IMPACT OF LIQUID-TO-GAS HYDROGEN MASS-TRANSFER ON SUBSTRATE CONVERSION EFFICIENCY OF AN UPFLOW ANAEROBIC SLUDGE BED AND FILTER REACTOR

Efficient anaerobic degradation may be completed only under low levels of dissolved hydrogen in the liquid surrounding the microorganisms. T his 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 parame ter for reactor monitoring by showing a good correlation with short-ch ain volatile fatty acid concentration, namely propionate, which was no t the case for the H-2 partial pressure. Biogas recycle was performed in a upflow anaerobic sludge bed and filter reactor. The effects of va rying the ratio of recycled-to-produced gas from 2:1 (9 l/l reactor pe r day) to 8:1 (85 l/l reactor per day) were studied. By increasing the liquid-gas interface with biogas recycling, the dissolved hydrogen co ncentration could be lowered from 1.1 to 0.4 mu M. Accordingly, the H- 2 sursaturation factor was also reduced, leading to an important impro vement 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 conc entration from 655 to 288 mg l(-1) and improved the soluble chemical o xygen demand removal by 10-15%. The main problem encountered was the s horter solid retention time, which could lend to undesirable biomass w ashout at high gas recycling ratio. This could be circumvented by impr oving the reactor design to reduce the turbulence within the biomass b ed.