A novel single continuous electrocoagulation process for treatment of licorice processing wastewater: Optimization of operating factors using RSM
Graphical abstract
Introduction
Licorice extract, obtained from the roots of Glycyrrhiza glabra plant, has a wide range of applications owing to its distinct sweet flavor and medicinal properties. Food and pharmaceutical industries are the major consumers of licorice extract. Licorice processing wastewater is characterized by high strength of soluble and insoluble chemical oxygen demand (COD), biochemical oxygen demand (BOD), organic materials, color, phosphate, nitrogen, and total suspended solids (TSS) (Fajardo et al., 2017; Terrazas et al., 2010). These characteristics make the licorice processing wastewater improper for recirculation into the process and direct discharge into municipal sewer systems and aquatic environments (Pajootan et al., 2012). Therefore, the licorice processing wastewater should be treated to a standard level before either being returned to the process or discarded into receiving environments. Nevertheless, there are just a few reports on the treatment of wastewater from licorice processing. A combined treatment including neutralization-regulation tank, upflow anaerobic sludge bed (UASB) reactor, settling tank and contact-oxidation tank was developed to finally obtain an effluent with COD of 200−500 mg/L which reaches the design requirement (Guo et al., 2010).
Performed a preliminary evaluation of anaerobic digestion and aerobic degradation for pretreatment of high-strength wastewater from licorice processing, followed by adsorption using powdered activating carbon (PAC) (Ramaswami et al., 2016). Only 15 % of COD was reduced by anaerobic digestion, whereas up to 80 % removal was achieved with activated sludge process. However, no significant color reduction was observed unless a high dosage of carbon (> 2 kg PAC/m3) was used. Various methods such as conventional biological technologies (Pirsaheb et al., 2009; Zinatizadeh and Ghaytooli, 2015), membrane bioreactors (Di Trapani et al., 2019), combined anaerobic-aerobic systems (Asadi et al., 2012), advanced oxidation processes (Ghasemi et al., 2016; Zangeneh et al., 2018) and integrated anaerobic-aerobic systems have been attempted to treat high-strength waster (Aziz et al., 2019; Chan et al., 2009; Shoukat et al., 2019). Biological technology is economical, but its effluent still contains high amount of color and refractory organic compounds. Therefore, physiochemical treatment methods including chemical coagulation (CC), flocculation, and electrocoagulation (EC) are widely used in industry to remove color and ensure that the treated effluent is more amenable to a secondary biological treatment (Birjandi et al., 2013; Drogui et al., 2007; Sardari et al., 2018). CC is a process in which coagulation and flocculation are induced via neutralization of negatively charged hydrophilic particles by dosing positively charged hydrolyzing metal (Chafi et al., 2011). The inherent disadvantage associated with CC is that it is an additive process in which expensive chemicals are used (Metcalf et al., 2003).
On the other hand, the effluent from CC cannot be reused due to high contents of dissolved components. Coagulation can also be accomplished through in situ formation of hydrolysable metal coagulants through electrolytic oxidation of metal anodes (e.g., iron or aluminum). Recently, electrocoagulation (EC) has become more popular as an alternative method to conventional CC for wastewater treatment. The complex process of EC consists of a multitude of synergistic mechanisms including chemistry, physics, and physicochemistry. In this process, coagulants generated by applying a potential difference can remove color, suspended and dissolved solids from wastewater (Huda et al., 2017).
In general, the whole electrocoagulation process involves three main successive steps: (i) oxidation of sacrificial anode and reduction of cathode resulting in metal ions (coagulating agents) release through anode electro-dissolution associated with O2 microbubbles generation and H2 gas evolution, respectively; (ii) aggregation of coagulants and destabilized pollutants to form larger flocs; (iii) removal of flocculated particles by sedimentation or through electroflotation by lifting them to the surface with the aim of micro-bubbles of H2 gas generated at the cathode (Gao et al., 2010b). Bearing in mind the abovementioned features of EC, it offers many benefits over traditional CC such as a compact treatment system, minimized sludge production, no need for supplementary addition of chemical coagulants, simplicity of design, uncomplicated and relatively inexpensive operation, short electrolysis and operating time, and comparatively low capital costs (Attour et al., 2014; Emamjomeh and Sivakumar, 2009; Khemila et al., 2018).
When using the iron, one of the most commonly used electrode material, in this process; the main reactions involved are as follows:
Anode:Fe(s) → Fe2+ (aq) + 2e−2H2O (l) → O2 (g) + 4H+ (aq) + 4e−
Cathode:2H2O (l) + 2e− → H2 (g) + 2OH− (aq)
In solution:Fe2+ (aq) + 2OH− (aq) ↔ Fe (OH)2(s)
If dissolved oxygen is presented in the solution, the reaction in solution would be:Fe2+ (aq) + 4H+ (aq) + O2 (g) → Fe3+ (aq) + 2H2O (l)Fe3+ (aq) + 3OH− (aq) ↔ Fe (OH)3(s)
EC has been applied successfully for color, organic matter, and suspended solids removal from various wastewaters such as textile wastewater (Bener et al., 2019; Merzouk et al., 2011), biodiesel wastewater (Chavalparit and Ongwandee, 2009), pulp and paper mill industry wastewater (Khansorthong and Hunsom, 2009), olive oil mill (Flores et al., 2017, 2018), pharmaceutical wastewater (Ahmadzadeh and Dolatabadi, 2018; Farhadi et al., 2012), algae dye removal (Gao et al., 2010a; Tumsri and Chavalparit, 2011), industrial wastewater (Gong et al., 2017), urban wastewater (Elazzouzi et al., 2017), oil refinery wastewater (Pérez et al., 2016), heavy metals (Akbal and Camcı, 2011), phosphate removal (Hashim et al., 2019), and landfill leachate (Oumar et al., 2016). To the best of our knowledge, there are no reports on the capability of EC for organics and color removal from licorice processing wastewater. Therefore, the aim of this study is to evaluate the operation of a new continuous EC reactor which consist of three parts: coagulation, sedimentation and flotation for the removal of a high concentration of COD and color from licorice industrial wastewater. The effect of operating and process parameters, such as current density (100–500 A m−2), inlet flow rate (11–16 ml.h−1), electrolysis time (30−90 min), electrolyte concentration (100−500 mg L−1), and mixing intensity (30–60 RPM) on the process performance of EC treatment system was investigated. Afterwards, RSM was employed to investigate and optimize the influence of these parameters on the treatment efficiency.
Section snippets
Wastewater characteristics
Industrial licorice wastewater from a licorice extract powder production plant (Zagros, Kermanshah, Iran) was sampled directly. Model wastewater was prepared by diluting the collected wastewater with tap water yielding COD concentrations of 1500−1600 mg/L. The prepared wastewater was stored at 4 °C in the dark prior to use. Table 1 shows the major characteristics of licorice wastewater used in this study.
Experimental setup and procedure
Electrocoagulation treatment was carried out in a single continuous reactor for both metal
Statistical analysis
In the EC process, process efficiency may be improved by the optimization of process and operating variables. Thus, following the characterization studies, the experimental data was processed with Design Expert10.0 software. Central composite design (CCD) and ANOVA analysis under RSM, the most common mathematical and statistical technique applied in industrial research for process development and RSM, the most common mathematical and statistical technique for process development (Nariyan et
Conclusion
An innovative continuous EC reactor was employed for the treatment of licorice processing wastewater. Central composite design with RSM was applied to find the optimal operating conditions. ANOVA results for the second-order polynomial models derived from the EC process indicated that these models are able to predict the responses with a good correlation between the empirical and predicted data. The electrolysis time and current density turned out to be the major operating factors influencing
Conflict of interest
None.
Acknowledgments
The authors would like to acknowledge Kermanshah’s Water and Wastewater Company, Iran for the financial support provided for this research work. The authors would also like to thank Razi University to provide the required facility to carry out the project.
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