Worldwide, water scarcity is problematic, e.g. in semi-arid regions with inadequate natural precipitation, and other water resources are needed for irrigation. Also in humid temperate regions as The Netherlands, seasonality causes periodic (summer) water shortage in agriculture. Crop yields may decrease with insufficient freshwater availability. Where in coastal areas insufficient freshwater availability may induce salinity of soil water with additional negative effects on yields, in higher sandy areas of our country drought damage for agriculture ranges from 200-500M€ in a dry and extremely dry year, respectively. For The Netherlands, climate change is expected to cause larger variability in the soil water balance and more frequent dry and wet conditions, both reducing yields. In view of competing claims by different societal sectors, water managing authorities and the agricultural sector need to develop means to better regulate water availability for all stakeholders. Replacing scarce fresh water by sewage treatment plant (STP) effluent and treated industrial effluent is considered for agricultural re-use.
Reuse of treated wastewater for agriculture
The quality of surface waters is often affected by Treated Waste Water/effluent (TWW), due to current direct emissions. Direct or indirect use of TWW as resource is not without risks due to contaminants such as pharmaceuticals, pathogens, illicit drugs, personal care products and biocides. These and others are called Contaminants of Emerging Concern (CEC), reflecting that they are not routinely monitored, there are indications of their environmental presence, there is uncertainty about persistence and toxicity, and legal environmental quality standards are lacking. So there is scarce information concerning their risks to human and environmental health, although they are found widespread in surface water and waste water, and many are not well removed during conventional waste water treatment techniques. This is a pressing problem, in view of the large number of substances used in society.
Direct use of TWW for irrigation has several advantages: (i) better control over soil moisture, hence better growing conditions for crops, (ii) reduced need for ground- or surface water extraction, and (iii) a reduced load of the CEC to surface water by in soil removal and smaller direct TWW-discharges.
For direct emission of TWW to surface water, resulting concentrations in the surface water can be modelled but emission by sub-irrigation may lead to reduced concentrations as CEC are degraded or fixed during soil passage before reaching the surface water. The reduction depends on soil properties, redox conditions, an adapted soil microbiome enhancing biodegradation, but also on the physico-chemical properties of the chemical. Using TWW for irrigation, several conditions such as retention time and water saturation can be managed within boundaries, so circumstances may be optimized for emission reduction.
As re-use of TWW might adversely affect food, soil and groundwater quality, hence human health or ecosystem, risks need to be assessed. Even though focused drip irrigation and subsurface irrigation via climate adaptive drainage systems should improve compared with direct discharge of TWW into surface water, CEC towards pristine groundwater is reason for concern.
Monitoring and modelling subirrigation with treated wastewater
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. To investigate such re-use, at two field sites that have climate adaptive drainage facilities TWW is introduced by sub-irrigation through the drainage system to store water until summer, when the crop needs it. Subject of research, executed by two PhD-students, is to investigate 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 (stand still-principle), operation should take the complicated 3 dimensional flow and transport routes into account. These routes can be highly dynamic, as they are also affected by erratic weather. Analysis of flow, CEC transport and degradation, and the supply of water and CECs to the root zone are monitored at both field sites. A characterisation of a broad selection of CECs present in the TWW in the laboratory is based on column experiments. To test the validity of derived sorption, mobility and degradation parameters, column experiments are done for different conditions. With numerical modelling, the agreement between model and column breakthrough is determined. Using the same parameters, the fate of CECs in the field is modelled and compared with observations. In a final step, the models are used for a risk and regional upscaling assessment.
All in all, central research questions that will be answered in this project are:
- How and to what degree can sub-irrigation with TWW contribute to reduced CEC emission to surface water as compared with current direct discharge, and so contribute to improved surface water quality?
- Which are the most effective and efficient dimensioning and operational management considerations that balance trade-offs in water supply, TWW re-use, and CEC emission reduction to surface water?
- Can trade-offs of TWW re-use in sub-irrigation be managed to avoid significant transport of CEC into the crop’s root zone and deeper groundwater?
- Which critical factors in upscaling TWW sub-irrigation to larger scales can be identified with this project?