UOSA treats wastewater from the Cities of Manassas and Manassas Park, Fairfax County, and Prince William County at their Millard H. Robbins Jr. Regional Water Reclamation Plant (RWRP). Treatment consists of screening, grit removal, primary clarification, primary effluent pumping, aerobic selectors, and biological treatment with secondary clarifiers. Tertiary treatment consists of rapid mix and flocculation, chemical clarification, two-stage recarbonation, flow equalization and pumping, filtration, granular activated carbon, disinfection, and dechlorination.
UOSA produced a “Nutrient Compliance Plan and Preliminary Engineering Report” (“PER”; 2/09) to develop and analyze alternatives for achieving compliance with regulatory nutrient waste load allocations (WLAs) that were issued in 2005. UOSA’s WLAs are 1,315,682 million pounds per year for total nitrogen (TN) and 16,446 pounds per year for total phosphorus (TP). UOSA’s existing TN-removal capability is insufficient to meet the new WLA, particularly as flows increase toward the 54 MGD plant design condition in future years. Therefore, the WLAs will require a significant increase in the TN-removal capability of the existing RWRP. The discharge permit already includes a TP limit of 0.1 mg/L; the new WLAs will not be of significant impact with regard to TP.
UOSA proposes a 3-Phase approach over the next decade to enhance TN removal capability in order to make cost-effective, incremental capital upgrades while meeting the WLA requirements. To ensure that plant performance consistently meets the permit regarding TN and TP, the following performance criteria were utilized in selecting the nutrient removal technology (NRT) upgrade alternative:
However, the receiving water quality (Occoquan Reservoir) is protected to a large extent by the RWRP’s discharge of nitrate-nitrogen, as shown though extensive scientific studies. When the nitrate undergoes natural denitrification in the reservoir, the liberated oxygen suppresses sediment phosphorus and ammonia-nitrogen releases and the resulting algae blooms that can severely impact this drinking water source. For this reason a consistent annual TN loading, predominately in the nitrate-nitrogen form and equal to the WLA poundage, is the primary objective of UOSA’s nitrogen control program. Although the TN concentration-based performance goal above was used in the NRT upgrade design, the performance limit in the pending Agreement is not keyed to achieving a specific annual average effluent TN concentration. Instead, the WLA annual TN poundage is the performance criteria, with the effluent concentration being back-calculated based on the total volume of flow treated over the calendar year.
The grant eligible project elements for Phase 1 [as presented in the PER and later modified per a Value Engineering (VE) Study] include the following:
- Side-stream equalization: Based on the VE Study, Phase 1 will include a Centrate Equalization Pump Station to collect and equalize centrate flows from the organic solids dewatering facility to minimize their impact on the biological treatment process.
- Convert aeration basins to a Modified Ludzak-Ettinger (MLE) configuration: Currently, these basins are designed for nitrification only, so the modifications will greatly increase their TN-removal capability. Because no new additional volume is being provided in Phase 1, these basins will still need to be able to operate in the nitrification-only mode.
In addition to the grant eligible components of the project, UOSA will also undertake rehabilitation and replacement improvements for several secondary treatment system units. The scopes of work for these improvements, which are ineligible for grant funding and not included in the Project Budget, were identified in the report, “Renewal and Replacement Study of Existing UOSA Assets” (CH2M Hill, 2/09). Future NRT phases identified in the PER are not included in the eligible scope of work for this grant agreement.