August 2006, Vol. 18, No.8

Plant Profile

French Broad River Water Reclamation Facility

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Location: Asheville, N.C.
Startup date: 1967
Service population: 135,000
Number of employees: 38
Design flow: 40 mgd (151,000 m3/d)
Average flow: 22 mgd (83,300 m3/d)
Peak flow: 65 mgd (246,000 m3/d)
Annual operating cost: $6 million

The French Broad River Water Reclamation Facility won the Gold Level National Association of Clean Water Agencies (NACWA; Washington, D.C.) Peak Performance Award for 5 consecutive years and last year won the Platinum Level award. Additionally, the Metropolitan Sewerage District of Buncombe County (MSD), which operates the plant, received a NACWA Excellence in Management Award in February 2004 for management of its facilities.

The 40-mgd (151,000-m3/d) facility uses primary and secondary microscreens, three stages of rotating biological contactors (RBCs), and sodium hypochlorite disinfection to treat its average flow of 22 mgd (83,300 m3/d). About 11 mgd (42,000 m3/d) comes from domestic sources, 2 mgd (7600 m3/d) comes from industrial sources, and 9 mgd (34,000 m3/d) comes from infiltration and inflow.

RBCs Abound
Flow enters the facility through a 66-in.-diameter (1650-mm–diameter) interceptor and passes through a bar screen and the grit and grease removal facility.

Next, seven 400-µm primary microscreens remove solids and floating material, and the flow passes on to the RBCs for biological treatment. MSD operates one of the world’s largest RBC plants. Flows pass through three RBC stages, which have 44, 72, and 36 individual RBC units, respectively. Biosolids from the RBCs, in the form of “sloughings,” are dewatered using gravity thickeners. Belt filter presses further dewater the slurry to an average of 25% solids. The cake is then burned in a fluidized-bed incinerator, which is the primary method of biosolids disposal. The ash is deposited into a lagoon on the plant property. (Alkaline stabilization can be used as a backup to incineration. This process produces “Nutrilime,” a Class A, Exceptional Quality biosolids product.)

After biological treatment, the liquid flow enters a four-celled intermediate clarifier with a combined volume of 2 million gal (7.6 millionL), followed by secondary microscreening through 18 27-µm screens.  Finally, sodium hypochlorite is used to disinfect the effluent, which is discharged into the intake flume for a hydroelectric facility where some of its energy is recovered for power generation. A portion of water from the French Broad River, the facility’s receiving stream, also passes through the same flume, from a dam located upstream of the effluent outfall.

Savings Plans
The facility uses several renewable resources and clever arrangements to reduce its expenses. The hydroelectric dam generates up to 2.4 MW of electricity using three turbines. The generation facility helps the reclamation facility offset its electrical power costs by about $33,000 per month. The original hydroelectric structures were built in the early 1900s and used to provide power to Asheville’s first trolley system. This system later was used to generate power until the 1960s, when it was decommissioned. The system was rehabilitated in the mid-1980s.
MSD also generates energy using methane from a nearby closed landfill. Gas is piped to the plant, where it powers two engine-driven generators, which together supply approximately 900 kW of power.

The facility also saves money by partnering with a local laboratory firm. In return for allowing the firm to use the MSD laboratory space, MSD receives all analytical lab work at no cost. MSD had found that its laboratory facilities were underutilized, and staffing the lab was an excessive cost. The lab company helped fill this niche, which saves MSD approximately $60,000 per year in laboratory testing costs.