Blending in Potable Reuse - Challenges of and Solutions to Meeting Multiple Water Quality Objectives

Authors:

  • Ben Stanford, William Becker, Jean Debroux, Stuart Khan, Wendell Khunjar , Stephanie Ishii

Priority considerations and recommended monitoring are dependent on the location at which DPR product water is blended into an existing water system, whether that be at the intake of a water treatment, within the water treatment plant, or in the distribution system.

The potential for rapid changes in blending ratios that may impact water quality must be considered when blending DPR product water into an existing system. These rapid changes may be driven by risk management, economics, or other factors.

When blending DPR product water into an existing system, there is a need to strike a balance between being within the appropriate range for an individual water quality parameter (e.g., chloride) and being within the appropriate range for indices that consider combinations of water quality parameters (e.g., chloride-to-sulfate mass ratio).

The range of potential blend ratios of DPR product water and other sources waters should be considered when planning for the stabilization of the blended product.

When using blended water supplies, another factor to consider is taste and odor concerns from consumers. Approaches to help mitigate these concerns include source specific flavor profiles, taste and odor control, and communication plans.

Considerations that are specific to DPR should drive planning, piloting, and monitoring. For example, it is known that residual ammonia in wastewater can impact downstream water treatment processes, such as disinfection. Thus, potential ammonia variability should be quantified and appropriate adaptations should be made during the planning and piloting phase, and ongoing monitoring should address the parameters needed to ensure proper treatment.

This presentation will discuss operational considerations unique to direct potable reuse (DPR), with a focus on the implications of blending DPR product water into an existing drinking water system. A wide range of DPR blending scenarios will be presented in order to highlight the potential impacts on distribution system stability and finished water quality. Planning, monitoring, and engineering strategies will be discussed as a means for addressing these various blending scenarios and associated implications.

In response to population growth, increased urban density, and varying climate, water agencies in the Unites States and internationally are exploring and implementing planned potable reuse of municipal wastewater, through indirect potable reuse (IPR) and DPR. Both IPR and DPR can provide a safe, reliable pathway for ensuring an adequate water supply, with careful consideration of operation being critical to sustained success. More specifically, the implications of relying on effective and consistent wastewater treatment as a means of source water protection, as well as the overall impacts of blending water supplies, must be acknowledged and addressed when considering potable reuse.

Incorporating potable reuse into a water supply portfolio presents many of the same challenges as selecting and blending any new water source into an existing system. Blending requires a high level of operational surety, appropriately sized storage and mixing zones, and a firm understanding of potential impacts on process performance and distribution system stability. Multiple tools and recommendations that have been developed in support of source water blending will be reviewed in the context of potable reuse. Additionally, evidence will be presented as to why there is a need to strike a balance between being within the appropriate range for an individual water quality parameter (e.g., chloride) and being within the appropriate range for indices that consider combinations of water quality parameters (e.g., chloride-to-sulfate mass ratio).

In addition to existing guidance, there are several factors that may be unique to DPR or at least require special consideration when evaluating the feasibility of blending DPR product water into an existing drinking water system. These factors span a wide range, including adaptation to rapid changes in blending ratios, maintenance of distribution system flow and pressure, impacts of nitrogen species on disinfection compliance and finished water quality, disinfection byproduct formation, corrosion control, and aesthetic concerns. The relationship between DPR and each of these factors will be discussed, as well as recommended pilot testing/planning evaluations and water quality monitoring for pre- and post-implementation of DPR blending to address these factors. Overall, this presentation aims to inform site-specific evaluations of potential DPR blending scenarios in order to support the safe and reliable use of potable reuse.

For more information, please contact the author at sishii@hazenandsawyer.com.

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Horizons Fall 2017 (pdf)

Horizons showcases significant water, wastewater, reuse, and stormwater projects and innovations that help our clients to achieve their goals, and can help you achieve yours. Articles are written by top engineers and process group leaders, demonstrating and explaining the beneficial application of a variety of technologies and tools.

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