Tuolumne Wastewater Treatment System

Sonora, CA


PACE performed a facility assessment and upgrade design for the 2.6 MGD Average Daily Flows (ADF) Sonora Regional Wastewater Treatment Plant (SRWWTP) to address the facility’s declining effluent performance. The existing SRWWTP features a conventional trickling filter secondary process that utilizes two trickling filters, followed by two secondary clarifiers, three aerated polishing ponds, and a chlorine disinfection system. Based on recent flows and loads, the influent flow limits identified in the WDRs are not exceeded, but the facility is at its design Biochemical Oxygen Demand (BOD) influent load limit, and is exceeding its design suspended solids loading criteria.


To improve the facility’s functionality and performance, PACE performed design upgrades to increase the capacity to 5.0 MGD of Max Day Flows (MDF) and 10.0 MGD Peak Hour Flows (PHF) and convert the secondary treatment process to an Extended Aeration Activated Sludge (EAAS) process utilizing the Parkson’s Biolac® mixing and aeration process. All wastewater flows are treated by a new headworks, primary screening and grit removal, new dual-train extended aeration activated sludge (EAAS) basins, new secondary clarifiers, new chlorine disinfection system, new effluent disk cloth filters and a new sludge dewatering facility. These upgrades also replace the existing polishing ponds and include a new administration and electrical building, a new headworks and sludge dewatering building, and a renovated digestion building where the existing anaerobic digesters were repurposed into aerobic digesters for solids processing. The facility has also been maintaining operation during construction by waiting to demolish the existing facility until the new facility is in full operation.


Key Features:

  • 90,000 yd3 earthwork in balanced cut-fill construction with enhanced stormwater conveyance and a finished grade above FEMA floodplain
  • High-quality process equipment including:
    • Parkson Biolac extended aeration configuration with variable freeboard for system equalization
    • Kaeser (Pillar) Mag-Bearing high-efficiency aeration blowers to provide constant dissolved oxygen
    • Ovivo submerged launderer clarifiers with tertiary feed pumps at constant flow downstream
    • Veolia Hydrotech 10-micron polyester filters for consistent low-turbidity effluent
    • FKC screw press for dry secondary thickened WAS dewatering
  • USDA grant-funded project includes 9,000 SF administration building, civil improvements, and recycled water fill station

Tuolumne Utilities District

Wastewater Treatment