Abstract
The natural water contamination because of the population growth, industrialization, urbanization, sewage from households, industries, institutions, and hospitals, among other things, has increased. After using in various ways, this water turns into wastewater, completing the hydrological cycle. Waterways that are contaminated pose numerous hazards to health and environment. As a result, contaminants must be removed. Coagulation is an effective basic chemical treatment technique that could be used to remediate such pollutants. The majority of people in rural areas are compelled to rely on easily accessible sources, which are typically of low quality and expose them to waterborne diseases, due to the high expense of chemical coagulant-treated water. Natural coagulant, a naturally occurring, plant-based coagulant, can then be used to reduce turbidity during the coagulation and flocculation stage of wastewater treatment. Chemical coagulants may be substituted with natural coagulants. It reduces turbidity while also being ecologically beneficial, serving a dual purpose. In order to solve turbidity concerns, substantial research is needed to find and adopt new techniques for water purification that are less expensive, need less energy, use fewer chemicals, and have a less negative impact on the environment. This study's objectives included evaluating the viability and efficacy of using natural coagulants in place of commonly used synthetic coagulants such aluminum sulphate as well as optimizing the coagulation procedure.
1. Introduction
Water is a necessity and an essential component of all the human and microbial activities. The hydrological cycle's most crucial element is water, which is also one of the requirements for all forms of life on Earth. Due to different environmental degradation activities, population expansion, climate change, rising standards of living, and urbanization [1], the world's water resources are steadily depleting [2]. The impending water crisis is the result of rapid population growth and haphazard waste disposal. Water scarcity has prompted more in-depth research into water and wastewater. As the urban population grows daily and untreated water is poured directly into sources of clean water, controlling and treating waste water has become a major concern. So, before introducing any waste water (from municipal, dairy, industrial, etc.) into the natural water resources, pretreatment of the water is particularly essential. Numerous procedures and technologies are being created to enhance water quality in order to fulfil the rising demand for water [3]. These inventions may be divided into three categories of therapeutic methods: physical, chemical, and biological. Adsorption [4], settling, medium, and membrane filtration, as well as UV treatments, are examples of physical approaches. Certain chemical processes which are used in wastewater treatment include coagulation [5], disinfection [6], oxidation [7], ion exchange [8], and softening processes [9]. Biodegradation by microorganisms [10], catalytic reduction [11], bioreactor processes [12], artificial wetlands [13], and phytoremediation [14] are a few examples of biological techniques. Additionally, in order to increase efficiency, some processes are merged with another [15].
The most popular method for removing dispersed particle matter and colloids from wastewater in basic water and wastewater treatment is coagulation [16]. Its main objective is to clear water bodies of suspended colloidal particles and reducing turbidity. In a chemical reactor, the process is typically carried out by introducing influent water or wastewater into a basin and physically mixing coagulant chemicals into it. Next, sedimentation to remove particles is carried out using gravity settling. Coagulants come in a variety of forms. Alum and ferric salts are the most frequently used chemical coagulants. However, there were a number of drawbacks to using these chemical coagulants, including the production of dangerously large amounts of sludge. Natural coagulants are another type of coagulant derived from plants. Natural coagulants are quite secure and kind to the environment. Animals, plants, and microbes may all extract it. Some plants can conduct some of the coagulation mechanisms, such as neutralizing the charge in colloidal particles and carrying out polymer bridging, which allows them to operate as coagulants.
2. Coagulants in Wastewater Treatment
Without the addition of coagulant agents, it is nearly impossible to eliminate the specific pollutants in wastewater. Colloidal or suspended particles have negative loads because stability prevents them from settling to the bottom, hence instability is necessary for them to flocculate in the bunch. Tiny floccules will form after coagulant annexation as a result of a reduction in the repellence between colloidal particles. The floccules will expand and cluster together, settle at the bottom, and become detached from the water suspension itself with the prolonged calm stirring [17, 18]. Since the nineteenth century, coagulation has been a simple method for water purification due to its comprehensible mechanics [19]; Jiang 2001).
3. Types of Natural Coagulants
Broad classification of natural coagulants is given in Figure 1;
Table 1 presents the types based on various categories.
4. Mechanism of Natural Coagulants in Waste Water Treatment
The idea that the dissolved particles and the polymer in a solution can interact was developed by the coagulation process because natural polymers feature a lot of charged functional groups in their polysaccharide chain, such as –OH, –COOH, and –NH. The four steps in coagulant processing include double layer compression, bridging mechanisms, charge neutralization, and sweep-floc mechanism [21]. Typically, a mixture of macromolecules including protein, carbohydrates, and lipids makes up natural coagulants. Polymers of polysaccharides and amino acids are frequently used as major building blocks [22].
In several practical situations, cationic polyelectrolytes are the most promising flocculants for dissolving negatively charged contaminated particles. Strong adsorptions are produced by an electrostatic interaction, which neutralizes the particle surface and may result in charge reversal. Furthermore, flocculation may occur resulting from particle surface charge reduction and decreased their electrical repulsion [23]. Numerous investigations on polyelectrolytes with maximal charge densities have shown to be effective simply because they have a good ability to transmit additional charge to the particle shell at a given dosage. Additionally, the “electrostatic patch mechanism” is a possibility that emerges when maximal charge density polyelectrolyte is absorbed on negative (–ve) surfaces with a relatively lower density of potential sites [23]. Particle attraction and flocculation are caused by the alternating positive and negative charges in the adsorbed polyelectrolyte chains. In Moringa oleifera, coagulating agents are cationic proteins with molecular masses of 12–14 kDa and isoelectric points (Pi) that are below 10–11. The researchers come to the further conclusion that adsorption and charge neutralization are the primary mechanisms that control the coagulation activity.
Many studies using coagulants such as Moringa oleifera, sago starch, and cassia seed gum have identified the bridging mechanism. Long chain polymers absorbed on particles can have head and tails that enter the solution via various techniques. A bridging mechanism needs a large area where polymer chain divisions can be attached and wrapped around other particles. Therefore, the significance of the most advantageous amount for the bridging mechanism is appropriate [23]. According to an earlier research study, the coagulating activity of the Nirmali microbe extract is enhanced by the presence of lipids, alkaloids with –COOH and –OH groups, and carbohydrates. Galactomannan and galactan, two polysaccharide particles extracted from Nirmali seeds, are capable of removing up to 80% of the turbidity in kaolin solution. The presence of numerous –OH adsorption sites alongside the galactomannan and galactan chains found in nirmali seed extracts results in an interparticle bridging effect. Due to a higher rate of aggregation as a result of increased solids concentration, sweep flocculation provides significantly more particle elimination than particle destabilization, as in the charge neutralization mechanism. Sweep flocculation is employed to remove suspended materials that are detrimental to the mesh composition. On the treatment of turbid water with chitosan (Chitosan has many appealing properties, including hydrophobicity, biocompatibility, biodegradability, nontoxicity, and the presence of highly reactive amino (-NH2) and hydroxyl (-OH) groups in its backbone, making it an effective adsorbent material for the removal of wastewater pollutants.), the same mechanism was proposed. The study found that when the pH is at 8.5, the sweeping mechanism dominates kaolinites removal [24].
5. Processing Steps of Natural Coagulants
Processing steps for natural coagulants involve three phases only. All of these phases are detailed using a flow chart. In the first phase, natural coagulants are pulverized manually or mechanically. Organic and alcoholic solvents were used in the second phase to remove the active agent. To treat water and wastewater, the final phase involves dialysis, lyophilization, ion exchange, and precipitation. Figure 2. shows processes’ phase of natural coagulants [25].
6. Comparison of Some Natural Coagulants’ Efficiency
With brief descriptions of their ideal circumstances, applications, and efficiency, a list of some of the plant-based coagulants explored as natural coagulants is compiled; Roselle seeds (Hibiscus sabdariffa) are rich in proteins (approx. 28%) and soluble in water. When in solution, they are uniformly positively charged. The negatively charged particles in the fluid that cause turbidity bind to these positively charged proteins. The study found that roselle seeds have the maximum turbidity removal performance for synthetic wastewater at pH 10 and 81 percent to 93 percent at pH 4 [26]. Moringa oleifera is one of the most effective plant extracts for water purification. Among other things, this method can remove biological oxygen demand (BOD), chemical oxygen demand (COD), turbidity, total dissolved solids (TDS), total coliform removal, algal removal, hardness, and total suspended solids (TSS). According to studies by the authors of [27], Moringa oleifera reduces total coliform by 96 percent in synthetic raw water and reduces turbidity from 100 NTU to 5.9 NTU, 5.9 NTU after dosing, and 5 NTU after filtration. Additionally, wastewater turbidity was reduced by 84 percent and COD by 46 percent in washing [28]. Turbidity, BOD, COD, hardness, TDS, and TSS are reduced in municipal wastewater by 61 percent, 65 percent, 55 percent, 25 percent, 69 percent, and 68 percent, respectively [29]. Hyacinth bean (Dolichos lablab) peels are characterized for use as a source of protein. Protein levels in hyacinth bean peels are moderate. With a dosage of 20 mg/L in synthetic water, turbidity removal efficiency is 99 percent [30]. Turbidity is reduced from 100 NTU (nephelometric turbidity units) to 11.1 NTU after dosing, filtration to 9.5 NTU, and total coliform removal is 89 percent in synthetic water [27]. Cactus is another powerful natural coagulant. Various research studies show that cactus species are effective at removing COD, turbidity, and colour. Cactus, for example, removes turbidity by 90–92 percent, COD by 90 percent, and colour by 99 percent in textile wastewaters at a dosage of 40 mg/L and pH 7.25 [31]. Nirmali seeds are yet another essential natural coagulant used to eliminate turbidity and total suspended particles (TSS). In laundry wastewater, it reduces turbidity and TSS by 96 and 76 percent, respectively [32]. Watermelon (Citrullus lanatus) is the recent approach for creating a powerful natural coagulant. Turbidity removal efficiency was 88% for tannery effluent and 98 percent for synthetic wastewater. TSS, BOD, and COD, as well as other physicochemical parameters of tannery wastewater, were significantly reduced. The COD removal efficiency was 50%, and the BOD content of the wastewater was reduced by 55%. When used as a coagulant, watermelon seeds comparatively reduced turbidity, BOD, COD and TSS in synthetic wastewater and tannery effluent [33].
Therefore, it can be argued that natural coagulants have a wide range of applications, including disinfection in addition to physical and biological water and wastewater treatment.
7. Conclusion
Artificial coagulants are frequently employed in the treatment of sewage, but when they are, they can have detrimental effects on human health, including memory loss, constipation, and lack of energy, colic, convulsions, and difficulties in learning. Therefore, using natural coagulants rather than chemical or synthetic ones can be quite advantageous. Natural coagulants can be obtained from a several types of natural sources, and they can be practical and affordable alternatives, when used as primary or supplementary coagulants. Natural and artificial coagulants come in two different varieties. In the treatment of wastewater, coagulants are used to eliminate different parameters. There are two categories of coagulant, i.e., plant-based coagulants and nonplant-based coagulants. The preparation of natural coagulants consists of three steps for the replacement of the coagulant dose in wastewater treatment. The treatment with biocoagulants represents an important development in a viable environment for the betterment of the ecosystem, particularly in less urbanized areas. In an attempt to clean wastewater, the ecofriendly coagulant is being used as a natural coagulant in the coagulation process. New coagulant processing techniques, such as composite polymerization and impregnation, can be used to create coagulants with increased capability. This review emphasized the numerous potential benefits of using natural coagulants derived from plants, animals, or biomass [34–42].
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.