume experiments were carried out underseveral width/depth ratios and Reynolds numbers, referring to the ranges of the parametersreported in an actual constructed wetland. The unsteady velocity field was measured in detailby PIV technique. Numerical simulations were conducted using quasi-3D model under thesame conditions as the flume experiments. Comparative analysis of flow velocity distribution,velocity fluctuation intensity, the relationships between HLE wave length and Reynoldsnumber, and between velocity fluctuation intensity and velocity difference in the modelwetland was conducted. In addition, the process of particles diffusing to vegetation area wereanalysed by numerical tracer experiments. Results showed that computation results agreedwith observation results very well in the longitudinal and horizontal velocity and otherdynamic changes, which verified the feasibility of quasi-3D model in SCW. In addition, thescale and intensity of HLE were effected by width/depth ratios and Reynolds numbers.In the case of SS movement study，the movement and precipitation of SS in single anddouble artificial short circuiting paths were calculated respectively, using the added SSmovement model. Under the single path condition, the volume and width of SS intrusion tovegetation areas were associated with the channel width/depth and flow rate, and the volumesand width of SS intrusion were increasing with width/depth reduction and flow rate increase.Under the double paths condition，the movement and precipitation of SS were simulatedunder different widths of central vegetation area, and the percentage of SS intrusion tovegetation area and distribution of SS in wetland were analysed. Results showed that under the double path condition, because the mutual interference of HLEs enhanced the substanceexchange between the paths and vegetation areas, the volumes of SS intrusion to vegetationarea were more than only single path exited in the constructed wetland. In addition, thevolumes of SS intrusion to vegetation areas were compared with width of central vegetationarea. Therefore, the purification efficiency of constructed wetland can be enhanced byappropriately adding the artificial short circuiting path and controlling the distribution of pathand vegetation.