Co-oxidation effects and mechanisms between sludge and alcohols (methanol, ethanol and isopropanol) in supercritical water

Sludge is a by-product of wastewater treatment process with high organic and nitrogen contents. Supercritical water oxidation (SCWO) rates of refractory species in sludge might be accelerated by reactive alcohols, which is referred to as a co-oxidation phenomenon. In this work, influences of three typical reactive alcohols (methanol, ethanol and isopropanol) on TOC and of NH₃–N removal rates of sludge by SCWO were analyzed and their co-oxidation products were characterized by GC–MS, FT–IR, EDS and TG–DTG methods. Additionally, to capture the nature of co-oxidation, HO₂^[rad] and OH^[rad] radicals released by alcohols were theoretically compared with detailed chemical kinetics models using the Chemkin Software. The results show that methanol, ethanol and isopropanol all exhibited co-oxidation accelerated effects on TOC and NH₃–N removal rates of sludge. One reason was that alcohols provided reactive HO₂^[rad] and OH^[rad] radicals. The other reason was that adding alcohols not only prevented forming recalcitrant products (non-nitrogen and nitrogen aromatic compounds) but also encouraged producing reactive products (non-nitrogen open chain compounds). Ethanol offered the best co-oxidation promotion effects on the removal of TOC and NH₃–N in the liquid products and also gave the lowest organic contents in the solid products, followed by isopropanol and methanol. This order was consistent with the difficulty of how the alcohols themselves could be oxidized in supercritical water since ethanol could support the highest amount of HO₂^[rad] radicals for sludge in the shortest time.