Innovative DAF Plant Meets City Sustainability Goals
- Peter H. Rice - City of Portsmouth, NH
- Michael Greeley and Matthew Valade - Hazen and Sawyer
The City of Portsmouth’s former Madbury water treatment plant was a 3.5 mgd conventional sedimentation and filtration plant constructed in the 1950s. At over 50 years old, the plant was starting to experience difficulty providing consistent and reliable treatment in accordance with current and potential future regulations. The City of Portsmouth piloted several treatment options, and decided to proceed with the design and construction of a new 4.0 mgd dissolved air flotation (DAF) and dual media filtration plant.
The project was constructed on the existing site, which is located in the rural setting of Madbury, NH, several miles outside of the City of Portsmouth. The 20 acre site is mostly covered with undisturbed vegetated land. Although clearing enough space for new construction would have been possible, there were several design constraints that greatly reduced the space available for construction. The new building was seeking LEED Certification, and was to be constructed within the requirements of the city’s recently adopted sustainability initiative. Both goals required that open, permeable space on the completed construction site be maximized, and disturbance of vegetated land be kept to a minimum. Additionally, the residuals management at the existing plant was inefficient and required large areas of the site to be available to accept the large sludge volumes produced by the aging plant. The existing plant provided a large portion of the city’s water demand, so taking the plant offline for any significant portion of time was not an option. The constraints required careful construction phasing coordination to construct several new process facilities in the same location as the existing process facilities they were replacing.
Construction commenced in May 2009 and the plant was placed online during the summer of 2011. During the startup phase, large volumes of water were required for disinfection, filter media installation and system testing. There was no available sewer connection at the remote site, so the contractor was limited to on-site disposal of the water. To overcome this obstacle, the contractor worked closely with the city personnel to operate the two plants in a variety of configurations, using the existing plant for retreatment of the testing water rather than disposing of it, resulting in both cost and schedule savings.
Since being placed online, the new plant has experienced increased filter run times and organics removal. Filter runs in the old plant would struggle to reach 40 hours during the summer months when organic matter in the raw water was at its highest. Run times at the new plant have been extended to 48-72 hours in the same raw water conditions, frequently at higher flow rates than produced at the old plant. Filter turbidities would tend to approach 0.3 ntu during the tail end of the filter runs at the old plant, but new filters remain steady throughout their filter run times, consistently operating well below 0.05 ntu. The new plant also displays improved organics removal, which will serve to help the city in meeting their regulatory disinfection byproduct compliance.
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