People in the trenchless industry in the middle Tennessee area are excited about hosting the 2012 No-Dig Show in Nashville in March. Here we take a look at the infrastructure Under Nashville to see how trenchless has helped deliver to the community, and serves as a significant part of the future.

The City of Nashville and Metro Water Services (MWS) are on the verge of a sewer system improvement program estimated to cost between $US1 billion and $US1.5 billion. MWS is a department of the city which operates the sewage collection and treatment system for most of Nashville, surrounding Davidson County and fringe areas of other counties near Nashville. MWS also treats and provides potable water for much of this area, and additionally operates the stormwater utility for the county.

Trenchless wastewater rehabilitation has played a major role in improving the sewage collection system over the past 20 years. This was highlighted during a previous No-Dig conference and numerous pumper equipment shows held in Nashville.

In 1993, the previous 15-year Nashville program was the recipient of the first Trenchless Technology Project of the Year – Rehabilitation. The winning Oak Valley Drive Project demonstrated that significant additional infiltration and inflow (I/I) reduction could be achieved by rehabilitating service laterals simultaneously with public sewer rehabilitation. Rehabilitation of service laterals and manholes in a comprehensive ‘system approach’ became a hallmark of this program.

In 2006, the Whites Creek Performance-Based I/I Reduction Contract, MWS – along with design-build partners Reynolds Inc. and Arcadis – was named the Trenchless Technology Project of the Year – Rehabilitation.

Recently published plans by the city show that trenchless techniques will play a significant part in the future.

Meeting the standards

Nashville began an aggressive program in 1990 to reduce the number and severity of separate sanitary overflows (SSOs) and combined sewer overflows (CSOs) following an earlier Order from the State of Tennessee. 15 years of work by the city resulted in major reductions of overflows and major improvements in the level of service for MWS customers.

One measure of improvement was recognised by state and federal regulators when 33 miles (approximately 53 km) of the Cumberland River were formally ‘de-listed’ during a ceremony in November 2002. Major reaches of the river were removed from the Environmental Protection Agency (EPA) 303d list, which is used to identify watercourses with significant environmental impairment. The de-listing meant that those reaches now meet EPA water quality standards.

Despite these improvements, further work was required, and an enforcement action initiated by the US EPA resulted in a Consent Order with Nashville that was approved by all parties in March 2009.

Initially, the city was required to develop and submit a Corrective Action Plan & Engineering Report (CAP/ER) and a Long-Term Control Plan (LTCP). Considering an extension of the original schedule allowing for the devastating flood in Nashville in May 2010, the city submitted these documents to the US EPA in September 2011. The CAP/ER (developed by the MWS engineering consultant, CDM) estimated a total of about 300 miles (approximately 483 km) of sewer rehabilitation work projected for the three main parts of the sewage collection system (served respectively by the three treatment plants operated by MWS shown in Table 1).

Table 1: rehabilitation work projected by treatment plant basin.

Wastewater treatment plant and basin | Rehabilitation upper limit* (feet)

• Central |1,251,000
• Whites Creek | 194,000
• Dry Creek |153,000
• Total |1,598,000

*Footage shown is the upper limit of pipe footage identified in the rehabilitation target areas. Actual rehabilitation footages will be based on achieving R-value reduction goals targeted determined through on-going flow monitoring.

Trenchless, Nashville and Mr Toosie

To get a more personal and close-up perspective, Assistant Director and Chief Engineer for MWS Cyrus Toosie allowed Trenchless International to interview him about his experiences with trenchless techniques in Nashville. When asked about past experiences with trenchless products in Nashville, Mr Toosie said that most of the sewer rehabilitation in Nashville has been conducted using trenchless methods for reducing I/I.

Nearly 240 miles (approximately 386 km) of sewers have been lined in the previous program – which represents almost 10 per cent of the system. Predominantly, Nashville used cured-in-place pipe (CIPP) type products, as well as fold-and-form liners.

“Generally CIPP products met our objectives better than other processes,” Mr Toosie said in regards to the early years.

The rehabilitation work (and upgrades to pumping stations, construction of detention tanks and upsizing some pipes) resulted in a reduction of SSO locations from 164 to 27, and a reduction of CSO locations from 32 to 9. The level of effectiveness of the rehabilitation work was measured objectively using over 100 long-term and hundreds of short-term flow monitors.

Mr Toosie said “We used the post-monitoring to measure the reduction of I/I and R-values (percentage of rainfall reaching the sewer pipes) to determine if an SSO could be terminated.

“Compared to dig-up and replace, trenchless rehabilitation is a whole lot less disruptive to the community.”

In regards to making decisions about products used in previous work Mr Toosie said “The advantages of CIPP for us were that there was no annular space, there was a strong bond in the host pipe, the material did not have a ‘memory’ like fold-and-form (at that time), and there was no opportunity for shearing at a butt joint (referring to HDPE pipe).”

Mr Toosie reflected that the previous program developed a definite preference for using CIPP. However, he pointed out that “We have been out of the business for about five years,” (referring to a lull in rehabilitation work by MWS from the end of the initial program and during the negotiation period for the current Consent Order).

“Now, we will go back to the drawing board and develop criteria and standards for the rehab work planned in the CAP/ER.”

Mr Toosie also said that this fresh look at the program would review new products developed or made available in recent years. As an example, he referred to the extensive, successful use of UV-cured CIPP in the City of Brentwood that has a satellite wastewater collection system connected to the Nashville system. UV-cured CIPP was not readily available during most of the prior Nashville programs.

Recently, MWS also used a PVC spiral panel type of liner for the first time to line 800 ft of a deteriorating 96 inch brick CSO outfall built in 1910. This installation downstream of the Schrader Lane CSO Regulator chamber was recently completed and a paper on this project will be presented during the 2012 NASTT No-Dig in March.

The overall strategy

Mr Toosie continued by explaining the overall strategy that formed the foundation of the CAP/ER. He said “I always think that it is important to have a strategic plan. It took three years to develop it (for the CAP/ER that was just submitted to EPA). Now we have a cook book, a road map.”

He explained the concepts behind developing the strategy “There are three solutions for reducing I/I and stopping overflows: sewer rehabilitation, additional conveyance capacity, and storage. It is like a triangle with these three points. Our job is to find the ‘sweet spot’. We need to get the R-value from a level of 20 per cent (for example) down to a level of 10 per cent. So, what does that require using these three techniques?”

Mr Toosie described the task of achieving a balance among the techniques for getting the best benefit for the cost.

“Overall, I think that sewer rehabilitation is effective. However, when we looked at the costs for the Central basin area, we found we could spend $US10 million on additional storage, and thereby eliminate a cost of $US100 million for rehabilitation. We tested the result with the computerised hydraulic model, and saw that the storage alternative would effectively handle the problem in that area,” Mr Toosie said.

Planning for the storm

The hydraulic model has been a critical tool used by CDM to evaluate the alternatives. The model allowed sophisticated testing of various storm sizes. The latest study is based on using the two-year, 24-hour dormant season storm design. This storm includes a peak intensity of 0.97 inches per hour and a 24-hour volume of 3.15 inches. Mr Toosie pointed out that the computer model was essential for identifying the effects of extreme conditions, such as two of the design storms occurring back-to-back. The final capacity and pumping requirements must be checked when conditions will not allow the facility to be completely drained from the first storm.

When this kind of analysis was conducted through all three of the collection system basins, the planners selected sewer rehabilitation as a major component of the overall program. Mr Toosie said that rehabilitation would cost about $US300 million and represent about 38 per cent of the money planned for the separate sewer system (or about 27 per cent of the overall program). He pointed out that these were still very broad estimates, and the MWS website cites the range as $US1billion to $US1.5 billion. Not being able to meet the schedule, even with a sufficient workforce, is Mr Toosie’s biggest worry.

Regarding projections of future problems, Mr Toosie said “It is too early to tell. We are just beginning and we have eleven years.” When asked about funding for such a massive program, Mr Toosie explained that in the near term, there is a lot of borrowing debt capacity, and that funding is now in place for the first three years. For the future, he said that his ‘gut feeling’ was that the system would borrow as much money as possible until a rate increase was needed.

Mr Toosie likes the idea of using sewer rehabilitation for asset management and sewer renewal, which is also a part of the Capacity Management, Operations & Maintenance (CMOM) requirement in the Consent Order.

A fresh look for the future

In planning for the next sewer rehabilitation projects, MWS will take a fresh look at available processes and techniques.

“Once all the rehab consultants are on board plus the construction manager and the program manager, then a focus group will be created from these players to evaluate and recommend the most cost-effective products and processes to serve MWS’ purposes,” Mr Toosie said.

This group will help establish the strategy for application of products and the criteria for selection.