Simulation of Conventional WWTPs Acting as Mediators in H2/CO2 Conversion into Methane

CO2-biomethanation was studied in the present manuscript by considering the direct injection of hydrogen into a conventional anaerobic digester treating sewage sludge within a simulated wastewater treatment plant (WWTP). The plant was simulated using Python software, and a Monte Carlo simulation was conducted to account for the high variability in the organic content of wastewater and the methane potential of sludge. Two scenarios were studied. The first case involves the use of an anaerobic digester to upgrade biogas, and the second case considers using the digester as a CO2-utilization unit, transforming captured CO2. Upgrading biogas and utilizing the extra methane to generate electricity within the same plant leads to a negative economic balance (first scenario). A hydrogen injection of 1 L H2/Lr d (volumetric H2 injection per liter of reactor per day) was required to transform the CO2 present in biogas into methane. The benefits associated with this approach resulted in lower savings regarding heat recovery from the electrolyzer, increased electricity production, and additional oxygen supply for the waste-activated sludge treatment system. Increasing the injection rate (second scenario) to values of 5 and 30 L H2/Lr d was also studied by considering the operation of the digester under thermophilic conditions. The latter assumptions benefited from the better economy of scale associated with larger installations. They allowed for obtaining enough savings regarding the fuel demand for sludge drying, in addition to the previous categories analyzed in the biogas upgrading case. However, the current electricity price makes the proposal unfeasible unless a lower price is set for hydrogen generation. A standard electricity price of 7.6 c€/kWh was assumed for the analysis, but the specific operation of producing hydrogen required a price below 3.0 c€/kWh to achieve profitability.