Investigation of UASB-reactors is complicated at laboratory-scale and operating many biological replicates to draw statistically significant conclusions is challenging. A simple syringe reactor system using off-the-shelf components was developed for the investigation of wastewater treatments with anaerobic granular sludge. As a proof of concept, the effect of hydrogen addition (in situ biomethanation) on the process performance and methane production was investigated with real wastewater of a paper company. The biogas production was highly reproducible and the removal of short-chain fatty acids and alcohols were easy to follow. Addition of H₂ gas as electron donor to upgrade biogas was easily implemented and after three cycles a significant negative effect on process performance was observed. Molecular community analysis revealed the predominance of both strict acetotrophic (Methanothrix) and hydrogenotrophic methanogens (Methanobacterium) as well as bacteria typically found in anaerobic wastewater systems including syntrophic microorganisms affiliated to the classes Clostridia, Syntrophobacteria and Syntrophia. H₂-addition had most probably negatively affected the syntrophic interactions due to thermodynamic constraints. The developed syringe reactor system was easy to use and very affordable lowering the barrier for widespread investigation of the process performance and microbial ecology of wastewater treatment based on anaerobic granular sludge.