The fluctuations were due to the changes in the feed volume during the acclimatisation period. The digester instability (decrease in COD removal efficiency and biogas production) that followed was caused by an OLR increase from 0.55 to 1.11 kg CODm−3 on day 83, which was maintained for 6 days. Feedingwas therefore stopped for about 23 days and resumed with an OLR of 0.46 kg CODm−3 day−1 on day 112 for 11 days followed by 0.63 kg CODm−3 day−1 on day 123 for 9 days corresponding to a HRT of 90 days due to the decreased volume of feed