| In the context of rapid urbanization,frequent climate extremes and continued industrialization,water shortage,water pollution and flooding have become urgent problems for urban development worldwide.The management and utilization of water resources have gradually attracted widespread attention,and China has proposed the policy of"sponge city"to deal with urban water problems.Roof rainwater has become one of the most suitable resources for development and utilization due to the advantages.Rooftop Rainwater Harvesting System(RHS)collects,stores,and utilizes rainwater,which can alleviate urban water crisis and prevent floods.The implementation of RHS helps to construct“sponge city”and provide guidance for other regions to alleviate water resource shortages and improve rainwater resource utilization technologies.Under the influence of climate change and water demand change,the design optimization of RHS and the uncertainty of actual engineering benefits are important factors that limit its widespread application.Based on the background of sponge city,this study starts from the research foundation of roof rainwater utilization,analyzes water quality,quantity,and water supply demand using national standards and government data,and outlines the composition and benefits of RHS.The article specifically analyzes the concentration and trend of pollutants in roof rainwater,designs scenarios to simulate operating conditions,and constructs the optimal design and benefit evaluation plan for the campus RHS.This study provides technical support for rainwater resource utilization models in water quality and resource deficient cities.The main research contents and conclusions are as follows:(1)Taking the roof rainfall of Yanta Campus of Chang’an University as the research object,six monitoring indicators were determined,including p H,TN,TP,COD,SS,and turbidity.Simultaneously,the average concentration(EMC)and coefficient of variation(_)of pollutants in rainfall runoff were used to analyze the concentration and trend of different pollutants.The results indicate that the p H of roof rainwater remains basically neutral.The concentration of pollutants is relatively high in the early stages of rainfall,and gradually decreases and eventually stabilizes with the extension of rainfall time.The range of each pollutant is 43.47-112.8mg/L,90.45-157.5mg/L,9.74-18mg/L,0.04-0.19mg/L,and41.8-72.4NTU,respectively.This study plotted the M(V)curve through the correlation between cumulative pollution load rate and cumulative runoff rate.It was found that pollutants have varying degrees of initial erosion effects,and the higher the rainfall intensity,the more significant the erosion effect.Finally,through the comprehensive analysis of different rainfall events using the pollutant trend method,it is determined that the initial abandoned flow rate of roof rainwater is 3mm,which can achieve a 52%-74%SS and COD pollution control effect.(2)Based on the rainfall data of Xi’an city from 1960 to 2020,a typical year of rainfall was selected,and a daily water balance simulation model was constructed to simulate the rainwater harvesting efficiency(WCE),water saving efficiency(WSE)and water supply reliability(R)under different roof rainwater utilization facility scales(0-250 m~3)and different water demand(toilet flushing,irrigation and comprehensive use).The results show that all operational indicators increase with the increase of water storage volume and level off beyond maximum.In terms of rainfall characteristics,the system operation performance shows a pattern of abundant water year>average year>dry water year.In addition,comparing the results of the three scenarios found that less water use would provide greater system efficiency,showing the lawn irrigation>indoor flushing>integrated use.Therefore,the RHS with higher rainfall,larger water storage capacity,and lower water demand has higher operational performance.We calculated the benefit cost ratio of RHS and found that the optimal design volume range is 25-70m~3.The design volume of 1-5a return period(45-92m~3)calculated by the design rainstorm method shows that the water balance model results are relatively small,and the rainwater utilization efficiency is also high.Finally,the volume optimization design method and application steps for on-site construction of RHS will be formed.(3)Taking Chang’an University Yanta campus as the study area,the roof rainwater harvesting scheme was planned based on environmental conditions,geographical location and water demand.At the same time,the campus is divided into five areas:I,II,III,IV,and V.The designed water storage capacity is 200m~3,400m~3,350m~3,200m~3,and 250m~3,and the annual rainwater usage is 1408.6m~3,2782.7m~3,2519.4m~3,1052.5m~3,and 1660m~3,respectively.The campus rainwater flood model was built using SWMM,we found that it can reduce surface runoff by 23.52%-29.92%and peak runoff by 15.79%-38.93%when the return period is1-5a.In addition,the RHS can also effectively improve regional sanitation conditions,increase residents’awareness of water conservation,and alleviate urban water resource shortages and water security crises,resulting in good economic,environmental,and social benefits. |
