Matching agricultural freshwater supply and demand: Recycling wastewater through subirrigation
Agriculture is confronted with increasing challenges associated with climate change such as prolonged periods of drought alternating with more intensive rainfall events, and more broadly with water scarcity issues which result in increasing crop damages. Strategies are being developed to control these risks and to secure long-term supplies of water. This requires new approaches to water management practices in which various sectors jointly search for solutions for their water needs and the needs of others.
In the agricultural sector, both farmers and water management authorities search for opportunities to manage risks of decreasing crop yields. Available groundwater sources for irrigation purposes are increasingly under pressure due to the regional coexistence of land use functions that are critical to groundwater levels or compete for available water. At the same time, treated wastewater (TWW) from industries and domestic wastewater treatment plants are quickly discharged via surface waters towards sea (Figure 1). Exploitation of these alternative water sources may be an effective strategy to balance regional water supply and agricultural water demand.
Figure 1: Supplying the Bolscherbeek with effluent from the Haaksbergen sewage treatment plant (left) exceeds the natural basic drainage (right) in the summer period (photo: Bas Worm, Water Board Vechtstromen, the Netherlands).
Controlled drainage systems allow to actively control groundwater levels and soil moisture conditions at an agricultural field. If an additional source of water is actively added to such systems, controlled drainage systems become infiltration systems, also called sub-irrigation systems. The goal of sub-irrigation is to raise the groundwater level and improve the soil moisture conditions for plant growth through capillary rise (Figure 2). Since 2015, a pilot project has been setup in Haaksbergen, in the eastern part of the Netherlands, in which TWW is applied to a corn field by sub-irrigation during the growing season using a climate adaptive drainage system. Re-use of TWW for sub-irrigation in agriculture serves the dual purpose of supplying water to crops and diminishing emissions of Contaminants of Emerging Concern (CECs) into surface water.
The research is investigating how sub-irrigation/drainage cycles can be optimally designed and operated, to raise groundwater levels towards the root zone and decrease CECs emission to surface water. As emission of CECs to the deeper groundwater or CEC uptake in crops should be negligible, operation should take all relevant flow and transport routes into account. These routes can be highly dynamic, as they are also affected by erratic weather. KWR has installed a field monitoring network at several locations in the vadose zone and the upper part of the local groundwater system, which enables them to measure vertical solute profiles in the soil water by taking samples. Analysis of flow, CEC transport and degradation, and the supply of water and CECs to the root zone are being monitored and modelled. Results show that the water availability for crops can be improved significantly. First chemical analyses indicate that remnants of medicines have not reached significantly to the root zone of the crop within a growing season. However, it reaches the shallow groundwater.
Figure 2: Sub-irrigation via drains with continuous water supply, with which the water table and the soil moisture regime could be affected actively.
Besides municipal wastewater treatment plants, industrial wastewater treatment plants produce water which is discharged to surface water. Since 2016, a similar pilot project to the Haaksbergen site has been setup in the southern part of the Netherlands, in which industrial TWW from the Bavaria Brewery is applied to a grass field by sub-irrigation during the growing season, using a controlled drainage system.
Direct use of TWW for irrigation presents several advantages:
- A better control over soil moisture, hence better growing conditions for crops;
- A reduced need for groundwater or surface water extraction;
- A reduced load of the CECs to surface water by soil removal and smaller direct TWW-discharges.
However, in order to assess whether a larger scale application of sub-irrigation with TWW from industrial or municipal wastewater treatment plants may be acceptable, it is very important to know the risks associated with the method, and to examine whether these can be avoided or reduced. Therefore, a good knowledge of freshwater resources availabilities, substances behaviour in the soil and potential distribution to crops and deeper sub-surface is essential. Only then can managers and policy makers be provided with reliable information about the pros and cons, so that they can make a careful, comprehensive assessment of opportunities and risks of reusing wastewater in agriculture.
Re-USe of Treated effluent for agriculture – RUST
This research, executed within the framework of the projects NWO RUST, Lumbricus and Water in the Circular Economy (WiCE), is a collaboration between the Netherlands Organisation for Scientific Research (NWO), KnowH2O, Waterschap Vechtstromen, Wageningen University and Research, Utrecht University, University of Amsterdam and KWR Watercycle Research Institute.
Dr.ir. Ruud P. Bartholomeus
KWR Watercycle Research Institute & Wageningen University and Research