Research Article
Optimization of sunlight photo-catalytic treatment coupled to a coagulation-flocculation process
https://doi.org/10.21203/rs.3.rs-1959503/v2
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The oleic industry is one of the most polluting in food industry sector. An easy treatment of this waste is Coagulation–flocculation. Photo-catalysis can be used as complementary treatment to improve the water quality and make it reused. We optimize this method to treat mill wastewater collected from the region of Mahdia, by varying the conditions and studying their effects. Organic pollutant were removed and the final quality of water were under the imposed norm NT 106-02.
Mill wastewater
photo-catalysis
coagulation
flocculation
optimization
For centuries, Mediterranean basin countries have been at the forefront of the international olive oil industry. In Tunisia, the total olive oil production capacity stands at 45,000 tons per day in average (ONAGRI, 2022). According to ONAGRI, National Observatory of Agriculture, Tunisian production of olive oil is about 400,000 tons in 2020, about 350,000 tons of which are dedicated to exportation (Elkadri, et al., 2022). Olive oil is, consequently, fundamental to the country’s economy. In addition to oil, this industry produces soap either directly from oil or by chemical extraction from pomace. However, according to the Tunisian Ministry of Environment, it also generates important quantities of olive mill wastewater (OMW), reaching 950,000 m3 per year (Sierra et al., 2022).
Many researchers (Elkadri et al., 2022, Sierra et al., 2022, Kovačević et al., 2022) consider this agro-industrial waste as one of the heaviest and most difficult to manage
This type of sludge is a dark viscous fluid with malodors. It consists of more than 80% of water and sugar, nitrogen, organic acids, polyphenols, polyalcohols, pectines, mucilage, tannins and inorganic substances (Zghari et al., 2018, Juárez et al., 2008). Their composition depends on many factors like region, climatic conditions, nature of the soil and even the trituration method (Aggoun et al., 2016). Residual oily substances of OMW float to the surface, inhibit evaporation, and create anaerobic conditions favorable for stench, which complicates the treatment of OMW and engenders several environmental impacts. The current techniques largely employed in the treatment of this type of waste could be divided into three types: physico-chemical, biological, and advanced oxidative methods. Even though the techniques under the last type are very efficient and enable a reduction in chemical oxygen demand (COD) by more than 70%, they are still very expensive and energy-demanding (Rivas et al., 2001, Ugurlu et al., 2007). Biological techniques are also efficient in terms of COD reduction, but they are very slow and difficult to manage (Yesilada et al., 1998 ; Jarboui et al., 2009). Physico-chemical methods seem to be very appropriate because of their low cost and simplicity. However, Boukhoubza et al.(2009), contested their efficiency compared to other methods (2009). Since rapid and easy intervention for the treatment of all these effects is required to reduce the environmental impact of OMW accumulation, we propose in this work to implement an efficient process combining coagulation-flocculation and sunlight photo-catalysis. In this study, we tried to determine the optimum conditions of this process by means of experiments using two types of design: mixture design and full factorial design.
2.1. Reagents and materials
An oil production factory in the Mahdia. OMW provides us olive mill wastewater.
Caustic soda solution was sed with concentration 1 M. sodium chloride (99%) Merck was used to prepare a 1.5 g.L− 1 solution.
The catalyst P25 was procured by Aldrich.
2.2 Physico-chemical analysis
OMW is characterized according the norms regrouped in the table 1
|
Parameter |
pH
|
Suspension matter |
Dry matter |
Ash contain |
COD |
|
Norm |
ISO 10523:2008 |
ISO 11923:1997 |
NF EN 12880 |
ISO 6107:2021 |
ISO 15705:2002 |
Coagulation is performed by mixing 40 mL of OMW vigorously with 10 or 0 mL of NaOH solution (A) or NaCl solution (B) or aloe vera gel (C). The proportions (A, B, and C) of the mixture used in this step are determined by means of full factoriel design . Table 2 and presents the corresponding experimental points. Coagulation-flocculation is ensured by 1 minute of rapid agitation of the coagulant and OMW. Then it was maintained under slow agitation for 1 hour. Then it was kept 7 days for decantation. The obtained solution is separated from the sludge and 20 mL of water is treated by 20 mg of P25 under sunlight for 10 hours.
The sludge is shaped into briquettes using a mold, and these briquettes are then left to undergo dehydration under the sun for a period of two days. During this time, the sun's heat effectively removes the moisture, resulting in the solidification and hardening of the briquettes. This process helps to enhance their durability and stability, making them suitable for various applications.
Table 2: experimental points
|
N° Essai |
F1 |
F2 |
F3 |
y1 (%) |
|
1 |
-1 |
-1 |
-1 |
46 |
|
2 |
1 |
-1 |
-1 |
48 |
|
3 |
-1 |
1 |
-1 |
49 |
|
4 |
1 |
1 |
-1 |
63 |
|
5 |
-1 |
-1 |
1 |
78 |
|
6 |
1 |
-1 |
1 |
84 |
|
7 |
-1 |
1 |
1 |
95 |
|
8 |
1 |
1 |
1 |
93 |
3.1. OMW characterisation
OMW is one of the complex polluting effluents. It is difficult to treat regarding the great quantity of pollutants in solid suspension. Treated OMW is characterised by a pH of 4.5 ± 0.01 and CDO 55.1 ± 0.02 (gO2 · L− 1). Dry matter is estimated to 103.0 ± 0.5 (g.L− 1) and ash contain to 15.5 ± 0.8 (g.L− 1).
3.2. Coagulation – flocculation trials
When considering the significance of the DM (dissolved matter) content in the effluent, we opted for coagulation as the treatment method. Coagulation proves effective in reducing solid suspensions to composted or pyrolyzed materials. Recent research has shown a growing trend towards the use of natural products in this process, with various natural elements gaining attention for their coagulation properties. Notable examples include chitosan (Patel and Vashir, 2013), the seed of moringa oleifera (Desta and Bote, 2021), and cactus (Shilpaa et al., 2012; Taa et al., 2016).
Cation concentration has been found to have a significant impact on coagulation, as demonstrated in the study conducted by Ndabigengesere and Narasiah (1996). Conversely, the effect of anions appears to be less significant. To explore the potential enhancement of aloe vera gel in the presence of NaCl or NaOH solutions, we investigated their combined effects. Lagoudianaki et al. (2007) have previously reported the positive impact of using these metals for odor reduction. Following coagulation treatment and decantation, the final pH ranges from 6 when aloe vera is used alone to 12 when only NaOH solution is employed. The quantity of sludge generated is minimal (around 48%) when the NaCl solution, NaOH solution, and aloe vera are used together, while it reaches a maximum of 70% when aloe vera is used alone.
However, it is important to note that this treatment alone is not sufficient. The liquid component of the effluent requires additional treatment as it still contains toxic compounds, primarily soluble phenols. Photocatalysis under sunlight has emerged as a low-cost and efficient method for wastewater treatment. In our study, we utilized P25 as the photocatalyst and allowed for an adsorption period of 30 minutes, followed by 10 hours of sunlight exposure. The resulting mixtures exhibited a final pH of approximately 7.50, with a midpoint DCO (chemical oxygen demand) of 10 ± 0.02 gO2 • L−1. This approach demonstrates promising results in further purifying the effluent and mitigating the presence of harmful substances.
3.3. Design of experiments results
Table 2 presents the obtained results of organic compounds degradation for each experimental condition.
We observe the best organic degradation with the experience 7 conducted in absence of NaOH.
Figure 1 of the Pareto diagram reveal that the aloe vera is the only influent factor. The Daniel diagram of the figure 2 show that the interaction between ionic force and aloevera is negative
For the decolorization response, it is clear that only the interaction existing between iron solution (B) proportions and dilution (X1) (p < 0.05) has a significant, and negative, influence on this response. However, for the organic degradation response, six terms are influencing this response significantly: three proportions of used components (p < 0.001), and three linear terms (with negative coefficients) of interaction existing between components proportions and dilution (X1) (p < 0.05).
Moreover, the corresponding models’ regressions used for both responses are evaluated with different statistical criteria (Table 3).
Table 3 regoups all the coefficient of the model
|
a1 |
2,5 |
|
a2 |
5,5 |
|
a3 |
18 |
|
a12 |
0,5 |
|
a13 |
-1,5 |
|
a23 |
1 |
|
a123 |
-2,5 |
the fit quality of the model applied to organic degradation seem is perfect we have a value of R2 = 1 and null residu.
The model is the following :
Y = 2.5 A + 5.5 B + 18C + 0.5 AB -1.5 AC + BC -2.5 ABC
Coagulation-flocculation is one of the simplest and lowest-cost methods. It ensures acceptable results mainly for wastewater charged with particles. However, its outcome is not satisfactory. That is why we opted in this study to combine coagulation-flocculation with sunlight photo-catalysis. The process was optimized by means full factorial design applied organic compound degradation. The moste influence factor is aloevera addition and ionic forces more influent than pH.
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Author contribution
Sahar Raissi : carrying out experiences and redaction
Fatma Fakhfakh : catalyst synthesis revision
Bilel Hadrich : design of experiences and redaction
Funding
This study was carried out with no financial support of any part.
Declaration of competing interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Availability of data and materials
All data generated or analysed during this study are included in this published article
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