Hampton Roads Sanitation District (HRSD) owns and operates the Virginia Initiative Plant (VIP) in Norfolk, VA which provides secondary treatment with biological nutrient removal (BNR). HRSD upgraded the plant in 2015 with the objective of providing additional nitrogen removal capabilities and improved biological phosphorus removal. A second goal of the upgrade was to increase the capacity of the plant from 80 MGD to 100 MGD, reducing the possibility of overflows during major storms.
Aeration satisfies oxygen demand while BioMix compressed gas mixing maintains mixed liquor suspension.
0,11
hp/1,000 FT3 mixing efficiency,
$75 000
annual O&M savings
Challenge
One driver for the upgrade was the reduction of nutrient discharges to the Chesapeake Bay. Virginia’s Phase 1 Watershed Implementation Plan for the bay established reduced nitrogen waste load allocation levels. Upgrading VIP was a major element in HRSD’s strategy to meet the new standards and improve local water quality.
To meet the government mandates and further reduce the nutrients discharged to the Chesapeake Bay, HRSD upgraded VIP to achieve annual average effluent concentrations of 5 mg/l total nitrogen and 1 mg/l total phosphorus. This was accomplished by converting the previous 3-stage nutrient removal process to a 5-stage enhanced biological nutrient removal (BNR) process by adding a second anoxic zone and a re-aeration zone.
The configuration supports two operating modes, a normal flow mode providing 5-stage BNR and a wet weather mode comprised of a 3-stage process. This added flexibility was enabled through the construction of new versatile bioreactors (VBRs) and secondary clarifiers. These units work in series with the VIP reactors to enhance nutrient removal during normal flow conditions and then transition to separate, parallel operation during high flow events.
The versatile bioreactors (VBRs) perform anoxic operation.
Solution
Providing energy efficiency and process flexibility by enabling the plant to decouple aeration from mixing, EnviroMix’s BioMix compressed gas mixing was selected for the VBRs, which are aerobic/anoxic swing zones and third stage aerobic zones.
BioMix provides supplemental mixing in the aerobic stage, enabling the diffused aeration system to match process oxygen demand without over-aeration. This minimizes excess DO in the mixed liquor return, preventing poisoning of the post anoxic zones and reducing costly methanol dosage. In the VBRs, during 5-stage BNR operation, BioMix operates independently from the aeration system to create anoxic conditions and maintain suspension of mixed liquor.
Results
By selecting the BioMix system, the plant realized:
- Annual O&M savings of $75,000 vs. submersible mixers.
- 20-year present worth savings of $1.35 million.
- Annual methanol savings of $125,000.