Natural agents as auxiliaries in water clarification: literature review and experimental evaluation

The increasing demand for water resources (primarily for human consumption and industrial and agricultural activity) is driven by socio-economic development, and population growth. Recent research has been focusing on alternative coagulants based on natural elements, as opposed to the commonly used iron and aluminum salts, for use in water and wastewater treatment processes. In this context, a thorough literature review on alternative coagulants and their application to water treatment processes has been conducted in this work. In addition, three experiments have been conducted with a widely used natural organic coagulant (Moringa oleifera). The alternative clarification system used is the helically coiled tube flocculator (HCTF), with high turbidity removal efficiency and low processing times. A comparative analysis of the turbidity reduction over time was performed with samples collected after 600, 900, 1200, 1500, 1800, 2100, 2400, and 2700 s from the hydraulic circuit. The process efficiency using the proposed alternative coagulant reached 95.3% (after 1800 s). The turbidity removal efficiency remained almost constant after 1800 s, with variations below 1%. These results prove that natural alternative agents can be powerful tools in the water treatment process, with efficiency values exceeding those obtained using chemicals (e.g., aluminum sulphate) as coagulant/ flocculant agents.


Introduction
Coagulation and flocculation are very important steps in water treatment processes. During coagulation, chemicals are commonly added to water in a rapid mixing stage, thereby enhancing particles destabilization and, consequently, flocs formation. The flocs formed are removed from water in the flotation/ sedimentation step. The quality of the treated water should satisfy established governmental regulations, to avoid causing damages to the environment and human health (Santos, Pereira, Santana, & Silva, 2011). In this context, the development of new techniques and products to improve process efficiency becomes necessary.
In addition to this, the increasing demand for water resources and their newly proposed treatment and reuse processes, allow the development of scientific knowledge about alternative coagulants. These can be used in water treatment processes, for both industrial activity (Santos et al., 2011) and human consumption (Arantes, Paterniani, Rodrigues, Hatori, & Pires, 2015), where water resources are essential.
Oenning Junior and Pawlowsky (2007) pointed out that reuse of water resources in industrial activities is driven by the reduction in production costs, which positively affects profits. Reusing water resources also reduces maintenance and damage costs caused by impurities present in the water operated in machines and tools. Another driving factor is the reduction of environmental impact during production. Indeed, reusing water can reduce its contamination and interference with its chemical (such as pH), and physical characteristics (such as temperature) when it is finally returned to the natural environment.
According to Vargas-Camareno and Romero-Esquivel (2006), the use of iron and aluminum salts based coagulants may cause neurological diseases such as Alzheimer's disease (Bongiovani, Konradt-Moraes, Bergamasco, Lourenço, & Tavares, 2010). Llopis and Díez (2002) highlight the risks of exposure to aluminum. On the contrary, reduction in the incidence of neurological diseases within a population is observed to be correlated to alternative coagulants of vegetable origin (Li, Zhang, Hu, Gan, & Li, 2016). Therefore, natural coagulant alternatives constitute an important prevention strategy, leading to reduced impact on public health.
In the literature review, we highlight the research carried out in Latin American and Southeast Asian countries, where developing countries such as Brazil and China are located (Huang, Liu, Li, & Yang, 2017). This way, the connection between an adequate use of natural resources and sustainable development applied to the industry can be perceived, since sustainable production policies impact a company's image.
Thus, efficiency of the water treatment process and, consequently, characteristics of the treated water are important factors in defining public health and sanitation policies, as well as appropriate extraction rates of natural resources for production.
Therefore, this paper aims at performing a thorough literature review on alternative coagulants and their application to water treatment processes. In addition, three experiments have been conducted with a widely used alternative coagulant: Moringa oleifera. Results from the literature proved that Moringa oleifera can be a powerful tool in the water treatment process (Muniz, Duarte, & Oliveira, 2015;Camacho, Sousa, Bergamasco, & Teixeira, 2017;Landázuri et al., 2018).
Consequently, the retrieved literature was sorted by publication date (ascendant sorting), using Microsoft Excel. In the resulting table, articles were further sorted by the journal of publication, impact factor (2017), and Qualis score (considering Engenharias III area, when available; otherwise, Engenharias I area was considered). Based on the results encountered in the literature, Moringa oleifera was chosen as the alternative coagulant to be studied, due to its high potential for the removal of impurities (Muniz et al., 2015), and its availability in Brazil. The concentration of the Moringa oleifera-based coagulant was determined through experiments performed in the chemical laboratory of Federal Institute of Espírito Santo (campus Cariacica).
In the first experiment, 1x10 -3 kg of Moringa oleifera (seeds in shell) and 8x10 -5 m 3 of distilled water were used to prepare the coagulant. They were blended at 157 rad s -1 (1500 rpm) for 60 s, as presented in Muniz et al. (2015). The obtained solution was then filtered. For the test, synthetic water was prepared mixing 5x10 -4 m 3 of natural water and 5x10 -4 kg of clay (bentonite) with magnetic stirring. At this stage, a sample of the test water was collected (1x10 -5 m 3 ), for comparison purposes.
Then, 1x10 -5 m 3 of the coagulant was added to the test water and the resulting solution was mixed at 126 rad s -1 (1200 rpm) for 60 s. Subsequently, the rotation speed of the magnetic stirrer was reduced to 21 rad s -1 (200 rpm) and maintained for 600 s. Three samples were collected after 900, 1800, and 2700 s, to be compared with the test water. Before collecting samples, the used beakers were weighed. All collected samples were dried to remove the remaining water and to measure the solids present in each sample.
In the second experiment, 1x10 -3 kg of Moringa oleifera (without shells) and 8x10 -5 m 3 of distilled water were used to prepare the coagulant, following the same procedure as the first experiment. Test water preparation and sample collection procedures remained unchanged, and four samples were thus prepared (one test water sample, and three proposed samples).
In the third experiment, similarly prepared coagulant solution and test water were evaluated in a hydraulic circuit. A schematic representation of the adopted hydraulic circuit is shown in Figure 1. The used flocculator was a helically coiled tube flocculator (HCTF), as presented by Oliveira and Teixeira (2017a), with a length of 10.53 m and a volume of 7.5x10 -4 m 3 . This type of flocculator was chosen due to its low cost and high efficiency (Oliveira & Teixeira, 2017a;2017b;. The experiment was performed on a solution of 0.130 m³ of test water and 1x10 -3 m 3 of coagulant solution. For the third experiment, process efficiency was evaluated by observing the differences between turbidity values before and after coagulant application. To determine the initial turbidity, five samples were collected, and their average value was adopted in all comparisons. The coagulant solution was the same as that used in the first experiment (seeds with shell). After applying the coagulant, samples were collected at the end of the hydraulic circuit. The final turbidity was measured for samples collected after 600, 900, 1200, 1500, 1800, 2100, 2400 and 2700 s. Table 1 presents the papers evaluated in this work to determine the alternative coagulant used in all experiments. From Table 1, the importance of Moringa oleifera can be easily highlighted, as it has been studied in more than 50% of presented papers. Table 2 was then created to compile important information regarding the selected papers (Table 1), including journals of publication, Qualis score, impact factor (JCR), number of citations in the performed research, and percentage of citations in the performed research.

Results and discussion
Due to the high percentage of citations in previous literature, Moringa oleifera was chosen to be evaluated in all experiments in this work.
Results of the first (seeds in shell) and second experiments (seeds without shells) are presented in Table 3 and 4, respectively. In these tables, a comparative analysis of bentonite mass reduction at each sample collection time is presented (after 900, 1800, and 2700 s); the comparison was done with the values for plain test water in every instance.
Data comparison shows that the highest efficiencies at all times were obtained using seeds in shell, where process efficiency reaches 78% at 2700 s. Additionally, the use of whole seeds of Moringa oleifera strongly contributes to two more aspects: first, it avoids the time-consuming process of removing seeds from their shells; secondly, it avoids disposal of removed shells in the natural environment.
Results of the third experiment (using the hydraulic circuit and seeds in shell) are presented in Table 5. The average value of the initial turbidity was 148 NTU, as shown for sample #1 (Table 5). Moreover, a comparative analysis of the turbidity reduction at each sample collection time is presented (after 600, 900, 1200, 1500, 1800, 2100, 2400, and 2700 s); the comparison was done with the initial turbidity value of test water in every instance.
As shown in Table 5, the efficiency obtained in the third experiment was higher than those in the first and second experiments. In the first experiment, the process efficiency peaked at 78.1%, whereas it only reached 43.8% in the second experiment. In the third experiment, however, the process efficiency reached a maximum of 95.3% (after 1800 s) and a minimum of 86.5%. Therefore, the lowest process efficiency obtained in the third experiment was higher than the best values obtained in the first two experiments. It is important to emphasize that the lowest process efficiency obtained in the third experiment (86.5%) was also higher than the highest value of process efficiency obtained by Oliveira and Teixeira (2017a) using aluminum sulphate (86.2%). Acta Scientiarum. Technology, v. 42, e44800, 2020 Table 1. Papers evaluated in this work.

Conclusion
This paper analyzed alternative coagulants studied in the literature and their application to water treatment processes. Based on the literature analysis, Moringa oleifera was chosen as the natural agent to be used in experiments, reaching a process efficiency about 95.3%. Furthermore, the lowest process efficiency obtained in experiments was higher than the best results obtained in the literature when using aluminum sulphate as coagulant. These results also prove that Moringa oleifera can be a powerful tool in water treatment processes, which can enhance high process efficiency while potentially lowering damages to the environment and human health.