Combined ammonia recovery and solid oxide fuel cell use at wastewater treatment plants for energy and greenhouse gas emission improvements

Current standard practice at wastewater treatment plants (WWTPs) involves the recycling of digestate liquor, produced from the anaerobic digestion of sludge, back into the treatment process. However, a significant amount of energy is required to enable biological breakdown of ammonia present in the liquor. This biological processing also results in the emission of damaging quantities of greenhouse gases, making diversion of liquor and recovery of ammonia a noteworthy option for improving the sustainability of wastewater treatment. This study presents a novel process which combines ammonia recovery from diverted digestate liquor for use (alongside biomethane) in a solid oxide fuel cell (SOFC) system for implementation at WWTPs. Aspen Plus V.8.8 and numerical steady state models have been developed, using data from a WWTP in West Yorkshire (UK) as a reference facility (750,000 p.e.). Aspen Plus simulations demonstrate an ability to recover 82% of ammoniacal nitrogen present in digestate liquor produced at the WWTP. The recovery process uses a series of stripping, absorption and flash separation units where water is recovered alongside ammonia. This facilitates effective internal steam methane reforming in the fuel cell with a molar steam:CH4 ratio of 2.5. The installation of the process at the WWTP used as a case of study has the potential to make significant impacts energetically and environmentally; findings suggest the treatment facility could transform from a net consumer of electricity to a net producer. The SOFC has been demonstrated to run at an electrical efficiency of 48%, with NH3 contributing 4.6% of its power output. It has also been demonstrated that 3.5 kg CO2e per person served by the WWTP could be mitigated a year due to a combination of emissions savings by diversion of ammonia from biological processing and lifecycle emissions associated with the lack of reliance on grid electricity.