Treatment and valorization of municipal solid waste gasification effluent through a combined advanced oxidation – microalgal phytoremediation approach

Abstract

Liquid effluents generated during the gasification process of municipal solid wastes (MSW) represent highly heterogenous and recalcitrant substrates, and thus require energy-intensive treatments prior to their discharge into the environment. However, these streams represent untapped carbon sources which can be converted, under the right conditions, to platform molecules, decreasing thus the elevated costs of current mitigation approaches. Thus, the present study describes a novel two-step chemical and biological effluent treatment process which co-generates microalgal biomass containing valuable functional components. The detoxification potential of ozone, ultrasonication and a combination of both of these advanced oxidation processes (AOPs) were initially evaluated using wastewater generated from scrubbing primary syngas produced from the gasification of MSW. The pretreated effluents were subsequently used to cultivate Chlorella vulgaris microalgae under various illumination (light/dark cycles of 24h/0h, 12h/12h and 0h/24h) and nutrient (supplementation of yeast extract) regimes, in order to assess their potential to further convert the remaining carbon into value-added functional biomolecules, such as photosynthetic pigments. The use of ozone, either individually or combined with ultrasounds, showed the best performances for the substrate detoxification prior to the biological treatment step. It was determined that 20 min of oxidative reaction using ozone coupled with ultrasounds was sufficient to degrade up to 70% of the total phenol compounds, 64% of the COD, and 75% of the color initially present in the gasification wastewater. The further microalgal phytoremediation and conversion experiments showed that 12 h of illumination per day, together with 0.5 g/L of yeast extract, generated the highest microalgae growth rate (0.29 d−1), biomass productivity (244 mg/L/d), photosynthetic pigment accumulation (12.0 and 4.5 mg/L of total chlorophyll and carotenoids, respectively) as well as total carbon removal (57 %). Therefore, the combination of AOPs with mixotrophic cultivation of C. vulgaris demonstrates for the first time an alternative to current treatment strategies of highly-recalcitrant industrial wastewaters, which includes the further valorization of the carbon available in these streams.