Optimization of Coagulation to Remove Turbidity from Surface Water Using Novel Nature-Based Plant Coagulant and Response Surface Methodology
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1. Introduction
Water pollution has become one of the major threats to the entire biosphere due to urbanization and the rapid expansion of industries [1]. Surface water, after treatment, is a common source of municipal water supplies. This supply is used for drinking purposes. For instance, in the USA, around 66%, and in Europe around 35–45%, of municipal water supplies are based on surface water sources [2]. In Pakistan, around 16 million people collect and consume water from insecure sources, including surface water and groundwater [3]. The quality of surface water declines due to organic and inorganic constituents from natural and anthropogenic sources [4]. Colloidal particles include organic material such as algae and inorganic material such as sand, silt, and sediments [5]. Many of the environmental problems are associated with the presence of heavy metals, total suspended solids, and turbidity in surface water [6]. Therefore, surface water contamination is a vital concern as it affects municipal, household, and agricultural activities.
The removal of turbidity, along with other impurities, from surface water using chemical coagulants is a vital part of surface water treatment processes. For this, coagulation–flocculation is widely used. It is considered reliable, economical, and efficient among all other physicochemical treatment technologies [7,8]. In general, coagulants are classified as natural and chemical. Chemical coagulants, e.g., alum, ferric chloride, and polyaluminum chloride (PACL), are used in water and wastewater treatment [7,9,10]. There are several drawbacks associated with the use of chemical coagulants in the water treatment process. Firstly, the carbon footprint associated with the use of chemical coagulants is a major concern. The use of alum as a coagulant contributes to an estimated 35% of the carbon footprint of a water treatment facility. Moreover, an excessive use of alum salt may lead to the development of Alzheimer’s disease in humans. Also, chemical coagulants result in a large volume of sludge production, a change in the water pH, and high procurement costs [11]. Another major concern associated with chemical coagulants is the disposal of the sludge produced. For example, in Australia, the cost of the disposal of alum sludge was estimated to be AUD 130 per ton [12].
Moreover, the presence of residual aluminum in drinking water reduces the water disinfection efficiency [13]. Acrylamide (a synthetic organic polymer) is found to be neurotoxic and carcinogenic. If iron (Fe) salts are used excessively for water treatment, these cause blood-colored stains and visible rust [14].
Therefore, considering the shortcomings of chemical coagulants, the trend has been shifted toward the use of natural bio-coagulants (BCs). BCs are low-cost, can be produced abundantly, and do not change the pH of treated water [15]. Moreover, BCs are non-toxic and produce less sludge, which makes them more effective compared to chemical coagulants [16].
Plant-based natural coagulants are produced from non-hazardous, renewable, degradable, and carbon-neutral sources [12]. Plant-based coagulants help to destabilize the colloidal particles and form micro and macro flocs through polymer bridging or charge neutralization. Macro flocs can be easily removed through settling, while micro flocs need the aid of filters or flocculants for their removal [17,18].
Natural coagulants are mostly plant-based. Moringa Oleifera, Tamarindus indica, tannin, Plantago major, Nirmali seeds, and Strychnos potatorum were studied as effective green coagulants for the treatment of textile wastewater [19]. The Moringa Oleifera seed extract is the most widely studied plant-based coagulant, having comparable efficiency to alum. The coagulation mechanisms for the Moringa seed extract are adsorption, charge neutralization, and interparticle bridging [11,20]. The responsible active coagulating agent in most plant-based coagulants is dimeric cationic protein. Moreover, common beans such as red maize, red beans, sugar maize, Phaseolus vulgaris, Opuntia stricta, and walnut shells have also been evaluated as BCs [21].
Chickpeas have been explored as a coagulant in comparison with Dolichols lablab and M. Oleifera. For the coagulant extract of chickpeas, maximum turbidity removal was observed at 85.89% in a clay suspension with an initial turbidity of 95 NTU at a dose of 100 mg/L. Moreover, it was concluded that the removal efficiency of all coagulants was higher for highly turbid water and low for low-turbidity water [22].
Given the drawbacks associated with chemical coagulants, there is a growing need to identify new bio-coagulants that possess good coagulation potential for water purification. Additionally, the availability and abundance of such coagulants must be taken into consideration to ensure their widespread availability. The understanding of the coagulation mechanisms in bio-based coagulants has received insufficient consideration, and there has been little attention given to the effect of pH on their efficacy [23]. Additionally, previous studies have explored the coagulation potential within a narrow or single range of pH, or have focused on fixing a single variable such as the pH or dose. Only a few studies have explored the effect of simultaneous variations in the pH range and coagulant dose to understand their combined effect. Therefore, the objective of this study was to explore the coagulation potential of plant-based natural coagulant Sorghum (seeds), which has not been explored previously. Sorghum coagulants can be used for small water supplies and water treatment at the household level, especially in rural areas that have limited accessibility to chemical coagulants. Aloe Vera was tested as a coagulant aid, to examine its role in enhancing turbidity removal.
2. Materials and Methods
2.1. Sample Collection and Characterization
Seven grab samples of surface water from the Bambawali–Ravi–Bedian canal, passing through Lahore, were collected at one-hour intervals from 9:00 a.m. to 4:00 p.m. Later, these were mixed well to form a composite sample. Bottles cleaned with distilled water were used to store the samples. The sample bottles were labeled and stored at 4 °C. The samples were characterized for quality parameters using standard methods, and these values are reported in Table 1.
2.2. Preparation of Coagulants
Coagulant solutions were prepared with Sorghum seed powder (great millet) and leaves of Aloe Vera, separately. Methods for preparing the stock solutions for each are defined below.
2.2.1. Preparation of Stock Solution for Sorghum Coagulant
Sorghum seeds were bought from the local market, dried for 48 h in direct sunlight, and then dried in an oven at 103 °C for 24 h. Dried seeds were blended in a mixer (Cambridge GC-5026) to obtain a powdered form of coagulant. The Sorghum powder was then sieved through mesh no. 40 to obtain uniformly sized particles that were smaller than 0.42 mm. Of the sieved powder, 2.5 g was dissolved in 1 L of distilled water and agitated using a magnetic stirrer for 30 min to liberate the active coagulant compounds [24]. The mixture was passed through Whatman filter paper no. 42 and the resultant filtrate was used as a stock solution. The stock solution’s concentration was 2.5 g/L, with a pH of 7.2. The stock solution was added to the canal water in appropriate proportions to obtain the coagulant concentrations of 10, 20, 40, 60, and 80 mg/L.
2.2.2. Preparation of Coagulant Aid
The leaves of Aloe Vera were available locally. These were collected and washed well to remove dust particles. The thick exterior skin was removed, and the pulp (a gel-like material) part was separated carefully. The pulp was blended with a beater to obtain a homogenous liquefied paste. Then, 10 mL of fresh liquefied paste was directly mixed into 990 mL of surface water to obtain a 1% (v/v) Aloe Vera gel concentration solution. Similarly, volumes of 8 mL, 6 mL, 4 mL, and 2 mL of the liquefied paste were directly added to 992 mL, 994 mL, 996 mL, and 998 mL of surface water to obtain concentrations of 0.8%, 0.6%, 0.4%, and 0.2%, respectively.
2.3. Design of Experiment Using Response Surface Methodology (RSM)
The design of the experiment was carried out using Design-Expert software (DOE version 12.0.1.0), and response surface methodology (RSM) was used to optimize the response variables (turbidity removal) against the treatment variables (pH and dose). A detailed design summary is presented in Table 2. RSM involves a combination of mathematical and statistical tools. It is extensively used to optimize output variables and for the design of experiments [7]. Experimental results were analyzed using various models (i.e., linear, quadratic, cubic, etc.). The most appropriate model was selected to evaluate the effects of the treatment variables on the response. Analysis of variance was further used to validate the statistical results of the response and suggested model. The desirability function was used for the optimization of the response variable.
Experiments were conducted in two trials. The first trial refers to the use of Sorghum alone and the second trial refers to the use of Aloe Vera as a coagulant aid with Sorghum. In the first trial, the pH (2–10) and coagulant dose (10–80 mg/L) were selected as treatment variables. For the second trial, the pH (2–10), Sorghum coagulant dose (10–80 mg/L), and Aloe Vera dose (0.2–1%) were selected as treatment variables, while turbidity removal (%) was set as a target (response) variable for both trials. The low values were coded as −1, representing the minimum pH value of 2, and the high values were coded as +1, corresponding to the maximum pH value of 10. Similarly, for the dosage, 10 mg/L was denoted as −1, and 80 mg/L was represented as +1. In total, 25 experimental runs were conducted for each trial.
2.4. Jar Test
A jar test apparatus (PB-900, Phipps and Birds, Richmond, Virginia USA) was used to simulate coagulation–flocculation–sedimentation. The beakers were filled with a sample of 1 L (surface water). The pH of the sample was adjusted to the desired level in each beaker using 1 M NaOH (Sigma Aldrich, St. Louis, MO, USA) and 1 M H2SO4 (Sigma Aldrich, St. Louis, MO, USA). Additionally, the desired coagulant dose was added to each beaker. Rapid mixing (200 rpm for 2 min) followed by slow mixing (30 rpm for 25 min) and settling for 30 min were selected as operational parameters for the jar test apparatus. In the second trial, the required volume of freshly prepared liquefied coagulant aid (Aloe Vera) was added to the beaker at the start of slow mixing. A turbidity meter (Hach 2100 AN Turbidimeter, Loveland, CO, USA) and a pH meter (Hach sensION+ 3 pH meter, Loveland, CO, USA) were used to measure the turbidity and pH of the samples, respectively. The blank samples were run for each pH level, and residual turbidity of each blank was measured at the end of each experiment. Figure 1 shows the experimental workflow diagram.
2.5. Fourier Transform Infrared Spectroscopy (FTIR) of Coagulant, Coagulant Aid, and Suspended Solids
FTIR analysis was performed for the Sorghum coagulant powder, the coagulant aid, i.e., Aloe Vera (before treatment), and the flocs produced after treatment using an Agilent Cary 600 series Fourier transform Infrared Spectrophotometer (Agilent Technologies, CA, USA). For sample processing, the flocs (separated through filtration) were carefully moved to a clean glass slide. Flocs were dried at room temperature for 24 h and shifted to an airtight jar. For analysis, a 1 mg sample was mixed with 150 mg KBr (analytical grade). A pellet was prepared using a 10 T press. The recorded spectral range was 600–4000 cm−1. These spectra were employed to identify the main functional groups present in the coagulants and flocs.
2.6. Scanning Electron Microscopy (SEM) of Coagulant, Coagulant Aid, and Sludge
Scanning electron microscopy (SEM, Hitachi S-37N, Hitachi High-Tech, Tokyo, Japan) images of coagulant powder, coagulant aid, and produced flocs were attained using a high magnification at a voltage of 15.0 kV to examine the surface morphology of the particles. Samples were gold sputter-coated before SEM analysis. The shape and the size of the flocs were studied using SEM images to determine the possible coagulation mechanisms.
2.7. Statistical Analysis
The experimental data were fitted to various statistical models (from linear to complex models) to identify the most appropriate model in terms of coagulant performance. The selected model was used to evaluate the effect of variables on the response. Analysis of variance (ANOVA) was performed on the developed model to determine the statistical significance and to check its suitability by comparing the predicted and actual values. Alpha (0.05) was used as the significant level.
4. Conclusions
This study reveals that the Sorghum coagulant was efficient in treating turbid surface water. Sorghum worked better at a lower pH of 2. However, a significant removal was also observed around the pH 7.1 of natural surface water, especially with the addition of the coagulant aid Aloe Vera. The maximum turbidity removal of 87.73% for Sorghum (great millet) was achieved at pH 2 at a dose of 40 mg/L, with a residual turbidity of 38 NTU, while the Sorghum coagulant along with coagulant aid (Aloe Vera) achieved turbidity removal up to 84.2% at pH 2.7 and Sorghum and Aloe Vera doses of 17.1 mg/L and 0.9%, respectively. At a lower pH, Aloe Vera as a coagulant aid is not an effective option as it reduces the turbidity removal efficiency. However, at a neutral pH of 7.1, it increased the turbidity removal efficiency significantly. From the ANOVAs of both trials, it was found that pH had a significant impact on turbidity removal instead of the coagulant dose. The coagulation mechanism was identified as adsorption due to the presence of the carboxylic, amine, and carbonyl groups in the Sorghum coagulant and amine in Aloe Vera. The estimated treatment cost were USD 50.8 (pH 2) and 59.8 (pH 7) for Trial 1, and USD 37.1 (pH 2.7) and 32.9 (pH 7) for Trial 2, to treat 1000 m3 of canal water. Future studies should conduct pilot-scale experiments, and life cycle assessments should also be carried out. Evaluating Sorghum’s effectiveness in treating acidic industrial wastewater is recommended due to its superior performance in acidic pH conditions. Further studies on using locally available natural bio-coagulants (BCs) are recommended.
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