Engineering Angle: Fit-for-Purpose Water Reuse And The Road Toward Water Security
By Art Umble, Wastewater Practice Leader for MWH Global in the Americas.
The future of water resources for safe and reliable consumptive uses is uncertain. Today, hundreds of watershed basins worldwide experience water scarcity during some part of any given year. Scarcity of a country’s water resource is expressed as the ratio of the total amount withdrawn to the total renewable water available. The water stress ratio in the U.S. is currently between 20 percent and 40 percent, implying that the condition is serious; it also implies that substantial, widespread water management programs must be implemented to balance supply and demand, including resolving potentially intense conflicts between competitive users.
Figure 1: Transitioning To A “One Water” Culture
Adapted from: Howe, C., et al (2013)
Many argue that the key to our sustainable water future is a fully integrated, “one water” culture, where every drop of available water is renewable, including complete recovery of treated wastewater. Research asserts that as a society develops, its culture around how water is used changes in response to society’s evolving needs. Eventually, a “one water” society emerges where supply and demand are in perpetual, sustainable balance; where the infrastructure supporting water’s many uses fully supports the myriad needs for water (see Figure 1). One critical element in achieving a “one water” society is the aggressive reuse of treated municipal and industrial wastewaters.
Managing water is not complicated. First, where practical, avoid its use. If unavoidable, reduce its use. With reduction optimized, apply reuse wherever practical. Finally, substitute reuse with complete recycle.
When communities delay addressing water scarcity, they are often forced to react by implementing the final step — direct potable recycling — leaving no time to plan and implement these logical and necessary transition steps. Instead, they are forced to “leap frog” over the crucial proactive steps in the “one water” transition process (see Figure 2). Because technological solutions exist for implementing safe and effective direct potable reuse, decision-makers may feel they have no option but to implement these solutions to address scarcity problems, often requiring huge, and perhaps unaffordable capital investments. But their public may not be psychologically “ready” for such a jump and strongly resist the change, further hindering movement to the “one water” solution.
Figure 2: Fit-for-Purpose Water
The reuse of water involves treating “used” water to a quality acceptable for the intended reuse while posing the least risk to the user. Examples of reuse include irrigation of specific agriculture (e.g., turf and tree farms, public parks and sports fields), fire flow testing, cooling tower blowdown, cement making, household landscapes, toilet flushing, and laundry, to name a few. All of these reuse options require a specific water quality dictating the level of treatment necessary to achieve that quality. In many cases, the reuse requires a quality much less than that of drinking water while remaining completely safe. This is possible because technologies can be manipulated to treat waters to specific quality endpoints. This is known as “fit-for-purpose” (FfP) reuse, and represents a critical step in the transition to a “one water” society.
The technical challenges in reuse have been solved. Unfortunately, barriers remain that hinder the advancement of FfP reuse as a crucial proactive transitional step toward a “one water” paradigm. One is regulatory. Rather than being performance-based, rules regulating reuse remain overly prescriptive, greatly restricting reuse flexibility. Furthermore, the regulations are inconsistent across state and local jurisdictions. Another barrier is institutional. The integrated management of water is hindered by siloed cultures in planning, finance, and engineering organizations, which limit collaborations critical to achieving the “one water” archetype. Still another is legal. Our current framework of laws surrounding the water environment is founded in principles of preserving the equilibria of environmental systems. Because climate change is rapidly disrupting these equilibria, the entire legal structure around water must change to reflect this. Finally, we must do better with our management of risk. Across the water reuse landscape, the public is highly intolerant of even the slightest system failure. Being awarded a second chance is often out of the question. Therefore, consistent, constant, creative, and transparent communication of the benefits of integrated water is perhaps the most crucial aspect of bringing reuse, and specifically, “fit-for-purpose” reuse, into the norm of accepted standards and best practices here in the U.S.
Without a doubt, the water picture in North America is changing and poses great challenge. The future sustainability of our water environment must be firmly built around a “one water” culture. Purposed reuse of water is a critical step in reaching this goal.