Hydrogen and methane production from swine wastewater using microbial electrolysis cells

The production of a useful and valuable product during swine wastewater treatment, such as hydrogen gas, could help to lower treatment costs. Hydrogen can theoretically be produced from wastewater by electrohydrogenesis in a microbial electrolysis cell (MEC) or by fermentation. Using a single-chamber MEC with a graphite-fiber brush anode, hydrogen gas was generated at 0.9-1.0 m³ m^-3 day^-1 H₂ using a full-strength or diluted swine wastewater. COD removals ranged from 8 to 29% in 20-h tests, and from 69 to 75% in longer tests (184 h) using full-strength wastewater. The gas produced was up to 77 ± 11% hydrogen, with overall recoveries of up to 28 ± 6% of the COD in the wastewater as hydrogen gas. Methane was also produced at a maximum of 13 ± 4% of total gas volume. The efficiency of hydrogen production, based on the electrical energy needed (but excluding the energy in the wastewater) compared to the energy of the hydrogen gas produced, was as high as 190 ± 39% in 42-h batch tests with undiluted wastewater, but was lower in longer batch tests of 184 h (91 ± 6%). Hydrogen gas could not be recovered in fermentation tests using wastewater with a heat-treated inoculum. Hydrogen production was shown to be possible by fermentation when the wastewater was sterilized, but this process would not be practical or energy efficient. We therefore conclude from these tests that MECs are an effective method for hydrogen recovery from swine wastewater treatment, although the process needs to be further evaluated for reducing methane production, increasing the efficiency of converting the organic matter into current, and increasing recovery of hydrogen gas produced at the cathode.