Climate protection in wastewater treatment plants

The plant in Altenrhein on Lake Constance is a prime example of why wastewater treatment plants emit harmful greenhouse gases and which measures can be implemented to counteract this.

The motto of the Altenrhein Wastewater Association (AVA) is very apt. "We clear things up" is a simple way of highlighting what a wastewater treatment plant does – namely taking something dirty and making it clean again. The fact that harmful greenhouse gases are emitted during this process was not known for a long time and thus also not examined. It has now become clear that the some 650 wastewater treatment plants in Switzerland are responsible for 1 to 3 per cent of national greenhouse gas emissions – mainly nitrous oxide. According to 14 longterm measurements carried out as part of the "N2Oara" project on various types of treatment plants, researchers from Eawag and the ETH were able to demonstrate in 2022 that around one-fifth of all nitrous oxide emissions in Switzerland originate from treatment plants.

The good news is that wastewater treatment plants can take various measures to either prevent or eliminate greenhouse gas emissions before they are released into the atmosphere. These climate protection measures are currently voluntary. They are in part very complex and are not cost-effective. However, there is the possibility of taking over the investment and operating costs of such measures. Together with its partners South Pole and Infraconcept, the KliK Foundation runs two support programmes for measures aimed at reducing methane and nitrous oxide emissions at wastewater treatment plants. The principle is for the KliK Foundation to purchase the certifications issued for emission reductions that have been achieved and certified by the Swiss Confederation. The plant run by the Altenrhein Wastewater Association takes part in both programmes. When visiting the plant, it quickly becomes apparent how complex and technical "clearing things up" is in reality.

Nitrous oxide (N₂O)
N₂O, also known as laughing gas, is the third most important greenhouse gas after carbon dioxide and methane. These gases have a different impact on the environment. The global warming potential of the different gases is measured uniformly in CO₂ equivalent (CO₂e). Due to its high global warming potential – 265 times higher than that of CO₂ over a 100-year period – nitrous oxide reduction has major leverage. In addition to N₂O, wastewater treatment plants also emit two other greenhouse gases (although on a much lower scale): carbon dioxide (CO₂) and methane (CH₄). The global warming potential of CH₄ is almost 30 times higher than that of CO₂.

Die grauen Behälter (oben) enthalten Faulwasserfilter aus Membranen (unten).

Wastewater treatment and greenhouse gas emissions

All wastewater treatment plants and their supply systems work according to a similar principle: The wastewater comes from our sanitary installations – together with wastewater from roads, public spaces and roofs – to the wastewater treatment plant through the sewage system. Here, the wastewater is first treated mechanically, then biologically and chemically, before being fed into the nearest moving body of water. In the case of Altenrhein, this is Lake Constance. On a cloudy yet dry Monday morning, Christoph Egli, Managing Director at AVA, leads us across the plant directly to the basin in which the biological cleaning takes place. The dirty water has already been cleaned of larger particles and sludge, but still contains dissolved pollutants – including nitrogen compounds. Billions of microorganisms now break these down with the addition of oxygen. This process causes the majority of nitrous oxide emissions.

Climate protection and fertiliser production

This is where the climate protection measures come into play. One possibility is additional chemical treatment of the dirty water in order to remove the nitrogen in the form of ammonia. The ammonia gas is then dissolved in sulphuric acid as ammonium sulphate – a highly effective liquid fertiliser used in agriculture. Known as stripping, this measure has been supported by the KliK Foundation since 2019. The stripping system at the plant in Altenrhein has been in operation since 2021 and features complex technology – known as the membrane procedure. Together with the wastewater treatment plants in Opfikon and Yverdon, Altenrhein is the only other plant in Switzerland to use such a procedure for the chemical treatment of dirty water.

We now leave the basin used for biological cleaning and take a look at the stripping plant in a neighbouring building. Stretching over two floors, the installations are extremely technical. You no longer have the impression that you are standing in a wastewater treatment plant. "This technology doesn’t have much in common with the normal work seen in a wastewater treatment plant," confirms Christoph Egli. It would be more at home in a chemical factory. Operation and maintenance thus pose a challenge. Looking after the plant needs a great deal of know-how. Furthermore, hazardous chemical substances are used that must be handled with great care. Christoph Egli shows us an open cylindrical container with countless finely layered membranes. These membranes consist of extremely thin threads that pass through the dirty water. As the membrane material is hydrophobic – in other words, repellent to water – only nitrogen escapes following the various chemical procedures, which is then bonded to form liquid fertiliser.

Stefan Binggeli (links) und Christoph Egli führen gemeinsam durch die Anlage.

Significant investments are paying off

As Christoph Egli explains, the stripping plant posed numerous challenges in Altenrhein. "On one hand, only very little of the technology had been established at the outset. There were few providers on the market. On the other hand, a plant like this needs a lot of space and a building with sufficient height. The financing also caused us some problems. Without the support programme from the KliK Foundation, financing of the plant would not have been possible. We are not yet able to sell the fertiliser that we produce in a profitable way."

The measures for reducing nitrous oxide emissions contribute to climate protection, with up to 68 per cent fewer emissions measured. Thanks to these emission reductions, the wastewater treatment plants also benefit from funding that covers the high investment costs. Moreover, revisions to the Waters Protection Ordinance are also being drawn up in Switzerland. Once these come into force, wastewater treatment plants will be obligated to introduce certain emission reduction measures. With this in mind, it is a good idea to be aware of the possible options at your own plant and to implement measures in good time.

Methane collection and other energy efficiency measures

Measures have also been implemented in Altenrhein to deal with the methane emissions that occur during treatment and fermentation of the sludge. These measures are also being supported by the KliK Foundation. The next, and last, stop on our tour takes us to where this takes place – namely up on the roof, where we find ourselves standing next to four imposing round towers featuring facades clad in Swiss wood. "This is where the sludge is stored and ferments," explains Christoph Egli as he takes us even higher to the roof of one of the towers. Instead of an opening where methane used to escape, the fermentation towers now all feature a dome that collects the gas. This is then fed through pipes into two in-house cogeneration plants, where it is used to produce electricity for plant operations and for district heating.

Moreover, the wastewater treatment plant in Altenrhein has also seen an increase in its energy efficiency. Wherever possible, it uses all sources of waste heat and energy. The cogeneration plants are the most important providers of heat at the plant. In order to utilise the recovered waste heat to its full potential and increase self-supply with waste heat from the engines, the plants have been equipped with additional heat pumps. The wastewater – with temperatures between 7 and 20 degrees – is also used as a source of heat. After cleaning, the wastewater passes a heat pump that extracts heat from the water. This is used for heating the fermentation towers and as a source of heat for drying the sludge. Last but not least, three of the flat roofs at the wastewater treatment plant are also equipped with a photovoltaic system.

Programme for nitrous oxide reduction
Nitrous oxide (N₂O) – also known as laughing gas – from wastewater treatment plants causes between 0.3 and 1.4 per cent of greenhouse gas emissions in Switzerland. The majority of emissions at wastewater treatment plants are caused in the process of biological treatment, and a smaller part in the burning of sludge. A range of measures are used to reduce nitrous oxide emissions, which are supported by the KliK Foundation. In addition to the stripping process detailed in the text, dynamic regulation and off-gas measurement (DynARA), the replacement of the SHARON procedure, and the combustion of waste heat (regenerative thermal oxidation, RTO) have also been supported since 2023. Infraconcept is the owner and developer of the programme.
Programme for methane reduction
In communal wastewater treatment plants, methane is emitted at various stages of the process – particularly when treating and storing sludge. The goal of the programme is to prevent these emissions by supporting the plant operators in implementing measures to reduce methane. The relevant process stages at the treatment plant are equipped with a gas collector (e.g. sludge stack cover). The flow of exhaust air is collected and then eliminated as part of a combustion process, fed into a biogas network, or used for the operation of cogeneration plants. 24 wastewater treat- ment plants are currently taking part in the support programme. The company South Pole is the owner and developer of the programme.