The phrase “Necessity is the mother of invention” has been definitively traced back to early 16th century England, and even attributed to Plato in the Latin form, “Mater artium necessitas.” In today’s world of water, necessity is also becoming a major factor in rising interest regarding potable water reuse. This is especially true in areas where changes in climate or usage demands have stressed traditional sources of supply, as evidenced by increasing numbers of applications worldwide. For those who work in a water-stressed environment, this article can provide added perspective on specific points of opportunity — and points of caution.
A Brief History Of Water Reuse
Early applications of water reuse were conservatively limited to non-potable use. These included irrigation applications ranging from agricultural to recreational use (e.g., golf courses, ballfields, etc.) and for industrial uses such as process water, boiler feedwater, and cooling water (including some recirculating cooling systems).
In many cases, parallel distribution systems were constructed to keep wastewater treatment plant (WWTP) effluent for non-potable reuse segregated from potable water produced by water treatment plants (WTPs). Additional disinfection steps have been added in non-potable water applications where exposure to pathogens is a concern based on the requirements of the application, or where recycled water is being redirected for potable water reuse.
As experience with the water reuse technology has heightened awareness, and as potable water demands have become more pressing, interest in potable water reuse applications has become more prevalent.
Similar Concept, Multiple Executions
In its recently published document, Mainstreaming Potable Water Reuse in the United States: Strategies for Leveling the Playing Field, the U.S. EPA has classified the practices it considers potable water reuse in very broad terms … “Potable water reuse is defined here as encompassing a continuum of applications ranging from different types of indirect potable reuse (e.g., systems in which communities use surface water or groundwater containing treated wastewater as a drinking water source) to flange-to-flange direct potable reuse (e.g., systems that introduce purified water directly back into an existing water supply system).”
For years, municipalities up and down the rivers have been “recycling” water discharged from an upstream WWTP, diluted by the river, and pumped through the intakes of downstream WTPs. Today, the EPA defines that as de facto water reuse. Surface waterways receiving such WWTP effluent that is not specifically treated for reuse are considered effluent-impacted waterways. They rely upon specific dilution factors to disperse the effluent, even as river flow rates change with the seasons.
Today, there are two emerging water reuse applications that impose full advanced treatment steps to further purify water before recycling it back for reuse. Indirect potable water reuse reintroduces treated water back into the regular hydrological cycle (surface water or groundwater) as an environmental buffer between WWTP discharge and eventual WTP intake. This indirect potable water reuse is experiencing increasing interest, especially in terms of aquifer regeneration. Direct potable water reuse has been approached more cautiously, going one step further with more sophisticated post-treatment steps designed to enable recycling into an engineered storage buffer or directly into drinking water intake streams.
How the water will be reused dictates the final disinfection treatments in the last step before final discharge. Treatments can vary depending on the constituents of concern in the reused water stream — pathogens (such as protozoa, bacteria, or viruses), trace organics, nitrates, total dissolved solids, metals, disinfection byproducts (DBPs), and others. Over time, such treatments have shifted from basic physical processes to more advanced processes such as advanced oxidation and membrane filtration. Improvements in familiar treatments such as granular activated carbon (GAC) are offering new options for removing problematic components — including pharmaceutical compounds and endocrine disruptors — at half the cost and carbon footprint of treatments such as reverse osmosis (RO).
While the EPA has not yet formulated federal regulations on potable water reuse, a number of states and localities have issued their own regulations. The EPA’s 2017 Potable Reuse Compendium identifies treatment requirements for direct potable water reuse in 3 states and indirect potable water reuse in 14 states (Table 3-2 on page 3-13). That document also profiles a number of international projects (Table 2-2 on page 2-8).
Multiple Considerations — Positive And Negative
As in conventional WWTPs, varied treatments and disinfection processes are available for water reuse — engineered physical systems, engineered chemical systems, engineered biological systems, and natural systems. Table 4.1 (page 69) from this downloadable water reuse document published by the National Academy of Sciences outlines the relative value of various processes for specific component removal.
Before deciding on a particular approach, however, it might be worthwhile to consider this Johns Hopkins University study on transformational products created as a byproduct of specific treatments. In the study, they looked at extended impacts related to using hydrogen peroxide and UV light to oxidize toxic compounds in the reuse water stream.
Beacons Of Hope: Innovative Efforts Showing The Way
While some water utilities remain able and content to satisfy water needs from traditional surface water and groundwater resources, others pressured by water scarcity or mounting costs in a changing environment are leading the quest for greater water independence through water reuse.
On the water-sensitive West Coast, Orange County is one example of using highly treated WWTP effluent to recharge its own aquifer and stave off saltwater intrusion from the nearby Pacific Ocean. On the East Coast, the Sustainable Water Initiative for Tomorrow (SWIFT) in Hampton Roads, VA is doing the same thing with the Potomac aquifer near the shores of the Chesapeake Bay.
Utilities interested in learning more about research into potable water reuse can download the Executive Summary of the Potable Reuse Research Compilation, a synthesis of findings from 34 projects that investigated the feasibility of direct potable reuse projects. Both are available from The Water Research Foundation.