Master Planning for Water Treatment Needs as Per Capita Demand Goes Down

Authors:

  • David Haas PE, Frank Rombardo PE, Benjamin Moss - Hazen and Sawyer
  • Lance Buchanan PE, Tom Ginn PE - Cobb County-Marietta Water Authority

To budget and adequately plan for future water treatment needs, Cobb County-Marietta Water Authority (CCMWA) undertook performing a comprehensive assessment of its two water treatment facilities, which include the 72-mgd Wyckoff WTP and 86-mgd Quarles WTP. The Quarles WTP, which includes two distinct treatment trains – Plant No. 1 (Q1), originally constructed over 60 years ago with a current capacity of 42 mgd, and Plant No. 2 (Q2), originally constructed in 1981 with a current capacity of 60.5 mgd (86 mgd is the maximum permitted combined flow that can be used).

One of the key focuses of this study was to determine what to do with the aging Q1 plant to make it more reliable. A total of eight alternatives were initially considered for the Q1 plant. Using both economic and non-economic criteria, these were screened down to two alternatives for further evaluation – either perform a comprehensive rehabilitation of Q1 or to demolish Q1 and rebuild it. The recommendation from this study was to demolish and reconstruct Q1 based on several key considerations, including the opportunity to design processes and structures with improved redundancy, reliability, and flexibility; to address existing hydraulic limitations at Q1 (current minimum recommended design standards); to consider processes that will help meet water quality goals and future regulatory requirements; to replace aging infrastructure and eliminate current vulnerabilities; and to serve as a foundation for reliable capacity beyond the planning period of the study.

To determine the appropriate initial size for the new Q1 plant, a summary-level review of water conservation trends expected in the next 25 years was performed, including an overview of factors most relevant to Cobb County that affect the per capita water demand, and comparison to per capita water demands in other parts of the US evidenced by impacts from water conservation, pricing, economic conditions, and increased fixture efficiency. CCMWA has seen their annual average daily per capita demand decrease from approximately 190 gallons per capita per day (gpcd) in the 1990’s to the present level of 100 gpcd with further reductions to 90 gpcd anticipated by year 2040. From this review, an assessment was made to help CCMWA refine its evaluations and forecasts of water demand and select the most appropriate initial capacity of the new Q1 plant.

This presentation will include discussion of how to effectively utilize facility assessments to evaluate existing infrastructure for rehabilitation or replacement for planning for future water treatment needs. It also will discuss how to select the appropriate size of new facilities in an age of decreasing water demands (evaluation of per capita use and factors contributing to changes in usage); and how to develop a design that provides flexibility for future expansions. Key components of the design approach, including the use of 4D modeling (time sequence of construction coupled with 3D Revit models of the new facilities) to identify sequencing constraints will also be highlighted.

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

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