Plug-flow or near plug flow wastewater systems have proven effective for treating both BOD/COD and nutrients including N & P. In a true plug flow system, the influent moves through the system in one direction, much like water in a pipe. Systems with a rectangular basin with length over 4x the width often resemble an "imperfect" plug flow system - where flow approximates a true plug flow system. While effective for treating wastewater, a plug flow system is subject to problems related to:
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.
Erik Rumbaugh has been involved in biological waste treatment for over 20 years. He has worked with industrial and municipal wastewater facilities to ensure optimal performance of their treatment systems. He is a founder of Aster Bio (www.asterbio.com) specializing in biological waste treatment.
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