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Guidelines for Engineered Storage Systems

Project: 12-06
Program: Principal

Funding Partner: Singapore PUB
Total Budget: $211,788 (Cash: $100,000, In-Kind cash and service: $ 111,788)

Principal Investigator: Andrew Salveson, Carollo Engineers


As communities move towards more direct potable reuse (incorporation of highly purified recycled water into a drinking water source without an environmental buffer), it will be imperative to provide utilities with guidance on sizing, design, and response procedures for potential changes in water quality in engineered storage systems as they will become increasingly important in overall water management. The availability of an engineered storage buffer is a key element in direct potable reuse using current treatment and monitoring techniques. With the purpose of an engineered storage buffer being to provide response time to identify treatment failures, implement appropriate actions, and a final monitoring point where the water quality can be validated for potable reuse before being introduced into drinking water.

In densely populated urban environments, it will be more cost effective to store advance-treated reclaimed water at or near the point of need to minimize energy consumption.

While it is well documented that drinking water quality may deteriorate in storage and distribution systems, changes in reclaimed water may be exacerbated by its relatively higher levels of nutrients and organic matter energy sources. However, there are no known existing guidelines on reclaimed water storage quality, except for requirements to maintain 5 mg/L free chlorine in the state of New South Wales (NSW Recycled Water Co-ordination Committee (RWCC). NSW Guidelines for Urban and Residential Use of Reclaimed Water, 1st May 1993).

What does exist are numerous guidelines and regulations for reclaimed water quality criteria for specific end uses, including criteria for aquifer storage and recovery (NRC, Water Reuse: Potential for Expanding the Nation’s Water Supply Through Reuse of Municipal Wastewater. National Research Council, Washington D.C., p. 363. 2012) and wastewater reuse (US-EPA, 2012 Guidelines for Water Reuse. USAID, USDA, NIFA compiled by CDM Smith for the U.S. Environmental Protection Agency, p. 341.) Nonetheless, it is unclear how best to manage engineered storage and water recovery systems. There is also a lack of research pertaining to reclaimed water quality changes over time in engineered storage and distribution systems.

Therefore, the engineered storage buffer must be of a sufficient capacity to allow for the protection of public health from inadequate treatment and for the measurement of specific constituents to be assured that the quality of the water provided meets all applicable public health standards. Guidance on the best configuration for storage facilities is needed in order to protect public health.

Goals and Objectives

The project will develop recommendations for optimizing engineered storage systems for direct potable reuse; this will be accomplished through examining current practices and existing research to generate a guidance document and report.

Research Approach

Task 1: Literature Review and Knowledge Transfer: This project will tap into a range of directly relevant research and experience. There are several ongoing WateReuse Research Foundation projects and recent reports that overlap with this effort, including the following:

  • WRRF 11-10: Evaluation of Risk Reduction Principles for Direct Potable Reuse
  • WRRF 10-05: Role of Retention Time and the Environmental Buffer of Indirect Potable Reuse Projects
  • WRRF 11-01: Monitoring for Reliability and Process Control of Potable Reuse Applications
  • WRRF 11-02: Equivalency of Advanced Treatment Trains for Potable Reuse
  • National Research Council (NRC). Water Reuse: Potential for Expanding the Nation’s Water Supply Through Reuse of Municipal Wastewater. The National Academies Press. 2012.
  • Tchobanoglous, G., H. Leverenz, M. Nellor, and J. Crook, Direct Potable Reuse – A Path Forward, WateReuse Research Foundation Report. 2011.
  • Law, I., The Future Direction for Potable Reuse, Water, Vol 35, No 8, 58-63, December 2008.

The literature reviews from these four projects and three reports will be updated based upon new information, but will not be repeated point-for-point. Key conclusions of these reports will be integrated into this project.

Key information will also be gathered from the participating utilities listed in this proposal further on. This key information includes performance data, mechanical reliability data, public outreach and perception issues/concerns, and utility perspectives on the value of the environmental buffer.

Additionally, water quality data will be collected from utility partners currently practicing IPR. The project team will also perform interviews with these utilities to develop case studies on the value of existing natural storage systems. The intent to gain knowledge from the case studies will demonstrate how RRT and monitoring through IPR can be improved, useful and/or successful.

Task 2: Designing Engineered Storage: The second major phase of the project consists of designing engineered storage buffers. One of the main design parameters of interest is the total retention time, which will be defined based on the water quality in the advanced treated water, the online monitoring accuracy of the advanced treatment processes, and the response time available for critical analytical methods and monitoring for the engineered storage system. Based on these design parameters, conceptual designs with associated planning-level costs will be developed. The intent is not to have a one-size-fits-all approach, but to have a method to balance treatment, monitoring, and engineered storage that could be applied to a range of applications and local regulations. These results will be developed in several steps, with input from WRRF and the utility partners.

A DPR brainstorming workshop is planned to sort through these engineering issues related to the design of engineered storage systems. At this workshop we will determine design and operating parameters, as well as monitoring schemes for a number of engineered buffer configurations. The results from this workshop will form a central part of the final guidance document.

Basic Design Concepts: During the workshop, various design configurations will be considered, such as those laid out by Tchobanoglous et al (2011), including:

  • above-ground or subsurface tanks or systems of tanks,
  • covered and lined surface storage reservoirs,
  • engineered subsurface aquifers, and
  • large diameter pipelines.

These basic configurations can then be enhanced by interior flow-directing structures such as vertical and/or horizontal baffles to ensure a particular flow path and thus tailor retention time distribution and level of mixing to desired design values. Designs will also consider addition of in-line treatment steps including chemical addition points at selected locations to achieve a chosen treatment objective.

Operational flexibility will also be considered, such that “side loops,” which may or may not include additional treatment steps, and some additional storage time may be made available on a temporary basis, if initial screening-level monitoring indicates a potential problem with water quality. That must be investigated further before the water progresses through the system on its regular path.

Monitoring System Design: Each design configuration will include a number of monitoring points. Monitoring parameters will be based on the conclusions from the research projects listed above that are currently underway, and will include a mixture of surrogate parameters for which online instant response monitoring is available (turbidity, potentially fluorescence, TOC), rapid (though not instantaneous) response monitoring for parameters more closely linked to pathogenic activity. For a future system, these monitoring approaches would be periodically calibrated by bench-top challenge testing. Periodic monitoring for the specific indicator organisms, and pathogenic organisms themselves would also be recommended as a part of quality assurance.

“Whole Plant” Design Considerations Directly Affect Engineered Storage Design: The extent to which risk reduction measures are implemented upstream and downstream of the engineered storage system materially affects the size, form, monitoring, and fail-safe requirements placed on the engineered storage system itself. Therefore, we will to address the design of engineered storage systems in this broader context by identifying trade-offs between additional safeguards placed on the existing treatment systems and larger storage capacity, such as longer RRT, and/or monitoring density for the engineered storage portion itself. Based on the review of information to be performed in Task 1 and to a certain extent, Task 3, these additional safeguards may take more traditional as well as non-traditional forms, such as:

  • source control programs to prevent the entry of toxics (or pathogen spikes) into the DPR system;
  • treatment process robustness and reliability, e.g., redundancy to reduce or eliminate the potential of a single point of failure;
  • front-to-back Hazard Analysis and Critical Control Point (HACCP) analysis of the full, integrated treatment system from wastewater source control to potable water delivery to identify and address critical control points for an integrated monitoring program; or
  • organizational aspects relating to specialized operator training, data integration, and personal and organizational accountability

Task 3: Examining Public Perception: Public perception will be examined with an opinion survey using a sample of customer email addresses supplied by our utility partners. Due to the nature of this sample, the results will not be necessarily be predictive or representative of the attitudes and opinions of the general public but it will give us important information.

The survey instrument will attempt to measure three separate aspects of public attitudes toward engineered storage and direct potable reuse. First, we will measure an unaided general attitude toward the concepts of engineered storage, in particular reactions to the safety, and efficacy of such systems and the public willingness to consume water produced by such a system without an environmental buffer. Second, we will test the effect of several short educational messages about engineered storage and direct potable reuse on effecting changes in attitudes and willingness to consume treated water. These informational messages will be based on current scientific knowledge, and may include both written and graphic explanations. Finally, we will provide information on the likelihood of water system failure and its possible effects on public health. The results of this final portion of the survey will test the extent to which changes in attitudes produced by the first set of educational messages are robust to possible real world challenges. We will normalize these challenges with what occurs in water treatment plants noting that unacknowledged reuse forms a part of most treatment systems.

The survey results will be analyzed for top line, or macro results, and to the extent possible further analyzed on the basis of demographic information, such as location, age, gender, and education.

Task 4: IPR to DPR Transition Case Studies: In the final major phase of the project, the data and design information developed during the first two phases will be synthesized into IPR-to-DPR transition scenarios based on the case studies of the participating utilities. These scenarios will be stand-alone assessments written for each participating utility and together will support the development of the final deliverable for the project, which will be a guidance manual for sizing and design of engineered storage systems for DPR that is practical, implementable, and provides sufficient detail to serve as a roadmap for future DPR systems.

Our approach is to perform utility case studies. Utilities have been selected that span a range of treatment approaches and potable reuse applications, including Upper Occoquan Service Authority, the West Basin Municipal Water District, the City of San Diego, El Paso Water Utilities,the City of Lubbock, and Singapore’s Public Utility Board (PUB). Windhoek, Namibia will provide insight and guidance based upon their existing DPR facility.

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