Pre-treatment can enhance anaerobic digestion of seaweed; however, seasonal variation in the biochemical composition of seaweed has a significant impact on the pre-treatment effect. In this study, various pre-treatments were employed for the brown seaweed Laminaria digitata harvested in March (with high ash content and low carbon to nitrogen (C:N) ratio) and September (with low ash content and high C:N ratio). Washing of L. digitata harvested in March with hot water (defined as 40 C) removed 54% of the ash and improved the volatile solids (VS) content by 31% leading to an improved biomethane yield of 282 L CH4 kg VS-1. This pre-treatment affected a 16% increase in biodegradability, reduced salt accumulation in the digestate by 54%, and increased specific methane yield per wet weight by 25%. This level of effect was not noted for seaweed harvested in September, when the biodegradability is higher. (C) 2017 Elsevier Ltd. All rights reserved.