Electrochemical Nutrient Recovery Enables Ammonia Toxicity Control and Biogas Desulfurization in Anaerobic Digestion

Organic waste streams can be valorized and reduced in volume with anaerobic digestion (AD). An often-encountered key issue however is the high ammonium (NH₄⁺) content of certain waste streams. Ammonia (NH₃), in equilibrium with NH₄⁺, is a toxic compound to the methanogenic community, which limits the organic loading rate and endangers process stability. An electrochemical system (ES) linked to a digester could, besides recovering this nutrient, decrease NH₃ toxicity through electrochemical extraction. Therefore, two digesters with and without ES attached in the recirculation loop were operated to test whether the ES could control NH₃ toxicity. During periods of high ammonium loading rates, the methane (CH₄) production of the ES-coupled reactor was up to 4.5 times higher compared to the control, which could be explained through simultaneous NH₄⁺ extraction and electrochemical pH control. A nitrogen flux of 47 g N m^–2 membrane d^–1 could be obtained in the ES-coupled reactor, resulting in a current and removal efficiency of 38 ± 5% and 28 ± 2%, respectively, at an electrochemical power input of 17 ± 2 kWh kg^–1 N. The anode also oxidized sulfide, resulting in a significantly lower H₂S emission via the biogas. Lastly, limited methanogenic community dynamics pointed to a nonselective influence of the different operational conditions