Linking metal bioavailability to methanogenic activity in anaerobic granular sludge: insights from depletion strategy

Introduction: Purified terephthalic acid (PTA) is a raw material used in the production of PET bottles. During PTA manufacturing processes, Co and Mn are used as catalysts, and PTA/metals-containing wastewater (WW) is generated. Fe is also added. During the anaerobic digestion (AD) of PTA-WW in two industrial-scale bioreactors, Co, Mn, and Fe accumulated in the anaerobic granular sludge (AGS). Generally, those metals sustain methanogenic activity during the AD process but may also promote a toxic effect when present in high concentrations in the sludge. The aim of this study is to understand the impact of the metal's bioavailability on the methanogenic activity of AGS. Experimental: Sequential batch reactors SBR1 and SBR2 were inoculated with AGS from two full-scale reactors treating PTA-WW. At the start-up, 18 small reactors (165 ml) were fed with pre-treated PTA-WW, with 1.85 gCOD/L, and HRT of 24h. The pre-treated PTA-WW consists of raw PTA-WW treated with NaOH at pH 11, promoting the removal of Co, Mn, and Fe by precipitation. AGS harvested from an entire bottle of both SBR was used to proceed with specific acetoclastic methanogenic activity (SAMA) and to assess the metal bioavailability by modified Tessier SEP, divided in exchangeable (EXG), carbonate (CARB), bound to organic matter/sulfides (OM/S) and residual (RES) phases. Metals were quantified by ICP-OES. A pulse of raw PTA-WW and the effect of EDTA were also assessed. Results and Discussion: The study showed that SAMA was sustained up to 0.43±0.02 gCH/gVS.d even in the absence of Co, Mn, and Fe during 162d of SBR operation. The SAMA was supported by the metal pool present in AGS, even though more than 98% of Co and 80% of Fe were retained in the OM/S and RES phases. The high mobility of EDTA complexed metals may improve diffusion into the granular matrix, boosting SAMA (0.51±0.01 gCH/gVS.d) by 18% in the inorganic sludge of SBR2. A pulse of raw PTA-WW after 120d of suppression showed an increase in methane production of 11% (SBR1) and 22% (SBR2). Conclusions: This study demonstrates that AD of PTA-WW can be reached with no extra supplementation of Co, Mn, and Fe supported by the metal pool present in the AGS. High SAMA was observed despite low Co and Fe bioavailability and in the absence of metal supplementation. These findings highlight the economic and operational benefits of adequate metal supplementation in the AD process of PTA-WW and suggest future recovery and pulse supplementation strategies.