Advanced Oxidative Techniques for the Treatment of Aqueous Liquid Effluents from Biomass Thermochemical Conversion Processes: A Review

This review explores advanced oxidative techniques of wet oxidation (WO) and catalytic wet oxidation (CWO) for treatment of primary aqueous effluents from hydrothermal liquefaction and pyrolysis (with subsequent upgrading) of biomass. Hydrothermal liquefaction and pyrolysis bio-oil upgrading processes produce an aqueous phase rich in organic carbon. However, the characterization of the compounds in such streams is limited and studies on their treatment are even more rare. Few studies have identified major compound families as phenols and smaller carboxylic acids. Often these compounds are overly toxic to existing wastewater treatment operations. Here, advanced oxidative techniques are reviewed with specific focus on how WO and CWO could affect some of these constituents such as phenols and carboxylic acids. Fundamental aspects of such reaction systems are reviewed including the most plausible oxidants (O2, O3, H2O2), the physical and chemical stages of oxidant interactions with the contaminants themselves, and their formation under non-catalytic and catalytic process conditions. Differing types of catalysts (homogeneous, heterogeneous, active carbons) and their novel formation such as carbon nanotubes are considered. Central to the oxidation mechanism, regardless of oxidant or presence of catalyst, is the formation of oxygen-reactive species such as hydroxyl free radicals. While WO processes do generate such oxidative free radicals, it is often under stringent operating conditions. Through the addition of metal catalysts, the oxidative process is successful at removing contaminants at significantly reduced temperatures and pressure. Continuous reactors have shown the best success of removal at both the bench and pilot plant scale. However, many are plagued by oxidant transport deficiencies. To improve transport, novel reactor schemes are emerging for the next generation of oxidative continuous systems. Advanced wet oxidation processes are well suited for the treatment of thermochemical aqueous phase products such that the effluent stream is suitable for processing in wastewater treatment plants or subsequent bioenergy plant water recycle.