Throughout the last century, many North American municipalities implemented water disinfection and filtration systems to protect residents from illnesses caused by microbial contamination. But in recent years, as populations grew, pollution increased and treatment plants aged, some municipalities were stricken by waterborne pathogens that are highly resistant to disinfection and can cause serious illness or death.
One of the worst events was back in April 1993 when an outbreak of microscopic Cryptosporidium spread throughout the water system in Milwaukee, Wis., making more than 400,000 ill and taking the lives of more than 100. While the tiny organisms were virtually undetectable to the city’s water customers, their effects were huge and far-reaching.
Twelve years later, the policy and procedure changes that occurred in the wake of this unfortunate event are still shaping how many North American municipalities approach water treatment. Many are taking a second look at their existing treatment plants and are considering advanced technologies such as immersed ultrafiltration membranes to enhance their current processes and provide additional public health protection.
Only one year after the Milwaukee crisis, the nearby city of Racine braced itself for trouble when its 40-mgd, dual-media water treatment plant was overwhelmed by high turbidity and large quantities of organic material. Water managers immediately issued a boil water order to the city and their quick action helped citizens to avoid any illnesses.
“We made several improvements to our water treatment facility following the boil water order,” said Mike Kosterman, superintendent of the Racine Water Utility (RWU). “The enhancements ensured that our conventional treatment plant would consistently meet all regulatory requirements; however, we still wanted to implement additional treatment technology that would provide an extra barrier against waterborne pathogens to the 110,000 people that we serve.”
The city considered several options for the additional treatment including UV, ozone and immersed membrane filtration. While each can provide additional benefits to the water treatment process in Racine, the membranes provided a selective barrier that physically blocks waterborne pathogens from passing into the treated water stream.
With this in mind, the RWU chose immersed membrane filtration as the best option for the process enhancements. In October 2000, the city began working with engineering and construction firm, CDM, to conduct pilot testing of two leading immersed, hollow-fiber membrane systems. Both systems were carefully considered based on cost, service, support, company experience, membrane performance and long-term operating costs.
ZENON ZeeWeed membranes were selected for the RWU based on the outstanding value and performance the system demonstrated during the on-site pilot testing. Because the RWU was adding membranes to the existing conventional system rather than replacing it, CDM engineers considered two locations for the membrane system: post-sedimentation or post-conventional filtration. A post-sedimentation strategy would enable the existing filter basins to be retrofitted with membranes, while a post-conventional filtration strategy would require the construction of new membrane tanks.
CDM compared the capital and operating costs of running the membranes in both scenarios. The study showed that conventionally filtered water provided a substantially better influent to the membranes, which would enable them to operate at a higher flux, and as a result, fewer membrane cassettes would be required. CDM concluded that it was cost-effective and provided greater treatment process redundancy to build the new membrane tanks that would use conventionally filtered water influent, rather than purchase a larger quantity of membranes to sufficiently treat the lower quality settled influent. “Efficient use of the existing facilities, combined with an innovative preselection process for procurement of the membrane system, resulted in significant cost savings for the city of Racine,” said Len Rago, CDM vice president. “At $0.40 per gallon of capacity, this project has achieved one of the lowest unit costs in the industry.”
Construction was also greatly simplified due to the plant design selected. When CDM began construction of the membrane building in November 2003, RWU customers were unaffected. The plant was able to continue operating with no reduction in capacity or water quality. Once the membrane system is complete in June 2005, and ready to begin operating, RWU operators will open the valves to the membrane process trains and the city will begin receiving high quality, ultrafiltered water.
New treatment process
The first stages of the water treatment plant will remain unchanged. The RWU will still draw water from Lake Michigan, and pass the raw water through a conventional water treatment system that includes flocculation, sedimentation and dual-media filtration. Powdered activated carbon can be fed either upstream of the dual granular media filters or directly upstream of the membranes to provide taste and odor control.
“Raw water turbidity can spike as high as 400 NTU when we get certain wind conditions or when heavily silted water from a nearby river enters the lake,” Kosterman said.
Following the conventional treatment process, the water will flow into the membrane process tanks. Seven process trains, each with six cassettes of membranes, will provide treatment for up to 50 mgd of water.
An eighth train was also constructed; however, it will remain idle until increased water demand requires its use. The additional tankage gives the RWU the ability to expand its water production simply by adding membrane cassettes in a just-in-time manner. At full capacity, the RWU will be able to provide up to 60 mgd of water to Racine and the surrounding area.
Each membrane cassette contains thousands of membrane fibers that hang loosely in the process tank. 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, and turbidity of less than 0.1 NTU. Rejected particles remain in the process tank and are periodically removed by backwashing.
Automated operation and cleaning
The operation of the membrane system is highly automated and fibers can be easily cleaned with a clean-in-place backpulsing process that forces permeate water back through the membranes. This dislodges any particles that may adhere to the membrane surfaces. The water used for backpulsing the system is recycled to the headworks of the plant to minimize water loss. Intermittent aeration of the membranes is also used to scour debris from the fibers.
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 off line for cleaning without disrupting the water production of the remaining trains.
Monitoring equipment ensures that each train meets turbidity and particle count levels and also indirectly verifies the integrity of the membranes. In the unlikely event that turbidity or particle counts rise, an alarm will notify the operator and the appropriate action can be taken. Membrane integrity is also directly monitored by automatic pressure hold tests. Each individual membrane cassette is tested three times per day to ensure that the membranes are achieving the required removal rate of microorganisms such as Giardia and Cryptosporidium.
“Additional structures and a great deal of equipment has been added to our facilities in the recent years without additional staffing,” Kosterman said.