**4. Characterization of industrial tannery wastewaters**

Tannery wastewater contains significant content of chemical substances, including toxic compounds. Thus, several parameters were carried out to characterize these tannery wastewaters. Among these parameters, there are physicochemical analyses such as pH, temperature, electrical conductivity, turbidity, suspended solids (SS), chemical oxygen demand (COD), sulphate ions (SO4 2−), nitrite (NO2 − ), nitrate (NO3 − ), ammonium (NH4 + ), orthophosphate (PO4 3−) and sulfide ions (S2−).

The pH was measured using a pH meter HANNA with a type electrode Senti X 22 according to NF T90.008 [25]. Conductivity and turbidity were measured by ORION type conductivity. Suspended solids (SS) were determined by centrifuging a wastewater volume according to standard NF T90.105 [25]. Sulfide ions were measured by the indirect method according to standard NF T 60–203 [25]. Ammonium, orthophosphate, nitrate, nitrite, sulfate, BOD5 and COD were carried out by the spectrophotometric method using a DR/2005HACH at a fixed wavelength and according to AFNOR standards issued by Rodier J. et al. [25]. The bacteriological parameters were also evaluated, especially total coliform (TC), fecal coliform (FC), fecal sterptocoques (SF), total aerobic mesophilic (FMAT) and staphylococci. The enumeration of these bacteria was performed using desoxycolate lactose, slanetz, lauriabertani and Chapman respectively [25].

In general, the average temperature of R1, R2, R3 and R4 was between 24 and 27°C. In fact, these effluents take usually the environmental temperature, because of the absence of heating or cooling operations. The **Figure 2** shows the results of pH, conductivity and suspended solids for the fourth rejects (R1, R2, R3 and R4). So, R1 was largely basic, R2 had also a basic pH, R3 was slightly basic, whereas R4 was acidic (**Figure 2a**). The use of sodium carbonate and bicarbonate, during the first operations, could explain this basic pH of R1, R2 and R3. Concerning R4, the use of acids, for the solubilization ofchromium salt, could explain its acidic pH. The values obtained are comparable to those found in previous work on wastewater from tanneries that have a weakly basic pH [4, 26–28].

As for the conductivity, its average value was around 10 and 30 ms/cm, and then it was largely exceeding the Moroccan standards [29]. The largest values are recorded for unhairing-liming reject (R1), deliming-bating reject (R3), and

#### **Figure 2.**

*Results of physical parameters within the fourth rejects of a modern tannery before and after settling: (a) pH; (b) conductivity; (c) suspending matter.*

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

chromium tanning reject (R4) (**Figure 2b**). These high values of conductivity show significant use of salt during these tanning operations (**Table 1**). For suspended solids (SS), they had a high amount, which achieved more than 5000 mg/L for all effluents (R1, R2, R3 and R4) (**Figure 2c**). However, R1 had the highest amount of SS; this could be justified by the huge organic matter (Proteins, hair, fat ….) eliminated from animal skin during this step.

Concerning nitrogen compounds, ammonium, nitrate and nitrite have been followed and their average concentration has shown in **Figure 3**. R1 had again a high concentration of nitrate, nitrite and ammonium compared to R2, R3 and R4 (**Figure 3a–c**). This could be explained by the use of ammonium during unhairingliming unit (**Table 1**). Indeed, nitrate amount was higher than nitrite and ammonium amount, this could be explained by the reduction of ammonium to nitrate passing by nitrite. This reduction could be through chemical reactions or bacteria such as *Nitrobacter, Nitrosomonas* … etc. [29]. On the other hand, these fourth rejects were conformed to Moroccan standards of discharge in term of nitrogen compounds [29]. These results of nitrate, nitrite and ammonium ions are consistent with those of some authors [30, 31].

**Figure 4** presents the result of phosphate ions. As shown, their amount was largely lower than the Moroccan standards reject for all tannery effluents (R1, R2, R3 and R4). Furthermore, R1 had the highest phosphate amount due to the use of some phosphate chemicals in this unit.

Concerning sulfate ions, **Figure 5a** shows that R3 and R4 were very loaded in sulfate than R1 and R2. This could be due to the use of sulfate chemicals in these units. Nevertheless, the tannery effluents were exceeded the Moroccan standards except for the reject R2 (**Figure 5a**). These high loads were due to the use of many

**Figure 3.**

*Nitrogen compounds of tannery effluents before and after settling: (a) nitrate; (b) nitrite; (c) ammonium.*

*Promising Techniques for Wastewater Treatment and Water Quality Assessment*

**Figure 4.** *Composition of the effluent studied before and after settling: Orthophosphate ions.*

**Figure 5.**

sulfate products during deliming and tanning units as chromium sulfate [32]. Concerning sulfide, the R1 had the highest load which approximates 1600 mg/L (**Figure 5b**). This huge amount could be justified by the use of the sulfide and sulfuric acid during the unhairing-liming step to eliminate hair of the animal skin. Indeed, the effluent of R1 is largely alkaline; which proves the presence of hydrosulfide HS- ions according to the Pourbaix diagram [33]. The results obtained are consistent with those found by [30] for the final rejection and those found by [34].

On the other hand, the organic load of the effluent is evaluated by measuring the chemical oxygen demand (COD) (**Figure 6a**) and biological oxygen demand BOD5 (**Figure 6b**). The **Figure 5** shows that R1 is very loaded in COD and BOD5 than others (R2, R3 and R4) exceeding Moroccan standards reject [29]; this could be justified by the high amount of chemicals used in this first unit and the organic matter eliminated as well. The same figure reveals that all effluents are non-biodegradable because of the report COD/BOD5, which was higher than 4 (**Figure 6c**). The concentrations found in COD are comparable to results obtained by several authors [30, 31].

Even if the characterization of these four rejects after settling shows a slight reduction of all physicochemical parameters, their amount did not meet Moroccan standards.

The microbiological analyses showed a low concentration of the total aerobic mesophilic flora (FMAT), which the average value was 300, 400 and 700 CFU/mL *Treatment of Tannery Effluent of Unit Bovine Hides' Unhairing Liming by the Precipitation DOI: http://dx.doi.org/10.5772/intechopen.97657*

**Figure 6.**

*Composition of the effluent studied before and after settling: (a) chemical oxygen demand; (b) biological oxygen demand; (c) ratio COD/BOD5.*

respectively for R1,R2 and R4 (**Table 2**). For pathogenic and fecal germs (Staphylococci, fecal streptococci, and the fecal coliform), **Table 2** reveals an absence of all of them. This could be explained by the high concentration of salts that may inhibit bacterial growth [35], and also by the toxicity of chromium which is present with a very high concentration [36]. However, we can conclude that the obtained germs (MTAF) may be halophilic and chromium bacteria.

In conclusion, tannery effluents are very complex, toxic and loaded in organic and inorganic matter, especially the first reject R1 of the unhairing-liming unit. Furthermore, R1 had a huge amount of sulfide, which could easily reduce to hydrogen sulfide under anaerobic conditions. As mentioned above, this toxic gas harms all living organisms, including human health. Therefore, the treatment of R1 has been very essential to remove sulfide ions.


*CFU, colony forming units; FC, fecal coliform; TC, Total coliform; SF, Fecal sterptocoques; FMAT, total aerobic mesophilic.*

#### **Table 2.**

*Microbiological characterization of the wastewater of different operating tanning units.*
