Anaerobic treatment of hemp thermomechanical pulping wastewater

Biological treatment is an indispensable instrument for water management of non-wood pulp mills, either as internal measure or end of pipe. In this paper, anaerobic treatment of wastewaters derived from the thermomechanical pulping (TMP) of hemp (Cannabis sativa L.) is described. Hemp stem wood and hemp bark TMP wastewaters were treated in laboratory scale upflow anaerobic sludge blanket (UASB) reactors. For both types of wastewater, maximal COD removal of 72% were obtained at loading rates of 13–16 g COD•l−1 d−1, providing 59–63% recovery of the influent COD as methane. The reactors continued to provide excellent COD removal efficiencies of 63–66% up to a loading rate of 27 g COD•l−1 d−1, being the highest loading rate tested. Batch toxicity assays revealed the absence of methanogenic inhibition by hemp TMP wastewaters, coinciding with the high acetoclastic activity of the reactor sludge of approximately 1 g COD•g VSS−1 d−1. Due to the relatively low molecular weight of hemp TMP lignin, its removal (measured as UV280) during anaerobic treatment was remarkably high and averaged 45 and 31% for the hemp stem wood and the hemp bark TMP UASB reactors, respectively. Gel permeation chromatography revealed that the lignin removed corresponded to the lowest molecular weight derivatives. Subsequent batch aerobic post-treatment led to a considerable increase of color levels and polymerization of the residual lignin to molecular weights in excess of 34 kD.