Situated on the Barren River, the city’s wastewater treatment plant (WWTP) collects wastewater from both the city of Bowling Green and outlying areas of Warren County, and discharges the treated effluent back into the river. Built in 1964, the facility was upgraded and renovated in 1976, and again in 1992, which was the last time the plant underwent significant improvements. Faced with an aging infrastructure, more stringent effluent requirements, and projected growth in the area, Bowling Green Municipal Utilities (BGMU) commissioned GS&P to lead the design and engineering processes required to assess treatment and residuals management alternatives for a new 15 million-gallon-per-day (mgd) wastewater treatment plant.
The project’s objectives included the design and construction of a new WWTP that complied with stricter biological nutrient removal requirements, had adequate capacity to treat both present and future wastewater flows, and provided a sustainable, cost-effective solution for biosolids handling and disposal.
“BGMU is a leader in environmental stewardship, but they recognized they weren’t immune to the adverse and unpredictable effects of an aging treatment plant,” says Ken Baker, senior vice president of GS&P’s water resources division. “In 2007, they determined the need to make improvements to the existing facility based on a 201 Facilities Plan report that projected the current plant wouldn’t have enough capacity to serve a growing population by 2025. Another key driver for the plant upgrade was the EPA’s requirements for discharge into the Barren River, which were becoming much tougher.
“Prior to this project, the treatment plant’s capacity was 10.6 million gallons per day. The 201 Facilities Plan report, which was required by the Kentucky Division of Water, determined that BGMU needed a plant that could handle 15 million gallons of sewage on an average day. So we had to find the means to expand the plant from 10.6 mgd to 15 mgd, and then provide the technology that would allow the plant to achieve the more stringent waste requirements.”
Led by GS&P, a team comprised of project managers, designers, engineers and operational and maintenance (O&M) personnel was formed in order to solicit vital input from BGMU staff during the design phase. The integrated team was challenged early and often to develop recommendations that met the criteria established by the basis of design. These criteria included: the use of energy-efficient equipment and existing tankage where practical; simplicity and reliability of operation and maintenance; 20-year life cycle costs; identifying a sustainable treatment process that could provide biological nutrient removal (BNR); utilizing the existing site and avoiding any construction in the floodplain; and minimizing disruption of existing plant operations during construction.
“We were adamant about engaging the client throughout the project—especially the O&M staff,” says Baker. “We wanted their input from day one on everything—from equipment and process selection, to how and where the plant was going to be built.”
“We conducted a series of workshops for BGMU, and had operational and maintenance staff accompany us on site visits to several different facilities to assess the operating performance of different wastewater treatment technologies,” adds Kristi Schnell, GS&P senior associate and water resources engineer. “This approach encouraged the staff to be engaged in the design and selection of equipment—as well as process controls, instrumentation and control systems—and ultimately gave BGMU more ownership in the final facility.
“You really have to listen to the people who are working in the field on a day-to-day basis. It’s often about the little things, such as having to bend down to work on pumps or to turn on valves, and in certain areas we came up with design solutions that placed apparatuses at eye-level so they would be easier for staff to access. It’s one thing to design something on paper, but if staff aren’t going to be able to run and maintain it, then there really is no point.”
During the design process, examination of FEMA Flood Insurance Rate Maps determined that approximately 30 percent of the existing 39-acre parcel of land was located in a floodplain. Due to the Special Flood Hazard Areas, there was nominal space to build an upgraded facility using bioreactors and secondary clarifiers as initially planned. To address this site constraint, it was ultimately decided that the sequencing batch reactor’s (SBR) extended aeration process provided the best overall solution for the plant upgrade. The largest SBR in the U.S. at the time of installation, the new reactor would provide more efficient wastewater processing for higher volumes, while offering a treatment process that could achieve BNR to reduce nitrogen and phosphorus concentrations in the effluent.
“We evaluated several options for the new plant and presented BGMU with five short-listed alternatives that we recommended for the site,” says Baker. “We decided on the SBR process not only because it’s small footprint would allow us to meet site constraints, but also because it could produce an effluent that would meet both current and anticipated EPA standards. It also offered the lowest life cycle costs, and operationally we knew it would respond well to stormwater flows.”
In addition to the selection of the SBR process, the GS&P team was able to utilize some of the plant’s existing infrastructure by using the remaining digester structures for the aerated sludge holding tanks.
Prior to GS&P’s upgrades to the plant, BGMU had to pay for trucks to haul waste material (known as dewatered cake) to the Ohio County Balefill, located approximately 60 miles northwest of the facility. In order to reduce the transportation fees associated with hauling the byproduct, the team evaluated several options to determine a long-term, sustainable solution, including composting, lime stabilization, microwave drying technology and thermal drying technology. All alternatives were assessed using a 20-year life cycle cost analysis that accounted for capital, operation and maintenance costs.
“Most wastewater treatment plants utilize landfills to dispose of their residuals,” says Schnell. “However, landfilling the residuals requires tipping fees and transportation costs, which can be quite expensive, and there’s limited space in a landfill. In BGMU’s case, the landfill operators couldn’t provide an assurance that they would continue to accept the residuals over the long term, and this uncertainty presented a major challenge. To tackle this issue, we went through an extensive evaluation process with BGMU’s management team and finally decided on utilizing a Therma-Flite indirect dryer because it would provide a long-lasting, sustainable residuals management plan.”
The largest thermal dryer in the U.S., as well as the first of its kind, the Therma-Flite IC-10,000 BIO-SCRU® dryer can process 10,000 pounds of water per hour or 12.5 dry tons of sustainable Class A biosolids per day, which can be used as an agricultural fertilizer supplement. Designed to address the needs of a medium-size municipality such as Bowling Green, the revolutionary biosolids dehydration system would also keep operator attention and maintenance costs to a minimum.
“The dryer arrived on site in August 2011 and was commissioned in June 2012,” says Schnell. “It weighed over 130,000 pounds, and Therma-Flite had to get special roadway permits to transport it from Benicia, California, to Bowling Green. The dryer only traveled on secondary highways, which meant that all the bridges had to be checked to make sure they would support the weight. While traveling through the Rocky Mountains, the dryer had to travel at 3-4 miles per hour.”
With the Therma-Flite dryer in place at BGMU’s plant, local farmers can now visit the facility and leave with tons of sustainable Class A biosolids to fertilize their crops and fields. Prior to the dryer’s installation, these farmers were forced to travel to either Louisville’s or Nashville’s wastewater treatment facilities to purchase a Class A product, or pay for the cost of commercial-grade fertilizer.
“BGMU has marketed its Class A biosolids as ‘BGreen,’ and they’re selling it at $23 a ton,” says Schnell. “As a comparison, the most commonly used commercial-grade fertilizer costs about $726 per ton. One full truck of commercial-grade fertilizer, which holds approximately 24 tons, costs $17,424. The equivalent amount of BGreen would be 7.4 trucks, and at $23 a ton that comes to $4,085, which is a $13,339 savings for the farmers.
“This new thermal dryer represents a huge environmental and economic improvement over the previous process because it promotes sustainable use of waste, and is projected to reduce BGMU’s residuals management costs by at least 30 percent per dry ton. Also, the BGreen product has proved to be very effective as a crop fertilizer, and demand is extremely high among farmers who recognize both the value of getting it locally and the value of the product.”
In addition to the leading-edge thermal drying and sequencing batch reactor technologies, the new design included new headworks and preliminary treatment processes, aerated sludge holding tanks, a new influent pump station, improved odor control mechanisms, a septage receiving station, high-efficiency turbo-compressor blowers, ultraviolet disinfection equipment and dewatering centrifuges. Updated instrumentation and controls allow operators overseeing the process to monitor and control operations with real-time data response technology.
Recipient of the Grand Award for the Water and Wastewater category in the American Council of Engineering Companies of Tennessee’s 2013 Engineering Excellence Awards competition, Bowling Green Wastewater Treatment Plant’s $40 million renovation and expansion features cost-effective and operator-friendly technologies that support higher treatment volumes, produce higher-quality effluent, and provide long-term sustainable biosolids management solutions.
“Bowling Green WWTP is now producing a better effluent and has the capacity to support the needs of the city of Bowling Green for years to come,” says Schnell. “With the new SBR process, the water quality is far superior compared to the old method, and visually it looks almost identical to what comes out of your faucet. So the plant is putting cleaner water back into the river than the water plant is drawing out.”
“GS&P’s design achieved every objective that we put forward,” says Mike Gardner, director of water and sewer services at BGMU. “The renovated plant will help us to meet existing and future requirements, and to embrace our commitment to environmentally conscious wastewater practices without sacrificing economic practicalities.”