**Table 3.**

*Treated wastewater quality.*

By referring to the discharge standards (**Table 3**), a gradual improvement in the physicochemical parameters of the water during the study period is observed. In fact, the high values of turbidity and SS observed during the January campaign were subsequently greatly reduced. This can be attributed to the appearance of the phenomenon of bulking (expansion of sludge) in the station during the month of January and its disappearance thereafter. The effluent remains difficult to biodegrade since the BOD/COD ratio is usually less than 0.3. The heavy metal contents are low, which leads to the conclusion that there are no industrial discharges in the station. Thus, if the physical and chemical qualities of the treated wastewater are generally close to the standards, the biological quality still remains high. Tertiary treatment could then complete the treatment process and leads to water quality that meets all the requirements.

#### **3.2 Tertiary treatment**

The improvement of the final quality of the effluent and in particular the biological quality is studied by applying membrane processes. Two axes are developed. First, improving the final water quality by testing different processes. Then, define the operating conditions which ensure the best flow of permeate.

#### *3.2.1 Qualitative study*

#### *3.2.1.1 MF-UF coupling*

The process is composed by two-stage (**Figure 1**). In the first, the effluent undergoes microfiltration with recovery of the permeate which is then treated in a second stage by ultrafiltration. Two tests were carried out for this process using each time different microfiltration membranes.

During the first test, microfiltration and ultrafiltration are carried out on a bench-top pilot equipped with membranes of pore size 0.1 μm and 15 KDa respectively. The main results obtained as well as the retention efficiency (RE) are reported in the **Table 4**.

Qualitatively, the first treatment with MF leads to an elimination of more than 90% of the turbidity and the SS. The COD and the BOD are also reduced to values lower than those of the Tunisian standard of discharge in the receiving environment (respectively 90 and 30 mg/L). After the second treatment with UF, most of the parameters analyzed undergo an additional reduction (**Table 4**). Moreover, UF is more suitable for removal COD particles [7]. For this test the analysis of biological parameters was not performed.

The evolution of the permeate flow during the filtration test shows that the flows are lower in MF than those obtained after MF-UF. Indeed, the values obtained are respectively in the order of 25L/h m2 and 150 L/h m<sup>2</sup> (**Figure 4**). In fact, the decrease of the permeate flux to more than the half during the first 20 minute of the filtration is caused by the clogging phenomenon of the membrane due to the colloidal fraction in the effluent. In addition, the importance of membrane fouling leads to the drop of MF flow to a relatively low value at the end of experiment. However, in the second step of UF; the permeate flux is higher despite the small size of membrane pores. In fact, the majority of particles and colloids have been already eliminated after the first step of MF.

In order to improve the permeate flux of the first microfiltration step, a membrane of greater porosity (0.2 μm) was used during a second test of the MF-UF coupling, while keeping the same characteristics of the UF membrane of the second stage. During this test, complete elimination of turbidity, MES and total

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


#### **Table 4.**

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

**Figure 4.** *Permeate flux decline for MF (0.1* μ*m) step and MF-UF step (small scale pilot).*

flora was observed. However, the reduction in COD was lower, not exceeding 40%. The existence of small organic particles, not filtred, may be the cause of the low reduction in COD. However, a significant improvement in the permeate flux of the MF was observed. Thus, stabilized flow rates of around 90 L/h m2 are obtained. In the second stage of UF, the same performance of the previous test is obtained, ie a stabilized flow rate of around 150 L/h m2 (**Figure 5**).

Likewise, significant clogging of the first stage MF membrane was observed. In order to limit the consequences of this problem, an additional pre-treatment step appears essential.

#### *3.2.1.2 Coagulation-MF coupling*

In this process, microfiltration was preceded by coagulation pretreatment (**Figure 1**). Alumina sulfate is chosen as the coagulant.

In order to optimize the dose of used coagulant, varying amounts of alumina sulfate are added (between 20 and 100 mg/L). Stirring is performed with a Jar Test. After settling, COD measurements are taken. The best reduction in COD is obtained with a dose of 40 mg/L (**Figure 6**).

Indeed, a dose of 40 mg/L of coagulant was added to the raw effluent. After stirring and settling for 24 hours, the water is microfiltered through a 0.2 μm membrane. This coagulation pretreatment has led to an improved of turbidity and a 35% reduction of COD value (**Table 5**). After MF, a reduction in SS, color and BOD values is observed with retention efficiency of 76%, 31% and 75% respectively.
