Launch Slideshow

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Power to the pupils

Power to the pupils

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    Dania Beach Public Services and partners celebrate the March 27 dedication of their award-winning water treatment plant. The facility earned LEED Gold certification by skillfully compensating for the energy requirements of nanofiltration. For example, the white overhang (far left) blocks direct sunlight but lets in enough natural light to illuminate 95% of the building. On the treatment side, variable-frequency drives were used with high-efficiency pump motors to lower power demands. Photo: CDM Smith

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    COMPENSATING FOR CONCENTRATE
    Broward County water has a high organics content — thus the decision by Dania Beach Public Services Department to pursue nanofiltration in parallel with the city's existing lime-softening system. Just one problem: Membrane processes retain 15% to 20% water, leaving behind high levels of waste concentrate. The solution is a three- or four-stage membrane system that uses concentrate from one stage as feed water for the next. Up to 95% of water can be recovered depending on plant configuration. Next challenge: Nanofiltered water is basically distilled water, so hardness and alkalinity must be returned before distribution. Mixing the permeate with lime-softened water, which has an excess of both, saves chemical costs. The concentrate that exits the plant is discharged to the sanitary sewer system. Operating at lower concentrate volumes reduces the need to buy water from the county, saving $100,000 annually. Diagram: CDM Smith

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    A LAKOS Separators and Filtration Solutions Model JPX-0650-L/FLG/S6 separator keeps sand from migrating into the cartridge filters and damaging the membranes. Photo: Frederick Bloetscher

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    GE Water & Process Technologies' Muni.Z 5-micron polypropylene filters elements in Fil-Trek Corp. Model S6GLH30-087-4-10F-E-E5 cartridge housings remove sand and particulates from source water before membrane filtration. Photo: Frederick Bloetscher

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    The consulting firm Biwater designed and built the membrane skid testing bank instrumentation. All electrical systems in the old lime-softening plant were replaced with higher-efficiency systems and all instrumentation was moved to one central control center in the new operations building. Photo: Frederick Bloetscher

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    A new chemical feed system was required to feed sulfuric acid and anti-scalant to raw water to reduce pH before nanofiltration. A custom-designed Indusco Environmental Services Inc. degasifier removes the carbon dioxide that nanofiltration produces to restore pH. After leaving the degasifier tower, the membrane permeate is mixed with the lime-softened water in a 50:50 ratio. Photo: Frederick Bloetscher

Faced with continuing growth and redevelopment, an aging water treatment plant, and increasingly stringent environmental regulations, Dania Beach, Fla., Public Services Department sought to design-build a new nanofiltration/reverse osmosis membrane process expansion to the city's existing 3-mgd lime-softening plant. The new treatment facility will meet future demand while improving both water quality and color. By using energy-efficient equipment, construction materials with high recycled content, and less water via xeriscaping and low-flow fixtures, the project also reflects the city's desire to provide sustainable infrastructure services.

The project — a collaboration between the city's public services department, a local university, and consulting engineers — shows how intellectual capital can be harnessed to produce cost-effective and long-lasting operational enhancements.

I'm a Florida Atlantic University College of Engineering and Computer Science professor who had been serving as a de facto consultant to Public Services for 10 years. When the department began drawing water from a new county wellfield, operators noticed it had much higher color and more organics than the city's wells, which were threatened with saltwater intrusion. Public Services needed to remove color, organics, and trihalomethane (THM) precursors while also increasing treatment capacity.

Nanofiltration was deemed to be the best process for meeting these goals. However, the reverse osmosis process presents two potential challenges for water-limited areas like Dania Beach: waste concentrate that must be disposed in an environmentally safe manner, and up to 20% water loss.

I assigned two groups of civil, environmental, and geomatics students the task of reducing concentrate volumes as their senior design project. Their solution had to be flexible enough to treat variable-quality raw water, achieve high system recovery, reduce energy demands, and minimize raw water and concentrate disposal costs.

They came up with an affordable innovation that can serve up to twice the city's population of 16,000 without additional capital investment: a 2-mgd, two-stage nanofiltration membrane system with convertible third- and fourth-stage reverse osmosis units. The convertible third stage operates as a single stage, allowing 91% to 92% recovery to meet concentrate disposal restrictions; or reconfigures to a third- and fourth-stage (2:1 array) to maximize water recovery and further lower operating costs. Pilot testing confirms that recovering up to 95% of raw water is possible.

After analyzing the process, site, and design issues to reduce power and water use, the students also suggested building a facility that could be certified Gold according to the U.S. Green Building Council's LEED standards, in part by generating a significant amount of power onsite from wind and solar sources. The university administered the application process with their help.

Fast track to stimulus funding

Public Services had a $8,820,923 Clean Water Revolving Loan to design and build the nanofiltration facility when the Florida Department of Environmental Protection offered to provide $2.55 million of that amount as an American Recovery and Reinvestment Act of 2009 “principal-forgiveness loan” (the law's term for a grant).

Public Services had identified design-build as a potential bidding option because the project-delivery method would give managers firm pricing for all costs within roughly four months. Design-bid-build, on the other hand, would've taken approximately one year for completion of detailed design while adding at least $500,000 in fees. The tight deadline for awarding stimulus funds clinched the decision for design-build.

Design-build was pursued using a two-step process. The first part was qualification-based. The contractor, engineer, and membrane constructor were identified and evaluated by a five-person selection committee consisting of city employees and employees from two nearby utilities that had recently built a nanofiltration facility. Of the seven qualifications packages the committee received, the two top-ranked firms were asked for cost proposals.