The traditional linear ‚ take, make, waste’ approach to managing water is increasingly proving to be unsustainable as it is leading to water stress (insufficient water supplies), unsustainable resource (energy and chemicals) consumption, the dispersion of nutrients into the aquatic environment (especially phosphorus), and financial instability in utilities. Different approaches are needed to achieve economic, environmental, and social sustainability.
 

Sustainable infrastructure and management authorities must be developed which will dramatically reduce net water withdrawals for urban uses; reduce water supply and waste management resource consumption, with a goal of energy neutrality; and significantly improve nutrient management. To that end, a diverse toolkit is available and is increasingly being applied to reduce net urban water abstraction from the environment, thereby helping to relieve urban water stress and reduce resource consumption and nutrient dispersal. This toolkit includes the following resources:
 

Stormwater management and rainwater harvesting
 

A diverse set of technologies exists to capture stormwater runoff and either treat it for introduction into the environment or contain it for later use. Components of these systems are generally distributed throughout the urban area and, consequently, are referred to as distributed or decentralized stormwater management. Stormwater capture offers significant potential to contribute to urban water supply in locations with modest population densities, or even in densely populated areas with high precipitation. For example, infiltrated stormwater recharging the groundwater can serve as a source of water for irrigation and other non-potable uses during dry periods.
 

Water conservation
 

Conservation also involves a combination of technologies and practices. Water conservation not only benefits water supply and treatment by reducing the quantity of water required, but it also reduces wastewater treatment needs. Reduced water and wastewater flows extend the life of conveyance and treatment facilities, which can contribute to the financial sustainability of water and wastewater utilities. This is provided reduced flows do not adversely affect revenues needed to operate these utilities.
 

Water reclamation and reuse
 

Reclamation and reuse are established practices that can dramatically reduce net withdrawal from the environment. Cost-effective treatment technology continues to evolve, capable of previously unknown performance levels, especially membrane technology. Increasingly stringent discharge standards are also enabling reclamation and reuse as the incremental cost for water reclamation is reduced compared to treatment and discharge options.
 

Energy management
 

Water reclamation and reuse may necessitate increased treatment levels which consume more energy. These consumption increases may be more than offset by reduced energy requirements for water supply, treatment, and distribution, potentially offering a net reduction in energy use. Systematic analysis of urban water management systems, including water supply, water and wastewater treatment, and conveyance, can result in water supply approaches which require less energy in aggregate.
 

The energy present in the wastewater stream, which consists of heat energy and the energy value of organic matter and nitrogen present due to pollutant discharges, can serve as an energy source. Three types of technologies are generally available to leverage the energy potential of the wastewater stream: (1) anaerobic biological which convert organic matter to biogas, (2) thermal technologies which combust (particulate) organic matter and extract thermal energy, and (3) microbial fuel cells.
 

Nutrient recovery
 

Wastewater management systems are critical links in global nutrient cycles, as a portion of the nutrients applied to grow crops for human consumption ultimately ends up in the wastewater stream. Phosphorus is the most problematic nutrient in the waste stream; to effectively recycle phosphorus, phosphate recovery from wastewater is needed. Technologies are available not only to remove phosphorus from the wastewater stream, but also to recover it in useful forms.
 

In general, phosphate can be recovered as either calcium phosphate or struvite and improvements continue to be made in the approaches and technologies used. Source reduction is an important priority in terms of phosphorus management. The mass of phosphorus in the wastewater stream has been reduced significantly in recent years as a result of the ban on phosphate in home laundry detergents, and it can be further reduced through bans for other products.
 

Source separation
 

This refers to the provision of multiple qualities of water for domestic and commercial use and the collection of separate wastewater streams with significantly different qualities. Dual distribution systems, which separately provide potable water and water for urban irrigation, are becoming common in water short locations. This allows different qualities of source water, with different levels of treatment, to supply these needs. The net result is that additional water supplies are made available to meet human needs. Separation of waste sources can facilitate water reclamation and reuse, organic matter conversion to energy, and nutrient recovery.
 

These approaches can be incorporated into urban water and resource management systems with improved performance characteristics, including significantly reduced urban water use, reduced energy consumption, and nutrient recovery. Centralized water reclamation and reuse systems, and systems incorporating decentralized elements (hybrid systems) have already demonstrated the capability to significantly reduce net urban water consumption.

Coupling hybrid or decentralized water management with source separation and centralized organic management and nutrient recovery offers the potential to achieve energy neutrality and significant nutrient recovery. Different qualities of urban water (potable, non-potable, and irrigation) would be supplied. Wastewater sources would be segregated to separate those streams containing a higher proportion of organic matter, and nutrients from less contaminated wastewater sources will greatly facilitate water, energy, and nutrient recovery.
 

Guiding principles can help formulate alternative systems, which can be analyzed to determine those which are the most sustainable. Economic evaluations need to consider systematic impacts and must be evaluated based on marginal reductions in urban water supply and wastewater management costs rather than average costs. These systems can be implemented within existing urban areas as development and re-development occurs. The greatest institutional barrier to implementation of improved systems is probably the stove-piping of the urban water management profession, a situation which can be addressed.
 

The toolkit elements described above can be implemented in a variety of configurations, ranging from centralized to decentralized. In a centralized system, potable water is produced in one, or a small number of, water treatment plants and distributed uniformly throughout the subject service area. Wastewater is collected and conveyed to one, or a small number of, wastewater treatment plants for treatment and disposal or reuse. In contrast, in distributed systems multiple potable and wastewater treatment facilities are provided throughout the service area. A hybrid configuration consists of centralized and decentralized components.
 

Adoption of these new approaches to urban water and resource management can lead to more sustainable solutions, defined as financially stable, using locally sustainable water supplies, energy neutral, providing responsible nutrient management, and with access to clean water and appropriate sanitation for all.