Water Treatment Plant Sets State Standard for Use of Filtration and Disinfection Technologies

Murfreesboro, Tenn., located near the geographic center of the Volunteer State, is best known for being the site of one of the Civil War's bloodiest battles the Battle of Stones River. The city was also the capital of the state in the early 1800s, before being replaced by nearby Nashville in 1826.

Today, Murfreesboro is better known as one of the country's fastest growing cities, with a population that has grown more than 120% since 1990. Middle Tennessee State University, which boasts the largest undergraduate enrollment in the state, also is located in Murfreesboro.

The city can now claim another distinction. In December 2008, its water and wastewater utility, Murfreesboro Water and Sewer Department, commissioned the 20-million-gal-per-day (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. The plant, which was constructed in 1967 and averaged 10.6 mgd prior to expansion, is also one of the only facilities in the world that has the capability to utilize granular activated carbon either before or after membrane filtration to enhance microbial reduction, improve taste and remove odors.

According to Alan Cranford, the water and sewer department's superintendent of water treatment, the plant's expansion and improvements, which began in 2005, were precipitated by three factors. "While we had upgraded the plant several times since its original commissioning, some of the equipment was 40 years old,” he said. “And with the rapidly growing population in this part of the state, we needed to expand the plant's capacity. Lastly, we wanted to be proactive about preparing for anticipated new EPA regulations relating to water treatment. For example, many water utilities are moving from sand filtration to membrane filtration to more effectively control organics and Cryptosporidium at lower cost."

Natural conditions, turbidity and water hardness in the Murfreesboro area posed two treatment challenges for the Stones River expansion project. The Stones River plan draws some of its source water from a stream that has elevated levels of turbidity during rain events. "During heavy rain storms, turbidity can rise dramatically," Cranford said. "Before the installation of the new plant, it was not uncommon to have finished water turbidity levels of 0.1 to 0.2 NTU."

In addition, the geology of the region resulted in naturally hard groundwater supplies. As a result, a lime softening technology had to be incorporated into the upgraded plant. "While western Tennessee's soil is very sandy, limestone deposits in middle and eastern parts of the state make for relatively hard water,” Cranford said. “This was a particular concern for restaurants and other food industry customers—even car washes. So we made the decision to incorporate an RDP Tekkem lime slaking system for a more cost-effective softening process at the plant."

The key companies involved in the plant's upgrade were the design engineer, Smith Seckman Reid Inc.; the general contractor, Building Crafts Inc.; the electrical contractor, Wolfe & Travis Electric Co.; and the systems integrator, M/R Systems.

The membrane filtration technology selected for the project was the Microza microfiltration membrane from Pall Corp., which features smooth inner and outer membrane skins with a highly porous symmetrical support structure giving high flow rates. In selecting the disinfection technology, Smith Seckman Reid recommended replacing the chlorine gas unit with onsite sodium hypochlorite generation. Safety was a big reason for the choice. "The Stones River plant is located near a Veterans Administration hospital, and many of their patients cannot be easily moved from the facility in the case of an emergency," said Cranford. "We felt it wasn't worth the potential risk to continue using chlorine gas. In addition, the hazmat safety training and ongoing vigilance required to comply with our EPA risk management plan were significant time consumers.

"When Smith Seckman Reid investigated the most appropriate on-site system for the Stones River plant, they considered simplicity of operation and maintenance. Selecting a system that had a proven track record in larger facilities was important, too."

The system selected was the ClorTec onsite sodium hypochlorite generating system from Severn Trent Services . The system includes two 900-lb-per-day hypochlorite generators that are kept in operation concurrently, giving the facility redundancy in the event one unit temporarily ceases operation; two 40-ton brine silos; and three 15,000-gal storage tanks.

The ClorTec system works by feeding softened water into a brine dissolver. A brine solution is formed, which is further diluted to a consistent 0.8% solution. The salt solution then is passed through electrolytic cells, which apply a low-voltage DC current to the brine to produce the sodium hypochlorite solution. The sodium hypochlorite solution, a chlorine equivalent, is then safely stored in the storage tanks. When it reaches the low-level set point, the system automatically restarts to replenish its supply. Three liquid feeders inject the sodium hypochlorite as needed into the distribution system to maintain the disinfection demand.

"The 0.8% solution is non-hazardous," said Cranford. "Had we selected bulk sodium hypochlorite, its concentration ranges from 12% to 15%, a solution strength that is considered a hazardous material and is subject to containment requirements."

Compared to gaseous chlorine, generating a sodium hypochlorite disinfectant at the plant without the use of chemicals eliminates potential hazards associated with transportation and storage. And unlike bulk sodium hypochlorite, which degrades with exposure to ultraviolet light, the 0.8% sodium hypochlorite solution is stable. And, according to Cranford, an economic evaluation of chlorine disinfection alternatives determined that onsite generation provided a less expensive life cycle cost for the Stones River facility than purchasing bulk sodium hypochlorite over a 20-year period.

Since the plant's re-commissioning in 2008, finished water turbidities have consistently been less than 0.02 NTU and total organic carbon reduction has improved by almost 33%—from 39% reduction to 55% reduction. The expanded facility has provided additional capacity for the rapidly growing community, while the facility upgrades have improved the security, operability and maintainability of the facility and improved the quality of the finished product.

"The ClorTec system has proven to be simple to use and easy to maintain. And we've been impressed at the consistency of sodium hypochlorite solution it has produced,” said Cranford. “Compared to other onsite systems we investigated, the consistency of the solution has been outstanding."

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