Application of rumen microorganisms for a high rate anaerobic digestion of papermill sludge

The anaerobic digestion of papermill sludge containing a high amount of inorganic matter was studied. Despite the presence of about 58% inorganic matter in the substrate, a high rate of hydrolysis and subsequent acid formation could be achieved in an acidogenic reactor which was inoculated with rumen microorganisms. Degradation efficiency of neutral detergent fibre amounted to 62% at high loading rate (34·2 g volatile solids per litre per day) and short solid retention time (51 h). In order to study the effect of accumulation of inorganic matter in the reactor, degradation efficiency was studied at various loading rates and solid retention times. An increase of solid retention time to 74 h resulted in a decreased degradation efficiency, probably due to an increased ash content in the reactor under these conditions. The effect of inorganic matter accumulation was also studied after coupling of the rumen derived acidogenic reactor to an upflow anaerobic sludge blanket-type methanogenic reactor. By using this two-stage digestion process an overall conversion of papermill sludge into biogas could be realized. The operation over about three months of the two-stage process was studied in terms of process stability, nutrient recycling and accumulation of inorganic matter.