Would Plant-Based Flocculants the Substituters for Sludge Dewatering!!: History
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Contributor:
Yosra Hadj Mansour
,
Bouthaina Othmani
,
Faouzi Ben Rebah
,
Wissem Mnif
,
Mongi Saoudi
,
Moncef Khadhraoui

Due to its high water content, sewage sludge dewatering is not just a simple operation; rather, it is a challenging task and a costly management process. Its final handling is usually preceded by several dewatering steps, and among them is the conditioning process known as the flocculation stage, which is carried out using synthetic chemical reagents. Despite the abilities of these additives to reduce sludge volume and extract its bound waters, they are suspected to cause serious environmental and health threats. Their substitution by natural and efficient additives originating from plant extracts could thus be a safe and an eco-friendly alternative, overcoming ecosystem damages.

  • sludge
  • dewatering
  • conditioning
  • chemical flocculants
  • plant-based flocculants

1. Introduction

With the aim of protecting ecosystems and ensuring the continuity of human life, great efforts have been devoted to wastewater remediation and its related technologies. Yet, despite the well-established paths therein and the effectiveness of conventionally applied wastewater treatment processes, it is believed that their associated sludge is one of the detrimental causes behind environmental and human health degradation, primarily if they are disposed of without efficient treatment means [1]. Furthermore, in addition to its unpleasant odor and high quantity of suspended solids (SS) content, the presence of large amounts of water therein renders its management more challenging, and its transport to the final drying beds is very costly. Its dewatering is hence a crucial step towards ensuring successful solid–liquid separation, reducing its volume, and alleviating its handling charge. To do so, the sludge is usually conditioned in advance to promote its associated water removal and extend the life of the mechanical devices used for dewatering (plate and membrane filter presses), which are precious and energy-consuming [2].
This latter task is usually performed via the supply of synthetic flocculants, also called synthetic conditioners, such as aluminum sulphate, poly-aluminum chloride, ferrous chloride, and polyacrylamide [3][4]. The key role of these chemicals is to agglomerate the suspended matter into large and settleable flocs and free them from the water to which they are bound. Consequently, the sludge volume will be reduced upon the release of the trapped water into solid particles, thus leading to an increase in its settling velocity and compactness [5]. However, in spite of their efficiency in facilitating solid–liquid separation, the effective application of these chemicals is doubtful, as it is reported to cause serious environmental burdens; thus, it is a growing human health concern [6].
In fact, due to their nonbiodegradability, metal salts and organic polymer residues may accumulate in the treated sludge and thus hinder its usage. Further, if valorized in agriculture, those residues may leach out into soil and groundwater, endangering fauna, flora, and human health [7]. Likewise, it is accepted that severe diseases such as Alzheimer’s disease and neurotoxic and carcinogenic illnesses have been ascribed to the extensive application of these chemicals in wastewater treatment and sludge dewatering [8][9]. Additionally, the high cost and non-availability of such chemicals, especially in developing countries, could represent the main hurdles to their supply. Taking these drawbacks and limitations into consideration, in sludge conditioning, replacing chemical flocculants with natural ones derived from plant extracts is both worth considering and deemed to be a safe and environmentally friendly option. In this vein, although some prior studies have highlighted the uses of some plant-based flocculants (such as Moringa [10], Cactus [11], and Okra [12]) as surrogates for their chemical equivalents, and although they have been evaluated for their sludge-dewatering performance, the application of green conditioners in this field, compared to the conventional synthetic flocculants that are extensively exploited in wastewater treatment plants, is still in its infancy, [13][14][15][16]. In order to persuade environmentalists and stakeholders to invest in the replacement of chemical flocculants with safe and green ones, additional works exploring natural flocculants in the dewatering process are required.
Thus, the present entry sheds light on the feasibility of using plant-based flocculants for sludge dewatering and offers a comparison of their performance with those of their chemical equivalents. Although previous reviews and studies mention other bioflocculants derived from animals, plants, or microorganisms, the majority of these works focused on sludge composition, conditioning, and dewatering methods [17][18][19]. It is also worth noting that flocculants originating from animals or microorganisms are difficult to acquire and require sophisticated technology and substantial precautions. Furthermore, most of the recent review articles presented a holistic summary of the coagulation/flocculation process in sludge dewatering [20], i.e., they reviewed the commonly used chemical and natural flocculants without comparing their dewatering capabilities. Few reviews have discussed and assessed the potential use of plant-based flocculants instead of chemical ones for sludge treatment [21].

2. Sludge Properties and the Necessity for Dewatering

Sludge is a suspension of thick semi-solid deposits (slurry, organic matters, trace metals and nutrients, etc.) and microorganisms, which form more than 96% of the sludge’s weight and volume [22]. This high water content can be classified into free and bound water. Free water is easily separated from the sludge by simple filtration or sedimentation. However, bound water, which consists of vicinal, interstitial, and internal water, is difficult to separate. According to its position in the sludge matrix, the vicinal water is held on the solids’ surface, the interstitial one is captured in the interstices, and the internal water is chemically bound to the solids [23]. This kind of water is deemed to be the main barrier for dewatering because it is tightly attached to the sludge’s constituents (microorganisms and organic and inorganic matter) and binds them together. As a result, sludge porosity decreases, thus preventing water withdrawal.
Otherwise, microorganisms and organic polymers, namely the extracellular polymeric substances (EPS), are considered to be the prime factors to maintain the sludge’s moisture due to their ability to trap water. Thus, based on the water distribution within, the EPS can be classified into three types, namely the soluble (S-EPS) and the loosely bound (LB-EPS) and tightly bound extracellular polymeric substances (TB-EPS) [24]. Despite its high bound water content, the EPS forms stable and viscous gel networks due to the prevalence of proteins and polysaccharides in their composition [25]. The ionization of these biopolymers imparts a negative surface charge to the sludge particles and leads to the aggregation of organic matter and microbial cells [26]. Consequently, important water volume is retained, and flexible flocs with low settleability are produced, resulting in dewatering difficulties. Considering the complex structure of sludge, dewatering is necessary to promote the further treatment, transportation, and disposal of sludge and to elucidate the causes of its exploitation.
Generally, sludge dewatering is carried out by conventional mechanical means using different pieces of equipment and methods, such as centrifuging, belt filter presses, plate and frame filters, etc. [27]. Although, the efficacy of these methods in eliminating free water, as well as a small portion of bound water and the dry solids produced after dewatering, does not exceed 40% [28]. Therefore, the sludge is typically subjected to a pretreatment stage by different conditioning technologies involving biological (enzyme treatment), physical (ultrasonic, thermal, and microwave treatments, filter aid applications), and chemical (acid or alkali, oxidation, and flocculation treatment) methods [29][30][31].
it is worth mentioning that long processing times and high energy requirements impede the real application of biological and physical conditioning methods on an industrial scale. So, for economic reasons, among all of the chemical conditioning approaches, coagulation/flocculation is the most applied one for sludge dewatering due to its rapidity, cost-effectiveness, and efficacy in separating water from solid particles [32]. Coagulation/flocculation is based on the addition of chemicals to aggregate the (SS) into large and compacted flocs, enabling the removal of the bound water of sludge and improving its settleability. Likewise, regarding wastewater treatment, the efficiency of the coagulation/flocculation process to assist in sludge dewatering depends on the nature and physicochemical properties of the used chemical coagulants/flocculants, which are generally aluminum salts, iron salts, and acrylamide-based synthetic polymers [33][34].

3. Chemical Flocculants for Improving Sludge Dewatering: Efficiency and Effects on the Environment and Human Health

The conventional synthetic flocculants, such as aluminum sulphate (Al2(SO4)3, aluminum chloride (AlCl3), polyaluminium chloride (PACl), ferric sulphate (Fe2(SO4)3, and ferric chloride (FeCl3), as well as cationic and anionic polyacrylamides (PAM), have been applied in sludge dewatering for a long time [14][35][36]. The efficacy of these chemicals has been assessed in several research works. For instance, in a study conducted by Masihi and Gholikandi [37], the dewatering performances of aluminum (Al2(SO4)3 and AlCl3) and iron (Fe2(SO4)3 and FeCl3) salts were evaluated with respect to their specific resistance to filtration (SRF), capillary suction time (CST), time to filter (TTF), and moisture content (MC) in treating an anaerobically digested sludge.
Otherwise, synthetic polymers, namely PAMs, have been commonly used in sludge conditioning to enhance its dewatering [38][39]. On account of their long chain and high charge density, polyacrylamide-based flocculants bridge the small solids together to form large and settleable flocs. Consequently, the separation of solids from water content in the sludge may occur, enabling efficient dewatering. In fact, the application of PAM led to important decreases in the dewatering properties (SRF, CST, and MC) of sewage sludge [40]. Likewise, in treating a waste-activated sludge, Wu et al. [41] found that prominent reductions in SRF (from 14.20 × 1012 m/kg to 0.40 × 1012 m/kg) and CST (from 225 s to 19.70 s) were achieved by a cationic PAM.
Based on all of this information, it can be concluded that synthetic flocculants certainly improve sludge dewatering. However, due to their chemical nature, the application of these flocculants requires many precautions as it can harm both the environment and human health [42][43]. In fact, as reported in the literature, the enhancement of sludge dewatering using metal salts or PAM as flocculants generally requires carefully controlled pH and a specific dosage to ensure the aggregation of solids and rule out the re-stabilization of sludge flocs [44]. Furthermore, admittedly, sludge treated by such chemicals is voluminous and acidic, thus impeding its transport and disposal [24]. Additionally, dewatering devices may be damaged as a result of the intensive use of corrosive ferric salts [45]. Moreover, due to their non-biodegradability, chemical residuals ascribed from synthetic flocculants may remain embedded in the dewatered sludge matrix, subsequently leading to secondary pollution and limiting the use of this sludge [46]. Consequently, sludge handling becomes much more costly as further treatment is required. Generally, serious environmental pollution is associated with with the use of these synthetic polymers. For example, the resulting alum sludge is a source of toxic aluminum that is harmful to both human and marine life. Aluminium may be associated with Alzheimer’s disease, and its toxicity towards fish has previously been reported [7][47]. However, obtaining an exact estimation of the toxic sludge produced in a region is difficult as various factors may control its production (the polymer type, water characteristics, sludge characteristics, the process used, etc.) [48].
Generally, the mechanisms of coagulation/flocculation include charge neutralization and the bridging effect, which are appropriate for sludge dewatering. The used polymers can efficiently destroy the relative stability of the charged particles and allow them to agglomerate into large flocs prior to sedimentation and mechanical dewatering. In the dewatering process, the resulting charge neutralization reduces the thickness of the hydrated shell of sludge particles and allows for the enhancement of free water content by compressing the electric double layer and weakening the sludge water surface tension. In deep dewatering, chemicals weaken the water-trapping capability of sludge by destroying the network structure of sludge. Moreover, the bridging effect is also essential in sludge dewatering [49][50][51].
Considering all of these adverse effects and limitations associated with the application of chemical flocculants as conditioners for sludge dewatering, looking for natural, efficient, and harmless surrogates has become the primary focus of environmentalists, not only to comply with the standard regulations but also to provide safe and costless sludge treatment, transport, and disposal. Thus, natural flocculants derived mainly from plant extracts have been evaluated as possible alternatives to enhance the dewatering process.

4. Plant-Based Flocculants for Sludge Dewatering: Efficiency and Comparison with Chemical Flocculants

Over the last few years, more than 57 plants have been identified as natural coagulants/flocculants for the treatment of various wastewaters [52]. Despite efforts to prove their efficiency in removing various water pollutants, the exploration of plant-based flocculants for sludge dewatering is still limited to only a few plants, such as Moringa, Cactus, Aloe, and Okra, which are believed to be the most effective sludge conditioners due to their active agent content [11][12][53][54]. Indeed, the application of these popular plants as green flocculants could be attributed to their reliable ability to aggregate sludge’s colloidal particles into big flocs, allowing for easy sedimentation and separation from the treated water [13][55][56]. Thus, this flocculating activity has piqued the interest of scholars and has spurred on many in the scientific community to exploit them in sludge dewatering and apply them as surrogates for chemical conditioners.

4.1. Moringa

Moringa, and chiefly the Moringa oleifera (MO) species, is the most investigated plant in the field of sludge dewatering. The MO seeds allow for the congregation of sludge’s solid particles into dense and settleable flocs. In other words, this natural flocculant enhances sludge dewatering due to its ability to strengthen solids by reducing its compressibility and improving its permeability to facilitate bound water release. To gain more insight into Moringa’s capacity to aid sludge dewatering, which compiles the previous research works that have assessed the variations in dewatering properties (such as the SRF and the CST) that occur when using Moringa as a flocculant. In fact, Rabea et al. [57] found that the powder of MO seeds decreased the SRF by up to 70%. This result denotes that filterability becomes greater via powder supplementation, hence promoting sludge water removal.
The significant dewatering performance of MO powder compared to MO water and salt extracts could be ascribed to the capacity of this powder to, once dissolved in the sludge matrix, release the cationic proteins that are responsible for its flocculating ability. Due to its low molecular weight and positive charge density, the cationic MO proteins neutralize the negatively charged solids to generate dense and compacted flocs that are readily settleable via gravitational decantation. On the other hand, the un-dissolved MO powder serves as an adsorbent to collect sludge solids and assist in its dewatering. In fact, it may produce rigid and porous solids of low compressibility and good permeability to enhance water removal.
Regarding MO water and salt extracts, researchers assume that their effectiveness is due to the total solubility of proteins as active agents. Along this line, Tat et al. [58] revealed that MO water extract was better than dry powder and salt (NaCl) extracts with respect to reducing SRF and CST. Also, in dewatering a sewage sludge using this natural flocculant, Mohammad et al. [59] found significant decreases in SRF (from 0.90 × 1012 m/kg to 3.64 × 1011 m/kg) and CST (from 9 s to 7.10 s). Nonetheless slightly better SRF (1.48 × 1011 m/kg) and CST (5.50 s) declines were observed using a synthetic cationic polyacrylamide (Zetag 8140).
Likewise, the MO seeds extracted by NaCl salt exhibited a comparable SRF reduction (72%) to that found using alum (79%) in a sludge dewatering process based on a drinking water treatment method [60]. These results are in agreement with those reported by Ghebremichael and Hultman [61].

4.2. Cactus and Aloe

In contrast to Moringa, a few studies have addressed sludge dewatering using Cactus and Aloe. To the best of the researchers' knowledge, only two research works have investigated the efficiency of these plants in assisting in the sludge dewatering process. For instance, in a study related to the application of Cactus (Opuntia ficus indica), Betatache et al. [11] reported that replacing a chemical flocculant with cactus juice for sewage sludge dewatering is feasible. In comparison with a few tested synthetic organic cationic (Chimfloc C4346), anionic (Sedipur AF 400), and non-ionic (Sedipur NF 102) polymers, as well as metal salts—namely alum (Al2SO4) and iron chloride (FeCl3)—the cactus juice showed the best dewatering performance. In fact, the SRF value obtained using the cactus juice (SRF = 0.17 × 1012 m/kg) was slightly less than those obtained using the cationic polymer (SRF = 0.3 × 1012 m/kg), the non-ionic polymer (SRF = 4 × 1012 m/kg), and alum (SRF = 1.3 × 1012 m/kg). This result implies that cactus-derived flocculants have the capacity to enhance sludge dewatering.
On the other hand, regarding the use of Aloe vera, Jaouadi et al. [54] appraised the efficiency of aloe gel as a flocculant to treat sewage sludge. They noticed that the addition of this natural flocculant as a conditioner promotes the raw sludge’s settleability, and an enhanced settling rate of 67.50% was achieved using 3 mL/L as a result of the aggregation of flocs. Moreover, it can be deduced that this improvement in particle strength allowed for the removal of the trapped bound water, hence facilitating solid–liquid separation.
Based on these promising findings, cactus and aloe vera have the potential to be bioflocculants for sludge dewatering, and their polysaccharide content and specifically their polygalacturonic acid content are regarded as the main agents responsible for their flocculating abilities [55]. Due to its long polymeric chains, polygalacturonic acid provides a bridge to adsorb the suspended sludge solids and binds them together in order to produce strong and dense aggregates. Consequently, the compressed solids enable the smooth withdrawal of bound water, facilitating sludge dewatering.

4.3. Okra

Like Cactus and Aloe, Okra (Abelmoschus esculentus) is a common polysaccharide-based flocculant widely applied for wastewater treatment [56][62]. However, its application in sludge treatment is still limited. To fill this gap, Lee’s group has explored the dewatering efficiency and feasibility of using Okra in lieu of conventional synthetic flocculants for sludge dewatering [12][63][64]. Scholars have paid great attention to assessing how the methods used to extract bioflocculants (conventional hydrothermal and microwave-assisted extraction) affect their dewatering abilities. In a preliminary study conducted by Lee et al. [12], Okra water extract and its oven-dried powder (acquired after conventional hydrothermal extraction) were evaluated in terms of their efficiency in dewatering synthetic kaolin sludge. Both natural flocculants showed significant SS removal and water recovery rates exceeding 98% and 68%, respectively. Further improvements in these dewatering properties were observed using microwave-extracted powder as salient SS removal (99%) and water recovery (75%) rates were attained [64]. The prominent dewatering capabilities of okra-based flocculants are merely attributed to the high solubility of their polysaccharides with the increase in extraction temperature when using microwave extraction. Likewise, compared to conventional synthetic flocculants, Okra showed higher SS removal and water recovery rates than a cationic and anionic PAM. According to these interesting findings recorded by Lee et al. [12][63][64], the dewatering efficiency of Okra, and specifically its microwave-extracted powder, as a bioflocculant makes it a relevant candidate to replace the commonly used chemical flocculants.

This entry is adapted from the peer-reviewed paper 10.3390/w15142602

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