Start-Up Challenges in A Slow Economy: When Less than Design Minimum Rules

The start-up of the Clark County Water Reclamation District (District) Moapa Valley Water Resource Center (MVWRC) presented several operational challenges. The influent flow and organic load was significantly less than the design minimum as the project schedule occurred during an unexpected economic turndown – that continues on today. In order to accelerate startup, the basins were seeded with thickened waste activated sludge from the District's Main Central Plant in Las Vegas. Instead of growing and stabilizing, the biomass essentially disappeared in a matter of weeks after seeding, resulting in a sharp decrease of the mixed liquor suspended solids concentration. Start-up troubleshooting indicated that a combination of solids settling, midge fly larvae invasion, uncontrolled solids wasting, and biofilm formation on tank walls caused the initial biomass instability. Inconsistent plant performance was also observed during start-up. Process modeling and field observations indicated that reduced organic load, excessive aeration, and biomass instability were the major causes of the inconsistent process performance. Process modifications were identified and implemented to address the start-up challenges experienced by District staff. Supplemental carbon was implemented on an emergency basis. The process configuration and aeration control were modified to minimize excessive aeration. The return activated sludge and mixed liquor internal recycle flows were adjusted to minimize solids settling in the structures. As a result of the start-up optimization effort, the MVWRC consistently exceeds both the design treatment goals and the mandated permit requirements.