Plant Profile: Upper Hocking Water Pollution Control Facility

The city of Lancaster, located near the Hocking River in south central Ohio, sought to expand the Upper Hocking Water Pollution Control Facility (UHWPCF) to address the growing needs of the community and anticipated population growth. The plant serves the north and west portions of the city and approximately 19,000 residents. Expected future industrial growth and the need to improve treatment capabilities by producing high-quality effluent to be used in manufacturing and other applications was another driving factor for expansion.

Modular Means Flexible

Plant superintendent Mike Nixon noted the city has been as been pleased with the operation of UHWPCF and that the original plant was designed in a modular manner to expand the system as flows increase, allowing the city to add to the previous design and double the capacity of the existing plant in a fairly simple manner.

The plant is current designed for a peak hourly flow of 8 million gallons per day (mgd) and an average flow of 2 mgd. Current average flow is 1.6 mgd. The UHWPCF will be expanded to treat 4 mgd with an average daily flow of 14 mgd doubling capacity to meet future requirements. Its expansion is expected to be commissioned in mid-2024.

Treatment Train

The treatment process starts at the Upper Hocking Pump Station where five variable frequency drive (VFD) pumps are sized to accommodate dry and wet weather flows. The facility includes odor control and emergency standby power facilities.

Two force mains — a 14-inch line and an 18-inch line and each approximately 14,300 feet long — convey flow from the pump station to the UHWPCF, providing capacity for peak flow and maintaining minimum scouring velocity at low flow. Preliminary treatment is sized for an 8 mgd peak flow and consists of two cylindrical drum screens with 2-millimeter perforations. A washer and compactor at each screen processes the screened material for landfill disposal. 

A 1-million-gallon circular equalization tank (EQ) with a pumped jet mixing system is used to limit peak flow to the biological process to 6 mgd. Screened flow in excess of 6 mgd is diverted to EQ. After wet weather flows subside, flow from the EQ is pumped into the aeration system influent.

Three vertical loop reactor (VLR) tanks with a total process volume of 866,000 gallons use a surface-type disc aerator and coarse bubble diffusers at the tank floor level. Three VFD process air blowers supply air to the VLRs, which can operate in parallel or in series and have internal mixed liquor return capability.

Liquid solids separation uses hollow-fiber membranes. There are three membrane tanks, each with 17 racks. The ultrafiltration membranes have a pore size of 0.1 microns. Three VFD membrane feed pumps supply mixed liquor from the VLRs to the membrane tanks. Three VFD blowers provide scouring air. To prevent solids from clogging the membranes, scouring air and mixed liquor are introduced at the tank floor directly below the membrane racks.

Three filtrate pumps pull clear liquid through the membranes and discharge it to disinfection, using sodium hypochlorite and citric acid for periodic cleaning cycles. A horizontal, low-pressure, high-output ultraviolet light system provides disinfection. A constant water surface elevation is maintained by an effluent weir.

A minimum dissolved oxygen concentration of 6 mg/L is maintained by a step-aeration system downstream from the disinfection system. Treated effluent is conveyed to the Hocking River through a 36-inch outfall pipe. While there is no phosphorous limit in the NPDES permit, alum or ferric chloride is available for phosphorous removal, in order to keep the effluent level around 1 mg/L.

In the Cannibal sludge minimization system, biological solids are destroyed through an interchange of mixed liquor flow between the VLR activated sludge process and interchange tanks. Air is provided to the tanks by two constant speed blowers and coarse bubble diffusers. A decanter in each tank thickens solids with the recycled water being returned to the VLRs. The oxidation reduction potential (ORP) of the interchange tanks is controlled by the extent of aeration provided to minimize biological solids yield.

Mixed liquor is processed through a 0.25-millimeter wedge wire cylindrical drum screen and then through grit cyclones to further reduce solids yield from the biological system by removal of inert and non-readily degradable solids. Solids are processed through a washer/compactor. Grit is dewatered by gravity for landfill disposal. Solids are wasted from the interchange tanks to a residuals storage tank aerated with a constant speed blower and coarse bubble diffusers. A decanter thickens the solids.

Waste solids are dewatered with a high-solids, horizontal, solid-bowl type centrifuge and conveyed to a dumpster for landfill disposal. The dewatering system includes variable speed sludge feed pumps and liquid polymer feed units.

Engineering & Designing for Expansion

The city chose Evoqua to provide for the expansion of the facility, which has been in operation since 2011. Arcadis Engineers provided engineering and design with Kokosing Industrial serving as the general contractor. The decision to expand the existing VLR system was considered the most cost-effective solution to double the treatment facility size.

Evoqua’s VLR system utilizes looped reactors in series enabling dissolved oxygen stratification to provide simultaneous nitrification and denitrification in a smaller footprint.

The process was adapted from Evoqua’s Orbal Multichannel Oxidation Ditch system with similar tankage but has been flipped on its side, using the same surface-mounted discs to provide mixing and to deliver oxygen with an upper and lower compartment separated by a horizontal baffle.

The VLR system is designed to be 20 to 30% lower in power costs than conventional biological nutrient removal (BNR) processes. It is comprised of two or more tanks with the first tank operating as an aerated anoxic reactor. The average daily flow would increase to 4 mgd with a peak hourly flow of 14 mgd by adding a second train of three tanks to the existing system.

“Because the plant was designed as modular expansions, the equipment was selected to allow for expansion in either one to four phases or some larger pieces like the centrifuge was designed to cover two phases,” said Denise Crews, environmental engineer with the city of Lancaster. “The modular design allows us to continue operations during construction with the only shut down needed to transfer treatment from the existing plant to seed the second phase.”

Crews said the plant is designed for a total of four phases and the two-phase expansion under construction is to be completed in July 2024.

“Further expansions will occur when flows increase,” she added. “The design for this expansion was fast-tracked for an industrial customer. Design started in February 2021, the project went out to bid in November 2021 and construction began in April 2022. It was a remarkable aggressive schedule made possible by the planned phases and the module design created by Arcadis.”