Brackish water pre-treatment method: selection of the study area and water sample testing

Water is an essential element for human activities, and it is used for domestic, agricultural, and industrial purposes. The existing water on the earth is about 332.5 million cubic miles, but 96% is salty and only 2.5% is fresh (Gleick 1993). Faced with the demographic and economic development that humanity has experienced, this resource is becoming increasingly scarce; around 40% of the world's population, which is predicted to increase to 60% by 2025, currently has severe water shortages (Ibrahim et al. 2017). Thus, the endowment of water resources in the world went from 9,317 m³/capita/year in 1980 to 5,500 m³/capita/year in 2020 (Official website of the world bank).

Morocco, in particular, suffers from water stress. In fact, the endowment of water resources in Morocco has decreased from 2,560 m³/capita/year in 1960 to 632 m³/capita/year in 2020. In addition, Morocco will have a deficit of nearly 2.3 billion m³/year by 2030 (Report by the Directorate of Financial 2020). Faced with this situation, the desalination of sea water and brackish water is among the solutions that have gained popularity in recent decades in several countries around the world (Jones et al. 2019). This technique remains a good alternative to conventional water supply (surface water, groundwater).

A major issue still remains to be resolved, that of access to water for rural populations in the most remote areas. In this sense the use of small desalination units based on renewable energy remains one of the most widespread solutions (Chafidz et al. 2016; Yoon et al. 2022). Their development requires, in particular, work in the field to locate sampling areas of the water to be desalinated and the analysis of these physio-chemical characteristics. This analysis constitutes an important step in characterizing water and then subsequently developing the appropriate pre-treatment methods (Maazouzi et al. 2013).

In this perspective, we will propose a brackish water pre-treatment method before desalination to serve the rural populations in Morocco who are suffering from lack of water. Considering the performance of this technique in terms of water quality (Hafez et al. 2009) and the importance of taking water characteristics into account before designing and validating the various techniques to be used, the choice of the deployment area for the desalination and analysis of water test samples reveals a crucial importance to be able to identify the populations to target, the analyses to prepare, and consequently the characteristics of the raw water that should be expected throughout the project (Gourai et al. 2015).

Through this study, we were able to identify potential provinces and prioritized based on predefined criteria to optimize sampling locations and install brackish water pre-treatment units before desalination. Starting with analysing brackish water samples, we focus on mineral content and physico-chemical characteristics, crucial for validating pre-treatment techniques before desalination. This aids in proposing a pilot model for pre-treatment techniques later on.

The objective of this section is to determine the provinces in which samples will be taken to analyse brackish water and to be able to subsequently adapt the water pre-treatment processes. This is done first through the identification of the selection criteria by the research group: the geographical proximity to the greater Casablanca area, the accessibility of the population to drinking water, the number of the targeted rural population, and the solar energy potential of the area.

To determine the target area, we decided to choose among the provinces of Morocco. For logistical and financial reasons, it was also decided to eliminate provinces with a distance of more than 200 km from Casablanca (National portal 2023). The potential provinces are then presented subsequently (Figure 1).

We included this criterion because of the financial costs of travel and the costs necessary to carry out the sampling. To control the financial aspects of the project, a maximum distance of 200 km from Casablanca was set (Table 1).

The number of rural populations in the study area is an important criterion as the project targets the largest possible rural population to ensure the impact of our solution will reach the maximum number of rural populations. Below is the number of rural populations in the provinces concerned. The data are taken from the general census established by the HCP (High Commission for Planning) in 2014 (HCP official website) (Table 2).

This mapping gives the global horizontal irradiation in kWh/m²/year of Morocco. We have selected the concerned provinces, and we have been able to summarize their respective irradiation in Table 3.

Before embarking on the water quality analysis of the samples which will come from the selected provinces and for reasons of convenience, we decided to launch a test to collect and analyse some samples in the commune of Benslimane at the level of four wells with different depths in meters (P1/200 m, P2/120 m, P2/20 m, and P3/15 m). It was also decided, for comparative purposes, to include another source of brackish water in the Tissa area and a sample of seawater in EL Mansouria.

Twenty parameters were measured, five of which were carried out in the field after calibration and/or validation of the portable equipment: temperature, conductivity, pH using a multi-parameter analyser Type CONSORT – Model C535, and turbidity using a turbidity meter Type HACH-Model 2100P. Table 4 summarizes the method used and the unit of measurement for each parameter analysed.

For nitrogenous elements, water samples are fixed with concentrated sulphuric acid in a 500-mL polyethylene bottle and with concentrated nitric acid for Na⁺ and K⁺ cations in a 250-mL polyethylene bottle. The other unfixed parameters were sampled in a 1 L polyethylene bottle. The bottles of water taken, in accordance with Moroccan standard 03.7.059 (Moroccan standard 2020), are labelled and then sent to the laboratory in a cool box at a low temperature of ±4 °C, accompanied by a card bearing all the necessary information, in particular the origin, date, and time of sampling. Water samples are taken, transported, and stored in accordance with the protocol of national drinking water office (commonly called ONEP) quality control laboratory and standard norms (ONEP 2007; ONEP 2008).

The methods used in the Laboratory of Natural Resources and Environment (LNRE) of the Polydisciplinary Faculty of Taza (FPT) are as follows: volumetry for bicarbonates, chlorides (Cl⁻), calcium, and magnesium (Ca²⁺ and Mg²⁺); molecular absorption spectrophotometry for sulphates, nitrates, nitrites, ammonium ions, and orthophosphates; and flame spectrophotometry for sodium and potassium (ABOUZAID and DUCHESNE 1984; Rodier 2009).

To summarize the results of each province by criteria, Table 5 represents an overview of the main statistics of our case study.

To show once again the deviation of each province value from the global variation of each criterion, we used the normalization technique to allow aggregation of criteria with numerical and comparable data.

In our case, we aim to select the provinces that have the best possible combination to minimize the distance to Casablanca and maximize the other criteria.

Table 6 summarizes the calculation of the normalized values. For the criterion of the distance, the formula of the Min-Max Scaler will be preceded by 1 minus the value found by the formula to show the punishing aspect to have a big value for the distance. For the rest, we write directly the result calculated only by the formula, and the final score for each province will be the sum of its normalized values for all criteria.

We note after the calculations made above that we can select the main provinces that have the highest scores (the top 6) to carry out several samples in each province. According to our study, these are the following provinces: Settat, El Jaddida, Khouribga, Berrechid, Sidi Bennour, and Benslimane.

From these figures, we can extract the following observations:

• For the sample from P1/200 m, the elements Cl⁻ and Na⁺ represent 73.31% with 2,128 and 1,686 mg/L, while represents 9.95% (518 mg/L) and represents 3.13% (163 mg/L), exceeding World Health Organization (WHO) thresholds and recommendations of 200 mg/L, 200, 250, and 50 mg/L, respectively (Diouf et al. 2022). The elements responsible for the hardness of the water (Mg²⁺ and Ca²⁺) represent 5.09% and 7.07%, respectively, with a concentration of 265 and 368 mg/L, exceeding WHO thresholds ranging from [50–150 mg/L] to [75–200 mg/L], while the rest of the elements are less than 1.42%.

• For the sample from P2/120 m, the elements Cl⁻ and Na⁺ represent 71.47% with 3,692 and 2,646 mg/L, while represents 9.73% (863 mg/L) and represents 2.51% (223 mg/L), exceeding WHO thresholds and recommendations already mentioned earlier. The elements responsible for the hardness of the water (Mg²⁺ and Ca²⁺) represent 7.96% and 7.21%, respectively, with a concentration of 706 and 640 mg/L, exceeding WHO thresholds ranging from [50 mg/L–150 mg/L] to [75 mg/L–200 mg/L], while the rest of the elements are less than 1.1%.

• For the sample from P2/20 m, the elements Cl⁻ and Na⁺ represent 61.83% with 989 and 789 mg/L, while represents 12.48% (359 mg/L) and represents 10.05% (289 mg/L), exceeding WHO thresholds and recommendations already mentioned. The elements responsible for the hardness of the water (Mg²⁺ and Ca²⁺) represent 7.34 and 7.23%, respectively, with a concentration of 211.2 and 208 mg/L, exceeding WHO thresholds and recommendations already mentioned, while the rest of the elements are less than 1.03%.

• For the sample from P3/15 m, the elements Cl⁻ and Na⁺ represent 43.71% with 823 and 667 mg/L, while represents 39.37% (1,342 mg/L) and represents 5.05% (172 mg/L), exceeding WHO thresholds and recommendations already mentioned. The elements responsible for the hardness of the water (Mg²⁺ and Ca²⁺) represent 6.25 and 4.69%, respectively, with a concentration of 213.1 and 160 mg/L, exceeding WHO threshold ranging from [50–150 mg/L] for Mg²⁺, while the rest of the elements are less than 0.89%.

The concentrations of the different components as well as their variations in relation to the depth of the wells can be justified by taking into account the location of the sampling zone and the following elements:

• A possible marine intrusion: As discussed by Diouf et al. (2022) and according to Bear et al. (1999) and Moujabber et al. (2006), based on the ratio of concentrations in mg/L of Ca²⁺ and Mg²⁺, the enrichment of Ca is given by Equation (2):

  

  (2)

According to Bear et al. (1999), high ratios of Ca enrichment (>1) could be regarded as saltwater intrusion. After calculating this ratio, we can see that it varies between 0.75 and 1.38 (P1/200 m: 1.38, P2/120 m: 0.91, P2/20 m: 0.98, P3/15 m: 0.75). We can also see that this rate generally increases with depth, which gives an indication of possible marine intrusion in the deep layers of the ground.

• Agricultural activities in the region using fertilizers, which increase the concentrations of elements such as and (Egboka 1984; Ju et al. 2006; Lwimbo et al. 2019)

From Figure 5, we can extract the following observations:

• For the Tissa sample, the elements Cl⁻ and Na⁺ represent 76.32% with 1,420 and 1,076 mg/L, respectively, while represents 12.17% (398 mg/L) and represents 4.34% (142 mg/L), exceeding WHO thresholds and recommendations already mentioned. The elements responsible for the hardness of the water (Mg²⁺ and Ca²⁺) represent 3.31 and 2.78%, respectively, with a concentration of 108.5 and 91.2 mg/L, while the rest of the elements are less than 1.04%.

• For the seawater sample, orders of magnitude of concentrations vary greatly compared to brackish water samples. The elements Cl⁻ and Na⁺ represent 85.40% with 20,620 and 10,089 mg/L, while represents 8.17% (2,940 mg/L) and represents 0.6% (219 mg/L), exceeding WHO thresholds and recommendations already mentioned. The elements responsible for the hardness of the water (Mg²⁺ and Ca²⁺) represent 3.97 and 1.11% respectively, with a concentration of 1,430 and 400 mg/L, exceeding WHO thresholds and recommendations already mentioned. The rest of the elements are less than 0.8%.

The main information that can be drawn from the figure is as follows:

• For conductivity, all the samples are above the WHO threshold of 1 mS/cm or that recommended by the Moroccan standard of 2.7 mS/cm (Moroccan standard 2020). It is clear that the conductivity of the seawater sample is very high because of its high dissolved salt content, whereas that of the brackish water samples is relatively low compared with seawater. According to the samples we have, the relationship between conductivity and well depth is not trivial and depends very much on the specific characteristics of the sampling site. It is the same case for salinity which exceeds the desirable threshold for drinking water of <1 g/L

• For turbidity, which measures the opacity of the water due to the presence of suspended particles, all the samples were below the threshold recommended by the Moroccan standard, which is the limit value of 5 NTU (Moroccan standard 2020). The pH measurement is also compliant between 6.5 and 8.5.

To validate the choice of our pre-treatment technique, we went through a set of pre-treatment techniques, and we were able to characterize them in terms of application and performance using the existing literature. Table 7 summarizes our relevant results.

From Table 7, we can see that the electrocoagulation (EC) technique is the pre-treatment technique which has more advantages whether in terms of pollutant elimination (Hardness, Turbidity) or even lower energy consumption. In addition, the suitability of the technique for our project given that it uses electricity which will be supplied by solar energy will contribute to reducing the carbon footprint of our technique. In addition, this technique is more compact and requires less human intervention which is not the case for other techniques which require the addition of chemicals.

EC is a technique that has been used to treat many types of water, and this is especially the case for wastewater from different types of industrial or even domestic wastewater, groundwater, brackish water, and many other applications for the elimination of particular pollutants. Several possibilities exist for the design of EC, but in general, the basic scheme of EC is the following (Figure 7).

Illustrative case study: In advancing the proposal for the simplified pilot model, it is imperative to establish operational hypotheses for the numerical application of this illustrative case study.

• - For the energy requirements of the unit:

• • • We admit an objective to produce potable water sufficient for a family, estimated at V = 20 L/day.

  • • In addition, we presume that the maximum pollutant removal efficiency has been experimentally validated for a specific set of factors influencing EC (detailed in Section 3.3.2.1). In our context, we consider U = 20 V, I = 3A, and t = 1 h as the parameters for this experimentation. In this case, the necessary electrical energy will be calculated according to Equation (4) as follows: Energy required = (20 × 3 × 1)/(0.02) = 3,000 W h/m³.day.

• - Calculation of the size of the photovoltaic generator:

For a PHOTOWATT panel from the PX1650 DE range with a power of 165 W_peak, we will have N = 6.

• - Simplified diagram of the pilot model:

This study focused on identifying candidate provinces in Morocco by considering various criteria, including distance, rural population, rates of non-access to drinking water, and solar irradiation. The selected areas were subject to brackish water sample collection to comprehensively understand their characteristics, essential for designing effective pre-treatment processes for water pre-treatment before desalination prototype. Analysing test samples revealed insights into mineral loads and physico-chemical characteristics, shedding light on the impact of soil location and human activities. Moreover, depth played a role in the presence of certain elements, such as and ⁠. Notably, the elements contributing to water hardness exceeded drinking water standards, necessitating further exploration of appropriate pre-treatment techniques through a simplified pilot model. This is what was done by a simplified EC pilot model.

We are grateful to the host laboratory LARILE (Laboratory of Advanced Research in Industrial and Logistic and Engineering), especially the OSIL team (Optimization of Industrial and Logistics Systems), LIEME (Laboratory of Electrochemistry Engineering, Modeling and Environment) and Laboratory of Natural Resources and Environment (LNRE) of the Polydisciplinary Faculty of Taza (FPT) for their support and help. This work was established within the framework of a joint research project between CNRST (National Center for Scientific and Technical Research) in Morocco and FRQ (Québec Research Funds) in Quebec.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.