The new technique reliably measures whether rivers or lakes have run out of air

International research shows that fresh water contaminated with fecal material can be determined faster and more reliably with the new technique. Credits: Jiao et al., Doi: 10.1126 / sciadv.abc7318. This work is licensed under CC BY-NC.

International research shows that fresh water contaminated with fecal material can be determined faster and more reliably with the new technique.

When wastewater from villages and towns flows into rivers and lakes, large amounts of fat, protein, sugar and other organic substances that contain carbon end up in nature along with fecal matter. These organic substances are broken down by oxygen-consuming bacteria. The greater the amount of wastewater, the better the bacteria thrive. This, however, means that the oxygen content in the water continues to decrease until finally the fish, shellfish or worms are literally left without air. This has created low-oxygen death zones in many rivers and lakes around the world.

So far, there has been no gold standard for measurements

To measure how much water is contaminated with organic matter in feces, government agencies and environmental researchers regularly take water samples. One of the widely used measurement methods uses a chemical reaction to determine the content of organic substances. As an international team of scientists now shows, this established method provides values ​​from which it is difficult to derive the actual degree of water pollution. Prof. Helmuth Thomas, director of the Hereon Institute for Carbon Cycles, is also a contributor to the study, which is now published in a scientific journal Scientific progress. “Therefore, we are introducing a new method to achieve much more reliable measurements in the future,” he says.

By the usual measurement method, water samples are mixed with the chemicals permanganate or dichromate. They are particularly reactive and decompose all organic matter in a short time. The amount of permanganate or dichromate consumed can then be used to determine the amount of organic matter in the water sample.

Experts call this measurement the “chemical need for oxygen,” the CCP. The problem with COD measurements is that they do not distinguish between organic matter that ends up in water in a canal and those that form naturally – such as lignin and humic acids – that are released when wood decays. This means that water pollution can hardly be distinguished from the natural content of organic substances.

“For the Han River in South Korea, for example, we have shown that pollution by organic matter from wastewater has decreased in the last twenty-five years. COD measurements continue to show high values ​​as before, ”says Helmuth Thomas,“ because here natural substances make up a large part of the organic matter in water. “

Complicated biological analysis

But how can more real pollution be measured more reliably? The biological method of measurement has been established here for decades, but is much more complex than the CCP method and is therefore less commonly used by government bodies and research institutions. In this case, a sample of water from a river or lake is taken and the oxygen content in the water is measured as the initial value. Another “parallel sample” was immediately hermetically sealed. Then this water sample is rested for five days. During this time, the bacteria break down the organic substance, gradually consuming oxygen in the water. After five days, the tank is opened and oxygen is measured. If the water contained a lot of organic matter, then the bacteria were particularly active. Oxygen consumption was then correspondingly high. Experts refer to “biological oxygen demand” (BOD) in this measurement.

“Measuring BOD is far more accurate than COD, because bacteria preferably break down small organic molecules from wastewater, but leave natural ones, such as lignin, intact,” says Thomas. However, BPK measurement also has its drawbacks. On the one hand, the BOD measurement takes five days, while the COD value is available after a few minutes. On the other hand, when filling, storing and measuring water samples, care must be taken not to let oxygen into the sample from outside air and not to falsify the measured value. “Only a few people with a lot of lab experience have mastered how they can fully measure BPK,” says Thomas. “Therefore, government bodies and researchers still prefer COD today despite greater uncertainties.”

Faster and more reliable method

Helmuth Thomas and his team therefore introduce an alternative method that improves conventional BOD measurement. The advantage of the method is that only one water sample is needed which closes immediately and measures oxygen consumption without disturbing the sample. Therefore, it is unnecessary to reopen the oxygen content sample after five days. This prevents the sample from coming into contact with atmospheric oxygen again.

With the new approach, optical fibers are inserted into the sample container as soon as the water sample is filled. Through this fiber, the oxygen content can be continuously measured directly in the sample using optical effects. Thomas says, “We can continuously measure the oxygen content and get a far more accurate picture of oxygen consumption by bacteria.”

The first tests showed that a significant result was available after only forty-eight hours, something that significantly speeds up the measurement of BOD. Overall, the optical method makes BOD measurements not only more reliable but also faster. Helmuth Thomas assumes that the new method will be established in the coming years as a new standard that will replace both CCP and classical BPK measurements. For example, in the future it will be possible to reliably determine whether water pollution control measures are indeed successful.

Reference: April 14, 2021, Scientific progress.
DOI: 10.1126 / sciadv.abc7318