The study, published in Environmental Science and Pollution Research by Anass Omor and colleagues working at Sidi Mohamed Ben Abdallah University in Fez and Cadi Ayyad University in Safi, tackles the effluent at its source: the unhairing-liming unit itself. This is a deliberate choice. Because the stream is so concentrated, diluting it with other tannery wastewater before treatment can blunt the effectiveness of both chemical and biological processes. By targeting the raw, sulfide-loaded liquor directly, the researchers could optimize each stage for the specific chemistry of the stream, then chain the two together into a system they argue is well suited to small- and medium-scale tanneries, particularly in developing countries where capital and skilled operation are often scarce.

The chemical heart of the process is ferric chloride, a cheap and widely available coagulant. When ferric iron meets dissolved sulfide under alkaline conditions, the chemistry is unforgiving to the pollutant: sulfide is precipitated out of solution as insoluble iron sulfide species, while the iron simultaneously hydrolyzes to form hydroxide flocs that sweep up colloidal and dissolved organic matter through coagulation and sweep-flocculation. The team systematically varied the dose of ferric chloride and the pH of the reaction, searching for the point where sulfide removal peaked without wasting reagent. The optimum landed at a ferric chloride concentration of 1.2 moles per liter operating at pH 8.5. Under those conditions, the chemical stage alone stripped 90.11 percent of the sulfide and 88.33 percent of the chemical oxygen demand, a measure of the total oxidizable organic load in the water.

Those numbers are impressive, but they were not enough. Moroccan discharge standards set hard limits on residual sulfide and organic content, and the chemically treated effluent still exceeded them. This is where the second stage earns its keep. The researchers fed the pretreated liquor into a sequencing batch reactor, a biological system that runs through discrete cycles of filling, aeration, settling, and decanting in a single tank rather than across a chain of separate basins. The SBR format is attractive for smaller installations because it demands less footprint, less piping, and simpler automation than conventional continuous-flow plants, and its batch nature gives operators flexibility to adjust cycle times as the incoming load fluctuates.

Operating at a deliberately high organic loading rate of 1.5 kilograms of chemical oxygen demand per cubic meter per day, the sequencing batch reactor polished what the chemistry had left behind. The acclimated microbial community oxidized the residual sulfide and consumed the remaining biodegradable organics, pushing cumulative removal across the combined system to 99.97 percent for sulfides and 99.60 percent for chemical oxygen demand. The final effluent met every applicable Moroccan regulatory threshold. To guard against the possibility that the striking results were a statistical fluke, the team applied one-way analysis of variance, which confirmed that the performance of the combined process was highly significant, with a p-value below 0.001.

Understanding what actually forms during the chemical stage mattered as much as the removal percentages, because the solid waste a process generates can determine whether it is practical in the real world. The researchers interrogated the precipitate with three complementary techniques. X-ray diffraction revealed the crystalline phases present, scanning electron microscopy exposed the particle morphology, and Fourier-transform infrared spectroscopy identified the chemical bonds within the solid. Together, the analyses pointed to gypsum, calcium sulfate dihydrate, as the primary precipitate. That finding is quietly consequential: gypsum is a benign, well-characterized material with established routes for reuse in the construction industry, and the favorable morphology observed under the microscope suggests the sludge would settle and dewater readily, easing the handling burden that often sinks otherwise promising precipitation processes.

The logic of the two-stage design reflects a hard truth about sulfide-laden wastewater. Biological treatment alone struggles when sulfide concentrations are extreme, because dissolved sulfide is toxic to the heterotrophic bacteria that do the work of degrading organic matter, and it exerts an immediate chemical oxygen demand that can overwhelm aeration systems. Conversely, chemical precipitation alone cannot economically drive sulfide and organic levels down to regulatory limits without enormous reagent doses. By letting ferric chloride absorb the first, most punishing shock load, the process protects the microbial community downstream, and by letting the reactor finish the job biologically, the design avoids the cost and sludge volume of pushing chemistry to full compliance. Each stage covers the other’s weakness.

The broader context gives the work its urgency. Tannery workers face documented occupational hazards ranging from respiratory disease to acute hydrogen sulfide exposure, and the effluents that leave tannery walls have been repeatedly linked to degradation of receiving waters, including marine environments near discharge points. Conventional approaches across the industry include electrocoagulation, advanced oxidation, membrane filtration, activated carbon adsorption, and various combinations of anaerobic and aerobic bioreactors, each with trade-offs in cost, energy demand, and operational complexity. What distinguishes the ferric chloride and sequencing batch reactor pairing is its accessibility: both components rely on equipment and reagents that a modest facility can procure and maintain, and the sequencing batch reactor in particular has a track record of inexpensive, efficient service on tannery effluents in Fez.

The researchers are candid that the chemical stage on its own falls short of the regulatory bar, and that honesty frames the practical message of the study. For a small tannery weighing its options, the choice is rarely between a perfect technology and a flawed one; it is between an affordable system that works and an unaffordable one that exists only on paper. The ferric chloride and SBR system, the authors argue, offers a robust and sustainable middle path, one that converts a hazardous stream into a compliant effluent while producing a gypsum-rich sludge with plausible routes to valorization rather than a legacy of toxic waste. In regions where leather processing is both an economic anchor and an environmental sore point, that combination of performance and pragmatism may matter more than any single removal percentage.

As the leather industry faces tightening environmental scrutiny worldwide, studies of this kind sketch what cleaner production could look like for the thousands of small tanneries that dominate the sector in developing economies. The Moroccan team’s results, grounded in careful optimization, statistical validation, and materials characterization, suggest that the path to compliance need not run through exotic reagents or energy-hungry advanced oxidation. Sometimes the answer is old chemistry doing heavy lifting in the front of the plant, patient microbes finishing the job at the back, and a treatment train designed around the realities of the people who must run it every day.

Subject of Research: Hybrid chemical and biological treatment of sulfide-rich tannery unhairing-liming wastewater

Article Title: Advanced treatment of effluents loaded with sulfide from unhairing-liming tannery unit: a combination of chemical and biological processes

Article References: Omor, A., Laidi, O., Jai, M. E., Rais, Z., & Elkarrach, K. (2026). Advanced treatment of effluents loaded with sulfide from unhairing-liming tannery unit: a combination of chemical and biological processes. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38293-8

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38293-8

Keywords: tannery wastewater, sulfide removal, unhairing-liming, ferric chloride, sequencing batch reactor, chemical oxygen demand, gypsum precipitate, wastewater treatment, Morocco, biological treatment, pollution remediation, environmental engineering

The stakes could hardly be higher. Nearly 90 percent of tanneries rely on chromium sulfate as their principal tanning agent, and around 35 percent of the chromium used in the process ends up in the effluent, either as hexavalent or trivalent chromium. Hexavalent chromium, Cr(VI), is the form that keeps toxicologists awake: it is roughly 100 times more toxic than Cr(III), it is highly mobile in water because of its hydrophilic character, and chronic exposure is linked to hemorrhaging, ulceration, severe skin irritation, chest pain, and lung cancer. The World Health Organization’s permissible limits are routinely exceeded in untreated discharges, and the study’s sampling region, Jajmau in Kanpur district, Uttar Pradesh, is home to dozens of tanneries whose untreated effluents have long been implicated in the pollution of the Ganges River.

The technology at the heart of the paper is a hybrid of two well-known ideas. In conventional electrocoagulation, a sacrificial iron anode dissolves under an applied current, releasing ferrous ions that act as in-situ coagulants, sweeping pollutants into settleable flocs. The Fenton reaction, meanwhile, uses ferrous ions to activate hydrogen peroxide, producing hydroxyl radicals, which are among the most aggressive oxidants known, capable of shredding organic molecules that resist biological treatment. The electrochemical-Fenton process merges the two: the iron electrodes generate Fe2+ electrochemically, and added hydrogen peroxide reacts with it inside the cell to produce hydroxyl radicals continuously. Because the coagulant and the oxidant are both generated in situ, the process avoids the dosing uncertainties and secondary pollution risks that plague conventional Fenton chemistry, where excess chemicals must be neutralized after treatment.

The chemistry unfolds in a coordinated sequence. At the anode, iron is oxidized to ferrous ions; the ferrous ions then react with hydrogen peroxide to yield ferric ions, hydroxyl radicals, and hydroxide. The radicals oxidize the organic load, driving down COD. Simultaneously, the hexavalent chromium is electrochemically reduced to the trivalent form, a reaction that consumes protons and electrons at the electrode surface. The newly formed Cr(III) then precipitates as insoluble chromium hydroxide, locking the metal into a solid phase that can be separated from the water. Acidic conditions favor every step of this cascade, which is why pH emerged as a decisive variable in the optimization.

To find those conditions, the researchers turned to response surface methodology, a statistical framework that maps how multiple operating variables interact, rather than testing them one at a time. Using a Box-Behnken design, they ran 30 experimental combinations spanning four factors: pH from 3 to 7, electrolysis time from 20 to 60 minutes, current density from 5 to 15 milliamperes per square centimeter, and hydrogen peroxide concentration from 200 to 800 milligrams per liter. The resulting quadratic models were exceptionally strong. The F-values of 274.22 for chromium removal and 299.81 for COD removal indicated models far too good to be explained by random noise, with a probability of chance below 0.01 percent. Predicted R-squared values of 0.9875 and 0.9821 tracked their adjusted counterparts within 0.02, and adequate precision ratios of 55.09 and 67.73 signaled robust signal-to-noise performance.

The parameter sweep revealed clear patterns. Removal efficiency for both pollutants rose from pH 3 to pH 5 and then declined toward pH 7, with the optimum at pH 5 delivering 93.78 percent chromium removal and 97.2 percent COD removal after 60 minutes of electrolysis. Below the optimum, insufficient hydroxide limits the precipitation of Cr(III); above it, ferric sludge formation and radical scavenging interfere with the Fenton cycle. Increasing hydrogen peroxide from 200 to 800 milligrams per lifted chromium removal from 73.58 to 93.78 percent and COD removal from 75 to 87.6 percent, because more peroxide means more hydroxyl radicals. Raising the current density to 5 milliamperes per square centimeter lifted chromium removal from 79.46 to 90.45 percent, driven by faster dissolution of ferrous ions from the anode. Longer electrolysis times helped steadily: extending treatment from 20 to 60 minutes at fixed conditions raised chromium removal from 53.4 to 81.25 percent and COD removal from 46.5 to 71.6 percent. Interestingly, current density variations within the tested range had less influence than time and peroxide dose.

The team also probed the mechanism by which chromium leaves the solution, fitting their kinetic data to adsorption models. The uptake of Cr(VI) onto the metal flocs followed pseudo-second-order kinetics, with an R-squared of 0.995, and the experimentally observed equilibrium capacity of 20.267 milligrams per gram matched the pseudo-second-order prediction of 18.145 milligrams per gram far better than the pseudo-first-order estimate of 4.228. Among the isotherms, the Langmuir model fit best, with an R-squared of 0.99 against 0.86 for the Freundlich model, indicating that chromium adsorbs as a monolayer on the floc surfaces with a maximum capacity of 25.39 milligrams per gram. The Langmuir constant of 0.34 liters per milligram and a favorable adsorption intensity of 0.36 pointed to strong binding between chromium species and the iron-based flocs, which is precisely what a treatment process needs to prevent re-release of the metal.

Under the globally optimal conditions identified by the model, pH 5, an electrode gap of 2 centimeters, one hour of electrolysis, a current density of 15 milliamperes per square centimeter, and 500 milligrams per liter of hydrogen peroxide, the predicted chromium removal of 92.06 percent was confirmed experimentally at 91.25 percent, and the predicted COD removal of 97.5 percent was matched by an actual value of 96.2 percent, with a model desirability of 1.0. Against India’s Central Pollution Control Board discharge standards, which cap hexavalent chromium at 0.1 milligrams per liter, total chromium at 2 milligrams per liter, and COD at 250 milligrams per liter, the treated effluent comfortably meets the COD requirement. The chromium concentrations, while slashed by more than an order of magnitude, remain above the stringent Cr(VI) limit, so the authors are candid that a secondary polishing step would be needed for full regulatory compliance. They position ECF as a powerful primary treatment that does the heavy lifting before final polishing.

How does the result stack up against the wider literature? Comparable electrochemical approaches have delivered 87 percent chromium removal from real tannery effluent via electrochemical peroxidation, roughly 90 percent removal from chromium-EDTA complexes using electro-peroxone, and near-complete removal in electrocoagulation of electroplating wastewater and electroprecipitation of real tannery effluent. A heterogeneous electro-Fenton cathode system achieved almost complete Cr(VI) removal within 30 minutes, while electro-Fenton followed by precipitation and titanium-anode indirect reduction reached above 95 percent and around 80 percent, respectively. The present work sits squarely within this competitive field, and its distinguishing strength is that it achieved 91.25 percent removal from genuine tannery wastewater under moderate operating parameters, with a validated statistical model rather than one-variable-at-a-time tuning.

The authors are appropriately measured about the road ahead. The study was conducted at laboratory scale, and real industrial effluents vary in composition from batch to batch, which could shift the optimum. Long-term electrode stability, energy consumption, and operational costs were not comprehensively evaluated, and these factors will determine whether the economics work at full scale. They call for pilot-scale validation, long-term performance assessment, and techno-economic analysis, along with integration of the ECF process with complementary technologies and exploration of renewable-energy-driven or green-oxidant variants. Even with those caveats, the message is compelling: a simple beaker-scale cell with iron plates, hydrogen peroxide, and a well-chosen set of operating conditions can strip the great majority of hexavalent chromium and organic load from some of the dirtiest industrial water in the world, using chemistry that generates its own reagents and leaves behind a separable solid. For the tannery clusters lining rivers like the Ganges, that combination of simplicity, scalability, and verified performance is exactly what a practical cleanup demands.

Subject of Research: Optimization of the electrochemical-Fenton process for removing hexavalent chromium and organic pollutants from real tannery wastewater

Article Title: Optimization of the electrochemical Fenton process for hexavalent chromium removal from tannery wastewater

Article References: Kumar, R., & Bajpai, M. (2026). Optimization of the electrochemical Fenton process for hexavalent chromium removal from tannery wastewater. Discover Green Chemistry, 1(1), Article 9. https://doi.org/10.1007/s44509-026-00012-4

Image Credits: AI Generated

DOI: 10.1007/s44509-026-00012-4

Keywords: hexavalent chromium, tannery wastewater, electrochemical-Fenton, advanced oxidation, hydrogen peroxide, response surface methodology, COD removal, adsorption kinetics, Langmuir isotherm, water treatment, green chemistry, industrial pollution

The research, led by Dhomas Indiwara Prana Jhouhanggir and colleagues working with the Environmental Agency of Bantul Regency in Yogyakarta, Indonesia, began with a sobering field assessment. Monthly monitoring of the centralized activated sludge treatment plant serving ten leather tanning industries revealed persistent compliance failures. Ammonia concentrations in the treated effluent ranged from 2.24 to 11.10 milligrams per liter throughout 2025, consistently exceeding the local discharge limit of 0.5 milligrams per liter. BOD5 and COD also spiked dramatically in some months, peaking at 225 and 487 milligrams per liter respectively in August, far above the regulatory thresholds of 50 and 110 milligrams per liter.

These failures are not surprising. Activated sludge systems depend on delicate microbial communities whose nitrifying bacteria are highly sensitive to the wild fluctuations in flow, salinity, pH, and toxic load that characterize tannery wastewater. Influent flow at the Bantul plant nearly doubled between January and February 2025, swinging from 437 to 868 cubic meters per month. During a three-day field assessment conducted alongside the laboratory experiments, the full-scale system removed only about 26 percent of influent ammonia, leaving 15.64 milligrams per liter in the effluent, and achieved just 24 percent removal of total dissolved solids, which remained far above the 2000 milligram per liter limit.

To address these weaknesses, the team turned to microalgae, organisms that photosynthesize, assimilate nitrogen and phosphorus directly into their cells, and tolerate environmental variability better than bacterial consortia. The researchers obtained a Chlorella vulgaris culture from the Jepara Brackishwater Aquaculture Fisheries Center and verified its identity twice over. Light microscopy showed the characteristic unicellular, spherical cells averaging 6.33 micrometers in diameter, while PCR amplification and sequencing of the 18S rRNA gene returned 100 percent query coverage and 100 percent sequence identity against reference strains in the NCBI GenBank database. A phylogenetic tree placed the isolate firmly within the authenticated C. vulgaris clade, closest to strains from South Korea, Cape Verde, and Malaysia, confirming a globally conserved genetic profile for this species.

The experimental heart of the study was a two-stage cultivation design. Tannery effluent collected from the equalization tank of the centralized plant was centrifuged to reduce turbidity and sterilized before inoculation with three different proportions of algal culture: 10, 20, and 30 percent by volume, each supplemented with Walne medium. Each stage lasted seven days at 24 degrees Celsius under continuous illumination of 5000 lux with aeration. Growth was tracked daily by optical density and fitted to the Gompertz kinetic model, which achieved coefficients of determination between 0.9906 and 0.9966 across all treatments, indicating an excellent fit to the observed growth dynamics.

The two-stage strategy proved decisive. In the first stage, the raw effluent’s high pollutant load, turbidity, and toxic constituents such as chromium and sulfide suppressed growth, particularly at lower inoculum densities where longer lag phases reflected slower acclimatization and greater sensitivity to ammonia inhibition. By the second stage, however, the first round of algal treatment had already removed much of the inhibitory load and clarified the medium, allowing better light penetration and faster growth. The maximum specific growth rate in stage two rose to 0.0765 cells per milliliter per day for the 30 percent treatment, statistically comparable to pure culture growth, and the lag phase shortened to just over half a day, indicating rapid adaptation.

Pollutant removal followed the same trajectory of improvement. Under the best-performing condition, CVG 30 percent, the two-stage system achieved 92.94 percent removal of BOD5, 91.60 percent removal of COD, 99.74 percent removal of total chromium, 93.31 percent removal of total nitrogen, and a striking 99.80 percent removal of ammonia. Sulfide removal exceeded 99.99 percent in every treatment, and total suspended solids fell by up to 97.60 percent. Critically, the final concentrations of chromium, BOD5, COD, and ammonia all fell within the applicable regulatory discharge limits, a feat the existing full-scale activated sludge system could not match for most of these parameters during the same period. Only total dissolved solids remained above the limit, at 3740 milligrams per liter, reflecting the inherent difficulty of biologically removing dissolved inorganic ions such as chloride, sodium, and sulfate.

The authors attribute this performance to the complementary mechanisms of phycoremediation. Dissolved pollutants first adhere to the algal cell surface through biosorption, involving adsorption, ion exchange, surface complexation, precipitation, and electrostatic interactions. Living cells then transport selected nutrients and contaminants intracellularly, where nitrogen from ammonia is incorporated into amino acids, proteins, and nucleic acids, directly linking pollutant removal to biomass production. Meanwhile, photosynthetic oxygen generation supports aerobic degradation of organic matter by associated microorganisms, and enzymatic transformations break down complex organics both outside and inside the cells. This metabolic flexibility explains why microalgae outperformed conventional biological treatment on the very pollutants, especially ammonia, that most plague activated sludge operations.

Beyond water cleaning, the study adds a waste-to-resource dimension by characterizing the fatty acid methyl ester profile of the biomass harvested from the best treatment. The lipid composition proved remarkably balanced: 28.41 percent saturated fatty acids, 35.17 percent monounsaturated fatty acids, and 36.41 percent polyunsaturated fatty acids. Methyl palmitate dominated the saturated fraction at 22.40 percent, a hallmark of biodiesel feedstocks. Methyl cis-11-eicosenoate, a long-chain monounsaturated ester with potential lubricity and even immunostimulatory pharmaceutical applications, reached 16.67 percent, an unusually high level that suggests wastewater cultivation may enhance long-chain monounsaturated lipid synthesis. Most notably, methyl linoleate accounted for 26.36 percent of the total, and the biomass also contained gamma-linolenic acid, eicosapentaenoic acid, and docosahexaenoic acid, omega fatty acids associated with cardiovascular protection, anti-inflammatory effects, and neurological development.

The researchers caution that fatty acid composition alone does not establish a specific commercial application, and that pilot and full-scale trials will be needed to evaluate long-term stability, energy requirements, and economic feasibility when the algal process is integrated with existing infrastructure. Because dissolved solids resist biological removal, a membrane polishing step such as ultrafiltration or reverse osmosis may still be required to meet the TDS standard. Nevertheless, the integrated assessment, spanning molecular strain verification, kinetic growth modeling, pollutant removal, field benchmarking against a real treatment plant, and biochemical biomass characterization, provides one of the most comprehensive demonstrations to date that two-stage microalgal cultivation can transform tannery effluent from a regulatory liability into a feedstock, closing the loop in one of the world’s most polluting industries.

Subject of Research: Two-stage microalgae cultivation for tannery industrial effluent treatment and biomass valorization

Article Title: Two-stage cultivation of microalgae C. vulgaris for tannery industrial effluent treatment: strain verification, growth performance, pollutant removal, and fame characterization

Article References: Jhouhanggir, D. I. P., Pertiwiningrum, A., Kollia, C., Dardavila, M. M., Lymperopoulou, T., Fitriyanto, N. A., Suyono, E. A., Abidin, M. Z., & Widiastuti, I. (2026). Two-stage cultivation of microalgae C. vulgaris for tannery industrial effluent treatment: strain verification, growth performance, pollutant removal, and FAME characterization. Case Studies in Chemical and Environmental Engineering, 14, Article 101476. https://doi.org/10.1016/j.cscee.2026.101476

Image Credits: AI Generated

DOI: 10.1016/j.cscee.2026.101476

Keywords: Chlorella vulgaris, tannery wastewater, phycoremediation, ammonia removal, chromium removal, two-stage cultivation, Gompertz model, FAME profile, biodiesel, activated sludge, wastewater treatment, biomass valorization