Experiment Research On Residential District Runoff Treatment Using Horizontal Subsurface Flow Wetlands

With speeding up of the process of urbanization development in China, the proportion of permeable land area becomes increasingly smaller. The paroxysmal, impulsive and randomicity urban non-point source pollution caused by storm runoff has become one of the major sources of receiving water pollution. Consequently, water pollution by non-point source has become one of the main problems to be urgently solved in present comprehensive treatment of urban water pollution. By making use of the valuable rainwater resource, not only can the conflict between supply and demand of urban water resource be relieved to a certain degree, the quantity of urban surface runoff can also be effectively reduced to delay the form of conflux, and consequently to lighten the pressure of rainwater Drainage facilities and to cut down anti-flood investment and flood loss. Increasing the urban rainwater use efficiency is an effective measure of water-saving cities.Through the analysis of runoff rainwater quality at two monitoring points at the campus of Chongqing University, this paper aims to explore community rainwater quality and the flush model of rainwater quality as related to the length of rain time. Horizontal subsurface flow wetland is designed and constructed according to rainwater runoff characteristics and its fluidity state is studied to understand the fluidity state of horizontal subsurface flow constructed wetland under steady flow and unsteady flow. Pilot-scale experiment about the rainwater runoff treatment effect and impulsive load treatment effect is carried out to find out the best mode and to analyze the critical factors affecting treatment effect. The part findings have been validated at the Palm Springs water quality maintaining project.â‘ Through rainwater runoff monitoring results, it is found out that the main pollutants of runoff are SS, CODcr, NH3-N and TP. The average concentration of these pollutants is low and causes only low-concentration wastewater. Runoff quality is related to the pollution degree of underlying cushion, road traffic, people flux and sunshine days before rain. Roof rainwater and road rainwater takes on obvious first flush effect, while the flush model of rainwater runoff pollutant basically conforms to the model of formula C t = C₀e^-K-t.â‘¡The fluidity state experiments indicate that the fluidity state of the horizontal subsurface flow wetland is dispersing model. The fluidity state of experiment II (D/UL=0.109) is closest to the ideal horizontal flow, and its fluidity state is equivalent to the one of CSTR model with six levels in series. The fluidity state of experiment III(D/UL=1.354) is closest to complete mixing model which is not in series of CSTR. The fluidity state of experiment I(D/UL=0.292) is situated between the ideal horizontal flow and the complete mixing model, but is more closer to the former.â‘¢The optimal combined plan is definited after 18 orthogonal experiments, the HRT is 36 hours, the depth is 400 mm, and the days before last operating is 8 days. After treatment, the outflow is clean and its average concentration of COD_crr,NH₃-N,TP,TN and turbidity are16.25 mg/L,0.87 mg/L,0.040 mg/L,1.15 mg/L &1.51 NTU, and satisfies the requirements of (GB/T18921—2002) and (GB/T18920-2002). Meanwhile through the additional 4 experiments about the influence level, we find that HRT is the restrictive element of the treatment of the horizontal subsurface flow wetland.â‘£Through the two different impact load experiments under 3 hours and 5 hours, we find that the horizontal subsurface flow wetland has stronger ability to bear the impact load of CODcr and TP, but the removal effect of NH3-N is not obvious, and its treatment effect are 14.78% & 6.99% respectively. So we can draw the conclusion that the treatment effect of complete mixing model which is not in series of CSTR in the artificial wetland under impact load.⑤The advantages of horizontal subsurface flow constructed wetland are outstanding. It has favorable treatment effect, low construction and operation cost, easy management and maintenance, and also nice landscape effect. At present, this technology has gradually been applied to water treatment of residential communities and parks. This will be a prosperous wastewater treatment technology and the development orientation of future water treatment.