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Article

Plant-Based Flocculants as Sustainable Conditioners for Enhanced Sewage Sludge Dewatering

by
Yosra Hadj Mansour
1,2,
Bouthaina Othmani
1,
Faouzi Ben Rebah
3,*,
Wissem Mnif
4,
Moncef Khadhraoui
1 and
Mongi Saoudi
2
1
LR16ES19 Laboratory for Environmental Engineering and Eco-Technology (GEET), National Engineering School of Sfax, University of Sfax, P.O. Box 1173, Sfax 3038, Tunisia
2
LR22ES01 Laboratory of Biomathematics, Faculty of Sciences of Sfax, Department of Mathematics, P.O. Box 1171, Sfax 3000, Tunisia
3
LR24ES12 Laboratory of Plant Physiology and Functional Genomics, Higher Institute of Biotechnology of Sfax (ISBS), University of Sfax, P.O. Box 263, Sfax 3000, Tunisia
4
Department of Chemistry, Faculty of Sciences at Bisha, University of Bisha, P.O. Box 199, Bisha 61922, Saudi Arabia
*
Author to whom correspondence should be addressed.
Water 2024, 16(20), 2949; https://doi.org/10.3390/w16202949
Submission received: 13 September 2024 / Revised: 12 October 2024 / Accepted: 14 October 2024 / Published: 16 October 2024
(This article belongs to the Special Issue Monitoring and Remediation of Contaminants in Soil and Water)
Browse Figures
Versions Notes

Abstract

:
With the aim to establish clean and sustainable sludge treatment, green conditioning using natural flocculants has recently gained a growing interest. In this study, a variety of plant materials, namely Moringa (Moringa oleifera) seeds, Fenugreek (Trigonella foenum-graecum) seeds, Potato (Solanum tuberosum) peels, Aloe (Aloe vera) leaves, Cactus (Opuntia ficus indica) cladodes, and Phragmites (Phragmites australis) stems, were evaluated for their potential bioflocculant activity in conditioning sewage sludge. They were thoroughly characterized to determine their active flocculating compounds. Sludge dewaterability was evaluated by assessing various sludge parameters, including specific resistance to filtration (SRF), dryness of filtration cake (DC), and total suspended solid removal (TSS) from sludge filtrate. The collected results from various physicochemical characterizations of plant materials suggest that the main flocculating agents are carbohydrates in Cactus and Fenugreek and proteins in Moringa, Potato, and Phragmites. Additionally, all tested plant-based flocculants demonstrated effective dewatering performance. Interestingly, compared to the chemical flocculant polyaluminum chloride, Moringa and Cactus showed superior conditioning effects, yielding the lowest SRF values and the highest DC. As a result, the use of these natural flocculants improved sewage sludge filterability, leading to a significant removal of total suspended solids from the filtrate. The conditioning properties of Moringa and Cactus can be attributed to their high protein and sugar content, which facilitates the effective separation of bound water from solids through charge neutralization and bridging mechanisms. Thus, green conditioning using plant-based flocculants, particularly Moringa and Cactus materials, presents a promising and eco-friendly approach to enhance sewage sludge dewatering for safer disposal and valorization.

1. Introduction

With rapid urbanization and economic growth, the capacity of wastewater treatment plants is expanding to meet stringent social and environmental regulations. Simultaneously, this has led to a substantial increase in sewage sludge production, raising economic, health, and environmental concerns. Globally, sewage sludge production is projected to double from 3.5 million metric tons/day in 2002 to 6.1 million metric tons/day by 2025, resulting in an estimated management cost of approximately $375 billion [1]. In general, sewage sludge is characterized by high water content, a substantial organic matter load, and various additional components, including microorganisms and minerals [2].
The water in sludge comprises both free and bound water. Unlike free water, which can be easily removed through gravity drainage, bound water is tightly held within the sludge matrix and closely associated with extracellular polymeric substances (EPS), making its removal a laborious process. Consequently, this substantial volume of water (both free and bound) complicates sludge management and disposal [3]. Therefore, dewatering is a crucial step in sludge treatment, facilitating effective solid-liquid separation and promoting secure and cost-effective storage, disposal, and/or valorization. To achieve this, sewage sludge must first be conditioned to modify its physical and chemical properties, enhancing bound water removal and ensuring the durability of mechanical dewatering equipment [3]. In fact, conditioning acts as a flocculation stage that improves the sludge settling rate and speed by increasing particle cohesiveness and size [4].
Sludge conditioning can be achieved through biological [5], physical [6], and chemical methods [7]. Among these, the chemical coagulation-flocculation method is the most commonly used method due to its simplicity, rapidity, and cost-effectiveness [8]. However, widely used synthetic coagulants and flocculants, such as aluminum and ferric salts, as well as organic polymers, are reported to have negative impacts on the environment and human health [9]. In fact, sludge treatment with these chemicals can produce large volumes of sludge that contain considerable moisture and toxic chemical residues [10]. For instance, metal salts can alter the sludge pH, leading to corrosion of the mechanical devices [11]. Additionally, these chemical conditioners can have harmful health effects due to their persistent chemical residues, which can enter the trophic chain when valorized as fertilizers or disposed of without treatment [12,13,14]. For these reasons, several authorities have banned the use of synthetic polymers in wastewater and sewage sludge treatment plants [15]. Consequently, there is a pressing need for low-cost, safe, and environmentally friendly flocculants. As a result, various natural flocculants derived from animals, microorganisms, and plants [2,8,16,17] have emerged as promising alternatives for sludge conditioning, replacing conventional chemical flocculants. Interestingly, due to their abundance and safety, various plant materials (from wood, shells, seeds, or leaves) have been investigated as green conditioners [18,19,20]. In this context, the present study evaluates the effectiveness of a range of plant-based flocculants for sewage sludge conditioning and dewatering. Six plants (Moringa, Fenugreek, Potato, Aloe, Cactus, and Phragmites) were selected as green conditioners due to their availability in Tunisia. This selection is based on their well-documented flocculating performance in water treatment [20,21,22,23,24,25,26]. For example, Moringa seeds have been shown to reduce water turbidity by 90% to 99%, making them a viable alternative to conventional flocculants [22]. The flocculation activity of Moringa is primarily due to the positive charge of its polyelectrolytes, which enables efficient binding of particles in suspension [23,24,25]. In the same context, the effectiveness of Fenugreek as a coagulant and Aloe as a flocculant was studied for treating palm oil mill effluent, revealing that the coagulation-flocculation process was primarily driven by inter-particle bridging as the dominant mechanism [27]. Although these plants have demonstrated efficacy in separating liquid and solid particulates in wastewater processing, their application as natural flocculants in sludge conditioning remains limited to laboratory-scale applications. Consequently, the six plant materials were characterized to identify their main active agents and elucidate their potential flocculation mechanisms. The sludge dewaterability was assessed by monitoring the specific resistance to filtration (SRF), dryness of filtration cake (DC), and the total suspended solid (TSS) removal from the filtrate. This diverse selection of plants aims to provide a comprehensive, secure, and environmentally friendly solution for sludge conditioning, ultimately enhancing sustainable dewatering. Additionally, it offers an eco-friendly, cost-effective alternative to conventional chemical flocculants, addressing the urgent need for safer and more sustainable sludge management practices.

2. Materials and Methods

2.1. Preparation of Plant-Based Flocculants

Six plant materials derived from Moringa (Moringa oleifera) seeds, Fenugreek (Trigonella foenum-graecum) seeds, Potato (Solanum tuberosum) peels, Aloe (Aloe vera) leaves, Cactus (Opuntia ficus indica) cladodes and Phragmites (Phragmites australis) stems (Figure 1) were selected as natural flocculants for sludge conditioning owing to their wide availability, economical cost, low toxicity, and biodegradability [28]. Fresh Cactus, Aloe, and Phragmites were harvested from a farm located in the Sfax region (Tunisia), while Fenugreek, Potato, and Moringa were purchased from a local market in Sfax. Plant samples were firstly cleansed with tap water to remove dirt and unwanted particles, cut into small slices when required, and dried at 40–60 °C. All plant materials were then ground into a fine powder (Figure 1) to be finally sieved under 100 µm and then packed and stored at 4 °C for transport and subsequent use.

2.2. Physicochemical Characterization of Plant-Based Flocculants

2.2.1. Preparation and pH Measurement of Plant Solutions

Plant solutions were prepared by dissolving 2 g of the plant powder in 50 mL of distilled water. After 20 min of agitation, the suspensions were filtered, and the pH was measured using a pH meter (Crison-GLP 22).

2.2.2. Determination of Organic Matter, Ash and Mineral Composition

Based on the work of Latimer (2012), 3 g of oven-dried plant samples were incinerated at 550 °C in a muffle furnace for 16 h to determine their organic matter and ash contents. Mineral composition in ash such as (Ca, Mg, Na, K) was quantified using an atomic absorption spectrometer (Thermo) after acid digestion of a known aliquot of ash.

2.2.3. Analysis of Bioactive Sugars

The extraction of bioactive molecules (mainly sugars) was carried out following the protocol established by Khattabi et al. (2022) [29] with minor modifications. 50 g of powdered plant materials were macerated in aqueous methanol (80:20; v/v) for 24 h. The solvent was then evaporated under reduced pressure using a vacuum pump at a temperature of 40 °C. The resulting residue was stored at 4 °C in a sterile container for further use. Extraction yields were calculated following the Equation (1):
E x t r a c t i o n   y i e l d % = W ₁ W ₀ × 100
where W1 is the weight of the obtained residue (g) and W0 the weight of the initial plant material powder (g)
Sugars were identified using high-performance liquid chromatography (HPLC 1260-Agilent) following the protocol of Zaky et al. (2017) [30]. The analysis employed a REZEX ROA-LC (8 µm) column maintained at a constant temperature of 55 °C. The mobile phase consisted of a 5 mM H2SO4 solution with a flow rate of 1 mL/min. A sample injection volume of 20 μL was used to detect sugar using a refractive index detector set at 50 °C. Sugars were identified and quantified based on standard samples of known sugars, with results expressed in mg/100 g of plant material.

2.2.4. Determination of Crude Protein Content

The crude protein content of the plant materials was estimated using Kjeldahl’s method, where an aliquot of 1 g was digested under acid conditions in the presence of a catalyst. The obtained solution was then titrated with standardized hydrochloric acid, and the protein content was increased by multiplying the obtained amount of nitrogen by a conversion factor of 6.25 [31].

2.2.5. FTIR Spectra and Zeta Potential Measurement

To identify the main functional groups, FTIR spectra of the powdered plant materials were recorded using a Perkin Elmer spectrometer in the range of 450–4000 cm−1. Additionally, the surface charges were assessed by measuring the zeta potential values. For this purpose, 0.08 g of plant powder was dissolved in 80 mL of ultrapure water. The suspensions were stirred using a magnetic stirrer for 1 h before being filtered and transferred to the cell for zeta potential measurements using a Zetasizer Nano-ZS (ZEN 3600).

2.3. Sewage Sludge Sampling

A secondary sludge was collected from an activated sludge municipal wastewater treatment plant in Santiago de Compostela, Galicia (NW Spain). This secondary activated sludge was obtained from the secondary settling tank. To prevent microbial decomposition and other alterations, the sewage sludge samples were stored at 4 °C, shielded from light exposure, during the characterization and conditioning tests.

2.4. Conditioning Experiments and Sludge Dewaterability Assessments

The conditioning experiments utilizing coagulation-flocculation tests were conducted using a Floclab jar test device (Figure 2). Four beakers, each with a capacity of 250 mL, were simultaneously filled with raw sewage sludge. The dosages of plant-based flocculants were determined based on literature and preliminary trials. The mixture was stirred vigorously at 200 rpm for 3 min, followed by slow stirring at 50 rpm for 20 min before undergoing dewaterability tests.
The dewaterability properties of both raw and conditioned sludge were assessed by measuring pH, water content (WC), supernatant total suspended solids (TSS), dryness of filtration cake (DC), and specific resistance to filtration (SRF). The sludge pH was measured using a pH meter (Crison-GLP 22). The WC of the raw sludge was determined based on Equation (2):
W C % = W ₀ − W ₁ W ₀ × 100
where W0 is the weight of the total raw sludge sample (g) and W1 is the weight of the sludge residue obtained after drying at 105 °C (g) and was called total solid (TS) expressed in g,
The TSS of supernatants was determined using the gravimetric method where a 0.45 μm filter paper was used to trap TSS from supernatant sludge liquid [32], and the TSS was calculated according to the following formula:
T S S   ( m g / L ) = A − B V × 1000
where, A (mg) is the weight of the dried filter paper after filtration, B (mg) is the weight of dried filter paper before filtration (mg) and V (mL) is the volume of the filtered sludge supernatant. The TSS removal rate was calculated using the equation below [33]:
T S S % = T S S ₀ − T S S ₁ T S S ₀ × 100
where TSS0 is the value of sewage sludge before conditioning and TSS1 is the value of sewage sludge after the conditioning process.
The DC was measured according to APHA Standard Methods [32]. The sludge cake obtained after filtration was dried at 105 °C for 2 h, and the DC was calculated using the following equation:
D C % = W 2 W 1 × 100
where W1 (g) is the weight of the cake collected via filtration after sludge conditioning, and W2 (g) is the weight of this collected cake dried at 105 °C for 2 h,
The SRF values were determined following Olivier et al. (2007) [34]. A fixed volume of both raw and treated sludge was filtered under a constant vacuum pressure, and the volume of the collected filtrate was recorded each 5 min. SRF was then determined using the following equation:
S R F   ( m / k g ) = b × 2 A × 2 P µ C
where b (s/m6) is the slope of the filtrate discharge curve (t/V) versus (V), A (m2) the filtration area, P (N/m2) the pressure applied, μ (N.s/m2) the filtrate viscosity as described by Psoch and Schiewer, 2008 and C (kg/m3) the ratio of the dry cake to filtrate volume.

3. Results and Discussion

3.1. Physico-Chemical Characterization of the Plant-Based Flocculants

3.1.1. Biochemical Composition of the Plant Extracts

As shown in Table 1, the plant materials exhibit mildly acidic properties, with pH values ranging from 5.0 to 6.5, influenced by their organic matter content. These green flocculants contain substantial organic matter, ranging from 87% to 97%. Generally, each plant material exhibits distinctive characteristics, as they originate from different botanical families. These results align with previous studies on Moringa [35,36], Fenugreek [37,38], and Potato [39], all reporting substantial organic matter content exceeding 94%. It is important to note that the organic matter content of plant material comprises the total lipids, proteins, and carbohydrates, which are considered responsible for the plant’s flocculating potential [40].
Interestingly, it was reported that proteins found in Moringa seeds [41] Fenugreek [42] might be the main active agent with high flocculating ability in sludge conditioning. Seeds are noted to be excellent reservoirs of proteins, and their commercial potential as flocculant sources needs further investigation [35]. At the same time, the flocculation capacity of Aloe [20], Cactus [21], and Potato [43] is attributed to the polysaccharides present in their mucilage which are considered major active compounds. However, to the best of our knowledge, no studies have been conducted on the use of Phragmites as a natural flocculant for sludge conditioning.
On the other hand, the main minerals content in the plant materials were Ca, Na, Mg, and K. The variations in these concentrations among the different plants may be attributed to various factors such as plant type, maturity stage, and growing conditions, including salinity, watering, and climate conditions [44]. Interestingly, the mucilaginous materials of Cactus and Aloe exhibited notable Ca contents of 56 mg/g dw and 22.5 mg/g dw, respectively. Moreover, Potato, Moringa, and Phragmites showed significant K levels of 35.1 mg/g dw, 34.1 mg/g dw, and 12.9 mg/g dw, respectively (Table 1). Indeed, it is important to indicate that the mineral composition (Ca, Na, Mg, and K) in the plant materials is supposed to enhance their flocculation performance. Wang et al. (2015) [45] reported that these cationic species may play a role in pollutant removal through double-layer compression and charge neutralization. Additionally, Hou et al. (2024) [46] suggested that these cations could disrupt the structure of extracellular polymeric substances (EPS), making them more hydrophobic and leading to the release of bound water. Furthermore, to gain deeper insight into the organic matter content of selected plants, particularly their polysaccharides, the six plant materials were characterized using HPLC.

3.1.2. HPLC Analysis

For all powdered plant materials, the aqueous-methanol extraction method reached a substantial yield of approximately 80% extractable matter using the binary solvent. Previous studies have reported similar methanolic extraction yields for Moringa oleifera, Aloe barbadensis, Ficus religiosa, and Acacia nilotica [47]. The HPLC chromatograms of the six plant materials, along with the standards for various sugars, are presented in Figure 3. By comparing with the standards, the biochemical compositions of the six plant materials were identified and quantified, as summarized in Table 2.
Based on the chromatographic data presented in Table 2, several identified molecules in the plant extracts were determined to be neutral sugars, including sucrose, glucose, fructose, and arabinose. Fructose and glucose were the major sugars in Aloe, consistent with findings reported by Zhang et al. (2018) [48]. In the case of Cactus, in addition to glucose and fructose, the extract exhibited a high sucrose concentration of 14.66 mg/100 g, supporting the results obtained by Kunyanga et al. (2014) [49]. This suggests that sucrose is predominant, while glucose and fructose are minor components of Cactus polysaccharides. Regarding the remaining plants, trace amounts of neutral sugars were detected in Phragmites, Moringa, Fenugreek, and Potato, unlike in Cactus and Aloe. For Phragmites, our findings align with those of Tursun et al. (2011) [50], reporting that sucrose is the dominant soluble sugar compared to glucose and fructose. In contrast, the sugars found in Moringa and Fenugreek were in low quantities, similar to the results reported by Compaoré et al. (2011) [51] and Lahuta et al. (2018) [52].
Considering the significant amounts of carbohydrates and proteins found in some of the investigated plants, along with the reviewed literature, it is supposed that the flocculation behavior of these plants is due to a synergy of carbohydrates, proteins, and associated mineral content, mainly Ca and K. This declaration is supported by the findings of Choudhary and Neogi (2017) [53], Lanan et al. (2021) [26], and Mnif and Ben Rebah (2023) [8]. Therefore, to further explore the functional groups of these carbohydrates and other compounds in the plant materials, FTIR analysis was conducted.

3.1.3. Fourier-Transform Infrared (FTIR) Spectroscopy

FTIR spectra are valuable for identifying functional groups in organic molecules. Figure 4 displays the FTIR spectra of the studied plant-based flocculants.
As shown in Figure 4, most of the plant materials exhibited similar bands with slight variations in intensity. As listed in Table 3, the main common bands observed include a broad band at 3298 cm−1, attributed to stretching vibrations of hydroxyl (–OH) groups and N–H groups of glycoproteins; bands around 2916 cm−1 and 2849 cm−1 recognized as C–H stretching vibration; bands at 1637–1647 cm−1 assigned to C=O vibration of the COO− an ionic form of carboxylic acids; band at 1508–1595 cm−1 corresponds to the vibration of the C=C groups of carboxylic acids and ketones, and a band at 1319 cm−1 that can be attributed to symmetric stretching vibrations of ionized carboxylic acid groups (COO−). Additionally, we identified an absorption band characteristic of C–O stretching vibration at 1025 cm−1.
The infrared spectra and HPLC chromatograms of the studied plant materials indicate that all detected bands and compounds are associated with carbohydrates and proteins.

3.1.4. Zeta Potential Analysis

The zeta potential (ζ), known as the electrostatic potential, is a crucial parameter for measuring the surface charge, stability of colloidal dispersion, and predicting the possible flocculation mechanism [54]. The zeta potential is highly dependent on the solution’s pH and typically falls within the range of −30 to +30 mV [55]. As shown in Table 4, the ζ values of the studied plant-based flocculants were negatively charged at their initial pH (around 6). This anionic feature indicates the presence of negatively charged carboxylic groups (–CO and –COOH) as confirmed by FTIR analysis (Figure 3) and would provide a site for sludge particle adhesion. Similarly, studies on tannin-based flocculants [56], okra [57], and cactus [58] have highlighted the significant impact of pH on zeta potential, influencing changes in the flocculation mechanism.

3.2. Raw Sewage Sludge Characterization

The characteristics of the raw sewage sludge presented in Table 5 indicate a significant moisture content of 86%, reflecting high water content. The pH of the sewage sludge was approximately 5.6, indicating its acidic nature, likely due to the fermentation processes producing acidic compounds. The total solids and the total supernatant suspended solids were 36 g/L and 51 mg/L, respectively (Table 5). Additionally, the sewage sludge showed a zeta potential (ζ) of −13.9 mV, suggesting that it contains stable, negatively charged particles. The SRF was found to be 6.70 × 1013 mg/kg, and the DC was 20.3%. The aspects of the sewage sludge before and after conditioning are illustrated in Figure 5. For comparative purposes, Table 5 also displays the characteristics of other reported sewage sludge. Figure 5 illustrates the appearance of sewage sludge before and after conditioning. Given its high water content and the presence of stable suspended solids (Table 5), effective dewatering of sewage sludge is crucial for plant operators to facilitate subsequent treatment, storage, and disposal.

3.3. Evaluation of the Dewatering Performance of the Plant-Based Flocculants

3.3.1. Effect of Sludge Conditioning on Specific Resistance to Filtration (SRF)

Figure 6 illustrates the effect of the chemical agent (polyaluminum chloride, PACl) and the natural flocculants on the specific resistance to filtration at dosages ranging from 500 mg/L to 2500 mg/L. As shown in Figure 6, the SER values decrease for all flocculants as the dosage increases, indicating improved sewage sludge dewatering. Notably, the SRF for the sludge conditioned with the chemical agent decreases sharply (3 folds), from 6.70 × 1013 m/kg to 2.2 × 1013 m/kg at a dose of 500 mg/L, before reaching equilibrium as the flocculant dosage increases further.
Regarding the natural materials, it can be observed that all plant-based flocculants demonstrated a reduction in SRF, mainly at the higher dosage of 2500 mg/L, likely due to their use in raw form. At this optimized dosage, the most effective sludge conditioning was achieved with Moringa (1.9 × 1013 m/Kg) and Cactus (2 × 1013 m/Kg). However, Fenugreek, Potato, Phargmites, and Aloe exhibited lower dewatering potential, despite a noticeable decline in SRF. The results obtained are in line with numerous previous studies [19,41,61]. Tat et al. (2010) [41] reported a significant reduction in SRF with 3000 mg/L of Moringa, dropping from 5.149 × 1012 m/Kg to 1.743 × 1012 m/Kg. Interestingly, the efficiency of Moringa powder in reducing the SRF of sludge has been reported to be comparable to that of commercial flocculants (Zetag 7653) [61]. Similarly, Betatache et al. (2014) [19] observed a substantial decrease in SRF using cactus juice, from 1.03 × 1013 m/kg to 0.17 × 1012 m/kg at a dosage of 4000 mg/kg, showing dewatering performance comparable to various chemical reagents (Chimfloc C4346, Sedipur AF 400, Sedipur NF 102, Al2SO4 and FeCl3). The dewatering efficiency of cactus and Aloe as bioflocculants makes them strong candidates to compete with commonly used chemical flocculants and various green materials, such as starch-grafted flocculants, which have recently shown a reduction in SRF from 2.31 × 1012 to 5.68 × 1011 [62]. Generally, the effectiveness of sludge dewaterability is predominantly influenced by the nature and biochemical characteristics of bioflocculants, as highlighted in the literature [63,64,65,66]. Although plant-based flocculants have demonstrated significant potential as green alternatives for sludge conditioning, their application has been investigated in only a limited number of studies.

3.3.2. Variation of Dryness of Filtration Cake upon Sludge Dewatering

The effectiveness of sludge dewatering can also be evaluated by measuring the DC under constant pressure [67]. In this context, Figure 7 shows the variation in DC at the optimized dosage of 2500 mg/L. The DC increased from 20.3% to 30% with the chemical agent. For natural flocculants Cactus, Moringa, Fenugreek, Potato, Phragmites, and Aloe, the increase followed this order: Moringa (35%) > Cactus (34%) > Potato (25%) > Fenugreek (24%) > Phragmites (23%) > Aloe (22%). The increase in DC reflects an improvement in dewaterability due to enhanced removal of bound water. This indicates that adding plant-based flocculants promotes the release of trapped bound water, thereby facilitating solid-liquid separation through floc aggregation. The higher dryness of the resulting cake points out that biopolymers contribute to the formation of larger and denser flocs, leading to an efficient settling. The release of bound water during floc agglomeration has been documented in dewatering processes, as reported by Katsiris and Kouzeli-Katsiri (1987) [68]. Additionally, Zemmouri et al. (2015) [69] observed a significant increase in DC to 17.31% using chitosan at 3 kg/t ds (dry solids) when conditioning municipal activated sludge. In comparison, synthetic polyelectrolytes such as Sed CF802 and FeCl3, at dosages of 2 kg/t ds and 4 kg/t ds, raised the DC to 15.78% and 18.78%, respectively.

3.3.3. Variation of Total Suspended Solids Removal during Sludge Conditioning

TSS are undissolved particles that remain finely dispersed in fluid, making them difficult to settle due to their electrical charge [15]. Knowing the TSS content in the supernatant of sludge is crucial for determining appropriate treatment processes, such as selecting effective coagulation and flocculation agents and for reducing the load on filtration systems. Typically, before flotation or decantation, TSS can undergo physicochemical destabilization using coagulants, followed by agglutination with flocculants [70].
Figure 8 shows the effect of sludge conditioning with PACl and various natural flocculants on TSS removal from the supernatant at the optimal dosage of 2500 mg/L. The results indicate that PACl conditioning achieved 43% TSS removal, comparable to Moringa and Cactus, which removed 40% and 44%, respectively. These findings are consistent with previous studies by Gold et al. (2016) [71] and Sethu et al. (2019) [72]. In contrast, Fenugreek, Potato, Phragmites, and Aloe exhibited lower TSS removal compared to Moringa and Cactus. Overall, sludge conditioning enhances particle settling and increases cake permeability, thereby improving the efficiency of downstream thickening and dewatering processes.

3.4. Flocculation Mechanisms in Sludge Dewatering Using Plant-Based Flocculants

According to the literature, flocculation can generally occur through several mechanisms: charge neutralization, bridging, sweep-flocculation, and adsorption (Lai et al., 2018) [73]. For plant-based flocculants, the most likely coagulation mechanism is charge neutralization, though bridging is also a common process that leads to the formation of complexes (particle-flocculant-particle) [42]. Freitas et al. (2015) [57] similarly noted that the polymeric structure of natural flocculants typically involves a combination of adsorption, charge neutralization, bridging, and the patch mechanism. Further, in the case of sludge, when a variation in zeta potential is observed, charge neutralization is likely the dominant mechanism, as described by Muyibi and Alfugara (2003) [74]. Additionally, Li et al. (2022) [62] explained that starch-grafted flocculants neutralize the negative charges in sludge, decompose EPS, increase hydrophobicity, and create channels for bound water release.
In this study, the functional groups (carboxyl, hydroxyl, and amine) identified in the plant materials, along with hydrogen bonds, are considered highly effective for flocculation, as they provide ample adsorption sites for pollutant binding. These functional groups, such as hydroxyl, carboxyl, and amino groups, attach to the surfaces of sludge particles, neutralizing their charge and reducing electrostatic repulsion. This promotes the formation of heavier flocs. Moreover, due to their large molecular size, plant-based flocculants can form bridges between adjacent sludge particles. This bridging effect leads to the creation of larger aggregates or flocs, enhancing particle cohesion and facilitating the aggregation process. As the flocs settle, water is separated from the sludge, resulting in a denser, more concentrated, and compacted sludge cake.
It is, therefore, presumed that bridging is the most likely flocculation mechanism for these plant-based flocculants. The presence of certain minerals in some plant materials may also enhance pollutant uptake through charge neutralization and the destruction of EPS. In this context, Othmani et al. (2020) [21] reported that cactus powder operates through a dual mechanism of bridging and neutralization, attributed to its high content of polysaccharides and cations. Similarly, Jaouadi et al. (2020) [20] found comparable results with Aloe extract. Furthermore, Ndabigengesere et al. (1995) [75] attributed the flocculation activity of Moringa to the presence of cationic proteins, which act through charge neutralization and adsorption. However, as noted by Bratby (2006) [76], there is no single mechanism of particle destabilization by coagulants and flocculants that is universally applicable. More research is needed to elucidate the mechanisms behind the conditioning and dewatering capabilities of plant-based flocculants.

4. Conclusions

The study focuses on evaluating the dewaterability performance of six plant-based flocculants for sludge conditioning. The plants were first subjected to various physicochemical characterizations, including FTIR, zeta potential, and HPLC analyses. Their dewatering potential was then assessed by measuring specific resistance to filtration, dryness of filtration cake, and the total suspended solid content in the supernatant.
From the laboratory tests, the following conclusions were drawn:
  • Flocculants like Moringa and Cactus exhibited a high content of carbohydrates (mainly glucose and sucrose), along with proteins and minerals such as calcium and potassium.
  • All the tested plant-based flocculants had similar FTIR spectra, showing characteristic peaks with the presence of carboxyl (–COOH), hydroxyl (–OH), and amino or amine (–NH2) functional groups.
  • Compared to the synthetic chemical flocculant (PACl), Moringa and Cactus demonstrated the most significant reduction in specific resistance to filtration (1.9 × 1013 m/Kg and (2 × 1013 m/Kg, respectively) and the highest dryness of filtration cake (35% and 34%, respectively) among the tested plants. However, the other plant materials also showed notable improvements in dewatering performance.
  • Based on FTIR data and chemical characterization, the flocculation mechanism of these plant-based flocculants is supposed to involve both bridging and charge neutralization.
  • This investigation contributes to the protection of the environment and human health while promoting sustainable practices in sewage sludge treatment and its safe valorization.
  • Perspectives and Future Works
The findings from this research highlight the efficacy of flocculants derived from plants such as Cactus and Moringa for sewage sludge dewatering, making them competitive alternatives to chemical reagents as safe conditioners. Future research should aim to discover novel and effective flocculants derived from other plants or food wastes that exhibit similar high dewatering efficiency. Additionally, the purification and stability of these plant materials is an important area for further investigation. Moreover, more work is needed to elucidate the flocculating mechanisms of plant-based flocculants.

Author Contributions

Conceptualization, Y.H.M., B.O., F.B.R., W.M., M.K. and M.S.; writing—original draft preparation, Y.H.M., B.O., M.K. and M.S.; writing—review and editing, F.B.R., W.M., M.K. and M.S.; visualization, F.B.R., W.M. and M.S.; supervision, F.B.R., W.M. and M.K.; project administration, M.K. and M.S.; funding acquisition, W.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Deanship of Graduate Studies and Scientific Research at University of Bisha-Saudi Arabia through the Fast-Track Research Support Program.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

The authors are thankful to the Deanship of Graduate Studies and Scientific Research at University of Bisha for supporting this work through the Fast-Track Research Support Program. The first author would like to acknowledge Erasmus + KA107 for providing him with a scholarship during his stay at the University of Santiago de Compostela (USC), Spain. She is deeply grateful to Almudena Hospido for her kind supervision during the stay as well to Ferreira Casal Miguel Ángel, Monica Dosil, Mar org, and Ovidio Álvarez, who gave a great hand in sludge sampling and plant characterization; thank you all.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Aleluia, J.; Ferrão, P. Assessing the costs of municipal solid waste treatment technologies in developing Asian countries. Waste Manag. 2017, 69, 592–608. [Google Scholar] [CrossRef] [PubMed]
  2. Bień, B.; Bień, J.D. Conditioning of Sewage Sludge with Physical, Chemical and Dual Methods to Improve Sewage Sludge Dewatering. Energies 2021, 14, 5079. [Google Scholar] [CrossRef]
  3. Hadj Mansour, Y.; Othmani, B.; Rebah, F.B.; Mnif, W.; Saoudi, M.; Khadhraoui, M. Could Plant-Based Flocculants Substitute the Conventional Synthetic Chemicals in the Sludge Dewatering Process? Water 2023, 15, 2602. [Google Scholar] [CrossRef]
  4. Shi, C.; Sun, W.; Sun, Y.; Chen, L.; Xu, Y.; Tang, M. Synthesis, Characterization, and Sludge Dewaterability Evaluation of the Chitosan-Based Flocculant CCPAD. Polymers 2019, 11, 95. [Google Scholar] [CrossRef] [PubMed]
  5. Álvarez-Torrellas, S.; Peres, J.A.; Gil-Álvarez, V.; Ovejero, G.; García, J. Effective adsorption of non-biodegradable pharmaceuticals from hospital wastewater with different carbon materials. Chem. Eng. J. 2017, 320, 319–329. [Google Scholar] [CrossRef]
  6. Egea-Corbacho Lopera, A.; Ruiz, S.G.; Alonso, J.M.Q. Removal of emerging contaminants from wastewater using reverse osmosis for its subsequent reuse: Pilot plant. J. Water Process Eng. 2019, 29, 100800. [Google Scholar] [CrossRef]
  7. Sousa, J.M.; Macedo, G.; Pedrosa, M.; Becerra-Castro, C.; Castro-Silva, S.; Pereira, M.F.R.; Silva, A.M.T.; Nunes, O.C.; Manaia, C.M. Ozonation and UV254nm radiation for the removal of microorganisms and antibiotic resistance genes from urban wastewater. J. Hazard. Mater. 2017, 323, 434–441. [Google Scholar] [CrossRef]
  8. Mnif, W.; Ben Rebah, F. Bioflocculants as Alternative to Synthetic Polymers to Enhance Wastewater Sludge Dewaterability: A Review. Energies 2023, 16, 3392. [Google Scholar] [CrossRef]
  9. Lin, Q.; Fan, M.; Peng, X.; Ma, J.; Zhang, Y.; Yu, F.; Wu, Z.; Liu, B. Response of Vallisneria natans to aluminum phytotoxicity and their synergistic effect on nitrogen, phosphorus change in sediments. J. Hazard. Mater. 2020, 400, 123167. [Google Scholar] [CrossRef]
  10. Oladoja, N.A. Headway on natural polymeric coagulants in water and wastewater treatment operations. J. Water Process Eng. 2015, 6, 174–192. [Google Scholar] [CrossRef]
  11. Ismail, N.; Abdullah, S.R.S.; Idris, M.; Kurniawan, S.B.; Halmi, M.I.E.; Al Sbani, N.H.; Jehawi, O.H.; Hasan, H.A. Applying rhizobacteria consortium for the enhancement of Scirpus grossus growth and phytoaccumulation of Fe and Al in pilot constructed wetlands. J. Environ. Manag. 2020, 267, 110643. [Google Scholar] [CrossRef] [PubMed]
  12. Exley, C.; Mold, M.J. Aluminium in human brain tissue: How much is too much? JBIC J. Biol. Inorg. Chem. 2019, 24, 1279–1282. [Google Scholar] [CrossRef] [PubMed]
  13. Mold, M.; Linhart, C.; Gómez-Ramírez, J.; Villegas-Lanau, A.; Exley, C. Aluminum and Amyloid-β in Familial Alzheimer’s Disease. J. Alzheimer’s Dis. 2020, 73, 1627–1635. [Google Scholar] [CrossRef] [PubMed]
  14. Mohammad, T.A.; Noor, M.J.M.M.; Ghazali, A.H. Assessment of using synthetic polymers in dewatering of sewage sludge. Desalination Water Treat. 2016, 57, 23308–23317. [Google Scholar] [CrossRef]
  15. Wan, C.; Alam, M.A.; Zhao, X.-Q.; Zhang, X.-Y.; Guo, S.-L.; Ho, S.-H.; Chang, J.-S.; Bai, F.-W. Current progress and future prospect of microalgal biomass harvest using various flocculation technologies. Bioresour. Technol. 2015, 184, 251–257. [Google Scholar] [CrossRef]
  16. Ge, D.; Bian, C.; Yuan, H.; Zhu, N. An in-depth study on the deep-dewatering mechanism of waste activated sludge by ozonation pre-oxidation and chitosan re-flocculation conditioning. Sci. Total Environ. 2020, 714, 136627. [Google Scholar] [CrossRef]
  17. Saleem, M.; Bachmann, R.T. A contemporary review on plant-based coagulants for applications in water treatment. J. Ind. Eng. Chem. 2019, 72, 281–297. [Google Scholar] [CrossRef]
  18. Abdulazeez, Q.; Jami, M.; Alam, M.; Iwata, M. Analysis of the Efficiency of Sludge Dewatering Using Moringa oleifera as Natural Phytocoagulant. Int. J. Res. Chem. Metall. Civ. Eng. 2015, 2, 111–117. [Google Scholar]
  19. Betatache, H.; Aouabed, A.; Drouiche, N.; Lounici, H. Conditioning of sewage sludge by prickly pear cactus (Opuntia ficus Indica) juice. Ecol. Eng. 2014, 70, 465–469. [Google Scholar] [CrossRef]
  20. Jaouadi, T.; Hajji, M.; Kasmi, M.; Kallel, A.; Chatti, A.; Hamzaoui, H.; Mnif, A.; Tizaoui, C.; Trabelsi, I. Aloe sp. leaf gel and water glass for municipal wastewater sludge treatment and odour removal. Water Sci. Technol. 2020, 81, 479–490. [Google Scholar] [CrossRef]
  21. Othmani, B.; Gamelas, J.A.F.; Rasteiro, M.G.; Khadhraoui, M. Characterization of Two Cactus Formulation-Based Flocculants and Investigation on Their Flocculating Ability for Cationic and Anionic Dyes Removal. Polymers 2020, 12, 1964. [Google Scholar] [CrossRef] [PubMed]
  22. Eichhorn, C.; Weckmüller, S.; Urban, W. Natural Flocculant from a Combination of Moringa oleifera Seeds and Cactus Cladodes (Opuntia ficus-indica) to Optimize Flocculation Properties. Water 2022, 14, 3570. [Google Scholar] [CrossRef]
  23. Hussain, G.; Haydar, S. Comparative Evaluation of Glycine max L. and Alum for Turbid Water Treatment. Water Air Soil Pollut. 2020, 231, 57. [Google Scholar] [CrossRef]
  24. Nkurunziza, T.; Nduwayezu, J.B.; Banadda, E.N.; Nhapi, I. The effect of turbidity levels and Moringa oleifera concentration on the effectiveness of coagulation in water treatment. Water Sci. Technol. 2009, 59, 1551–1558. [Google Scholar] [CrossRef] [PubMed]
  25. Xiong, B.; Piechowicz, B.; Wang, Z.; Marinaro, R.; Clement, E.; Carlin, T.; Uliana, A.; Kumar, M.; Velegol, S.B. Moringa oleifera f-sand Filters for Sustainable Water Purification. Environ. Sci. Technol. Lett. 2018, 5, 38–42. [Google Scholar] [CrossRef]
  26. Lanan, F.A.B.M.; Selvarajoo, A.; Sethu, V.; Arumugasamy, S.K. Utilisation of natural plant-based fenugreek (Trigonella foenum-graecum) coagulant and okra (Abelmoschus escluentus) flocculant for palm oil mill effluent (POME) treatment. J. Environ. Chem. Eng. 2021, 9, 104667. [Google Scholar] [CrossRef]
  27. Lim, K.S.; Sethu, V.; Selvarajoo, A. Natural plant materials as coagulant and flocculants for the treatment of palm oil mill effluent. Mater. Today Proc. 2022, 48, 871–887. [Google Scholar] [CrossRef]
  28. Zhang, H.; Guan, G.; Lou, T.; Wang, X. High performance, cost-effective and ecofriendly flocculant synthesized by grafting carboxymethyl cellulose and alginate with itaconic acid. Int. J. Biol. Macromol. 2023, 231, 123305. [Google Scholar] [CrossRef]
  29. Khattabi, L.; Boudiar, T.; Bouhenna, M.M.; Chettoum, A.; Chebrouk, F.; Chader, H.; Lozano-Sánchez, J.; Segura-Carretero, A.; Nieto, G.; Akkal, S. RP-HPLC-ESI-QTOF-MS Qualitative Profiling, Antioxidant, Anti-Enzymatic, Anti-Inflammatory, and Non-Cytotoxic Properties of Ephedra alata Monjauzeana. Foods 2022, 11, 145. [Google Scholar] [CrossRef]
  30. Zaky, A.S.; Pensupa, N.; Andrade-Eiroa, Á.; Tucker, G.A.; Du, C. A new HPLC method for simultaneously measuring chloride, sugars, organic acids and alcohols in food samples. J. Food Compos. Anal. 2017, 56, 25–33. [Google Scholar] [CrossRef]
  31. Chapman, H.D.; Pratt, P.F. Methods of analysis for soils, plants and waters. Soil Sci. 1962, 93, 68. [Google Scholar] [CrossRef]
  32. Rice, E.W.; Bridgewater, L.; American Public Health Association. Standard Methods for the Examination of Water and Wastewater; American Public Health Association: Washington, DC, USA, 2012. [Google Scholar]
  33. Owodunni, A.A.; Ismail, S. Revolutionary technique for sustainable plant-based green coagulants in industrial wastewater treatment—A review. J. Water Process Eng. 2021, 42, 102096. [Google Scholar] [CrossRef]
  34. Olivier, J.; Vaxelaire, J.; Vorobiev, E. Modelling of Cake Filtration: An Overview. Sep. Sci. Technol. 2007, 42, 1667–1700. [Google Scholar] [CrossRef]
  35. Khinchi, R. Proximate analysis and chemical composition of Moringa oleifera seeds and its use in broilers diet. Int. J. Chem. Stud. 2018, 6, 563–566. [Google Scholar]
  36. Maghfiroh, W.; Saefurahman, G.; Hidayatuloh, S.; Kawaroe, M. Harvesting effectiveness of Chlorella sp. biomass using different flocculation treatments of Moringa oleifera extract and pH conditions. IOP Conf. Ser. Earth Environ. Sci. 2018, 209, 012014. [Google Scholar] [CrossRef]
  37. Mabrouki, S.; Omri, B.; Abdouli, H.; Hiar, K.; Gamaoun, W.; Triki, M.A. Chemical, Functional and Nutritional Characteristics of raw, autoclaved and germinated fenugreek seeds. J. New Sci. 2015, 16, 541–551. [Google Scholar]
  38. Chu, R.L.; Vasanthi, S.; Anurita, S. Aloe vera as a natural flocculant for palm oil mill effluent (POME) treatment—Characterisation and optimisation studies. IOP Conf. Series. Mater. Sci. Eng. 2021, 1195, 012035. [Google Scholar] [CrossRef]
  39. Javed, A.; Ahmad, A.; Tahir, A.; Shabbir, U.; Nouman, M.; Hameed, A. Potato peel waste—Its nutraceutical, industrial and biotechnological applacations. AIMS Agri. Food 2019, 4, 807–823. [Google Scholar] [CrossRef]
  40. Anhwange, B.; Ajibola, V.O.; Oniye, S. Amino Acids compositions of the Seeds of Moringa oleifera (LAM) Detarium mirocarpum (Guill & Perr) Bauhinia monandra (Linn). ChemClass J. 2004, 93, 9–13. [Google Scholar]
  41. Tat, W.K.; Idris, A.; Noor, M.J.M.M.; Mohamed, T.A.; Ghazali, A.H.; Muyibi, S.A. Optimization study on sewage sludge conditioning using Moringa oleifera seeds. Desalination Water Treat. 2010, 16, 402–410. [Google Scholar] [CrossRef]
  42. Lee, C.; Chong, M.; Robinson, J.; Binner, E. A Review on Development and Application of Plant-Based Bioflocculants and Grafted Bioflocculants. Ind. Eng. Chem. Res. 2014, 53, 18357–18369. [Google Scholar] [CrossRef]
  43. Zafar, M.S.; Tausif, M.; Mohsin, M.; Ahmad, S.W.; Zia-ul-Haq, M. Potato Starch as a Coagulant for Dye Removal from Textile Wastewater. Water Air Soil Pollut. 2015, 226, 244. [Google Scholar] [CrossRef]
  44. El-Mostafa, K.; El Kharrassi, Y.; Badreddine, A.; Andreoletti, P.; Vamecq, J.; El Kebbaj, M.; Latruffe, N.; Lizard, G.; Nasser, B.; Cherkaoui-Malki, M. Nopal Cactus (Opuntia ficus-indica) as a Source of Bioactive Compounds for Nutrition, Health and Disease. Molecules 2014, 19, 14879–14901. [Google Scholar] [CrossRef] [PubMed]
  45. Wang, Z.; Shen, L.; Zhuang, X.; Shi, J.; Wang, Y.; He, N.; Chang, Y.-I. Flocculation Characterization of a Bioflocculant from Bacillus licheniformis. Ind. Eng. Chem. Res. 2015, 54, 2894–2901. [Google Scholar] [CrossRef]
  46. Hou, J.; Hong, C.; Ling, W.; Hu, J.; Feng, W.; Xing, Y.; Wang, Y.; Zhao, C.; Feng, L. Research progress in improving sludge dewaterability: Sludge characteristics, chemical conditioning and influencing factors. J. Environ. Manag. 2024, 351, 119863. [Google Scholar] [CrossRef]
  47. Sultana, B.; Anwar, F.; Ashraf, M. Effect of Extraction Solvent/Technique on the Antioxidant Activity of Selected Medicinal Plant Extracts. Molecules 2009, 14, 2167–2180. [Google Scholar] [CrossRef]
  48. Zhang, Y.; Bao, Z.; Ye, X.; Xie, Z.; He, K.; Mergens, B.; Li, W.; Yatcilla, M.; Zheng, Q. Chemical Investigation of Major Constituents in Aloe vera Leaves and Several Commercial Aloe Juice Powders. J. AOAC Int. 2018, 101, 1741–1751. [Google Scholar] [CrossRef] [PubMed]
  49. Kunyanga, C.N.; Vellingiri, V.; Imungi, K.J. Nutritional quality, phytochemical composition and health protective effects of an under-utilized prickly cactus fruit (Opuntia stricta Haw.) collected from Kenya. Afr. J. Food Agric. Nutr. Dev. 2014, 14, 9561–9577. [Google Scholar] [CrossRef]
  50. Tursun, N.; Seyithanoglu, M.; Uygur, F.N.; Elibuyuk, I.O.; Elibuyuk, E.A. Seasonal dynamics of soluble carbohydrates in rhizomes of Phragmites australis and Typha latifolia. Flora Morphol. Distrib. Funct. Ecol. Plants 2011, 206, 731–735. [Google Scholar] [CrossRef]
  51. Compaoré, W.R.; Nikièma, P.A.; Bassolé, H.I.N.; Savadogo, A.; Mouecoucou, J. Chemical composition and antioxidative properties of seeds of Moringa oleifera and pulps of Parkia biglobosa and Adansonia digitata commonly used in food fortification in Burkina Faso. Curr. Res. J. Biol. Sci. 2011, 3, 64–72. [Google Scholar]
  52. Lahuta, L.B.; Szablińska, J.; Ciak, M.; Górecki, R.J. The occurrence and accumulation of d-pinitol in fenugreek (Trigonella foenum graecum L.). Acta Physiol. Plant 2018, 40, 155. [Google Scholar] [CrossRef]
  53. Choudhary, M.; Neogi, S. A natural coagulant protein from Moringa oleifera: Isolation, characterization, and potential use for water treatment. Mater. Res. Express 2017, 4, 105502. [Google Scholar] [CrossRef]
  54. Ramlee, A.; Som, A.M.; Puasa, W.; Amani, H.; Hamid, A. Coagulation-flocculation mechanism and characterisation of Hylocereus undatus foliage as a natural coagulant in industrial wastewater treatment. Chem. Pap. 2023, 77, 6083–6093. [Google Scholar] [CrossRef]
  55. Murdock, R.C.; Braydich-Stolle, L.; Schrand, A.M.; Schlager, J.J.; Hussain, S.M. Characterization of Nanomaterial Dispersion in Solution Prior to In Vitro Exposure Using Dynamic Light Scattering Technique. Toxicol. Sci. 2008, 101, 239–253. [Google Scholar] [CrossRef]
  56. Junior, A.T.; Din, S.; Sebastien, N.Y. Optimization of Coagulation/Flocculation Treatment of Brewery Wastewater Employing Organic Flocculant Based of Vegetable Tannin. Water Air Soil Pollut. 2019, 230, 202. [Google Scholar] [CrossRef]
  57. Freitas, T.K.F.S.; Oliveira, V.M.; De Souza, M.T.F.; Geraldino, H.C.L.; Almeida, V.C.; Fávaro, S.L.; Garcia, J.C. Optimization of coagulation-flocculation process for treatment of industrial textile wastewater using okra (A. esculentus) mucilage as natural coagulant. Ind. Crops Prod. 2015, 76, 538–544. [Google Scholar] [CrossRef]
  58. Choudhary, M.; Ray, M.B.; Neogi, S. Evaluation of the potential application of cactus (Opuntia ficus-indica) as a bio-coagulant for pre-treatment of oil sands process-affected water. Sep. Purif. Technol. 2019, 209, 714–724. [Google Scholar] [CrossRef]
  59. Pöykiö, R.; Watkins, G.; Dahl, O. Characterisation of Municipal Sewage Sludge as a Soil Improver and a Fertilizer Product. Ecol. Chem. Eng. S 2019, 26, 547–557. [Google Scholar] [CrossRef]
  60. Yang, Y.; Zhou, K.; Tian, G.; Liu, B.; Jiang, Z.; Bian, B. Feasibility of improving wastewater sludge dewaterability by combination of cationic polyacrylamide and synthetic fibers for resource utilization. Sep. Purif. Technol. 2023, 306, 122620. [Google Scholar] [CrossRef]
  61. Wai, K.T.; Idris, A.; Johari, M.M.N.M.; Mohammad, T.A.; Ghazali, A.H.; Muyibi, S.A. Evaluation on different forms of Moringa oleifera seeds dosing on sewage sludge conditioning. Desalination Water Treat. 2009, 10, 87–94. [Google Scholar] [CrossRef]
  62. Li, L.; Peng, C.; Zhan, Z.; Ma, F.; Zhang, J. A novel treatment for amelioration of sludge dewaterability using green starch-grafted flocculant and realized mechanism. Sep. Purif. Technol. 2022, 301, 122060. [Google Scholar] [CrossRef]
  63. Hamman, J.H. Composition and applications of Aloe veraleaf gel. Molecules 2008, 13, 1599–1616. [Google Scholar] [CrossRef] [PubMed]
  64. Vijayaraghavan, G.; Sivakumar, T.; Kumar, A.V. Application of plant based coagulants for waste water treatment. Int. J. Adv. Eng. Res. Stud. 2011, 1, 88–92. [Google Scholar]
  65. Sepúlveda, E.; Sáenz, C.; Aliaga, E.; Aceituno, C. Extraction and characterization of mucilage in Opuntia spp. J. Arid Environ. 2007, 68, 534–545. [Google Scholar] [CrossRef]
  66. Shebek, K.; Schantz, A.B.; Sines, I.; Lauser, K.; Velegol, S.; Kumar, M. The flocculating cationic polypetide from Moringa oleifera seeds damages bacterial cell membranes by causing membrane fusion. Langmuir 2015, 31, 4496–4502. [Google Scholar] [CrossRef]
  67. Pambou, Y.B.; Fraikin, L.; Salmon, T.; Crine, M.; Léonard, A. Enhanced sludge dewatering and drying comparison of two linear polyelectrolytes co-conditioning with polyaluminum chloride. Desalination Water Treat. 2016, 57, 27989–28006. [Google Scholar] [CrossRef]
  68. Katsiris, N.; Kouzeli-Katsiri, A. Bound water content of biological sludges in relation to filtration and dewatering. Water Res. 1987, 21, 1319–1327. [Google Scholar] [CrossRef]
  69. Zemmouri, H.; Mameri, N.; Lounici, H. Chitosan use in chemical conditioning for dewatering municipal-activated sludge. Water Sci. Technol. 2015, 71, 810–816. [Google Scholar] [CrossRef] [PubMed]
  70. Katrivesis, F.K.; Karela, A.D.; Papadakis, V.G.; Paraskeva, C.A. Revisiting of coagulation-flocculation processes in the production of potable water. J. Water Process Eng. 2019, 27, 193–204. [Google Scholar] [CrossRef]
  71. Gold, M.; Dayer, P.; Faye, M.C.A.S.; Clair, G.; Seck, A.; Niang, S.; Morgenroth, E.; Strande, L. Locally produced natural conditioners for dewatering of fecal sludge. Environ. Technol. 2016, 37, 2802–2814. [Google Scholar] [CrossRef]
  72. Sethu, V.; Selvarajoo, A.; Lee, C.; Ganesan, P.; Goh, L.; Mok, X. Opuntia cactus as a novel bio-coagulant for the treatment of Palm Oil Mill Effluent (POME). Prog. Energy Environ. 2019, 9, 11–26. [Google Scholar]
  73. Lai, H.; Fang, H.; Huang, L.; He, G.; Reible, D. A review on sediment bioflocculation: Dynamics, influencing factors and modeling. Sci. Total Environ. 2018, 642, 1184–1200. [Google Scholar] [CrossRef] [PubMed]
  74. Muyibi, S.; Alfugara, A. Treatment of surface water with Moringa Oleifera seed extract and alum—A comparative study using a pilot scale water treatment plant. Int. J. Environ. Stud. 2003, 60, 617–626. [Google Scholar] [CrossRef]
  75. Ndabigengesere, A.; Narasiah, K.S.; Talbot, B.G. Active agents and mechanism of coagulation of turbid waters using Moringa oleifera. Water Res. 1995, 29, 703–710. [Google Scholar] [CrossRef]
  76. Bratby, J. Coagulation and Flocculation in Water and Wastewater Treatment; IWA Publishing: London, UK, 2006; 421p. [Google Scholar]
Water 16 02949 g001
Figure 1. Powders of plant materials.
Figure 1. Powders of plant materials.
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Figure 2. Jar-test for coagulation-flocculation experiments.
Figure 2. Jar-test for coagulation-flocculation experiments.
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Figure 3. HPLC chromatograms of plant materials and the standards. Each letter is assigned to the appropriate chromatogram where (A) standards, (B) Moringa, (C) Fenugreek, (D) Potato, (E) Aloe, (F) Cactus, and (G) Phragmites. The standards involve neutral sugars: peak 1 = Sucrose; peak 2 = Glucose; peak 3 = Fructose; peak 4 = Arabinose.
Figure 3. HPLC chromatograms of plant materials and the standards. Each letter is assigned to the appropriate chromatogram where (A) standards, (B) Moringa, (C) Fenugreek, (D) Potato, (E) Aloe, (F) Cactus, and (G) Phragmites. The standards involve neutral sugars: peak 1 = Sucrose; peak 2 = Glucose; peak 3 = Fructose; peak 4 = Arabinose.
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Figure 4. FTIR spectra of plant materials.
Figure 4. FTIR spectra of plant materials.
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Figure 5. Sewage sludge (a) before and (b) after conditioning.
Figure 5. Sewage sludge (a) before and (b) after conditioning.
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Figure 6. Effect of different doses of flocculants on SRF.
Figure 6. Effect of different doses of flocculants on SRF.
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Figure 7. Dryness of filtration cake upon sludge conditioning.
Figure 7. Dryness of filtration cake upon sludge conditioning.
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Figure 8. Total suspended solids removal upon sludge conditioning.
Figure 8. Total suspended solids removal upon sludge conditioning.
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Table 1. Chemical and biochemical characterization of plant materials.
Table 1. Chemical and biochemical characterization of plant materials.
PlantspHOrganic Matter
(%)		Proteins
(%)		Ca
(mg/g dw)		Mg
(mg/g dw)		Na
(mg/g dw)		K
(mg/g dw)
Fenugreek6.597.027.650.601.4015.052.46
Moringa5.995.025.330.324.710.3412.90
Potato6.594.210.190.112.251.8035.10
Phragmites6.290.78.180.720.710.7334.10
Cactus5.789.07.356.024.90.9011.95
Aloe 5.887.63.7622.59.300.7812.75
Note: dw: dry weight.
Table 2. Sugar composition of plant materials (in mg/100 g).
Table 2. Sugar composition of plant materials (in mg/100 g).
Peak Retention Time (min)Identified Compound in the Plant ExtractCactusAloe PhragmitesMoringaFenugreekPotato
4.9Sucrose4.660.884.853.971.180.85
5.8Glucose10.2415.121.360.900.24nd
6.3Fructose10.5929.81.180.56nd0.23
6.8Arabinosendnd0.80ndndnd
Note: nd: not detected.
Table 3. Main functional groups present in the compounds of plant materials.
Table 3. Main functional groups present in the compounds of plant materials.
Wave Number (cm−1)Functional Chemical Groups
3298Hydroxyl group OH/N-H bands
2916–2921C–H symmetric stretching in CH2
1637–1647Carbonyl function C=O of the COO– ionic form of carboxylic acids/Ionized COOH (COO– symmetric stretching)
1508–1595C=C groups of carboxylic acids and ketones
1319CH3 primary aromatic amines/Ionized COOH (COO– symmetric stretching)
1025C–O–C/C–N stretching vibration of amine groups
Table 4. Zeta potential and pH of the plant-based flocculants.
Table 4. Zeta potential and pH of the plant-based flocculants.
PlantsCactus MoringaAloe Potato FenugreekPhragmites
ζ (mV)−7.62−8.03−12.4−26.8−25.4−24.8
pH6.056.055.956.56.56.37
Table 5. Characteristics of the raw sewage sludge.
Table 5. Characteristics of the raw sewage sludge.
ParameterUnitPresent StudyBetatache et al.,
2014 [19]		Pöykiö et al.,
2019 [59]		Yang et al., 2023 [60]
WC%~8695.3876.496.2
pH-5.67.166.1nd
TSg/L3645.423.6nd
TSSmg/L51ndndnd
Zeta Potential (ζ)mV−13.9ndnd−12.5
SRFm/kg6.7 × 10131.03 × 1013ndnd
DC%20.34.62ndnd
Note: nd: not determined.
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MDPI and ACS Style

Hadj Mansour, Y.; Othmani, B.; Ben Rebah, F.; Mnif, W.; Khadhraoui, M.; Saoudi, M. Plant-Based Flocculants as Sustainable Conditioners for Enhanced Sewage Sludge Dewatering. Water 2024, 16, 2949. https://doi.org/10.3390/w16202949

AMA Style

Hadj Mansour Y, Othmani B, Ben Rebah F, Mnif W, Khadhraoui M, Saoudi M. Plant-Based Flocculants as Sustainable Conditioners for Enhanced Sewage Sludge Dewatering. Water. 2024; 16(20):2949. https://doi.org/10.3390/w16202949

Chicago/Turabian Style

Hadj Mansour, Yosra, Bouthaina Othmani, Faouzi Ben Rebah, Wissem Mnif, Moncef Khadhraoui, and Mongi Saoudi. 2024. "Plant-Based Flocculants as Sustainable Conditioners for Enhanced Sewage Sludge Dewatering" Water 16, no. 20: 2949. https://doi.org/10.3390/w16202949

APA Style

Hadj Mansour, Y., Othmani, B., Ben Rebah, F., Mnif, W., Khadhraoui, M., & Saoudi, M. (2024). Plant-Based Flocculants as Sustainable Conditioners for Enhanced Sewage Sludge Dewatering. Water, 16(20), 2949. https://doi.org/10.3390/w16202949

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