When the service population grows, the treatment plant must expand — nothing exciting there. But with its latest upgrade, completed in June 2007, the Town of Abingdon, Va., decided to leapfrog over state requirements. The town installed more than $2 million in voluntary facilities at its Wolf Creek Water Reclamation Facility to produce exceptional quality effluent.
Planners sought to create a state-of-the-art facility to be good neighbors, as well as good environmental stewards. High-quality effluent benefits local residents along Wolf Creek, as well as the downstream South Holston Lake, which serves as a water source for Bristol, Va., and Bristol, Tenn. The lake also is a major recreational attraction for the area.
Influent comes to the Wolf Creek facility through the area’s 162.5-km (101-mi) collection system. At the facility, preliminary treatment removes inert material from the flow. An influent pump station, which was expanded and modernized during the upgrade, pushes the flow into two primary settling tanks.
Following settling, the flow enters the plant’s two activated sludge aeration trains consisting of four cells in each train, including an anoxic zone. During the upgrade, the activated sludge system was modified to add denitrification. The modified Ludzack–Ettinger denitrification process has reduced the effluent total nitrogen level by about 60%.
After aeration, final clarifiers allow time for solids to settle from the effluent. Next, the clarified effluent passes through a tertiary filter. The filtration is a purely voluntary process for the Wolf Creek facility. The filter consistently provides an exceptional quality effluent, reducing total suspended solids (TSS) to less than 2.0 mg/L.
The filter also ensures high-quality effluent during plant upsets or equipment failures. The highest individual effluent TSS value recorded since the new facility went on-line was 11.1 mg/L. On that day, the flow processed was 23,500 m3/d (6.2 mgd), with an instantaneous maximum flow of 37,900 m3/d (10 mgd).
An ultraviolet-light system, consisting of three in-line, 12-bulb units, provides disinfection to the final effluent. The ultraviolet system replaced the old chlorine gas system, which posed a potential danger to the environment and community through the possibility of a chlorine gas leak.
Before it’s released into Wolf Creek, most of the facility’s effluent is aerated using a cascade aerator. The remaining portion of the effluent flows through two microhydroelectric generators that provide 1 kW of power to the facility.
The solids from the facility’s primary settling tanks and final clarifiers are treated in an anaerobic digester system consisting of a primary and a secondary digester equipped with a gas holder. The digesters have a combined volume of 1817 m3 (480,000 gal) and can treat up to 121 m3/d (32,000 gal/d) of combined primary and secondary sludge.
During the upgrade, mixers were added to the digesters to enhance digestion, thereby increasing gas production and reducing the amount of solids to be disposed of. The digested sludge is dewatered to about 21% solids by two solid bowl centrifuges. The solids cake is either trucked to a landfill or composted with recycled wood chips via the aerated static-pile composting method. The finished compost is designated Class A biosolids compost, which allows it to be sold to individuals for landscaping and soil improvement.
The solids sent to the landfill also get a second life. The landfill itself uses a methane gas recovery system and sells the methane. Studies have shown that mixing the moist solids cake with refuse increases methane production, so the landfill sees the solids as an asset.
In addition to upgrading existing processes, Abingdon took advantage of its expansion project to add grease-receiving facilities at Wolf Creek. This addition enables the town to accept grease-trap waste from area restaurants — keeping it out of the collection system. Previously, grease had to be hauled to Roanoke, Va., or Knoxville, Tenn. — about 210 km (130 mi) away. That long haul led to high transportation costs, which were passed along to the restaurant patrons.
Now, this wastestream is processed through a septage-receiving station to remove inert material and then treated in the facility’s anaerobic digesters. The grease is ideally suited for anaerobic treatment and spurs methane gas production. The methane gas production during the last year was in excess of 204,000 m3 (7.2 million ft3), with a Btu equivalent of 190,000 L (50,000 gal) of liquefied petroleum gas. It has an estimated value of $100,000.
The Wolf Creek facility uses its digester gas to heat its digester and produce 60 kW of electrical power using two microturbine generators in a combined heat and power configuration.
The microturbines are a part of a larger energy recovery project funded by federal stimulus dollars. Other components of that project are a geothermal heating and cooling system, a 2-kW grid-connected solar power system, and an energy management system. The geothermal heat pumps utilize the facility’s final effluent as a heating and cooling exchange medium. The overall energy recovery system should reduce the annual electrical cost by approximately 20% — about $30,000.
The results of the upgrade are obvious when comparing pre- and post-upgrade effluent quality numbers. For instance, during the first 3 months of operation, the TSS and ammonia discharges were more than 90% below the facility’s permitted limits. The table (above) shows the positive change in effluent parameters.
In addition to the treatment upgrades, the expansion also included a new administration building that has a space for employee training, as well as an environmental education center for the community.
Tour groups receive a video presentation about the facility, a walking tour of the grounds, and informational material to take home. The public awareness program is generating interest in the facility, helping to educate the public about the operations that take place, and emphasizing the importance of reclaiming wastewater.
The facility has hosted several groups and organizations. Most recently, a group from the Glade Middle School, located a few miles from the facility, visited. Two representatives from the Virginia Department of Environmental Quality arranged to be onsite to assist the facility staff in leading the kids on a stream survey and collection of various aquatic insects for observation. The kids learned about the anatomy of the collected specimens, the habitat of each, stream quality, stream testing, and how human actions, such as polluting the stream with pesticides and oil, can affect the insects.
What’s more, the town accomplished this entire project on a budget. Through careful planning and taking advantage of a zero percent interest-rate loan from the Virginia Revolving Loan Fund, the town kept residents’ sewer rates affordable.
Despite serving only 9900 customers, Abingdon fully funds its operation, maintenance, and debt service from the sale of sewer service while maintaining affordable rates. The in-town monthly base rate is $23.07 for 7600 L (2000 gal) and $4.06 per 3800 L (1000 gal) thereafter. So, a household using 18,900 L (5000 gal) would pay $35.25, which is 1.14% of median household income for Abingdon.
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