Biogas Quality Improvement Using Water Wash and Phosphorus Recovery as Struvite in Jones Island Wwtp

Abstract

Raw biogas from anaerobic digestion has a methane content of 50 to 60% and a carbon dioxide content of 40 to 50% on a molar basis (Bortoluzzi, Gatti, Sogni, & Consonni, 2014). Milwaukee Metropolitan Sewerage District (MMSD)’s Jones Island Waste Water Treatment Plant (WWTP) uses the biogas supplied from South Shore WWTP in drying Milorganite, a slow-release phosphate fertilizer. But with only 45% methane, the gas cannot be used for sophisticated purposes. To maximize its potential as energy source, the methane content must be upgraded to its market competitor natural gas. Based on simulation results from Aspen Plus software - High Pressure Water Scrubbing (HPWS) or water wash seems to be the best option. The process requires running the impure gas through pressurized water. Based on Henry’s law, CO2 is dissolved easily because of low partial pressure. The integrated process doesn’t need additional water or pressure, as it can use wastewater from WWTP and the gas is already supplied at an optimal pressure. It can also remove most of the H2S, present as a trace amount in the biogas. Furthermore, struvite, a better-quality phosphate fertilizer can be recovered with adequate aeration and adding NaOH. From simulation results, the methane content can be improved up to 98.7 % at pressures up to 150 psi.