Treatment of tannery wastewater by infiltration/percolation process using natural clay combined solid wastes

This work aims to treat the raw effluents from a tannery using raw coal waste (fly ash and bottom ash) and bentonite clay as less expensive adsorbents and percolating agents. Their combination constitutes effective filters to retain heavy metals, dyes, and organic matter. After analysis of the raw wastewater, a significant pollutant load was detected; the chemical oxygen demand (COD) is around 3,400 mg center dot L-1, the biological oxygen demand about 240 mg center dot L-1 and suspended matter of 490 mg center dot L-1. These waters are also heavily loaded with heavy metals especially chromium with a high value of 981 mg center dot L-1, and they have an acidic pH of 4.2, which greatly exceed the specific limit discharges values, applicable to discharges from Moroccan tannery industries. Their treatment with fly ash, bottom ash, and bentonite clay leads to an almost complete elimination of Cr, Fe and Zn with a discoloration of 96% where the pH becomes neutral to slightly alkaline. The COD analysis gives an abatement rate of more than 72% while suspended matters are considerably reduced with a turbidity rate of 99%. Thus, the treated water can be used for irrigation or as wash water in industries.