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.
The utility chose the latter because it wouldn't require any new building, thus reducing costs, and because it would incorporate the existing piping. It also offers the greatest flexibility for meeting future regulations and the ability to be easily automated. After conducting side-by-side tests with products from two manufacturers, the utility chose the Memcor CS submerged system from Siemens Water Technologies because it offered the lowest 20-year lifecycle cost.
To use submerged membranes in place of the existing filter system, the utility:
- Installed submerged membranes in the existing filter basins
- Increased the pretreatment capacity from 7.5 mgd to 15 mgd without a parallel treatment train
- Installed a new rapid-mix basin
- Converted the ozone contact basin and the first third of the sedimentation basins into flocculation basins
- Added plate settlers to the remaining portions of the sedimentation basin
- Used the waste wash water reclamation building to house the ancillary membrane equipment and the wash water plate settler
- Upgraded the waste wash water basin for year-round use
- Increased the intake and high service pump station capacities to 15 mgd
- Switched disinfection from chlorine gas to bulk sodium hypochlorite
- Eliminated ozone by using powdered activated carbon and potassium permanganate for taste and odor control.
The upgrade had to be done during the winter when the plant wasn't in use, so speedy installation was critical. Construction began August 2004 and the plant was on line by April 2005. Since then, it's met its capacity and performance needs. Should the plant need to be expanded again, new modules may be added to the membrane basins quickly and fairly inexpensively.
Manitowoc and Kennewick are just two utilities whose need to expand capacity in a limited space, timeframe, and budget made low-pressure membrane filtration a viable option. Outstanding water quality and the ability to adapt to rapidly changing raw water conditions are the icing on the cake.
— Gallagher is director of global process technology for Siemens Water Technologies, Shrewsbury, Mass.
Environmentally friendly filtration
Mississippi's state capital spares 40 acres of wetlands.
In 1914, when Jackson, Miss., built its first drinking water treatment plant, it was standard operating procedure to discharge untreated residuals into local waterways. A second plant, built in 1992, did the same until a residuals handling facility was added in 1997.
By 2006, the Mississippi Department of Environmental Quality was threatening to fine the city $25,000/day for using the Pearl River as an effluent filter, demand was close to overwhelming the plants' combined 67-mgd capacity, and neither could meet the EPA's Long Term 2 (LT2) Enhanced Surface Water Treatment Rule.
In the process of overcoming these challenges, the public works department became one of the first in the nation to use submerged membranes for filtration.
The $52 million project to renovate the older plant and add a submerged membrane system to the newer plant was the city's most expensive, necessitating a bond issue to be paid off through higher rates. But the investment is paying off in lower manganese, trihalomethanes, and haloacetic acids levels, not to mention regulatory compliance.
Meanwhile, the older plant has been modified to send treatment residuals to the sanitary sewer. As a bonus, the membranes doubled the newer plant's capacity to 50 mgd without having to build into adjacent wetlands the city had bought in anticipation of a physical expansion.
The department looked at three membrane-filtration systems, two small-footprint flocculation/sedimentation systems, ozone, and granular-activated carbon filtration before choosing the ZeeWeed system. Made by GE-Zenon Environmental Inc., the system was ideal for the plant expansion project because it eliminates the need for sedimentation. To reduce the likelihood of finger-pointing on issues related to the membranes' 10-year warranty, the company also supplied all major upstream pumps and mixers as well as the computer control system, which uses GE-Fanuc human-machine interface software.
“Pilot testing is essential because different membrane systems respond to different waters differently,” says Donald Bach, PE, senior civil engineer for the department. “Visit other plants to see how different installed systems operate and how their operators view their systems.”
Today, the wetlands remain untouched, the Pearl River no longer receives untreated residuals, operators spend much less time physically monitoring operations, residents drink better water, and the membrane system is less expensive to operate than the conventional system. That's a win-win situation for everyone.
— Stephanie Johnston