Taking urban water system as an example, it should be an integration of all water elements within a city, including natural part, namely the natural waters within the urban watershed, such as rivers, streams, and lakes, as well as waters under the surface (groundwater), and artificial (engineering) part, namely the water-related infrastructural facilities built in the urban area, such as those for water supply, wastewater collection and treatment, and its final disposal. The source water for the city is usually from natural waters, and after use for various purposes the used water returns to natural waters. Therefore, the whole urban water system forms a water cycle in the urban watershed, and sustainable management of the whole water cycle is the basic principle.
Water System Management
An urban water system can be viewed as an integration of four subsystems: source water system for provision of source water for water supply, water supply system for purification of the source water to the quality meeting drinking water requirement (usually in a water treatment plant) and distribution to users in the urban area (usually through a distribution pipe network), wastewater system for collection of the used water from users (usually through a collection pipe network) and treatment of the collected wastewater to meet discharge requirement (usually in a wastewater treatment plant), and drainage system for final disposal of the treated wastewater and discharging surface runoff during precipitation.
For the sustainable management of the urban water system with these subsystems, the tasks mainly include the followings:
Water Supply System is principally a system of engineered hydrologic and hydraulic components that provide water supply. Sustainable management of a water supply system include the management of a series of components including the raw water collection point, water purification facilities, water storage facilities, additional water pressurizing components, water distribution network and so on. The objectives are to sustain each of these components in a good operational condition, and ensure the whole system working well under all circumstances.
Wastewater System is downstream of the water supply system in a city. It is also a system of engineered hydraulic components that facilitate water use, receive and finally dispose the used water. Sewer or wastewater collection network and wastewater treatment facilities are the most important components of an urban wastewater system. The main part of such a system is made up of large pipes (the sewers) that convey the sewage from the point of production to the point of treatment or discharge. In some cities, municipal wastewater may also be carried together with stormwater, in a so-called “combined sewer system” in contrast to the “separate systems” where sewage is carried separately in sewers and runoff from streets is carried in storm drains. However, during high precipitation periods a sewer system may experience overflow event, which forces untreated wastewater to flow directly to receiving waters.
Urban Flood Management refers to the management of flood events in cities and surrounding areas. Urban flooding occurs for several reasons and has a wide variety of impacts, and there are correspondingly various strategies to mitigate those impacts. The most common way is to mitigate urban flooding via urban drainage systems, which transport storm water away from streets and businesses and into appropriate storage and drainage areas. Another traditional urban flooding management strategy is gray infrastructure, which is a set of infrastructure types (including dams and seawalls) traditionally constructed of concrete or other impervious materials and designed to prevent the flow of water. An alternative to gray infrastructure is green infrastructure, which refers to a set of strategies for absorbing and storing stormwater at or close to the location where it falls. Green infrastructure includes many types of vegetation, large open areas with pervious surfaces, and even rainwater collection devices. Since the ratio of pervious to impervious surfaces across an area is important in flooding management, understanding and altering land use and the proportion of land allocated to different purposes/use types is important in flood management planning. In particular, increasing the percent of land dedicated to open, vegetated space can be helpful in providing an absorption and storage area for storm runoff.
Water Quantity Management
Water quantity management relies on a combination of policies, at national and sub-national levels of government, to better manage demand for water, promote water use efficiency and allocate water, which varies across seasons and geographically, across uses where it is most needed. Water demand policies are usually developed by taking into account short and long-term projections and uncertainties while incorporating social, economic and ecological functions. Ecologically sustainable limits are often, but not always, linked to water management plans, which limits water use at environmentally sustainable levels by determining long-term sustainable diversion limits for both surface and groundwater resources.
In addition to environmental sustainability, the design of water allocation regimes can incorporate economic efficiency and social equity objectives. To support economically efficient use of water resources, the allocation regimes allow transfer of water entitlements between users, so water can be used for higher value uses. The key tasks of water quantity management usually include:
Water Quota Management is the important task of water quantity management. Water quota is the standard of quantity of water intake per capita, or per area, or per product in unit time. It is also a comprehensive reflection of the users’ water demand and levels of water-saving and water management. A water quota is the water consumption limit under a certain condition. From the perspective of government management, a water quota is a defined standard water quantity usage enforced to be followed by all users. There are usually three main concepts of water quotas, namely, the design quota, statistical quota and management quota. The design quota is often specified for guiding the design of facilities that meet the user’s water demand. The statistical quota is often used for water demand predictions that reflect the current and future situations of water use. The management quota is set for managing practical water use. The objective of water quota management is to implement an index system which quantifies quota indicators, and defines the reasonable scale of water corresponding to accountable factors. This index system integrates regional water allocation (in macro scale) with water demand corresponding to water users (in micro scale).
Water Supply and Demand Management is toward a balance between the amount of water supplied and the demand for real water use. The capability of water provision through a water supply system often closely relates to the availability of freshwater sources, which may fluctuate with climate change or even keep decreasing as reported in many countries and regions. Contrast to this, to the user side water demand may increase with population growth, improvement of living conditions, and economic development. How to take a balance between water supply and demand is always an issue to be studied and resolved. Sometimes supply cannot meet local demand, which creates conflict. Climatic factors, such as drought and global warming, water management practices, and over-exploitation place pressure upon water supplies. Diversion of water resources to increase water supplies often has an adverse impact on water quality and the local ecology. Water conservation measures and water management based on sound scientific principles are needed to avert a water crisis.
Water Quality Management
The availability of waters for various uses is much decided by water quality. Terminologically, water quality refers to the chemical, physical, and biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against which compliance, generally achieved through treatment of the water, can be assessed. The most common standards used to monitor and assess water quality convey the health of ecosystems, safety of human contact, extend of water pollution and condition of drinking water.
Water quality has a significant impact on water supply and oftentimes determines supply options. For human consumption, the water should be free from pathogens and with the concentrations of inorganic and organic chemicals below the limit of hazardous to human health. Water quality criteria and/or standards are also set for other purposes of water use. For example, dissolved ions, such as high concentrations of calcium and magnesium, may affect the suitability of water for a range of industrial and domestic purposes. Even for environmental water quality relating water bodies such as lakes, rivers, and oceans, water quality standards are also set according to different environmental conditions, ecosystems, and intended human uses, especially regarding toxic substances. The most common practices of water quantity management include:
Drinking Water Quality is usually managed following a drinking water safety plan that identifies credible risks from catchment (source water) to consumer (tap water user), prioritizes those risks and puts in place controls to mitigate them. It also requires processes to verify the effectiveness of the management control systems put in place and the quality of the water produced. Drinking water quality management firstly requires a system wide assessment of risks from the source to the tap, and then the identification and monitoring of the most effective control points to reduce identified risks, and finally the development of effective management control systems and operational plans to deal with both routine and abnormal operating conditions. Recognition must be given to the potential for serious events to occur and provision made for management of these events. Drinking water quality management is largely based on a set of drinking water quality guidelines or standards which are health-based targets regarding inorganic, and organic impurities possibly existing in water. Thus, the system assessment is to determine whether the drinking-water supply (from source through treatment to the point of consumption) as a whole can deliver water of a quality that meets the health-based targets.
Wastewater Treatment is to remove contaminants from wastewater and convert it into an effluent that can be returned to the natural water cycle. Once returned to the nature, the effluent creates an acceptable impact on the environment. Therefore, the objective of wastewater treatment is for the protection of environmental water quality. There are several kinds of wastewater which are treated at the appropriate type of wastewater treatment plants including domestic wastewater or sewage treatment, industrial wastewater treatment, and other types of wastewater treatment such as that for agricultural wastewater and leachate treatment. In most domestic wastewater treatment plants, major pollutants to be removed include suspended solids, organic matter, and nutrients. Therefore, solid/liquid separation and biological oxidation are commonly used processes. Industrial wastewater often contains other toxic inorganic and/or organic substances which cannot be easily removed by conventional wastewater treatment methods. In such cases, more sophisticated treatment processes, such as chemical oxidation and adsorption have to be considered. Even for domestic wastewater treatment, there are growing concerns on the hazardous effects of micropollutants, such as ECDs, PPCPs and pesticides residual in the treatment effluent on the environment. More and more advanced treatment units are thus added to the conventional processes for their effective removal. There are also growing needs for treated water reuse to mitigate water shortage. Such kind of reclaimed water should meet the quality requirement for safe use.
Water Pollutant Load Control is a strategy widely adopted for the protection of water environment. Each of the natural and artificial waterbodies has its watershed. It inevitably receives certain inflow of pollutants from either point source (treated and/or untreated wastewater discharge) or non-point source (surface runoff). When the total pollutant load from the catchment area of a waterbody is beyond its capacity of self-purification, water quality will be deteriorated. By definition, pollutant load is the mass of a pollutant that is discharged into a water body during a period of time. Different from the concentration limit of a pollutant in the discharged effluent from a wastewater treatment plant based on national and/or local regulations, there is usually no regulation available for the determination of allowable pollutant load. For this reason, wastes discharge following the discharge regulation may not mean that the receiving water will not be polluted. In some countries, areawide “Total Pollutant Load Control System” has been implemented regarding the protection of waterbodies, especially the closed or semi-closed waterbodies such as lakes, reservoirs, and bays. The determination of the total pollutant load needs an overall evaluation of the inflow of pollutant mass from all sources, the carry capacity of the waterbody, the seasonal variation of hydraulic conditions, and the ultimate goal of environmental water protection.