Conventional wastewater treatment plants were never designed to strip out recalcitrant organic micropollutants such as pharmaceuticals, and trace amounts routinely slip through into receiving waters. Advanced oxidation processes (AOPs) attack these stubborn compounds by generating highly reactive free radicals that fragment contaminant molecules. But chemical degradation alone does not guarantee safety. As contaminants break down, they spawn transformation products (TPs) that can, in some cases, be as toxic as—or more toxic than—the parent compounds. The research team, led by Lucas Gustavo da Costa and Alam Gustavo Trovó of the Federal University of Uberlândia, together with collaborators at the Oswaldo Cruz Institute, set out to answer a deceptively simple question: does measurable chemical removal of micropollutants actually translate into reduced biological harm?

The centerpiece of their work is the H₂O₂/S₂O₈²⁻/UVC process, a hybrid system that combines hydrogen peroxide and persulfate with short-wave ultraviolet C radiation. UVC photons cleave both oxidants simultaneously, generating hydroxyl radicals (HO•) and sulfate radicals (SO₄•⁻) in the same reaction volume. This dual-radical strategy outperforms systems relying on either oxidant alone, or on peroxymonosulfate (HSO₅⁻), which is costlier and demands more energy for activation. The hybrid route also resists interference from inorganic species commonly found in real effluents, making it an economically attractive candidate for deployment at full-scale treatment plants. The process had already been chemically optimized in the team’s earlier work using multivariate mixture design and rotatable central composite design experiments; the new study is its first integrated biological stress test.

The experiments used real municipal effluent from a wastewater treatment plant in Uberlândia, Minas Gerais, sampled after the plant’s final tertiary treatment stage of coagulation-flocculation with ferric chloride and flotation. The effluent was enriched with three pharmaceuticals representing different therapeutic classes: colchicine (COL, an antimitotic agent), nitazoxanide (NTZ, an antiparasitic), and sulfamethoxazole (SMX, a widely detected antibiotic), each at 325 nmol L⁻¹—equivalent to 130, 100, and 82 micrograms per liter, respectively. That concentration was deliberately chosen as high enough for direct HPLC–DAD analysis without preconcentration, yet low enough to be environmentally representative. Treatment was carried out in an amber glass reactor irradiated by two 8-watt UVC mercury lamps emitting at 254 nm, with a measured irradiance of 4.9 W m⁻². Residual oxidants were neutralized with sodium thiosulfate before any biological testing, ensuring that observed effects could not be attributed to leftover peroxide chemistry.

The ecotoxicological battery spanned multiple trophic levels and levels of biological organization. Oxidative stress was assessed in the earthworm Eisenia andrei by measuring malondialdehyde (MDA), a marker of lipid peroxidation, and protein carbonylation, an irreversible oxidative modification of proteins. Phytotoxicity was evaluated using lettuce (Lactuca sativa) seeds and a growth index combining germination rate and root elongation. Cytotoxicity and genotoxicity were quantified in onion (Allium cepa) root meristems through the mitotic index and the frequency of chromosomal and nuclear aberrations—micronuclei, chromosome breaks, stickiness, bridges, and nuclear buds—across 5,000 cells per sample. Finally, growth inhibition was tested in two environmentally relevant microbes: Azospirillum brasilense, a plant growth-promoting bacterium used in soybean cultivation, and Saccharomyces cerevisiae, a yeast that persists in soils and fermentative niches.

The untreated enriched effluent told a worrying story. It exhibited substantial genotoxicity in onion cells—21%, approaching the 24% seen with the positive control, methyl methanesulfonate—and severe phytotoxicity, with lettuce growth indices far below the 80% threshold that signals absence of toxicity. Even the unspiked effluent itself suppressed lettuce growth, achieving a growth index of only 47%, evidence that bioactive compounds survive conventional treatment. In earthworms, individual aqueous solutions of NTZ and SMX raised MDA levels by 37.2% and 23.5% respectively, while the three-compound mixture increased lipid peroxidation by 38.0%, pointing to additive or synergistic oxidative stress mechanisms involving reactive oxygen species and Fenton-type chemistry within cells.

The picture changed decisively after oxidation. Following 10 minutes of UVC-driven treatment—the point at which 80% chemical degradation had been achieved, matching the minimum removal target proposed in the European Union’s COM(2022)541 directive—genotoxicity fell to 10%, and after 20 minutes, corresponding to the limit of quantification for the parent compounds, it dropped further to 6%, a statistically significant reduction. The plant growth index climbed above 80%, crossing from toxic territory into biocompatibility. The mitotic index in onion cells remained statistically indistinguishable from the deionized-water control (around 39–40% versus 40%), demonstrating that the process generated no cytotoxic byproducts capable of arresting cell division. And crucially for agricultural applications, neither the treated effluent nor its transformation products inhibited growth of A. brasilense or S. cerevisiae.

The mechanistic details are instructive. In the untreated enriched effluent, micronuclei—membrane-bound DNA fragments expelled from the nucleus during flawed cell division—were among the most frequent aberrations, indicating clastogenic and aneugenic damage. Colchicine exposure predictably produced C-metaphase accumulation, a signature of its tubulin-binding, spindle-disrupting mechanism first described in Allium nearly a century ago. SMX depressed the mitotic index and induced chromosomal damage, consistent with prior findings in Vicia faba. NTZ, whose genotoxicity had never before been reported, produced chromosome breaks, stickiness, and nuclear buds. After oxidation, the overall aberration burden fell sharply, particularly micronuclei and stickiness, although a few bridges and polyploid cells persisted even at 20 minutes—a reminder that some transformation products or resistant residuals may linger.

The microbial results came with a subtlety. Reduced A. brasilense growth after treatment initially looked like a red flag, but the team attributes it to depletion of assimilable organic matter, which serves as radical scavenger during oxidation and as a nutrient source afterward—not to toxicity. The negative control likewise grew slowly. This distinction matters because AOPs are designed to mineralize organic carbon, and a nutrient-poorer medium should not be misread as a more toxic one. For S. cerevisiae, no inhibition occurred at any stage of treatment, although NTZ alone significantly depressed yeast optical density, reinforcing the compound’s cell-cycle interference potential even at nanomolar-scale exposures.

The study’s broader significance lies in its methodological stance. By adopting an “effect-driven approach,” the team evaluated the toxicity of whole reaction mixtures without needing to isolate and identify individual transformation products—an impractical task given that many degradation byproducts are not commercially available. Their findings align with a growing consensus that treatment efficacy must be judged not merely by parent-compound removal but by the nature and reactivity of the resulting transformation products. Biochemical biomarkers such as MDA and protein carbonylation detected sublethal disturbances that mortality-based endpoints would have missed entirely, providing early warnings of cellular distress at environmentally relevant concentrations.

One caveat deserves attention: while lipid peroxidation declined after treatment, protein carbonylation rose slightly—1.05-fold above control—suggesting some transformation products may still provoke protein oxidation even as lipid damage subsides. The authors flag this discrepancy and call for future identification of the specific TPs responsible, alongside expanded organism panels, longer-term exposure studies, and pilot-scale validation across different environmental matrices.

The regulatory context sharpens the urgency. The EU’s proposed urban wastewater treatment directive mandates at least 80% removal of specified organic micropollutants and microbiological control for agricultural reuse, yet it conspicuously omits eco-compatibility assessment of the treated water itself. This study supplies exactly that missing dimension, showing that a process satisfying the chemical benchmark also delivers measurable biological benefit—lower genotoxicity, restored plant growth, intact cell division, and unharmed beneficial microbes. As water scarcity intensifies globally and reuse becomes less optional, the Brazilian team’s integrated bioassay framework offers a template for ensuring that the water farmers irrigate with is not merely chemically compliant, but genuinely ecologically safe.

As cities search for ways to conserve water and recycle nutrients, a hidden problem is emerging in the fields that feed them: traces of medicines can move from wastewater, animal manure and sewage-derived fertilizers into edible plants. A comprehensive review in Environmental Chemistry Letters finds that pharmaceutical residues are being detected across the water–soil–plant system, including in vegetables, fruits, roots and grains. The amounts reaching edible tissues are generally small—typically less than 1 percent of the total pharmaceutical load introduced into farmland—but the researchers warn that the apparent dilution does not eliminate concern. Some compounds persist in soil, some are readily absorbed by roots, and others are transformed inside plants into metabolites whose toxicity and behavior remain poorly understood. The findings place a new question at the center of agricultural water recycling: can circular farming systems close resource loops without quietly opening a pathway for drug residues into the food chain?

The pressure to reuse unconventional water and fertilizers is growing rapidly. Agriculture consumes roughly 70 percent of available freshwater worldwide, while climate change, urbanization and population growth are intensifying competition for water and arable land. Reclaimed wastewater is already widely used for irrigation in arid and semiarid regions; in Israel, it accounts for more than half of the water used in agricultural irrigation. Wastewater treatment also generates biosolids, while livestock farming produces manure rich in nutrients and organic matter. Applying these materials to soil can improve fertility, water retention and crop yields. Yet pharmaceuticals consumed by people and animals are not always completely removed or metabolized before reaching wastewater treatment plants, manure or sewage sludge. More than 11,000 pharmaceutical compounds are in use, and rising consumption of antidepressants, antidiabetic drugs, lipid regulators and other medicines means that agricultural systems may receive an increasingly diverse chemical mixture.

The review, by Lúcia H. M. L. M. Santos, Sara Rodríguez-Mozaz and Gianluigi Buttiglieri, brings together evidence from field studies rather than relying only on laboratory or hydroponic experiments. That distinction matters because real agricultural soils are chemically and biologically complex. Once a pharmaceutical enters a field, it may dissolve in soil pore water, bind to clay or organic matter, move with infiltrating water, break down through microbial or chemical reactions, or be absorbed by plant roots. The fraction available for uptake is therefore not simply the concentration measured in bulk soil. Soil pH, clay content, organic matter, moisture, temperature and microbial communities all influence whether a compound remains mobile or becomes temporarily trapped. Sorption can reduce uptake, but it is often reversible: a contaminant attached to soil particles may later be released into pore water during irrigation or rainfall, creating a delayed exposure route for crops planted months or years later.

The chemical structure of a drug also determines how readily it enters a plant. Neutral compounds generally cross root-cell membranes more easily than charged molecules, while polarity, water solubility, molecular size and hydrophobicity influence movement through plant tissues. Compounds with intermediate lipophilicity can be particularly mobile because they are soluble enough to travel in water but sufficiently compatible with cell membranes to pass into tissues. Carbamazepine, an antiepileptic and psychiatric drug, is a prominent example. It remains largely neutral across a broad pH range, has a logKow near 2.45–2.77, and can persist in soil for more than 60 days. Those properties help explain why it is repeatedly detected in crops irrigated with reclaimed water. Active transport may complicate the picture further: membrane proteins can sometimes carry ionic pharmaceuticals into cells even when passive diffusion would predict limited uptake.

The pattern of accumulation across a crop is uneven, but leafy vegetables appear to be the most exposed edible tissues. Across field studies involving reclaimed wastewater, the general order was leaves, then fruits, roots and cereal grains. Leafy greens such as lettuce, spinach, cabbage, leeks, parsley and arugula maintain extensive, actively transpiring tissues and are consumed directly, leaving little opportunity for contaminants to be excluded before harvest. In a survey of 445 commercial fields in Israel, leaves contained an average of about seven pharmaceutical compounds, compared with roughly six in roots, four in fruits and two in tubers. In vegetables irrigated with water dominated by treated wastewater in Jordan, total pharmaceutical concentrations reached 347 nanograms per gram dry weight in lettuce, 204 in parsley and 147 in arugula. Carbamazepine reached 215.7 nanograms per gram dry weight in lettuce. By contrast, cereal grains generally showed concentrations one to two orders of magnitude lower than other edible organs, although low accumulation does not mean zero exposure.

The route by which recycled material reaches a field appears to be as important as the concentration of the contaminants themselves. Reclaimed wastewater is applied repeatedly throughout a growing season, sometimes daily during hot summers, continuously adding pharmaceuticals in dissolved form—the form most accessible to roots. Biosolids and manure are usually applied once or twice a year, often before planting, and deliver contaminants largely bound to solid organic matter. That difference helps explain why crops grown in biosolid-amended soils generally contained fewer pharmaceuticals and lower concentrations than crops irrigated with reclaimed wastewater. Biosolids can increase organic matter, alter pH and stimulate microbial activity, creating additional sites for sorption and degradation. In some experiments, increasing the biosolid application rate reduced the uptake and movement of radiolabeled carbamazepine through edible vegetables. Even so, biosolids can also act as a long-term reservoir, gradually releasing residues as their organic components decompose.

Manure presents a different concern because veterinary medicines—especially antibiotics—are frequently excreted by livestock in urine and feces. Tetracyclines, sulfonamides and fluoroquinolones have been measured in manure at concentrations reaching 183,500, 32,700 and 24,700 nanograms per gram dry weight, respectively. Their strong attraction to soil can make agricultural land a persistent reservoir, while repeated applications may maintain exposure over many growing seasons. Field studies have detected antibiotics in lettuce, spinach, tomatoes, peanuts, cauliflower and other crops raised in manure-amended soils. The type of manure matters: crops grown with pig slurry or horse manure often contained higher antibiotic levels than those fertilized with poultry or cattle manure. In one comparison, norfloxacin concentrations ranged from 93.6 micrograms per kilogram dry weight in tomato fruit to 411 micrograms per kilogram in spinach leaves. Fruits sometimes showed the greatest bioaccumulation factors for antibiotics, while sex hormones reached a bioaccumulation factor of up to 30 in some root vegetables.

Perhaps the most unsettling finding is that measuring only the original drug may underestimate what is present in a crop. Plants are not passive filters; they metabolize foreign chemicals using enzyme systems that resemble detoxification pathways in other organisms. Phase I reactions such as oxidation, hydrolysis and hydroxylation can make a pharmaceutical more polar and mobile. Phase II reactions attach sugars, sulfates, glutathione or amino acids to the altered compound. Some conjugates are then locked into cell walls or vacuoles as so-called Phase III residues, becoming difficult to extract and detect. But these transformations do not automatically make a compound harmless. Carbamazepine metabolites, including 10,11-epoxycarbamazepine, are frequently found in crop tissues and can sometimes exceed the concentration of the parent drug. Certain metabolites retain biological activity, while others may be more persistent or toxic. Phase II conjugates can also be broken apart during digestion, potentially releasing the parent compound or an active intermediate. Because these substances occur at trace levels in chemically complicated plant tissues, advanced high-resolution mass spectrometry and non-targeted screening will be needed to find them reliably.

The review does not conclude that eating a particular vegetable is demonstrably dangerous, and the authors emphasize that field data remain limited compared with laboratory experiments. Instead, it identifies a monitoring gap that could become more important as water scarcity makes agricultural reuse unavoidable. Existing water-reuse standards often focus on conventional measures such as pathogens, suspended solids, pH and biochemical oxygen demand, while pharmaceuticals and other emerging contaminants may receive attention only when regulators judge that clear evidence of risk exists. The researchers argue for identifying compounds most likely to accumulate, developing models that predict concentrations in individual plant tissues, and prioritizing drugs or metabolites with low acceptable daily intakes or high toxicological concern. Drip irrigation can reduce direct contact between reclaimed water and edible plant surfaces, although it does not remove root uptake. Sprinkler systems may deposit residues on leaves and fruits, adding a second exposure route. Circular agriculture can still deliver major environmental benefits, but its safety depends on tracking not only what enters the field, but also what survives the journey from wastewater or manure into the food on our plates.

With global water demand projected to surpass supply in the coming decades, the urgency for sustainable water management strategies is palpable. Agriculture consumes an estimated 70% of the world’s freshwater resources, a staggering figure that underscores the necessity for alternatives. Traditional irrigation methods are no longer sustainable in many regions, prompting a shift toward treated wastewater as a solution. The researchers highlight that, with appropriate treatment, wastewater can yield comparable quality water suitable for agricultural use.

The team’s analysis categorizes several secondary wastewater treatment techniques, assessing their efficacy, cost, and impact on water quality. Techniques such as activated sludge processes, membrane bioreactors, and constructed wetlands are all evaluated for their potential to produce high-quality irrigation water. In each case, the researchers delve into the technical aspects, discussing their operational mechanisms and efficiency in removing contaminants.

Activated sludge processes have long been a cornerstone of wastewater treatment. This aeration-driven method promotes the growth of microorganisms that break down organic matter. The authors elucidate how variations within this technique can enhance its effectiveness for irrigation purposes, particularly by optimizing aeration and retention times. When executed correctly, this method can yield water that meets or exceeds agricultural standards.

Another treatment process analyzed is the membrane bioreactor (MBR) technology, which integrates biological treatment with membrane filtration. The results of this technique present a fascinating juxtaposition of efficacy and cost. While MBRs are often more expensive to implement, they excel at removing even the smallest contaminants, making their output particularly appealing for agriculture in regions with stringent water quality requirements.

Constructed wetlands emerged as a natural and cost-effective alternative in the study. This method creatively utilizes natural processes to treat wastewater through vegetation, soil, and microbial interactions. The researchers discuss the benefits of constructed wetlands, which not only purify water but also provide essential habitat for diverse wildlife. Such systems promise a dual benefit: water treatment and biodiversity conservation, offering an intriguing model for sustainable water use.

Beyond these processes, the study also examines the viability of integrating multiple treatment techniques for synergistic effects. By combining methodologies, the potential to achieve superior water quality emerges, which could be transformative for irrigation practices. The researchers advocate for a holistic approach, recommending that future irrigation water solutions consider local contexts and resource availability.

One of the significant findings of El Baradei and his colleagues was the relationship between cost and efficiency. While advanced technologies like MBR offer high-quality outputs, their upfront investment challenges widespread adoption in developing countries. The study advises policymakers to consider not only the initial costs but also the long-term savings associated with utilizing treated wastewater for irrigation, particularly in water-scarce regions.

The implications of adopting treated wastewater for irrigation extend beyond agriculture. Reducing reliance on freshwater sources allows for more sustainable water management practices overall. Furthermore, when treated wastewater re-enters the natural water cycle as irrigation returns seep back into groundwater, the researchers propose that this could enhance local aquifers and promote ecosystem resilience.

As the study gains traction within academic and environmental circles, it prompts a reevaluation of existing water management policies. Policymakers are urged to consider more integrative frameworks that recognize the value of treated wastewater. Sustained public awareness campaigns would also be crucial to mitigate the social stigma associated with using wastewater in agriculture.

Emerging from the COVID-19 pandemic, there is a renewed focus on resilient food systems. The insights from this research align perfectly with the global push toward sustainability and food security, signaling an encouraging trend among scientists, farmers, and policymakers alike. As nations grapple with the realities of climate change and water scarcity, the adoption of treated wastewater could serve as a critical building block in constructing a sustainable agricultural future.

In conclusion, the comparative analysis by El Baradei, Basiouny, and Hazem lays the groundwork for future explorations in wastewater treatment. By presenting compelling evidence that shows the feasibility and utility of secondary wastewater treatment as a resource for irrigation, the study makes a persuasive case for its consideration in agricultural practices globally. As challenging as the water crisis appears, the innovative approaches outlined herein shine a glimmer of hope in addressing one of humanity’s most pressing issues.

The future of sustainable agriculture, empowered by reuse principles and advanced wastewater treatment technologies, is destined for transformation. With adequate investment, policy support, and public engagement, treated wastewater could indeed become the lifeblood of a new irrigation revolution, fostering both ecological balance and agricultural resilience in the face of an uncertain future.

Subject of Research: Wastewater treatment techniques for irrigation.

Article Title: Different secondary wastewater treatment techniques as potential irrigation water resources: a comparative analysis and case study.

Article References:

El Baradei, S.A., Basiouny, M.I. & Hazem, N. Different secondary wastewater treatment techniques as potential irrigation water resources: a comparative analysis and case study.
Discov Sustain (2025). https://doi.org/10.1007/s43621-025-01221-w

Image Credits: AI Generated

DOI:

Keywords: Wastewater treatment, irrigation, agriculture, sustainability, water scarcity, activated sludge, membrane bioreactors, constructed wetlands.