Recovery of Ammonia during Food Waste Anaerobic Digestion Using a pH-Swing Electrochemical Process

Abstract

Anaerobic digestion (AD) is a biological process for breaking down organic materials in an oxygen-free environment by microorganisms. This process has been proposed as a promising solution for diverting food waste from landfills as well as producing biogas and recovering nutrients from food waste to use as a fertilizer. A major challenge during AD is that ammonia created by microorganisms inhibits microbial methane production. Therefore, ammonia removal during AD can improve the overall performance, and yield high-quality ammonia for fertilizer reblending or other uses. Previous ammonia recovery systems have relied on the use of high amounts of base to elevate the solution pH for converting ammonium into ammonia. While electrodialysis (ED) is a promising alternative to increase pH by electrochemical reaction without the need for chemical, a high voltage (>2.5 V) is required for electrochemical water splitting. To enable energy-efficient recovery of ammonia, a proton-mediated electrochemical reaction was implemented using hydroquinone (HQ)/benzoquinone (BQ) as a model redox couple. The proton-mediated redox couple undergoes electrochemical reduction with H+ uptake (increase the pH) and oxidation with H+ release (decrease the pH). A three-channel flow cell separated by cation exchange membranes was constructed and the channel located at the middle was fed by synthetic digestate while applying a constant current of 10 A m-2. Ammonia flux achieved was 132 g N m-2 d-1 with a solution pH of 9.5. The resulting energy consumption of 1.6 kWh (kg N)-1 was less than a half of that needed in ED due to the low cell voltage requirement (<1 V). The separated ammonium was captured in an acidic solution (0.05 M H2SO4) using a membrane contactor at a flux of 49 g N m-2 d-1, resulting in 4.3 kWh kg N-1 based on the energy consumed for the electrochemical separation. These results support that the use of proton-mediated redox couple can enable energy-efficient recovery of ammonia during food waste AD.