Effect of Dewatering on Fecal Coliforms and Methanogens in Anaerobically Digested Biosolids

Following centrifugal dewatering, fecal coliform levels may quickly rise by many orders of magnitude. This may be due to reactivation or by regrowth processes, or possible both. In this research, we show that regrowth may lead to these rapid increases when no reactivation is apparent. This regrowth can be reproduced by laboratory centrifugation, suggesting the final solids concentration is more important than the shear forces in determining the regrowth. We then examined the possible mechanisms leading to the regrowth, focusing on the changes in solute and solids changes and characteristics. Results indicated that the fecal coliform regrowth cannot be attributed to the removal of inhibitory substance or introduction of autoinducer and nutrient during centrifugation. Increases in total solids levels from centrifugation increase the fecal coliform levels but decrease the methanogenic activity; this cannot be attributed to changes of water activity, a parameter correlated with microbial growth in food science research. Addition of a specific inhibitor of methanogenic populations has a similar effect as increased solids concentrations, reducing methanogenic activity and accelerating FC regrowth. Thus it is the solids concentration that impairs methanogens, facilitating regrowth of fecal coliform bacteria.