This study was undertaken to examine the benefits of pressure-assisted ozonation (PAO) in enhancing solids reduction and biogas production in anaerobic digestion. The results showed significant improvements in both. With unacclimated inoculum at varied food-to-inoculum (F/I) ratios of 0.5-2, solids and COD reductions were improved by PAO, as well as by increased F/I ratios even without PAO, which would warrant further optimization of the F/I ratio for an unacclimated inoculum. With acclimated inocula at F/I ratio of 0.8, volatile suspended solids reduction and biogas production were improved by up to 60% and 800%, respectively, when the AS had been subjected to 20 cycles of PAO. In extended operation in plants where acclimated anaerobes are encountered, PAO pretreatment offers improved digestion of AS in terms of solids and COD removals and biogas production. (C) 2013 Elsevier Ltd. All rights reserved.