*3.2.2 Comparative study of the different processes*

The quality of the different treated water from the various processes tested is compared to the initial quality of the effluents as well as to Tunisian standards for reuse in irrigation (**Table 6**).

**Figure 5.** *Permeate flux decline for MF (0.2* μ*m) step and MF-UF step (small scale pilot).*

**Figure 6.** *Optimization of coagulant dose.*

#### *Tertiary Treatment for Safely Treated Wastewater Reuse DOI: http://dx.doi.org/10.5772/intechopen.94872*


#### **Table 5.**

*Treated wastewater quality after coagulation and coagulation-MF treatment.*


#### **Table 6.**

*Quality of raw effluent and treated by the different processes.*

Membrane techniques (MF and UF) do not have a great influence on EC and TDS. Their selectivity is far from stopping mono- and bivalent ions. However, they are effective in removing turbidity and SS. The use of these techniques also leads

to an improvement in color, especially after use of UF which provides effluent whitening [28].

In addition, more than 50% of COD and BOD are eliminated during water treatment by the various methods used. The ranges of variation of these values are quite wide and reflect the influence of the quality of the water to be treated on these two parameters. In particular, the residual COD values may reflect the existence of small particles that escape filtration [29]. In general, the final quality of the treated wastewater, whether by the membrane technique or by coagulation-MF coupling, meets Tunisian standards for agricultural irrigation (NT 106–03).

It was also found that the involvement of a membrane technique in the treatment process eliminates the total flora from the treated water. This is because the size of bacteria on the one hand and the clogging of the surface of the membranes used on the other hand combine to greatly reduce the passage of bacteria through the pores of the membranes. However, there can be easy contamination of treated water due to the presence of nutrients such as nitrates and phosphorus [15, 28].

The results obtained also make it possible to observe that there is a reduction in the concentrations of heavy metals in the treated water despite their low concentrations in the initial effluent. These results can be attributed not to the membrane technique used but rather to the retention of organic colloids with which metals are generally associated [14].

All the results obtained show that, despite the variation in the quality of the water collected at the outlet of the Mahrès station, additional treatment by membrane filtration allows the elimination of the total flora and the improvement of other physico-chemicals parameters. In fact, the use of microfiltration alone ensures good quality treated water, complying with standards and can be reused without restriction. On the other hand, the coupling of MF to UF or to coagulation rather has an effect on the quantity of treated water and not on the quality.

#### *3.2.3 Quantitative study*

The optimization of the operating parameters is carried out on a semi-industrial pilot equipped with the same type of membrane with a porosity of 0.1 μm and a filtering surface of 800 cm2 . Two optimization trials were performed. The first is to do a single microfiltration step while a coagulation-MF coupling was tested in the second test.

The MF test is carried out with an initial volume of 20 L and under the following operating conditions: a transmembrane pressure TMP of 0.85 bar, a circulation speed U of 2.25 m/s and at room temperature. The initial flux is very high, around 800 L/hm<sup>2</sup> which, after 40 min, stabilizes at a value of 200 L/hm2 . The volume reduction factor (FRV) reaches a value of around 3.5, thus reducing the volume treated to 5.7 L.

For the second MF-coagulation test, 36 L of wastewater was pretreated by adding 40 mg/L of alumina sulfate. The supernatant is then microfiltered under the same operating conditions as the previous test. After 70 minutes, 31 liters of permeate are recovered which corresponds to an FRV of around 8.5. The stabilized permeate flux is approximately 200 L/hm<sup>2</sup> (**Figure 7**).

It appears that the coagulation step led to, on the one hand, reduce the major part of the colloids present in the raw effluent and on the other hand, to achieve very high FRV values. This coupling therefore results in an improvement in permeate flow of around 36% compared to MF alone (**Figure 7**). However, the introduction of this step in an overall sanitation process introduces two drawbacks, one relates to the use of coagulant and the other to the contact time required for this operation.

*Tertiary Treatment for Safely Treated Wastewater Reuse DOI: http://dx.doi.org/10.5772/intechopen.94872*

**Figure 7.** *Permeate flux decline for MF and coagulation-MF coupling (semi-industrial pilot).*

It was also observed that during the coagulation-MF coupling, the unclogging of the membrane became easier than before. In fact, the membrane returns to its initial state, after a simple circulation of distilled water. Indeed, it was found that coagulation is the more efficient pretreatment before filtration processes [30–32]

### **4. Conclusion**

The results obtained show that, despite the variation in the quality of the treated wastewater, additional treatment involving a membrane separation technique (MF) made it possible to eliminate the microbiological danger and the improvement of other parameters (biological and physicochemical). Coupling this technique with another process (coagulation or UF) does not lead to a significant improvement in the quality of the treated water. However, such a coupling can have an influence on the amount of water to be treated. Indeed, the pretreatment with coagulation before microfiltration improves the permeate flow and decrease membrane fouling compared to MF alone.

The integration of this microfiltration step on the scale of the wastewater treatment plant makes it possible to increase the available reserves of good quality water and to widen the fields of their uses. In fact, the unrestricted reuse of this treated water for the irrigation of crops of high economic profitability makes it possible to amortize investment costs while guaranteeing the health security of farmers.

*Promising Techniques for Wastewater Treatment and Water Quality Assessment*
