By Janine Witko, P.E.
From the March/April 2016 Issue
On a global level, concerns are rising about the efficient use of water resources and how these supplies can be conserved and reused. Over the last two decades, extreme drought events in the United States—extending from the southeast to California and the Pacific Northwest—have vividly illustrated the imperative for technologies that increase water usage efficiency as well as strategies and methods to reduce demand.
While there are numerous ways to reduce water demand and reuse water, there are challenges in their implementation. Facility executives benefit by understanding these issues to balance the complex relationships between resource management, technical and regulatory feasibility, social responsibility, and cost. In the United States, many regulators are still in the process of developing the requirements for water and wastewater reuse, and their potential application for direct or indirect potable supply.
There are multiple benefits associated with demand reduction and reuse. These include:
- Reduce average dry-weather flows (non-precipitation periods when reusing sewage or stormwater), which leads to delayed or deferred infrastructure investment.
- The collection and reuse of stormwater can reduce pollutants from entering waterways.
- Localized reuse options add to the resiliency of buildings and local communities through the ability to generate and supply water (particularly for uses like irrigation) when there might not be another available supply (or one which is unable to used during drought periods).
Performing a proactive, overall water management analysis is immensely informative for all concerned parties. The first step is to identify and quantify the demands for potable and non-potable waters. The team can then assess the different types of strategies that could be implemented to reduce water demand, as well as energy demand.
For proposed facilities or developments, the infrastructure needed to support rainwater collection, gray water collection, and reuse can be designed into the buildings and grounds easier and more efficiently than in existing facilities, whereas retrofitting plumbing, mechanical systems, or piping can be problematic and costly. Consideration needs to be given to space availability for additional piping and costs associated with separating systems.
At a project in the United Kingdom, the original concept called to employ groundwater to supply such non-potable demands as toilet flushing, track washdown, landscape irrigation and athletic field maintenance, and combined heat and power (CHP) process water. A proposal was developed to substitute recycled backwater from a nearby sewer outfall for the groundwater. While an energy intensive, extensive water treatment scheme was required to implement this plan, the result—a 40 % reduction in water demand—proved to be a preferred solution.
Projects pursuing LEED green building certification are required to incorporate water reduction features in their designs to qualify for the LEED rating. Site developers, large and small, have several options to conserve or reuse water and to minimize stormwater runoff.
One of these options—low-flow plumbing fixtures—can be installed in facilities to lower water consumption. Increasingly, building codes require low-flow fixtures in new developments or for upgrades of major properties.
Meanwhile, stormwater collection and reuse is one of the most readily achievable means of potable water demand reduction. Stormwater collection facilities can be used to both minimize runoff and provide a source of non-potable supply. However, these have space and maintenance requirements, and treatment might be required, based on the ultimate use. Stormwater can be used for many non-potable applications, including landscape irrigation, sanitary systems, and mechanical purposes.
My firm, Arup, has developed a rainwater cistern-sizing tool that triangulates rain gauge data, calculates irrigation demands, and iterates daily cistern mass balance calculations keyed to tank-size increments. This can be used to optimize the size of a rainwater collection system. Creative use of native landscaping and xeriscaping can also help maximize on-site stormwater resources.
Reusing gray or black (wastewater) water is another avenue that can contribute to a facility’s overall water efficiency. Typically, to reuse gray or black water requires some type of purifying treatment. The level of treatment is often dictated by the ultimate use of the water and the anticipated amount of human exposure to it.
Arup developed an efficient reuse system at Yarra Park in Melbourne, Australia. In this case, there was a commitment to restoring the park to pre-drought conditions. The Melbourne Cricket Club raised the bar on the plan, and proposed providing an alternative water supply for the park. The challenge was that there was limited regulatory context that directly addressed a viable solution for the site: sewer mining, or reuse of black water. Compounding the situation was that Arup had been asked to design a treatment system that met the stringent local Victoria EPA Class A standard. The water treatment scheme required included screening, bioreactors, ultrafiltration, and disinfection (see diagram). Although it is a labor and energy intensive process, the studies proved that this treatment allowed the demand for potable water to be reduced by 50%.
Witko is the Americas Water Business Leader for Arup, located in the New York City office. She is a licensed professional engineer with over 30 years of diverse water related experience including planning, hydrologic and hydraulic modeling studies, detailed design and permitting, and construction support.
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