# A sustainable and environmentally friendly approach for wastewater treatment using natural flocculants: pH effect and optimization

Saba Abdolalian, Bahman Nazari, Mohsen Taghavijeloudar

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Saba Abdolalian, Bahman Nazari, Mohsen Taghavijeloudar. A sustainable and environmentally friendly approach for wastewater treatment using natural flocculants: pH effect and optimization. 2nd international congress on Civil Engineering, Architecture, Building Materials and Environment, Nov 2022, Helsinki, Finland. hal-04363452

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# A sustainable and environmentally friendly approach for wastewater treatment using natural flocculants: pH effect and optimization

Saba Abdolalian¹*, Bahman Nazari², Mohsen Taghavijeloudar³

Department of Civil and Environmental Engineering, Babol Noshirvani University of Technology, Babol, Iran. Email: Saba.abdolalian71@gmail.com

Department of Chemical Engineering, Babol Noshirvani University of Technology, Babol, Iran. Email: bn.nazari@gmail.com

Department of Civil and Environmental Engineering, Seoul National University, Seoul, Korea. Email: mohsen@snu.ac.kr

## Abstract

Coagulation and flocculation are one of the most important solid-liquid separation processes that are used in wastewater treatment. In this study, the performance of *Pistacia terebinthus*, as an eco-friendly coagulant for wastewater treatment was evaluated. To do this, different dosages of *Pistacia terebinthus* (0.5, 1, 1.5, and 2 g/L) were applied to wastewater samples to remove chemical oxygen demand (COD) and turbidity. Additionally, the effect of various initial pH (3, 5, 7, 9, 11) on the flocculant performance and settling time was investigated. Moreover, zeta potential analysis was conducted to find the main mechanism of treatment. The results showed that in the optimal condition of NF dosage= 2 g/L and pH=11 after 60 minutes of settling time, maximum removal efficiencies of 95% and 98% were obtained for COD and turbidity, respectively. Moreover, it can be concluded that the main mechanism of the *Pistacia terebinthus* as a natural flocculant with pH adjustment is bridging combined with charge neutralization.

**Keywords:** Wastewater, Flocculation, *Pistacia terebinthus*, pH adjustment.
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**1. Introduction**

With the rapid growth of civilization and industrialization, a large amount of municipal and industrial wastewater is generated and the energy demand for retreating it has increased [1-3]. Raw wastewater is a complicated mixture of water and varied pollutants such as organic matter, suspended solids (SS), nutrients, heavy metals (HM), turbidity, and chemical oxygen demand (COD) [4]. Discharging untreated wastewater directly to the environment will pose serious risks to the ecosystem in general and to human health in particular. For instance, discharged wastewater into surface waters such as rivers and lakes reduces photosynthesis and the amount of dissolved oxygen. Moreover, wastewater pollutants encourage algal blooms in water bodies leading to the deaths of many species [5]. Additionally, toxic organisms and other chemicals in wastewater cause physiological changes in human bodies, harm their immune system, and cause several diseases. Therefore, to prevent human and environmental damage, it is necessary to remove these pollutants from wastewater and comply with the standards imposed by the legislation requirement [6].

So far, various methods have been proposed for wastewater treatment [7]. Primary, secondary, and tertiary/advanced treatment are three main techniques used to treat wastewater. Water is separated from suspended particles, floating, and settleable materials using primary methods (filtration, sedimentation and floating). biodegradable contaminants is removed using secondary methods (activated sludge process or anaerobic digestion) non-biodegradable waste is removed using tertiary methods (membrane, algae treatment, and advanced oxidation processes) [8]. However, most of these methods include drawbacks that reduce their effectiveness such as high energy and cost demand, requires skilled operator, excess sludge leading to the generation of secondary pollution with low COD removal efficiency and are not economically acceptable for large-scale effluents [9, 10]. Meanwhile, coagulation and flocculation offer a promising solution to wastewater treatment facilities which can remove a wide range of pollutants such as TSS, COD, and turbidity. The coagulation and flocculation process works by neutralizing suspended particles that are negatively charged. Therefore, the destabilized particles agglomerate to make up bigger flocs [11]. There are two types of coagulants (i) inorganic coagulants including Aluminum and Iron Coagulants such as aluminum sulphate, sodium aluminate and ferric sulphate (ii) organic coagulants including Polyamines and Polytannate. However, commercially available chemical coagulants are expensive and toxic for the living environment. Natural polymers are cost-effective and environmental friendly [12].

Several studies have reported the use of natural flocculants for wastewater treatment. Surjana and Maize seed powder and Chitosan applied in a study for removing Congo red dye [13]. In another research, 97% turbidity and 85% COD removal was obtained using a low amount of the okra mucilage [14]. Subramonian et al (2014) investigated the effect of
<figure><img src="image_4.png" /></figure>

natural *Cassia obtusifolia* seed gum in treatment of industrial effluent. The results showed that in their recommended conditions (initial pH 5, 0.75 g/L dosage, 10 rpm and 10 min slow-mixing, and 1 min settling time), *Cassia obtusifolia* gum removed high TSS and COD and the coagulation efficiency of natural flocculant was comparable to alum [15]. Rice starch was used in coagulation-flocculation process for industrial wastewater treatment which have exhibited good performance on the removal of suspended solids and organic load [16]. Lim et al (2021) investigates the use of fenugreek seeds to treat POME and found that it has great potential for solids removal process [17]. Whereas, natural flocculant have some shortcoming such as being locally available and in many cases, achieving higher performance requires a higher dose. Indeed, introducing new, cheap flocculant with high coagulation performance can be applied as an effective alternative coagulating/flocculating agent [18].

*Pistacia terebinthus* is one of the 20 *Pistacia* species widely distributed in the Mediterranean region and Asia. This plant has different biological activities and is used in traditional cures of burns and asthma. It has antioxidant and antimicrobial property too [19]. The aim of this research is to evaluate the application of a novel plant-based *Pistacia terebinthus* as a natural flocculent. To do this, different dosages of *Pistacia terebinthus* powder were assessed for treating industrial pulp and paper wastewater with high range of COD and turbidity. Additionally, effect of pH adjustment on the *Pistacia terebinthus* performance was investigated. The flocculation conditions were optimized to obtain the best flocculation conditions. Furthermore, Zeta potential analysis were used to find *Pistacia terebinthus* mechanism.

## 2. Materials and methods

### 2-1-Materials

The wastewater samples were obtained from a pulp and paper company in Babol, Mazandaran, Iran. The properties of wastewater samples are given in Table 1. The pH of the sample was adjusted using solutions of NaOH and HCl. All the chemicals such as NaOH and HCl for pH adjustments and the chemicals that were used in COD measurements, were provided from Merck Company, Germany and were of analytical grade. The *Pistacia terebinthus* as natural flocculant were bought from a local market and were crushed and separated from the shell, then pulverized and powdered. The stock soluble was obtained by mixing 5 g of the *Pistacia terebinthus* powder with 5.84 g of NaCl and solvate with 100 mL deionized water. Then uniform mixture was attained by exposing ultrasound radiation with power of 70% for 6 minute. Finally, the solution was filtered. The *Pistacia terebinthus* seeds and final prepared solution is demonstrated in Figure 1.

<table><caption>Table 1: Characteristics of the pulp and paper wastewater sample in this study</caption><thead><tr><td>Parameter</td><td>Unit</td><td>Value</td></tr></thead></table>
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<table><tr><td>pH</td><td>-</td><td>6.7</td></tr><tr><td>COD</td><td>mg/L</td><td>1250</td></tr><tr><td>Turbidity</td><td>NTU</td><td>530</td></tr></table>

<figure><img src="image_6.png" /><figcaption>Figure 1: The *pistacia terebinthus* seeds and stock solution</figcaption></figure>

## 2-2-procedure

Coagulation process was performed in a jar test apparatus equipped with six beakers. Each of the 250 L beaker was filled with approximately 200 mL of wastewater. Various dosages of *pistacia terebinthus* (0.5, 1, 1.5, 2, 2.5 g/L) were added into the beaker. Different initial pH of the wastewater (3, 5, 7, 9, 11) was adjusted to evaluate the effect of pH on flocculants performance. Each beaker was agitated at 30 rpm for 30 minutes for flocculation mixing and 30 minutes for settlement. After that, top of the beaker would form a supernatant layer while sludge will settle at the bottom of the beaker. The turbidity and COD of the supernatants were measured and recorded through standard methods. The removal efficiencies of COD and turbidity calculated through following equation:

$$Removal efficiency = \frac{(C_0 - C_t)}{C_0} \times 100$$

where, $C_0$ and $C_t$ were the concentrations of the COD and turbidity in the wastewater before and after coagulation-flocculation treatment, respectively.

## 2-3-Analytical methods

The COD were measured by absorbance methods at 600 nm wavelengths using a spectrophotometer (2800 UV/VIS UNICO, China) according to the of 5220 C colorimetric methods of book entitled Standard Methods for the Examination of Water and Wastewater [20]. Turbidity was measured using a portable turbidity meter (HANNA HI9370, USA). Furthermore, the wastewater samples zeta potential and pH was measured using a Zetasizer
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(Nano-ZS, Malvern) and a pH meter (WA-2017SD LUTRON, Japan), respectively. All the experiments were performed in triplicates to avoid errors.

## 3.Result and discussion

### 3-1-COD and turbidity removal of *Pistacia terebinthus* natural flocculants

performance of different dosages of *Pistacia terebinthus* as a natural flocculants for the removal of COD and turbidity from pulp and paper wastewater in pH of 7 is shown in Figure2. As can be seen from Figure2a, *Pistacia terebinthus* showed COD removal efficiency of 67% at the optimum dosage of 1.5 g/L. COD removal is attributed to the settlement of organic materials suspended in wastewater as results of coagulation-flocculation process [21]. Figure2b shows the effect of *Pistacia terebinthus* dosage on the PPWW turbidity removal. Maximum turbidity removal of 87% were obtained by applying 1.5 g/L dosage of *Pistacia terebinthus*. However, after adding more coagulants over the optimal dosages, a deterioration was observed in the COD and turbidity removal efficiencies.

<figure><img src="image_8.png" /><figcaption>Figure2: Effect of *pistacia terebinthus* dosage on a) COD and b) turbidity removal efficiency</figcaption></figure>

### 3-2-Effect of pH on *pistacia terebinthus* performance

Figure3 demonstrate the effect of pH adjustment on removal of COD and turbidity from wastewater in different dosages of natural flocculants. According to Figure3, pH adjustment has a positive effect on both COD and turbidity removal efficiency. In acidic condition (pH = 3, natural flocculant dosage = 2 g/L), 81% and 91% removal efficiencies were obtained for COD and turbidity, respectively. Also, in alkaline condition (pH=11, natural flocculant dosage = 2 g/L), 95% and 96% removal efficiencies were observed for COD and turbidity, respectively. At lower pH, multiple flocculation-coagulation mechanisms of charge neutralization and particle-bridging cause more flocculation. However, at higher pH,
<figure><img src="image_9.png" /></figure>

sweeping mechanism combined with the particle-birding mechanism resulted in a better flocculation and removal efficiency.

<figure><img src="image_10.png" /><figcaption>Figure 3: Effect of pH on *pistacia terebinthus* performance (a) COD and (b) Turbidity</figcaption></figure>

3-3- Effect of pH and dosage on settling time

Figure 4 shows the sedimentation time for obtaining maximum COD and turbidity removal efficiencies by *Pistacia terebinthus* natural, flocculant at different dosages and pH. Without any treatment, slightly removal efficiencies were obtained after 60 min sedimentation (control black line). The fastest sedimentation time belongs to optimum condition of NF = 2 g/L, pH = 9 (5 min) due to synergy effect between pH and dosage of NF and combination of two mechanism of sweeping mechanism combined with the particle-birding. The fastest sedimentation time was observed as following order: NF = 2 g/L, pH = 11 (5 min) > NF = 2 g/L, pH = 3 (15 min) > NF = 0.5 g/L, pH = 11 (25 min) > NF = 0.5 g/L, pH = 3 (40 min) > NF=1.5 g/L, pH = 7 (45 min) > Control (60 min).
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>Figure 4: Settling time for maximum (a) COD and (b) Turbidity removal efficiencies</figcaption></figure>

3-4-Mechanistical analysis

Zeta Potential (ZP) provides the measurement on the electrostatic charges of particles that affects the attraction and repulsion forces between their particles [22]. Zeta potentials of the wastewater obtained from various samples after treatment with different concentrations of *Pistacia terebinthus* (0, 0.5, 1, 1.5, 2, 2.5 g/L) were recorded and the results are shown in Figure 5. Zeta potential studies show an increasing trend when the dosages of cationic natural flocculent increased which is favorable for flocculation process. As can be seen in Figure 5, the poly-cationic natural flocculent of *Pistacia terebinthus* could not significantly influence on ZP. According to Figure 5, even adding 2.5 g/L of *Pistacia terebinthus* increased ZP from -12.3 to – 0.5 mV (still negative). It was observed that maximum flocculation occurred when the value of zeta potential was almost near to zero. The results suggested that charge neutralization might not be the main flocculation mechanism of *Pistacia terebinthus*. Given the relatively high removal efficiencies that *Pistacia terebinthus* showed in Figure 2 and 3, there should be another flocculation mechanism such as bridging combined with charge neutralization to justify this behavior.

<figure><img src="image_13.png" /><figcaption>Figure 5: Effect of *Pistacia terebinthus* natural flocculant on wastewater zeta potential</figcaption></figure>
<figure><img src="image_14.png" /></figure>

## 4. Conclusions

The main objective of this research, was to evaluate the performance of *Pistacia terebinthus* as a natural flocculant. The results showed that 67% and 87% removal efficiencies were achieved for COD and turbidity, respectively. Additionally, the coagulation performance was enhanced with the pH adjustment. Findings concluded that the optimum conditions for the COD and turbidity removal in wastewater, were at 2 g/L of *Pistacia terebinthus* dosage and pH of 11, and 60 min sedimentation time. The maximum removal efficiencies of 95% and 98% were observed for COD and turbidity, respectively. Besides, the zeta potential analysis has demonstrated that the interparticle bridging due to presence of hydroxyl in high pH and charge neutralization resulted in high removal efficiencies. Therefore, considering the cost and dose of presently used flocculants in the treatment, its easy availability, simple process of preparation, it can be an effective alternative for chemical flocculants for COD and turbidity removal.

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