2024-01-01 14:24:27
As a teenager, Koen Pelsma thought that it was fish that made the bubbles on the water of a canal. Then Pelsma would walk his dog in his hometown of Roosendaal, he would see bubbles popping all the time, and his fellow citizens would say: ‘There you will have the carp again’.
As a newly promoted microbiologist, Pelsma now knows better. “Those bubbles, those are methane emissions.” It’s not just cows that emit this powerful greenhouse gas, or leaking gas pipes in Turkmenistan. Methane also bubbles out of the canals in Dutch cities, a gas that warms the earth much faster than CO2. Organic material is rotting there and causes emissions, especially in the summer.
When Pelsma started his PhD research four years ago, he noticed that hardly any research had been done into this. “I could only find one paper, from Australia.” In the Netherlands no measurements had been taken in the many canal cities. “For water emissions, we mainly look at rural areas, for example at ditches near agricultural land. Research into canals is often about sewage contamination and human safety.”
While Radboud researchers already discovered in 2019 that the city pond near Nijmegen produced significant emissions, probably because nutrients from city life end up in it, which can rot.
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Not included in climate reports
After four years with test tubes on boats in Amsterdam, Leiden and Middelburg, among others, Pelsma has now launched the first major study into emissions from Dutch canals. While agriculture (and especially dairy farming) accounts for about 70 percent of Dutch methane emissions, the canals probably account for somewhere between 0.1 and 2 percent. “That’s not much. But more than has previously been demonstrated. And it is not included in the current climate reports.”
Methane expert Hugo Denier van der Gon (TNO) thinks it is a ‘good and useful’ study. Globally, a significant portion of methane emissions come from gas wetlands in the tropics, where organic matter lies rotting in the water, microorganisms eat it and emit it as methane.
Research into these types of processes is also being conducted in lakes, swamps and ditches in the Netherlands, but the canals were a blind spot. Van der Gon thinks this is because canals have become so much cleaner over the last two hundred years. “They used to be the city’s sewer and gases would continuously bubble up. You heard non-stop bubbles when you walked past it and became deathly ill if you fell into it. Now we are swimming in it.”
The canals are still not spotless, and the sewers also sometimes overflow. As a microbiologist, Pelsma was especially curious about what is happening beneath the water surface. “It is a dynamic environment: people throw food in it, sail boats. What do microorganisms do after a pride, when a lot of beer ends up in the canal for a day? Do they convert that into nitrous oxide, or nitrogen?” The microbiologist himself found creatures in the water that convert organic material into methane, as well as slime on the quay that eats and breaks down that methane. “A natural filter system.” Pelsma used measuring equipment to determine that these processes account for up to 0.13 percent of Dutch methane emissions.
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My device went into tilt
But by far the largest amount of methane comes from the bubbles that float to the top. “Especially in the summer, when the water is warm.” Methane then escapes far too quickly for the slime, and in far too high concentrations for Pelsma to measure. “My device then went into tilt.”
Pelsma’s study still has some shortcomings, and is mainly a starting point for further research into canal emissions. For example, the PhD student mainly measured in the spring, while emissions are greatest in the summer. Pelsma himself now works as a consultant at Deltares and cannot continue his passion for canals for the time being. “I hope that the next PhD student will measure those bubbles.” Who knows, he suggests, canals may turn out to cause up to 4 percent of Dutch methane emissions. “That wouldn’t surprise me.”
Can cities do something about emissions from their canals? Denier van der Gon emphasizes that the focus should be on agriculture. “That’s where the vast majority of the methane comes from, so let’s take action there.” Pelsma sees opportunities in cities to grow extra methane-filtering slime using artificial structures on the canal walls. But it makes more sense to continue to improve water quality. “This solves various problems: safety for people, the quality of life for fish and those emissions.”
Residents of cities can also contribute. Do not throw an apple in the canal, but in the trash can, so that it cannot rot and gas. And go sailing. “Boats mix oxygen in the water. This oxygen does not break down organic material into methane, but into the less strong CO2.” Then take a simple electric whisper boat, says Pelsma. It is counterproductive with a polluting sloop.
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