Meeting Strict Summer Permit Requirements on Day One - Start-up of the Western Wake Regional WRF
- Chris White, PE, Barry Bickerstaff, PE, Katya Bilyk, PE, David Wankmuller – Hazen and Sawyer
- Damon Forney – Town of Cary
The population of western Wake County, NC has grown steadily, drawing increasing amounts from the Cape Fear River Basin for potable water supply. However, most of the treated wastewater from the growing population is discharged to the Neuse River Basin. To significantly reduce the interbasin transfer, a new water reclamation facility and associated conveyance systems were constructed in southwestern Wake County with a discharge point in the Cape Fear River Basin.
Construction of the Western Wake Regional Water Reclamation Facility (WWRWRF) began in 2011. The treatment plant, influent pump stations, effluent pump station, and approximately 20 miles of pipeline were placed in service during the summer of 2014. Stricter summer permit limits were applicable and initial plant discharge parameters below the monthly average permit limits was desired.
To meet NPDES permit requirements on initial discharge, a start-up plan was developed using BioWin process simulation software. Initial influent flow was projected to be approximately 0.75 mgd. Excess basin capacity of the 18 mgd facility was utilized to store the biological treatment process effluent until satisfactory nutrient removal was obtained. Required solids seeding prior to influent flow was estimated with the process modeling software to achieve a mixed liquor suspended solids concentration of ~600 mg/L. Detailed process sampling and nitrification batch tests tracked the biological activity of the plant throughout the seeding period. Batch testing indicated an initial nitrification rate of 9.4 mg NH3-N / g VSS / hr, but subsequent tests revealed approximately a 75% rate decrease at the end of the first week of seeding. Additional seeding was required with batch testing confirming acceptable nitrification rates as wastewater arrived at the plant. Chemical dosing was available to supply supplemental carbon, metal salt for phosphorus removal, polymer to address settling issues, and caustic solution to adjust alkalinity. As wastewater flow entered the plant, process parameters were adjusted to address alkalinity and carbon deficiencies. Intermittent influent flow also caused dissolved oxygen control challenges that were addressed with operational strategies as well as control loop tuning.
Process modeling defined a starting point to efficiently start-up a greenfield facility. Options in the start-up plan allowed operations staff to quickly resolve issues and make process adjustments to discharge effluent on the first day in compliance with the monthly average permit limits.
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