Hydrophobic membranes for ammonia recovery from digestates in microbial electrolysis cells: Assessment of different configurations

Abstract

The combination of hydrophobic membranes and microbial electrolysis cells (MEC) was assessed in two different configurations in order to recover ammonia from anaerobically digested pig slurry. Polytetrafluorethylene (PTFE) hydrophobic membranes were inserted both in an H-type three-chamber cell (MEC-H) and a two-chamber sandwich configuration MEC (MEC-S), both fitted with a cationic exchange membrane (CEM) dividing the anode and cathode compartments. The use of electrochemical techniques such as electrochemical impedance spectroscopy was applied to monitor the increase of the biofilm on the anode, related to the decrease of the charge transfer resistance. When operated in the higher organic loading rate (28 ± 5 kg COD m−3 d−1) the current density produced in the MEC-H was 1.40 ± 0.71 A m−2, compared to 0.61 ± 0.28 A m−2 in the MEC-S. The flux of ammonium through the CEM in the MEC-H was of 3.4 ± 1.2 g N m−2 h−1. Regarding the recovery of ammonia through the hydrophobic membrane, the flux of ammonia was of 1.5 and 0.7 g N m−2 h−1 in the MEC-H and MEC-S, respectively, mainly governed by the pH value and the ammonia concentration of the catholyte. The combination of MEC with hydrophobic membranes reveals as a suitable technology for the recovery of ammonia and treatment of high strength wastewater such as livestock.