Projects

Designing for Harford County’s Future: Expansion of the Abingdon WTP

The Harford County Department of Public Works is a facing challenges associated with a rapidly growing population and limitations in water supply and treatment capacity. By increasing the capacity of the Abingdon Water Treatment Plant from 10 to 20 mgd, the immediate demands for finished drinking water could be met while allowing time for the County to take a broader look at the supply and treatment issues over the longer term. Hazen and Sawyer evaluated technologies, performed facility planning, and developed cost-effective and innovative design documents to help the County meet their immediate needs for finished drinking water.

Project Outcomes and Benefits

• Identify the most appropriate water treatment technologies to enable the plant to treat the near-pristine water from Loch Raven Reservoir as well as the more difficult-to-treat water from the Susquehanna River.
• Develop a plan for the facility that takes into account the long-term needs of the Abingdon WTP, including advanced treatment technologies to address potential future regulations (e.g., UV disinfection for Cryptosporidium inactivation and granular activated carbon adsorbers for endocrine disrupter and organic matter removal).
• Address the issue of near-zero discharge for residuals generated by the plant, working with the Maryland Department of the Environment to allow for recycle of spent backwash water.
• Perform bench-scale treatability testing to optimize the chemical treatment regime for the expanded plant and identify critical process loading rate criteria.

Hazen and Sawyer provided facility planning, preliminary engineering, final design, bid and construction-phase services for an expansion from 10 mgd to 20 mgd (with planning for buildout to 40 mgd) at the Abingdon Water Treatment Plant (WTP) in Harford County, MD. The Abingdon WTP, which treats water from either the Loch Raven Reservoir or the Susquehanna River, was originally constructed in the early 1990s using the Tricon bouyant-media upflow solids contact clarification process. Based on increasing demands in the County’s distribution system as well as the Base Realignment and Closure (BRAC) program, the capacity of the existing plant must be increased by year 2010.

The two raw water supplies are very different with respect to water quality, with the Susquehanna River being higher in turbidity and natural organic matter concentration. This difference is especially glaring when the Deer Creek Pump Station is first started-up. The Deer Creek PS is used to pump raw water from the Susquehanna River into the raw water pipeline serving the Abingdon WTP and City of Baltimore. Because this station sits unused for long periods of time, the water becomes stagnant and has historically been difficult to treat. Hazen and Sawyer performed bench-scale treatability testing to identify the chemical treatment regime and process design criteria required to ensure that the expanded plant can deliver drinking water in compliance with the Safe Drinking Water Act regulations regardless of the source.

The design of the project was completed in 2008, and bids from prequalified contractors were opened, with the low bid being within 1% of the engineer’s estimate. Design features of the project included:

• Detailed construction sequencing and shutdown scheduling to permit operation of the existing 10 mgd plant while expanding to 20 mgd.
• Plant architectural and basin design to complement the colonial architecture of the existing plant and surrounding area.
• New rapid mix, 4-stage flocculators, and plate settler basins.
• New filters, designed for 10 mgd (with of filter out of service) and provisions for future high-rating and the addition of GAC.
• Bidding documents and design drawings providing for the use of two different types of sludge collection equipment and filter underdrain systems.
• New clearwell with interior baffles, with provisions for removing half of the clearwell for maintenance and inspection while the other half remain in service.
• Provisions for future advanced treatment: UV Disinfection and GAC contactors.
• New enclosed raw water strainers, and new variable speed high service pumps with cone valves, and a new surge protection system for the two pressure zone high service system.

In addition to the above, the design includes a unique backwash recycle process utilizing an innovative design that allows for filters to be physically disconnected from the main process and used for filtering plant backwash water during high recycle rate periods.

The complexities associated with yard piping at an existing facility necessitated the use of three dimensional (3-D) CAD in preparing the plans and profiles for the various pipelines required to connect the various treatment facilities together. The previous plant design stubbed-out several pipelines for future connections and the “repurposing” of these future connections for use on the new design project provided for significant cost savings due to easier pipeline construction.

Harford County was working on a new system model and facility plan for their First Zone, which included removal of the Abingdon Elevated Water Storage Tank (EWST) from the distribution system for use solely as backwash water storage on the plant site. Since the Abingdon EWST had previously served a useful function in offsetting surge due to finished water pumping station shutdowns and/or power loss scenarios, Hazen and Sawyer needed to perform surge modeling of the First Zone and associated pumping units to identify what problems might be caused by the removal of the tank. The results of the surge modeling indicated that a hydropneumatic surge tank (bladder-style) on the plant site would be required to avoid both detrimental high and low pressure surges in the distribution system.