Iron Chemistry Meets Bacteria to Strip Toxic Sulfide from Tannery Wastewater

Tanneries have always carried an unmistakable signature, and much of it comes from the unhairing-liming stage, where hides are soaked in a potent alkaline bath of lime and sodium sulfide to dissolve hair and loosen the epidermis. The liquid that drains away from this step is among the most aggressive industrial effluents anywhere: it can carry sulfide concentrations high enough to release lethal hydrogen sulfide gas on contact with acid, alongside a heavy load of dissolved organic matter that smothers rivers and wrecks biological treatment plants. In Morocco, where a large leather sector clusters around historic cities such as Fez, this wastewater has long posed a stubborn problem for regulators and operators alike. Now a team of Moroccan researchers reports a hybrid treatment train that pairs a simple chemical precipitation step with a biological sequencing batch reactor, achieving removal efficiencies above 99 percent for both sulfide and chemical oxygen demand, and bringing the treated water fully within Moroccan discharge standards.

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