- Low D.O. near inlet due to high soluble BOD creating very high F/M conditions
- Filamentous bacterial growth due to low DO and low F/M (found in extended aeration plug flow systems)
- Toxicity issues caused by influent not being diluted as in a complete mix reactor. (This is why most industrial influents are not designed as plug flow)
To monitor step feed effectiveness, look at D.O. profiles along the basin length. Aim for consistent D.O. residuals along the first 50% of the basin. We want 80% of the soluble BOD removal to occur in the first part of the basin if we also need to remove ammonia nitrogen. For optimal ammonia oxidation, you need to have at least a 2 mg/L D.O. residual in the ammonia removal zone.
To use step-feed effectively, operators must run DO residuals and oxygen uptake rates at multiple points. Use microscopic exam and SV30 tests to closely monitor filamentous organisms. By using good monitoring, step-feed can help with systems that often experience bulking.