The Future of Biogas in Wastewater Treatment

David Hume (Mott MacDonald), Chris Thurston (Watercare Services Ltd.), James Newton (Mott MacDonald), Michael Whittome (Mott MacDonald)

As the water sector focusses increasingly on Net Zero strategies, there is a greater impetus on the best use of resources available at wastewater treatment plants. Biogas is generated from recovered waste and is considered a renewable energy source. There is growing interest in biogas, believed to be an essential fuel in the transition away from the world's reliance on fossil fuels. Improvements in biogas treatment technologies, new types of fuels and the desire to decarbonise are aspects that require consideration in the path forward for Watercare. 

Many treatment plants in Aotearoa generate biogas through anaerobic digestion (AD), and then use this for heating and generating electricity. But as the electricity supply in Aotearoa is increasingly decarbonised, the benefit in terms of greenhouse gas (GHG) emissions reduction is rapidly diminishing. This paradoxical problem has led Watercare to consider: What is the best use of their biogas resource into the future?

Watercare produces biogas through the anaerobic digestion of collected solids at its most significant wastewater treatment sites at Māngere and Rosedale. The biogas is collected and combusted using combined heat and power (CHP) co-generation engines to produce heat and power for the treatment works as well as to minimise GHG emissions by combusting rather than emitting methane. Some biogas is also combusted in hot water boilers to provide additional process heat. As a result, biogas is a valued resource in lowering power costs and reducing the reliance on imported fuel and energy.

Watercare has set ambitious greenhouse gas reduction targets, including a 50% reduction in operational emissions (equivalent to carbon savings of approx. 70,000 tCO2e/year) by 2030. Through Watercare’s Decarbonisation Roadmap it was identified that there is the potential to achieve higher GHG emissions reduction by using some or all of the biogas produced at Māngere and Rosedale for alternative uses than the current co-gen engines to generate electricity and heat, such as to produce biomethane, which can be injected into the gas grid in order to displace fossil natural gas with its relatively high GHG emissions. 

There are a range of potential opportunities for the use of biogas from wastewater AD, that are in various stages of application around the world, including:
• biogas upgrading to biomethane for grid injection,
• generation of green hydrogen, and
• recovery of a high value carbon dioxide side-stream from the biogas upgrading.

Mott MacDonald has worked with Watercare and potential technology partners to review the required technologies, potential carbon reduction opportunities, costs and revenues, and risks to site electricity resilience of using biogas in an alternative way to business as usual. Te Ao Māori principles have been considered at the outset and have guided the discussions. This paper discusses the future use of biogas in the Aotearoa New Zealand context, the primary drivers, the required technologies, and the risks and opportunities.