Electrochemical treatment of simulated beet sugar factory wastewater

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Abstract

Electrochemical treatment of simulated beet sugar factory wastewater was studied as an alternative treatment method for the first time in literature. Through the preliminary batch runs, appropriate electrode material was determined as iron due to high removal efficiency of chemical oxygen demand, COD, and turbidity. The effect of operational conditions, applied voltage, electrolyte concentration and waste concentration on COD removal percent and initial COD removal rate were investigated through response surface methodology, RSM. In the set of runs, highest COD removal and COD initial removal rate were realized as 86.36% and 43.65 mg/L min, respectively, after 8 h at the applied voltage of 12 V, 100% waste concentration with 50 g/L NaCl. Treatment conditions were optimized by RSM where applied voltage was kept in the range, electrolyte concentration was minimized, waste concentration, COD removal percent and COD initial removal rate were maximized at 25 °C. Optimum conditions at 25 °C were estimated as 12 V applied voltage, 100% waste concentration and 33.05 g/L electrolyte concentration to achieve 79.66% and 33.69 mg/L min for COD removal and COD initial removal rate, respectively. Kinetic investigations denoted that reaction order of electrochemical treatment reaction was 1.2 with the activation energy of 5.17 kJ/mol. These results support the applicability of electrochemical treatment to the beet sugar factory wastewater as an alternative advanced wastewater treatment method with further research.

Introduction

The beet sugar factory wastewater with high organic load has a strong potential to create serious environmental pollution problems if discharged before treatment. Typical levels of biochemical oxygen demand, BOD5, are 4000–7000 mg/L in untreated effluent from beet processing, while wastewater has chemical oxygen demand, COD, of up to 10,000 mg/L. In addition to the sugars and organic materials arriving with the beet, wastewater resulting from the washing of incoming raw materials may also contain crop pests, pesticide residues, and pathogens [1].

Conventional treatment methods of sugar factory wastewater include preliminary filtration of solids, sedimentation for suspended solids reduction, flow and load equalization and advanced biological treatment, which is typically anaerobic followed by aerobic treatment and nutrient removal [1], [2]. Utilization of wastewater ponds (lagooning) [3], [4], [5] is commonly preferred as an economic process for the sugar factory wastewater, since the discharge of treated water takes place only when the required degree of removal is reached [6]. However, large space requirements [3], risk of unpleasant and annoying odor emissions in the spring and nearly summer [7], contamination risk of ground water in the event of inadequate sealing of the pond's floor [3], [8], [9] and undesirable massive development of algal production during the vegetative period [3] are main disadvantages of the lagooning. Aerated ponds are also considered for the treatment of sugar factory wastewater requiring less space and residence time than lagooning, but oxygen consumption and hydraulic retention time, HRT, can be high and excess land requirement may still exist [3].

In last two decades, combined anaerobic and aerobic treatments of sugar industry wastewater have been approved as an acceptable process due to high performance of COD removal, relatively small energy requirements, an almost insignificant production of excess sludge, low odor emissions, less land usage, compact system structure and energy recovery [3], [10]. However, high rate systems especially upflow anaerobic sludge blanket, UASB, expanded granular sludge blanket, EGSB, and fluidized bed reactor, FBR, systems started to be preferable anaerobic technologies for soluble wastewater because of the increased pollution loading rate and relatively decreased HRT [11], [12], [13], [14]. Nevertheless, all conventional biological treatment systems for sugar factory wastewaters may not be appropriate and feasible due to large land space requirement as well as high capital and operational costs [3], [10], [15].

Conversion of sugar wastewater to produce energy in fuel cells currently represents an alternative way for wastewater treatment. However, existing transition metal-catalyzed fuel cells cannot be used to generate electrical power from carbohydrates [16]. Microbiological fuel cells, MFC, with a life time of more than 5 years [17] can use carbohydrates, but limited electrochemical COD removal efficiency around 20%, low current (<0.1 A) and voltage output (0.5–0.7 V) and low power density (40 W/m3) are the current disadvantages of MFCs implying that MFC technology needs to be improved for industrial applications [18], [19].

Electrochemical treatment may be considered as an economical alternative process under the conditions when conventional treatment methods fail to reduce pollution [20]. The electrochemical treatment is considered as one of the advanced oxidation processes, potentially a powerful method of pollution control, offering high removal efficiencies in compact reactors with simple equipments for control and operation of the process. Electrochemical processes generally have lower temperature requirement than those of other equivalent non-electrochemical treatments and adding electrolyte solutes to increase the conductivity of wastewater is usually preferred. The treatment process would be relatively non-specific, that is, applicable to a variety of contaminants but capable of preventing the production of unwanted side-products [21]. In recent years there has been a growing interest in the treatment of industrial effluents by electrochemical methods as an alternative to traditional biological treatments [22]. Many researchers had investigated the electrochemical oxidation of various types of wastewater containing phenol [23], [24], [25], cyanides [26], nuclear wastes [27], human wastes [28], cigarette industry wastewater [29], textile wastewater [30] and tannery wastewater [31]. Nevertheless, there are few studies dealing with electrochemical treatment of food-processing industrial wastewaters such as deproteinated whey wastewater [32], coke-plant wastewater [33], coffee curing wastewater [34], olive oil wastewater [35], olive mill wastewater [36], [37], [38], [39], [40], [41], [42], green table olive processing wastewater [43], starchy wastewater [44], distillery industry wastewater [45], [46], [47], beer brewery wastewater [48] and vinasse wastewater from beet molasses [49].

In recent years, studies have been carried out to determine the feasibility and to optimize the electrochemical treatment technologies with response surface methodology, RSM. The RSM is an important branch of experimental design and a critical technology in developing new processes, optimizing their performance, and improving design and formulation of new products. The most popular class of RSM is second order central composite design, CCD. The CCD is an effective design that is ideal for sequential experimentation and allows a reasonable amount of information for testing lack of fit while not involving an unusually large number of design points [50], [51], [52]. In the optimization process, the responses can be simply related to chosen factors by linear or quadratic models. A quadratic model, which also includes the linear model, is given asη=β0+j=1kβjxj+j=1kβjjxj2+i<j=2kβijxixj+eiwhere η is the response, xi and xj are variables (i = 1 to k), β0 is the constant coefficient, βj, βjj and βij (i and j = 1 to k) are interaction coefficients of linear, quadratic and the second-order terms, respectively, k is the number of independent parameters (=3 in this study) and ei is the error. RSM has already been used to optimize the electrochemical treatment of deproteinated whey wastewater [32], industrial paint wastewater [53], textile dye wastewater [54], [55], electrochemical removal of mercury ions from wastewater [56], sodium from fermented food composts [57], chromium from industrial wastewater [58] and chromium-contaminated waters [59]. Kaminari et al. [60] have also used RSM to study the effects of operational parameters involved in designing fluidized-bed electrochemical reactors for the electrochemical removal of lead from industrial wastewater [60].

Generally, two mechanisms are considered to be responsible for electrochemical degradation of organic matter, R [35]:

  • (a)

    Direct anodic oxidation where the pollutants are adsorbed on the anode surface, S, and destroyed by the anodic electron transfer reactions.

  • (b)

    Indirect oxidation in the liquid bulk mediated by the oxidants that are formed electrochemically; such as chlorine, hypochlorite, hydroxyl radicals, ozone and hydrogen peroxide. Anodic water discharge results in the formation of hydroxyl radicals which are adsorbed on the anode surface and can then oxidize the organic matter [35]:H2O + S  S[OHradical dot] + H+ + eR + S[OHradical dot]  S + RO + H+ + eIn the presence of NaCl, chlorohydroxyl radicals are also formed on the anode surface and then oxidize the organic matter:H2O + S + Cl  S[ClOHradical dot] + H+ + 2eR + S[ClOHradical dot]  S + RO + H+ + ClReactions between water and radicals near the anode can yield molecular oxygen, free chlorine and hydrogen peroxide [35]:H2O + S[OHradical dot]  S + O2 + 3H+ + 3eH2O + S[ClOHradical dot] + Cl  S + O2 + Cl2 + 3H+ + 4eH2O + S[OHradical dot]  S + H2O2 + H+ + eFurthermore, hypochloric acid can be formed during electrochemical reaction byH2O + Cl  HClO + H+ + 2e

Therefore, direct anodic oxidation of organics through reactions (3), (4), (5) results in reduced COD as well as the formation of primary oxidants such as oxygen, chlorine, hypochloric acid and hydrogen peroxide [35].

In neutral to moderate pH solutions, a cycle of chloride–chlorine–hypochlorite–chloride occurs, causing the initial concentration of chlorides to remain stable. In strong alkaline solutions, the cycle of chloride–chlorine–chloride is blocked due to the production of stable ClO3. At low pH, chlorides are reduced with the production of free chlorine; while at high pH, the chlorides are oxidized and chlorates are produced [49], [61].

Tasaka and Tojo [62] have suggested that in alkaline solution the anodic oxidation of ClO proceeds under the consecutive first order reactions:ClO + 2OH  ClO2 + H2O + 2eClO2 + 2OH  ClO3 + H2O + 2ewhere the rate constant of Eq. (10) is much less than the one of Eq. (11). The electrochemical oxidation of hypochlorous acid is difficult to investigate due to its high oxidation potential and the conversion of HClO into Cl2 [63]. The electrochemical oxidation reaction of HClO and HClO2 areHClO + H2O  HClO2 + 2H+ + 2eHClO2 + H2O  ClO3 + 3H+ + 2eA related oxidation reaction, the electrochemical oxidation of ClO3 has been studied extensively [64]:ClO3 + H2O  ClO4 + 2H+ + 2eAnalogous to chlorate formation in alkaline solution, it is likely that a bimolecular reaction of HClO is responsible for chlorate formation in acidic solution. The proposed reactions are [63]:a slow reaction: 2HClO  ClO2 + 2H+ + Clanda fast one: HClO + ClO2   ClO3 + H+ + ClFree chlorine and oxygen can further react on the anode yielding secondary oxidants such as chlorine dioxide and ozone, respectively:H2O + S[ClOHradical dot] + Cl2  S + ClO2 + 3H+ + 2Cl + eO2 + S[OHradical dot]  S + O3 + H+ + ePrimary and secondary oxidants are quite stable and migrate in the solution bulk where they indirectly oxidize the pollutants [35]. On the other hand, the main cathodic reaction is the evolution of hydrogen but as a side reaction, there is a possibility of the cathodic reductions of hypochlorite and chlorate to form chloride radical [65].

In this study, an attempt was made to achieve electrochemical treatment of simulated beet sugar factory wastewater in the presence of supporting electrolyte with adequate electrode material, being first in literature. The operational conditions were then optimized using RSM by Design-Expert® 6 (trial version). The batch runs were designed in accordance with the CCD and sugar factory wastewater was simulated for the standardization of the wastewater throughout the study. Three factors; waste concentration, applied voltage and electrolyte concentration were selected as the primary independent variables, while COD removal percent and COD initial removal rate were the responses (dependent variables) in the design and optimization of the treatment process. Reaction kinetics of the treatment have also been studied at different waste concentrations and temperatures so as to fine-tune the process.

Section snippets

Chemicals and materials

In order to standardize the beet sugar factory wastewater in the runs, the beet sugar factory wastewater was synthetically prepared based on the real process information from the Turkish Sugar Factories Corp., Ereğli Sugar Factories Campaign Report of years 2000/2002 [66]. Average parameter values of the generated wastewater for the period of two campaign years are presented in Table 1, which are also similar to those of 1995/1996 campaign [2]. As given in Table 1; COD (sugar), nitrogen,

Selection of electrode material

The efficiency and end products of electrochemical waste treatment are profoundly dependent on the nature of anodic material [29], [69], [70]. In the study, iron, carbon and stainless steel SS304 were tried for electrode material due to their low cost and commercialized features.

The COD and turbidity percent, and pH time profiles of batch experiments were presented in Fig. 2(a), (b) and (c), for carbon, stainless steel and iron electrodes, respectively, at 25 °C, 8 V applied voltage, 100% waste

Conclusion

The electrochemical treatment of simulated beet sugar factory wastewater was investigated with iron electrodes in the presence of NaCl electrolyte, for the first time in literature. The effects of operating parameters of applied voltage, waste and electrolyte concentrations on COD removal percent and COD initial removal rate were elucidated through batch runs. COD removal percent and COD initial removal rate changed in the range 6.98–86.36% and 0.38–43.65 mg/L min, respectively, depending on

Acknowledgements

Sugar Institute and Ankara Mechanical Factory of Turkish Sugar Factories Corporation (Turkey) are greatly acknowledged for providing their facilities to carry out this work and for technical assistance. This study was also supported by the Project Management Unit of Akdeniz University, Antalya, Turkey.

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    Current address: School of Engineering and Science, Jacobs University Bremen, Campus Ring 1, 28759, Bremen, Germany.

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