Root Force

Knoxville Utilities Board combines accelerated system upgrades with a comprehensive maintenance program that includes effective root control.
Root Force
KUB Wastewater System Asset Management and Planning team leader James Koontz, left, and Water Systems engineering manager Mark Rauhuff are shown in front of a water KUB system map.

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The Knoxville Utilities Board (KUB) of Knoxville, Tenn., is engaged in an assertive program to renew its wastewater infrastructure. The effort consists of a two-pronged attack. The first focuses on system enhancement, rehabilitation and replacement. The second concentrates on operating and maintaining the system, including an emphasis on effective root control.

The utility was founded in 1939 as a provider of water and electrical services. Gas service was added in 1945, and KUB assumed wastewater services from the City of Knoxville in 1987. KUB is an independent agency of the city and funds its operations through ratepayer revenues.

By the late 1990s, KUB was committed to system improvement through capital wastewater system replacements and a management, operation and maintenance (MOM) program. However, a consent decree signed with the US Environmental Protection Agency in early 2005 bound the utility to address chronic sanitary sewer overflows (SSOs).

The utility’s first program designed specifically to address SSOs was Partners Acting for a Cleaner Environment, a 10-year, $530 million program launched in October 2004 in anticipation of the consent decree. Dubbed PACE10, the program is composed of more than 130 projects focusing primarily on infrastructure improvements.

The consent decree also prescribed a series of maintenance and capital programs and required the utility to establish a continuing sewer system assessment program (CSSAP).

KUB operates more than 1,300 miles of wastewater infrastructure, with pipes ranging between 6 and 84 inches in diameter. The older pipes date back as far as a century and are made of brick, clay and concrete. Newer pipes are made of PVC, HDPE, ductile iron and fiberglass-reinforced concrete.

“We don’t have a problem with combined storm and sanitary sewer overflows, because the systems were designed with separated flows,” says Mark Rauhuff, water systems engineering manager with KUB. “One neighborhood that had combined flows was separated in 1995.”

Assessment cycle

The CSSAP has targeted a 12-year cycle for complete assessment of the system. The program achieves its goals through multiple approaches including CCTV camera inspection, smoke testing, manhole inspections and flow monitoring.

System modeling resulting from the CSSAP confirmed what earlier studies had already indicated.

“SSOs were largely related to a lack of system capacity and driven by inflow and infiltration (I&I) caused by heavy wet weather events,” says Rauhuff. “However, a smaller number of dry weather SSOs often followed wet weather events as grease and solids were forced downstream to create new blockages. The primary approach of PACE10 regarding SSOs is to address the situation with a mixture of pipe replacement and rehabilitation, an increase in wastewater storage capacity and treatment plant upgrades.”

Over the history of the wastewater system, the need for increases in collection system capacity and storage are a direct result of customer growth and an aging system that has resulted in increasing I&I.

“The original design featured trunk sewers sized for the needs of the community served,” says Rauhuff. “As they’re being replaced, we’re increasing their capacity. We’ve also built six wet-weather wastewater storage basins with 33 million gallons of total storage.”

Four of the units, totaling a 21-million-gallon capacity, serve the system during regular wet weather. Two others, with a combined capacity of 12 million gallons, serve two of the four wastewater treatment plants during extreme wet weather. A series of flow monitors inform the SCADA system when water levels rise, efficiently diverting flow to the storage tanks.

KUB is also upgrading its Kuwahee and Fourth Creek wastewater plants to optimize treatment capacity during peak wet-weather flows.

Replacing pipe

The ambitious goal of the pipe replacement program is 2 percent of the system per year, amounting  to about 25 miles of sewer line annually and 218 miles to date.

Contractors perform most of the dig-and-replace pipe work, and also take on most of the rehabilitation contracts.

“From both a budgeting and scheduling perspective, it makes more sense for us to keep in-house crews for emergency work which they can give their uninterrupted priority,” says James Koontz, KUB’s wastewater system asset management and planning team leader.

The pipe rehabilitation program employs some trenchless technologies, including pipe bursting and epoxy cured-in-place-pipe lining.

“At one point we were also using a grout product,” says Koontz. “It was a good application, but it required extreme precision in applying it in just the right location. However, we feel that we’re often getting more value by putting in new pipe than using grout point repairs in most applications.”

As more detailed system information is collected, KUB has become able to focus and prioritize its sewer maintenance efforts.

“Our MOM effort has switched from a reactive model to a proactive blockage abatement program based upon predictive condition assessment investigations,” says Koontz. “If previous inspections or blockages have indicated a problem area, we may return there to inspect and clean as often as every month.”

Emphasis on root control

Root control forms an important component of KUB’s sewer maintenance program. The largest blockages are found in sewer easements, since the roots of native trees typically do not stretch underneath roadways.

“We’ve tried chemical and we’ve tried mechanical,” says Koontz. “However, at this point we’re doing far more mechanical than chemical.”

KUB contracted chemical root control services about 10 years ago, but found that the treatments offered diminishing returns. The foaming agents destroyed roots as promised and increased sewer capacity.

“However, to stay on top of the roots, you have to keep applying and reapplying the chemical treatment,” says Koontz. “We found, through visual inspection, that once you begin to eliminate the section of root where it enters the pipe, you’re pulling the finger out of the dam so to speak and increasing inflow from outside the pipe. Chemical treatment is still on the table, however it’s been several years since we’ve opted to use it.”

Most of the current root control program involves hydraulically propelled cutting equipment. In some cases, root cutting releases other blockages, such as debris and grease trapped in the roots. Hydraulic flushing and vacuuming is followed by a CCTV inspection to ensure that the debris has been cleared.

“Our KUB maintenance crews are equipped to perform root removal, but we don’t typically put them on scheduled maintenance runs,” says Koontz. “Routine root control is contracted, while our crews perform root removal as part of emergency blockage calls.”

Removing roots from laterals

The root removal program includes chasing roots into lower laterals up to the customer’s property line.

“Sometimes roots even grow into the mains from laterals,” says Koontz. “If we find that roots continue to be a problem further down the lateral, or if smoke testing and CCTV inspection indicate structural defect, we have a policy to ensure the customer clears the pipe or replaces the lateral. In the early years of the program when a large number of lateral replacements were occurring, we assisted lower income customers with costs through a grant program.”

The utility performs a cost-benefit analysis on any pipe segments that require frequent root removal.

“We’d rather use the capital budget to replace that length of pipe than to clear it repeatedly,” says Rauhuff.

Prevailing over SSOs

Applying both infrastructure improvements and diligent maintenance, KUB has reduced SSOs from more than 400 in 2004 to fewer than 100 in 2012, with most of the remaining events related to extreme rainfall. Detailed information about each incident is available 24/7 on the utility’s website.

“Utilities sometimes forget to make an extra effort to communicate with their customers as various programs are initiated,” says Rauhuff. “In 2012 alone, we sent out 19,000 letters about construction projects, including 11,000 about PACE10 initiatives. Customers can understand and appreciate the need for rate increases when they’re alerted to the work and can see KUB’s success in reducing SSOs.”

PACE10 is being extended beyond its 10-year original scope to the conclusion of the wastewater treatment plant upgrades in 2021, at which point an expected $650 million will have been spent to meet its targets. The sewer infrastructure program will then join KUB’s Century II program at a renewal rate of 2 percent annually.

“PACE10 is on time and on budget, and we continue to meet each of the terms and schedules of the consent decree,” says Rauhuff. “Of the 126 projects that must be completed by June 2013, we’ve already completed 125. We’re definitely on top of the program.”


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