Wet oxidation pre-treatment of woody yard waste: Parameter optimization and enzymatic digestibility for ethanol production

Woody yard waste with high lignin content (22% of dry matter (DM)) was subjected to wet oxidation pre-treatment for subsequent enzymatic conversion and fermentation. The effects of temperature (185-200 degreesC), oxygen pressure (3-12 bar) and addition of sodium carbonate (0-3.3 g per 100 g DM biomass) on enzymatic cellulose and hemicellulose (xylan) convertibility were studied. The enzymatic cellulose conversion was highest after wet oxidation for 15 min at 185 degreesC with addition of 12 bars of oxygen and 3.3 g Na2CO3 per 100g waste. At 25 FPU (filter paper unit) cellulase g(-1) DM added, 58-67% and 80-83% of the cellulose and hemicellulose contained in the waste were converted into monomeric sugars. The cellulose conversion efficiency during a simultaneous saccharification and fermentation (SSF) assay at 10% DM was 79% for the highest enzyme loading (25 FPU g(-1) DM) while 69% conversion efficiency was still reached at 15 FPU g(-1) DM. Total carbohydrate recoveries were high (91-100% for cellulose and 72-100% for hemicellulose) and up to 49% of the original lignin and 79% of the hemicellulose could be solubilized during wet oxidation treatment and converted into carboxylic acids mainly (total carboxylic acids = 3.1-7.4% on DM basis).