Brief Project Description
The existing Middle River Regional Sewage Treatment Plant has a combined design flow of 6.8 MGD with 6.0 MGD of the biological treatment capacity in the oxidation ditch, and 0.8 MGD of the capacity in the adjacent rotating biological contactor (RBC; formerly the Verona WWTP). The oxidation ditch process includes screening, dual concentric triple channel aeration basins, three circular secondary clarifiers, and ultra violet (UV) disinfection prior to discharge to the Middle River. The RBC process consists of primary clarification, RBC, secondary clarification, and disinfection by the oxidation ditch UV equipment. The plants share a solids handling process which consists of aerobic digestion, holding tanks, and two belt filter presses. Solids can be either land applied in liquid form, or dewatered for landfill disposal. The facility discharges to the Middle River, which is a tributary of the Shenandoah and Potomac Rivers.
Under the proposal submitted for nutrient removal cost share, the existing oxidation ditch process will be upgraded to achieve enhanced nutrient removal (ENR) at the permitted capacity of 6.8 MGD. The Verona RBC process will be decommissioned. The future discharge permit for this facility will limit the annual average effluent discharges of total nitrogen (TN) and total phosphorus (TP) to 82,839 pounds per year for TN and 6,213 pounds per year of TP, which is equivalent to annual average effluent concentrations of 4.0 mg/L for TN and 0.30 mg/L for TP at the 6.8 MGD design flow.
To achieve the future effluent requirements, the oxidation ditch used for biological treatment will be retrofitted to create a spiral plug-flow path to be operated in a 4-stage Bardenpho configuration (anoxic, aerobic, anoxic, aerobic zones). The existing brush rotors will be replaced with fine bubble diffused aeration, and new blowers will be installed to supply the aerobic zones. Submersible mixers will be installed to provide mixing in anoxic zones and during periods when process air requirements are low. Methanol will be added to the second anoxic zone as a supplemental carbon source for the denitrification process. Ferric chloride will be added prior to the secondary clarifiers to assist with phosphorus removal. The mixed liquor concentration in the biological basins will be increased to achieve the desired nitrogen removal within the existing tankage. A fourth secondary clarifier will be constructed to handle the higher solids loading from the biological process.
Other modifications to the liquids treatment processes of the existing facility will include the installation of grit removal equipment. The equipment will be arranged to allow room for an anaerobic zone to be constructed in the future. Head losses between the grit basin and the oxidation ditches will be minimized by the addition of a second parallel 24-inch pipeline.
In addition to permanent alum and methanol feed and storage facilities, chemical feed facilities for sodium hydroxide, polymer, sodium hypochlorite, and polyaluminum chloride will be constructed. These chemicals will be used on a non-routine basis to correct for upsets in pH, nitrification, or solids settling, and also for control of filamentous growth.
Two additional belt filter presses, rated for 300 lb/hr, will be installed for removal of biosolids. This will provide adequate dewatering capacity needed for the additional nutrient removal, and will also provide needed redundancy.
Cloth disk filtration will be installed for solids polishing of the effluent prior to disinfection and discharge.
In summary, the ENR upgrade will consist of the following main elements:
- Installation of grit removal facilities (not grant eligible)
- Oxidation ditch modifications and blower installation
- Construction of a fourth clarifier
- Installation of cloth disk polishing filters
- Installation of two additional solids presses and building
- Construction of a methanol building with containment area and feed system
- Construction of a metal salt storage and feed system