Efficient anaerobic degradation may be completed only under low levels of dissolved hydrogen in the liquid surrounding the microorganisms. This restraint can be intensified by the limitations of liquid-to-gas H-2 mass transfer, which results in H-2 accumulation in the bulk liquid of the reactor. Dissolved hydrogen proved to be an interesting parameter for reactor monitoring by showing a good correlation with short-chain volatile fatty acid concentration, namely propionate, which was not the case for the H-2 partial pressure. Biogas recycle was performed in a upflow anaerobic sludge bed and filter reactor. The effects of varying the ratio of recycled-to-produced gas from 2:1 (9 l/l reactor per day) to 8:1 (85 l/l reactor per day) were studied. By increasing the liquid-gas interface with biogas recycling, the dissolved hydrogen concentration could be lowered from 1.1 to 0.4 mu M. Accordingly, the H-2 sursaturation factor was also reduced, leading to an important improvement of the H-2 mass transfer rate, which reached 20.86 h(-1) (+/-9.79) at a 8:1 gas recycling ratio, compared to 0.72 h(-1) (+/-0.24)for the control experiment. Gas recycling also lowered the propionate concentration from 655 to 288 mg l(-1) and improved the soluble chemical oxygen demand removal by 10-15%. The main problem encountered was the shorter solid retention time, which could lend to undesirable biomass washout at high gas recycling ratio. This could be circumvented by improving the reactor design to reduce the turbulence within the biomass bed.