Anaerobic treatment of deinking sludge: Methane production and organic matter degradation

Deinking sludge (DS) is a residue from the waste paper recycling industry. It is a by-product originating from the deinking process which is designed to remove inks and other impurities from waste papers to recover usable fibers. The aim of this study was to investigate the possibility of anaerobic digestion (AD) of DS in order to produce methane (CH₄) by degrading organic matter. DS differs clearly from common AD substrates due to its specific composition. The focus was laid on comparing various inocula in order to find appropriate microbial consortia. Three inocula from different origins were investigated. After sludge and inocula characterization, batch AD at a mesophilic (37 °C) condition was performed for 21 days to determine biogas and CH₄ potentials as well as sludge biodegradability in comparison to cellulose. The highest average CH₄ yield achieved in the 21 days of the batch experiment was 167 NmL/g organic dry matter (oDM). However, the CH₄ potentials from all experiments did not have a wide range (average 160 NmL/g oDM; standard deviation ±5.0 NmL/g oDM). The highest organic matter degradation achieved was 31%. It can be stated that conventional AD inocula are usable to degrade DS, but that a significant part of the oDM was anaerobically not degradable. The overall CH₄ yields were lower compared to top AD substrates such as energy crops, but in a similar range like residue-based AD substrates such as manure. Since actual DS management is cost-intensive and affecting the profitability of waste paper industry significantly, AD as a management option with an energetically valuable output is a very promising option.