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Treatment of high salinity wastewater using CWPO process for reuse

  • Yakun Zhuo , Mei Sheng , Xueke Liang and Guomin Cao EMAIL logo
Published/Copyright: April 11, 2017
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Journal of Advanced Oxidation Technologies
From the journal Journal of Advanced Oxidation Technologies Volume 20 Issue 2

Abstract

A high salinity wastewater from epoxy resin was treated with the catalytic wet peroxide oxidation (CWPO) process, so that it can be reused as the chlor-alkali process feedstock. Both bench and pilot scale trials were conducted out in this research. The effect of oxidant (hydrogen peroxide) and catalyst (ferrous sulfate) dosages, and their dosing methods, pH value, temperature, and reaction time on TOC removal by the CWPO process were evaluated through bench experiment. The obtained optimal reaction conditions for the CWPO process were as following: H2O2 dosage = 0.735 M, Fe2+ dosage = 0.027 M, temperature = 90ºC, pH = 3.0–3.5, and reaction time = 200 min. Multiple additions of oxidant and catalyst significantly enhanced TOC removal compared to adding the same total dosage in one step. In a pilot trail, 735 moles of H2O2 and 27 moles of Fe2+ were continuously added to a 1000 L wastewater over 3 hours while the pH and temperature of the reaction solution were automatically controlled at 3.3 ± 0.5 and 90 ± 2℃, respectively, the wastewater TOC values were reduced to less than 150 mg/L from 2500–2700 mg/L, which satisfies the influent TOC limit (200 mg/L) of the diaphragm electrolytic cell. In addition, both the iron ion and sulfate ion concentrations in the pilot effluent were less than their influent limits of the diaphragm electrolytic cell, thus the treated wastewater had been successfully applied in a chlor-alkali plant for production chlorine and caustic soda.

Keywords: wastewater; epoxy resin; catalytic wet peroxide oxidation (CWPO); salinity; reuse; chlor-alkali process

1 Introduction

A large amount of wastewater contains high concentration salinity and organic pollutants have being generated in the production process of epoxy resin, epichlorohydrin, vinylidene chloride, vanillin, hydrazine hydrate, glyphosate and etc. For example, the sodium chloride (NaCl) and total organic carbon (TOC) concentrations of the wastewater from epoxy resin production are about 20 % (w/w) and 2700 mg/L, respectively, and the wastewater generated in vinylidene chloride production process contains about 20 % (w/w) NaCl and 4500 mg/L TOC, respectively.

Many methods for treating the high salinity wastewater have been applied in industrial. However no single technology has been considered as a good method, as each has significant drawbacks. Biological wastewater treatemtnt process is the most cost effective method compared to other processes. However, based on bacteria tolerance to sodium chloride, non-halophiles grow optimally at sodium chloride content less than 2 % [1], so the high salinity wastewater must be diluted with a lot of fresh water before biological treatment. Dilution not only consumes a large amount of fresh water, but also increases the volume of wastewater discharged. On the other hand, the evaporations and crystallization is a common processes for separation of salt and water [2], in which multi-effect evaporation and mechanical vapor recompression have been used to pretreat the high salinity organic wastewater at some enterprises in China. But, the crystallized salt from the evaporation process contains a lot of organic pollutants, which should be disposed as a hazardous waste according to Chinese regulation. However, the hazardous waste disposal cost is very expensive. Thus, an alternative process should be developed for treating the high salinity wastewater.

Advanced oxidation processes (AOPs), a popular method for removing soluble refractory organics in wastewater, have been successfully used for different industries [3–5], and could provide a promising solution for high salinity wastewater treatment. During advanced oxidation processes, the hydroxyl radical (·OH) was generated, which is one of most active oxidants known with the exception of fluorien. The hydroxyl radical can be used to destroy most of refractoyy constituents and oxidizing them to CO2 and H2O [6, 7]. Among AOPs, ozonization is a useful oxidation mehod, but ozone solubility in water is low, and its lifetime is short [8]. Furthermore, the solubility of ozone will be decreased as the salt concentraion increased, which makes the efficiencies of ozone utilization to decrease obviously [9]. H2O2/UV needs UV radiation, but the molar extinction coefficient of H2O2 is very low, which leads to quite high operation cost. Overall, the power cost due to the UV lamps is up to 50 60 % of the total cost of wastewater oxidation [10]. As to wet air oxidation (WAO) or catalytic wet air oxidation (CWAO), they employ excessive temperatures from 150 to 300 ºC and pressures up to 200 bars and thus have high investment and operating costs [11].

Recently, the catalytic wet peroxide oxidation (CWPO) process has been paid much attention by reaecrchers. In CWPO process, the powerful hydroxyl radicals were also generated from H2O2 in the presence of catalyst such as Fe or Cu and Mn etc. The use of this method is very attractive since it can work under relatively mild conditions (e. g., atmospheric pressure and several tens of degrees Celsius), and its capital investment costs is much lower than that of WAO or CWAO processes [12]. CWPO has been largely studied and used in the treatment of wastewaters in the pharmaceuticals, pesticides, metallurgy and other industries [13, 14], and the TOC removal in the CWPO processes is similar to that of the WAO or CWAO processes. Thus, it seems appropriate to look at removing the organic pollutants from high salinity wastewater through the CWPO process. Furthermore, the treated high salinity wastewater can be reused as a raw material for a chlor-alkali process, which is desirable for both eco-friendly and cost-effectively.

The main production methods of chlorine and caustic sodia are diaphragm and membrane cell electrolysis, using NaCl solution as feedstock in China. Diaphragm cells have the advantage of operating with less pure brine than required by membrane cells. Hence, diaphragm cells are more proper for reusing the waste brine as feed than membrane cells. Based on the data provided by a chlor-alkali plant in Jiangsu province, China, the concentrations of Ca + Mg, SO42−, Fe, NH4+, SS, and TOC in the purified brine for diaphragm cells feed should be less than 6 mg/L, 10 g/L, 10 mg/L, 1 mg/L, 1 mg/L, and 200 mg/L, respectively. The analyzed results showed that the concentrations of Ca+Mg, SO42+, Fe, NH4+, and SS in the high salanity wastewater from epoxy resin production were less than their corresponding maximum limits in the purified brine for diaphragm cells feed, but the TOC level was much higher than its limit. Therefore, as long as the TOC concentration of the high salinity wastewater could be degraded to less then 200 mg/L; the treated high salinity wastewater could potentially be reused as diaphragm cell electrolysis feedstock for producing of chlorine gas and sodium hydroxide. Of course, introducing external impurities into the high salinity wastewater during the TOC removal process should be avoided, or, if some impurities are introduced, for instance SO42+ and iron from the CWPO process, their concentrations in the treated wastewater should still be less than their corresponding limits.

When treating the high salinity wastewater from epoxy resin production by the CWPO process, only oxidant (H2O2), catalyst (FeSO4∙7H2O) and pH control agents (HCl and NaOH) are added to the wastewater. Oxidant (H2O2) is consumed during the reaction process, or decomposed into H2O and O2. The iron ions in the reaction solution would be precipitated after neutralization with sodium hydroxide, and monitoring shows that the concentration of iron ions in the supernatant are between 3.0 and 4.0 mg/L, which is lower than its limit. In addition, the calculated SO42− concentration in the supernatant is lower than 10 g/L (max limit) based on the catalyst dosage. Furthermore, the supernatant is filtered through a filter membrane before it enters the electrolytic cell, and thus, the filtrate’s SS can be regarded as zero. Therefore, as long as the wastewater TOC value could be degraded to lower than 200 mg/L in the CWPO process, the treated wastewater can be used as raw material in diaphragm electrolysis. Preliminary tests confirmed that the TOC value of wastewater from epoxy resin production can be degraded to less than 200 mg/L by the CWPO process. Thus, in theory, it should be possible to both treat the high salinity wastewater discharged from epoxy resin synthesis process with the CWPO process and use the treated water in chlor-alkali production. However, for this to be carried out in practice, the processing conditions has to be optimized and the associated costs minimized.

Therefore, in this research we looked at in detail the treatment of high salinity wastewater from epoxy resin production by the CWPO process. First, the CWPO process conditions, such as temperature, pH, the dosages of oxidant and catalyst, the dosing method of oxidant and catalyst, were optimized through bench experiments to minimize the operating cost so as to make this process cost effective. Second, a pilot trial was carried out on site to verify the results of bench testing. Finally, a case of full-scale application of CWPO process to treat the high salinity organic wastewater from epoxy resin production was presented.

2 Experimental and Methods

2.1 Wastewater

The wastewater used in the experiment was provided by the Jiangsu Yangnong Kumho Chemical Co. Ltd. located in Jiangsu province, China. The main pollutants are organics and salt in the wastewater, and its total organic carbon (TOC) and salt (NaCl) concentrations were about 2500–2700 mg/L, and 18.2–20.5 % by weight, respectively.

2.2 Chemicals

For the bench scale CWPO experiment, hydrogen peroxide (H2O2, 30 %), sodium hydroxide, and hydrochloric acid were analytical-grade reagents purchased from Shanghai Lingfeng Chemical Regent Co. Ltd. Ferrous sulfate heptahydrate was analytical-grade reagents purchased from Sinopharm Chemical Regent Co. Ltd. H2O2 was used directly, and sodium hydroxide, hydrochloric acid and ferrous sulfate were prepared into a solution (2 M, 2 M and 0.5 M, respectively).

For the pilot scale CWPO experiment, hydrogen peroxide (27.5 %), sodium hydroxide (30 %), hydrochloric acid (30 %) and ferrous sulfate heptahydrate (98 %) were industrial-grade reagents. (Although using ferrous sulfate as catalyst will increase an unwanted impurity in the treated wastewater, the SO42− concentration in the treated wastewater was less than its maximum limit, and ferrous sulfate is cheaper than ferrous chloride, so ferrous sulfate was still chosen as catalyst in this research.) Hydrogen peroxide was used directly. Both sodium hydroxide and hydrochloric acid were diluted to a concentration of 15 %, and ferrous sulfate were prepared into a solution(0.5 M).

2.3 Bench scale CWPO

A 500 ml three-neck round-bottom flask was used as the reactor of CWPO process in bench scale experiments. The reactor was equipped with a mechanical agitator in the center neck, and the other two necks were for pH control and adding chemical reagents respectively. The temperature of the reaction was controlled by a thermostatic water bath. The wastewater sample was acidified with HCl solution to pH = 3.5 (except for pH optimization), then pour 200 ml acidifying wastewater sample into the reactor. While the solution was agitated, H2O2 and FeSO4 solutions were added into the reactor at various time intervals. The pH value of the reacion solution was adjusted with NaOH or HCl solution every 20 min. After all the H2O2 and FeSO4 solution was added, the resultant solution was continuously mixed and allowed to react for 60 min. Samples were taken at regular intervals. The taked samples was neutralized to pH = 7.0–8.0 with NaOH soltuion, then settling for 120 min. The supernatant was filtered with 0.45 μm millipore filter, and the filtrate was collected and analyzed for TOC.

2.4 Pilot scale CWPO

For pilot scale CWPO experiments, 1000 liter epoxy resin wastewater was pumped into a glass-lined reactor with a capacity of 1500 liter, and acidified to pH 3 4 (see Figure 1). The wastewater temperature was kept at 90 ± 2ºC by the jacket heating. H2O2 and Fe2+ were continuously and slowly added into the reactor with metering pumps over 3 hours, while the reaction solution pH value was automatically controlled at 3.3 ± 0.5. Stirring and heating were continued for 60 min after the complete addition of oxidant and catalyst. Then, the reaction solution pH value was adjusted to 7.0 8.0. Subsequently, the reaction solution was discharged to the sedimentation tank for settling for 120 min. The sample was taken from supernatant of sedimentation tank for TOC analysis. Then, the supernatant would be filtrated and reused as the feed material of a diaphragm electrolytic cell in the subsequent test.

Figure 1 Schematic diagram of the pilot trial process of CWPO.
Figure 1

Schematic diagram of the pilot trial process of CWPO.

1- wastewater tank, 2- wastewater pump, 3- H2O2 tank, 4- H2O2 metering pump, 5- reactor, 6- FeSO4 metering pump, 7- FeSO4 solution tank, 8- HCl metering pump, 9- HCl solution tank, 10- NaOH metering pump, 11- NaOH solution tank, 12- settling tank

2.5 Analysis

TOC of the samples was analyzed by means of an Elementar Liqui TOC analyzer (Germany). Residual H2O2 concentraions in the samples were determined by iodometric titration [15]. The total iron concentraion in the filtrate was analyzed by atomic absorption spectroscopy (AAS, novAA, Germany).

3 Result and discussion

3.1 Optimization of the CWPO process

3.1.1 Optimization of the dosing method

CWPO process is essentially a Fenton reagent oxidation, except that its reaction temperature is higher. The higher the temperature, the faster the H2O2 decomposition into O2 and H2O is, and thus the corresponding utilization efficiency of H2O2 is lower. Therefore, controlling the oxidant and catalyst dosing methods in the CWPO process is more important than in the Fenton oxidation. For this reason, the test first investigated the effect of H2O2 and Fe2+ dosing methods on TOC removal, and the results were showed in Figure 2.

Figure 2 Effect of dosing method of H2O2 and Fe2+ on TOC removal during CWPO process (Reaction conditions: T = 90ºC, pH = 3.0, [H2O2] = 0.98 M, [Fe2+] = 0.027 M).
Figure 2

Effect of dosing method of H2O2 and Fe2+ on TOC removal during CWPO process (Reaction conditions: T = 90ºC, pH = 3.0, [H2O2] = 0.98 M, [Fe2+] = 0.027 M).

The TOC removal progresses of the two experiments was showed on Figure 2. In one experiment all oxidant (H2O2) and catalyst (Fe2+) were added into the reactor in initial satge, while in other one the oxidant (H2O2) and catalyst (Fe2+) were added every 10 min with the same total addition as the first experiment. Obviously, the TOC removal progress of the multiple-step dosed was slower than the one-step dosed in the initial phase of oxidation. But the final TOC removal efficiency of multiple-step dosed was significantly higher than the one-step dosed.

The amount of residual H2O2 and Fe2+ in the reaction solution are key factors for TOC removal. Because more hydroxyl radicals are formed as H2O2 and Fe2+ concentration increases (eqs 1–3) [16]. Moreover, it should be noticed that when H2O2 and Fe2+ concentration increased, scavenging of hydroxyl radicals will occur (eqs 2, 4–6) [17].

Fe2++H2O2→Fe3++OH−+HO⋅(1)
Fe2++HO⋅→Fe3++OH−(2)
HO⋅+RH→Oxidationproducts(3)
H2O2+HO⋅→HO2⋅+H2O(4)
HO2⋅+HO⋅→H2O+O2(5)
HO2⋅→O2−⋅+H+(6)

The residual H2O2 concentration in reaction solution of one-step dosed process had been shown in Figure 3, which matched the performance of TOC degradation in Figure 2. During the first 25 min the one-step dosed CWPO process, the residual hydrogen peroxide concentration was declined quickly and a great number of hydroxyl radicals was occured. The organic contaminants was oxidized by the hydroxyl radicals, and the organic-radical species were generated, which was further oxizied to carbon dioxide and resulting in the decrease of TOC concentration within a short perod of time (eq. 3). As a result, the TOC removal efficience achieved 62 % just 20 min later. In the meantime, the other parts of the hydroxyl radicals reacted with H2O2, HO2·and Fe2+, which also lead to the low residual hydrogen peroxide content (eqs 2, 4and 5). After approximately 30 min, the reaction reached equilibrium, which is accompanied by a parallel decrease of the residual hydroxyl radical content. From 30 to 200 min, only about 2 % TOC was removed, and the final TOC removal efficiency was about 64 %.

Figure 3 Change of the residual H2O2 concentration during CWPO process for one-step addition.
Figure 3

Change of the residual H2O2 concentration during CWPO process for one-step addition.

For the multiple-step dosed CWPO process, the inital phase TOC removal was obviously lower then one-step dosed process due to low initial H2O2 and Fe2+ contents. Of course, some hydroxyl radicals scavenging reactions connected with H2O2 and Fe2+ were not promoted. Maintaining moderate content of H2O2 and Fe2+ in reactor was conducive to produce more hydroxyl radicals for CWPO process. And the final TOC removal efficiency reached to 97.8 %. It is proved that the residual hydroxyl radicals was the mine factor to mineralized organic contaminates. Therefroe, to achieve a higher TOC removal in CWPO process, the oxidant (H2O2) and catalyst (Fe2+ ) shoud be dosed using multiple-step method.

3.1.2 Optimization of oxidant and catalyst dosing method

As the frequency of adding hydrogen peroxide and ferrous sulphate was from 1 to 20 times within 250 min of reaction time, the TOC removal efficiency increased by 35.3 % (see Figure 4). And as the interval of feeding hydrogen peroxide and ferrous sulphate was changed from 2 min to 10 min, the TOC removal efficiency just increased by 3.3 % (see Figure 5), moreover, choosing 7 min and 10 min as interval of feeding possessed very similar TOC removal efficiency (97.6 % and 97.7 %, respectively).

Figure 4 Effect of the frequencies of dosing H2O2 and Fe2+ on TOC removal during CWPO process (Reaction conditions: T = 90ºC, pH = 3.0, [H2O2] = 0.98 M, [Fe2+] = 0.027 M).
Figure 4

Effect of the frequencies of dosing H2O2 and Fe2+ on TOC removal during CWPO process (Reaction conditions: T = 90ºC, pH = 3.0, [H2O2] = 0.98 M, [Fe2+] = 0.027 M).

Figure 5 Effect of the interval of adding H2O2 and Fe2+ on TOC removal during CWPO process (Reaction conditions: T = 90ºC, pH = 3.0, [H2O2] = 0.98 M, [Fe2+] = 0.027 M).
Figure 5

Effect of the interval of adding H2O2 and Fe2+ on TOC removal during CWPO process (Reaction conditions: T = 90ºC, pH = 3.0, [H2O2] = 0.98 M, [Fe2+] = 0.027 M).

The results show that the residual H2O2 and Fe2+ concentration was a key factor effect of the TOC degradation. The higher frequencies that were used to dose hydrogen peroxide and ferrous sulphate, the more uniform residual hydroxyl radical content will be. And hydroxyl radical scavenging reactions will be minimized at the same time. In addition, increasing the time interval between dosages increases the TOC removal up to a certain amount (7 min in this case). This suggests that the organic contaminants can be effectively oxidized in seven minute interval, and thus increasing the interval beyond that was almost not help for further increaseing TOC removal.

3.1.3 Effect of pH value on CWPO process

The pH value change of the reaction solution at various times was presented in Figure 6, and the pH value was’t adjusted during the whole process. It can be observed from Figure 6that pH value dropped sharply at early stage, due to the generation of some acidic products as a result of organics oxidation and hydrogen ion (Reaction 6) [18]. Afterwards, the further oxidation of these intermediates resulted in the slight increase of the pH value.

Figure 6 The reaction solution pH variation during CWPO process (Reaction conditions: T = 80ºCinitial pH = 3.0, [H2O2] = 0.98 M, [Fe2+] = 0.027 M).
Figure 6

The reaction solution pH variation during CWPO process (Reaction conditions: T = 80ºCinitial pH = 3.0, [H2O2] = 0.98 M, [Fe2+] = 0.027 M).

For the same initial pH = 3, comparing the one which pH controlled during the CWPO process with the other one which there was not any control of pH, the finial TOC removal of the controlled (97.4 %) was higher than the no control (95.3 %). Hence further experiments were carried out to test the effect of pH on TOC removal, during which pH was adjusted to the determined value every 20 min (with hydrochloric acid or sodium hydroxide). The relationship between the pH value and TOC removal was shown in Figure 7. As the pH value increased from 1.5 to 3.0, the TOC removal slowly increased; when pH value was equal to or over 4.0, the TOC removal decreased significancy. Therefore, the optimal pH value for the CWPO process was nearby 3.0, which corresponds to the maximum concentration of the active Fe2+ species and to the lowest rate of H2O2 parasitic decomposition [19]. On the other hand, when the pH value increases, the insoluble ferric hydroxides would be formed and precipitated, then the regeneration ferrous ion may be inhibited (eq. 7). As a result, the production of hydroxyl radicals were interrupted while the decomposition of H2O2 becomes preponderant [20].

Fe3++H2O2→Fe3+⋯O2H2++H+→Fe2++HO2⋅+H+(7)
Figure 7 Effect of pH on TOC removal during CWPO process (pH was adjusted in every 20 min) （Reaction conditions: T = 80ºC, [H2O2] = 0.98 M, [Fe2+] = 0.027 M）.
Figure 7

Effect of pH on TOC removal during CWPO process (pH was adjusted in every 20 min) （Reaction conditions: T = 80ºC, [H2O2] = 0.98 M, [Fe2+] = 0.027 M）.

It’s worth noting that for very acidic conditions, the TOC remvoal also decreases in spite of iron species remaining soluble, because high content of hydrogen ion can sequester hydroxyl radicals [21]. Hence, a further research was carried out at pH value equal to 3.0.

3.1.4 Effect of temperature on CWPO process

The operating temperature is known to be another important parameter in the CWPO process. The impact of reaction temperature to the TOC removal was studied by adjusting the temperature from ambient to 90ºC. As shown in Figure 8, the TOC removal was only 65.8 % as the reaction temperature was 25ºC, where a lot of hydrogen peroxide was still present at the end of the experiment. This result indicated that the inactivity of the catalyst is not because of low residual oxdant concentration, but due to the slow generating hydroxy radicals. As expected, when the reaction temperature was increased, the TOC removal was rising. For example, as the reaction temperature rose to 40ºC, the TOC removal has risen to 89.2 %; when the temperature rose to 90ºC, the TOC removal rate as high as 97.6 %. Increasing the reaction temperature, the rate of H2O2 conversion into hydroxyl radicals is accelerated; in addition, the quantity of H2O2 available to scavenge these hydroxyl radicals was reduced accordingly [22]. In fact, the original temperature of the wastewater was near 90℃ when discharged from the steam stripping tower for solvent recovery, so a 90ºC reaction temperature was applied in the pilot trial and the full-scale application.

Figure 8 Effect of temperature on TOC removal during CWPO process （Reaction conditions: pH = 3.0, [H2O2] = 0.98 M, [Fe2+] = 0.027 M）.
Figure 8

Effect of temperature on TOC removal during CWPO process （Reaction conditions: pH = 3.0, [H2O2] = 0.98 M, [Fe2+] = 0.027 M）.

3.1.5 Effect of Fe2+ and H2O2 dose on CWPO process

Both of catalyst and oxidant dose had influences on the CWPO process, which are also the main factors that determine the cost of running [23], and the experimental results were presented in Figure 9and Figure 10, respectively. Figure 9showed that the TOC removal raised quickly from 44.8 % to 92.9 % when ferrous sulphate dose increased from 0.009 M to 0.01325 M, and then slowly increased to 97.7 % as the dose was between 0.01325 to 0.027 M. The oxidation mechansim could be that the hydroxyl radical are generated through the interaction between ferrous sulphate and hydrogen peroxide. Therefore, the increase of ferrous sulphate dose will help to generate more hydroxyl radicals, which will lead to an increase of TOC removal. But when more ferrous sulphate was introduced to the system, there was no further TOC removal obtained. The higher ferrous sulphate content could favor the reaction between ferrous ion and hydroxyl radicals (eq. 2), so that ferric ion content is increased and hydrogen peroxide concentration is decreased. In addition, the higher ferric ion concentration contributed to the increase of hydrogen peroxide decomposition (eqs 1and 6) [24].

Figure 9 Effect of Fe2+ dose on TOC removal during CWPO process （Reaction conditions: T = 90ºCpH = 3.0, [H2O2] = 0.98 M）.
Figure 9

Effect of Fe2+ dose on TOC removal during CWPO process （Reaction conditions: T = 90ºCpH = 3.0, [H2O2] = 0.98 M）.

Figure 10 Effect of H2O2 dose on TOC removal during CWPO process （Reaction conditions: T = 90ºCpH = 3.0, [Fe2+] = 0.027 M）.
Figure 10

Effect of H2O2 dose on TOC removal during CWPO process （Reaction conditions: T = 90ºCpH = 3.0, [Fe2+] = 0.027 M）.

As showed in Figure 10, the TOC removal efficiency was increased as the hydrogen peroxide dosage was increased. And the TOC removal efficiency was 93.6 % (residual TOC of 163 mg/L) when 0.735 M H2O2 was dosed. It should be noticed that at a higher H2O2 dose, not all the oxidant is efficiently used for TOC mineralization due to the formation of intermediate compounds more refractory to be oxidized or even scavenging reactions of the hydroxyl radicals [15]. In addation, the hydroperoxyl radicals (HO2·) would be produced throuht the reaction of the generated hydroxyl radicals with excess of hydrogen peroxide. The hydroperoxyl radicals are much less reactive and do not contribute to the oxidative degradation of the organic substrate which occurs only by reaction with hydroxyl radicals [25].

Therefore, it is important to optimize the applied dose of catalyst (ferrous sulphate) and oxidant (hydrogne peroxide) to maximize TOC removal of the CWPO process and minimize the operation cost. Considering the requirement for TOC (<200 mg/L), the optimal ferrous sluphate dosage and hydrogen peroxide dosage were 0.027 M and 0.735 M, respectively.

3.2 Pilot trial

In order to determine the feasibility of the industrial treatment of epoxy resin wastewater, an additional pilot trial was performed under the optimized reaction condition (T = 90 ± 2 ºC, pH = 3.3 ± 0.5, [H2O2] = 0.735 M, [Fe2+] = 0.027 M) with adding oxidant and catalyst continuously for 1000 liter wastewater. The repeated tests result showed that the TOC removal efficiency could be stabilized at 93 % or more, namely that the effluent TOC concentration was less than 200 mg/L. In addition, the iron ion concentration in the pilot effluent was less than its limits of the diaphragm electrolytic cell. Broadly, the results obtained from the pilot trial confirm that the CWPO process is effective in the reduction of TOC and is a very interesting alternative for epoxy resin wastewater treatment.

3.3 Operating cost estimation

For the implementation of the treatment of epoxy resin wastewater by CWPO technology, it is not only necessary to degrade TOC to less than 200 mg/L, but also to do so at an acceptable operating cost, such as less than RMB 100/m3 (or $ 16/m3). Thus, the operating cost of CWPO process was evaluated based the pilot trial, and the result was showed in Table 1. As the temperature of the wastewater is near 90ºC, the heating cost was ignored.

Table 1

Operating cost of CWPO process.

ReagentBasisPrice

(US $)		DoseOperating cost

(US $/m3)
H2O2 (27.5 %)kg0.13490.8 kg/m312.167
FeSO4∙7H2Okg0.0817.65 kg/m30.620
NaOH (30 %)kg0.1266.7 kg/m30.844
HCl (30 %)kg0.04815.0 kg/m30.720
Sludge[*]kg0.0483.6 kg/m30.173
ElectricitykWh0.0133.0kWh/m30.039
Total14.563

As showed in Table 1, the total operating cost of CWPO process was approximately $14.56/m3, which was lower than the acceptable cost of the enterprise. Therefore, the CWPO process can be used as an alternative method for the epoxy resin wastewater treatment and reuse.

3.4 Full-scale application

In 2013, a full-scale CWPO device was designed and built for treating the high-salinity wastewater from epoxy resin production in Jiangsu province, China, and successfully put into commercial operation. The designed capacity of the CWPO device was 500 m3/d, and has four series of reactor (each effective volume was 20 m3). The running conditions were as following: the wastewater flow 20 m3/h, H2O2 (27.5 %, w/w) dose 1600 kg/h, FeSO4∙7H2O dose 125 kg/h, reaction temperature 90 95℃. The effluent of CWPO device was neutralized with NaOH solution and settled for 2 h in a sedimentation tank, the supernatant was filtrated through a microporous membrane filtration, and then the filtrate was pumped to the diaphragm electrolytic cells for producing chlorine and caustic soda. So far, both CWPO device and diaphragm electrolysis device have been stable operating for more than three and half years, achieving the aim of high-salinity wastewater recycling and utilization.

4 Conclusion

When the high salinity wastewater from epoxy resin synthesis was treated by CWPO process, its TOC concentration could be decreased to less than 200 mg/L, and could be reused as a raw material of a chloro-alkali process. The results from the bench scale experiment showed that the optimum H2O2 dose, Fe2+ dose, pH value, and reaction temperature, were 0.735 M, 0.027 M, 3.0, and 90ºC, respectively. A significant improvement of the TOC removal efficiency was obatined by multiple addition methods of H2O2 and Fe2+. Also controlling the pH value during the process considerably increased the TOC removal efficiency. In a pilot trail, with the controlled reaction conditions (pH = 3.3 ± 0.5, T = 90 ± 2ºC), as 735 mole hydrogen peroxide and 27 mole ferrous ion were continuously added to a 1000 liter epoxy resin wastewater in 3 hours, the TOC removal efficiency could be stabilized at 93 % or more and the operating cost was approximately $14.28/m3. Thus it is recommended to treat the high salinity wastewater from epoxy resin synthesis with CWPO and recycle the treated wastewater to a chloro-alkali process for reuse.

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Published Online: 2017-4-11

© 2017 Walter de Gruyter GmbH, Berlin/Boston

Articles in the same Issue

  2. Editorial
  4. Excitation Kinetics of Oxygen O(1D) State in Low-Pressure Oxygen Plasma and the Effect of Electron Energy Distribution Function
  6. Using amino-functionalized Fe3O4-WO3 nanoparticles for diazinon removal from synthetic and real water samples in presence of UV irradiation
  8. Treatment of high salinity wastewater using CWPO process for reuse
  10. Electrochemical Advanced Oxidation Processes (EAOP) to degrade per- and polyfluoroalkyl substances (PFASs)
  12. Effect of feedstock impurities on activity and selectivity of V-Mo-Nb-Te-Ox catalyst in ethane oxidative dehydrogenation
  14. Photocatalytic Degradation of Azo Dyes Over Semiconductors Supported on Polyethylene Terephthalate and Polystyrene Substrates
  16. Effects of calcination temperature on sol-gel synthesis of porous La2Ti2O7 photocatalyst on degradation of Reactive Brilliant Red X3B
  18. ClO2-oxidation-based demulsification of oil-water transition layer in oilfields: An experimental study
  20. Semi-permanent hair dyes degradation at W/WO3 photoanode under controlled current density assisted by visible light
  22. Degradation of PVA (polyvinyl alcohol) in wastewater by advanced oxidation processes
  24. Degradation of imidacloprid insecticide in a binary mixture with propylene glycol by conventional fenton process
  26. Gemini surfactant-assisted synthesis of BiOBr with superior visible light-induced photocatalytic activity towards RhB degradation
  28. Photocatalytic paraquat degradation over TiO2 modified by hydrothermal technique in alkaline solution
  30. Enhancement of Profenofos Remediation Using Stimulated Bioaugmentation Technique
  32. Mechanistic insight on the sonolytic degradation of phenol at interface and bulk using additives
  34. Biosolubilization of low-grade rock phosphate by mixed thermophilic iron-oxidizing bacteria
  36. Degradation of methyl orange using dielectric barrier discharge water falling film reactor
  38. Rapid prediction of hydrogen peroxide concentration eletrogenerated with boron doped diamond electrodes
https://doi.org/10.1515/jaots-2017-0024
Keywords for this article
wastewater; epoxy resin; catalytic wet peroxide oxidation (CWPO); salinity; reuse; chlor-alkali process

Articles in the same Issue

  2. Editorial
  4. Excitation Kinetics of Oxygen O(1D) State in Low-Pressure Oxygen Plasma and the Effect of Electron Energy Distribution Function
  6. Using amino-functionalized Fe3O4-WO3 nanoparticles for diazinon removal from synthetic and real water samples in presence of UV irradiation
  8. Treatment of high salinity wastewater using CWPO process for reuse
  10. Electrochemical Advanced Oxidation Processes (EAOP) to degrade per- and polyfluoroalkyl substances (PFASs)
  12. Effect of feedstock impurities on activity and selectivity of V-Mo-Nb-Te-Ox catalyst in ethane oxidative dehydrogenation
  14. Photocatalytic Degradation of Azo Dyes Over Semiconductors Supported on Polyethylene Terephthalate and Polystyrene Substrates
  16. Effects of calcination temperature on sol-gel synthesis of porous La2Ti2O7 photocatalyst on degradation of Reactive Brilliant Red X3B
  18. ClO2-oxidation-based demulsification of oil-water transition layer in oilfields: An experimental study
  20. Semi-permanent hair dyes degradation at W/WO3 photoanode under controlled current density assisted by visible light
  22. Degradation of PVA (polyvinyl alcohol) in wastewater by advanced oxidation processes
  24. Degradation of imidacloprid insecticide in a binary mixture with propylene glycol by conventional fenton process
  26. Gemini surfactant-assisted synthesis of BiOBr with superior visible light-induced photocatalytic activity towards RhB degradation
  28. Photocatalytic paraquat degradation over TiO2 modified by hydrothermal technique in alkaline solution
  30. Enhancement of Profenofos Remediation Using Stimulated Bioaugmentation Technique
  32. Mechanistic insight on the sonolytic degradation of phenol at interface and bulk using additives
  34. Biosolubilization of low-grade rock phosphate by mixed thermophilic iron-oxidizing bacteria
  36. Degradation of methyl orange using dielectric barrier discharge water falling film reactor
  38. Rapid prediction of hydrogen peroxide concentration eletrogenerated with boron doped diamond electrodes
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