Influence of free ammonia extraction in methane production from human urine

Abstract

Human urine has a high chemical oxygen demand (COD) content which makes anaerobic treatments potentially appropriate for the management of yellow waters, allowing for energy recovery. However, its high N content makes this treatment challenging. The present work studied the viability of performing an anaerobic digestion process for COD valorization on a real (not synthetic) urine stream at laboratory scale. To deal with nitrogen inhibition, two different ammonia extraction systems were proposed and tested. With them, a proper evolution of acidogenesis and methanogenesis was observed. Nitrogen was recovered in the form of ammonium sulphate, which could be used for agriculture, in two different ways: ammonia extraction from the urine stream before feeding the reactor and in situ extraction in the reactor. The first method, which proved to be a better strategy consisted in a desorption process (NaOH addition, air bubbling and acid (H2SO4) absorption column, HCl for final pH adjustment) whereas the in situ extraction in the reactor consisted of an acid (H2SO4) absorption column installed in the biogas recycling line of both reactors. Stable methane production over 220 mL/g COD was achieved and methane content in the biogas was stable around 71%.