Aloe steudneri gel as natural flocculant for textile wastewater treatment

Water is a crucial component of life, its uses range from drinking and hygiene to industrial processing. But the depletion of this vital resource has become a global concern due to uncontrolled anthropogenic influence. Poor environmental conditions arising from inadequate or non-existing wastewater management pose significant threats to human health, well-being and economic activity. On average, high-income countries treat about 70% of generated wastewater, while low-income countries treat only 8% (United Nations 2017). Agricultural and industrial activities pose a major pollution threat to waters, and within industry textile manufacturing leads by being the largest polluter in the world (Babu et al. 2007). Major pollutants in textile wastewaters are high suspended solids (SS), chemical oxygen demand (COD), heat, colour, acidity, and other soluble substances (Mondal et al. 2017).

In Ethiopia, the textile industry sector is undergoing a major development, growing on average by 51% in the last 5–6 years (Pols 2015), but this change has a serious negative impact in terms of pollution. Yirgalem Addis textile factory is one of the industries with complex composition of wastewater containing high values of 5-day biological oxygen demand (BOD₅), COD, total suspended solids (TSS), lead (Pb), and turbidity. As a solution, the industry implemented a treatment plant with conventional coagulation-flocculation, using synthetic coagulants and flocculants at a high cost.

In addition to its high cost and availability problems, conventional synthetic polyacrylamide is used as a flocculation agent, even though there are concerns around its toxicity because of un-polymerized carcinogenic monomer present in it (Xu et al. 2014). As a result, research has been conducted on the performance of natural plant extracts in water and wastewater clarification in order to substitute the synthetic flocculant. Plant-based coagulants such as mustard seed extract (Bodlund et al. 2014), rice starch (Teh et al. 2014), guar gum (Mukherjee et al. 2013), banana stem juice (Ku Hamid 2013), Moringa oleifera (Muthuraman & Sasikala 2014), Cassia obtusifolia seed gum (Subramonian et al. 2014) and others have been studied. But when it comes to applicability, one universal natural flocculant is not available because they are limited in certain areas and some are limited by seasons. Therefore, each country should evaluate the potential as well as to develop appropriate methods for natural polymer production. This research evaluates the efficiency of Aloe steudneri gel which is a succulent plant belonging to liliacea family as flocculant in treatment of textile wastewater.

The study was conducted in Addis Ababa Science and Technology University, Environmental Engineering department laboratory from August to November 2017. The samples were taken from a Buba hill which is partially covered by Aloe vera steudneri species (Sebsebe & Nordal 2010). The hill is found in Chancho town at about 40 km north of Addis Ababa city (Figure 1).

Samples of the mature fresh Aloe vera 30–40 cm long were selected and collected in polyethylene plastic bags. It was then washed and split in half by using a knife and the slimy thick gel was recovered in a 1liter beaker. The remaining parts were dried at 70 °C for 24 hours in an oven for further phytochemical analysis.

50 ml of recovered gel was introduced into 500 ml of distilled water and stirred using a magnetic stirrer for half an hour. Then, the solution was strained through a sieve of 25 mm. Finally, the filtrate was collected and stored in refrigerator not exceeding 48 hours to avoid spoilage by microorganisms.

Effluent samples were collected from Yirgalem Addis textile factory plc., located in Addis Ababa around Nifas Silk Lafto sub city in the area commonly referred to as Adei Ababa area. Composite samples of the wastewater were collected and the analysis conducted in 24 hours.

The pH and temperature were measured using a pH meter and digital thermometer (WTW 340i) respectively. Turbidity was measured by a portable turbidometer (HACH 2100Q) and dissolved oxygen (DO) by WTW oxi 3310. A drying oven MEMMERT, digital sieving equipment and mortar were used in the preparation of dried Aloe vera for phytochemical screening. The test of flocculation was carried out using a flocculator PRAGATI with six stations for the jar test with a pipette tube to extract the supernatant. BOD₅, COD, and Pb content of the wastewater were analyzed using Oxitop automatic, dichromate method and atomic adsorption spectrometry. All analyses of spot samples were conducted in accordance with Standard Methods (APHA 1998) at room temperature.

The phytochemical screening was based on the coloring reactions and the precipitation reactions of the chemical compounds in the Aloe vera gel solution.

0.5 g of Aloe vera gel was stirred with about 10 ml of distilled water and then filtered. A few drops of 1% ferric chloride solution were added to 2 ml of the filtrate. Occurrence of a blue-black precipitate indicated the presence of tannins (Buvaneshwari et al. 2017).

About 25 g of Aloe vera gel was dissolved in distilled water, filtered, and then 2 ml of 10% aqueous sodium hydroxide was added to produce a yellow coloration. A change in color from yellow to colorless on addition of dilute hydrochloride acid was an indication for the presence of flavonoids (Buvaneshwari et al. 2017).

2 ml extract was diluted with 20 ml distilled water, and the solution was mixed for half an hour. Then the solution was allowed to stand for 2 hours. Formation of about 1 cm layer of foam indicated the presence of saponins (Buvaneshwari et al. 2017).

About 0.2 g of each portion was shaken with 10 ml of benzene and filtered. Then 5 ml of 10% ammonia solution was added to the filtrate and shaken. Appearance of a pink, red or violet color in the ammonnical (lower) phase was taken as the presence of free quinones (Sofowora 1993).

Swelling and fat adsorption capacity were determined according to the standard methods adopted from AOAC (Association of Official Analytical Chemists) for both the Aloe vera gel and the standard Magnafloc 333 flocculant.

So, absorbency was calculated as grams of water per gram of polymers (Aloe vera or polyacrylamide) g/g.

Wd is the dry weighed of the original gel (before adsorbing the oil).

Tf = the turbidity of treated wastewater in NTU

Microsoft Excel 2013 was used to carry out statistical analyses, develop the box and whisker plots, and figures. The significant differences among samples were analyzed using the Mann-Whitney U-test at 5% significance level.

Table 1 presents the effluent characteristics of the Yirgalem Addis textile factory, the wastewater from which has a significant amount of lead, which is above the permissible value, and high amounts of COD, BOD₅ and TSS, and a very low amount of dissolved oxygen (DO), which is undesirable. From the values of COD and BOD₅ it can be observed that the wastewater is mainly composed of organic matter since BOD₅ is more than 80% of the COD value.

The result obtained from dried Aloe vera steudneri leaves showed the presence of secondary metabolites such as tannins, saponins, flavonoids and quinones, which may be related to the plant's medicinal properties. These results are comparable to (Irma et al. 2015) for aloe species from Benin, (Mariappan & Shanthi 2012) for aloe vera species found in India, and (Moses et al. 2014) who used Aloe vera harvested in Uganda (Table 2).

Natural flocculants' performance is always attributed to the specific compounds present in them, which gives the natural extract a distinct character in flocculation performance. These characteristics are a high molecular weight for the settling of suspended particles and water absorption or particle adhering character for the bridging mechanism.

Table 3 shows that the swelling capacity of Aloe vera gel was higher than Magnafloc. This may have originated from the greater availability of monomer molecules in the vicinity of the chain propagation sites of Aloe vera gel radicals.

The swelling capacity or the absorbance of the Aloe vera gel at 1.9 g of H₂O/g of polymer is comparable with the high efficiency flocculant swelling capacity of graft copolymers studied by (Pandey et al. 2017 and Giri et al. 2016), which were 2.2 and 2.5 g H₂O/g of polymer, respectively.

The fat adsorption capacity shows that Aloe vera is much superior by fiber content than Magnafloc, which increases its ability to adsorb the fat content present. Thus the fiber content of the Aloe vera gel may play a significant role in reducing the highly organic content of wastewater.

Even if the pH of the raw wastewater is in a permissible range (6–8), it may be changed considerably during the treatment process by the flocculant used. This leads to additional expenses by adding chemicals to adjust the pH of the treated wastewater. Hence, industries need flocculants that do not significantly vary the pH of the wastewater (Figure 2).

As can be seen in Figure 2, as flocculant dosage increased from 10 ml to 60 ml, the pH of the wastewater was reduced from 6.5 to 5.6 for AVS-floc, and 6.6 to 6.1 for Magnafloc. There was a slight decrease in pH for both cases, but in the case of AVS-floc (natural) the pH change was higher than that of the Magnafloc.

Figure 3 shows the natural flocculant (AVS-floc) efficiency is highly influenced by the pH of wastewater while the synthetic (Magnafloc) is little influenced by the pH change compared to the AVS-floc. As shown below, at a lower pH value, the flocculation efficiency of the AVS-floc was less than 50%, it increased from pH 6 to 7, and then decreased as the pH increased. Despite the relatively stable nature of the synthetic (Magnafloc), both work best at pH 7.

This may be due to complications caused by H⁺ ions in acid region and OH⁻ ion in basic region, which react and compete with the adsorption site of the flocculant (Reddy et al. 2012).

As shown in Figure 4, the change in mixing time directly influenced the flocculants' performance in turbidity removal. The mixing time was varied at 10, 15, 20, 25, 30 and 35 minutes and optimum pH of 7 while other variables were kept constant (mixing speed = 40 rpm and flocculant dosage = 15 g).

In the case of the synthetic Magnafloc, the removal efficiency slightly increased as mixing time increased from 10 to 15 then to 20 minutes. But after 20 minutes, the turbidity removal efficiency reached 68.9%, but there was no change after increasing the mixing time to 35 minutes. Hence, the optimum mixing time for Magnafloc was 20 minutes. In the case of the natural AVS-floc, the turbidity removal efficiency increased as mixing time increased up to 25 minutes. After 25 minutes, the removal efficiency reached 67.3%, but there was no change with increase in time, showing that it is the optimum length of time for mixing.

The optimum mixing speed for the two flocculants (AVS-floc and Magnafloc) determined where removal efficiency reached its peak. The mixing speed was varied at 40, 50, 60, 70, 80 and 90 rpm, pH was kept constant at 7, flocculant dosage was kept constant at 10 mg, and mixing time was kept constant at 25 and 20 minutes for AVS-floc and Magnafloc respectively (Figure 5).

As shown in the above figure, the removal efficiency increased as mixing speed increased and reached its peak at 60 rpm for both flocculants (AVS-floc: 81.57% and Magnafloc: 83.15%).

Then as mixing speed is further increased the removal efficiency starts to decline, this may be due to re-suspension of precipitated particles which were formed as aluminium hydroxide precipitate during the coagulation stage.

As can be seen in Figure 6, as flocculant dosage increased from 10 ml to 30 ml, the removal efficiency reached 91.57%, its peak, for the case of AVS-floc and 92.1% for the Magnafloc. Further increment of AVS-floc dosage decreased the removal efficiency. However, the removal efficiency increased on further increment of Magnafloc dosage and its peak efficiency, 92.7%, was achieved at 37 ml but decreased after this point (Figure 6).

The decrease in removal efficiency after the optimum point may be due to re- stabilization of dispersed solid induced. Generally, there was no significant differences (α<0.05) for different flocculant dosage, mixing time and speed between the two flocculants. However, during pH variation, there was significant difference (α > 0.05).

The performance comparisons of AVS-floc and Magnafloc for other wastewater parameters (COD, BOD₅, TSS, DO and Pb) were conducted at optimal conditions as shown in the Table 4.

The comparison of untreated and treated wastewater for DO, pH and temperature is shown in the Table 5 and the removal efficiencies of COD, BOD₅, TSS, turbidity content and Pb for both flocculants presented in Figure 7 below. However, the removal efficiencies for COD, BOD₅, turbidity and Pb were not significantly different (alpha < 0.05).

The present study proved that Aloe steudneri gel was a promising and effective natural coagulant in substituting Magnafloc. The removal efficiencies for COD, BOD₅, TSS, turbidity and Pb were 76.8%, 83.5%, 57.9%, 92.3% and 77% using AVS-floc, and 78%, 89%, 51%, 92.7% and 72% using Magnafloc. In addition to this, there were no significance differences for the parameters except pH variation.

The manuscript is possible through the financial support from Ethiopian road authority.