Combination of sonophotolysis and aerobic activated sludge processes for treatment of synthetic pharmaceutical wastewater

Chemical Engineering Journal 255 (2014) 411–423

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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Combination of sonophotolysis and aerobic activated sludge processes for treatment of synthetic pharmaceutical wastewater Amir Mowla, Mehrab Mehrvar ⇑, Ramdhane Dhib Department of Chemical Engineering, Ryerson University, 350 Victoria Street, Toronto, Ontario M5B 2K3, Canada

h i g h l i g h t s  Aerobic activated sludge process was only able for partial TOC removal from a SPWW.  UV/US/H2O2 process at optimum condition was able to remove more than 90% TOC in SPWW.  Effects of various operational parameters on the UV/US/H2O2 process were studied.  Combined UV/US/H2O2 and aerobic AS resulted in higher mineralization while lower oxidant consumption.  Combined processes reduced retention time in both sonophotoreactor and bioreactor.

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Article history: Received 16 April 2014 Received in revised form 11 June 2014 Accepted 13 June 2014 Available online 21 June 2014 Keywords: Sonophotolysis Activated sludge Synthetic pharmaceutical wastewater Total organic carbon removal Combined processes Advanced oxidation processes

a b s t r a c t The performance and effectiveness of sonophotolytic process, aerobic activated sludge (AS) process, and their combination in reduction of total organic carbon (TOC), chemical oxygen demand (COD), and biological oxygen demand (BOD) from a synthetic pharmaceutical wastewater (SPWW) are evaluated. Batch mode experiments are performed to obtain optimal experimental operating conditions of the sonophotolytic process. An ultrasonic power of 140 W, initial pH solution of 2, and air flow rate of 3 L min1 are found to be optimal operating conditions. The initial optimum molar ratio of H2O2/TOC was found to be 13.77 for the sonophotolytic process operated in batch mode. In continuous mode, a 90% TOC reduction was obtained in the sonophotolytic process after 180 min retention time, whereas only 67% in an aerobic AS process for retention time of 48 h. However, combined sonophotolytic and aerobic AS processes improved the biodegradability of the SPWW with 98% TOC and 99% COD removal while reducing the retention time in sonophotoreactor and aerobic AS bioreactor to 120 min and 24 h, respectively. Besides, the consumption of H2O2 was reduced significantly in the combined processes. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Pharmaceutical industries are characterized by a large number of products, processes, plant sizes as well as the magnitude and the quality of produced wastewater. For manufacturing each type of product, several processes and raw materials may be required [1]. During the last few decades, the production and consumption of pharmaceutical compounds have been increased significantly, mainly due to the developments in medical science and also the considerable growth in the world population. Nowadays, a huge amount of medicines is manufactured each year for human and animal consumptions [2]. Therefore, an enormous amount of wastewater is generated in pharmaceutical industries [3]. Pharmaceutical wastewaters are generally categorized as one of the main ⇑ Corresponding author. Tel.: +1 416 9795000x6555; fax: +1 416 9795083. E-mail address: [email protected] (M. Mehrvar). http://dx.doi.org/10.1016/j.cej.2014.06.064 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.

complex and toxic industrial wastewaters with high BOD, COD, total suspended solid (TSS), toxicity and odor as well as low BOD/COD ratio. Moreover, wastewater from pharmaceutical industry might contain various amounts of organic solvents, catalysts, raw materials, and reaction intermediates which make their treatment procedure complicated [1,4,5]. Most treatment methods of pharmaceutical wastewater are physico-chemical and conventional biological processes. Coagulation-flocculation and activated carbon adsorption are frequent examples of physico-chemical mechanisms. Suarez et al. [6] applied coagulation-flocculation as pre-treatment for hospital wastewater. The treatment was able to reduce TSS by about 92% and COD up to 70%. However, the removal of most pharmaceutical components such as antibiotics were marginal. Activated carbon in both powdered (PAC) and granular (GAC) forms was also used for the removal of micropollutants. More than 90% removal of estrogens was reported by both GAC and PAC processes [7]. Also, up

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to 90% removal of endocrine disrupting material by PAC was observed [8]. Biological methods are known as the most common and cost-effective choices of the treatment. In the case of industrial pharmaceutical wastewater, aerobic AS process with long hydraulic retention time (HRT) is a very frequent treatment option [9]. Membrane bioreactors (MBR) are also aerobic technologies which have been used alone or in combination with AS process to treat pharmaceutical wastewaters. About 99% COD and 95% BOD of a real pharmaceutical manufacturing wastewater was removed by MBR [10,11]. Even though conventional biological methods are economical choice of treatment, several types of industrial wastewater such as those from petrochemical, pharmaceutical, leather, dye, pulp and paper and pesticide manufacturing plants contain considerable amount of organic compounds which are nonbiodegradable and refractory to microorganisms applied in biological treatment systems. These pollutants cannot be removed by conventional wastewater treatment plants and the standard regulations cannot be reached. Also, the release of these substances into the environment and their presence in drinking water may have harmful effects on both humans and ecosystems [12–14]. Considering the aforementioned issues, additional treatment steps seem to be indispensable. Among technologies used to remove nonbiodegradable substances, advanced oxidation processes (AOPs) are influential treatment methods for degrading recalcitrant materials or mineralizing stable, inhibitory, or toxic contaminants [15]. AOPs are of great interest and used by several researchers to treat different types of pollutants during past few decades [16–22]. AOPs such as UV/H2O2, Fenton, etc. could be described as an oxidation method based on the intermediacy of highly reactive species such as hydroxyl radicals (OH) in a procedure leading to the degradation of target contaminants [23]. The application of ultrasound irradiation (US) or sonolysis in water and wastewater treatment has received a lot of attention in recent years and several studies have been reported [24]. Several advantages of sonolytic process such as avoiding consumption of chemical oxidants or catalysts, safety, and lower demand for the clarification of aqueous solution, make their application simple and desirable [25]. Sonochemical reactions are principally due to a phenomenon named acoustic cavitation. The phenomenon is the process of formation, expansion, and sudden implosion of gas microbubbles. The acoustic cavitation leads to the generation of high local pressure (as high as 1000 atm) and high temperature (as high as 5000 K). It is known that under these extreme conditions, the pyrolysis of water molecules results in the formation of hydroxyl radicals as follows (Eq. (1)) [26,27]: US

H2 O ! H þ OH

ð1Þ

Generally, US waves at frequencies in the range of 20-1000 kHz can produce cavitation in aqueous solutions [28]. The cavitation acts as a means of concentrating the diffusing energy of ultrasound into microbubbles. During sonolysis, three types of sonochemical reactions can take place. First, the pyrolytic reactions which happen due to the high pressure and temperature inside the cavitation bubbles; second, the free radical attack which is performed by the produced reactive radicals in the interfacial area between the bubbles and the liquid phase, and third, the generation of hydroxyl radicals in the liquid bulk solution [29,30]. Organics components with low solubility and/or high volatility are expected to go through fast sonochemical degradation since they have a tendency to accumulate inside or around the gas–liquid interface. Therefore, sonolysis may be a proper method for the removal of pharmaceutical micropollutants. Even though AOPs are very effective in treating almost all organic compounds, some flaws prevent their commercial applica-

tions. The high requirement of oxidant/catalyst dosage, high electrical power consumption, and precise pH adjustment are some of these drawbacks which make operational cost of AOPs high [31]. Therefore, to overcome the aforementioned problems and to find efficient and economical treatment, the combination of advanced oxidation and biological processes as a potential alternative has attracted attention of many researchers. Carballa et al. [32] combined ozonation and anaerobic digestion for the removal of 11 pharmaceutical components and reported that the ozonation pretreatment improved the efficiency of the biological post treatment. In another study, Sitori et al. [33] achieved 95% dissolved organic carbon removal (DOC) from an industrial pharmaceutical wastewater by combining solar photo-Fenton and biological treatments. Fenton was also combined with sequential batch reactor to treat a real pharmaceutical wastewater containing two antibiotics where 89% COD removal was achieved [34]. In these studies, generally, AOPs are applied as a pre-treatment to degrade refractory compounds and to improve the biodegradability level of the wastewater. The produced biodegradable intermediates could be mineralized in a subsequent low cost biological step. Finding the optimum retention time of the wastewater in an AOP reactor is a challenging issue. On one side, in order to reduce the cost of AOPs, lower dosage of chemicals and lower retention times should be applied to achieve small percentage of mineralization. On the other hand, a very low mineralization causes the formation of intermediates which are still toxic and similar to the parent compounds. Therefore, the selection of the point to transfer the effluent of an AOP reactor to the bioreactor should be performed carefully. Two factors are important in combined processes, the biodegradability of the wastewater after photochemical oxidation and the presence of residual oxidants such as H2O2, which are inhibitory to microorganism in biological treatment systems. In this study, the remediation of a synthetic pharmaceutical wastewater was carried out by means of a sonophotolytic process (UV/US/H2O2) alone and sonophotolysis as a pre-treatment for the aerobic AS process. The effects of operating parameters, such as US power, H2O2 concentration, pH, and HRT in both sonophotolysis and aerobic AS processes were investigated. Furthermore, the effluent of the sonophotoreactor was analyzed to evaluate the changes in the biodegradability of the wastewater as well as residual concentration of H2O2. Based on the results obtained, the optimal HRT for both the sonophotoreactor and the bioreactor are found and the combined treatment was performed under optimal operating conditions. To the best of the authors’ knowledge, this is one of the first reports studying the combination of the sonophotolytic process and biological treatment without using H2O2 neutralizer. Results of this study can help having an efficient treatment of industrial pharmaceutical wastewater.

2. Materials and methods 2.1. Materials The SPWW was prepared based on a list of components reported in a study by Badawy et al. [35]. The components were detected in the wastewater generated by a pharmaceutical and chemical company in Cairo, Egypt. The wastewater contained chloramphenicol, diclofenac, salicylic acid, and paracetamol which were the main products of the production plant. Also, some byproducts including p-aminophenol, nitrobenzene, benzoic acid, and phenol were detected in the raw wastewater [35]. Three sets of concentrations in distilled water were chosen to conduct the experimental runs. Characteristics of these three sets are shown in Table 1. The 30% (w/w) H2O2 (Sigma–Aldrich) was used as received. Also, NaOH and H2SO4 solutions (VWR, Canada) were

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A. Mowla et al. / Chemical Engineering Journal 255 (2014) 411–423 Table 1 Characteristics of the SPWW used in this study. Compound

Molecular formula

Molecular weight (g mol1)

Concentration in 1st SPWW (mg L1)

Concentration in 2nd SPWW (mg L1)

Concentration in 3rd SPWW (mg L1)

4-Aminophenol Paracetamol Phenol Chloramphenicol Benzoic acid Salicylic acid Diclofenac sodium Nitrobenzene TOC (mg L1) TN (mg L1) COD (mg L1)

C6H7NO C8H9NO2 C6H6O C11H12Cl2N2O5 C7H6O2 C7H7O3Na C14H11Cl2NO2Na

109.13 151.17 94.11 323.132 122.12 160.11 318.1

6.25 2.5 12.5 7.5 6.25 28.75 0.5

12.5 5 25 15 12.5 57.5 1

25 10 50 30 25 115 2

C6H5NO2

123.06

7.5 44.83 ± 0.25 2.56 ± 0.08 127 ± 1

15 89.75 ± 0.37 5.12 ± 0.08 252 ± 1

30 179.33 ± 0.35 10.25 ± 0.12 515 ± 1

used for pH adjustment while required. For the biological process experiments, aerobic sludge seed was obtained from Ashbridges Bay Wastewater Treatment Plant, Toronto, Canada. The inoculum was acclimatized to the SPWW samples. 2.2. Experimental setup and procedure Fig. 1 depicts the schematic diagram of the experimental setup. The sonophotoreactor is an airlift external loop reactor. The riser is 9.72 cm in diameter and 110 cm in height. The height and the diameter of the downcomer are 90 and 3.25 cm, respectively. Also, the total volume of the photoreactor is 7 L. The sonophotoreactor was employed in both batch and continuous modes. As shown in Fig. 1, the setup is equipped with a single ended UV lamp (Ushio America Inc.) and a commercial ultrasonic processor (Branson, S250D Sonifier). The UV lamp, located in the centerline of the riser, is 84.6 cm in height and 1.55 cm in diameter with the wavelength of 253.7 nm and 13 W output power. The sonifier had a 13 mm diameter tip which was capable of working in continuous and pulse modes. It had a constant frequency of 20 kHz and a variable output power up to 200 W. All experiments were performed with sonifier at continuous mode. Since the sonophotoreactor was an airlift external loop reactor, the contents of the wastewater were circulated during the sonication time. Therefore, all parts of the wastewater were exposed to sonication at various time periods both in batch and continuous modes. There was also a perforated circular tube air sparger 5 cm above the reactor bottom. The

temperature in the reactor was also monitored. During batch mode experiments, the reactor was initially filled with the wastewater and while required with the oxidant. Then the UV and/or US radiations was started and after the desired time, samples were taken for analysis. Therefore, there was no input/output to the system during the radiation period. In the continuous mode experiments, the inlet flow of the wastewater was adjusted with desired residence time. This means that the wastewater continuously entered and left the reactor. The samples were taken from effluents of the reactors. The aerobic AS bioreactor was a continuous flow completely mixed activated sludge reactor with an effective working volume of 25.5 L under ambient condition. The activated sludge reactor was composed of two parts; the aeration tank and the clarifier for separation of liquid and biomass. Diffused air was used to provide aeration and also to mix the content of the bioreactor. The concentration of dissolved oxygen (DO) must be kept over 2 mg L1 in order to prevent anaerobic reactions [36]. Before conducting experiments, 8 L of the aerobic sludge seed with initial suspended solid concentration of 1800–1900 mg L1, were loaded into the aerobic AS bioreactor and was acclimatized with the SPWW samples. The inoculum was acclimatized by feeding the SPWW continuously into the bioreactor. The flow rate of the inlet SPWW for the acclimatization process was set at 20 mL min1. Acclimatization period was performed for 28 days. This period could be divided into four periods of 7 days. The influent concentration of the SPWW to the bioreactor was increased

1-Riser 2-Downcomer 3-UV lamp 4-US horn 5- US processor 6-Feed tank 7-Perlistatic pump 8-Sonophotoreactor outlet 9-Compressed air 10-Aerobic AS bioreactor 11- Air flow Meter

Fig. 1. Schematic diagram of the external loop air lift sonophotoreactor and aerobic activated sludge (AS) reactor.

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gradually. In the first period (1st to 7th day), the initial TOC of the SPWW was set to 22.5 mg L1. The initial TOC was increased to 44.83, 89.75, and 179.33 mg L1 on the 8th, 15th, 22nd day, respectively. Nutrients were fed to the reactor as well to maintain the COD:N:P molar ratio of 100:5:1 [37]. The nutrient medium consisted of KH2PO4, K2HPO4, NaHPO47H2O, NH4Cl, MgSO4, FeCl3 and CaCl2 [38]. During 28 days, samples were collected from the bioreactor to measure the mixed liquor suspended solid (MLSS) and mixed liquor volatile suspended solid (MLVSS) concentrations. These parameters were used to determine the growth of microorganism and to observe the acclimatization process. During experiments, DO and temperature were measured by using a DO meter (YSI 58 Dissolved Oxygen Meter) and the pH was also measured using a pH meter (Thermo Scientific, Ottawa, Ontario, Orion 230A+). By changing the air flow rate, the DO was maintained at least 2 mg L1. The TOC and total nitrogen (TN) were measured by a TOC/TN analyzer (Apollo 9000, Teledyne Tekmar, USA). The concentrations of MLSS, MLVSS, BOD5 and COD were measured according to the American Public Health Association [38]. The residual concentration of H2O2 in the effluent of the sonophotoreactor was measured using 2,9-dimethyl-1,10-phenathroline (DMP) (Alfa Aesar (USA)) method as described in the open literature [39]. The method had a detection limit of 0.8 lM. 3. Results and discussion 3.1. Assessment of parameters’ changes during the processes The initial temperature of the SPWW was in the range of 24.8– 25.3 °C. Effluents of the sonophotoreactor had temperature increase up to 33.1 °C. For the activated sludge reactor, during both acclimatization period and experiments, the temperature varied between 24.5 °C and 25.6 °C. Therefore, there was a temperature increase in the effluents of the sonophotoreactor during experiments, which is obviously due to the heat generation caused by UV lamp and US probe. However, due to the SPWW circulation in the external loop air lift sonophotoreactor, the effect of temperature increase which was small, could be partially neutralized. In order to evaluate the thermal evaporation of the SPWW,

experiments were performed at 25 °C and 30 °C. However, less than 2% TOC reduction was observed after 2 h. Therefore, thermal evaporation did not contribute significantly to the total removal rate and therefore, this effect was neglected. The initial pH of the SPWW was in the range of 3.80–4.0. Before introducing the SPWW to the bioreactor, the pH was increased to 7 using NaOH solution, which is the ideal range for the growth of microorganisms. During the acclimatization of the biomass, the pH values in the biological reactor were fluctuating significantly. This may be due to metabolism and enzyme reactions during the growth of the microorganisms. Also, according to the literature, the rate of production or consumption of hydrogen ions might be affected by the changes in the CO2 production rate, ammonia removal rate, and phosphorus removal rate [40]. The pH values in the aerobic AS bioreactor were in the range of 5.53–7.81. On the other hand, during the experiments, the pH values in the aeration tank varied between 6.82 and 6.95. While conducting the combined processes experiments, the pH of the effluents from the sonophotoreactor was adjusted to 7–7.50. The DO was also monitored in the bioreactor. In the aeration tank DO was in the range of 2.1–3.4 mg L1. 3.2. MLSS and MLVSS concentrations of aerobic activated sludge Fig. 2 shows concentration profiles of the MLSS and MLVSS in the aerobic AS bioreactor. As this figure shows, there is a quick rate of adaptation of the biomass to conditions in the aeration tank. The trend in Fig. 2 indicates a rapid growth of microorganism until reaching a plateau which is the stabilization phase. As shown in the figure, by increasing the inlet TOC to the system due to the elevated food availability, the rate of reproduction of microorganisms increased. However, after the second week, only the microorganisms that got adapted to the higher dosage of pharmaceutical components survived and their accumulation decreased. Finally, After 28 days of acclimatization, a considerable amount of sludge was produced, therefore, the MLSS and MLVSS of the AS were reached to approximately 3200 and 2300 mg L1, respectively. The goal of the acclimatization period was to adapt the microorganisms to the components of the SPWW. Therefore, during this period, some reduction of the TOC in the SPWW was expected. Experiments were started after reaching a steady state phase in the growth

Fig. 2. Evolution of mixed liquor suspended solid (MLSS) and mixed liquor volatile suspended solid (MLVSS) in the aerobic AS process.

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curve of microorganisms. The results are in accordance with previous studies [41,42]. 3.3. Treatment of SPWW using aerobic activated sludge process Biological treatment using the aerobic AS process in continuous mode in a laboratory scale was studied to treat the SPWW. Four HRT of 12, 24, 36 and 48 h were used with the TOC loading rates of 0.93–15 mg L1 h1 and TN loading rates of 0.064–0.85 mg L1 h1. The experimental results for TOC and TN removals are depicted

415

in Figs. 3 and 4, respectively. The optimum retention time for aerobic AS process was 24 h since there was no significant changes in the TOC removal for retention times over 24 h. The TOC removal after 24 h retention time was 65%, 69%, and 73% for the three SPWW initial concentrations. For HRT of 48 h, 67%, 71%, and 76% TOC reduction were observed which showed only a slight improvement. Also, in the case of TN removal, the results were in the ranges of 32– 60% and 52–65% for HRT of 24 and 48 h, respectively. Furthermore, it was also observed that at higher influent TOC and TN concentrations, the TOC and TN removal rates were higher.

Fig. 3. Total organic carbon (TOC) removal for different synthetic pharmaceutical wastewater (SPWW) concentrations at various HRT using aerobic activated sludge treatment in continuous mode without recirculation.

Fig. 4. Total nitrogen (TN) removal for different synthetic pharmaceutical wastewater (SPWW) concentrations at various HRT using aerobic activated sludge treatment in continuous mode without recirculation.

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The treatment ability of the aerobic AS process was not satisfactory since more than 95% TOC removal was the goal to meet the standard regulations. These unsatisfactory results were attributed to the existence of nonbiodegradable components. In order to achieve high removal efficiency, the application of advanced oxidation technologies as pre-treatment seems crucial. However, in order to make the treatment process practically economical, the combination of AOPs with biological process could be a good alternative. 3.4. TOC removal in SPWW using UV/US/H2O2 process alone in batch recirculation mode The slow mineralization rate is a major problem associated with the application of sonolysis in wastewater treatment [43,44]. Also, the formation of toxic intermediates, which are sometimes more toxic than parent components, is a great concern while applying photolysis [45]. On the other hand, during the sonophotolytic process, due to the simultaneous US and UV irradiation, more reactive radicals are produced and consequently the rate of degradation would increase [46]. Additionally, this elevated rate of mineralization causes the reduction of the formation of intermediate components. Operating factors such as US frequency and the output power, the pH, and the type, and the amount of dissolved gas have important role in a sonophotolytic process [2]. Additionally, the UV light intensity and the concentration of H2O2 have key role in photolysis process. In this section, batch mode experiments were performed to determine the operating parameters for the maximum efficiency for the treatment of the SPWW. 3.4.1. Optimal H2O2 dosage for the UV/US/H2O2 process Several studies reported that the addition of H2O2 to UV or US irradiation or their combination (US/UV) increases the treatment efficiency [47–49]. However, it should be considered that there is an optimum concentration of H2O2 that should be determined carefully. Overdose of this oxidant causes a reduction in organics removal effectiveness due to the recombination of hydroxyl

radicals (OH) as well as the reaction of produced OH with the excess H2O2 molecules to generate other radicals, such as hydroperoxyl radicals (HO2) which have less oxidizing power than OH (Reactions (2) and (3)) [50]. Increasing the cost of the process is another problem associated with the overdose of H2O2. The low oxidant concentration, on the other hand, leads to the lack of OH in the solution and decreases the degradation effectiveness. 

OH þ  OH ! H2 O2

ð2Þ

H2 O2 þ  OH ! H2 O þ HO2

ð3Þ

In order to determine the optimum dosage of H2O2, various concentrations in the range of 0–3000 mg L1 were used. Three wastewater samples with the initial TOC concentrations of 44.83, 89.75, and 179.33 mg L1 were used for experiments in the batch recirculation mode for 90 min reaction time. The results are shown in Fig. 5. All curves in Fig. 5 show the same trend and indicate an enhancement in the TOC removal by increasing H2O2 concentrations up to an optimum dosage. The maximum TOC removal for the samples with TOC of 44.83 mg L1 was 63.95% after 90 min using 1750 mg L1 H2O2. The optimal oxidant dosage for the other two SPWW with higher initial TOC was found to be 2250 mg L1 which eventuated in 44.72% and 19.8% TOC removal, respectively. Fig. 5 also confirms that at a higher organic loading of the wastewater, the TOC removal capacity decreases due to the presence of more organic matter ready to compete for reaction with OH. In order to declare the results in a more practical form, it is recommended to determine the optimal molar ratio of [H2O2]/[TOC] [17,47]. The ratio of [H2O2]/[TOC] is an important parameter to optimize the wastewater treatment which could be used to adjust the H2O2 concentration based on the concentrations of organic matters present at any given time. Consequently, this factor assists to maximize the efficiency and diminish chemical and electrical expenses. Fig. 6 presents the optimal initial molar ratio of [H2O2]/[TOC] for three wastewater samples. Molar ratio of 13.77 was found as the optimum value for the SPWW with the initial TOC of 44.83 mg L1. The results are in accordance with data found in the open literature, where the molar ratios between 0 and 100

Fig. 5. Optimal initial concentration of H2O2 for different synthetic pharmaceutical wastewater (SPWW) concentrations after 90 min treatment by UV/US/H2O2 process in batch mode (pH = 3.9, US power = 140 W, and air flow rate = 2 L min1).

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Fig. 6. Effect of the initial molar ratio of [H2O2]o/[TOC]o on the treatment of different synthetic pharmaceutical wastewater (SPWW) concentrations within the UV/US/H2O2 process in batch mode (pH = 3.9, US power = 140 W, and air flow rate = 2 L min1).

were reported [17,51]. As shown in Fig. 6, UV/US process (no H2O2), reduced the TOC concentration of all the SPWW samples by less than 8%. This is shown in Fig. 6 where [H2O2]/[TOC]o is zero. The SPWW was also treated using UV alone (the results are not shown). The UV alone was only able to reduce the TOC concentration between 0 with [TOC]o = 179.33 mg L1 and 7% with [TOC]o = 44.83 mg L1. 3.4.2. Effect of ultrasonic power on TOC removal The effect of ultrasonic power on the UV/US/H2O2 process was also studied in the batch recirculation mode. Output powers of 20,

60, 100 and 140 W were applied with the initial TOC of 44.83 mg L1. Optimum initial H2O2 concentration of 1750 mg L1 was used while other parameters kept constant (pH = 3.9, air flow rate = 2 L min1). As Fig. 7 shows, increasing the US power from 20 to 140 W improved the TOC removal by about 19%. Elevated US power causes higher rate of breakage of H2O2 molecules in the aqueous solution [52]. Consequently, the concentration of hydroxyl radicals was increased. Furthermore, an increase in the ultrasonic power contributed to higher mixing intensity due to the turbulence and microstreaming which are generated during the cavitational microbubble collapse. To recap, higher ultrasonic power results in higher number of

Fig. 7. Ultrasonic power effect on total organic carbon (TOC) removal during batch US/UV/H2O2 process ([TOC]o = 44.83 mg L1, [H2O2]o = 1750 mg L1, pH = 3.9, and air flow rate = 2 L min1).

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cavitation, number of microbubble generated, formation of hydroxyl radicals, mass transfer, and more degradation of pollutants [53–55]. The results of this study as well as data from literature confirm the importance of US output power on competence of both sonolytic and sonophotolytic processes.

3.4.3. Effect of initial pH on TOC removal The initial pH of solution is a critical parameter which affects the efficiency of many AOPs. The data regarding the effect of pH on the UV/US/H2O2 process is limited. Durán et al. [28] reported an increase in TOC removal while increasing pH from 2 to 8 for the treatment of food processing industry wastewater treatment using the UV/US/H2O2 process. Another recent report by Xu et al. [46] studied the degradation of dimethyl phthalate by US/UV process and stated a systematic reduction in dimethyl phthalate degradation by increasing the pH in the range of 2–10. In the present study, the TOC removal was observed under the sonophotolytic process by changing pH in the range of 2–8. The optimal initial H2O2 concentration of 1750 mg L1 and optimal US power of 140 W were applied while the air flow rate was kept at 2 L min1. The results are shown in Fig. 8. It was found that by increasing the pH, the TOC reduction was decreased. An explanation to this trend may be due to the reduction of oxidation potential of hydroxyl radicals while pH is elevated [56]. In addition, the fast consumption of hydroxyl radicals in alkaline medium must be considered (Reaction (4)). 

OH þ OH ¢ H2 O þ O

ð4Þ 10

1

in ionic form in pH values greater than 3 [58–60]. Therefore, greater degradation rate could be expected at higher acidic condition. 3.4.4. Effect of air flow rate on TOC removal Air flow rates in the range of 0–5 L min1 were applied while other parameters adjusted to their optimal values found in previous steps ([H2O2] = 1750 mg L1, US Power = 140 W, and pH = 2). The results are demonstrated in Fig. 9. Increasing the air flow rate from 0 to 3 L min1 improved the TOC removal from 45% to 73%. Further increase in the air flow rate did not affect the TOC removal significantly and around 57% TOC removal was achieved for flow rates of 4 and 5 L min1. Several researchers stated a positive effect of gas sparging on the enhancement of sonochemical process [55,61]. Adverse effect of high air flow rates (4 and 5 L min1) could be described by considering the lower residence time of organics in the sonophotoreactor which is caused by higher liquid circulation during the treatment with elevated air flow rates. 3.4.5. TN removal in SPWW using UV/US/H2O2 process In order to observe the ability of the sonophotolytic process on TN removal, the SPWW with initial TN of 2.52 mg L1 was treated under sonophotolysis. All optimal condition found in previous sections ([H2O2] = 1750 mg L1, US Power = 140 W, and pH = 2, air flow rate = 3 L min1) were applied. After 90 min, less than 3% TN removal was achieved. These poor results imply the disability of the UV/US/H2O2 process in the elimination of nitrogenous compounds.

1

In Reaction (4), kforward and kbackward are 1.2  10 M s and 9.3  107 s1, respectively [57]. The effect of pH on the degradation rates in sonophotolytic process is also reliant on the state of the contaminant molecules, whether the pollutants are present as ionic species or as molecules. Several studies reported that the degradation rate is low at the pH range which the pollutant is in its ionized form [54,57]. This behavior is due to the fact that components are nonvolatile and more stabilized at their ionized form. Therefore, they react with OH only at microbubble surfaces. On the other hand, in molecular form, they can enter the vapor phase; consequently, they decompose by both thermolytic cleavage and reaction with OH in aqueous solution. In current study, the pKa value of most of the components are more than 3 (phenol: 9.95; diclofenac sodium: 3.80; salicylic acid: 3; paracetamol: 9.9; and benzoic acid: 4.2) which implies that these compounds are mostly

It was observed that the SPWW samples were approximately nonbiodegradable and the aerobic AS process was only able for the partial treatment of the wastewater. On the other hand, the UV/US/H2O2 process using the optimal operating conditions showed a significant efficiency in the TOC removal in a short period of time. However, achieving complete removal of pollutants requires longer reaction time and higher consumption of chemicals which make the treatment expensive. Therefore, the combination of the two processes is studied in order to accomplish greater removal of TOC, COD, and TN while trying to make the treatment economical by reducing the usage of chemicals and finding an

Fig. 8. Effect of initial pH of the synthetic pharmaceutical wastewater (SPWW) on total organic carbon (TOC) removal after 90 min in batch mode by US/UV/H2O2 process ([TOC]o = 44.83 mg L1, [H2O2]o = 1750 mg L1, US power = 140 W, and air flow rate: 2 L min1).

Fig. 9. Effect of air flow rate on total organic carbon (TOC) removal after 90 min in batch mode by US/UV/H2O2 process ([TOC]o = 44.83 mg L1, [H2O2]o = 1750 mg L1, US power = 140 W, and pH = 2).

3.5. Combined UV/US/H2O2 and aerobic AS processes for treatment of SPWW

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optimum retention time for transferring the wastewater from sonophotoreactor to the bioreactor. The experiments were performed in continuous mode to investigate the biodegradability of treated SPWW for different HRT under the UV/US/H2O2 process. Continuous mode studies were performed only on the SPWW with the inlet TOC of 44.83 mg L1. Biodegradability studies were carried out based on the evolution of BOD5/COD ratio and the average oxidation state (AOS) of the wastewater samples during the treatment. Additionally, since the effluent of the sonophotoreactor would be introduced to the bioreactor, the concentration of H2O2 should be minimized. Hence, the concentration profile of H2O2 was determined during the UV/US/ H2O2 process. According to the results from biodegradability studies and H2O2 concentration profile, the optimum retention time to transfer of the SPWW from the sonophotoreactor to the bioreactor was determined to be 120 min. In the next step, using the results found from continuous mode studies, the SPWW was pre-treated in the sonophotoreactor and the effluent was introduced to the aerobic AS bioreactor.

using 250 mg L1 H2O2 is marginal. Only 4% TOC removal was observed. Therefore, further studies were focused on inlet H2O2 concentrations of 500, 750, 1000, 1250, 1500 and 1750 mg L1.

3.5.2. Biodegradability studies Common parameters to study the biodegradability of a wastewater include the ratio of BOD5/COD or BOD5/TOC [23,63,64]. Typically, the BOD5/COD ratio of 0.4 or more in a wastewater sample implies that the sample could be considered as biodegradable [23]. Another parameter is the AOS which is a measure of the oxidation state of the wastewater [33,65]. The AOS is a very helpful parameter to estimate the oxidation degree of mixed solutions and provides indirect data about biodegradability of the solutions. AOS may vary between 4 (for CO2, the most oxidized state of carbon) and 4 (for CH4, the most reduced state of carbon). The AOS could be calculated as follows [62]:

AOS ¼ 3.5.1. UV/US/H2O2 as pre-treatment in continuous mode According to the results from batch mode studies, continuous mode experiments were conducted using US power of 140 W, pH 2, air flow rate of 3 L min1, and various H2O2 concentrations at different HRT from 30 to 180 min. The TOC profile is depicted in Fig. 10. The results are in accordance with batch mode experiments. Increasing the inlet H2O2 concentration increased the TOC removal. Under the optimum condition, approximately 90% TOC removal was observed which confirmed the high treatment ability of the UV/US/H2O2 process. In order to achieve high treatment efficiency in bioreactors while combining with AOPs, the effluent of the AOP reactor should be biodegradable. Also, it has been proven that high concentrations of hydrogen peroxide are harmful to bacterial activity [62]. To find an optimum point to transfer the wastewater from sonophotoreactor to the aerobic AS bioreactor, the biodegradability and residual hydrogen peroxide concentration in the effluent of the sonophotoreactor need to be determined. As Fig. 10 shows, the TOC removal

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4 ðTOC  CODÞ TOC

ð5Þ

The TOC and COD are in molar concentrations in Eq. (5). The evolution of the BOD5/COD ratio and the AOS for the wastewater with inlet TOC of 44.83 mg L1 were investigated during the UV/ US/H2O2 process. Results are illustrated in Figs. 11 and 12. Increasing the HRT from 0 to 120 min improved the AOS significantly. However, the AOS remained almost constant for HRT of 150 and 180 min. The elevation of the AOS implies that the intermediates which are formed during the treatment are oxidized easier [66]. Constant AOS after 120 min HRT means that the chemistry of the produced intermediates does not change notably after that time. The BOD5/COD ratio of the SPWW was also improved by increasing HRT during the UV/US/H2O2 process. Applying HRT of 120 min for all experiment except the one with inlet oxidant concentration of 500 mg L1 made the BOD5/COD ratio cross 0.4 which means the wastewater samples could be considered biodegradable. However, samples with higher inlet concentration of the oxidant became biodegradable in lower HRT.

Fig. 10. Total organic carbon (TOC) removal using different H2O2 concentrations during continuous mode UV/US/H2O2 process ([TOC]in = 44.83 mg L1, pH = 2, US power = 140 W, and air flow rate = 3 L min1).

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Fig. 11. Evolution of average oxidation state during the UV/US/H2O2 process at various HRT, ([TOC]in = 44.83 mg L1, pH = 2,US power = 140 W, and air flow rate = 3 L min1).

Fig. 12. Evolution of BOD5/COD ratio during the UV/US/H2O2 process at various HRT,([TOC]in = 44.83 mg L1, pH = 2, US power = 140 W, and air flow rate = 3 L min1).

3.5.3. Residual H2O2 concentration profile As mentioned earlier, high concentrations of H2O2 affect the performance of bacterial communities negatively. In many studies concerning the application of H2O2 in AOPs prior to the biological treatment, the residual H2O2 was removed or neutralized from the wastewater before introducing it to bioreactor. Several components are used to remove H2O2 such as catalase enzyme [17,35,49,67] and sodium sulfite [68]. Continuous addition of

catalase to the wastewater, especially in the industrial scale, would increase the treatment cost significantly. In order to avoid using catalase, the residual concentration of H2O2 should be minimized in the effluent of the AOP reactor. This may achieve whether by decreasing the initial dosage of the oxidant or increasing the HRT. Fig. 13 illustrates the residual H2O2 concentration under the UV/ US/H2O2 process at various HRT. The trend of residual oxidant concentration is approximately the same for all experiments. Reducing

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the inlet dosage of H2O2 caused lower residual concentration in the effluent. Using the optimum dosage of the oxidant found earlier (1750 mg L1) resulted in more than 130 and 110 mg L1 H2O2 in the effluent of sonophotoreactor after 120 and 180 min HRT, respectively. In the case of inlet H2O2 concentrations of 750 and 500 mg L1, experiments with HRT of 120 min and more, less than 11 mg L1 H2O2 in the effluent were observed. This low concentration caused in a low efficiency of the treatment which can be seen in the TOC removal curve (Fig. 10). Therefore, there was no need

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for retention times higher than 120 min while working with 500 and 750 mg L1 H2O2. It has been reported in the open literature that very low concentrations of H2O2 does not cause drastic problems on microorganisms employed in biological treatment [62,69]. Also, Laera et al. [3] reported that H2O2 in low range of 3–7 mg L1 did not affect the membrane bioreactor biomass while integrated with the UV/ H2O2 process. These findings would be useful to determine the stage that the wastewater samples could be transferred from the sonophotoreactor to the aerobic AS process.

Fig. 13. Residual H2O2 concentration in the effluent of the sonophotoreactor at various HRT, ([TOC]in = 44.83 mg L1, pH = 2, US power = 140 W, and air flow rate = 3 L min1).

Fig. 14. Comparison of total organic carbon (TOC) and chemical oxygen demand (COD) removal using different alternatives in continuous mode without recycling, including the UV/US/H2O2 process alone in continuous mode, aerobic AS process alone in continuous mode and combination of both processes ([TOC]in = 44.83 mg L1, [COD]in = 127 mg L1, air flow rate in sonophotoreactor = 2 L min1, pH = 2, [H2O2]in = 750 mg L1,US power = 140 W, HRT in sonophotoreactor = 120 min, and HRT in bioreactor = 24 h).

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3.5.4. Combined UV/US/H2O2 and aerobic AS processes for SPWW treatment Biodegradability studies demonstrated that except the experiment with inlet oxidant dosage of 500 mg L1, after 120 min retention time, the wastewater samples became biodegradable and significant efficiency from aerobic AS process could be expected. Additionally, considering residual H2O2 profile and data found in the open literature regarding the tolerance of microorganisms to H2O2, it seems that only effluents from experiments with inlet H2O2 concentrations of 750 and 500 mg L1 and the retention time more than 120 min could be transferred to the bioreactor without having adverse effect on microorganisms. Since experiments with 500 mg L1 oxidant did not yield an acceptable TOC removal, 750 mg L1 H2O2 was chosen for combined experiments. Also, HRT of 120 min was selected due to limited residual H2O2 and satisfactory BOD5/COD ratio. For the aerobic AS process, the HRT of 24 h was chosen for combined processes. In the final step of this study, the SPWW with inlet TOC loading of 44.83 mg L1 was first treated under the UV/US/H2O2 process with the HRT of 120 min. The inlet H2O2 concentration was 750 mg L1. Other operational parameters were adjusted to the optimum values found in batch experiments (pH = 2, US power = 140 W, air flow rate = 3 L min1). The pre-treated wastewater was transferred to the bioreactor and the flow rate was adjusted to 24 h retention time in the bioreactor. The results are shown in Fig. 14. The TOC and COD removal were 98% and 99%, respectively. The inlet molar ratio of [H2O2]/[TOC] in the combined processes was 5.9 which showed a significant reduction compared to the 13.77 optimal value found in the UV/US/H2O2 process alone. From the 98% TOC removal, about 31% was due to the sonophotolytic process and the rest was attributed to the aerobic AS process. In the case of COD reduction, 43.5% was eliminated in the sonophotoreactor and 55.5% was deducted in the bioreactor. The results confirm that the combination of advanced oxidation and aerobic AS processes could contribute to a high treatment efficiency and reduce the chemical dosage consumption. In addition, 39% TN removal was observed. Almost all TN removal was occurred in the aerobic AS process. However, higher TN removal was achieved in the combined processes compared to the aerobic AS process alone. This may be due to change in the structure of nitrogenous compounds. Also, the increased COD availability and removal in the effluent of the UV/US/H2O2 process, has led to higher demand for nitrogenous sources in the biological process.

4. Conclusions The efficiencies of the UV/US/H2O2 process, the aerobic AS process, and their combination for the treatment of a nonbiodegradable SPWW were studied. The biological system was able to remove TOC in the range of 65–73% under 24 h HRT. Increasing the HRT to 48 h did not make any significant change on the efficiency of the treatment. The ability of the biological process alone to treat the SPWW was not sufficient since more than 95% TOC removal was set as the goal of this study. A set of experiments in batch recirculation mode was performed to determine the optimum operational parameters of the UV/US/H2O2 process. An ultrasonic power of 140 W, initial pH solution of 2, and airflow rate of 3 L min1 were found as optimal values. Also, in case of H2O2 concentration, 1750 mg L1 for the wastewater with initial TOC of 44.83 mg L1 resulted in highest degradation which contributed to optimum experimental [H2O2]/ [TOC] molar ratio of 13.77. The sonophotolytic process in continuous mode using the optimum operating condition under 180 min HRT resulted in more than 90% TOC removal. In order to reduce

the consumption of oxidants and the retention time in the AOP reactor, combined processes were studied. According to the data from biodegradability studies and also those from H2O2 concentration profile, 750 mg L1 of the oxidant was selected to be used in the combined processes. The HRT in the sonophotoreactor and bioreactor was selected as 120 min and 24 h, respectively. In the combined processes, the SPWW was pre-treated in the sonophotoreactor using the optimum operational parameters and 750 mg L1 H2O2. The effluent was collected and transferred to the bioreactor. Combined UV/US/H2O2 and aerobic AS processes were able to treat the SPWW successfully. Over 98% TOC and 99% COD removal were observed. The consumption of the oxidant was reduced considerably. Acknowledgments The financial support of Natural Sciences and Engineering Research Council of Canada (NSERC) and Ryerson University is deeply appreciated. Also, the authors would like to thank Ashbridges Bay Wastewater Treatment Plant in Toronto, Canada, for providing the aerobic activated sludge. References [1] S.K. Gupta, S.K. Gupta, Y. Hung, Treatment of Pharmaceutical Wastes, in: L.K. Wang, Y. Hung, H.H. Lo, C. Yapijakis (Eds.), Waste Treatment in the Process Industries, Taylor and Francis, Boca Raton, 2006, pp. 167–233. [2] M. Klavarioti, D. Mantzavinos, D. Kassinos, Removal of residual pharmaceuticals from aqueous systems by advanced oxidation processes, Environ. Int. 35 (2009) 402–417. [3] G. Laera, D. Cassano, A. Lopez, A. Pinto, A. Pollice, G. Ricco, G. Mascolo, Removal of organics and degradation products from industrial wastewater by a membrane bioreactor integrated with ozone or UV/H2O2 treatment, Environ. Sci. Technol. 46 (2011) 1010–1018. [4] H.F. Schröder, Substance-specific detection and pursuit of non-eliminable compounds during biological treatment of waste water from the pharmaceutical industry, Waste Manage. 19 (1999) 111–123. [5] D. Sreekanth, D. Sivaramakrishna, V. Himabindu, Y. Anjaneyulu, Thermophilic treatment of bulk drug pharmaceutical industrial wastewaters by using hybrid up flow anaerobic sludge blanket reactor, Bioresour. Technol. 100 (2009) 2534–2539. [6] S. Suarez, J.M. Lema, F. Omil, Pre-treatment of hospital wastewater by coagulation–flocculation and flotation, Bioresour. Technol. 100 (2009) 2138– 2146. [7] A. Schäfer, L. Nghiem, T. Waite, Removal of the natural hormone estrone from aqueous solutions using nanofiltration and reverse osmosis, Environ. Sci. Technol. 37 (2003) 182–188. [8] S.A. Snyder, S. Adham, A.M. Redding, F.S. Cannon, J. DeCarolis, J. Oppenheimer, E.C. Wert, Y. Yoon, Role of membranes and activated carbon in the removal of endocrine disruptors and pharmaceuticals, Desalination 202 (2007) 156–181. [9] N.A. Oz, O. Ince, B.K. Ince, Effect of wastewater composition on methanogenic activity in an anaerobic reactor, J. Environ. Sci. Heal. A39 (2004) 2941–2953. [10] C. Chang, J. Chang, S. Vigneswaran, J. Kandasamy, Pharmaceutical wastewater treatment by membrane bioreactor process – a case study in southern Taiwan, Desalination 234 (2008) 393–401. [11] N.S.A. Mutamim, Z.Z. Noor, M.A.A. Hassan, G. Olsson, Application of membrane bioreactor technology in treating high strength industrial wastewater: a performance review, Desalination 305 (2012) 1–11. [12] B. Strenn, M. Clara, O. Gans, N. Kreuzinger, Carbamazepine, diclofenac, ibuprofen and bezafibrate—investigations on the behaviour of selected pharmaceuticals during wastewater treatment, Water Sci. Technol. 50 (2004) 269–276. [13] A. Battimelli, D. Loisel, D. Garcia-Bernet, H. Carrere, J. Delgenes, Combined ozone pretreatment and biological processes for removal of colored and biorefractory compounds in wastewater from molasses fermentation industries, J. Chem. Technol. Biotechnol. 85 (2010) 968–975. [14] A. Mannucci, G. Munz, G. Mori, C. Lubello, Anaerobic treatment of vegetable tannery wastewaters: a review, Desalination 264 (2010) 1–8. [15] O. Legrini, E. Oliveros, A. Braun, Photochemical processes for water treatment, Chem. Rev. 93 (1993) 671–698. [16] S.H. Venhuis, M. Mehrvar, Photolytic treatment of aqueous linear alkylbenzenesulfonate, J. Environ. Sci. Health A 40 (2005) 1731–1739. [17] G.B. Tabrizi, M. Mehrvar, Integration of advanced oxidation technologies and biological processes: recent developments, trends, and advances, J. Environ. Sci. Health A 39 (2004) 3029–3081. [18] S. Sanches, M.T. Barreto Crespo, V.J. Pereira, Drinking water treatment of priority pesticides using low pressure UV photolysis and advanced oxidation processes, Water Res. 44 (2010) 1809–1818.

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