Combined homogeneous and heterogeneous advanced oxidation process for the treatment of tannery wastewaters

Tanneries are typically characterized as pollution-intensive industrial complexes that generate widely varying, high-strength wastewaters (Chowdhury et al. 2013). The variability of tannery wastewaters arises not only from the fill and draw-type operation associated with tanning processes, but also from the different procedures used for hide preparation, tanning and finishing. These procedures are dictated by the kind of raw hides employed and the required characteristics of the finished product. The tanning industry also has a very high toxic intensity per unit of output. During the tanning process at least about 0.3 kg of chemicals are added per kilogram of skin/hides (Durai & Rajasimman 2011). Tanneries generate wastewater in the range of 30–35 litre per kilogram of skin/hide processed with variable pH (6–10.5), total suspended solids (TSS) (526–2,865 mg/L), biochemical oxygen demand (BOD) (100–2,906 mg/L), chemical oxygen demand (COD) (2,102–11,153 mg/L) (Lofrano et al. 2013). One of the refractory groups of chemicals in tannery wastewater derives mainly from tannins. Tannins are characterized by complex chemical structures, because they are composed of an extended set of chemicals such as phenol, naphthalene, formaldehyde and melamine-based syntans, and acrylic resins. The BOD₅/COD ratio of tannins is also lower than other compounds. However, it is worth noting that no one of these compounds showed a BOD₅/COD ratio higher than 0.4, indicating a very low biodegradability for each of them (Lofrano et al. 2013).

The main treatment processes used at tannery plants are primary clarification (sedimentation), secondary treatment (activated sludge process) and/or tertiary processes (membrane processes as ultrafiltration). The activated sludge process has been the most common wastewater treatment process for the removal of organics in our country; however, it is inefficient for the removal of recalcitrant organics and micro-pollutants in tannery wastewater. The mixture of compounds used in the tannery process can be released into the environment because the compounds remain even after conventional treatment (activated sludge process, aerobic sequencing batch reactor, anaerobic filters, upflow anaerobic sludge blanket and membrane bioreactor) and negatively affect organisms and the environment. The wastewater of tanneries has also been associated with a huge foaming problem on surface waters (Durai & Rajasimman 2011). The high concentrations of pollutants with low biodegradability in tannery wastewater represent a serious and actual technological and environmental challenge. Therefore, many researchers have attempted to develop new technologies for complementing or even replacing some of these treatments (Khansorthong & Hunsom 2009).

Advanced oxidation processes (AOPs) can be considered an effective alternative and have become the most widely used treatment technologies for organic pollutants not treatable by conventional technologies due to their high chemical stability and/or low biodegradability. AOPs involve the generation of highly reactive radical species, mainly the hydroxyl radical (OH^•) to oxidize the organic matter to the end products of water and carbon dioxide (Trabelsi-Souissi et al. 2013). The use of sunlight as the source of irradiation to perform AOPs reduces the processing costs and makes them more affordable for commercial use as a water treatment technology (Babuponnusami & Muthukumar 2012). These processes can be used before or after conventional treatment or even as the principal stage, depending on the characteristics of the wastewater and the quality requirements of the treated wastewater (Zgajnar et al. 2011).

Among various AOPs, Fenton reagent (Fe²⁺ + H₂O₂) and photocatalysis (TiO₂ + UV) have been intensively investigated for environmental applications during the last decades. They have both been recognized as ‘green technologies’. Above all, the Fenton reaction can be initiated in conditions of normal temperature and pressure with no external energy, thus it can hopefully be utilized in water in situ remediation areas (Eskelinen et al. 2010). In the photocatalytic research field, some catalysts, for example, TiO₂, CdS and GaP have proved to possess high activity with solar light irradiation, thus endless clean solar energy is a powerful support for this technology (Sauer et al. 2006; Rauf et al. 2011). However, the classic Fe²⁺-based Fenton system requires operation at pH < 3.0 to prevent the precipitation of Fe²⁺ and Fe³⁺. Usually, a large excess of Fe salts in traditional Fenton systems cannot be recycled, producing large amounts of chemical sludge (Su et al. 2012; Hermosilla et al. 2012). The major drawback of TiO₂/UV photocatalysis was that its high efficiency occurred only under UV irradiation, which can use only 3–4% of the solar energy reaching the earth (Portjanskaja et al. 2009). Thus, in order to extend the practical applications of industrial and environmental interest, some efforts have been made to develop a synergistic reaction to enhance the Fenton reaction and TiO₂ photocatalysis. For instance, when Fenton is combined with the TiO₂ photocatalysis reaction, the efficacies of both the systems for the production of radicals are enhanced (Chen et al. 2013). The conduction band (CB) electrons and photoenergy can accelerate the ferric and ferrous iron cycles (Fe²⁺/Fe³⁺); at the same time, Fenton reagent (Fe²⁺/H₂O₂) inhibits the electron–hole recombination by scavenging CB electrons of photo-excited TiO₂ to increase the lifetime of the valence band hole and subsequently generate more radicals. Moreover, electrons from TiO₂ decompose more oxidants to radicals, which can promote the Fenton reaction efficiency drastically (Kim et al. 2012).

The present study demonstrates that the synergistic combination of TiO₂ photocatalysis (solar/TiO₂) and Fenton reaction (Fe²⁺/H₂O₂) produces a marked kinetic enhancement of the degradation of tannery wastewaters at neutral pH conditions, which is detrimental to Fenton oxidation.

Tannery wastewaters were obtained from a tannery near Erode, Tamil Nadu, India. The tests were performed with primary treated wastewater (collected at the outlet of the plain sedimentation tank). The sample was collected in plastic cans that were transported to a laboratory and stored at 4 °C. The physicochemical characteristics of the wastewater, determined using standard methods, are listed in Table 1. After the primary treatment, the BOD₅ to COD ratio was 0.40, indicating the non-biodegradable character of the wastewater and the possible presence of minimally biodegradable chemical substances, which undermine biological treatment (Lofrano et al. 2013). Even after complete treatment, the treated wastewater did not satisfy the minimum national standards for discharge (MINAS 2007). Upgrade of the tannery's existing tannery wastewater treatment system is needed.

All reagents used in this experiment were of analytical grade and used as received without further purification. The chemicals used in this study are ferrous sulphate heptahydrate (FeSO₄.7H₂O), hydrogen peroxide (H₂O₂ 30% w/w), sodium thiosulphate (Na₂O₃S₂), sulphuric acid (H₂SO₄), potassium dichromate (Cr₂K₂O₇), mercuric sulphate (HgSO₄), ferrous ammonium sulphate (Fe(NH₄)₂(SO₄)₂·6H₂O), sodium hydroxide (NaOH), and sodium sulphite (Na₂SO₃), and were purchased from Merck (India). The photocatalyst employed was commercial TiO₂ (P25), and was supplied by Degussa (Germany). According to the manufacturer's specifications, P25 has an elementary particle size of 30 nm, a Brunauer–Emmett–Teller (BET) specific surface area of 50 m²/g and crystalline mode comprising 80% anatase and 20% rutile.

All photocatalytic experiments were carried out at Anna University Campus, Tirunelveli, India (8°44′N 77°44′E). The experiments were conducted using a laboratory-scale reactor system made from borosilicate glass having a size of 0.3 m (length) × 0.2 m (width) × 0.04 m (depth) operated in the batch mode. The irradiation surface area was 0.06 m². Irradiation was carried out in the open air with continuous aeration by a pump to provide oxygen and for the complete mixing of the reaction mixture. In all cases, 1 L of reaction mixture was irradiated. The reactor was exposed to strong solar irradiation from January to April between 11 a.m. and 3 p.m. The solar ultraviolet radiation was measured by a global UV radiometer which provided data in terms of incident energy per surface area (ultraviolet intensity 32 ± 2 W/m²). Initially, the pH of the wastewater was adjusted to a predetermined value (2–8) by adding sulphuric acid or sodium hydroxide. Then the required amount of ferrous sulphate (0–1 g/L) and TiO₂ (0–0.25 g/L) was added to the wastewater and stirred well to enhance the homogeneity of wastewater during the reaction. H₂O₂ (0–2.2 g/L) was added (considered the beginning of the experiment) and the mixture was solar irradiated. The required amount of sample was withdrawn from the reactor at selected time intervals. Immediately after collecting each sample, sodium sulphite solution (approximately 0.5 mL per 10 mL wastewater sample) was added to quench the oxidation reaction for H₂O₂ decomposition, and the pH was raised by adding sodium hydroxide to precipitate iron salt and TiO₂ particles. All experiments were performed in triplicate and averages were reported. The COD and BOD₃ of the samples were carried out as per Standard Methods (APHA 2005).

The Box–Behnken design (BBD) statistical experiment had been used on the advanced oxidation of pollutants. BBD is the optimization of analytical methods. Hence, BBD was used to investigate the effects of important operating parameters in the degradation of tannery wastewaters and to optimize the system. This is a type of response surface method (RSM) which is based on three level incomplete factorial designs. It requires fewer runs when compared to other RSM designs, making its application more economical (Masomboon et al. 2010). Basically, this optimization process involves three major steps: (1) performing the statistically designed experiments, (2) estimating the coefficients in a mathematical model and (3) predicting the response and checking the adequacy of the model. The Box–Behnken experimental design is used for evaluation of a dependent variable as functions of independent variables. The pH, TiO₂, Fe²⁺ and H₂O₂ concentrations were considered as independent variables. The COD removal efficiency was considered as a dependent variable in the Box–Behnken statistical design method. The pH was varied between 6 and 8, the TiO₂ concentration was varied between 0.10 g/L and 0.30 g/L, the Fe²⁺ concentration was varied between 0.25 g/L and 0.75 g/L and the H₂O₂ concentration was varied between 1.60 g/L and 2.00 g/L. A total of 29 experiments were employed in this analysis, using Design Expert Version 9.0.2 software (Stat-Ease, Inc., Minneapolis, USA). To simplify the recording of the conditions of each experiment and the processing of the experimental data, the factor levels were selected taking the high level as +1 and the lower level as −1.

The regression coefficients of the model describing the COD removal are summarized in Table 3. Analysis of variance (ANOVA) was applied in order to evaluate the significance of the fit of the developed model. P < 0.05 indicates that the model terms are significant at 95% confidence level or more. For the model predicted by Equation (1), P was less than 0.0001, indicating that it was significant for describing the COD removal efficiency. The lack of fit value 5.77 is not significant at the P value > 0.05. For a model to be successfully used for prediction, the lack of fit should be insignificant and the response surfaces sufficiently explained by the regression equation. A positive effect of a factor means that the response is improved when the factor level increases and a negative effect of the factor means that the response is not improved when the factor level increases. The factors that had relatively less effect on COD removal efficiency were the TiO₂ concentration (P = 0.0131) and the interactions between Fe²⁺ concentration and TiO₂ concentration (P = 0.1445) and between pH and TiO₂ concentration (P = 0.0139). As can be seen in Equation (1), the positive effects were the individual factors of TiO₂ concentration, pH and Fe²⁺ concentration, in this order. The negative effects were the double interaction between Fe²⁺ concentration and H₂O₂ concentration, the double interaction between pH and TiO₂ concentration and the factor of H₂O₂ concentration, as well as the double interaction between TiO₂ concentration and H₂O₂ concentration. The high correlation between actual experimental data and the model for COD removal (R² = 0.9785) showed that 97.85% of the variability observed in data can be explained by the model built for COD removal efficiency, leaving only 2.15% of variability owing to random error. Consequently, the quadratic model given by Equation (1) can be used to predict the COD removal efficiency, create and explore the response surface and find the optimal conditions of the process.

This equation shows the amount of additional BOD produced by the combined process. The synergetic biodegradability enhancement effect of this combined homogeneous and heterogeneous advanced oxidation process is based on the BOD₅/COD ratio. At 1 hour treatment the BOD₅/COD ratio was 0.50, 0.50 and 0.70 for solar/TiO₂/H₂O₂ process, solar/Fe²⁺/H₂O₂ process and solar/TiO₂/Fe²⁺/H₂O₂ process, respectively. Based on the BOD₅/COD ratio, the synergetic biodegradability enhancement effect of the combined process was 0.70. When compared to rate of degradation, the combined homogeneous and heterogeneous process is more effective than the individual processes.

Cost analysis was carried out by analysing the degradation process under optimal conditions. The operating cost for treating tannery wastewater by solar/TiO₂/Fe²⁺/H₂O₂ process has been evaluated. Table 5 presents the solar/TiO₂/Fe²⁺/H₂O₂ process operating and annual treatment costs per m³ of treated water necessary to remove 96% of COD. An annual treatment cost of US$21.34/m³ of treated water was obtained.

The degradation of tannery wastewater has been studied by means of the combined homogeneous and heterogeneous AOP. Box–Behnken design results confirmed that the TiO₂ concentration, pH and Fe²⁺ concentration positively affected COD removal efficiency in the solar/TiO₂/Fe²⁺/H₂O₂ process. Based on experimental results, an empirical relationship between the COD removal efficiency and independent variables was obtained and expressed by the quadratic model Equation (1). The effect of experimental parameters on the COD removal efficiency of tannery wastewater was established by the response surfaces of the developed model. The optimal operation parameters for the solar/TiO₂/Fe²⁺/H₂O₂ process for degradation of tannery wastewater were pH = 7, TiO₂ = 0.2 g/L, Fe²⁺ = 0.5 g/L and H₂O₂ = 1.8 g/L. Under this condition, 96% COD removal efficiency was achieved after 4 hours of treatment. The kinetic study indicated that the degradation kinetics of tannery wastewater followed the first-order kinetic. When compared to the rate of degradation, the combined homogeneous and heterogeneous process is more effective than the individual processes. The combined process is a useful tool for enhancing the biodegradability of tannery wastewater in biodegradation of wastewater which enters into a biological wastewater treatment process. The obtained results showed the feasibility of combined advanced oxidation processes to achieve suitable water qualities for internal reuse.