Steady-State Anaerobic Co-Digestion of Aqueous Condensate From Municipal Sludge Pyrolysis

Abstract

Anaerobic digestion (AD) of condensate or aqueous pyrolysis liquid (APL) derived from municipal wastewater solids was successfully achieved both as a sole substrate and as a co-digestate with synthetic sludge, overcoming toxicity challenges previously associated with APL degradation. Key strategies that enhanced APL conversion to methane included optimizing the solids retention time (SRT) and organic loading rate (OLR) to mitigate APL toxicity, using an acclimated inoculum, and employing APL ozonation prior to digestion. Gas chromatography–mass spectrometry analysis confirmed APL constituents were removed in the process. Inoculum biomass from an industrial waste digester (IB) exhibited better performance in APL degradation compared to inoculum from a municipal digester (MB). APL ozonation enhanced methane production in IB-inoculated co-digesters, achieving 98% of the maximum stoichiometric methane. Microbial community analysis showed that hydrogenotrophic methanogens predominated in syntrophy with acetate oxidizing bacteria in IB-inoculated reactors, whereas both acetoclastic and hydrogenotrophic methanogens were present in MB-inoculated co-digesters. This study demonstrates that APL can be digested alone or as a co-substrate, emphasizing the importance of appropriate SRT, OLR, and inoculum selection. Co-digestion could be a viable strategy for wastewater resource recovery facilities that operate digesters for sludge treatment and may incorporate wastewater solids pyrolysis in the future.