3.1 Industrial Wastewater Characterization
The results of the physicochemical characterization are summarized in Table 1.
Table 1
Table 1: Physico-chemical characterization of raw vegetable oil refinery wastewater.
Parameter
|
Raw wastewater
|
pH
|
5,42
|
Conductivity (µs/cm)
|
19764.1±2.09
|
Dissolved O2 (mg/L)
|
0.17
|
Turbidity (NTU)
|
≥4000
|
TSS (mg/L)
|
7892 ±1.76
|
COD (mg/L)
|
23040±2.07
|
TP (mg/L)
|
75,34±0.08
|
NH4+ (mg/L)
|
13.49±0.02
|
NO3- (mg/L)
|
256,41±0.09
|
Polyphenols (mg/L)
|
128,57±1.09
|
254(*50)
|
2,598±0.02
|
Color
|
2,923±0.02
|
oils and fats (mg/L)
|
32760±3.04
|
The results showed that the effluent is characterized by an acid pH of about 5.42, which agrees with the results found by Preethi et al. (2020). In addition, vegetable oil refinery wastewater (VORW) is rich in suspended solids with a content of 7.9 g/L. These suspended solids results are slightly higher than those cited in the literature (Kastali et al. 2021). The effluent used in this study is characterized by very high turbidity of over 4000 NTU. The value found in terms of turbidity is higher than that found by Khouni et al. (2020). Generally, due to the manufacturing process, the physicochemical characterization of wastewater from vegetable oil refineries varies from one effluent to another. In addition, VORWs are highly loaded with oils and fats with a concentration of 32.8 g/L, which exceeds In Generals. VORWs are known for their high oil and fat content due to large amounts of free fatty acids in the raw production material (Jameel et al. 2011). Table 1 shows that the organic load concentrations are also very high. The COD value reached 23040 mg/L, almost similar to that found by Dhanke and Wagh (2020).
Moreover, the effluent is characterized by the presence of complex elements, in particular polyphenols 129 mg/L. Venturi et al. (2017) have shown that the wastewater collected while refining olive oil has very high levels of phenolic compounds, mainly represented by non-flavonoid combinations. Indeed, these values indicate that organic compounds are not easily subjected to biological treatment and that a physicochemical process is necessary. Total phosphorus (TP), ammonium (NH4+), and nitrate (NO3-) are respectively in the order of 75 mg/L, 13 mg/L, and 256 g/L (Table 1). The physicochemical analyses revealed that almost all parameters exceed Moroccan standards indicating that the VORW are heavily loaded with organic and chemical pollutants and need to be further treated before being discharged into the receiving environment.
3.2 Turbidity removal
The reduction of the turbidity of wastewater from vegetable oil refineries by flotation in an acidic or basic medium has been tested. Figure 2 shows the turbidity removal efficiency as a function of pH. The best removal efficiencies were obtained for acidic pH (4 to 1), ranging from 95% to 97%. Basic flotation did not show good efficiency in turbidity removal. On the other hand, the results obtained agree with Abidin et al. (2013), which showed a removal efficiency of about 99% at pH 3. Generally, turbidity in wastewater is due to total suspended solids (TSS), such as clay, silt, finely divided organic and inorganic matter, soluble and colored organic compounds, plankton, and other microscopic organisms (Kastali et al. 2021). Turbidity removal is strongly linked to suspended solids removal. Flotation is a technique that removes suspended solids, oil, and grease. During flotation, air bubbles attach to the particles, so the conglomerate of solids and bubbles floats to the water's surface, where it can be scraped away (Steinke and Barjenbruch 2010). Wastewater pH has been identified as one of the parameters influencing wastewater treatment efficiency (Mandal 2014). In addition, Abidin et al. (2013) have shown that the percentage of turbidity removal decreases with increasing pH. Al-Maamari et al. (2012) have shown that flotation removes 57 to 78% of the turbidity. At the same time, dissolved air flotation showed a turbidity removal efficiency of 57.7% for the palm oil mill wastewater (Faisal et al. 2016). Cifuentes-Cabezas et al. (2021) found a 41.9% turbidity removal efficiency by applying the flotation process as a pretreatment of olive oil-washing wastewater.
3.3 COD removal
The acid and basic flotation resulted in a variable reduction of COD at different pH values (ranging from 1 to 8) Figure 3. Increasing the pH reduced about 37.5% COD at pH = 6. This removal efficiency decreased and reached 16% for pH 7 and 8. Acidic pH showed COD removal efficiencies varying around 80% at pH (2 and 2.5). Flotation is a very efficient method of liquid-solid separation and has a definite advantage for removing low-density particles that tend to float (Xing et al. 2018). COD removal could be due to the flotation of suspended solids, characterized by low density (Farhadi et al. 2012). Generally, the pH value controls the charge density and stabilizes suspended solids (López-Maldonado et al. 2014). The surface charge density of the particles tends to decrease under strongly acidic pH conditions due to the increase of the ionic strength, which compresses the thickness of the electric double layer (Singh et al. 2004). Dewi, Sari, and Syafruddin (2017) have shown that flotation is the most efficient method to remove COD from palm oil liquid waste. Rattanapan et al. (2011)used acidification by two acids, HCl and H2SO4, to pretest biodiesel wastewater. During one day, the COD removal efficiency was higher than 50%. Faisal et al. (2016) studied dissolved air flotation for a retention time of five days and an airflow of 11 L/min. They achieved a COD removal rate of approximately 35.5% from palm oil-rich wastewater.
3.4 Polyphenols and 254 nm removal
Polyphenols are environmentally toxic substances (Hamimed and Kthiri 2022). Vegetable oil refineries are one of the primary industries that often release toxic substances into the environment (Haddaji et al. 2022).
Chemical flotation resulted in a variable reduction of polyphenols and absorbance at 254 nm at different pH (1-8). Basic flotation resulted in the removal of 59.73% and 66.3% of polyphenols, respectively, for pH 6 and 8. The highest removal efficiencies are between 86.6% and 81.5% in the acidic pH range of 1 to 4 (figure 4). Due to aromatic substances, chemical flotation reduced the 254 nm absorbance (Peng et al. 2018). At pH 2 the elimination efficiency is about 90.53%. While at pH 8 this performance is approximately 15.74% (figure 5). The removal of phenolic compounds depends on their hydrophilic-hydrophobic characteristics. Simple compounds, such as phenolic alcohols, are hydrophilic, whereas high molecular weight polyphenolic compounds, characterized by a robust polyaromatic property, are generally hydrophobic (Khattabi Rifi et al. 2021). Phenolic compounds such as curcumin at pH 3 are predicted to be the most hydrophobic (Zembyla et al. 2018). Applying flotation as a pretreatment of olive oil washing wastewater removed only 5% of total phenolic compounds (Cifuentes-Cabezas et al. 2021). Furthermore, the combination of natural flotation and anaerobic-aerobic treatment of olive mill wastewater showed a 25.2% reduction in these compounds (Khattabi Rifi et al. 2021).
3.5 Phosphorus and nitrate removal
Among the main problems facing aquatic ecosystems is the discharge of large quantities of chemicals such as phosphorus and nitrates (Goswami et al. 2022). These substances lead to the disturbance of aquatic life by triggering the phenomenon of eutrophication (Romanelli et al. 2020). Removing this type of pollutants from wastewater before discharge into the receiving environment has become necessary. Therefore, chemical flotation in the presence of the acidic or basic medium was tested to evaluate the removal of nitrate and phosphorus from VORW. The results showed a reduction of about 99% of phosphorus in the pH range from 1 to 4. Basic flotation led to a phosphorus removal efficiency of about 85% for pH 6 and 7.
For nitrates, the flotation in the acid medium in the presence of HCl allowed reaching 97,5% of nitrates removal at a pH equal to 1,5. The minimum efficiency is about 71.56% in pH 8 (figure 7). Djouadi Belkada et al. (2018) have shown a better removal of nitrates in acidic environments. In addition, Parastar et al. (2013) showed that the optimum nitrate removal performance (95.5%) in an aqueous solution was achieved at an acidic pH. Chemical flotation is more effective in removing nitrate from VORW than other treatment techniques. Khouni et al. (2020) found a removal efficiency of 30.4% nitrates and 95.9% phosphorus using coagulation-flocculation as a treatment technique. In the combination of natural flotation and anaerobic-aerobic treatment of olive mill wastewater, a 93.9% reduction of nitrates was reported (Khattabi Rifi et al. 2021).
3.6 Oils and greases removal
Oils and fats (O&G) cause ecological damage to aquatic organisms, plants, and animals. They are also mutagenic and carcinogenic for human beings (Bestawy et al. 2020). Oils and fats form a layer on the water's surface, reducing biological activity due to decreased dissolved oxygen (Anamika et al. 2019). The presence of high concentrations of oils and fats in wastewater creates many problems in wastewater treatment plants due to their treatment difficulties and issues with the equipment of wastewater treatment plants (Chaudhari et al. 2021). Figure 8 illustrates the removal of oil and grease from VORW after treatment by chemical flotation at different pH. The results represent an almost total removal of oils and fats in an acid environment with a yield that can reach 97% and 99% at pH between 1 and 1.5. Generally, this pH affects the carboxyl function (COO-) on the surface of oil droplets, which become natural charges (Rattanapan et al. 2011). Then the oil droplets move closer together and flocculate. The large oil droplets from the flocculation rise to the surface, removing grease and oil from the wastewater (Rattanapan et al. 2011). They treated the biodiesel wastewater using an acidic pH of 3. The results showed an oil and grease removal efficiency of over 80%. Zhang et al. (2006) have shown that the acidification process of coal gasification wastewater with pure hydrochloric acid can reduce oil and grease with an efficiency of about 57.4%. In contrast, basic flotation led to 69% oil removal at pH 8. According to Cammarota and Freire. (2006), basic flotation reduces the average particle size of the fats by 73% of the initial average size, which facilitates the flotation process of these particles.
3.7 Color removal
The color of treated wastewater is generally considered an accurate indicator of Pollution (Sözen et al. 2020). The color in VORW is usually related to the pigments in the oil sources. These vegetable pigments mainly comprise carotene molecules (Godoy et al. 2020). Chemical flotation was tested to evaluate its effectiveness in removing color from VORW. Figure 9 shows the removal of VORW color by chemical flotation at different pH. Flotation has shown a more than 80% reduction in an acidic medium and can reach 96% and 98% yields at pH between 1 and 1.5. For pH 6 and 7, color removal rates decreased between 67% and 74%. A meager removal rate of about 3% was noted at pH=8. However, a lower efficiency of 50% was obtained by Yao et al. (2021), who used dissolved ozone flotation as a pretreatment process to remove color in a wastewater treatment plant. In addition, applying flotation as a pretreatment of olive oil washing wastewater reduced 40% of the color (Cifuentes-Cabezas et al. 2021). Eliminating color is related to removing COD, oil, and grease, as Bakraouy et al. (2017) show.
4. Statistical analysis
To interpret the results obtained for the different physicochemical parameters, a statistical study was carried out using a multivariate analysis method (principal component analysis (PCA)) (Rifi et al. 2022). PCA is a statistical test part of a group of factorial studies. It consists of transforming the initial quantitative variables correlated between them into new quantitative variables, uncorrelated (Granato et al. 2018). Multivariate statistical methods, mainly principal component analysis (PCA), are the most widely used strategy for process monitoring in wastewater treatment plants (WWTPs) (Liu et al. 2014). These monitoring strategies can identify faults and obtain better control performance for the data sets. Data analysis by PCA method was applied for turbidity, COD, polyphenols, oils and fats, Absorbance 254 nm, phosphorus, nitrates, and color for different pH. Figure 10 represents the principal component analysis of the samples treated using HCL as a flotation agent. The variation in the factorial axes C1 (78.59%) and C2 (18.96%) was significantly significant. The results show that almost all the variance is represented by the first component, with a positive correlation between turbidity, polyphenols, Abs 254nm, phosphorus, oils and fats, and color, while nitrates and COD were negatively correlated. Figure 11 represents the statistical analysis (PCA) of the different physicochemical parameters of the samples treated with NaOH. The figure shows that component 1 has a variance of 68.7%, and component 2 has a variance of 31.31%. These results show the effectiveness of treatment by chemical flotation in an acidic or basic medium to remove organic matter, oils, and greases. The latter constitute serious problems in the management and treatment of industrial wastewater. Therefore, this study's results can benefit researchers and industries in addressing the challenge of treating wastewater from vegetable oil refining. The proposed solution could be used in small and medium enterprises. It is considered a simple, economical, and effective method.