Alkaline pulping liquors are problematic for anaerobic treatment due to their toxicity to methanogens and their relatively large fraction of inert lignin. In previous research, toxicity due to wood extractives was shown to be highly eliminated during aerobic wastewater treatment, but not during anaerobic treatment. These observations have led to the proposal of a detoxification strategy denominated upfront dilution, based on the sequenced anaerobic-aerobic treatment of the pulping liquor, recirculating the aerobic effluent to dilute the incoming influent to sub-toxic concentrations. In this study, the treatment of highly toxic hemp stem wood black liquor (HSWBL) in lab-scale UASB reactors with upfront dilution was compared with direct anaerobic treatment and with direct aerobic treatment. Direct anaerobic treatment of 12 g COD/l HSWBL led to almost complete inhibition of the methanogenic activity within 14 d. However, recirculation of 75% of the aerobic post-treatment effluent for upfront dilution of the toxic HSWBL, enabled anaerobic treatment at loading rates up to 21.5 g COD/l(UASB) . d without significant inhibition of the methanogenic activity. Extensive detoxification was confirmed during anaerobic-aerobic treatment of 20 g COD/l HSWBL recirculating 86% of the aerobic effluent. COD and BOD removal was 72% and 97%, respectively, after anaerobic-aerobic treatment at an overall loading rate of 3.6 g COD/l d, while 30-35% of the incoming COD was recovered as methane. Batch-assays demonstrated significant detoxification after anaerobic-aerobic treatment of HSWBL. Treatment efficiencies and detoxification during anaerobic-aerobic and aerobic treatment were similar. However, the anaerobic-aerobic treatment system provided 50% lower surplus sludge production, production of 0.16 m(3) methane/kg CODremoved as an energy source, less nutrient dosage and substantial reductions in aeration costs. During anaerobic-aerobic treatment as well as aerobic treatment significant lignin removal was obtained, ranging from 28-58%. Lignin removal could be attributed to biodegradation of low molecular weight lignin (MW < 2.2 kD). The lignin which survived biological treatment was extensively polymerized to MW of > 34 kD.