Peak Interest: Sensitivity to Design Flows in Evaluating Large-Plant MBR Retrofits

The Hampton Roads Sanitation District (HRSD) is upgrading their 18 mgd Army Base Treatment Plant (ABTP) from secondary treatment to a nutrient removal process to achieve effluent limits of 5.0 mg/L total nitrogen (TN) and 1.0 mg/L total phosphorus (TP). The upgrade also considers potential future effluent limits of 3.0 mg/L TN and 0.3 mg/L TP. Conventional activated sludge (CAS) biological nutrient removal process alternatives were compared on a life-cycle cost basis to a membrane bioreactor (MBR) biological nutrient removal process. Process design was based on meeting the projected TN and TP effluent limits at the 18 mgd design flow and at the minimum winter temperature of 12° C. Design criteria also included a 45 mgd peak flow rate. The cost evaluation determined that CAS nutrient removal upgrades would be more cost-effective than the MBR retrofit. The evaluation also showed that while costs of the CAS nutrient removal process upgrade were driven predominantly by biological process design criteria, costs of the MBR retrofit were found to be extremely sensitive to the design peak flow and membrane flux rates and the level of nutrient removal ultimately required.Peak flow rates through the membranes could be reduced rationally by considering the distribution of the hourly influent flows to the ABTP. An analysis of the magnitude and duration of peak hourly flows showed that using a portion of existing process tank volume for flow equalization would allow a reduction in the membrane peak design flow from 45 mgd to 30 mgd and a corresponding reduction in total membrane area required. Reducing the peak membrane design flow brought the capital costs of the MBR retrofit in line with the CAS process alternative at the more stringent 3.0 mg/L TN and 0.3 mg/L TP Level Two effluent targets. However, power and membrane replacement costs made the MBR retrofit less cost effective than the CAS nutrient removal alternative on a life-cycle cost basis. For the ABTP upgrade, it was determined that potential construction-related and process performance benefits of the MBR retrofit did not outweigh the life-cycle cost advantage of the CAS alternative. However, the analysis did reveal that the relative costs of MBR retrofits can be distorted if the significant cost impact and sensitivity to peak design flow are ignored, so much so that MBRs are rejected out of hand based on cost alone with little consideration of their potential benefits.