The system had been built in the 1960s and was showing its age. Replacement parts were difficult to find and the plant suffered from increasingly frequent breakdowns. The outdated plant did not even meet state water quality regulations.The Southern Appalachian town of Boone, North Carolina, is home to Appalachian State College, several small ski resorts and a growing permanent population. The community has 14,000 permanent residents and 12,000 college residents, yet in 1996 its wastewater treatment plant was based on 1920s technology and had reached 80 percent of its hydraulic capacity. The system had been built in the 1960s and was showing its age. Replacement parts were difficult to find and the plant suffered from increasingly frequent breakdowns. The outdated plant did not even meet state water quality regulations.
North Carolina had also designated the South Fork trout stream as an "Outstanding Resource Water," thus lowering its requirement levels for pollutants in the water. Since Boone's wastewater plant discharges into this stream, the designation made it necessary to improve the plant's effluent. The facility's bio-chemical oxygen demand (BOD) limit would be reduced from 23.6 to 5 mg/L, its ammonia limit from 16.6 to 2 mg/L and its total suspended solids limit from 21.8 to 20 mg/L.
At the same time, Boone officials wanted to increase discharge capacity from 3.2 to 4.82 mgd. A significant upgrade and expansion would be the only way for the plant to meet these new regulations and capacity level.
Town officials decided to upgrade the 30-year-old plant to technology that would meet today's water quality standards. The original renovations were designed simply to meet state requirements. When the project was completed six months early and under budget in May 1998, it produced a cleaner effluent than anyone had originally expected.
"We were able to come out with a project that has been running a year and a half now or longer and doing great," George Sudderth, director of public utilities, said.
According to Steve Breedlove, construction manager for Hobbs, Upchurch and Associates, the project was a major upgrade to the plant. Boone officials hired Hobbs, Upchurch and Associates as the engineers for the project and Thamer Construction as the contractors. The design included demolishing most of the old plant but keeping the aerobic digesters, reworking the headworks and adding influent pumps and an aerated grit and grease removal system.
The original plan called for an existing digester to be changed over to a high rate aeration system that would accelerate the plant's sludge digestion. This plan would have restricted most of the demolition work until after the new sludge drying equipment was up and running and would have delayed most of the construction. Demolishing existing structures, erecting new structures and pouring more than 8,000 yards of concrete would have pushed the project into the winter months in an area that can get more than 110 inches of snow in a season. Boone's ski resorts also increase its winter population, making it even more difficult to make changes during this season. An alternative schedule was developed.
Another problem was sludge storage during construction. The old plant had used beds to dry the sludge. Sludge sat in these beds until it dried over time. However, these sand beds needed to be demolished to expand the new plant. Waiting for the existing sludge to dry in beds meant delaying construction and no one had the time to wait for the sludge to dry this way. A temporary belt press was brought in to quickly dry the sludge so the sand beds could be demolished and construction continued without delay. The new plant uses a permanent belt press instead of sludge beds.
The plant's surrounding terrain was another obstacle to construction. Using a crane to place forms and equipment materials in the oxidation ditches was almost impossible because of the topography. Thamer Construction strategically placed a 40-ton capacity Koehring crane directly on the oxidation ditch slab.
"The crane was placed almost dead center of the ditch and it could reach almost anything without moving," Jim Barbee, Boone wastewater treatment plant superintendent, said. Before this placement, the crane could not reach the other side of the ditch. Now it could easily access all sides of the ditch, speeding up production. This decision and placement was one of the keys to completing the project early.
More than 70 changes were made during the project for improvements at a cost of $407,000 in change orders. According to Sudderth, less than $100,000 was spent after sales tax returns. The project had a $1.3 million contingency fund for changes, therefore they were actually under budget. Some of these changes included construction scheduling to keep concrete pours from occurring during the winter, placing the 40-ton crane on the new oxidation ditch slab to access construction areas more easily and decreasing the concrete slab thicknesses because of existing hard rock foundations.
One money-saving change was the use of stainless steel pipe instead of ductile iron for the tertiary treatment. While the original plan had called for ductile iron, Thamer Construction negotiated a good price for better grade stainless steel pipe for the project.
The Finished Product
When the project was finished, pollutant levels did more than just meet state discharge requirements. Discharge levels were reduced to readings lower than team members had expected at the beginning of the project. The utility is discharging BOD at 1.6 ppm as opposed to 3.5 ppm and suspended solids of 1.37 ppm instead of 6.5 ppm. The plant's discharge capacity was expanded from 3.2 mgd to 4.8 mgd as planned.
"Our effluent is cleaner than we anticipated," Sudderth said. "I think everyone is surprised that the plant is doing as well as it's doing." In the year and a half since the project's completion, the plant has continued to improve as operator's knowledge about facility operation has increased.
The project replaced the old trickling filter secondary treatment system with an aerated oxidation ditch system using deep-bed tertiary filters and ultraviolet disinfection. The new design centers around extended air oxidation ditch technology utilizing 4,000 fine bubble air diffusers and 100 coarse air bubble diffusers. The sludge is converted to biosolids with a system capable of processing 7.2 dry tons of material per day. The system produces a Class A grade of biosolids suitable for use in horticulture and landscaping. The final product is a granular, soil-like material.
The old plant was a trickling filter plant capable only of secondary treatment. This technology allowed the facility to perform only a biological treatment process. The upgraded plant has tertiary treatment capabilities as well. After using clarifiers in the biological step, a sand filter treatment removes even more pollutants from the effluent.
Barbee said this additional step provides two major improvements: reducing pollutants such as BOD and ammonia and eliminating chlorine. "The primary difference is the reduction of these pollutants," Barbee said. "Our levels for BOD and ammonia often times are below detection. It's much lower than we ever anticipated."
Producing a cleaner effluent was a major impetus in the plant's design, said Breedlove. He added that now the sewer water often is cleaner than the river water.
Teamwork and Cooperation
Breedlove attributes the plant upgrade's success to the "cooperation between town of Boone officials and the engineers and contractors working together."
"The teamwork was definitely there," Thamer Project Manager Ken Cote said. According to Cote, the owner, engineer and contractor all provided input, moving the project forward differently than if the responsibility for different parts of the project rested on different people's shoulders.
"I think they knew that this plant could be special and everybody took a lot of interest in it," Breedlove said. "[It was] a huge cooperative effort from everybody. Any problems that were there were resolved right on the spot."
Sudderth, Barbee, representatives from Hobbs, Upchurch and Associates and representatives from Thamer Construction held monthly meetings to track the project's progress. These meetings "kept everybody on the same page," Sudderth said, and allowed the owner, engineer and contractor to provide input on all parts of the expansion. In addition, a town council member attended the meetings. Therefore, the Boone Council was apprised of all change order information as it happened and was able to act more quickly on it.
"[There was] much more cooperation and communication and teamwork than I've ever worked with before," Cote said.