Pretreatment of a sugar containing wastewater from a date processing plant was tested for COD removal in an anaerobic pilot filter over a period of eight months. The filter was operated in the upflow mode and randomly packed with a plastic pipe carrier material. Initial start and restart after a 6-week break, steady state performance on seven different COD load levels ranging from 1.6 to 25 kg COD/m3·d, and filter behavior during reactor acidification were observed. In all examined experimental cases, the anaerobic filter rapidly adapted (5-7 days) to new operating conditions, thus indicating its high process flexibility. Below COD loads of 20 kg COD/m3·d, a COD removal of 90% or more was achieved. The specific CH4 gas production was measured at 0.40-0.53 m3 CH4/kg COD removed, and the specific sludge production was quantified at 0.1 kg particulate COD/kg COD removed throughout the experimental periods. Biomass distribution revealed that 60% was attached to the packing media and 40% was suspended in the pore space. Based on COD profiles along the filter height, conclusions on the reactor flow pattern could be drawn. With the help of a simple reactor model, the experimental results could be used to simulate other process conditions for design considerations.