Evaluation and Implementation of BNR Upgrade at North River Wastewater Treatment Plant
Harrisonburg-Rockingham Regional Sewer Authority, Mt. Crawford, VA

The North River Wastewater Treatment Plant (NRWWTP) is a 16-MGC secondary treatment facility currently operating to achieve seasonal TKN limits. The plant has no effluent phosphorus limits. In 1998, the Harrisonburg-Rockingham Regional Sewer Authority (HRRSA) obtained a $2.5 million Water Quality Improvement Grant through the Potomac Tributary Nutrient Reduction Strategy of 1997. To quality for the grant, HRRSA agreed to voluntarily achieve annual effluent limits of 8 mg/1 total nitrogen and 1.5 mg/1 phosphorus.

HRSSA selected HSMM and another consultant to evaluate alternative biological nutrient removal (BNR) opportunities at the plant and implement the recommended alternative. HSMM prepared construction documents and provided construction-phase services. The team focused on BNR processes that required little new construction and would result in minimal additional sludge production.

The processes evaluated included Modified Johannesburg, 3-Stage Bardenpho Process, and Modified University of Cape Town Process. Using primary sludge fermentation and acetic acid, the team also evaluated four BNR processes that use volatile fatty acid addition. Each process was evaluated to determine the best use of available aeration basin volume and existing structures. The selected Modified Johannesburg process uses a pre-anoxic zone (to address raw sewage nitrates), an anaerobic one with volatile fatty acid fad, an anoxic zone, a swing anoxic/aerobic zone, and an anaerobic zone. During the high flow periods, a portion of the wastewater flow is diverted around the pre-anoxic and anaerobic zones.

Process modification included sludge separation, aeration basin modifications, and unit hydraulic capacity modification. For the sludge separation, primary sludge was separated from waste activated sludges and final clarifier scum for pumping to a new primary sludge fermenter. The fermenter was constructed in a previously abandoned gravity sludge thickener tank. In a second thickener tank, the team built a DAF thickener to receive waste activated sludge and final clarifier scum.

To get the most efficient use of the aeration basins, the project team baffled the eight existing basin to create the various zones used in the Modified Johannesburg process. This process uses new submerged mixers to mix the unaerated zones and provides internal nitrite recycle pumps. To accommodate, the increased mixed liquor and return sludge rates required for BNR, the team modified the final clarifiers and enlarged the existing return activated sludge pumps. The team also expanded the PLC-based instrumentation system to accommodate monitoring of the new BNR process equipment and systems.

The BNR plant has been fully operational since early 2000 and has met the nitrogen and phosphorus limits since that time.