Rapid Volume Expansion – an Investigation Into Digester Overflows and Safety

Introduction: For decades, plant operators from certain wastewater treatment plants have been plagued with anaerobic digester foaming. Foaming within an anaerobic digester can present significant problems for a utility including: fouling of gas lines, over pressurization of the digester through clogged pressure relief valves (PRVs), off-site odors, foam/sludge spillage from overflows, and loss of digester cover buoyancy on floating cover digesters. Digester foaming of floating cover digesters often lifts the floating cover and results in spillage of sludge, and an example of such an occurrence is shown in Figure 1. Under certain conditions, particularly with fixed covers, foaming events have been reported to cause structural damage to digesters.Figure 1 - Digester Foaming Event on a Floating Cover DigesterAnalysis of these events has led the authors to hypothesize that foaming in a commonly understood sense may not be the cause of such events, but rather a phenomenon that can be generally termed as a “rapid volume expansion.” The digestion mixing systems have often been associated with foaming problems, and in particular, gas mixing systems have been shown to exacerbate the problem. With gas mixing, the increased rising gas bubbles can attach to foaming agents, accelerating the physical process already occurring with rising gas bubbles generated within the digester. For this reason, mechanical mixing systems have been postulated by some to be the best technology to minimize foaming problems.In addition to reducing gas recycle, in some cases, mechanical mixers can be configured such that foam and scum at the digester liquid surface is incorporated back into the digester fluid. There are several other solutions to foaming within the anaerobic digestion process itself, including continuous surface withdrawal, foam suppression sprays, emergency overflow features, and the submerged fixed cover design and operation. These design features offer significantly greater control of digester operation while minimizing foaming potential. Despite these advantages, the industry has recently found some serious cases of digester “foaming” issues that may instead be related to rapid volume expansion and not traditional foaming. In fact mechanical mixers under certain operating conditions may actually exacerbate or cause rapid volume expansion.Cases of rapid expansion problems have been identified in many digesters. The most alarming events and observations are when digester contents appear to be rapidly expanding in volume (within 15 minutes), causing digester overflows and in some cases structural damage to the digester. The cause of these rapid expansion events is thought to be related to a change in gas holdup, the volume of biogas retained in the digester liquids. The authors have postulated three primary causes for rapid expansion phenomenon, all related to rapid changes in gas holdup: sudden changes to digester mixer operation; batch (slug) digester feeding or related conditions during digester startup; and sudden gas (CO2) release during sudden loss of head space gas pressure. For example, the mixing direction of mechanical draft tube mixers can have a direct impact on gas holdup. Mechanical draft tube mixers are capable of mixing in the upward and downward directions. During steady state conditions this is not a problem. However, case studies have identified that during sudden changes in mixing direction, the changes in the equilibrium of bubble behavior in the digester liquid is responsible for changes in gas holdup that may result in rapid expansion events.Approach and Conclusions This paper documents several case studies where rapid expansion of digester contents has occurred. These case studies indicate that bubble fluid dynamics and gas holdup related to digester operation are the likely cause of these events. To address this hypothesis, the physical components of the complex interactions of bubble formation and rise through the digester volume were defined and preliminary mathematical models of discrete bubble behaviors were developed to estimate components of gas holdup and the potential for rapid expansion. The mathematical models relate the complex flow behavior of liquid, gas, and particles associated with digested sludge and fluid dynamics of bubbles: bubble shape, bubble rise velocity, viscosity, surface tension, temperature, and pressure. The behavior of digester sludge as a non-Newtonian fluid is discussed in further detail as the properties of this fluid have a significant impact on bubble behavior. Using these models, estimates were made of potential volume of rapid expansion under defined conditions. Recommendations are made with respect to the need for further research and further model development. Finally, this paper offers some mitigating actions that we as designers and operators can undertake to minimize or prevent the risk of this phenomenon from occurring.