Are We Over-Mixing Our Digesters?

The purpose of this paper is to investigate the question: “Are we over-mixing our anaerobic digesters?” The authors believe that it is timely and appropriate to challenge the existing approaches to selection and design of mixing systems for anaerobic digesters. The application of anaerobic digesters has transitioned to new operating territory as thicker digester feedstocks are generated, codigestion projects introduce more complex feedstocks, and new configurations and more energy-intensive mixing systems are applied. The introduction of low-energy linear motion (LMM) mixing systems has provided a contra-point to the pumped hydraulic jet nozzle mix systems that have been popular in the last decade, but these applications are still in their infancy and their effectiveness is little understood.What has emerged from the new horizons for anaerobic digestion is a proliferation of rapid rise foam formation events. These are events that may not involve filamentous organisms, but from unanticipated circumstances. This paper contains the results of surveys made at four wastewater treatment plants with the assistance and encouragement of the management and superintendents at these facilities: Austin, TX (Hornsby Bend); Dallas, TX (Southside); Columbus, OH (Southerly); and Douglas L. Smith Middle Basin in Johnson County, KS.The results of the surveys revealed that there are multiple interactions that have resulted in rapid rise foam formation, including at times the presence of filamentous organisms from the biological treatment system, the continuity (or discontinuity) of digester feeding, and the addition of co-digestates that stimulate biogas formation, over-mixing. These interactions are revealed in the survey results presented below.