Pure wastewater for irrigation: prevention is better than cure

Attention must be paid to the accumulation of salts and other harmful substances in the soil. Not only are these bad for plants and our health, but in the long run, if not managed properly, they can damage the soil itself.

In June 2022, Aquafin suddenly stopped providing treated wastewater from wastewater treatment plants to farmers. In 2023 he is gone Water reuse regulation, a new European Union regulation, comes into force. She wanted to promote the sustainable use of circular water in agriculture by clarifying the quality requirements that this water must meet. A limited sample from 22 to 23 in Flanders showed that levels of PFAS and fecal bacteria in several wastewater treatment plants were above the limit value of European regulations. After all, PFAS are not effectively removed from water using current purification technologies. In addition to micropollutants, many questions remain about the effects of other, less problematic components of treated wastewater when they accumulate in soil: salts and organic matter.

Treated wastewater has been widely used elsewhere in the world for some time. This concerns not only treated wastewater, but also water used in industrial processes. Worldwide, treated wastewater is used for irrigation on approximately 10% of the irrigated area. Chile, Mexico, Israel, Egypt and Cyprus are just a few of the many countries where treated wastewater is regularly used, each with its own regulations regarding its quality. Therefore there is already experience and there should be many plots of land on which the effect of long-term use of this type of water on soil health can be studied and calculated.

Lin Wang, a researcher at Ghent University and ILVO, investigated what we can learn from studies conducted in countries where people have been irrigating with treated wastewater for some time. After an initial search, I found nearly 400 scientific studies, but it quickly became clear that the majority of studies did not delve into long-term effects on soil health or were not conducted in real fields. Only about 24 studies have addressed these aspects. So we still know little about the impact of this type of water on soil health. More research is needed!

Statistical analysis of selected studies showed that irrigation with treated wastewater always increases the salt and sodium content in the soil. “This is worrying, because too much salt is harmful to plants and the soil structure itself can be affected as well. “Soil salinization can have irreversible consequences, causing the soil to weaken and become unsuitable for agriculture,” says Wang. Naturally, salts accumulate less when the water undergoes further purification steps.

There are also many organic compounds in pure wastewater. This can be positive, because in this way we add organic matter to the soil. Studies also showed this increase in organic matter, but at the same time it was found that the soil was less permeable to water. Organic matter usually has a positive effect on the structure of the soil and thus on the way it absorbs and retains water. It turns out that this does not apply to irrigation with pure wastewater, perhaps because small organic molecules cause clogging of surface pores and also become hydrophobic during drought.

It is therefore right that our governments are cautious about allowing this type of water on a large scale to irrigate fields. However, over-regulation also poses a risk, because this water source will become increasingly important due to climate change, which means we will have to deal with droughts more often. Every drop counts! There are also many good examples, such as irrigation with wastewater from the frozen vegetable company Ardo. So we have to look for a sustainable solution to reuse water more.