Steven Beightol

Anaerobically digested biosolids odor generation and pathogen indicator regrowth after dewatering.

The objective of this research was to investigate whether a preferential stimulation of microorganisms in anaerobically digested biosolids can occur after dewatering and if it can lead to pathogen indicator regrowth and odor generation upon storage. Laboratory incubation simulating biosolids storage indicates that both odorant generation, based on total volatile organic sulfur compound concentrations (TVOSCs) and pathogen indicator regrowth, based on fecal coliform densities follow similar formation and reduction patterns. The formation and reduction patterns of both odor compounds and fecal coliforms imply that groups of microorganism are induced if shearing disturbance is imposed during dewatering, but a secondary stabilization can be achieved soon after 1-2 weeks of storage. The occurrence of the induction is likely the microbial response to substrate release and environmental changes, such as oxygen, resulting from centrifuge shearing. The new conditions favor the growth of fecal coliforms and odor producing bacteria, and therefore, results in the observed fecal coliforms regrowth and odor accumulation during subsequent storage. However, when both substrate and oxygen deplete, a secondary stabilization can be achieved, and both odor and fecal coliforms density will drop.

Understanding the Role of Mixing and Viscosity in Rapid Volume Expansion Due to Gas Holdup in Anaerobic Digesters

The objective of this research was to better understand the role of mixing and viscosity in rapid volume expansion (RVE) due to gas holdup in anaerobic digesters. Most RVE research has focused on the absence of mixing, which is the worst-case scenario for anaerobic digestion facilities; however, comparatively little work has investigated the effects of different mixing intensities on RVE. To prevent the retention of biogas bubbles in suspension, commonly known as gas holdup, bubbles must rise through digester contents and evolve into the headspace of the digester at a rate equivalent to the nucleation and growth of biogas bubbles. As bubble rise velocity slows, the rate at which bubbles are nucleating and growing will exceed the rate at which they are evolving into the headspace. This will lead to a larger overall volume of biogas retained in suspension resulting in gas holdup and RVE. Due to the shear thinning and non-Newtonian behavior of digester contents, the extent of RVE should decrease as mixing increases because, assuming bubble rise velocity can be modeled using Stoke’s Law, an increase in mixing will lead to a decrease in viscosity and increase in bubble rise velocity. To assess this hypothesis, this study considered two different digester samples with RVE tests conducted at different mixing intensities. As expected, volume expansion decreased as mixing intensity increased with a good linear correlation between viscosity and volume expansion. This work can be applied to identify a ‘critical’ shear rate at which volume expansion begins to occur for designing systems that effectively prevent RVE and ensure proper, adequate mixing.