Replacing Aging Infrastructure in Times of Decreasing Demands
- David Haas PE, Frank Rombardo PE, Benjamin Moss - Hazen and Sawyer
- Lance Buchanan PE, Tom Ginn PE - Cobb County-Marietta Water Authority
Cobb County-Marietta Water Authority (CCMWA) owns and operates the James E. Quarles Water Treatment Plant. Within the facility, Quarles Plant No. 1 (Q1), was originally constructed over 60 years ago and has a capacity of 42 mgd based on eight trains rated at 5.25 mgd each. Quarles Plant No. 2 (Q2) was originally constructed in 1981 and over the years has been expanded to its current capacity of 60.5 mgd based on 11 parallel treatment trains each rated at 5.5 mgd. The maximum permitted treatment capacity of the facility (Q1 and Q2 combined) is 86 mgd.
CCMWA undertook a comprehensive assessment of all of its treatment facilities in 2012. 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.
The new Q1 is being designed as a conventional treatment plant consisting of rapid mix, flocculation, sedimentation, and dual media gravity filtration. The new Q1 plant is being design with an initial capacity of 33 mgd with infrastructure in place to allow the potential for future expansion up to 66 mgd. Filtered water from Q1 will be piped to the existing post mix basin where chlorine, fluoride, and lime are dosed to the combined Q1 and Q2 filtered water. A new Q1 high service pump station is also being constructed as a part of this project to operate in parallel with the existing Q2 pump station. The new Q1 will be constructed in the footprint of the existing Q1 requiring detailed construction sequencing plans to be followed to maintain approximately one-third of the existing Q1 while the new plant is being constructed. The design of this project will be completed in December 2016, prior to ACE17. Construction will begin in 2017 and last approximately three years.
This presentation will include discussion of how to effectively utilize facility assessments to evaluate existing infrastructure for rehabilitation or replacement; 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.
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