Although less than 2% of the nation's drinking water plants use membranes for filtration, Manitowoc Public Utilities in Wisconsin and the Kennewick Public Works Department in Washington are using the technology to produce additional clean water without significant additional expense.
Located in one of the state's highest-growth areas, the Milwaukee suburb of Manitowoc was treating 10 mgd with slow sand filtration. To keep pace with population growth, the utility decommissioned the filters and built a new plant that uses submerged membranes to purify water.
Then the utility signed an agreement with the Central Brown County Water Authority to provide 8 mgd. Because the new plant couldn't provide that much, the utility retrofitted the mothballed filter basins to accommodate a 20-mgd submerged membrane system. The system is so compact that an additional 20-mgd capacity can be added without expanding the plant's footprint.
The $8.5 million retrofit took 18 months to complete. Water from Lake Michigan is prescreened before it flows into the membrane basins. A small amount of chlorine is added to the effluent for disinfection, and the water is stored in a clear well before being pumped 65 miles to the authority's distribution network.
Besides maximizing the plant's footprint, the new system reduces staffing requirements as well as chemical usage and residuals production, and simplifies process adjustments during changes in raw water quality.
Although drinking water plants have used membrane filters since the late 1980s, the technology wasn't accepted until the early 1990s. Few managers want to be the first to try out any new technology. But with the systems producing water that meets the EPA's Long Term 2 (LT2) Enhanced Surface Water Treatment Rule (see table on page 37), more managers are choosing membrane systems for both new construction and upgrades.A BOX WITHIN A BOX
In most cases, the capacity of existing media filters can be greatly extended without building a new facility. Expansion is limited only by the amount of water that can be brought to the facility and the capacity of the existing infrastructure.
Most media filters are limited to a filtration rate of 5 gallons/minute/square foot (gpm/ft2) or less. Submerged membrane systems, on the other hand, achieve equivalent filtration rates greater than 15 gpm/ft2, increasing capacity by three times when the entire filter box is reused.
Not all existing media filters are good candidates for conversion, however. Certain plant configurations may require complex pipe work and ancillary system designs that may not provide the level of access for maintenance or inspection expected in a modern water treatment facility.
Conversion is easier when the new membrane filters can fit within the existing hydraulic profile with minimum modifications to the existing concrete, or when modifications don't prevent adjacent media filters from operating while the membranes are being installed.
About four years ago, the Kennewick Public Works Department doubled the capacity of its 7.5-mgd filtration plant in the same footprint despite limited space in the pipe gallery.
The department selected engineering firm HDR to investigate three options for expanding capacity: build a mirror-image of the existing plant, use a high-rate sedimentation/filtration process, or use submerged membranes in place of the existing granular media filtration process.