Potatoes are desperate for treated sewage

Erik Vriezekolk, a Water Technology researcher at Wageningen Food & Biobased Research, first gave treated sewage to three crops — potatoes, onions and pears — two years ago. The water came from a sewage treatment plant that treats both household wastewater — from toilets, showers and washing machines — and wastewater from businesses and care institutions. This water undergoes an extensive purification process, after which it is considered clean enough to be discharged into streams and rivers. But if it is good enough for nature, is it also safe enough for the irrigation of food crops? Vriezekolk is investigating this in EffluentFit4Food, , a project in which he is working with provincial authorities, water boards, agricultural organizations and companies that supply water technology.

Farmers barely make any use of water from sewage treatment plants at present because this is not allowed by law. They do make plentiful use of water from ditches and streams and of groundwater to water their crops, but this is increasingly problematic during dry periods. In some areas, the groundwater table falls drastically during such droughts, and households or nature are then given precedence over agriculture. That means farmers can’t water their land for a while. Drought in coastal provinces leads to an additional problem as saline seawater is then able to penetrate the dry soil. That harms the crops. ‘That is why we are looking for alternative sources of water.’

Vriezekolk: ‘The government is currently looking at how to design legislation to allow the reuse of water for irrigation. Our project provides input for that.’ A key advantage of water from sewage treatment is that this water is available year round. But it is not completely clean, despite all the purification steps in the treatment plants. It is certainly not as clean as groundwater, which has been filtered through the sand.

Water that comes from sewage treatment plants still contains low concentrations of compounds and organisms that are harmful to the environment and possibly also to human health. These are heavy metals, bacteria, pesticides, drugs, hormones and medicines. The medicines include painkillers, beta-blockers and antipsychotics, for instance. The current study looks at all these substances, with the exception of heavy metals and bacteria. A total of 80 such substances were found.

There are many where we don’t know exactly what quantities are harmful to health, but additional purification processes would probably make the water safer. To test this, Vriezekolk compared irrigation using water taken directly from the sewage treatment plant with that using water that had undergone additional purification with the aid of ozone and UV. These techniques are also sometimes used to purify drinking water. ‘It turns out the extra purification of the water from sewage treatment plants is very effective: 50 substances are no longer found at all in the additionally purified water and concentrations of the other 30 compounds had fallen by between 20 and nearly 100 per cent.’

The sewage water that had undergone this extra purification was tested on potatoes, onions and pears. The plants were in pots at an experimental station close to Wageningen. ‘We decided on a setup in which the plants were given maximum exposure to the water we were testing,’ explains Vriezekolk. ‘So the soil the plants were in had a very coarse structure, for example. That makes it easy for the plants to absorb the water and compounds it contains. Sea clay, for instance, is much more impenetrable. Furthermore, the plants were watered exclusively with the test water whereas in practice they would receive rainwater too. In other words, first we are testing a higher exposure than we would expect in practice.’ 

At the end of the growing season, Vriezekolk and his colleagues harvested the potatoes, onions and pears. Then they drew up a protocol for testing whether the plant material contained the various compounds. To do this, they ground the crops to a pulp and performed extractions with various solvents to determine which substances were in the crop. Vriezekolk: ‘Developing a method like this is a question of making improvements again and again in several iterations. Now we are fairly certain we have the best possible method for accurately testing the plant material for the presence of the various compounds. This method can be used in practice in the future.’

It turned out the crops did absorb small quantities of the study compounds via the treated water. The potatoes that were given water taken directly from the sewage treatment plant contained 11 of the 80 compounds. Eight were from medicines used by humans, such as the antidepressant desvenlafaxine and the painkiller and drug metabolite desmetramadol (O-DSMT). Far fewer compounds were left in the water after the additional purification, and they were only found in the plants in very small quantities. ‘In conclusion, you see a difference between irrigation using water treated only in the sewage treatment plant and water that received an extra treatment with ozone and UV,’ says Vriezekolk. 

Of the three crops, uptake of the compounds was lowest in pears. This may be related to the greater distance the water has to travel to reach the fruit from the soil. However, onions also absorbed far less than potatoes. Vriezekolk does not yet know why that is exactly, but there are clearly big differences between crops.

The experiment is the first step towards practical implementation. Vriezekolk hopes to follow this with a pilot project in collaboration with a group of farmers. ‘That will let us test properly on the farm how water from sewage treatment plants can be used to irrigate crops. This will be in clay soil in Zeeland, and applying irrigation only as necessary when it hasn’t rained for a long time.’ Other questions also need answering. What health implications do the detected concentrations of the compounds have? How many potatoes do you need to eat before it’s harmful? There are also logistical questions. How can the water be transported to where it is needed and how can it be stored safely?

Vriezekolk sees his research as providing a basis for future policy formulation. ‘If we end up with extreme dry weather more often, this water could be a vital resource. That’s why it is important to investigate now whether it can be used safely on farms.’  

• Naar een veilige en duurzame irrigatie met effluent uit rioolwaterzuiveringen (RWZI’s)