Reduced nitrous oxide emissions thanks to tailored technologies

Though we were unaware of it just a few years ago, it is now clear that many wastewater treatment plants generate nitrous oxide emissions, some of which are very high. All in all, we can assume that nitrous oxide emissions are responsible for around two-thirds of the greenhouse gas emissions at Swiss wastewater treatment plants. The plants currently have four options for reducing these emissions. On one hand, there is the approach of preventing the formation of nitrous oxide. This includes separate chemical treatment of the dirty water (stripping), dynamic regulation and off-gas measurement (DynARA) and the replacement of the SHARON procedure. On the other hand, there is the possibility of burning the gas at very high temperatures after it has been created. Not every measure is suitable for every plant.

When Infraconcept started its mission to reduce nitrous oxide emissions at wastewater treatment plants in 2017 together with its partners Eawag and the Altenrhein plant, research in this field was in its infancy. Separate chemical treatment of the dirty water (stripping) was the only technology that was known and effective at the time. Since then, new emission sources have been discovered and new technologies developed. Since 2019, stripping has been registered at the FOEN for funding by the KliK Foundation. In 2022, we started revising the programme to include the three new measures detailed above. These were included in the support programme one year later.

We developed them together with Eawag and in model plants according to concrete project ideas, where we were able to carry out tests, measurements and analyses.

By opening the door to additional technologies, we wanted to enable further treatment plants to take effective climate protection measures and to become part of the support programme. In particular, many wastewater treatment plants are unable to implement stripping as their facilities are too small. The expansion of the programme has given the branch a real boost. Six further treatment plants have now registered and are currently working on putting projects into action.

Initially, we estimated a possible saving of 15,000 tonnes of CO₂ equivalent per year through the support programme. However, we are already seeing that the potential for climate protection can be significantly increased with the three new measures. In the coming years, we expect to see emission reductions of 40,000 tonnes of CO₂ equivalent per year, and more.

The chemical treatment of dirty water is suitable for large-scale, regional treatment plants with central sludge disposal – in other words, those with particularly large volumes of third-party sludge. DynARA is suitable for activated sludge plants of all sizes with a denitrification capacity below 65 per cent. Replacement of the SHARON procedure is suitable for existing treatment of dirty water using the SHARON procedure and combustion for fixed-bed biological systems and two-stage anammox plants.

Compared to other support programmes, the technologies in the nitrous oxide programme are complex. Each wastewater treatment plant is one of a kind and works differently. Furthermore, nitrous oxide has a very complex dynamic in terms of its creation and prevention. We support the wastewater treatment plant in analysing and choosing the best possible technology for their individual needs, and also in planning the project. Due to the high degree of complexity, this advice is crucial in ensuring the measure is a good fit for the plant, works properly and is eligible for funding. Very important here is that the registration for the support programme must take place before any investments are made!

Each measure has its own complexities, whether financial – especially stripping and combustion – or in operation, as is the case with DynARA. The plants need trained operating personnel on site and, in some cases, external support from engineering offices. Providing this knowhow is a challenge of its very own.

In the field of nitrous oxide, the technologies that are now known and well established are already part of the support programme – the goal now is to put them into action. I see potential in the combustion processes and in the development of separation technologies.