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Step-Feeding influent - the why and when to use multiple inlets for adding influent wastewater

11/19/2017

 
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A conventional plug flow activated sludge system
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:

  • 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)

For systems with any of the above problems, a simple work-around can help restore good biological activity in the sytem. By modifying the inlet to introduce influent at multiple points along the length of the basin, you can avoid the dissolved oxygen drop caused by high soluble BOD and also reduce substrate toxicity by lowering concentrations via dilution. Step feed helps maintain a consistent F/M ratio throughout the basin which also helps with low F/M filaments.

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.
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    Author

    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.

    View my profile on LinkedIn

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