The Potato Is Hungry For Purified Sewage Water

Due to climate change, farmers are increasingly facing water shortages. Irrigating with water from wastewater treatment plants could be a solution; it's available year-round. But is irrigating with sewage water also safe? Water technologist Erik Vriezekolk is taking the first steps to investigate.

Two years ago, researcher Erik Vriezekolk of Wageningen Food & Biobased Research introduced purified sewage water to three crops for the first time: potatoes, onions, and pears. The water came from a wastewater treatment plant (WWTP), which treats both household wastewater—from toilets, showers, and washing machines—and wastewater from businesses and healthcare institutions. After extensive purification, this water is considered clean enough to discharge into streams and rivers. But if it's suitable for nature, is it also safe enough to irrigate food crops? To investigate this, Vriezekolk is collaborating with provincial governments, water boards, agricultural organizations, and water technology companies in the EffluentFit4Food project .

Farmers aren't yet using water from the WWTP, as this is legally prohibited. They do, however, rely heavily on water from ditches and streams, as well as groundwater, to water their crops. This is becoming increasingly problematic, especially during dry periods. In some areas, the water table drops so sharply that households or nature conservation are prioritized over agriculture. Farmers are then temporarily prohibited from watering. In coastal provinces, drought poses an additional problem. Salty seawater penetrates the dry soil, damaging crops. "That's why we're looking at other water sources."

If wastewater is suitable for nature, is it also safe to use it to irrigate food crops?

Vriezekolk: "The government is currently exploring how to legally regulate the reuse of water for irrigation. Our project is providing knowledge for this." Water from the wastewater treatment plant has a significant advantage: it is available year-round. But despite all the purification steps in the WWTP, this water isn't completely clean. It's certainly not as clean as groundwater, which has been further filtered through sand.

80 substances in WWTP water
Water from the WWTP contains low concentrations of substances and organisms that can be harmful to the environment and possibly also to health. These include heavy metals, bacteria, pesticides, drugs, hormones, and medications. The latter category includes, for example, painkillers, beta blockers, and antipsychotics. Heavy metals and bacteria were not included in this study. Of the remaining substances, 80 were found.

50 substances disappeared, 30 others were greatly reduced by additional purification

According to Vriezekolk, the exact concentrations of many substances that pose a health risk are unknown, but additional purification steps are expected to improve safety. Vriezekolk compared water that came directly from the WWTP with water that had been further purified with ozone and UV. These techniques are also used—when necessary—to disinfect drinking water. "Additional purification of the water from the WWTP proved to be quite effective: 50 substances were no longer present in the further purified water, and the concentration of the other 30 decreased by between 20 and 100 percent."

Measuring what the plant absorbs
Vriezekolk then studied the effect of irrigation with this extra-purified sewage on potatoes, onions, and pears. "We chose a scenario in which the plants were maximally exposed to the test water. The soil in which the plants were growing was very coarse in texture. This promotes the absorption of water and nutrients. Marine clay, for example, is much less permeable. Moreover, the crops only received test water, while in reality, crops also absorb rainwater. So, we initially tested for higher exposure than we expect in practice," Vriezekolk explains.

At the end of the growing season, Vriezekolk and his colleagues harvested the potatoes, onions, and pears. They then developed a protocol to test the plant material for the presence of various substances. They ground the crops into a pulp and used extraction methods with different solutions to determine which substances were present in the crop. Vriezekolk: "Developing such a method is a process of continuous improvement; a lot of trial and error. Now we are quite certain that we have the best possible method for accurately testing plant material for various substances. This can be used in practice in the future."

Extra purification makes a difference
The crops were found to absorb small amounts of the tested substances through the water. The potatoes that received water directly from the WWTP still contained eleven of the 80 substances. Eight of these originated from medications, such as the antidepressant Desvenlafaxine and the painkiller and drug Desmetramadol (O-DSMT). However, thanks to the additional purification steps, only four substances were found in the crops, and these occurred only in very low concentrations. "So you see the difference between irrigating with water that had only been purified in the WWTP and water that had been additionally treated with ozone and UV," says Vriezekolk.

Of the three crops, the pear absorbed the least nutrients, which may be due to the longer path water takes from the soil to reach the pear. The onion also absorbed much less than the potato. Vriezekolk doesn't yet know exactly why, but what is clear is that not all crops are the same.

Basis for policy making
The experiment is a first step towards practical application. In a subsequent study, Vriezekolk hopes to set up a pilot with several farmers. "Then we can actually test in the field how water from the WWTP can be used as a water source for crops. For example, on the Zeeland clay, with irrigation only when necessary, because there's been a prolonged rainfall gap." At the same time, other open questions remain. What do the found concentrations of substances mean for health? How many potatoes do you have to eat before it becomes harmful? Logistical questions also remain unanswered. How do you get the water to the right place, and how do you store it safely when necessary?

If we experience more droughts in the future, this water could be vital

Vriezekolk sees his research as a foundation for future policymaking. "If we experience more extreme droughts in the future, this water could be vital. That's why it's good that we're now investigating whether it can be used safely on land."