Digester Cover Selection and Safety: Balancing Risk and Reward

Anaerobic Digesters are playing a key role in the increasing trend for Water Resource Recovery Facilities to become more energy independent. Addition of novel feedstocks for codigestion can lead to new stresses on old equipment. Resulting foaming and rapid volume expansion events can be a major nuisance in gas piping but their effects can damage or even destroy digester covers. As part of these projects, it is critical that existing digester covers are upgraded and new covers properly selected and designed with appropriate features to ensure proper and safe operation.Unfortunately, there are many stories of digester cover problems and failures throughout the industry and some have even been fatal. While it is not possible to discuss every situation individually, some consistent vulnerabilities emerge for different cover types and materials of construction. Designers and operators should have a sound understanding of potential failure modes to ensure safety measures and procedures be incorporated to reduce the risk of failure.In some cases, the safety equipment may provide a false sense of security. Explosions are certainly a risk in and around digesters and mitigating this risk gets a lot of attention during the design of nearby electrical and safety equipment. However, overpressurization events are more common so both risks need attention and prevention.The Little Patuxent Water Reclamation Plant in Howard County, Maryland has recently experienced a digester cover selection process firsthand for three new digesters that will be added (in 2017) as part of a biosolids processing upgrade from lime stabilization to belt drying. In addition to a review of the cover selection process, this paper also discusses instrumentation and equipment employed to help reduce the risks that have contributed to failures at other facilities.Through awareness of prior modes of failure and vulnerabilities (risk) associated with different cover types, designers will learn how certain additional safety features can play a key role in decreasing the instance of digester cover failures, despite the increasing number of operating digesters.