Located on the shores of Lake Michigan and about an hour south of Green Bay, the city of Manitowoc, Wis., is also positioned in one of the highest-growing population areas in the state. Nearly all of the water the city uses comes from Lake Michigan.
Up until 1999, Manitowoc Public Utilities (MPU) used traditional media filters that had a treatment capacity of 10 million gal per day (mgd).
Due to the population growth, MPU decided they needed to update their facilities with membrane filtration technology. At the time, Memcor Products offered MPU a CMF microfiltration facility that used pressurized membrane technology to filter water. The municipality built the new microfiltration plant in a new building and decommissioned its media filters as well as the building in which they were housed. This facility took approximately three years to complete, from pilot testing to start-up, and cost about $7 million.
Choosing to retrofit
The utility had added a single skid in 2000 and again in 2003 to keep pace with the growing population, but it turned out MPU found itself needing to expand again.
In July 2004, MPU signed a wholesale water agreement with the Central Brown County Water Authority (CBCWA), a joint water authority in northeastern Wisconsin consisting of six communities that needed to replace their groundwater sources. The member communities of the CBCWA did not, at the time, meet the U.S. Environmental Protection Agency’s water quality guidelines due to the high radium levels in its current water supply, the St. Peter’s aquifer. Faced with a mandate by the Wisconsin Department of Natural Resources, the CBCWA entered into the agreement with MPU. The water demands of the CBCWA were about 8 mgd, and the pressurized membrane microfiltration plant at MPU did not have that kind of excess capacity.
The utility turned again to USFilter Memcor Products (since acquired by Siemens Water Technologies) for help in June 2005.
In order to save space, MPU decided to retrofit the media filters that had been vacant for five years with the Memcor CS system, a submerged membrane filtration system. Using the existing basins, the engineers and construction teams were able to fit a 20-mgd capacity into the space that once would have produced up to 10 mgd from the media filters. The new technology is so compact that the building also has a future capacity of an additional 20 mgd (40 mgd total) should the need arise.
“MPU has always kept abreast of the changing membrane technologies, and the submerged system seemed to fit our need. Our conventional filter beds were not being used, so it seemed natural to retrofit a submerged plant into our existing space,” said Rob Michaelson, PE, water system manager of MPU. “We were also aware of the energy and chemical efficiencies related to the submerged filtration.”
Engineered and constructed in 18 months, at a cost of roughly $8.5 million, the submerged membranes reduced staffing requirements, simplified process adjustments during changes in raw water quality, reduced chemical usage and residuals production and minimized the treatment facility’s footprint. The new plant was up and running the first week of April 2007.
“The daily membrane capacity is 20 mgd, but with the physical layout of the plant, we can add 10 mgd in the future without making a whole lot of modifications,” said Nilaksh Kothari, PE, general manager of MPU and incoming president of AWWA. “So we spent a lot of capital dollars up front, but structurally it made a lot more sense to do it that way.”
Yet, while one would think retrofitting submerged membranes into an existing facility would save a large amount of time and money as opposed to building a plant from scratch, in reality the savings don’t amount to much, according to Mark White, PE, BCEE, and principal engineer for CDM and the submerged membrane project at Manitowoc.
“Due to the fact there was no room on the site for a stand-alone facility, significant modifications were required in the existing structure,” he said. “As a result, we estimate that the cost and time savings from reusing the existing structure was minimal compared to building a new stand-alone facility.”
Water from Lake Michigan is now lifted from lake level to pass through a pre-screen and then supply the submerged membranes. A small amount of chlorine is added to the water after it is filtered through the membranes for chemical disinfection and then that water is stored in a clear well. Membrane-treated water is pumped through 65 miles of transmission network to the CBCWA’s distribution network.
The two systems
The current system at MPU is unique in that it simultaneously operates both pressurized (11 mgd) and submerged (20-plus mgd) membrane filtration systems. It is only Memcor’s second customer to do so; the other is a utility in Erie, Colo.
Michaelson said the MPU’s water system operators are applying their knowledge of running a pressurized system to the new submerged system, as the two processes are fundamentally the same.
“We intend on operating the two plants in a lead/lag situation, to ensure that each plant can back up the other during peak flows or planned maintenance. We have yet to see any disadvantages with running the two plants together,” Michaelson said.
Pressurized and submerged systems are the two main configurations associated with hollow fiber, low-pressure membrane systems. Applications like groundwater and pretreatment to RO systems may work well with pressurized configurations, which typically operate in a closed environment and pressurize feedwater up to 40 psi. Submerged configurations, which operate in an open tank design where feedwater flows by gravity into the membrane cell, may work better for conventional plant retrofits because they easily fit into existing filter bays and have smaller footprints.
While there is a long-standing myth that pressurized membranes are able to handle high solids and water fluctuations better than submerged configurations, that is not necessarily the case. Some manufacturers claim that a higher allowable pressure differential allows a greater threshold to handle solids, but this is a bit misleading, according to Siemens Water Technologies. In 1998 during the demonstration of Siemens/Memcor’s submerged technology, both the pressurized and submerged configurations withstood the same flux and backwash interval. The pressurized system operated up to 22 psi and the submerged up to 12 psi. The results showed that the performance between pressurized and submerged systems resulted in nearly identical cleaning intervals.
Both types have been cost-effectively retrofit into existing facilities across the U.S., White said. He added that submerged membranes allowed for implementation of the new facilities in a smaller footprint than skid-mounted pressure membranes. This was important due to the severe site constraints at MPU, as discussed above.
According to White, engineers and consultants for the project worked to customize the membrane system in order to improve the layout and meet the needs of MPU. Areas of customization included:
- Vertical turbine pumps used in place of conventional horizontal pumps to reduce the facility foot- print, protect the pump motors from flooding and improve the system hydraulics; and
- An innovative “swing cell” back- wash management system which allowed one of the membrane cells to operate as either a primary or secondary membrane cell, treating either plant raw water or membrane backwash water.
Kothari’s final words about the membrane retrofit at MPU were that the utility has greatly benefited from membrane technology in general and specifically the small footprint and efficient filtration of the submerged membrane facility. Yet, he said, while there are lower capital costs with membrane technology versus conventional treatment methods, there can be costly preventative maintenance methods associated with membranes—especially because the type that were installed MPU need to be replaced about every four to five years.
“Speaking about polypropylene membranes in particular, one thing I would recommend for utilities that are looking into membrane technology is that they should perform pilot studies on used membranes that are maybe a year to 18 months old,” Kothari said. “Brand new polypropylene membranes work extremely well, but we have noticed there is decline in flux the more membranes are utilized.”