Laughing gas that is harmful to the climate – Swiss sewage treatment plants do not clarify enough – Knowledge


Sewage treatment plants emit enormous amounts of climate-damaging nitrous oxide – but our WWTPs don’t even eliminate half of their nitrogen. Countermeasures are now being put in place.

What Swiss sewage treatment plants produce in terms of climate-damaging gases, de facto nitrous oxide, is not a “quantité négligeable”.

On the contrary: the nitrous oxide from the clarifiers of the wastewater treatment plant (ARA) alone corresponds to a good one percent of the total greenhouse gas emissions in Switzerland, as a recent study by the Swiss water research institute EAWAG showed.

Stricter regulations in the EU

But the sewage treatment plants in Switzerland also have a problem with the nitrogen that causes nitrous oxide – a bigger problem than the countries of the EU.

In the EU, according to one, sewage treatment plants have to Report of the European Environment Agency EEA Eliminate at least 75 percent of their nitrogen – a target that countries like Austria and Germany meet.

In Switzerland, however, the 700 to 800 ARA eliminate significantly fewer. According to the FOEN, just 47 percent. The rest flows into streams and lakes.

What happens in an ARA?

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In a wastewater treatment plant (WWTP), the faeces are roughly removed with rakes. After that, bacteria in particular take over the cleaning. They eat the broth and turn it into clean stuff or stuff that can be further cleaned in other tanks with different bacterial mixes.

At the end, in the secondary clarifier, the little cleaning helpers are allowed to sink to the ground using gravity. So you can separate them from the clarified water. Part of this “activated sludge” is retained for the next work assignment. Most of it is incinerated as sewage sludge.

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Nitrogen pollutes water bodies

Every year, 18,300 tons of nitrogen in the form of nitrate are released into the water from the ARA. That is less than the 32,000 tons of nitrate that flow into water bodies from overfertilized agricultural soils. But enough to cause fish kills and algae blooms or to pollute the groundwater in some areas. And: Large amounts of nitrogen in the clarifiers lead to high nitrous oxide emissions.

ARA also under political pressure

“The sewage treatment plant operators and responsible authorities have to think about how they can get the problems under control quickly,” says Adriano Joss from the water research institute EAWAG.

There is also political pressure: the National Council and the Council of States have one Motion adopted, which obliges the ARA to significantly reduce its nitrogen inputs. The Federal Office for the Environment (FOEN) is currently working on amending the Water Protection Ordinance accordingly.

What is laughing gas?

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Nitrous oxide (N2O) is a highly potent greenhouse gas. Although it is by no means as common as CO2, it is 300 times more harmful to the climate. In sewage treatment plants, nitrous oxide is produced by bacteria that break down and convert nitrogen.

But how can the environmental and climate impact of nitrous oxide and nitrogen from the ARA be reduced? According to the FOEN, the decisive factor in the sewage treatment plants is “the standard of development of the biological treatment stage”.

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In concrete terms, the bacteria in the sewage treatment plants are intended to ensure that nitrogen is eliminated “as extensively as possible”. “Some plants will therefore have to be built larger in the future.” Experts estimate that with such an expansion, today’s nitrous oxide emissions could be roughly halved.

Less laughing gas thanks to “ball bacteria”?

With a new process, the ARA’s nitrous oxide emissions should even drop to almost zero. Among other things, this is being researched at EAWAG by Nicolas Derlon.

In the sewage treatment plant, bacteria are used that clump together into small globules. Such granule bacteria work more efficiently than today’s flaky ones. You should be able to eliminate more nitrogen.

The low nitrous oxide emissions were confirmed in a plant in the Netherlands. However, the process is still in development. It also has to prove itself in Switzerland, where there are three pilot plants today.

Implementation takes years

One thing is certain: the nitrous oxide from the clarifier will not disappear into thin air overnight. It will probably take about ten years to halve the nitrous oxide emissions and ensure more efficient nitrogen reduction in the ARA.

Sewage treatment plants are large and durable. “It is realistic to expand or convert a plant after a lifetime of 20 years,” says Eberhard Morgenroth from EAWAG and ETH Zurich.

An earlier demolition makes little sense in an overall ecological view, including the gray energy in the building material.

Of course, with a view to climate change, one could push for a faster reduction of nitrous oxide. But sewage treatment plants cost a lot of money – money that the authorities are talking about. Ultimately, therefore, it is a political decision how quickly progress can be made with cleaner ARA.

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