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At the turn of the millennium, the City of Little Rock, Arkansas, faced a daunting challenge: its aging sanitary sewer system struggled to keep up with the city's growth and increasing demand for capacity. The collection system had languished, and many assets were at the end of their remaining useful life. As a result, even mild wet-weather rainfall events would overwhelm the system, leading to a significant number of sanitary sewer overflows (SSO). This prompted the Sierra Club and the Arkansas Department of Energy and Environment to impose a consent administrative order (CAO) on the City’s utilities branch, Little Rock Water Reclamation Authority (LRWRA), setting the stage for one of the most significant sanitary sewer system improvement programs in the country.
Located on the south side of the Arkansas River, the city of Little Rock’s sanitary sewer system dates back to the late 1800s. The collection system took advantage of the area’s topography to convey flows using gravity, a theme which persists through to today, where approximately 95 percent of the collection system is still comprised of gravity lines. During the last 140 years, the collection system has expanded and matured, now encompassing over 1,400 miles of two- to 72-inch-diameter pipeline, 32 pump stations, and almost 40,000 manholes that convey flows to three water reclamation facilities (WRF).
Little Rock Water Reclamation Authority knew their system needed improvement. In 2001, they developed a plan to address issues and ensure system efficacy, the 2001 System Evaluation and Capacity Assurance Plan (SECAP). This is where the first roadblock came into play.
Like many communities, LRWRA’s management of its sewer system became an issue of dollars in versus dollars out. The funding structure LRWRA had in place was insufficient to meet the maintenance and improvement demands of a growing and aging system.
The second roadblock was inflow and infiltration (I/I), any non-sewage flows that enter the collection system, which inflates operational costs in three ways.
1. Capital improvement programs require substantial amounts of money to eliminate defective sewer assets to eliminate entry points for I/I to enter the sewer system.
2. Increasing downstream flows adds costs by increasing the capacity needed to convey the flows
3. Higher volumes of water arriving at treatment facilities mean paying to treat more water. Diluted sewage is also more difficult to treat, requiring additional time and resources.
RJN Group Project Manager and Senior Vice President Daniel Jackson, PE, stated that the Authority had a significant hill to climb at the onset of the program due to the capital which was going to be necessary to construct the capacity projects as well as reduce I/I thru significant rehabilitation projects.
“The City of Little Rock was experiencing north of 300 wet-weather SSO’s during rainfall events at the beginning of the order and blending a solution of capacity projects along with I/I reduction programs within different sewersheds of the City was a balancing act to make sure the right combination of projects were moved forward as cost-effectively as possible.”
The CAO required LRWRA to assess and improve its collection system to mitigate both dry and wet-weather SSO’s. Additionally, stringent annual reporting requirements were imposed that mandated the community to track and report on the initiative's progress. LRWRA needed an economical, long-term plan to regain and improve its collection system’s hydraulic performance.
In 2008, RJN Group was engaged to provide specialist hydraulic modeling and program manager planning services to review and update the Little Rock wastewater system evaluation and capacity assurance plan (SECAP), originally developed in 2001. RJN Group began a massive hydraulic modeling effort to develop an all-pipe hydraulic model to analyze lift station performance, overall system capacities, and peak hydraulic capacity at the three WWTPs. RJN Group then used the model to evaluate a myriad of alternatives to achieve the appropriate system capacity through a blend of cost-effective I/I removal and capacity improvements.
Improving the model to include all pipes was dynamic in being able to evaluate alternative solutions more accurately. The original hydraulic model only encompassed pipelines that were ≥12 inches in diameter. RJN Group performed flow monitoring to support the calibration of the model and verify assumptions and increased the model size by 440%, adding approximately 900 miles of additional pipe. Data was collected to calibrate the model from 86 flow meter sites and nine rain gauges.
From left: Pre-RJN Model, Current RJN Model
With the comprehensive and calibrated hydraulic model serving as a compass, RJN Group assessed the effectiveness of the 2001 SECAP and developed an updated SECAP in 2010. Since 2010, RJN has continually updated and recalibrated the model to reflect I/I reduction and system augmentation projects and continues to serve as an invaluable tool for assessing SECAP modifications, post-storm hydraulic diagnostics, and operational optimization.
Innovative Solutions to Major Problems
One of the major initial findings from the modeling initiative was identifying massive backflow conditions causing numerous SSOs.
The Rock Creek Interceptor surchar
ged, causing backflow conditions into the Brodie Creek Interceptor, which ultimately overflowed into the Fourche Interceptor. The backflow conditions also removed the line’s ability to flush sediments, which built up and began to fill the pipelines, further compounding the problem.
To remedy this, RJN recommended a diversion line to counteract the surcharge conditions and double the capacity of the existing Scott Hamilton Peak Flow Attenuation Facility to 61 MG. The addition of a 36th street to Mabelvale Pike outfall Installed a 54”, 11,000 LF diversion line for wet-weather incidents routed flows from the Rock Creek interceptor to a 68 MG lift station, where it was pumped into the 61 MG storage facility. This innovative approach allowed LRWRA to maximize its current infrastructure while eliminating about 35 SSO locations directly and more indirectly.
Scott Hamilton Peak Flow Facility
The key to LRWRA’s success was the accuracy of the underlying data. When RJN Group fine-tuned the model, a systematic, data-driven program was developed that helped the Authority be removed from the CAO.
Jackson said that the hydraulic model’s accuracy was the key to effecting change through a series of related efforts that added up to major change.
“Over the 15 years we have been working with LRWRA, the hydraulic model was the greatest tool in evaluating alternatives before design and construction, understanding how if certain amounts of I/I was removed how dramatically the cost-savings could potentially be from constructing large-scale capacity projects.”
The SECAP contained a series of recommendations that LRWRA could use as a roadmap to enhance its system, from maintenance and operations to expanding infrastructure to increase capacity.
“The plan was very intricated and intertwined,” Jackson said. “All of the small pieces had to be implemented to facilitate major changes, with no single action being able to achieve the results alone. We applaud LRWRA for having the commitment and passion to follow through on such an expansive plan.”
A Pivotal Moment
In 2015, LRWRA's leadership changed. The appetite to add additional assets diminished, and a focus on renewing and improving existing infrastructure was adopted. This shift eliminated two major construction recommendations—a seven-million-gallon storage facility and additionally a four-million-gallon in-line storage. This was a risky strategy by the Authority being able to remove the necessary I/I from the system to eliminate the storage requirements, however they were successful in doing so by implementing specifications and processes to the rehabilitation of the sewer system to achieve these results. Also, this change in philosophy in eliminating SSO’s allowed millions of dollars allotted to storage to be redirected to renewing assets that had reached their design life and needed replacement or repair.
LRWRA decided to conduct extensive sanitary sewer evaluation surveys (SSES) encompassing field condition assessments to identify system defects for I/I remediation, including:
A River Runs Through It
RJN crews found one of the most serious defects in Rose Creek, where an elevated manhole had partially collapsed. This created a major opening that allowed the creek to flow directly into the 36-inch line and accounted for millions of gallons of inflow any time the creek levels raised. Conservative estimates based on pre- and post-improvement flow monitoring quantified the inflow from this defect as 0.5 MGD; however, since the creek served as an unlimited source of inflow, the actual impact on the system was much higher.
Another problem identified during the SSES was systemwide root intrusions that created capacity constraints and blockages. A proactive maintenance plan was recommended to mitigate and manage the root issues, which LRWRA successfully implemented.
The findings from the field assessments were utilized to update and prioritize the capital improvement plan recommendations. The alternative recommendations from the SECAP helped further advance the effort. Leaning into the existing alternative recommendations saved time and money and allowed LRWRA to continue to make progress in improving its collection system and meeting the requirements of the CAO.
As of March 2024, LRWRA has been successfully removed from its CAO by investing approximately $575 million to revitalize its sanitary sewer collection system, completely mitigating all SSOs.
SSO Mitigation Progress 2003 to 2023
The utility must continue to analyze the impact of improvements and report findings to the Arkansas Department of Energy and Environment; however, from the basins that have been analyzed so far, the average reduction in I/I that LRWRA has achieved is a very impressive 30 percent.
The key to the project’s success was the commitment that all LRWRA personnel put forth in such a long and complex program. RJN is so thankful to the Little Rock Water Reclamation Authority for allowing our team of engineers to work on such an important and accomplished project.
Little Rock Water Reclamation Authority CEO Jean Block said the program's success was a team effort.
“The successful resolution of this 20-year compliance and improvement process is a testament to the expertise and dedication of the team,” she said.