BEYOND CONVENTIONAL DIGESTION? An Industry Dilemma

There is a movement underway in the US to upgrade the treatment performance of anaerobic digestion, particularly through process variations of conventional mesophilic anaerobic digestion. The level of interest is reflected in the number of pilot and demonstration studies underway, projects under design or construction, and new facilities in the early years of operation. Most of these projects are aimed at meeting the requirements for Class A pathogen reduction under the US EPA Part 503 regulations. This paper provides a review of the US and United Kingdom (UK) experience to-date with advanced digestion processes, and shares observations on research findings and their implications for facility design, operation and maintenance.Several of the recently conceived process variations such as two-phase digestion and temperature-staged digestion show considerable promise, but how a digestion facility would have to be designed and operated to ensure consistent production of Class A biosolids is still unclear. Little operating experience has been logged to-date with these newer forms of anaerobic digestion, and much is yet to be learned about their effects on biosolids dewatering, nutrient recycle streams, residual odors, and facility operation and maintenance. These uncertainties pose a dilemma for POTWs contemplating a digester improvement project, or in imminent need of a solution for producing Class A biosolids.Advanced digestion and management of a dewatered, Class A biosolids product may be a good fit for the needs of many POTWs. But are we expecting too much from anaerobic digestion alone? Or would coupling conventional mesophilic digestion with another process such as heat drying be a better solution? Only time (and experience) will tell.