Brief Project Description
The Henrico County Water Reclamation Facility has a design flow of 75 million gallons per day (MGD) and is capable of Biological Nutrient Removal (BNR), which was cost-shared under a prior WQIF grant agreement (#440-S-00-07). The existing liquid treatment process consists of screening, grit removal, primary clarification, activated sludge BNR, secondary clarification, filtration, and chlorination/dechlorination prior to discharge into the James River. Solids handling is accomplished by gravity belt thickening, anaerobic digestion, dewatering via centrifuges and land application.
Under a proposal for upgrade to Enhanced Nutrient Removal (ENR), the County will complete construction of the following components, commonly referred to as Phase 7. The County obtained a Certificate to Construct for Phase 7 on September 24, 2007.
BNR Tanks 9 & 10
The existing BNR Tanks 9 & 10 have a very large aerobic volume sufficient for complete nitrification. These tanks lack the anoxic environment to provide for effective denitrification; therefore, these reactors will be retrofitted to convert the existing 3-stage process into a 5-stage process to provide more efficient denitrification.
A secondary anoxic zone will be constructed at the downstream end of the existing aerobic zone, Cell 7. This will improve the basin denitrification thereby lowering the nitrate concentration in the effluent. The secondary anoxic zone will be a swing zone allowing for both aerated and unaerated/mixed operation.
A re-aeration zone will also be constructed at the downstream end of Cell 7. This zone will re-aerate the mixed liquor after it has passed through the secondary anoxic zone and before it is distributed to the secondary clarifier distribution channel.
The Nitrate Recycle (NRCY) pump station will be modified to allow for a suction intake from the downstream end of the remaining aerobic zone and the new re-aeration zone. The NRCY piping will have to be relocated to accommodate the new secondary anoxic and re-aeration zones.
BNR Tanks 11 & 12
Two new BNR reactors will be installed at the plant. These tanks will be identical to those installed during the previous Phase 2 plant expansion and will provide a 5-stage process, enabling the plant to achieve very efficient nutrient removal.
These new tanks will be capable of running as either a 5-Stage or a 3-Stage process. The system will be highly flexible, providing a wide array of treatment capabilities, depending on loading and climatic conditions. This flexibility allows the process to adapt for more efficient nutrient removal and to reduce chemical and energy requirements.
Primary clarifier effluent can be distributed entirely or partially into Cells 1, 2, 3, or 4. Return Activated Sludge (RAS) is discharged into Cell 1, an anaerobic zone, to provide conditions promoting enhanced biological phosphorus removal. Cells 2 and 3 can be operated as either an anaerobic zone, for biological phosphorus removal, or an anoxic zone for nitrogen removal depending on the treatment objective. Cells 4 and 5 are swing zones, capable of operating as anoxic zones, to increase denitrification volume, or aerobic zones, to increase the aerated volume for nitrification and CBOD removal. Cells 6 through 10 are equipped with diffused aeration systems for providing a dedicated aerobic environment for nitrification and BOD removal. Cells 11 and 12 can run as aerobic or anoxic zones, which provide a larger aerobic volume, or a secondary anoxic zone, for a higher degree of nitrogen removal. Cell 13 is the final cell and is used to oxidize any remaining ammonia and strip nitrogen gas, depending on the operating mode. The aeration in Cell 13 also ensures adequate dissolved oxygen required by the secondary clarifiers. Two NRCY pumps recycle nitrified mixed liquor from Cell 10 or Cell 13 to the anoxic Cells (2, 3 or 4) for denitrification. These tanks will be capable of running identically to the existing BNR Tanks 5–8.
Even with the proposed modifications to the BNR tanks and filters, Henrico County WRF may not be able to meet the annual average total nitrogen requirement of 5.0 mg/L without chemical addition. To ensure compliance, the plant will need to supplement biological nutrient removal with chemicals to either assist the organisms biologically or remove nutrients chemically. Nitrogen removal can be optimized through the addition of a carbon source in the secondary anoxic zones of the BNR basins. There are several options for supplemental carbon addition including methanol, acetate, and glucose. Henrico County WRF will utilize the safest, most effective and most cost efficient option.
Sodium hydroxide (caustic) is added to the treatment process as needed to increase alkalinity of the primary effluent feed to the BNR basins and increase the pH of the final effluent. Caustic is added to the primary effluent prior to discharge into the primary effluent distribution channel to ensure adequate alkalinity is available for nitrification in the BNR basins. Caustic is added to the filter effluent channel to ensure the final effluent pH is not less than 6.0.
Prior Eligible Work
In addition to the NRT upgrades described above comprising Phase 7, WQIF cost share will be provided for two NRT components included in previous construction under Phase 4. This includes four NRCY pumps used in BNR Tanks 9 & 10 and the fine bubble diffusers installed in BNR Tanks 1-4. These costs were not included in the prior WQIF Grant #440-S-00-07.