Nitrous oxide is and has been going under the radar for quite some time in the big climate debate despite its determining role, and on a global scale it is still overshadowed by carbon dioxide and methane. Nitrous oxide is documented to have great effect on our climate, but we still need more knowledge on the extent of emissions and how to reduce them.
In Denmark, the wish to reduce nitrous oxide emissions is reflected in politics as well. Following an ambition of having a climate- and energy neutral water sector by 2030, the Danish Environmental Agency has encouraged Danish water utilities to collect and report their data to the Paris Agreement. That includes data on the current as well as expected energy consumption and greenhouse gas emissions. After 2025, the Danish Government also expect to introduce taxes on nitrous oxide emissions from wastewater treatment plants with at least 25.000 PE. These initiatives will hopefully motivate the utilities to monitor and decrease their emissions.
Nitrous oxide is developed during the biological treatment process and released into our atmosphere. The amount of nitrous oxide developed and released varies greatly from treatment plant to treatment plant. Because of this, more Danish wastewater treatment plants are starting to monitor their nitrous oxide emissions and collect data.
Internationally, the Danish Water Sector is in the lead when it comes to working with nitrous oxide emissions. Several big treatment plants are actively working towards decreasing their emissions by monitoring nitrous oxide activity, and Danish legislations also support the ambition. Despite these initiatives, there is still a big need to measure the extend of nitrous oxide emissions and to establish strategies on how to decrease these emissions in an efficient and financially sound way.
‘We do have some data on nitrous oxide emissions from Danish wastewater treatment plant, but in Denmark we don’t have a standardized way of designing and operating treatment plants. Because of the big differences from plant to plant, the data we have can’t be used to conclude on nitrous oxide emissions in general. Big variation in design and operation means big difference in nitrous oxide emissions. That means we don’t know how much of the overall nitrous oxide emission comes from wastewater treatment plants, or when the big fluctuations happen. So, we need more data,’ explains Sylvie Braekevelt, Wastewater consultant from Ramboll Water.
There are several ways to measure nitrous oxide emissions on wastewater treatment plants, but Unisense Environment are the only ones to offer an operationally stable and commercially available device that can measure the level of nitrous oxide directly in the wastewater. Based on numbers from Unisense, the current number of installations of monitoring devices are less than 500 on a global scale. However, there is an increase in the demand of this type of technology, which is good news according to Sylvie Braekevelt. Sylvie believes that the first step is to expand data monitoring to more treatment plants and ensure that data is monitored over longer periods of time, in Denmark and globally.
‘Previously, process engineers designing treatment plants was not interested in including nitrous oxide emissions as a parameter in their design calculations. The focus was rather on purification efficiency, energy consumption, price, and process optimization. Luckily, nitrous oxide is now slowly being established as a parameter that we need to consider and optimise. One of the ways to decrease the nitrous oxide emissions is optimising the process management. With data you can gain an overview of the load fluctuations and thereby predict future fluctuations and adapt the aeration along the way. Meaning that the process management can be optimised to adapt to the changing needs from day to day.’
In Ramboll, we also work directly with the treatment plants to help them monitor and report their emissions. In Gilleleje’s new 50.000 PE treatment plant, north of Copenhagen, we have established nitrous oxide monitoring in the two large process tanks. Under construction of the plant as well as during process management, there has been a great focus on decreasing the level of nitrous oxide.
‘With its 8,3 meter deep process tanks, the new treatment plant in Gilleleje is bigger and deeper than the usual municipal treatment plant, which together with process management optimisation, has a great impact on lowering the nitrous oxide emissions’, civil engineer Bjarne Hjorth Pedersen, Market Manager in Ramboll Water’s wastewater treatment department and project manager on Ramboll’s deliverables for Gilleleje Treatment Plant.
Though the data from Gilleleje Treatment Plant are not quite ready to conclude majorly upon, they have so far confirmed the expectations that was the reason behind the design of the treatment plant. Working with online monitoring devices has also provided new and interesting information that was not considered in the design phase and now can be used for further process management optimisation.
The water sector is thus quite far ahead with technology for online monitoring and collection of data. In 2022, the amount of data in Denmark is expected to be big enough to conclude upon and thus share useful knowledge. Methods for decreasing nitrous oxide is, however, still under development.
This article will be followed by a separate and more detailed article about the data from Gilleleje Treatment Plant.
Virtual tour of Gilleleje WWTP