Suitability of aluminum material on sugar industry wastewater with chemical and electrochemical treatment processes
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Aluminum is a valuable material, which can be used for water and wastewater treatment. It exists in metal as well as in salt form. The efficiency of water and wastewater treatment depends upon the technology applied to treat. Sugarcane industry is coming under those industries which have a large amount of freshwater and release large amount of effluent. The goal of this research work is to study the behavior of aluminum metal and salt for the treatment of sugar industry wastewater on chemical oxidation and electrochemical oxidation. The effect of pH, dosing, temperature and catalysis on metal and salt has been also studied with both treatment methods. The results show that maximum 90% of chemical oxygen demand and 94% of color removal can be achieved with an aluminum electrode (electrocoagulation) at optimum conditions, pH 7, current density 178 A/m2, electrode distance 20 mm, and salt solution 0.5 M NaCl. In the same way, 81% chemical oxygen demand and 85% color removal were achieved with alum for the 0.5 M lime solution, at 50 mM mass loading, 21 °C operating temperature and optimum pH of 7, respectively. The sludge generated after treatment was also analyzed with settling filtration, thermal, FTIR and SEM.
KeywordsAnode material Chemical oxygen demand Effluent Optimization Sludge
Aluminum is the most abundant metal and the third most abundant element in the earth’s crust, after oxygen and silicon. It makes up about 8% by weight of the earth’s solid surface and it never occurs as a free element in nature . It is a light, conductive, corrosion-resistant metal with a strong affinity for oxygen. This combination of properties has made it a widely used material, with applications in the aerospace, architectural construction and marine industries, as well as in wastewater treatment . The aluminum salts with chloride, nitrate and sulfate are highly soluble and form a range of dissolved species on contact with water. The fate and behavior of aluminum in the aquatic environment are very complex . Aluminum speciation, which refers to the partitioning of aluminum among different physical and chemical forms, and aluminum solubility are affected by a wide variety of environmental parameters, including pH, solution temperature, and dissolved organic carbon (DOC) content. Metals in solution may be present as dissolved complexes, as “free” or aquo ions, in association with particles, as colloids or as solids in the process of precipitation .
In wastewater treatment, it can be used as an electrode in the electrocoagulation method, chemical salt as coagulant and sludge as red mud . Aluminum salts are widely used as coagulants in water treatment process due to the effectiveness in removing a broad range of impurities, including colloidal particles and dissolved organic substances [54, 60]. The mode of action is generally explained in terms of two distinct mechanisms: charge neutralization of colloids by cationic hydrolysis products and incorporation of impurities in an amorphous hydroxide precipitate. The relative importance of these mechanisms depends on factors such as pH and dosage . Alternative coagulants, based on pre-hydrolysed forms of aluminum are more effective than the traditional additives in many cases, but their mode of action is not completely understood, especially with regard to the role of charge neutralization and hydroxide precipitation. Electrocoagulation is another way to treat the water and wastewater in which aluminum is used as an electrode . In the EC process, coagulation is generated in situ by electrolytic oxidation of an appropriate anode material . During this process, charged ionic species are removed from wastewater by allowing it to react with an ion having opposite charge, or with flock metallic hydroxides generated within the effluent [16, 21].
Aluminum in form plate has been used to treat the textile industry wastewater , dye wastewater , heavy metal , sewage water , petrochemical wastewater , dairy wastewater , hospital wastewater , etc. and in form of salt for leachate , dairy wastewater , pesticide wastewater , and tannery wastewater . Aluminum is easily available, economically fit and highly efficient as compared to other metals for industrial effluent and drinking water treatment . Sugar industry is one major high amount water consumer and discharges large quantity of wastewater . Hence, with this information, an experimental was conducted to treat the sugar industry effluent with aluminum salt and electrode.
The main aim of this research work is to evaluate the performance and effectiveness of aluminum (salt and electrodes) to treat the sugar industry wastewater by the chemical and electro-oxidation techniques. The experimental operating parameters, effect of initial pH, current density applied, and distance between electrodes were scrutinized for sugar industry wastewater.
Materials and methods
Characteristic of sugar industry wastewater
Characteristics of EC (Al plate)
Material (anode and cathode)
Rectangular may be square
7.5 cm × 7.5 cm
Effective electrode surface area
10.7 × 10.7 × 13.7
Coagulation method is carried out in Jar test apparatus 0.20 dm3 of SIWW was taken in a 0.50-dm3 glass beaker. The pH of the effluent was noted and the initial pH (pH0) was adjusted by adding aqueous NaOH (1 M) or H2SO4 (1 M) solution in the range of pH 2–10. A known amount from 20 to 50 mM of the coagulant was added to the effluent and flash-mixed for 5 min by a magnetic stirrer and, thereafter, slowly mixed for 30 min. The effluent sample was then taken in a glass cylinder and kept quiescent for 6 h. The supernatant liquor was centrifuged and analyzed for its COD value. These steps were repeated at different dosages of the coagulant. The filtration characteristics of the solid residue formed in the treated effluent were studied using an ordinary zero haze grades.
The COD of the samples was determined by the standard dichromate reflux method . The chloride concentration was determined by the standard titrimetric Volhard method . Sulfates and the phosphates were determined using standard methods . The concentrations of the metal ions in the filtrate and the residue were determined using an atomic absorption spectrometer (GBC, Model Avanta, Australia). The protein content was determined by the Bradford method . The color of the sample was measured in terms of the absorbance at λ = 420 nm using a UV–Vis spectrophotometer (Model Lambda 35) from Perkin Elmer Instruments, Switzerland. The residual organics in the treated effluent were analyzed by Fourier transform infrared (FTIR) spectroscopy (Bruker Optics, VERTEX80). After the electrolysis experiment, the microstructure and thermal characteristics of the by-products, i.e., the sludge, were also examined by field emission scanning electron microscopy (FE-SEM, HITACHI, S-4700) and thermogravimetric analysis (TGA, SHIMADZU, DTG-60H). Filter paper (pore size 7–11 µm) was supplied by S.D. Fine Chemicals Ltd. The filter paper was supported on a ceramic Buchner funnel and gravity filtration was carried out.
Results and discussion
Mechanism with metal
Aluminum metal has been used as electrodes successfully in EC systems to treat water and wastewater. During electrochemical reactions, metal electrodes dissolved and form coagulant species with metal hydroxides . Metal anode dissolution is accompanied by hydrogen gas evolution at cathodes, bubble capture and floating, and suspended solid formation, thus removing contaminants [8, 9]. When aluminum electrodes are used, the electrochemical reactions occurring are the following.
Polymeric species such as Al2(OH)24+ and Al (OH)52+.
Amorphous and less soluble species such as Al(OH)3 by Eq. 11 and Al2O3.
Considering only mononuclear specification, the concentration of the various Al forms present in solution was calculated by Holt et al.  depending on pH. Al complexes acting as coagulants are adsorbed on oil particles and thus neutralize the colloidal charges, resulting in destabilization of the emulsion. This phenomenon is similar to the action of chemical coagulants in conventional chemical treatment. Hydrogen bubbles formed at the cathode can adsorb on the flocculated species and induce their flotation. The bubbles formed also reduce fouling of the cathode surface which could occur due to the formation of deposits. Sometimes, NaCl is usually employed to increase the conductivity of the water or the wastewater to be treated. The presence of the chloride ion in solution has been reported to decrease passivation of the aluminum surface and thereby increase the efficiency of electrocoagulation processes . The chloride ion has been attributed a role in the pitting corrosion of the metal surface.
Mechanism with salt
In aluminum derivatives, alum (Al2(SO4)3·nH2O) is a cheap and regular salt, and widely used as a coagulant in water and wastewater treatment. The aqueous chemistry of aluminum is complex and upon the addition of an aluminum coagulant in water treatment, multiple reaction pathways are possible. The type of interactions between the chemical coagulant and contaminants determines the mechanism of coagulation. The predominance mechanisms observed during conventional coagulation with metal coagulants are adsorption charge neutralization and sweep coagulation. For aluminum salts, the mechanism of coagulation is controlled by the hydrolysis species . The positively charged polyhydroxy complexes such as [Al8(OH)20]4+ in the pH range between 4 and 7 are the effective flocculants. Oversaturation and formation of amorphous hydroxide precipitate [Al(OH)3](s) enmesh colloidal particles in a “sweep floc”. Many other monomeric and polymeric species reported are
[Al(OH)]2+, [Al(OH)2]+, [Al2(OH)2]4+, and [Al(OH)4]−.
[Al6(OH)15]3+, [Al7(OH)17]4+, [Al8(OH)20]4+, and [Al13(OH)34]5+.
The alum as the coagulant is capable of achieving significant organic removal. The pH of the water during coagulation has profound influences on the effectiveness of coagulation for organic removal. Organic removal is much better in slightly acidic condition. For water of higher organic content, the optimum pH is displaced to slightly more acidic values.
Effect of initial pH on COD and color removal
The efficiency of alum coagulant in the treatment of SIWW was also tested at different pH values. It was observed that with increase in pH from pH 2 to 7, COD reduction increased to 23%, 36%, 47%, 55%, 64% and 70% while color reduced to 24%, 40%, 50%, 58%, 69%, and 75% at pH 2, 3, 4, 5, 6, and 7, respectively. The maximum COD and color reduced to 70% and 75% at pH 7. When process was carried out from pH 8 to 10, the COD reduced to 67%, 65%, and 60% and color reduced to 66, 61, and 59%, respectively, at pH 8, 9 and 10 as shown in Fig. 2c, d. The aluminum cation compound reacts with negatively charged colloid of wastewater and neutralized it. Impurities and colloids can be adsorbed onto the hydroxide precipitate. The colloids present in the solution can be either entrapped inside the hydroxide flocs formed, or enmesh to the surface of hydroxides. The enmeshment is defined as sweep coagulation [1, 2, 3, 4, 5, 6, 27].
Effect of dosing (current density and mass loading)
Available data indicate that the COD and color reduction efficiency increase from 68 to 78% and 70 to 82% by increasing the alum dose from 20 to 50 mM shown in Fig. 3c, d. Further increase in dose decreased COD and color removal. The predominant removal mechanism at low doses of alum is adsorption and charge neutralization . However, at high doses of coagulant is sweep floc coagulation by enmeshment in the aluminum hydroxide precipitate . Further increase of the alum dose from 50 to 60 mM decreases COD reduction by 78% and color reduction by 72%, respectively. Therefore, the optimum dose of alum that enhanced maximum removal of COD and color was taken as 50 mM .
Effect of temperature
The effect of temperature also has been studied for coagulation, which is shown in Fig. 4b. The experiment was carried out at different temperatures from 15 to 31 °C. The result shows that at 21 °C and 6 h retention time, 78% COD and 82% color reductions were found. The reduction of COD and color was deceased when at low temperature and high temperature of the sample. It may be because the variation of sample temperature during coagulation may lead to non-representative floc formation and the convection currents will interfere with settling [1, 2, 3, 4, 5, 6, 27].
Effect of catalyst
To examine the effect of catalyst for the coagulation process, lime was used at 0.3 M and 0.5 M of dosing, which is shown in Fig. 5b. Generally, lime is used for increasing or decreasing the pH of the wastewater as the acidic nature of alum always reduces the pH of sample and lime increases the pH . By this, combined effect of mass loading at pH 7 was studied. The result shows that 81% COD and 85% color reductions were found at 4 h of retention time as compared to without dosing of lime (73% COD and 78% color reductions at 6 h retention time) and 0.3 M dosing lime (79% COD and 83.5% color reductions). This attributes to alum, which reacts instantaneously and will proceed to other end products in the presence of lime. Lime also reduced the retention time for the coagulation process to 2 h as compared to without lime sample. Further increase in lime dose will decrease the COD and color reduction, due to an increase in pH of the sample. In place of lime, caustic soda and soda ash are also used to improve the alkalinity in water treatment plant [44, 51].
Analysis of residue obtained after EC (Al plate) at different pH
Weight of residue (kg/m3)
Tough to grind and flakey
Easy to grind and flakey
Approximated drying period (h)
COD reduction (%)
Color reduction (%)
Filterability of the treated wastewater
Kp × 10−12 (s/m6)
β × 10−6 (s/m3)
α × 10−14 (m/kg)
Rm × 10−12
Fourier infrared transform study
Scanning electron micrograph
The scanning electron morphologies of metal- and salt-treated sludge (dried) are shown in Fig. 8a, b. The sample that was treated with aluminum electrode has smaller particle sizes as compared to aluminum salt-treated sample. It indicates high porosity, inhomogeneous surface and particles varying in shape. Alum-treated sample sludge has bigger particle size and is easy to grind. The electrooxidation eliminates various pollutants in the wastewater by oxidation; meanwhile, the aluminum compound particles are formed including pollutants. When the aluminum compound particles are agglomerated with each other, the organics may also be included in the aggregates .
To treat the 1.4 dm3 of SIWW, energy consumption is approximately 22.85 kWh/h at an energy price in the Indian market = INR 8.75/kWh.
Cost of energy per m3 of SIWW treated = INR 8.75/kWh × 22.85 kWh = INR 199.93.
Aluminum required per m3 of SIWW = 628 g/m3.
Cost of aluminum sheets on bulk purchase (cleaning, cutting and placing) = INR 84/kg.
Cost of aluminum per m3 of SIWW treated = 0.628 kg × INR 84/kg = INR 52.75/m3.
Cost of energy + cost of electrode = 62.85 + 52.75 = INR 252.68 ($ = 3.60) per m3 of SIWW treated.
To improve the industrial water use, water quantity and quality need to be considered. To avoid the unwanted release of contaminants to the environment, zero effluent discharge, which involves water reuse, recycling and regeneration, is the ultimate goal. It is concluded that aluminum compound has a good role in wastewater treatment, especially it shows high-performance addition with catalysis. Maximum 90% (220 mg/L) COD and 94% of color reduction were found with 05 M NaCl catalysis, 20 mm electrode distances, 178 A/m3 current density at optimum pH 7 with electrochemical oxidation. It also shows 81% COD and 85% color reduction with 0.5 M lime solution, 50 mM mass loading, and 21 °C operating temperature at optimum pH 7 with chemical oxidation. Aluminum electrode shows good settling and filtration characteristics as compared to alum. The sludge (dried) generated after electrochemical treatment is small, flakey and grindable, which can be easily disposed. Otherwise, it can be used as fertilizer for agricultural crops. At last, electrocoagulation treatment is found to be more efficient as compared to chemical coagulation treatment.
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