Industrial wastewater causes several environmental and health issues due to its composition, and hydrogen sulfide is one of the main contaminants from various industrial activities, including tannery wastewater treatment plants. This study aimed to evaluate the application of a modified tannin to remove hydrogen sulfide, chemical oxygen demand, total nitrogen, color, and turbidity from tannery wastewater and to reduce the atmospheric emission of sulfides. The wastewater was treated using modified tannin, aluminum sulfate, and a combination of both. Gaseous sulfide emissions were analyzed qualitatively using paper soaked in lead acetate. The qualitative methodology of hydrogen sulfide in the atmosphere was efficient to identify the most critical areas in the tannery. The wastewater treatment tests showed that the best option was the combination of both coagulants, in a 50:50 ratio, resulting in a total coagulant dosage of 3,000 mg/L. The removal efficiencies achieved were 13.8% sulfides, 23.3% chemical oxygen demand, 9.1% total nitrogen, 97.3% turbidity, and 96.4% color. This treatment technique promotes the replacement of 50% of the aluminum sulfate currently used in the tannery by a non-toxic coagulant and does not require chemicals to adjust the pH of the wastewater.