Extracellular polymeric substances (EPS) extracted from aerobic granular sludge (AGS) have recently gained commercial interest as a valuable resource. The alkaline EPS extraction yields roughly 25% of soluble EPS, and about 75% of solid alkaline residual sludge. This study aims to give the first outlook to implement treatment of this alkaline residual sludge with anaerobic digestion (AD) at haloalkaline conditions (pH 9.5, Na+ [0.6 M]). Batch AD enrichment experiments were performed with an inoculum of haloalkaline anaerobic microorganisms, originating from soda lake sediments. The residual sludge was successfully digested and 40-50% of the theoretical production of biogas was obtained. A higher biodegradability was obtained compared to the digestion of AGS at haloalkaline conditions. Due to the alkalinity of the medium, the majority of the CO2 remained in solution, which yielded a high methane content in the gas phase, on average 98%. Environmental factors such as the cation composition of the medium, the salt concentration and substrate preparation showed a negative influence on the biogas yield. In addition, the first implications for continuous AD operation are discussed.