Improving Methane Yields of Semi-continuous Anaerobic Digestion of Deinking Sludge from Wastepaper Recycling

Abstract

The purpose of the study was to investigate the role of trace elements (TEs) supplementation in connection with the organic load rate (OLR) and the hydraulic retention time (HRT) on methane (CH4) yields in semi-continuous anaerobic digestion of deinking sludge (DS) in order to judge the possibility to use DS as alternative energy source in wastepaper recycling companies. After substrate characterization, a semi-continuous anaerobic digestion was performed for 74 days using 2-L stirred tank reactors at a mesophilic (37 °C) condition, whereas one of them was supplemented with a mixture of TEs (cobalt, nickel, molybdenum and selenium). The feeding periods had a duration of 4, 7 or 8 days. The highest cumulative CH4 yield after a feeding period was found to be 151 NmL/g oDM after TE supplementation. The TEs-supplementation has resulted in 42% increase in CH4 yield compared to the non-supplemented variant. The increase took place at the end of the experiment. Earlier feeding periods with similar conditions regarding HRT and OLR, did not show an increase. This can be explained by the fact that TEs supplementation requires a longer adaption time for the microorganism. TEs addition has shown a strongly positive impact on CH4 yields. Our results showed that it is possible to receive the same yields in semi-continuous, more practice-close operation, compared to previous potential determination experiments under batch conditions. A further optimization potential is seen by optimizing TEs supplementation in connection with VOA level in the reactor, optimizing C/N ratio and lowering the duration of the feeding intervals.