| Water-scarce cities often face problems of urban landscape water quality deterioration because of insufficient surface water(SW)for replenishment.Therefore,the need to find alternative water resources for meeting these water quality requirements cannot be overemphasized.Because of its stability and controllability,reclaimed water(RW)from wastewater treatment plant effluents have been widely used as a supply source for urban water bodies.However,due to the relatively high concentrations of nutrients contained in RW,the quality of receiving water bodies is difficult to control and most often does not meet the requirements of surface water environmental quality standards(Grade IV or V),which limits the utilization of RW as a source for replenishment.On the other hand,since urban water bodies are mostly used for aesthetic and recreational purposes,evaluation of their quality should be based on aesthetic aspects.In addition,the low concentration of suspended solids in RW could improve the landscape water quality,whereas the high concentrations of nutrients,such as nitrogen and phosphorus,could easily cause rapid algae growth to deteriorate it.Therefore,due to the high nutrients concentrations and low turbidity of RW,it is necessary to evaluate the effects of RW replenishment on the quality of urban waters and consider their benefit for landscape quality improvement.Thus,this study highlights the need to pay more attention to the landscape quality of urban waters.Firstly,control indicators of landscape quality in urban waters was proposed by deductive analysis,and the main pollutants affecting the indicators were analyzed.Subsequently,a numerical landscape water quality model was developed by the characterization of the physical and biological processes of pollutants occurring in urban water bodies.The basic assumptions of the modelling effort,as indicated above,was that the low concentration of suspended solids and the high concentrations of nutrients in RW.Therefore,the model was simulated to elucidate the effect of RW replenishment on the aesthetic quality of urban water bodies and to assess the feasibility of using RW from the viewpoint of landscape quality control.Moreover,an optimization scheme was established for RW replenishment and the improvement of the quality in urban water bodies.Finally,the urban ponds in Xi'an City,which are supplemented by RW,were taken as an example to evaluate the proposed water resources allocation scheme for guaranteeing their landscape quality.Knowledge of these effects can be helpful to the further use of RW as a water source for replenishing urban water bodies and relieve the water shortage problem.The main outputs of this work are as follows:(1)Proposal of water transparency as a control indicator of landscape quality in urban water bodies.Due to the shortage of SW and the high concentrations of nutrient in RW,there is no overstating that the water quality of urban water bodies is difficult to meet surface water environmental quality standards(Grade IV or V).However,it should be noted that the standard is mainly to ensure the health of the water environment,but the functions of urban water bodies is to provide an attractive urban scenery.Thus,it was not practical and reasonable to evaluate the water quality of urban water bodies by those standards.Otherwise,it may lead to a situation of their “overprotection.” Therefore,it is proposed rather to evaluate the quality of urban water bodies using visual indicators,such as color and turbidity.Therefore,by evaluating the effects of pollutants on the landscape quality(chromaticity and turbidity)and transparency of urban water bodies,transparency(measured by Secci disc,SD)was identified to be a useful control index of the landscape quality.(2)Development of a landscape quality model for urban water bodies.According to the connotation of transparency and its calculation method,and based on the characteristics of pollutants in urban water bodies,three kinds of pollutants were found to be the main factors that affected the transparency of urban waters,namely algae,residues and inorganic suspended solids.Consequently,a calculation method was proposed by analyzing the variations of these pollutants in water bodies.For algae,it is determinate by the process of growth,death and capture by zooplankton,whereas for residues,it is the death of algae and zooplankton,the capture of algae but unutilized by zooplankton,and the mineralization and sedimentation of residues.For inorganic suspended matter,it is the interaction with sediment.Furthermore,as urban water bodies are generally shallow,the resuspension process of sediment would occur easily.Therefore,the release of inorganic suspended solids,nutrients and residues from sediment was emphasized in the model.Then,a closed-loop landscape quality module was formulated numerically by the biotransformation processes of those pollutants in water bodies with the different forms of carbon,nitrogen and phosphorus.Finally,using MIKE3 software and by embedding its hydrodynamic and quality modules,a landscape quality model for characterizing urban water bodies was developed.(3)Optimization of RW supply scheme and strategies for landscape quality control was carried out by scenario analysis of the landscape quality model.Firstly,based on the types of urban waters,Hancheng Lake and Xingqing Lake were used as the representative of channel-type and lake-type urban water bodies,respectively.Then,calibration and validation of the model using long-term measured data from the selected urban waters were carried out,for which good degrees of fitting of those indicators were obtained.Based on this,the effects of replenishment water(quality and quantity)on the transparency and water quality in the two urban water bodies were studied by scenario analysis.Findings indicate a maximization of the advantages of the low-suspended solids content of RW to offset the effect on algae growth(caused by its high concentration of nutrients)on the transparency through adjustment of the RW proportion.Consequently,the required inflow for replenishment was reduced and an optimized supply scheme of RW replenishment was developed.Nonetheless,the proportion of RW and the total inflow required for replenishment were different for the various types of urban water bodies.Taking SD?70 cm as an example,for Hancheng Lake and Xingqing Lake,the required proportions of RW were 35-45% and 25-35% in May-October to achieve the minimum total water inflow,respectively.In January-April,85-100% and 75-100% of RW were required,respectively.Moreover,under the same conditions,the total water inflow required for replenishment of Xingqing Lake is greater than that of Hancheng Lake(the corresponding hydraulic retention time could be greater than 35.7%).Through the analysis of the optimization schemes for the two types of urban water bodies,strategies for landscape quality control was proposed,including a unitized RW rationale,optimized flow pattern and shortened hydraulic retention time.(4)Water resource allocation schemes for ensuring the landscape quality of multiple urban ponds were studied with Xi'an city as an example.For many ponds in Xi'an city,the utilization of RW for urban ponds replenishment has been proposed.A target landscape quality of SD?70 cm was obtained through statistical analysis of the measurement data of 14 ponds in Xi'an city with the population distribution method.Subsequently,a water resources allocation model of Xi'an city was established with WEAP software,and the quality and quantity of water sources for replenishment of each pond were obtained.Then,the transparency of each pond was simulated by the landscape quality model,which was then used to evaluate the landscape quality of the ponds.The results of water source configuration and water quality simulations by different scenarios indicate a decrease in the total water inflow required for the urban ponds with RW as the water source for replenishment.Also,the transparency of the ponds increases by an average of 13 cm,which improves the landscape quality of those ponds.However,it is projected that between the years 2020 and 2030,there will still be some ponds with low transparency after replenishment with RW(the average is about 45 cm).The simulation shows that the landscape quality of those ponds would be improved by utilizing RW rationally or by cascading it through connected ponds in series.Appropriate utilization of RW was achieved by taking advantage of the lower SS concentration for SD improvement in RW,which increased the transparency of the test ponds by 12 cm and decreased the total inflow required by 7.6%.Also,the water inflow required could be reduced by up to 63.3% by connecting the ponds in series according to the topography,whereby the transparency of all ponds in the city achieved the target values at a 10.2% decrease in the total inflow required.Therefore,with Xi'an city as an example,it is demonstrated that RW utilization and their rational allocation,as well as the connection of ponds in series is an effective measure to improve the landscape quality of urban water bodies in water deficient cities. |
PDF Full Text Request