Performance of up-flow anaerobic fixed bed reactor of the treatment of sugar beet pulp lixiviation in a thermophilic range

•SBPL as substrate; high content of microorganisms in SBPL.•Improve biogas productivity and the organic matter removal.•Different OLRs and hydraulic retention times (HRTs) were carried out.•The greatest efficiency; 90% COD removal for an OLR of 4.3kg COD/m3d.•OLR was directly correlated with active microbial biomass. The acclimatization and performance study of lixiviation of sugar beet pulp are carried out in upflow anaerobic fixed bed reactor in thermophilic range of temperature (55°C). Several hydraulic retention time is conducted (11, 8, 6, 4, 2, and 1.5days). The performance study showed that Chemical Oxygen Demand removal efficiency is 90% for 6days-HRT. While COD removal efficiency was reduced within the range of 74.3% and 59.4% in others HRT. Organic loading rates greater than 10kg COD/m3d in influent, (2days-HRT), produces a destabilization of the process due to total acidity accumulation in reactors although is the HRT with highest methane production. The results showed that an increase in OLR was directly correlated with active biomass inside reactor but not with the amount in microbial community. The bacterial concentration inside the reactor is strongly influenced by the content of microorganisms in the lixiviation of sugar beet pulp.