**5. Treatment of the unhairing-liming unit wastewater by precipitation**

Several processes have been studied for the treatment of tannery wastewater, using simple and advanced methods. These processes include physicochemical treatments such as electrochemical methods [37, 38], filtration [28, 39], ion exchange [40], membrane filtration [41, 42], precipitation [43, 44], coagulation [5, 45], solvent extraction [46, 47], reverse osmosis [48, 49], adsorption [45, 50] and aerobic or anaerobic biological systems [51–53].

However, the high operating costs, the large amount of used chemicals, and the production of sludge are the main disadvantages of traditional chemical processing [2, 54]. On the other hand, advanced treatment techniques, such as reverse osmosis, ion exchange and membrane filtration are very expensive and generate another waste [54–56].

In fact, a dechromatization station was performed to remove the chromium from R4 of tannery industries in Doukkarat area in Fez city, Morocco. Nevertheless, there is not any plant to reduce sulfide toxicity in this Moroccan city. Thus, the elimination of sulfide ions is mandatory, but the removing process should be nonexpensive and efficient. As mentioned above, R1 is non-biodegradable, and then, the physicochemical treatment is the best adequate treatment.

The chemical precipitation process is relatively simple and inexpensive. There are many precipitant agents such as ferric chloride, aluminum sulfate … etc. [21]. The principle of this treatment is based on the production of the insoluble complex from pollutants and chemical agent. Furthermore, the conventional chemical precipitation processes include hydroxide precipitation [57] and sulfide precipitation [58, 59]. According to the literature, ferric chlorides can react with sulfide ions to produce a complex compound. For this reason, chemical precipitation using ferric chloride may be a great process to remove sulfide ions from R1.

Khatoon et al. [60], showed that COD and chromium could be treated by coagulation with an elimination rate of 38 to 46% for the suspended matter and 30 to 37% for the Total COD. The chromium elimination rate is 74 to 99% for an initial concentration of 12 mg/L using a coagulant dose of 800 mg/L with an optimal pH of around 7.5. This study showed also that ferric chloride gave better results than aluminum sulfate.

Other studies [61, 62] consist of the elimination of sulfur compounds from unhairing-liming effluent after a preliminary settling for one hour, following by filtration in a sintered glass. This glass had a porosity of 10 microns and a diameter of 70 mm.

For the treatment of tannery effluent [61, 63], particularly unhairing-liming effluent, a volume of a FeCl3 solution was gradually added to 200 ml of this effluent until the formation of an insoluble complex. Afterward, these two phases (Liquid/ solid) are separated mechanically and the liquid phase was only analyzed.

This treatment is based on the reduction of sulfide ions by ferric chloride FeCl3 in a slightly basic medium according to these reactions, which were established by those authors [20, 64–67].

$$\text{2Fe}^{3+} + \text{HS}^{-} \rightarrow \text{2Fe}^{2+} + \text{S}^{0} + \text{H}^{\*} \tag{1}$$

$$\text{Fe}^{2+} + \text{HS}^{-} \rightarrow \text{FeS} + \text{H}^{\*} \tag{2}$$

$$2\text{Fe}^{3+} + 3\text{HS}^{-} \rightarrow 2\text{FeS} + \text{S}^{0} + 3\text{H}^{\*} \tag{3}$$

$$\text{FeS} + \text{S}^0 \rightarrow \text{FeS}\_2 \tag{4}$$

*Treatment of Tannery Effluent of Unit Bovine Hides' Unhairing Liming by the Precipitation DOI: http://dx.doi.org/10.5772/intechopen.97657*

#### **Figure 7.**

*(a) Effect of pH on the removal of sulfide ions and COD of effluent unhairing-liming; (b) elimination rate of sulfide ions in terms of the concentration of ferric ions at the pH of the medium (operating conditions: pH = 8.5, T = 24°C, [S2−]0 = 1570.94 mg/L).*

According to the first Eq. (1), ferric ions react with sulfide ions to produce elemental sulfur. Afterward, the product ferrous ions will also react with sulfide ions to produce FeS precipitate. Otherwise, the reaction between ferric ions and sulfide ions may produce FeS and elemental sulfur according to the third Eq. (3). Finally, the FeS is converted to pyrite (FeS2) according to the fourth reaction (4). The precipitation depends strongly on the medium pH and the concentration of ions ferric [37–40].

**Figure 7a** reveals that the best removal rate of sulfide ions and the chemical oxygen demand was at the pH 8.5. Meanwhile, **Figure 7b** shows that the abatement rate of sulfide ions and COD increases when ferric ions (Fe3+) concentrations increase too, and then this removal stabilizes at a value around 85% and 90% respectively for sulfide ions and COD, starting a ferric ions concentration of 1.4 mol/L. This could be explained by the high presence of hydrogen sulfide ions (HS− )at pH of 8.5 according to the Pourbaix diagram [33].

The adjustment of pH effluent was performed by the addition of sulfuric acid (H2SO4) at a concentration of 1 N to obtain pH values between 7 and 12. For optimization of ferric chloride concentration, different concentrations were carried out ranging from 0.2 to 1.8 mol/L and using a pH effluent of 8.5 (**Figure 7b**).

The results show that the COD and sulfide ions had the same evolution of elimination depending on the pH and concentration of ferric ions. COD removal reached 90% at pH 7, 8 and 11 for ferric ion concentrations of 1, 1.2 and 0.8 mol/L respectively. As to sulfide ions, their removals achieved 90% and 84% at pH 7 and 8 using ferric ion concentrations of 1.6 and 1.8 mol/L respectively.

#### **6. Conclusion**

The main objective of this chapter was the characterization of different effluents of a modern tannery, giving a Moroccan modern tannery of Fez city as an example, and the treatment of unhairing-liming effluent, which was very loaded in sulfide.

The physicochemical characterization, of the fourth rejects of this modern tannery, showed a huge organic and inorganic pollution of these effluents, particularly unhairing-liming effluent that is largely alkaline and characterized by a huge organic and mineral pollution such as sulfides. However, the biological characterization revealed that these four effluents were empty from fecal and pathogenic germs due to their high inorganic toxicity. Otherwise, chemical precipitation using ferric chloride could remove a big amount of COD and sulfide ions during the

#### *Promising Techniques for Wastewater Treatment and Water Quality Assessment*

treatment of unhairing-liming effluent. The abatement rate of sulfide ions reached 90% using a pH effluent of 8.5 and a ferric ions concentration of 1.4 mol/L.

In conclusion, the treatment of unhairing-liming wastewater could contribute to the protection of wildlife from the toxicity of sulfide ions through the reduction of the emission of greenhouse gases (Hydrogen Sulfide). Furthermore, chemical precipitation may be the best treatment for this type of effluent due to the high sulfide removal.
