Using high-flux membranes to handle increased demand & high turbidity
In 2008, the city of Murfreesboro, Tenn., experienced a population surge that caused a strain on water resources. Located just outside of Nashville, the city rapidly grew, and the water treatment plant required additional capacity as it was regularly hitting 80% of peak demand.
As the need to increase treatment capacity became more critical, the Murfreesboro Water Resources Department used this as an opportunity to expand and update aging equipment at its Stones River Water Treatment Plant, which averaged 10.6 million gallons per day (mgd) prior to expansion. The upgrade also gave Murfreesboro the opportunity to proactively prepare for anticipated new U.S. EPA regulations relating to water quality.
When a coagulant pump failed during construction, settled water turbidity increased to 78 ntu. The installed membranes treated that water to within standards of 0.02 and 0.04 ntu.
Getting the Most out of Upgrades
Originally constructed in 1967, the facility’s gravity filters began to meet their life expectancy and required replacement. As the Murfreesboro Water Resources Department began to qualify new filtration systems, officials sought a technologically advanced solution capable of providing an absolute barrier to Cryptosporidium and Giardia lamblia—a task that the old gravity filters were not equipped to handle.
Natural conditions, turbidity and water hardness in the Murfreesboro area posed additional treatment challenges for the Stones River Water Treatment Plant expansion project. The plant, which draws some of its source water from a river that has elevated levels of turbidity during rain events, previously saw finished water turbidity levels below 0.3 nephelometric turbidity units (ntu) during heavy rainstorms. Due to the natural limestone formations, lime softening technology reduced the hardness, iron and manganese from the raw water. The plant’s latest expansion included a comprehensive upgrade of the softening technology and provided pretreatment for the membrane filtration system.
When the plant update and expansion project initially was conceived, membranes were coming to the forefront as the most reliable, highest-quality filtration technology. This was appealing to the Murfreesboro Water Resources Department as the city wanted to be on the cutting edge with a top-of-the-line water treatment plant and technology capable of meeting treatment requirements. Compared to sand filtration, which the Stones River Water Treatment Plant previously used, membrane filtration enabled the plant to more effectively control Cryptosporidium and Giardia lamblia at a lower cost, and with an absolute barrier to the protozoans.
Suitable Membrane Solution
After evaluating numerous membrane filtration systems, Pall Water’s Aria Flex low-pressure, hollow-fiber system was selected to meet high safety standards for water quality and operator safety.
In December 2008, the Murfreesboro Water Resources Department commissioned the 20-mgd Stones River Water Treatment Plant, enlarging and upgrading the facility to be the largest membrane filtration plant, the largest onsite sodium hypochlorite generation facility and the only lime softening water treatment plant in Tennessee at the time. The plant also is one of the only facilities in the world that uses granular activated carbon either before or after membrane filtration to enhance microbial reduction, improve taste and remove odors.
Low-pressure, hollow-fiber membranes were used to meet safety standards for water quality while also providing treatment to effectively control Cryptosporidium and Giardia lamblia.
Smash Spikes in Turbidity
When the upgraded Stones River Water Treatment Plant opened its doors in 2008, it was operating at the highest flux rate in the country. The plant chose Pall Water’s membrane because it could run at 87 gal per sq ft per day. With this flux rate, Murfreesboro could optimize performance and achieve its desired water capacity using fewer membrane modules. This resulted in both upfront purchase and lifetime savings, as there are fewer membranes that will need to be replaced.
The strength of the membranes was tested when the coagulant pump failed during construction, causing increased settled water turbidity of 78 ntu, instead of the 1 to 10 ntu normally seen by the plant. Even with turbidity this high, the membrane system consistently provided a high quality of water between 0.02 to 0.04 ntu. Beyond handling increased turbidity and challenging feed water, the membranes provide an absolute barrier to pathogenic bacteria and superior filtration of any remaining particles that escape sedimentation. This reliability has been a game-changer compared to the facility’s previous gravity filter. Alan Cranford, water treatment plant manager at the Murfreesboro Water Resources Department, no longer has to worry about anything passing through the filters.
“The quality of Pall Water’s membranes and the Aria Flex system overall has been extremely impressive. In nearly 10 years of operation we have had very little instances of breakage or repair compared to our peers that are using different membrane providers. Our membranes have just as much life in them today as they did on day one,” Cranford said.
With clean-in-place conducted monthly, the membranes recover and perform at the same standards they did when they were placed in service in 2008, he added. Additionally, Cranford noted the membrane’s ability to quickly recover from fouling, the ease of maintaining and cleaning the system, and reduced fiber breakage were the main benefits of the system. They also aid the Stones River Water Treatment Plant in remaining compliant with ever-changing water quality regulations.
Qualifying Whether Membranes Will Sink or Swim
Initially intrigued by the opportunity to reduce membrane costs when it was time to evaluate replacement bids in 2015, the Stones River Water Treatment Plant swapped out one train of Pall Water membranes with a competitor’s membranes, as the company was willing to match the warranty.
Initially, 15 of 46 competitor modules could not pass an integrity test on startup and could not be repaired. These 15 modules were replaced 30 days into the trial. The replacement membranes in service were operationally comparable, although seven months into the testing phase, the membranes experienced drops in permeability, and flux rates fell within hours of cleaning.
The continued decline of the membranes led the Stones River Water Treatment Plant to remove replacement modules and reinstall the previously used hollow-fiber membranes throughout the plant. In Spring 2017, the membranes were reinstated and the filtration system continues to operate.
Making a Name for Membranes
The Stones River Water Treatment Plant joined with only a select few drinking water facilities in pioneering membrane filtration for the state of Tennessee. There now are approximately 50 membrane systems at drinking water facilities in Tennessee. The success of the Murfreesboro Stones River Water Treatment Plant set a standard for membrane filtration in Tennessee, and several utilities in the region look to the city for guidance.
Membrane durability and the high flux rates of the system helped the Murfreesboro Water Resources Department transform the Stones River Water Treatment Plant by doubling capacity to meet increased population needs. Since the plant’s re-commissioning in 2008, finished water turbidities have consistently been lower than 0.02 ntu, and total organic carbon reduction has improved from 39% to 55%.
The expanded facility has provided additional capacity for the rapidly growing community, and the upgrades have created an absolute barrier to Cryptosporidium and Giardia lamblia to better protect residents from harmful pathogens. With the membranes, the city of Murfreesboro successfully meets drinking water requirements and provides high-quality drinking water to its residents.