Research and development in the wastewater sector have shown that offsetting greenhouse gas emissions through improved efficiency and resource recovery is possible, but efforts beyond science and engineering are necessary to achieve net zero.
Wastewater treatment is responsible for a relatively small percentage of global CO2 emissions, but it is one of the largest sources of methane and nitrous oxide, which are estimated at 6.9% and 8.1%, respectively, according to the US EPA.
Making the wastewater sector more sustainable, something that authorities around the globe are encouraging, will require reducing and eventually neutralizing those emissions. However, that is easier said than done, not just because reducing emissions per se is hard, but also because even determining what contributes to the emissions is complex.
Photo credit: Ivan Bandura
Two articles in this issue of Nature Water explore this complexity and provide indications on how to improve matters on the path to net zero. The Review by Zhiyong Jason Ren and co-authors examines in detail how different components of wastewater treatment contribute to the emission of greenhouse gases. First, there’s the emission from the actual wastewater treatment process and for the energy used to operate plants. Then, there’s the emission related to the energy purchased externally. Finally, there are emissions related to the value chain, like those occurring during the construction of sewers or the production of chemicals. What makes the path to net zero emission particularly arduous is the complicated way in which such emissions are measured and accounted for. For example, as Ren and co-authors emphasize, only the first two types of emissions are usually monitored and declared, which can substantially underestimate the overall emission.
The Comment by Maria Salvetti discusses the path to net zero emission with a focus on the European Union and the Urban Wastewater Treatment Directive. The legal target for this document is to achieve energy neutrality of wastewater by 2045. As both articles highlight, energy neutrality is the first, essential step to eventually achieve net zero emissions. The main way to achieve energy neutrality is to compensate the energy used to operate treatment plants with the utilization of the energy embedded in wastewater itself, for example with the production of biogas. Eventually, the production of carbon free energy combined with higher energy efficiency, increasing utilization of renewable energy and the optimization of treatment processes can lead to net zero emissions. Although ambitious, the next zero target can be achieved. For example, as recently as September 2024 it was reported that the Danish wastewater utility BIOFOS, operating three major plants in Copenhagen, has achieved climate neutrality.
Achievements like that by BIOFOS are essential steps, but making this the norm will require a long time and huge efforts. One way forward is to consider additional approaches that go beyond the direct improvements in existing infrastructure. As Ren and colleagues suggest, greenhouse emissions are eventually directly related to the amount of wastewater that reaches treatment plants. Thus, promoting reductions in water use and waste can indirectly reduce emissions.
Another, perhaps more drastic approach is to rapidly transition to a more decentralized wastewater infrastructure. Avoiding the construction and running of a large sewage network will inevitably reduce greenhouse gas emissions. That is perhaps oversimplistic, and more work is needed to evaluate the benefits of urban decentralized systems including but not limited to emissions, as well as their feasibility. In this context, the Article by Manel Garrido-Baserba and co-authors in this issue presents a model of a representative decentralized system, evaluating the economic feasibility and showing its potential positive impact on resource recovery and environmental impact.
Net zero in the wastewater sector is achievable, but the road is long. Arguably the biggest obstacle on the way is the slowness with which the wastewater industry evolves, which is admittedly common to all activities based on large infrastructures and is not just a consequence of technical challenges. As highlighted by Maria Salvetti, the role of regulators is and will remain essential in ensuring that targets are met, and cooperation between all stakeholders, from the water and energy utilities to authorities and the public will be fundamental.
Read The tortuous path towards net zero emissions in the wastewater sector