Influence of free ammonia extraction in methane production from human urine

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%.