Regulation Scheme And Technical And Economic Analysis Of Urban Rainwater Source River In Northern China

China is currently in the process of rapid urbanization,with the rapid increase in urban population and discharge quantity,resulting in natural surface morphology change.The black and odorous phenomenon of urban rain-source river due to the lack of natural runoff,poor self-purification capacity and large amount of sewage and other reasons,makes a negative impact on the public's sensory experience and living environment.Because of the complexity of black and odorous water bodies' formation and the difficulty of treatment,remediation project has problems of temporary solution,huge cost and unsustainable economy.Therefore,it is of great significance to study and formulate a long-term economic and ecological sustainable remediation program of urban rain-source black and odorous river based on China's current specific situation.Based on the case of Hejia Stream in Harbin,this paper analyzes current situation of the study area and studies the remediation plans according to the actual condition of river basin.First of all,through the analysis of the status quo of the river course and the current replenishment program,it is clear that the primary goal is to ensure ecological water quantity and quality: the east stream uses a decentralized sewage treatment system to treat wastewater in situ,reusing for river landscape and reducing the surface water replenishment ratio;west stream upstream utilizes rainwater to recharge river water,transforming free land for rainwater harvesting facilities,combined with a variety of green infrastructure to effectively control non-point source pollution;west stream downstream takes measurements of the sewage treatment plant standard improvement,and increasing depth of processing units to guarentee the landscape water quality.(1)Based on the feasibility of water supply plan in Hejia Stream,calculation results of ecological demand water quantity are as follows: Hejia Stream ecological water demand is 95,090 m3/d,of which the upperstream of west stream water supply is 10,020 m3/d,west downstream is 67,630 m3/d,east stream is 17,440 m3/d.The factors contributing to ecological water demand most are river volume and residence time,among which residence time is directly related to the water quality of the supply water.Next,the treatment process is determined,according to the water quality and removal rate of decentralized wastewater treatment and rainwater utilization.Wastewater treatment uses MBR as the main process,having advanteges of small footprint,high degree of automation,and satisfied treatment effeciency.As rainwater is of poor biodegradability,it is treated by PAC as coagulant through physic-chemistry method.At the same time,treatment process is combined with a variety of sponge urban ecological measures,through the initial rain volume abandoned tank,water storage module storage and coagulation,sedimentation,purification,a variety of processes,to achieve river water replenishment and control of non-point source pollution purposes.(2)The cost-benefit analysis method is used to analyze the new strategy.The east stream adopts the scheme of both decentralized sewage treatment of reclaimed water and surface water to recharge the river.The amount of domestic sewage can be 10,200 t/d.Analysis results of decentralized processing facilities shows that the fixed cost is 25.9 million yuan,the total operating cost is 2.622 million yuan/year,and total benefit is 6.795 million yuan/year.It can be calculated that the average fixed cost of 0.57 yuan/t,which is lower than the average level of China's centralized sewage treatment plant,the average operating cost of 1.75 yuan/t,and the average income of 4.53 yuan/t.In the discount rate range of 6% to 10%,the net present value decreased from 31.5241 million yuan to 13.4272 million yuan,indicating new strategy is economically feasible.The upperstream of west stream utilizes rainwater for replenishment,with the catchment area of 10 km2.The rainwater harvesting potential result is 11,500 t/d,which is larger than that of the ecological water demand.The analysis of the cost-benefit of rainwater collection and utilization indicates that,the fixed cost is 5.4 million yuan,the total operating cost is 0.62 million yuan/year,and the total benefit is 6.7986 million yuan/year.It can be calculated the average fixed cost of 1.20 yuan/t,the average operating cost of 0.41 yuan/t,the average income of 4.53 yuan/t.Economic benefits mainly come from the benefits of saving water resources,followed by benefits of eliminating pollution emissions.In the discount rate range of 6% to 10%,the net present value is reduced from 31.5241 million yuan to 13.4272 million yuan,which indicates that the scheme is cost-effective and economically sustainable.The research results have certain reference significance for government decision-making process.(3)In order to evaluate comprehensive benefits of the remediation program,the evaluation index system is established and the analytic hierarchy process is used for weight calculation.After the non-dimensional treatment of the indicators of each scheme,the comprehensive benefit result is obtained.The weight results show that among economic,social and ecological benefits,experts and residents think that the most important weight of is social benefits weighting 0.56,followed by ecological benefits of 0.35,the lowest is the economic benefits of 0.09;the highest weight index indicators are providing good living environment,river water quality improvement and health risks reduction,with weights of 0.375,0.266 and 0.185,respectively.The comprehensive benefit results show that scenario III which is the new strategy proposed in this paper,the comprehensive benefit value is 0.893,which is higher than the benefit value of the situation II of 0.701.However,it is still necessary to carry out detailed project budget and health risk of reclaimed water evaluation.