As the world's population continues to grow and our cities and suburbs expand, with more buildings to accommodate the growth, we are facing a major challenge to manage and protect our valuable water resources. This is becoming especially critical under the added pressure of climate change.
The conventional and common approach to water management involves supplying potable water and disposing of waste - and stormwater. It is becoming increasingly apparent that this conventional, centralized approach must change in response to urbanization trends and climate predictions. As the availability of clean freshwater resources is diminished, it is becoming clear that in particular grey- and stormwater should be viewed as alternative and valuable sources of water, and not just inconveniences needing to be dealt with.
Consequently, rainwater collection and reuse and the reuse of highly treated greywater are attracting increased attention. They are most-commonly provided for non potable end uses but potable reuse is also gaining traction.
Many communities have begun to implement simple and relatively inexpensive water-reuse projects, such as irrigating golf courses and parks, and the benefits of these are seen almost immediately. Benefits from widespread adoption include improved reliability and drought resistance of the water supply, utility savings in infrastructure maintenance and deferred capital expenses, and the immediate rewards of green and lush outdoor recreation spaces.
There are also environmental advantages to consider. The use of reclaimed water ensures the transfer of nutrients to beneficial uses rather than discharging them into receiving waterways. Contamination of bays and lakes with nutrient-rich stormwater and wastewater increases the risk of eutrophication (excessively high nutrient concentrations), the consequences of which include low dissolved oxygen concentrations, algal blooms, and declining shellfish populations and periodic fish kills.
Increased and more widespread use of reclaimed water typically brings with it greater financial, technical, and institutional challenges. Upfront capital costs may be higher, but payback on these water-reclamation systems may be realized sooner by building owners, depending on the region. Regulators and citizens express concerns about the safety of using reclaimed water for domestic purposes because of the perceived risks and uncertainties. These challenges have limited the application of water reuse in the past. However, 21st-century technology, for example the AQUALOOP system, exists to safely produce recycled water at a relatively low costs. Combined with the increased need to do so, we should now be striving for new paradigms in water supply and management.
Water efficiency, conservation
Significant reductions in water use can be made by insisting that water-efficient fixtures meet building code, instead of installing older-style high-flow units. These include water-efficient dual-flush toilets, low-flow showerheads, low-flush urinals, flush less urinals, low-flow faucets, and flow restrictors.
External landscaping with minimal water requirements also promotes water savings. Landscaping can be designed with the use of drought-tolerant species, garden mulch techniques that minimize water losses, and subsurface irrigation methods including automated irrigation systems with rain and moisture sensors to ensure optimum application of irrigation water.
To understand where water savings can be made, a detailed water-balance model of the project should be carried out during the design phase. This process will identify all possible water savings by incorporating uncomplicated water-efficiency strategies into the development design. Using this approach, the use of water-efficient fixtures and water-conservation design strategies can reduce an average retail or commercial development's overall water use by more than 40%.
Identifying water-reuse opportunities
Opportunities associated with the design and installation of water-reuse systems will vary significantly from one building project to the next. Analysis of the water-balance model at the outset of the project will reveal any opportunities for alternate water supply, water conservation, and water recycling. Water-balance modeling will also reveal the availability of rainwater and wastewater that could be captured and treated for reuse.
All aspects of building operations that involve water should be investigated and potential sources of water identified. Both internal and external water sources should be considered.
Treatment infrastructure, which often includes filtration and disinfection, will be required to ensure water is of a suitable quality for its intended use. Relatively clean water sources, such as roof water, are relatively straightforward to collect and are suitable for non-potable use with little pretreatment. Ligh contaminated greywater require a higher degree of treatment to make them fit for use. Greywater recycling methods usually have greater treatment costs than rainwater collection and reuse, but they also have the added advantage of year-round water-source availability—so they are not reliant on seasonal rainfall patterns and climate.
Supplying reclaimed water to non-potable applications where the risk of human contact is very low can be an excellent way of offsetting the consumption of potable-water sources. For example, with cooling tower water, the level of direct human contact is minimal. However, to reduce the risk of illness caused by airborne and waterborne pathogens, the level of disinfection prior to reuse should be in line with the risks involved. Non-potable demands include toilet and urinal flushing, irrigation, and cooling towers. Potable demands include water for kitchen use, showers, and sinks It is important to understand the relative proportions of potable and non-potable demand, and this can be achieved through the water-balancing process.
In office complexes, the amount of water used for toilet and urinal flushing is proportionally higher than the amount used in showers. In apartment-, hotel- and rest home buildings, the opposite is true—water used for bathing and showering exceeds the toilet-flushing demand (see Figure 1).
Buildings and complexes with water-cooled boilers or cooling towers for air conditioning require large volumes of water to feed these systems. Properties with large gardens and lawns tend to use a significant portion of their water budgets for outdoor irrigation.
The current level of public acceptance and regulations in some areas mean little opportunity for the use of harvested or recycled water for potable use. To ensure water reuse is an option, the engineer must manage potential public health, operational, and environmental risks associated with the specific application. The primary risks associated with the use of reclaimed water are:
- Public health, which refers to inadvertent contact with the water and subsequent ingestion of pathogens and toxins, particularly relevant in the health and immunocompromised sectors.
- Environmental, which refers to the accumulation of contaminants in irrigated soils and plants or detrimental hydrological effects as a result of water extraction.
- Availability, which is dependent on rainfall patterns and catchment activities, sizing of treatment plants and buffer tanks, and maintenance schedules and equipment downtime.
- Operational, such as potential clogging and fouling of distribution systems and mechanical equipment with suspended and precipitated solids, and staining of toilet bowls.
Given the different treatment requirements of the various rain- and greywater sources and the potentially different end-use water qualities required, a detailed water-balance and feasibility assessment should be undertaken in all system designs.
- Peter Harbour, PhD, and Robyn Overall, PhD, CJ Arms and Associates
- How is water used? - Dr Lee Bint (BRANZ Sustainable Building Scientist)
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- Tags: attenuation, Greywater, Rainwater, Recycling, Stormwater retention, Stromwater infiltration, Sustainable, Water Supply, water treatment