**Abstract**

The tertiary treatment of resulting water from a conventional biological treatment process is envisaged in the aim to obtain a high quality of water that can be reused for different purposes. This treatment is based on the integration of the membrane-based technologies in the total process of wastewater treatment. The experimental studies are carried out on a small pilot, equipped with different mineral membranes of micro and ultrafiltration. These membranes are used for the different tested processes (MF, MF-UF and cogulation-MF). The results obtained make it possible to attend a complete elimination of the total flora and an additional reduction of the other parameters such as turbidity, suspended matter, COD and BOD. Tests on a large scale are then carried out on a semi-industrial pilot, equipped with the same type of membranes. The optimization of the operating conditions made allow the obtaining under the conditions of transmembrane pressure 0.85 bar, a cross flow velocity of 2.25m/s and with ambient temperature a filtrate flux of about 200 L/hm <sup>2</sup> . The coupling of a stage of coagulation in the membrane process allows the reduction of the effect of the membrane fouling and an improvement of 36% of the filtrate flux.

**Keywords:** treated wastewater, tertiary treatment, microfiltration, ultrafiltration, reuse

### **1. Introduction**

Population growth and economic development are putting pressure on water resources, especially in arid regions. Indeed, most MENA countries will have annual renewable water resources of less than 1000 m3 /capita by the year 2025, according to estimates and projections of country-based populations and annual renewable water resources [1]. Moreover, the majority of MENA countries were classified as having a water deficit in 2010 (less than 500 m3 /capita) [1]. Consequently, there is a need for new non-conventional water resources, such as water desalination, wastewater and rain harvesting, to meet the increasing demand. Wastewater reuse is gaining increasing attention for groundwater recharge and irrigation, since agriculture is the dominant water user in the region [2, 3].

Indeed, wastewater reuse for irrigation offers some attractive environmental and socioeconomic benefits [4–6]. In fact, the irrigation with treated wastewater leads to supply nutrients as fertilizer [7] as well as improvement crop production during the dry season [8, 9]. However, planners are aware of the potential disadvantages of wastewater reuse for irrigation which are, aside from pathogenic contamination

of irrigated crops, mainly related to their specific chemical composition being somewhat different from most natural waters used in irrigation [10]. Wastewater generally contains high concentrations of suspended and dissolved solids, both organic and inorganic (e.g. chloride, sodium, boron and selected heavy metals), that are added to wastewater during domestic and industrial usage [11]. Most of the salts added are only partially removed during conventional sewage treatment (secondary and tertiary), so they remain in the irrigation water [12]. Its content of trace elements, pathogens and high nitrogen may present a risk to the receiving environment. Additional treatment, particularly at the microbiological level, therefore appears necessary to ensure both user safety and reduce the impact on the receiving environment [13]. In this regard, membrane processes appear very promising for the complementary treatment of treated wastewater (TWW) [14]. Indeed, there is growing interest in direct filtration of wastewater treatment process for water reclamation to ease global water shortages [15–19]. Nowadays, integrated membrane systems treatment is becoming widely popular due to its feasibility, process reliability, commercial availability, relative insensitivity in case of wastewater treatment and lower operating costs [20]. Especially, direct filtration using ceramic membrane has been considered as an attractive option due to properties of ceramic membrane (e.g. a high durability and a high chemical resistance) [21–23].

In Tunisia, the reuse of treated wastewater (mainly secondary treated wastewater) for agricultural purpose is restricted for forages crops irrigation only. In Sfax (center east of Tunisia) where the average annual potential evaporation of 1200 mm, combined with the low rainfall and high temperatures, irrigation proves to be essential for crop production. Therefore, the treated wastewater has been used for forages irrigation since 1989. This practice had led to soil fertility improvement [24]. By contrast, an increase of soil salinity [25] and metallics elements accumulation has been detected [26]. In order to minimize health and environmental risks and for unrestricted irrigation reuse, the final treated wastewater quality should be improved.

The aim of this work is to study the feasibility of membrane techniques application, in particular microfiltration and ultrafiltration, for the tertiary treatment of the treated wastewater. Our study involves qualitative optimization trials, to define the appropriate treatment process. Three methods were tested, MF alone, MF-UF coupling and coagulation-MF coupling. Other tests to optimize the operating conditions and permeate flow are carried out, to evaluate the selected membrane process.
