Combatting global water shortages: Domestic wastewater in underground irrigation

The world’s freshwater supply is in trouble. Agricultural intensification alone accounts for 69% of global freshwater withdrawal, and is putting increasing pressure on our precious and limited resources. Agricultural water shortages are not only increasing in arid areas, but also in more temperate climates. So what are alternatives for meeting both the current and future global agricultural water demand?

Thanks to the soil’s natural purification system, the reuse of treated domestic wastewater in underground irrigation may release some of the pressure on the world’s freshwater supply. But is it safe enough? In her PhD research, Dominique Narain-Ford is exploring its potential.

Polluted wastewater often discharged directly into freshwater sources

At the moment, treated domestic wastewater is mostly directly discharged into surface waters such as rivers and lakes. Conventional wastewater treatment plants are unable to remove many pollutants. This means that treated wastewater contains pharmaceuticals, antibiotic resistant bacteria and a wide range of other chemicals.

The result is surface water that negatively impacts the environment and needs complex treatment to make it fit for consumption. This is particularly a problem during low flow conditions in the summer with usually high irrigation demand, small surface waters can consist primarily of treated wastewater.

Treated domestic wastewater contains a cocktail of chemicals and is mostly directly discharged into surface waters such as rivers and lakes

Use of treated domestic wastewater in agriculture already widespread

Treated domestic wastewater also contains many nutrients that can be used as fertilizer. Its use in aboveground irrigation, sometimes even untreated, is already widespread, particularly in Mediterranean countries, where the availability of freshwater is limited.

However, drawbacks do exist. "Fieldworkers and crops come into direct contact with contaminated water during aboveground irrigation," explains Narain-Ford. "This can pose serious health risks".

Soil acts as a buffer in underground irrigation

When underground irrigation (sub-irrigation) is used to supply treated domestic wastewater to crops there is no direct and potentially hazardous human contact with the water. It also makes optimal use of soil processes that filter, buffer, break down and minimize the spread of chemicals.

Risks not well understood

Despite its promise, the environmental and public health implications associated with reusing treated domestic wastewater for underground irrigation are not fully understood. What exactly happens to the pollutants entering the groundwater? Do they end up in the crops or deeper groundwater?

Under field and laboratory conditions, Dominique Narain-Ford is exploring the fate of a wide mixture of chemicals and antibiotic resistance during soil passage and how they disperse. The field site at Haaksbergen is unique because it’s the only site in the Netherlands that directly reuses domestic wastewater for agricultural irrigation.

The field site at Haaksbergen is unique because it’s the only site in the Netherlands that directly reuses domestic wastewater for agricultural irrigation.

Insights will improve risk assessment tools

These insights will improve risk assessment tools for assessing the opportunities and limitations of treated domestic wastewater reuse in underground irrigation systems. “With this research I hope to provide perspectives on the reuse of treated domestic wastewater in underground irrigation to reduce global water stress caused by the agriculture sector,” says Narain-Ford.

Dominique Narain-Ford is a PhD researcher at the Copernicus Institute of Sustainable Development, Utrecht University. She is also an external PhD researcher at the Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam and a guest researcher at KWR Water Research Institute.