Treatment of refractory organic pollutants in industrial wastewater by wet air oxidation

2.1 WAO of phenol and substituted phenol

Joglekar et al. (1991) carried out the WAO of aqueous solutions of phenol and substituted phenols in a 1 L stainless steel autoclave at temperatures in the range of 150–180 °C. The oxygen partial pressure was varied from 0.3 to 1.5 MPa and the initial phenol concentration was 200 mg/L. The data based on bench scale showed the WAO of phenols can achieve destruction efficiencies exceeding 99.9%. The reduction of COD during oxidation of all phenols was also greater than 90%. Eugenia et al. (2008) investigated WAO as a suitable precursor for the biological treatment of industrial wastewaters that contained high concentrations of phenol, o-cresol or 2-chlorophenol in a semi-batch reactor at temperatures of 215, 240 and 265 °C. The oxygen partial pressure was at 0.2 and 0.9 MPa. Total organic carbon (TOC) abatement was upto 86%. Farhad (1982) carried out the WAO of aqueous solutions of phenol in a continuous stirred tank reactor at temperatures in the range of 25–250 °C. The oxygen partial pressure was varied from 5.5 to 15.2 MPa and the initial phenol concentration was varied from 1200 to 4500 mg/L. The effect of process variables: temperature, pressure, liquid flow rate, air flow rate and agitation speed on the gas holdup ratio in the reactor was also studied. Phenol conversion in some runs was as high as 96.83%. Lin and Chuang (1994) employed WAO for treating high concentration chemical wastewater containing phenol and/or phenolic compounds. Experimental results indicate that over 90% removal of phenol or phenolic compounds can be efficiently achieved in the WAO process. It is found in the present studies that the combined process, if appropriately operated, is capable of drastically reducing the COD concentration of the high concentration chemical wastewater to meet the safe discharge requirement. Zhang et al. (1987) investigated the WAO for the purification of the phenol–water and the wastewater from coal gasification in a 2 L autoclave at temperature from 180 to 250 °C, oxygen pressure of 0.98–3.43 MPa, and initial concentrations of phenol at 3540, 7866 mg/L and COD at 9278, 22928 mg/L, respectively. 60–90% of the phenol and 35–55% of the COD were removed in 90 min at temperature from 190 to 250 °C and oxygen pressure of 0.98 MPa. Tang et al. (1995) carried out the WAO of high concentration phenol solution in a 1 L batch autoclave under following conditions: initial COD range from 7804 to 87002 mg/L; reaction temperature from 150 to 250 °C; oxygen partial pressure from 0.7 to 5.0 MPa. Experimental results showed that the WAO process was suitable for wide initial concentration range and the organic substances decomposed quickly. Within 30 min, COD removal reached 52.9–90% and phenol decreased by 86–99%. Fu et al. (2004) studied liquid-phase oxidation of aqueous solutions of phenol, nitrobenzene and aniline employing oxygen as the oxidant. The influence of temperature (160–220 °C) on WAO of these compounds was investigated. The order of the reactivity of these three compounds in fixed time is: phenol > aniline > nitrobenzene. Xu et al. (2003) treated Orthochlorophenol wastewater by various WAO methods. Parameters such as reaction temperature and oxygen pressure were investigated in terms of chemical oxygen demand (COD_Cr) removal rate. Experimental results showed that the WAO process without catalyst is also effective for the treatment of the Orthochlorophenol wastewater, upto 80% of the initial COD_Cr was removed by wet air oxidation at 270 °C with stoichiometric oxygen supply.

Portela Miguélez et al. (1997) made experiments by the WAO of aqueous solutions of phenol in a batch reactor at temperatures between 150 and 300 °C and pressures from 10 to 20 MPa. The initial phenol concentrations were between 460 and 1650 mg/L and the initial oxygen concentration was always above 800% excess. The reduction of COD during oxidation was measured and the influence of pressure and temperature on the induction time was also studied. The activation energy of oxidation reaction is 34.4 kJ/mol. Kolaczkowski et al. (1997) carried out the WAO of phenol at moderate temperatures (T = 200 °C) and pressures (3.0 MPa total pressure), and the oxidation of phenol has been achieved with upto 95% destruction in less than 30 min. An increase in either oxygen concentration or temperature has a positive influence on reaction rate and the influence of pH is shown to be complex.

2.2 WAO of dyes

Removal of dyes in textile wastewater has attracted a lot of interest because of the growing concern about residual color which is closely associated with toxicity and esthetics of the discharged effluent. Dyes exhibit a high resistance to microbial degradation because they retain color and structural integrity under exposure to sunlight, soil, and bacteria. In particular, azo dyes are readily converted into hazardous aromatic amines under anoxic conditions (Ganesh et al., 1994; Ekici et al., 2001). Some studies on the WAO of dyes are summarized in Table 2.

Zhou and He (2007) compared the degradation behaviors of an azo dye, cationic red X-GRL by three clean advanced oxidation processes. It was found that the WAO process was able to readily remove color and even at the low temperature of 120 °C, the color could be almost completely removed at 60 min, and the COD removal by WAO was about 35.4% at 120 min. Chen et al. (1999) used advanced oxidation methods to degrade reactive dyes at high concentrations in aqueous solutions. The removal of TOC and color by WAO was very sensitive to the reaction temperature. At 150 °C, the removal of 77% TOC and 90% color was obtained in less than 30 min. Hu et al. (2001) studied the WAO of printing and dyeing wastewater. The model simulations are in good agreement with experimental data. The results indicate that a certain fraction of organic pollutants in the printing and dyeing wastewater could not be removed even at elevated temperature and prolonged reaction time. The ratio of degradable organic matter is found independent of temperature. Lei and Wang (1999) applied WAO to degrade reactive dyes at high concentration. Experimental results showed that the removal of TOC and color was found to be very sensitive to the reaction temperature, and the color removal was also found to be closely related to the removal of TOC until an equilibrium TOC was reached. With increasing the reaction temperature to 150 °C, 80% TOC and 90% color were removed under the operating conditions without the employed catalyst within half an hour.

Zeng et al. (2005) studied WAO of highly concentrated dispersing blue dye wastewater conducted in a 2 L autoclave batch reactor. The results showed that the temperature was the key factor. When the temperature was 255 °C, 72.1% COD_Cr was removed after 2 h. Dong et al. (2007) carried out WAO to degrade azo dye reactive red 2BF, the impact of which including temperature, reaction time, oxygen partial pressure, KBrO₃ dosage and pH value was investigated. When temperature was 160 °C, oxygen partial pressure 0.8 MPa, pH 4, n(KBrO₃): n(Reactive Red 2BF) = 0.5:1 was used for degradation, the red color of 2BF (400 mg/L) was degraded to colorless after reaction for 6 h, and the TOC removal rate was 52%. Chen et al. (2003) presented the results of a study on the treatment of printing and dyeing wastewater from the textile industry by WAO. Experiments were conducted in a 2 L autoclave batch reactor at temperatures between 150 °C and 300 °C, oxygen partial pressure of 1.69 MPa. Experimental data indicate that a certain amount of COD in the printing and dyeing wastewater can be removed by simple WAO. The results obtained are useful for the design of WAO reactor and for the prediction of WAO performance in treating printing and dyeing wastewater. Zhang (2009) treated methylene blue water solution by WAO. And the mechanism of methylene blue solution in WAO was researched. The results showed that not only the reaction included the two phases: the anterior quick reaction and the upper slow reaction, but also the color removal rate of the water sample was higher than the COD removal rate. Finally, methylene blue was translated into dimethylaniline and dimethylaniline was translated into quinone and dimethyl amine. Liu et al. (2001) investigated the WAO of printing and dying wastewater. A COD removal of 56% was obtained after the wastewater was treated with WAO at 300 °C for 120 min. Furthermore, the biodegradability of the wastewater was improved from 0.04 to 0.40. Dai et al. (2006) used the response surface methodology (RSM) to model and optimize the WAO process using one azo dye cationic red X-GRL as model pollutant. Mono-factor experiments were carried out to select the relatively important factors, based on which the process was optimized by RSM under the following conditions: 60–180 °C, 0–2 stoichiometric oxygen amount, 100–700 r/min agitating rate, pH 2–10 and 500–2931 mg/L pollutant. Almost 100% of the dye removal rate was obtained under the optimized operation conditions by RSM. Lei et al. (2007) investigated the decolorization of a strong colored azo dye solution of cationic red X-GRL by WAO under relatively mild conditions. It showed that temperature influenced the dye removal obviously. The increasing of the oxygen accelerated the dye removal and the increasing removal rate slowed down after the oxygen amount got the stoichiometric amount. In an alkaline solution, the dye removal was more thorough than that of neutral and acid solution.

3.1 Wet air oxidation of emulsified wastewater

Emulsified wastewater usually comes from the mechanical industry. Its value of COD_Cr is normally 5000–100,000 mg/L. It contains all kinds of organic matters such as surfactants, additives and mineral oils. The emulsified wastewater belongs to typical highly concentrated hardly biodegradable organic wastewater. Normal treating methods concentrated on common physical or chemical methods such as chemical demulsification, electrolytic floatation, separation with membrane, but they have apparent shortcoming: bringing secondary pollution and the cost of operation too high.

Tang et al. (2003) carried out the WAO of the highly concentrated emulsified wastewater containing non-ionic surfactant performed in a 2 L high-pressure batch autoclave. The result indicated that the application of WAO had a good performance in the treatment of emulsified wastewater. And COD of the wastewater was reduced by 86.4% after 2 h oxidation at 220 °C. Zhao et al. (2002) studied the WAO of the emulsification wastewater performed in a 2-L high-pressure batch autoclave. According to the experimental results, temperature, oxygen partial pressure, organic concentration in the wastewater and other operation conditions had influence on oxidation efficiency to a different extent, among which temperature was the key factor, and 86% of COD_Cr in the wastewater was removed. Tang et al. (2004) investigated WAO of high strength refractory emulsification wastewater performed in a 2 L high pressure batch autoclave. The experimental results indicated that rising temperature impelled the reaction leaning to oxidizing into the end result directly, making middle result oxidize at higher speed too, and making the removal rate of COD_Cr and TOC rise notably, upto 86.4%. Zeng et al. (2004) studied WAO of highly concentrated emulsification wastewater was performed in a 2 L high-pressure batch autoclave, with emphasis on the removal effect of COD_Cr, TOC and influence factors. The experimental results indicated that the temperature was the key factor and the best feasible temperature was 220 °C, and that 86.4% of COD_Cr and 79.5% of TOC.

3.2 Wet air oxidation of TNT red water

2,4,6-Trinitrotoluene (TNT) is an important high explosive widely used in many fields. During its production, load, assemble and pack operations, a large amount of TNT wastewater is produced, which can cause environmental pollution and do harm to public health if it is discharged without effective treatment (Zhao et al., 2010).

Lu et al. (2007) treated TNT red water by WAO. The results indicated that when the reaction temperature is 200 °C, residence time 10 min, oxygen pressure 4 MPa, and the pH value 4, the COD value of TNT red water decreases from 56500 mg/L to 1072.5 mg/L, the removal efficiency is more than 98.10%. Hao et al. (1993a) presented the results of rate studies for the evaluation of temperature, partial oxygen pressure, initial red water concentration and salt concentration on WAO performance. Results show the higher the temperature and/or pressure, the more the contaminant reduction. Hao et al. (1993b) previously demonstrated that WAO is an effective process to treat diluted TNT red water at a temperature of 340 °C and oxygen partial pressure of 0.8 MPa. Results show the WAO removal efficiency to be a function primarily of temperature and, to a lesser extent, of oxygen partial pressure. TOC and COD removals at 260 °C and 0.62 MPa oxygen partial pressure were 77% and 91%, respectively.

3.3 Wet air oxidation of other wastewater

Lin and Ho (1997) investigated the WAO of chemical wastewater containing a high concentration of various organic acids and inorganic components. Experimental results indicated that over 50% reduction of COD concentration of the wastewater can be easily achieved in 60 min of WAO treatment operating at 225 °C, 3 MPa and a 1.01 min/L air flow rate. Lei and Sun (2000) described the application of WAO to the treatment of desizing wastewater from two textile companies. The effects of temperature, pressure and reaction time were explored extensively. More than 90% COD_Cr reduction and 80% TOC removal have been obtained. Verenich et al. (2000) undertook experimental research into the WAO of concentrated wastewaters from paper mills. The results showed COD and TOC removal increases of 50% to 80% at temperatures from 160 to 200 °C. The best results with respect to the improvement of biodegradability were observed under milder conditions. Li et al. (2010) pretreated tetrahydrofuran production wastewater by WAO. When the reaction temperature is 250 °C and the reaction pressure is 5.0 MPa, BOD₅/COD is increased from 0.18 to 0.67, which indicates that the biodegradability of the wastewater is greatly improved. The water quality of the effluent can meet the third grade discharge standards of GB8978-1996. Ge et al. (2005) introduced the reaction mechanism and reaction condition of WAO in the paper. The investigations of a series of fat wastewater treatment technology conditions are carried out with testing devices. It is observed that the COD stripping rate can reach over 90% at the reaction temperature of 200 °C and the reaction pressure of 3 MPa. This method can be used in engineering as a pretreatment means. Zhang et al. (2010) treated the organic wastewater through the WAO technology. The results showed that at the condition of 250 °C, Po₂ = 3.5 MPa, reaction time of 2 h and diluted multiples of 0.5 times (COD = 100,000 mg/L around), the removal rate of COD was 56%, the BOD₅/COD increased to 0.63 and the biochemical nature of wastewater was improved greatly, thus providing a good condition for the successive biological treatment. Zhang et al. (2007) used the WAO of degradation-resistant pesticide wastewater with high salinity. The results indicated that 98.0% removal rate of COD and over 99.0% removal rate of chroma were achieved under the following reaction conditions: temperature 280 °C, partial pressure of oxygen 4.2 MPa, initial pH value of the system 2.0 and reaction time 150 min.

4.1 Reaction mechanisms and pathways

The definite mechanisms and reaction pathways for WAO have not been established even for a pure compound because the oxidation of organic compounds goes through very complicated routes and leads to the formation of various intermediates. Generally, the final intermediates are short-chain organic acids such as acetic acid, etc. (Day et al., 1973; Mantzavinos et al., 1997a,b). The identification of intermediates and by-products formed during oxidation is essential to evaluate the toxicity and to point out possible refractory compounds.

Most of the studies have reported that the chemical reaction of WAO chiefly proceeds via free radical reactions. Several free radical reactions consisting of initiation, propagation, and termination of free radical have been proposed to take place during the WAO of various organic compounds (Bhargava et al., 2006; Mantzavinos et al., 1997a,b; Patterson et al., 2001; Robert et al., 2002). Robert et al. (2002) provided the direct experimental evidence for the presence of free radical intermediates in the WAO of cellulose using electron spin resonance (ESR) spectroscopy coupled with the spin trapping technique. Some studies gave the indirect evidences of free radical reactions. Co-oxidation is one of them, which involves the oxidation of an organic compound by free radical intermediates generated from another organic compound (Raffainer and von Rohr, 2001; Birchmeier et al., 2000; Mantzavinos et al., 1996). Another is to hinder or promote the free radical reactions using inhibitors such as t-butanol (Vaidya and Mahajani, 2002; Stoffler and Luft, 1999) or promoters like hydrogen peroxide (Vaidya and Mahajani, 2002).

Most of the studies on reaction pathway for WAO have employed phenol as a model compound because it is considered to be an intermediate in the oxidation of higher molecular weight aromatic compounds (Matjaž and Janez, 1996; Santos et al., 2001). Phenol oxidation follows an extremely complex pathway composed with parallel and consecutive reactions. Devlin and Harris (1984) proposed the mechanism for non-catalytic oxidation of phenol in which phenol is firstly oxidized to dihydroxybenzenes, i.e. hydroquinone and catechol, which are converted into benzoquinones. Rings of benzoquinones are then opened with the formation of appropriate acids which are further oxidized to short-chain carboxylic acids. Joglekar et al. (1991) studied WAO of aqueous solutions of phenol and substituted phenols. The result showed that the oxidation of phenols in water involves a free-radical mechanism and proceeds in two steps. The conditions have been found under which the overall oxidation reaction becomes reaction controlled or mass transfer controlled. Rivas et al. (1998) proposed a model for the WAO of phenol based on elementary radical reactions that are known to occur in the oxidation process. By applying the model to associated experimental data, unknown rate constants have been calculated using numerical techniques to solve the system of differential equations that describes the proposed mechanism. The principles described in the paper may be applied to understand the WAO mechanism of other organic compounds.

4.2 Reaction kinetics

It is desired that the kinetic models correspond reasonably with reaction mechanism and experimental data. Reaction parameters including reaction temperature, oxygen partial pressure, reaction time, solution pH, etc. influence the reaction rate. Studies on the kinetics of WAO have been extensively performed by many researchers. Jean (1989) proposed semi-empirical model WAO and believed WAO of sewage sludge is a first-order reaction, the reaction kinetics model is: $$\mathit{da}/\mathit{dt}=k_{\text{a}}·a$$ $$\text{db}/\text{dt}=k_{\text{b}}·b$$

with a being the concentrations of easily oxidized organic matter, b the concentration of not easily oxidized organic matter, k_a and k_b the reaction rate constant.

In recent years, Eftaxias et al. (2001) proposed a theoretical model for the WAO process: $$\text{dc}/\text{dt}=K_0{e}^{-E_{\text{a}}/\mathit{RT}}{[C]}^m{[O]}^n$$

with K₀ being the pre-exponential factor, E_a the activation energy(kJ/mol), T the temperature (K), C the organic matter concentration(mol/L), O the oxidant concentration (mol/L), t the reaction time(s), m, n the reaction order, R the gas constant [8.314 J/(mol·K)].

Although the kinetic models proposed for pure compounds are useful for the understanding of reaction mechanisms, they are not sufficient for real wastewaters containing a mixture of organic compounds. More detailed kinetic models are required to describe the reaction rate observed in the WAO of real wastewaters. Therefore, the rate laws have to be expressed by means of lumped parameters such as TOC and COD. For non-catalytic WAO, Li et al. (1991) proposed the general lumped kinetic model (GLKM), also named the triangular scheme, based on the simplified reaction scheme with acetic acid as a rate-limiting intermediate (Fig. 1). In the GLKM, all the organics are divided into three characteristic lumps, namely parent compounds and unstable intermediates except acetic acid (lump A), refractory intermediates, acetic acid being a typical representative (lump B), and end products (lump C). k₁, k₂, and k₃ are apparent constants for the chemical reaction rates and are dependent on reaction temperature and dissolved oxygen concentration. The GLKM has been used to analyze the experimental kinetic data in several studies (López et al., 1999; Jin et al., 2004).

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Figure 1 Generalized lumped kinetic model (GLKM).

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