Anaerobic digestion at high-pH and alkalinity for biomethane production: Insights into methane yield, biomethane purity, and process performance

Abstract

• Continuous reactor operation for biomethanation of waste sludge at pH 9.3. • Transition from hydrolysis-limited to a methanogenesis-limited AD process at the highest OLR tested. • No influence of ammonia toxicity observed (up to 14 mM tested). • pH profile can be reasonably well predicted through a chemical model. • pH influenced methanogenic community composition. The role of high-pH conditions in anaerobic digestion (AD) has traditionally been confined to it's use in pre-treatment processes. However, operating AD at elevated pH and alkalinity offers significant advantages, including in-situ upgrading of biogas to biomethane. This study examines the potential and scalability of AD under these conditions (pH ∼ 9.3; alkalinity ∼ 0.5 eq/L). The substrate used was the alkaline waste generated from the extraction of extracellular polymeric substances (EPS) from aerobic granular sludge (AGS), and the inoculum used was a haloalkaliphile microbial community from soda lake sediments. To evaluate the system’s performance, the organic loading rate (OLR) was incrementally increased. The highest methane production obtained was 8.4 ± 0.1 mL/day/g VSadded at a hydraulic retention time (HRT) of 15 days and an OLR of 1 kg VS /day/m 3 . At this loading rate, methanogenesis became the rate limiting conversion. The maximum volatile solids conversion was 48.1 ± 1.1 %. Throughout the reactor operation, methane purity in the biogas consistently exceeded 90 % peaking at 96.0 ± 0.2 %, showcasing the potential for in-situ biogas purification under these conditions. In addition, no ammonia inhibition was observed, even with free-ammonia (NH 3 ) concentrations reaching up to 14 mM. This study underscores the potential of high-pH anaerobic digestion as a sustainable method for both waste treatment and energy recovery.

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