Improved methanogenesis from aqueous pyrolysis liquid (APL) by inoculum selection and pre-ozonation†

Aqueous pyrolysis liquid (APL) from municipal wastewater solids (WWS) pyrolysis has a high chemical oxygen demand and diverse organics (N-heterocyclic, aromatic, and phenolic compounds) that can potentially introduce hazardous chemicals to the environment or result in disposal expenses. Therefore, new APL management and energy recovery strategies would be beneficial. The anaerobic degradability of APLs originating from WWS pyrolyzed at 500 and 700 °C was investigated under varying pre-ozonation conditions and by using four distinct inocula. The 700 °C APL was more toxic to anaerobic microorganisms in batch anaerobic toxicity assays and pre-ozonation decreased the toxicity as demonstrated by increased subsequent methane production rates (10–55% increase). The 500 °C APL did not show toxicity under the conditions tested and ozonation had limited impact on subsequent methane production. Microbial communities of two of the inocula demonstrated small shifts during APL digestion compared to the other two, suggesting potential acclimation of the initial inoculum to APL-like constituents. This study highlights that both APL pre-ozonation and type of inocula can significantly affect the anaerobic toxicity response to APL. While anaerobic treatment of municipal wastewater-derived APL is viable, pyrolysis temperature, organic loading rates, pretreatment and microbial community composition play important roles in biogas production.