‘We need to be far more careful with water’
4 oktober 2023
What exactly was the subject of your research?
‘I researched whether treated waste water from waste water treatment plants can be used for sub-surface irrigation of agricultural land. If treated waste water from these plants is used in aboveground irrigation, a relatively large amount of water evaporates. And the substances that are still in the water can end up on the plant. Because not all substances and microorganisms can be filtered out of the water; treated waste water still contains medicine residues, PFAS, E-coli and residues from cosmetic products. We wanted to find out whether, with sub-surface irrigation, some of these residues would be broken down in the soil.’
So, treated waste water from a waste water treatment plant is still dirty?
‘A waste water treatment plant filters out the majority of waste from the sewage. These components then sink to the bottom, together with many other potentially harmful substances, ending up in the sludge. With the existing methods, it is true, there are some substances, bacteria and other pollutants that we can’t filter out.’
And, as a result, treatment plants discharge polluted water?
‘It’s getting more and more difficult to purify water effectively because so many different substances that don’t belong to it end up in it. At the same time, over the next few years, dry periods will increase in the Netherlands. So we have to be far more careful with the way we deal with water. Not only in terms of quantity, by retaining and storing more water, but also in terms of quality. We don’t have enough good quality water.’
How does sub-surface irrigation come into it?
‘Firstly, we investigated whether sub-surface irrigation is effective, because you inject the water into the soil rather than spraying it on the plant. To do this, we used the drainage pipes that were already there, about a metre below the surface, where there is also shallow groundwater. Supplying water to a system like this is known as reverse drainage, or sub-surface irrigation. This allowed us to supplement the shallow groundwater during the dry season. Secondly, we were interested in what would happen to the untreated substances from the waste water, which could pose a risk if they were to end up in the root zone of the plant or in the deeper groundwater.’
Is sub-surface irrigation effective?
‘It really depends on the circumstances, not all areas and soil types are equally suited. For higher sandy soils, where the trial field for my research was located, sub-surface irrigation can offer a solution. With a maximum irrigation zone of five kilometres, in the Netherlands this method could supply an eighth of the water requirements of agriculture.’
With sub-surface irrigation, would a lot of untreated substances from the waste water end up in the soil?
‘It’s difficult to say. In our research we saw that, during the irrigation period, some substances accumulated at the points where the water dripped from the pipe into the soil. When it then rained in autumn and winter, those substances washed into the stream into which the waste water treatment plant also discharges. In other words the system essentially rinsed itself clean. The substances were also partially decomposed. By calculating the dilution factor using a test substance, we found that there are definitely decomposition processes taking place in the soil. This mainly happens when it is extremely dry, whereas, clearly, with irrigation you maintain the groundwater level.’
The soil can purify water?
‘We don’t exactly know yet for these conditions. Sandy soils, like dunes or riverbanks, are already being used for water purification. In that case, decomposition generally takes place under sub-aerobic conditions, whereby oxygen is available. Our trial setup primarily involved anaerobic conditions, whereby hardly any oxygen is available This can give rise to other breakdown products, that we don’t yet know about. Follow-up research will indicate what exactly these are. One way or the other, it is clear that nowhere near all substances decompose. PFAS, for example, just stays in the water, we can’t get rid of it using sub-surface irrigation systems.’