Brief Project Description
New facilities will be constructed to expand the Town’s existing 3.0 MGD plant to 6.0 MGD. The new facilities will also incorporate Nutrient Removal Technology (NRT) to enable the Town’s plant to meet the total nitrogen and total phosphorus effluent limitations under several nutrient discharge control regulations adopted by the State Water Control Board.
At the time that nutrient waste load allocations (WLAs) were being determined for this facility under amendments to the Water Quality Management Planning (WQMP) Regulation (9 VAC 25-720), the Town proposed to expand its existing 3.0 MGD plant to the highest design flow tier in the discharge permit - which was 4.5 MGD - and to have a Certificate to Operate before December 2010 for that expanded capacity. The amended WQMP Regulation reflected this approach with a conditional footnote for the WLAs. The Town also requested a WLA for a design capacity of 6 MGD; but, the Water Control Board responded that a WLA for 6 MGD at the Culpeper Plant would not be provided at that time. Subsequently, the Town has designed a facility for 6.0 MGD; however, the WQIF grant eligibility for all NRT components is limited to 90%. This is due to a design flow in excess of what the funding agency (DEQ) deemed to be “reasonable and necessary” for the project’s 20 year design life and is based on best professional judgment, a comparison of the estimated capital costs for expanded design flows of 4.5 and 6.0 MGD, and a significant amount of negotiation between the Grantee and Department staff.
The Town has selected two options to meet their plant expansion and upgrade needs. Option 1 and Option 2 are both included in Exhibit B, Project Budget. In brief, the expansion and upgrade shown in Option 1 includes two secondary clarifiers (which is the Town’s preference) and Option 2 includes one secondary clarifier. The Town would like the additional secondary clarifier built in order to accommodate a future facility expansion to 9.0 MGD. The Town’s bid process will require that the additional clarifier be bid as a separate item, in order to provide the Town with a lower cost alternative to the overall bid. After the bids are received the Town will evaluate the alternatives and select the preferred Option.
Components of the facility upgrade/expansion which are eligible for WQIF cost share include the following:
- Construct a new 6.0 MG flow equalization basin
- Construct a concrete splitter box
- Construct two new bioreactor tanks, including recycle pumps
- Construct two new secondary clarifiers
- Modify one return activated sludge (RAS) pump station and construct a new RAS pump station
- Construct a rapid mix facility
- Construct a denitrification filter treatment process
- Convert old chlorine contact tanks to backwash storage tanks
- Construct a gravity belt sludge thickener building and a sludge storage facility
- Construct an Alum storage and feed facility
- Construct a carbon storage and feed facility
- Digester modifications (previously completed project)
- Instrumentation and control and electrical upgrades
- A portion of the aeration system driven by the need to nitrify more efficiently and year round.
Items ineligible for WQIF cost share include:
- Intermediate pump station
- Ultraviolet (UV) disinfection
Influent Flow Equalization:
The current three lagoons have a capacity of 3.0 MGD; however, they do not provide adequate capacity for the new design flow of 6.0 MGD. Existing lagoon number three will be replaced with a new primary effluent equalization basin with a capacity of 6.0 MGD. The other lagoons will be reused as backwash reclaiming storage areas. A new pump station will be added to divert primary effluent to the new basin. The primary effluent will then drain by gravity to the nutrient removal basins.
Nutrient Removal Technology:
Two biological nutrient removal tanks are proposed, which are five stage tanks having a total tank volume of 3.97 MG. A splitter box will also be added. The first stage consists of an anaerobic zone, followed by anoxic 1, aerobic, anoxic 2 and a reaeration zone. Alum will be added before the final clarifier for phosphorous removal. Supplemental carbon (methanol) and alum will be added before the effluent enters the denitrification filters to reduce total nitrogen and total phosphorous levels. Blowers and fine bubble diffusers will be installed in the nitrification and reaeration zones. The existing alum storage tanks and chemical feed pumps will be replaced. New chemical feed points will be added and supplemental carbon feed facilities will be constructed.
Two 105 – foot diameter, 16-foot deep, secondary clarifiers are proposed to enable the facility to eventually increase the design flow to 9.0 MGD. The second of two clarifiers is being bid as an alternate deductive item; grant eligibility is limited to only one clarifier. Piping will be provided for feeding alum downstream of the clarifiers. A splitter box will be constructed to include coarse bubble diffusers.
Return Activated Sludge (RAS) Pump Station:
A new RAS pump station is needed to serve the new clarifiers. A slab-on-grade building will be constructed to house the RAS pumps. Three pumps will be installed if Option 1 is constructed and two pumps will be installed if Option 2 is implemented. The new pumps will be self-priming and provide a firm capacity of 18 MGD.
Flash Mix, Flocculation and Sedimentation Basins:
The facility currently includes flash mix, flocculation and sedimentation treatment downstream of secondary treatment and upstream of tertiary filtration. Alum will be added before the final clarifier for phosphorous removal. Methanol or alternate carbon source and alum will be added before the effluent enters the denitrification filters, for total nitrogen and total phosphorous polishing. The existing alum storage tanks and chemical feed pumps will be replaced. New chemical feed points will be added and supplemental carbon feed facilities will be constructed.
The existing cloth media filter will be removed and the building that houses it demolished. This area is needed for expansion of the existing deep bed filters. The existing cloth filter is in acceptable condition and may be reused in the future.
New filters will be constructed next to the existing deep bed filters. Six new filters will be constructed to provide a total filter area of 2,052 square feet. Each filter will be 9’6” by 36” with 6’ of sand over 1.5’ of supporting gravel.
The existing chlorine contact tanks will be converted to backwash storage tanks and covered to control algae growth.
The existing gravity thickener will be converted to a primary sludge thickener. Two new 1.0 meter gravity belt thickeners will be added and housed in a new building.
At the design maximum monthly flow of 6.0 MGD, the Town’s engineer estimated total solids production of 13,479 lbs/day. The sludge handling and digestion processes were each sized based on this predicted sludge production rate. Modifications to the existing digester will be made to accommodate the expected sludge production rates.
The capacity of the existing belt filter press (BFP) was evaluated by the design engineer and it was decided the capacity of the existing BFP should be sufficient for sludge dewatering at the expanded design flow.