Several plug-flow anaerobic digesters have been constructed on dairy farms in New York State in recent years primarily for odor control and also for combined heat and power generation. One long-term concern with these systems is the accumulation of non-digestible solids in the digestion cell. Non-digestible solids, made up of fixed solids and/or non-biologically degradable volatile solids can enter a manure treatment system; typical sources include manure, animal feed, bedding, and stones and sand tracked in by equipment tires. Typically an anaerobic digester (AD) is designed for a target daily loading rate and hydraulic retention time (HRT). If a significant portion fills with non-digestible materials over time and lowers the HRT, biogas production may be reduced and the digester may not perform as desired. A mesophilic plug-flow anaerobic digester in New York State was temporarily decommissioned in March of 2006 to perform emergency repairs to the internal heating system. This untimely need for repair provided an opportunity to extensively analyze the digester contents, both immediately after removing the flexible membrane cover in an undisturbed state and also during the cleanout process.