# Hybrid Coagulation–Membrane Filtration Techniques for Sustainable Soap Wastewater Treatment

Leila Merabti, Ouassila Benhabiles, Zahia Tigrine, Mounir Mellal, Nadia Chekir, Sarah Mahidine, Djilali Tassalit, Merouane Bentchikou, Amira Douadi, Linda Jammeli, et al.

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## To cite this version:

Leila Merabti, Ouassila Benhabiles, Zahia Tigrine, Mounir Mellal, Nadia Chekir, et al. Hybrid Coagulation–Membrane Filtration Techniques for Sustainable Soap Wastewater Treatment. Water, 2025, 17 (10), pp.1411. 10.3390/w17101411. hal-05105004

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Article

# Hybrid Coagulation–Membrane Filtration Techniques for Sustainable Soap Wastewater Treatment

Leila Merabti <fnref n="1" />, Ouassila Benhabiles <fnref n="1" /> ID, Zahia Tigrine <fnref n="1" />, Mounir Mellal <fnref n="2" />, Nadia Chekir <fnref n="2" /> ID, Sarah Mahidine <fnref n="1" />, Djilali Tassalit <fnref n="1" /> ID, Merouane Bentchikou <fnref n="2" /> ID, Amira Douadi <fnref n="2" />, Linda Jammeli <fnref n="3" />, Imen Khouni <fnref n="3" /> ID and Seif El Islam Lebouachera <fnref n="4" />,*, ID

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Academic Editor: Yung-Tse Hung,
Issam A. Al-Khatib, Rehab O.
Abdel Rahman, Tsuyoshi Imai
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Received: 20 March 2025

Revised: 1 May 2025

Accepted: 6 May 2025

Published: 8 May 2025

**Citation:** Merabti, L.; Benhabiles, O.; Tigrine, Z.; Mellal, M.; Chekir, N.; Mahidine, S.; Tassalit, D.; Bentchikou, M.; Douadi, A.; Jammeli, L.; et al. Hybrid Coagulation–Membrane Filtration Techniques for Sustainable Soap Wastewater Treatment. *Water* **2025**, *17*, 1411. https://doi.org/10.3390/w17101411

**Copyright:** © 2025 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
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(https://creativecommons.org/licenses/by/4.0/).

**Abstract:** Wastewater from soap production often contains high levels of organic pollutants, exceeding regulatory discharge limits and posing significant environmental concerns. This study investigates a two-stage treatment approach integrating ferric chloride ($FeCl_3$)-based coagulation–flocculation with membrane filtration to enhance wastewater purification efficiency. This method is one of the appropriate treatment techniques to reduce water pollution. Thus, numerous Jar test trials have been carried out in order to determine the optimal conditions and parameters that make it possible to reduce suspended solids. Key water quality parameters, including chemical oxygen demand (COD), pH, and turbidity, were monitored to assess process performance. Optimization experiments identified optimal coagulation–flocculation conditions, achieving a substantial COD reduction from 9200 mg/L to 351 mg/L significantly improving water quality. However, the treated effluent still failed to meet reuse standards, necessitating further purification. A subsequent membrane filtration stage was implemented, achieving a significant decrease in turbidity to 0.85 Ntu and a turbidity removal efficiency of 99.97%, indicating high treatment efficiency. The final COD of the collected water was 58 mg/L, well below regulatory limits. This hybrid treatment approach offers a highly effective and sustainable solution for soap wastewater management, supporting environmental protection and resource recovery.

**Keywords:** wastewater treatment; ferric chloride; soap industry; coagulation; flocculation; membrane filtration; water reuse

## 1. Introduction

Global water resources are increasingly strained due to population growth, industrial expansion, and agricultural activities, leading to pollution and the urgent need for effective wastewater treatment. Recycling treated wastewater is vital for water conservation

<footnote n="1">¹ Unité de Développement des Équipements Solaires (UDES), Centre de Développement des Énergies Renouvelables (CDER), Tipaza 42415, Algeria; leila.merabti2013@gmail.com (L.M.); benhabiles.ouassila@gmail.com (O.B.); phyzahia@yahoo.fr (Z.T.); msarah.ms@gmail.com (S.M.); tassalit2003@gmail.com (D.T.)</footnote>

<footnote n="2">² Laboratory of Matter's Valorization and Recycling for Sustainable Development, Faculty of Mechanical and Process Engineering, Université des Sciences et de la Technologie Houari-Boumédiène, Bab-Ezzouar, Algiers 16111, Algeria; m_rinmou@yahoo.fr (M.M.); nchekir@yahoo.fr (N.C.); merouanebentchy@gmail.com (M.B.); amiradouadi1502@gmail.com (A.D.)</footnote>

<footnote n="3">³ Laboratoire des Eaux Usées et Environnement (LabeauE), Centre de Recherches et des Technologies des Eaux (CERTE), Soliman 273-8020, Tunisia; lindajammeli@gmail.com (L.J.); imen.khouni.certe@gmail.com (I.K.)</footnote>

<footnote n="4">⁴ Institut des Sciences Analytiques et de Physico-Chimie Pour l'Environnement et les Matériaux, IPREM, UMR 5254, CNRS Université de Pau et des Pays de L'Adour/E2S, 2 Avenue P. Angot, Technopôle Hélioparc, 64000 Pau, France</footnote>

<footnote>* Correspondence: s.lebouachera@univ-pau.fr</footnote>
and ecosystem preservation. In Algeria, this is reflected in the implementation of over 137 treatment plants reclaiming approximately 16 million cubic meters annually [1]. Among the major industrial polluters is the soap and detergent industry, which consumes large volumes of water and generates wastewater with high variability and pollution loads, including temperature, color, suspended solids, biochemical oxygen demand (BOD), chemical oxygen demand (COD), and surfactants [2]. These pollutants are difficult to remove due to their colloidal nature, surface charge, and small size, often rendering conventional treatment methods ineffective.

Pollutants are challenging to remove due to their surface charge and small size. Various methods, including coagulation–flocculation, ion exchange, precipitation, adsorption, biological processes, and advanced oxidation, have been employed to eliminate colloidal particles from wastewater [3–5]. However, conventional methods such as biological treatment, sedimentation, or simple filtration often fall short when dealing with complex soap wastewater due to high concentrations of surfactants, oils, and other resistant compounds [4]. Among all treatment methods, coagulation–flocculation is one of the oldest and most essential for most water and wastewater treatments. This process has gained increased attention as it has proven effective in removing various contaminants, including toxic organic matter, heavy metals, and viruses [5–7]. Due to its high efficiency in contaminant removal, operational simplicity, effectiveness, and affordability, this process has been widely applied to the treatment of factory effluents [8,9]. It involves a two-phase process aimed at removing stable colloids from water by forming larger aggregates, separable by sedimentation. The first phase involves adding a coagulant to the water to destabilize the particles by reducing the repulsive forces between the colloids through neutralization of their negative charges. The second phase involves flocculating the destabilized particles, forming larger aggregates separable by sedimentation [10]. Nevertheless, suspended particles vary considerably in terms of source, composition, charge, size, shape, and density. The appropriate selection of coagulants and the correct application of coagulation and flocculation processes depend on understanding the complex interaction between these factors [11]. To overcome these limitations and promote green chemistry within wastewater treatment, hybrid processes integrating coagulation–flocculation with membrane filtration have emerged as promising solutions for enhanced overall performance. This synergy offers several key advantages; it improves pollutant removal by capitalizing on the strengths of each technique but also aligns with the principles of sustainability [12]. Coagulation reduces bulk pollutants and membrane fouling, while membrane filtration ensures high-level removal of residual turbidity, organic matter, and surfactants. It also enhances virus removal and demonstrates significant improvements in treating specific industrial wastewaters like soapy and laundry effluents [12–14]. Notably, the development and implementation of sustainable green solvents in membrane technology and related processes are crucial for minimizing environmental impact and fostering a green environment [15].

Numerous studies have explored coagulation–flocculation for treating soap and detergent wastewater, demonstrating varying degrees of success. Aboulhassan et al. [16] demonstrated that coagulation–flocculation with FeCl₃ can be effectively used to treat soap factory wastewater, eliminating surfactants and COD. The treatment was optimal between pH 7 and 9, achieving 99% surfactant removal and 88% COD reduction. Additionally, the 5-day biochemical oxygen demand BOD₅/COD ratio increased from 0.17 to 0.41. Sharghi et al. [17] optimized FeCl₃ dosage with anionic flocculants, which resulted in 80.8% COD removal and good turbidity removal. The optimal condition was 1000 mg/L of FeCl₃ with an anionic flocculant, improving biodegradability. Maung et al. [18] studied highly polluted soap industry wastewater, finding that it exceeded Indian discharge standards (BOD 43,000 mg/L, COD 99,200 mg/L). They proposed two treatment plant
designs including coagulation–flocculation with alum: one using anaerobic and activated sludge biological treatment, and the other using aerobic biological treatment. Both designs successfully reduced BOD to 347 mg/L, thus meeting discharge standards. Ali et al. [19] tested two flocculants, aluminum chloride-polyacrylamide (AlCl₃-PAM) and oat grafted with polymethyl methacrylate (OAT-g-PMMA), for treating soapy wastewater. The first one removed 82% of COD and 81.5% of TSS, compared to 63% and 66% for oat grafted with polymethyl methacrylate (OAT-g-PMMA). The treated effluents met Egyptian standards for irrigation.

However, significant variations in wastewater composition and treatment conditions highlight the need for further optimization. Hybrid processes, such as coagulation coupled with membrane filtration, have shown promise in improving treatment efficiency by addressing membrane fouling and enhancing organic matter removal [20–22]. Combining coagulation with membrane filtration enhances virus removal by incorporating them into larger flocs, improving membrane retention and creating a robust hygienic barrier [23]. This hybrid approach also reduces membrane fouling and increases the removal of organic matter that would otherwise permeate the membrane [24]. Louhichi et al. [25] demonstrated that combining coagulation–flocculation (CF) with ultrafiltration (UF) using a 5 kDa membrane significantly enhances the treatment of soapy water effluents. While CF alone achieved 99% turbidity and 27% COD removal, the CF/UF hybrid process resulted in 100% turbidity and 88% COD removal, displaying its strong potential. Huang et al. [26] developed a multi-stage laundry wastewater treatment process, combining 140 mg/L natural coagulant for coagulation–flocculation, 10 h equilibrium activated carbon adsorption and 60.51 L h⁻¹ m⁻² microfiltration. The combined process effectively improved water quality, with removals of 99.9% for color, 80% for COD, 92.9% for surfactants, and 99.4% for turbidity.

This study aims to evaluate the effectiveness of combined treatment processes for wastewater from the soap industry with high surfactant content. These processes include coagulation–flocculation, followed by membrane filtration, with the goal of obtaining high-quality water. Another more specific objective of this work is to determine the appropriate coagulant, its proper dosage, and to study the optimal experimental conditions for the pollutant removal capacity of wastewater from soap production. The industrial effluent addressed here is highly alkaline, rich in surfactants and organic matter, and presents a unique challenge due to its complex chemical matrix. Moreover, we utilize a lab-prepared microfiltration membrane synthesized by the phase inversion technique with a green solvent system. By specifically addressing the intricate characteristics of soap industry wastewater and utilizing a sustainably produced membrane, this research contributes novel insights to the application of hybrid coagulation/flocculation and membrane filtration for this specific industrial effluent, offering practical guidance for optimizing its treatment. This integrated approach, driven by the principles of green chemistry, underscores the importance of innovative solutions in membrane technology for achieving sustainable wastewater treatment.

## 2. Materials and Methods

### 2.1. *Source of Wastewater*

For our study, effluents were collected from a soap production factory “PROLIPOS” located in Ain Mlila in the east of Algeria, producing large quantities of effluent during the production process by saponification. Industrial soap manufacturing wastewater presents a complex chemical profile. Characterized by a highly basic sodium hydroxide solution and saponification byproducts, both contributing to elevated pH levels [27]. The wastewater also contains a mixture of residual vegetable oils, unconverted fatty acids,
and unrecovered soap compounds, indicating incomplete reactions and product wastage, alongside dissolved sodium and chloride ions originating from various stages of the manufacturing process.

Following the physicochemical characterization of raw wastewater (Table 1), a significant chemical pollutant load was identified in the soap effluent. Notably, turbidity exceeded regulatory limits, the pH was alkaline, and the chemical oxygen demand (COD) reached a substantial 9200 mg/L far surpassing the Algerian discharge standard of 120 mg/L [28]. Similarly, the biochemical oxygen demand (BOD<sub>5</sub>) was measured at 662 mg/L, also exceeding regulatory standards (35 mg/L) [28], indicating a high concentration of organic pollutants in the raw wastewater.

<table><caption>**Table 1.** Raw water physicochemical properties.</caption><thead><tr><td>Temperature (°C)</td><td>pH</td><td>Salinity (g/L)</td><td>Conductivity (ms/cm)</td><td>Turbidity (NTU)</td><td>COD (mg/L)</td><td>BOD<sub>5</sub> (mg/L)</td></tr></thead><tbody><tr><td>27.3 °C</td><td>12.83</td><td>2.6</td><td>5.4</td><td>3235</td><td>9200</td><td>662</td></tr></tbody></table>

## 2.2. Coagulation–Floculation

The efficiency of coagulation–flocculation is affected by several critical parameters, including temperature, pH, coagulant type, dosage, and mixing intensity [27]. To optimize these parameters for treating our wastewater, a series of Jar test experiments [29] were conducted, focusing on the performance of iron(III) chloride (FeCl<sub>3</sub>) as a coagulant. These tests evaluated the impact of varying conditions on chemical oxygen demand (COD) reduction, pH adjustment, and turbidity removal. The primary reagents employed in the coagulation–flocculation process (Figure 1) were Aluminum Sulfate (Al<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>) and Iron(III) Chloride (FeCl<sub>3</sub>); MERCK brand.

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Reagents employed in the coagulation–flocculation process.</figcaption></figure>

A standard Jar test procedure, following the guidelines outlined by Iwuozor [29], was employed to optimize the coagulation–flocculation treatment of soap factory effluent. The experiments were conducted using a six-beaker “Lovibond” model MT 740 Jar tester, with each one-liter beaker equipped with a rotating paddle connected to a common motor, ensuring uniform mixing across all samples (Figure 2). The Jar test procedure involves three distinct stages: coagulation, flocculation, and sedimentation.

1. Coagulation: Rapid mixing was applied at 150 rpm for 5 min immediately following the addition of the FeCl<sub>3</sub> coagulant.

2. Flocculation: Slow mixing was then carried out at 30 RPM for 20 min to promote floc growth.

3. Sedimentation: The samples were allowed to settle without mixing for 30 min.
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Jar test apparatus—Lovibond model MT 740.</figcaption></figure>

The coagulants tested in this study were Aluminum Sulfate ($Al_2(SO_4)_3$) and Ferri Chloride ($FeCl_3$), both widely used in industrial wastewater treatment. No additional polymeric flocculants were applied to isolate the performance of each coagulant. The experiments investigated a range of coagulant dosages: 0.8 g/L, 1 g/L, 1.2 g/L, 1.4 g/L, and 1.6 g/L. The initial characteristics of the soap factory effluent were as follows: COD = 9200 mg/L, $BOD_5$ = 662 mg/L, turbidity = 3235 NTU, and pH = 12.83. All experiments were conducted at an ambient temperature range of 22–25 °C.

After the 30 min sedimentation period, 20 mL supernatant samples was carefully siphoned from the middle depth of each beaker to avoid disturbing the settled sludge. These samples were then analyzed for COD and $BOD_5$ according to the Standard Methods for the Examination of Water and Wastewater.

## 2.3. *Membrane Filtration*

In this study, membrane filtration is implemented as part of a hybrid process, following coagulation–flocculation, to enhance the treatment of the soap factory wastewater collected. This combined approach serves as a polishing step, further improving the characteristics of the pretreated water and ensuring the achievement of our specific treatment objectives.

The selection of membrane type, thickness, and operating pressure are critical parameters for determining the suitability of a membrane for effective water treatment. Membranes are categorized based on their structure, materials, and manufacturing processes [30]. The membrane employed in this study is an organic microfiltration membrane, synthesized by using the phase inversion technique [31]. This method facilitates the production of filtration membranes from a collodion solution. The solution contains triethyl phosphate (TEP) used as an alternative solvent, in which a blend of polymers, including poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVDF) and other additives such as polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG), are dissolved [32,33]. Triethyl phosphate (TEP) has been used as green and no harmful solvent, in accordance with the concept of sustainable development and green chemistry principles [34]. TEP is not teratogenic, carcinogenic, or mutagenic, and as stated in its safety datasheet “contains no components considered to be either persistent, bioaccumulative and toxic, or very persistent and very bioaccumulative at levels of 0.1% or higher” [35].

The membrane was previously characterized in terms of morphology using Scanning Electron Microscopy (SEM), average pore diameter, contact angle, thickness, and mechanical strength [31]. The results of these characterizations are compiled in Table 2.
<table><caption>**Table 2.** Membrane Caracteristics [32].</caption><thead><tr><th>Membrane Code</th><th>Thickness (mm)</th><th>Porosity (%)</th><th>Mean Pore Diameter (µm)</th><th>Young's Modulus (N/mm²)</th><th>Lengthening at Break (%)</th><th>Contact Angle (°)</th></tr></thead><tbody><tr><td>M-0.12</td><td>0.063 ± 0.003</td><td>79.48 ± 1.03</td><td>0.35</td><td>114.33 ± 4.88</td><td>28.16 ± 1.40</td><td>104.6 ± 3.3</td></tr></tbody></table>

The experimental setup used is a laboratory-scale test bench (Figure 3) equipped with a crossflow filtration cell with viewport for photocatalytic application designed by DeltaE SRL (Rende (CS) Italy) (Figure 4). The system consisted of a 2 L capacity feed reservoir, an ESMATEC-IDEX peristaltic pump and valves to ensure the circulation of the water to be filtered, linked together with polymeric flexible pipes. Two pressure gauges are placed at the inlet (P1) and outlet (P2) of the filtration cell. All experiments were carried out at room temperature.

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Membrane filtration setup.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 4.** Crossflow membrane filtration cell.</figcaption></figure>

Figure 5 illustrates the experimental setup employed in this study. Filtration experiments were conducted with the following process parameters: a feed volume of 1 L, a flow rate of approximately 2 L per minute, and a pressure range of 0.5 to 2 bars. A flat sheet membrane with a 47 mm diameter, providing an effective surface area of 0.0013 m², was supported within the filtration cell by a stainless-steel grid. Before each filtration test, the membrane was cleaned by recirculating purified water through it for 30 min. The feed solution for the filtration experiments consisted of water that had undergone coagulation-flocculation treatment. During filtration, the retentate was recirculated back to the feed reservoir, while the permeate (filtered treated water) was collected and its characteristics measured.
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Schematic of the crossflow system [33].</figcaption></figure>

# 3. Results and Discussion

## 3.1. Coagulation Flocculation Treatment

### 3.1.1. Selection of the Appropriate Coagulant

The experimental study is divided into two main parts. The first part focuses on characterizing the soap manufacturing wastewater and conducting experiments using the coagulation-flocculation technique to identify a suitable coagulant for our specific effluent. This will be followed by a parametric investigation of the operating conditions for the coagulation-flocculation process to optimize treatment of our wastewater. The second part presents the results of membrane separation treatment conducted in a crossflow reactor.

Before optimizing the operating conditions by a parametric study, a feasibility test was carried out to allow the selection of the appropriate coagulant. The first preliminary tests were carried out, on the Jar test device, with two types of coagulants $Al_2(SO_4)_3$ and $FeCl_3$ at a concentration of 1 g/L, under the same operating conditions:

* Coagulation speed: 150 rpm, coagulation time: 5 min;

* Flocculation speed: 30 rpm, flocculation time: 20 min;

* Settling time: 30 min.

The evaluation of the coagulation-flocculation performance was carried out by monitoring the turbidity and pH values before and after treatment. The preliminary treatment results for Ferric Chloride ($FeCl_3$) and Aluminum Sulfate ($Al_2(SO_4)_3$) are presented in Figures 6 and 7, respectively. A notable difference in treated water quality is evident between the two coagulants. Specifically, the Ferric Chloride treatment demonstrated a significantly superior outcome. The corresponding turbidity and pH removal efficiencies for both coagulants are summarized in Table 3.

<figure><img src="image_9.png" /><figcaption>**Figure 6.** Preliminary treatment results for Ferric Chloride ($FeCl_3$). (a) Raw water; (b) After coagulation-flocculation; (c) After settling.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 7.** Preliminary treatment results for Aluminum Sulfate (Al₂(SO₄)₃). (a) Raw water; (b) After coagulation-flocculation; (c) after Settling.</figcaption></figure>

<table><caption>**Table 3.** Preliminary treatment results.</caption><thead><tr><th>Coagulants</th><th>pH Before</th><th>pH After</th><th>Turbidity Before (NTU)</th><th>Turbidity After (NTU)</th><th>Turbidity Removal Efficiency (%)</th></tr></thead><tbody><tr><td>Al<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub></td><td>12.83</td><td>6.07</td><td>3235</td><td>1531</td><td>32.67%</td></tr><tr><td>FeCl<sub>3</sub></td><td>12.83</td><td>8.80</td><td>3235</td><td>1025</td><td>68%</td></tr></tbody></table>

The removal efficiency was calculated using the following formula [36]:

$$ \text{Efficiency}(\%) = [(C_0 - C_1)/C_0] \times 100 \quad (1) $$

where C₀ is the initial concentration and C₁ is the final concentration after treatment.

The data indicate a substantial reduction in turbidity when using FeCl₃ compared to Al₂(SO₄)₃. Given its superior performance in turbidity reduction, FeCl₃ was selected for subsequent parametric studies.

### 3.1.2. Parametric Study of Coagulation-Flocculation

To optimize the coagulation-flocculation process for effective pollution removal from soap factory effluent, a comprehensive parametric study was conducted. This involved systematically varying key operational parameters, including coagulant concentration, coagulation speed, flocculation speed, coagulation time, and flocculation time. The resulting measurement data, obtained after treatment with Ferric Chloride (FeCl₃), are presented below.

#### * Coagulant dose

The first parameter is the coagulant dose. Experiments were conducted under consistent coagulation-flocculation operating conditions: a coagulation speed of 150 rpm for 5 min, a flocculation speed of 30 rpm for 20 min, and a Settling time fixed at 30 min.

The results show, in Figure 8a, a significant decrease in turbidity corresponding to an optimal coagulant concentration of C = 1.2 g/L. Turbidity initially decreases with decreasing pH due to better charge neutralization of colloids. However, at very low pH, flocc destabilization or solubilization may occur, causing turbidity to rise again. In Figure 8b, as the coagulant dose increases, more colloids are neutralized and aggregated. However, beyond 1.2 g/L, overdosing may lead to charge reversal or excess sludge formation, which reduces removal efficiency. The removal rate reached 99.68%, which indicates that the treatment was carried out under optimal conditions.
<figure><img src="image_11.png" /><figcaption>**Figure 8.** Concentration effect (a) on pH and turbidity (b) on turbidity removals.</figcaption></figure>

We conducted the rest of the experiments using the optimal dosage of 1.2 g/L. We sequentially varied the coagulation time, coagulation speed, and flocculation time parameters while keeping the remaining parameters (coagulation speed, flocculation speed, and flocculation time) constant. Settling time was consistently maintained at 30 min.

* **Coagulation time and speed**

We conduct these experiments using an optimal coagulant dosage of 1.2 g/L. Initially, we determined the optimal coagulation time. Subsequently, we varied the coagulation time while maintaining the following fixed parameters:

* Coagulation speed: 150 rpm;

* Flocculation speed: 30 rpm;

* Flocculation time: 20 min;

* Settling time was held constant at 30 min.

In Figure 9a, turbidity decreases significantly as pH drops in the initial coagulation phase (2–4 min), suggesting a lower pH promotes coagulation, likely through enhanced coagulant hydrolysis and charge neutralization. Subsequently, turbidity stabilizes at a low level despite further pH fluctuations in the acidic range. This stabilization indicates that optimal coagulation conditions are likely reached, and further pH reduction does not improve removal, possibly due to shifts in coagulant species or exceeding the ideal pH range.

<figure><img src="image_12.png" /><figcaption>**Figure 9.** Effect of coagulation time (a) on pH and turbidity (b) on turbidity removals.</figcaption></figure>

Figure 9b shows that turbidity removal efficiency increases with coagulation time, rising from 95.6% at 2 min to a peak of 99.8% at 6 min. Extending the time to 8 min provides no significant further improvement, indicating an optimal coagulation time around 6 min under these conditions.
Optimization of coagulation speed is performed using a fixed coagulant dosage of 1.2 g/L and a coagulation time of 5 min. The coagulation speed parameter is varied while maintaining the following constants:

* Flocculation speed: 30 rpm;

* Flocculation time: 20 min;

* Settling time: 30 min.

The data presented in Figure 10a reveal that a complex interplay between coagulation speed and pH on turbidity. There appears to be an optimal coagulation speed of around 150 rpm in this experiment, which coincides with a pH of around 5.82, resulting in the lowest turbidity. Deviating from this speed, especially increasing it significantly, leads to higher turbidity, possibly due to increased shear forces disrupting floc formation. The changing pH with coagulation speed also plays a role, initially seeming to favor coagulation as it decreases but then potentially hindering it at very low values.

<figure><img src="image_13.png" /><figcaption>**Figure 10.** Effect of coagulation speed (a) on pH and turbidity (b) on turbidity removals.</figcaption></figure>

Figure 10b demonstrates that there is an optimal coagulation speed for maximizing turbidity removal. In this case, it appears to be around 150 rpm. Below this speed, mixing might be insufficient for effective coagulant distribution and particle destabilization. Above this optimal speed, the increased shear forces become detrimental, hindering floc growth and reducing the overall removal efficiency. This visually confirms the text's conclusion that the coagulation process has a low sensitivity to variations in agitation rate around the optimal point, and that 150 rpm was selected as a good balance for efficient operation.

* **Flocculation time and speed**

The impact of flocculation time was assessed by varying the duration between 10 and 40 min. During these experiments, the following parameters that remained constant are as follows:

* Coagulant dosage (1.2 g/L);

* Coagulation speed (150 rpm, 5 min);

* Flocculation speed (30 rpm);

* Settling time (30 min).

Figure 11a clearly shows that for this specific system, there appears to be an optimal flocculation time around 20 min, which resulted in the lowest turbidity. Deviating from this optimal time, either shorter or longer, leads to higher turbidity. Shorter times are likely insufficient for proper floc growth, while longer times, especially beyond 20 min, seem to cause floc breakup due to prolonged mixing, regardless of the pH fluctuations observed.
<figure><img src="image_14.png" /><figcaption>**Figure 11.** Effect of flocculation time (a) on pH and turbidity (b) on turbidity removals.</figcaption></figure>

Figure 11b demonstrates that a 20 min flocculation period resulted in the peak removal rate of 99.87%, indicating a highly efficient process at this specific duration. This peak performance suggests that at 20 min, the flocs formed were optimally sized and dense, allowing for maximum particle capture and subsequent settling. Consequently, 20 min was selected as the optimal flocculation time for this particular treatment process.

To examine the effect of flocculation speed on the treatment process, a series of experiments were conducted. The optimal coagulant dosage was maintained at a constant 1.2 g/L, ensuring that variations in performance were solely attributable to changes in flocculation speed. To provide a controlled experimental environment, other critical parameters were held fixed at optimal results found in the previous experiments. The coagulation speed was set at 150 rpm, the coagulation time at 5 min, the flocculation time at 20 min, and the settling time at 30 min. By systematically altering the flocculation speed while keeping these parameters constant, we aimed to determine the optimal agitation rate for maximizing floc formation and subsequent removal efficiency.

The data presented in Figure 12a clearly indicate that flocculation speed significantly impacts process results. It reveals a non-linear relationship between flocculation speed, pH, and turbidity. The optimal turbidity is around 30 rpm. Increasing the speed to 40 rpm is very detrimental. However, further increasing the speed to 50 rpm unexpectedly leads to a significant reduction in turbidity, accompanied by a rise in pH. Finally, increasing the speed to 60 rpm shows a slight increase in turbidity again. This complex behavior highlights that the optimal flocculation speed is a balance between promoting floc collision and growth and avoiding excessive shear that can cause floc breakup, and this balance can be influenced by the changing pH conditions.

<figure><img src="image_15.png" /><figcaption>**Figure 12.** Effect of flocculation speed (a) on pH and turbidity (b) on turbidity removals.</figcaption></figure>
Figure 12b illustrates that a flocculation speed of 30 rpm achieves the peak turbidity removal efficiency. Moving away from this optimum, particularly increasing the speed to 40 rpm, significantly reduces performance. A further rise to 50 rpm leads to a partial recovery in removal, before a subsequent decrease at 60 rpm. This non-linear relationship emphasizes the importance of finding the right balance in flocculation speed to maximize particle aggregation and settling.

Consequently, a speed of 30 rpm was taken as the optimal value, providing ideal agitation, low enough to prevent floc breakage, but sufficient for aggregation. With the optimal parameters for coagulation and flocculation now defined, we proceed to evaluating the sedimentation time, using the previously determined optimal conditions.

* Sedimentation time

Sedimentation is a critical stage in this treatment process, directly impacting the overall process's efficiency. As illustrated in Figure 13a, we observe a significant initial decrease in turbidity from 7.27 NTU at 30 min to 4.16 NTU at 60 min. This indicates that a substantial amount of suspended particles settles out within the first half-hour of sedimentation. Following this sharp decline, there is a slight increase in turbidity to 5.07 NTU at 90 min, which could potentially be due to minor disturbances or re-suspension, although it is followed by a continued gradual decrease. Over longer sedimentation times, the turbidity steadily declines, reaching its lowest observed value of 3.28 NTU at 1440 min (24 h). This highlights that extending the sedimentation period generally leads to clearer water as more particulate matter settles out under gravity.

<figure><img src="image_16.png" /><figcaption>**Figure 13.** Effect of sedimentation time (a) on pH and turbidity (b) on turbidity removals.</figcaption></figure>

For the evolution of pH with sedimentation time, starting at 5.79 at 30 min, the pH decreases to 5.59 at 60 min. Subsequently, the trend reverses, and the pH gradually increases over time, reaching 6.38 at 1440 min. This suggests that the sedimentation process influences the chemical characteristics of the water, leading to a less acidic condition with prolonged settling. Further investigation is warranted by the initial pH decrease and subsequent increase. This one should focus on the chemical reactions during sedimentation, considering the potential for acidic and alkaline components to settle at different rates.

By examining Figure 13b, we can directly quantify the effectiveness of sedimentation at different time intervals. The high removal efficiency at 30 min supports favorable results at this time, representing a good compromise between treatment effectiveness and operational speed. The subsequent slight dip and then increase in efficiency at longer times provide a more nuanced understanding of the settling dynamics. The eventual plateauing of the removal efficiency suggests that extending the sedimentation time beyond 24 h yields diminishing returns in terms of turbidity removal. Consequently, to maximize process effectiveness, a sedimentation time of 30 min is recommended for practical applications, balancing performance with operational efficiency.
In summary, the coagulation-flocculation treatment demonstrably achieved a significant reduction in turbidity, decreasing from 3235 NTU to 3.28 NTU, and a substantial reduction in chemical oxygen demand (COD), from 9950 mg/L to 351 mg/L. This marked improvement in water clarity, visually confirmed in Figure 14, which compares untreated and treated soapy effluent, indicates a highly successful treatment outcome. The optimal operating conditions, identified by the lowest turbidity values, are detailed in Table 4.

<figure><img src="image_17.png" /><figcaption>**Figure 14.** Soap effluent before and after coagulation-flocculation treatment.</figcaption></figure>

<table><caption>**Table 4.** Characteristics of treated water by coagulation-flocculation.</caption><thead><tr><td>pH</td><td>Turbidity (NTU)</td><td>COD (mg/L)</td></tr></thead><tbody><tr><td>6.38</td><td>3.28</td><td>351</td></tr></tbody></table>

Furthermore, the use of Ferric Chloride as the coagulant in the process resulted in a high removal efficiency of 99.89%. In addition to the very satisfactory treatment efficiency, the process yielded a visually clear water product. The optimal operating parameters of this outcome, including coagulant dosage, flocculation speed and time, and sedimentation time, are summarized in Table 5.

<table><caption>**Table 5.** Optimal parameters of coagulation-flocculation treatment.</caption><thead><tr><td>Coagulant Dose (g/L)</td><td>Coagulation Time (min)</td><td>Coagulation Speed (rpm)</td><td>Flocculation Time (min)</td><td>Flocculation Speed (rpm)</td><td>Sedimentation Time (min)</td></tr></thead><tbody><tr><td>1.2</td><td>5</td><td>150</td><td>20</td><td>30</td><td>30</td></tr></tbody></table>

Our research validates the effectiveness of coagulation-flocculation with FeCl₃ for treating soap manufacturing wastewater, characterized by high turbidity and COD [16]. The selection of FeCl₃ as a superior coagulant to Al₂(SO₄)₃ aligns with previous observations on surfactant laden wastewater, where iron based coagulants are more effective for colloidal destabilization and floc formation [16,17,25]. Through parameter optimization, we achieved significant reductions, decreasing turbidity by 99.89% to 3.28 NTU and reducing COD to 351 mg/L. These results align with findings from comparable studies on detergent wastewater treatment [16,17] and underscore the potential of this method. Furthermore, the achieved effluent quality supports the use of coagulation-flocculation as a crucial pretreatment step before membrane filtration to minimize fouling [25,26], although subsequent treatment is necessary to address the remaining COD.

## 3.2. Membrane Filtration Treatment

Although iron chloride coagulation-flocculation effectively improved water clarity and quality, the resulting COD level (351 mg O₂/L) still exceeds Algerian reuse standards [28].
Consequently, while the optimized parameters from the coagulation–flocculation will be used in further experiments, membrane filtration is required as a secondary treatment. This additional step will ensure the treated water meets the necessary discharge or reuse standards.

To evaluate the stability and performance of the chosen membrane for this secondary treatment, a permeability test was conducted using a tangential flow filtration cell. Recognizing that membrane flux can be influenced by applied pressure and initial pore compaction, the membrane was first stabilized by pre-filtering pure water. Following this stabilization, the membrane's permeability was determined by measuring the pure water flux (J) at varying working pressures (P) of 0.5, 1.0, and 1.5 bar.

The results of this permeability test are presented in Figure 15, which illustrates the pure water flux as a function of the working pressure across the membrane. The pure water flux exhibits a linear relationship with the applied working pressure, with the trend line passing through the origin. This observed linear behavior is consistent with Darcy's law, which describes the flow of a fluid through a porous medium under pressure. This conformity to Darcy's law indicates stable and predictable performance of the membrane under the tested pressure range.

<figure><img src="image_18.png" /><figcaption>Figure 15. Pure water flux as a function of working pressure.</figcaption></figure>

The calculated average pure water permeability of the membrane was found to be 90.16 L/m²·h·bar for a 12.5 cm² flat membrane section. This permeability value confirms the membrane's suitability for microfiltration, suggesting an adequate capacity to allow the passage of water at the tested pressures. The stable and predictable performance, as evidenced by the linear relationship between flux and pressure, is a crucial factor in the potential long-term effectiveness of this membrane in the secondary treatment stage.

For the membrane treatment, the pressure was fixed at one bar. We carried out filtration tests of the soap factory water treated by Ferric Chloride coagulation–flocculation. The permeate was collected every 30 min. We measured the pH and turbidity as pollution parameters for each volume collected. The results showed (Figure 16) that starting at approximately 3.3 NTU at the 30 min, the turbidity exhibits a gradual decrease over the filtration period, reaching around 0.9 NTU at 150 min. This suggests that the membrane continues to effectively remove residual suspended solids or colloidal matter that were not entirely eliminated during the preceding coagulation–flocculation stage. The decreasing turbidity indicates an ongoing purification process by the membrane. Regarding pH, the initial measurement at 30 min was approximately 6.4. Following a slight drop to about 6.3 at 60 min, the pH exhibited a further gradual decrease, settling at approximately 6.1 by the end of the 150 min of filtration. This trend suggests that the membrane filtration process has a subtle influence on the pH of the treated water. The gradual decrease in pH could be attributed to the selective passage or retention of certain ionic species by the membrane. Consequently, results highlight the dynamic nature of membrane filtration and emphasize the importance of evaluating its performance over extended operational
durations. The continued reduction in turbidity suggests that longer filtration times under these conditions could lead to even higher permeate quality in terms of clarity. Similarly, the gradual decrease in pH warrants consideration in the context of the intended reuse or discharge standards for the treated water.

<figure><img src="image_19.png" /><figcaption>**Figure 16.** pH and Turbidity as a function of time after membrane filtration.</figcaption></figure>

Table 6 elucidates the substantial turbidity reduction achieved by the hybrid coagulation–flocculation/microfiltration (CF-MF) system. This demonstrates a marked decrease in the pollution load from the initial raw soap factory wastewater, following both coagulation–flocculation and subsequent membrane filtration. The overall turbidity removal efficiency reached 99.97%. The chemical oxygen demand (COD) of the permeate also exhibited a significant reduction, decreasing from 351 mg/L to 58 mg/L after membrane filtration.

<table><caption>**Table 6.** Results of CF-MF hybrid treatment.</caption><thead><tr><th>Parameters</th><th>Raw Water</th><th>CF Treatment</th><th>MF Treatment</th><th>Standards [20]</th></tr></thead><tbody><tr><td>Turbidity (NTU)</td><td>3235</td><td>3.28</td><td>0.90</td><td>-</td></tr><tr><td>Turbidity removal efficiency</td><td>-</td><td>99.89</td><td>99.97</td><td>-</td></tr><tr><td>pH</td><td>12.83</td><td>6.38</td><td>6.10</td><td>6.5–8.5</td></tr><tr><td>COD (mg/L)</td><td>9200</td><td>351</td><td>58</td><td>120</td></tr></tbody></table>

The resulting chemical oxygen demand (COD) value, achieved after the coagulation–flocculation and membrane filtration processes, now falls within the permissible limits mandated by environmental discharge standards. This compliance signifies a substantial reduction in organic pollutants, ensuring that the treated wastewater meets regulatory requirements and minimizes its potential impact on receiving ecosystems.

Our study employs a coagulation–flocculation (CF) and membrane filtration (MF) hybrid system for treating heavily polluted soap wastewater, highlighting CF as a vital pretreatment for reducing turbidity and COD similarly to other studies [25]. We further detail the optimization of the CF stage with $FeCl_3$. In contrast to more complex treatments [26] for laundry effluent, our simpler CF-MF approach effectively polishes the pretreated effluent, demonstrating the synergy between the two stages in achieving COD levels [25,26]. The study’s findings on membrane permeability and flux behavior are consistent with general MF principles for soap wastewater treatment [25].

Furthermore, as depicted in Figure 17, the membrane filtration stage effectively demonstrated its ability to selectively retain Ferric Chloride, used in the pretreatment process. This selective retention is a crucial attribute of the membrane, particularly in the context of finishing treatment. The presence of residual Ferric Chloride in treated wastewater
can lead to adverse environmental effects, including pH imbalances and increased metal concentrations. By effectively preventing the carryover of Ferric Chloride, the membrane filtration process not only enhances the overall treatment efficiency but also mitigates the risks associated with residual coagulant, thereby contributing to a more sustainable and environmentally sound wastewater treatment solution.

<figure><img src="image_20.png" /><figcaption>**Figure 17.** Ferric Chloride Residue on Membrane After Hybrid CF-MF Treatment.</figcaption></figure>

**4. Conclusions**

This study successfully demonstrated the efficacy of a hybrid coagulation–flocculation (CF) and membrane filtration (MF) system for treating highly polluted wastewater from a soap manufacturing industry. The initial characterization of the wastewater revealed significant levels of turbidity, COD, and BOD, exceeding Algerian discharge standards, necessitating a robust treatment approach. Through systematic optimization of the CF pro- cess, using Ferric Chloride as the coagulant, optimal operating parameters were identified, resulting in a 99.89% turbidity removal and a substantial reduction in COD. However, the residual COD still surpassed discharge regulations, highlighting the need for a secondary treatment. The subsequent MF stage, utilizing a microfiltration membrane, further polished the effluent, achieving an impressive 99.97% overall turbidity removal and reducing COD to within acceptable discharge limits. The novelty of this work lies in its successful application of this combined CF-MF system optimized specifically for the challenging characteristics of soap industry wastewater with its high surfactant and alkalinity content to achieve com- pliance with stringent Algerian discharge standards. Notably, the membrane effectively retained residual Ferric Chloride, demonstrating its capability as a reliable finishing treat- ment. This work emphasizes the significant potential of tailored combined CF-MF systems for the soap manufacturing industry to achieve high-quality treated wastewater, comply with strict discharge regulations, and contribute to sustainable water resource management within industrial contexts. The findings of this study offer valuable insights for optimizing wastewater treatment processes in similar industries, thereby promoting environmental sustainability and adherence to regulatory requirements. Moving forward, future research will focus on a comprehensive exploration of membrane parameter optimization within this hybrid system. This will involve investigating the influence of various membrane characteristics, such as pore size, material, and operating conditions, to further enhance the system’s efficiency, reduce operational costs, and minimize membrane fouling, ultimately striving for even more sustainable and economically viable wastewater treatment solutions.

**Author Contributions:** Methodology, L.M., O.B., M.M., N.C. and Z.T.; Validation, L.M., D.T. and Z.T.; Formal analysis, L.M., Z.T., M.M., N.C., S.M. and I.K.; Investigation, L.M., O.B., Z.T., S.M., A.D. and L.J.; Resources, M.M., N.C., S.M., D.T., S.E.I.L. and I.K.; Data curation, L.M., O.B., M.B., A.D. and L.J.; Writing—original draft, L.M.; Supervision, S.E.I.L.; Project administration, S.E.I.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.
**Data Availability Statement:** The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

**Conflicts of Interest:** The authors declare no conflicts of interest.

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