Expanding Capacity and Quality
How does a water treatment plant (WTP) expand when community growth has pushed residential and commercial development right to the facility’s fence line? This was the question that the Draper Irrigation Co. asked when examining how to increase capacity and improve water quality at its WaterPro WTP in Draper, Utah.
WaterPro began examining upgrade options in 1999, hoping to find a solution that could expand its treatment capacity from 2.8 million gal per day (mgd) to 6 mgd and provide high-quality water that would consistently meet the U.S. Environmental Protection Agency’s (EPA) Long-Term 1 Enhanced Surface Water Treatment Rule (LT1ESWTR).
Although the existing conventional trimedia filters provided adequate treatment under normal flow conditions, melting snow in the spring and heavy rains caused turbidity spikes that threatened to overwhelm the aging plant’s filtration system.
Granular media basins were retrofitted with ZeeWeed membranes,
increasing capacity from 2.8 mgd to 6 mgd and improving treated water quality.
WaterPro worked with consulting firm Epic Engineering to examine several options including conventional granular media systems, sandballasted high-rate settling and membranes.
Despite budget levels that were set for conventional treatment alternatives, WaterPro continued to strongly consider membranes for their proven ability to produce high-quality water in a very compact plant layout, and for consistent reduction of high turbidity under virtually all conditions.
WaterPro selected GE’s ZeeWeed ultrafiltration (UF) membranes in October 2002 for the retrofit of its granular media basins. Compact footprint, high-quality water, resistance to turbidity and cost were the main factors for the selection. The configuration of the ZeeWeed 1000 membranes was also a key factor in selecting the membranes for the retrofit since the modular cassettes provided the best fit into the plant’s existing granular filter media basins.
The retrofit was considerably more cost-effective than constructing a new conventional treatment plant even though new piping was required along with permeate pumps, backpulse pumps and other associated membrane system equipment. Construction estimates also showed that the UF system could be retrofitted into the existing building at a cost of more than 20% below competitive pressurized membrane systems.
As part of the retrofit, Epic also optimized the layout of the existing building to accommodate a new compressor room, pipe gallery, basement storage and chemical room.
WaterPro treats water from several mountain streams, which first flows through a sedimentation basin to settle out large solids. From these basins, the raw water flows to two trains of membrane CS-WPRO-MUNDW-EN 1106 NA tanks, each with five cassettes of ZeeWeed 1000 UF membranes.
Each cassette holds 63 membrane elements, but can hold up to 72 elements—allowing future treatment capacity of up to 8.4 mgd.
Thousands of membrane fibers are loosely suspended in each membrane cassette and a slight vacuum is applied to the end of each membrane fiber to draw water through microscopic pores and into the hollow fibers. With a nominal pore size of 0.02 microns, the membranes form a physical barrier to suspended solids and provide greater than 4-log removal of pathogens such as Giardia and Cryptosporidium. Filtered water is collected from each fiber into a central header. Rejected particles remain in the process tank and are discharged to new drying beds where they are captured for disposal. Excess backwash water is decanted and sent to the company’s pressure irrigation system. In a final step, chlorine and fluoride are added to the treated water before it is released to the WaterPro potable water distribution system.
The operation of the system is highly automated with computer controls for permeation, system monitoring and maintenance. The membrane fibers can be easily cleaned with a backpulsing process that forces permeate water back through the membranes and dislodges any particles that may adhere to the outside of the membranes. When necessary, in-situ chemical cleaning can be automatically performed if membrane fouling reduces permeability below a specified performance level. During this process, one train can be taken offline for cleaning while the remaining train continues producing water.
Capital and Operational Efficiencies
The automated operation of the plant means that a full-time operator is not required. The plant’s SCADA system lets the operator monitor the plant from virtually any offsite location. Further savings are realized in chemical costs since the direct filtration of raw water eliminates the need for coagulant.
The onsite hydro-generating station provides further efficiencies by supplying electricity to the WTP. To drive the turbine, a portion of the water from the mountain creeks is directed into a conduit that channels water to the turbine. Once past the turbine, the water continues into the plant for treatment. Even during low stream flows, the turbine can generate two to three times more power than the WTP requires, and the surplus is sold to the local power grid.
“Even though we expect water treatment regulations to become stricter, we’re not worried because ZeeWeed produces water that exceeds state and federal drinking water requirements," said David Gardner, development manager for WaterPro. “Our ZeeWeed membranes will help us to meet more stringent regulations in the future. Cryptosporidium oocysts are about 200 times larger than the pores in our UF membrane fibers. The membranes not only block these particles from passing into the distribution system, but also ensure that our 26,000 customers have reliable, long-term protection from pathogenic microorganisms and harmful chlorination byproducts in their water.”