给水排水专业英语教材原文.docxVIP

四：The looped geometry is also favored from the water quality aspect, as it would reduce the water age. The pipe sizes and distribution system layouts are important factors for minimizing the water age. Due to the multidirectional flow patterns and also variations in reduced water age. The advantages and diadvantages of looped water distribution systems are given in Table 4.1.Advantages:Minimize loss of services, as main breaks can be isolated due to multidirectional flow to demand points.Reliability for fire protection is higher due to redundancy in the system.Likely to meet increase in demand-higher capacity and lower velocities.Better residual chlorine due to inline mixing and fewer dead ends Reduced water age.Diadvantages:Higher capital costHigher operational and maintenance costSkilled operation It has been described in the literature that the looped water distribution system, designed with least-cost consideration only , are converted into a tree-like structure resulting in the disappearance of the original geometry in the final design. Loops are provided for system reliability. Thus, a design based on least-cost considerations only defeats the basic purpose of loops provision in the network. A method for the design of a looped water distribution system is described. This method maintains the loop configuration of the network by bringing all the pipes of the network in the optimization problem formulation, although it is also based on least-cost consideration only. 五:Wastewater Collection and Sewerage System DesignWastewater may be classified into the following components: Domestic or sanitary wastewater. Wastewater discharged from residences, commercial, and institutional facilities.Industrial wastewater. Wastewater discharged from industries .Infiltration and inflow. Water that enters the sewer system from groundwater infiltration, storm water that enters from roof drains, foundation drains, and submerged manholes.Storm water. Runoff from rainfall and snow melt.Water consumption and wastewater production change with the seasons, the days of the week, and the hours of the day.Industrial wastewater can pose serious hazards to municipal system because the collection and treatment systems have not been designed to carry or treat them. The wastes can damage sewers and interfere with the operation of treatment plants.The depletion and degradation of urban water resources recently has led to the advocacy of a sustainable urban water system, characterized by lower water consumption, preservation of natural drainage, reduced generation of wastewater through water conservation and reuse, advanced water pollution control, and preservation and/or enhancement of the receiving water ecosystem.Basic elements of the urban water system are shown in Figures 5.1 Three major water pollution control components are identified: urban drainage (conveying both surface runoff and municipal wastewater), sewage treatment plants, and the receiving waters. The interdependency and connectivity among these components are shown by arrows indicating hydraulic transport by either gravity or pumping. For simplicity, only the major pathways of flow and pollutants are shown. Other transport modes, such as the mechanical removal of solids or sludge from various elements of the system, have been omitted.During the past century, two types of urban drainage systems have evolved-combined and separate. The combined system conveys both surface runoff and municipal wastewater in a single pipe. In dry weather, the flow is transported to the sewage treatment plant and treated. In wet weather, as the runoff inflow into the combined sewers increases, the capacity of the collection system and of the treatment plant is exceeded and the excess flows are allowed to escape from the collection system into the receiving water in the form of the so-called combined sewer overflows.In the separate sewer system, surface runoff is transported by storm sewers and discharged into the receiving waters, and municipal wastewaters are transported by sanitary sewers to the sewage treatment plant and treated prior to discharge into the receiving waters.Both drainage system exist in many variations. Interactions among catchments drainage, sewers, sewage treatment plants, and the receiving waters are shown in Figures5.1a and 5.1b. The interactions between storm sewer discharge and the receiving waters are particularly strong and related to the impact of urbanization on the hydrologic cycle. During urban development, urban surfaces are covered by impervious elements such as rooftops, streets, sidewalks and parking lots, and soils become consolidated by land use activities. Consequently, natural rainfall abstractions caused by vegetative canopy, depressions and infiltration into the ground are reduced and a higher proportion of rainwater is converted into direct runoff. Fast concentration of runoff on impervious surfaces, together with typical hydraulic improvements such as gutters, storm sewers and drains, result in the increased incidence and magnitude of floods. This impact is further aggravated by the straightening, deepening and lining of streams in urban areas.Although the storm water and sewage are conveyed separately in separate system, some cross-connections are hardly avoidable. Influx of municipal sewage into separate storm sewers contributes to the pollution of storm water and the influx of storm water into separate storm sewers contributes to the pollution of storm water and the influx of storm water into sanitary sewers increases the flow rates ,which may exceed the Sewage Treatment Plant capacity and lead to sewage bypasses. Sources of such influx include cross-connection between the sanitary and storm sewers. There are also connections between sanitary and storm sewers and the groundwater-in the form of infiltration of groundwater(undesirably increasing flow rates), and sewer exfiltration leading to the pollution of groundwater. In a well designed and maintained separate sewer system, the cross-connections between storm and sanitary sewers are avoided, sewers are watertight to prevent infiltration and the interactions between storm water and sewage treatment plants are thereby minimized. The main remaining interactions are those between storm water or STP effluent discharge and the receiving waters. Wet weather flow produce hydraulic and pollution shocks on the treatment plants which, while they do not affect the mechanical part part of properly designed facilities, do affect especially the nitrification and denitrification processes of biological treatment by shortening the reaction time, reducing the return sludge flow and diminishing the biomass as sludge is flushed into the final clarifier. Furthermore, the biomass reactions are rather sensitive to fluctuations of concentration, temperature and pH value. All these factors can lead to reduced treatment efficiencies and increased discharge of pollutants into the receiving waters.In combined sewer systems, the interactions among the three major components are even stronger than in separated systems. In dry weather , the combined system functions like the separate system-the only flow generated is municipal sewage which is transported to the sewage treatment plant for treatment. In wet weather , surface runoff enters directly combined sewers. When the capacity of the system is exceeded, flows are either discharged directly into the receiving waters with very adverse impacts, or enter CSO control facilities , which also interact with the STP.The pollution characteristics of CSOs, while somewhat similar to those of storm water, are strongly affected by domestic sewage and sewer sludge washout from combined sewers. Consequently , CSOs are particularly significant sources of solids, biodegradable organic matter, nutrients and faecal bacteria . Their impacts on the receiving waters are similar to those described in the preceding section , but much stronger in terms of oxygen depletion, eutrophication and increased productivity, and faecal pollution. It is desirable, therefore, to control CSOs prior to their discharge into the receiving waters. Such control facilities are typically operated in conjunction with the sewage treatment plant. 六：Stormwater collection and sewer design Historically, many communities elected to collect storm water and wastewater in combined sewers and convey the peak dry weather flow to the wastewater treatment plant while large surges of storm water were diverted directly to surface water bodies. The resulting mixture of sewage and storm water has major adverse impacts on the receiving bodies of water. Current regulations prohibit this combination in new facilities. Design procedures for storm sewers The design of storm sewers includes a number of steps, including the establishment of design objectives , preparation of input data, computation of runoff flows, and the sizing and layout of drainage elements. Design objectives. Design objectives are generally established during the planning process. Traditionally, such objectives dealt only with runoff quantity and specified the desired levels of protection against flooding by specifying return periods of design rainfall events. The minor drainage , which includes underground sewers and small open channels and provides local convenience and prevention of water ponding , is typically designed for short return periods, from 1 to 10 years. The major drainage system conveys flood flows through urban areas and includes large sewers , the natural drainage system, swales, streets and other overland routes. This system is generally designed for return periods from 50 to 100 years. Thus, there is more interest in defining drainage water quality objectives in terms of the receiving waters, quite often outside of the mixing zone. Additionally, in the ecosystem approach , traditional water quality objectives for the receiving stream, typically derived from the water users, are expanded for ecological protection and enhancement of the stream, further increasing the expectations on runoff quality. Thus, the water quality objectives for urban runoff will probably continue are driven by the water quality conditions in the receiving waters, established locally for specific watercourses. Design flows and pollutant loads Using the established design objectives, runoff flows and their associated pollutant loads must be determined at various points in the drainage network. In current practice, various levels of sophistication are employed, depending on the design objectives and the size of the drainage area. Calculations of flows start with the preparation of rainfall data inputs used to calculate runoff hydrographs at inlets to the drainage network. These inlet hydrographs are then routed through the transport network. Rainfall data are used in urban runoff calculations in various forms, depending on the design approach and computational procedure used. The most common forms include intensity-duration-frequency curves, synthetic design storms, historical design storms, and actual or synthetic long-term rainfall records. Although the literature on rainfall data for storm sewer design is rather extensive the issues of uncertainties in rainfall inputs and their impact on calculated runoff flow have not been fully resolved. Uncertainties include errors in point measurements, and impacts of temporal and spatial distributions on estimates of catchment rainfall. Other uncertainties are introduced in the analysis of observed data by using various assumptions which may or may not be valid. It should be noted , however, that the impact of rainfall data uncertainties on calculated runoff flows is somewhat reduced by the fact that the catchment functions as a filter which dampens out some perturbations (real or spurious) in rainfall inputs. IDF curves were among the earliest rainfall inputs used in runoff calculations, particularly as inputs to the rational method. The IDF curves are derived from rainfall records suing frequency analysis, usually applied to annual rainfall maxima of durations from 5 minutes to 24 hours. The main shortcoming of the IDF curves is the assumed constancy of rainfall intensities, which makes them unsuitable for computations of runoff hydrographs. The development of synthetic design storms was necessitated by the need to use time-varying rainfall inputs to calculate runoff hydrographs. Design storms are generally defined as rainfall events developed for the design of specific facilities , such as sewer pipes or retention basins. Design storms are associated with certain return periods, and the calculated flow values are commonly(and erroneously) presumed to have the same return period as the storm. In spite of theoretical shortcomings of the design storm concept and evident uncertainties in return periods of various parameters of the calculated hydrographs, design storms are widely accepted in practice. The criticism of synthetic storms as “events that never occurred” led to the adoption of historical design storms, which have the advantage that their severity and the resulting magnitudes of runoff flows and flood damages are well documented. Some shortcomings of the synthetic design storms, such as uncertainty regarding the storm return period and antecedent moisture, apply here as well. Furthermore, long-term climate change and its impacts on precipitation undermine the inherent assumption that the statistics of rainfall records containing these historical storms will not change. The limitations of design storms and related single or multiple event simulations can be avoided by establishing design flows from frequency analysis of simulated runoff records. In this case, precipitation is converted into flows using one of the continuous simulation models described in the section on models. Rainfall inputs in these simulations are actual rainfall records or records simulated by rainfall models (the former are more readily accepted by practitioners). Simulated runoff records, in terms of water quantity and quality , are well suited for analysis of water quality in he receiving waters. Difficulties with long-term variations in precipitation and establishment of joint probabilities of runoff discharges and conditions in the receiving waters are unavoidable. Design of sewer system structures Sewer pipes-after computation of runoff flows and their characteristics, the layout of the sewer system is selected and its elements are designed. In general, each pipe is designed individually as an open-channel in which the design flow depth should be less than 0.85 of the pipe diameter . Following the sizing of individual sewers, the system functioning is checked to assess its overall performance and whether any damages would occur when the pipe surcharging would cause damage at those points the hydraulic grade line rises above the elevations of basements, or underpasses. Sewer inlets are key elements in drainage design which divide runoff flow into two components-surface flow and subsurface flow conveyed by sewers. Temporary storage on the catchment surface is utilized to reduce flow peaks in sewers, and sewer surcharging is avoided by placing a limit on the maximum flow conveyed through inlets. Other considerations in inlet design include bicycle safety and pedestrian convenience-gutter flow over pedestrian of water. As a result the density of inlets in urban areas is fairly high, certainly higher than required for road drainage for road drainage. This further emphasizes the need to restrict inlet capacities. The ultimate goal of storm water management is to provide adequate services in urban areas, while maintaining storm water flows and volumes at levels comparable to predevelopment conditions and controlling pollutant fluxes. Such a goal should be achieved under a number of constrains, including the given physical constraints (local physiography ), cost-effectiveness , acceptable future maintenance burden, and a neutral impact on the environment. 八:Coagulation and flocculationThere are three types of substances which can be found in water. These substances are chemicals in solution, colloidal solids, and suspended solids. Coagulation/flocculation will remove colloidal and suspended solids. In the water treatment industry, the terms coagulation and flocculation imply different mechanisms. Although the words “coagulation” and “flocculation” are often sued interchangeably they refer to two distinct processes. Coagulation indicates the process through which colloidal particles and very fine solid suspensions are destabilized so that they can begin to agglomerate if the conditions are appropriate. Flocculation refers to the process by which destabilized particles actually conglomerate into larger aggregates so that they can be separated from the wastewater.Coagulation The particulates in raw water, which contribute to color and turbidity, are mainly clays, silts, viruses, bacteria , humic acids ,mineral (including asbestos, silicates, silica, and radioactive particles), and organic particulates. At pH levels above 4.0, such particles or molecules are generally negatively charged. Coagulation is employed for the removal of waste materials in suspended or colloidal form. Colloids are presented by particles over a range of 0.1-1.0nm. these particles do not settle out on standing and cannot be removed by conventional physical treatment processes. Colloids present in wastewater can be either hydrophobic or hydrophilic. The hydrophobic colloids possess no affinity for the liquid medium and lack stability in the presence o