Deficit irrigation with treated wastewater improves crop quality and sustainability in semiarid farming

Chemical analyses showed that the reclaimed water met irrigation safety standards for salinity, sodium adsorption ratio (SAR), and microbiological contaminants. Concentrations of heavy metals and thermotolerant coliforms were well within acceptable thresholds, underscoring the feasibility of reclaimed wastewater for agricultural reuse. Monthly monitoring was conducted to track water quality and ensure safety throughout the experiment.

With water scarcity threatening agricultural productivity in semiarid regions, researchers are turning to non-conventional water sources and climate-resilient crops to secure sustainable food systems. A newly published study titled “Domestic Reclaimed Water for Circular Agriculture: Improving Agronomic Performance of Sweet Sorghum in a Semiarid Tropical Climate” in Sustainability provides scientific evidence supporting the use of treated domestic wastewater to irrigate sweet sorghum under varying water regimes.

Led by researchers from Brazil’s Federal Rural University of Pernambuco and the National Institute of the Semiarid Region, the study assessed how different irrigation depths using reclaimed water impacted the growth, yield, and water use efficiency of sweet sorghum grown in field conditions. By targeting one of Brazil’s most water-limited agricultural zones, the work responds to urgent challenges around climate adaptation, circular resource use, and sustainable crop production.

Can reclaimed wastewater sustainably replace freshwater in crop irrigation?

The study is based in Parnamirim, Pernambuco - a region characterized by high temperatures, variable rainfall, and growing water insecurity. Domestic wastewater, after treatment in a local sewage plant, was used to irrigate sorghum through a drip system. Five irrigation regimes, ranging from 50% to 150% of the crop’s evapotranspiration needs (ETc), were applied over two consecutive cycles: a main crop and a ratoon (regrowth) crop.

Chemical analyses showed that the reclaimed water met irrigation safety standards for salinity, sodium adsorption ratio (SAR), and microbiological contaminants. Concentrations of heavy metals and thermotolerant coliforms were well within acceptable thresholds, underscoring the feasibility of reclaimed wastewater for agricultural reuse. Monthly monitoring was conducted to track water quality and ensure safety throughout the experiment.

Researchers found that sorghum irrigated with treated domestic wastewater responded differently depending on the water regime. Lower irrigation levels (50% and 75% ETc) promoted higher water use efficiency and sugar content (measured as °Brix), while higher irrigation volumes (125% and 150% ETc) reduced efficiency and, in some cases, productivity. These findings make a strong case for using reclaimed water under deficit irrigation regimes, deliberately applying less water than the full ETc, in water-scarce regions.

How does irrigation depth affect biomass, water use and crop quality?

The study measured a broad range of physiological and agronomic variables, including plant height, stem diameter, leaf area, fresh biomass, water use efficiency (WUE), and °Brix levels in extracted juice. In the first crop cycle, differences in growth and yield across irrigation treatments were modest, likely due to high rainfall that diluted the effects of the treatments. However, in the ratoon crop, the impact of irrigation depth was more pronounced.

Fresh biomass in the ratoon crop peaked at moderate irrigation depths, around 100% of ETc, and declined when water was applied in excess. Similarly, WUE followed a downward trend at higher irrigation volumes, dropping by over 50% in the highest irrigation treatment (150% ETc) compared to the lowest (50% ETc). These findings point to a non-linear relationship between water application and productivity, where over-irrigation leads to resource inefficiencies and potential yield penalties.

Sugar concentration also responded inversely to water volume. In both harvests, °Brix levels were highest under deficit irrigation (14.23% and 12.77% for first and second cuts, respectively, under 50% ETc), and lowest under surplus irrigation (as low as 9.77% in the second cut with 150% ETc). This suggests that moderate water stress may induce physiological responses that enhance sugar accumulation, potentially improving the quality of forage and silage.

Notably, the ratoon crop exhibited lower biomass and growth metrics overall, with reductions in height, stem diameter, leaf number, and total mass compared to the main crop. The researchers attributed this to increased tillering, intraspecific competition, and nutrient limitations from the absence of additional fertilization in the second cycle.

What are the broader implications for climate-resilient agriculture?

The results support a growing consensus in agricultural sustainability: maximizing efficiency does not require maximizing input. Sweet sorghum, a drought-tolerant and fast-growing forage crop, demonstrated strong adaptability to reclaimed water and moderate irrigation, making it well-suited for semiarid zones. Moreover, the findings align with circular economy principles by turning treated domestic wastewater into a productive resource that addresses water scarcity, reduces environmental discharge, and provides nutritional inputs to crops.

The study also highlights the importance of precision irrigation and careful water quality monitoring. While the treated wastewater used in this research met all safety criteria, the authors caution that long-term reuse in other contexts could pose risks from heavy metals or emerging contaminants if not properly managed. Regulatory oversight and site-specific evaluation are key to ensuring that reclaimed water remains safe and effective for crop irrigation.

On the environmental front, the practice offers dual benefits: conserving freshwater supplies and minimizing the ecological burden of wastewater disposal. On the economic front, it reduces dependence on synthetic fertilizers due to the nutrient content of the effluent. Socially, it promotes food security and rural resilience in regions vulnerable to climate shocks.