Brief Project Description
Grantee: Stafford County
Grant: #440-S-10-06 (Aquia WWTF-Phase II)
Stafford County was awarded a Water Quality Improvement Fund grant in 1998 (Agreement #440-S-98-09), and received a total of $304,242 in cost-share for the installation of nutrient reduction technology at their Aquia Wastewater Treatment Facility (WWTF). The project consisted of the prior construction of a Schreiber biological process (on-line in 1992; 4.2 MGD design capacity) and modifications to a parallel treatment train (aerated lagoon system; rated at 2.3 MGD) to achieve biological nitrogen removal for the plant’s total permitted capacity of 6.5 MGD.
Stafford County was awarded a second WQIF grant in 2007 (Agreement #440-S-08-07) and received a total of $1,857,407 in cost share for the prior construction of a nutrient reduction technology (NRT) improvement project. The project was designed to manage increased hydraulic and organic loadings, and to maintain a total nitrogen (TN) design goal of 8.0 mg/L at the Aquia WWTF at the treatment capacity of 6.5 MGD.
Under a proposal for additional nutrient removal to meet new discharge regulations for the Chesapeake Bay and its tributaries, the WWTF will be further upgraded to achieve State of the Art (SOA) treatment with annual average nutrient concentrations of 3.0 mg/l total nitrogen and 0.18 mg/l total phosphorus. The design capacity will be expanded to 10.0 MGD in accordance with 20 year growth projections. Since the referenced concentrations are only compatible with the County’s waste load allocation (WLA, annual mass load allowance) up to an 8.0 MGD flow with the proposed technology, a nutrient offset plan is in place to keep the County in compliance with the WLA at the higher design flow. It should be noted that some of the integral components involved in the upgrade are sized at 12.0 MGD due to cost-effectiveness and speculation on future flows. Grant eligibility determinations take this factor into account, limiting eligibility to the reasonable and necessary expansion to 10.0 MGD for a 20-year useful service life.
Major components and unit processes of the NRT project include:
- Demolition (partially grant eligible): Demolition of the existing lagoons and smaller secondary clarifiers to facilitate construction of the new NRT facilities within the same footprint.
- Head works (not grant eligible): Upgrading and expansion of influent screening; addition of a third channel to split flow; raising the sidewalls of the existing flow channels; and replacement of existing 36” piping.
- New Biological Tank (partially grant eligible): Construction of a third counter-current oxidation ditch (Schreiber train), identical to the two existing units which facilitates a future plant expansion without increasing the footprint. This addition increases the design capacity to 12.0 MGD.
- Existing Biological Tank Upgrades (partially grant eligible): Installation of new blowers, fine bubble diffusers, and appurtenances in the two existing Schreiber tanks as well as new rotating bridge mechanisms which are at or near their design life.
- Secondary Anoxic Tanks (grant eligible): Installation of three denitrification tanks (one per train) between the Schreiber trains and the secondary clarifiers. Each tank includes a vertical mixer, 2 blowers, a set of diffusers, 2 inline nitrate probes, and analyzers. The three tanks are sized to accomplish TN reduction by endogenous respiration and are consistent with what is needed for a 12.0 MGD flow.
- Secondary Clarifier (partially grant eligible): Construction of a third clarifier at the proposed design capacity of 12.0 MGD.
- Chemical Feed (grant eligible): Construction of bulk chemical storage and feed facilities for supplemental carbon addition (acetic acid or similar). One new feed pump will also be installed at the existing alum feed facility.
- Waste Activated Sludge (WAS), Return Activated Sludge (RAS), Nitrified Recycle (NRCY) Pumping (partially grant eligible) – New RAS screw pumps for the new clarifier; two new WAS pumps, and three new internal recycle pumps.
- New Secondary Anoxic Influent Pump Station (grant eligible): Installation of five new pumps to lift flow to the new secondary anoxic reactors.
- Filter Pump Station (partially grant eligible): Addition of a new screw lift pump station, with three new screw pumps, to carry clarified effluent to the tertiary filtration building.
- Outfall Modifications (not grant eligible): New second parallel effluent flow metering channel and 36” discharge line to the existing outfall.
- Generators (partially grant eligible): Two new standby generators with automatic transfer switches.
- Instrumentation and Electrical (partially grant eligible): Upgrades to the existing systems to accommodate the new and enhanced components.