The objective of this study is to highlight the opportunities for positive interaction between the integrated water system and hydrogen technologies, with reference to wastewater treatment plants.
The possibilities for using green hydrogen within wastewater treatment plants include the biological upgrade of biogas to biomethane; the CO2 contained in the biogas can be reduced to CH4 in the presence of green hydrogen. This reaction (known as the Sabatier process) is performed by microorganisms present in the conventional anaerobic digestion process.
It is advantageous to install photovoltaic plants and use their electrical energy to cover the site’s self-consumption needs. Once self-consumption is met, any excess self-produced electricity can be used to power green hydrogen and biofuel production systems.
In particular, it is reasonable to convert hydrogen into methane through methanation processes that reuse the available CO2 from biogas upgrading processes.
The case study focused on the Bresso (MI) plant, managed by Gruppo CAP SpA, and provided the following main results:
• The plant could reduce indirect emissions by about 28% with the installation of a 2000 kWp PV system, a contribution that increases to 38% if the PV capacity is raised to 4000 kWp;
• In the case of a 2000 kWp PV system, the surplus would be very limited, making the installation of an electrolyzer and a PtG system not cost-effective. Conversely, if the PV capacity increases to 4000 kWp, the amount of non-self-consumable electricity would grow, allowing for more hours of operation of the electrolyzer.
In the best-case scenario of the analysis conducted, the plant components had the following characteristics: the electrolyzer has a power of 500 kWe (with a maximum production of just under 100 Nm3/h of H2), hydrogen is stored in cylinders with a capacity of 360 kgH2, the methanator has a nominal capacity of 24 Nm3/h of CH4, and the PtG system produces a total of 37 tons of CH4 per year, with an additional 42 tons produced through the use of ozone to support sludge treatment.
From the perspective of CO2 emissions reduction compared to the initial conditions (2080 tCO2/year), the PtG system contributes an 11% reduction (-230 tCO2/year) due to the production of additional biomethane. This 11% adds to the 38% reduction in emissions from the PV system. The optimized cost of avoided CO2 is around €180/tCO2.
In conclusion, many wastewater treatment plants with anaerobic digestion in the integrated water system could provide a particularly advantageous application for the production and use of hydrogen in Power-to-Gas systems, benefiting from:
• The availability of space for installing additional RES capacity, mainly photovoltaic;
• The availability of CO2 for the methanation process;
• The synergies from recovering and using the oxygen co-produced by the electrolyzer onsite.