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Research Article
Effect of Photo-Fenton Operating Conditions on the Performance of Photo-Fenton-SBR Process for Recalcitrant Wastewater Treatment

Emad S. Elmolla and Malay Chaudhuri
 
ABSTRACT
The study was undertaken to examine the effect of H2O2/Fe2+ molar ratio as one of the important operating conditions on the performance of combined photo-Fenton-SBR process for treatment of recalcitrant (antibiotic) wastewater. The SBR was fed with photo-Fenton-treated antibiotic wastewater under five H2O2/Fe2+ molar ratio (10, 20, 50, 100 and 150). The results indicated that, as H2O2/Fe2+ molar ratio decreases (increase of Fe2+ concentration), BOD5/COD ratio of the photo-Fenton-treated effluent increases and SBR and overall efficiency increases. The SBR efficiency in terms of sCOD removal was observed to be very sensitive to BOD5/COD ratio below 0.40. It decreased from 69±1% at BOD5/COD ratio of 0.49±0.01 to 44±1% at 0.19±0.02. In this study, the best H2O2/Fe2+ molar ratio for treatment of the antibiotic wastewater was observed to equal 20. At photo-Fenton-SBR operating conditions (H2O2/Fe2+ molar ratio 20, H2O2/COD molar ratio 2.5, pH 3 and 30 min photo-Fenton irradiation time and 24 h hydraulic retention time in SBR), the combined photo-Fenton-SBR efficiency (overall) was 86% as sCOD and final sCOD was 56±2 mg L-1, which meets the requirements of the discharge standard.
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Emad S. Elmolla and Malay Chaudhuri, 2010. Effect of Photo-Fenton Operating Conditions on the Performance of Photo-Fenton-SBR Process for Recalcitrant Wastewater Treatment. Journal of Applied Sciences, 10: 3236-3242.

DOI: 10.3923/jas.2010.3236.3242

URL: http://scialert.net/abstract/?doi=jas.2010.3236.3242
 
Received: September 06, 2010; Accepted: September 06, 2010; Published: October 19, 2010

INTRODUCTION

Pharmaceutical compounds including antibiotics and other drugs have been observed in the aquatic environment. These compounds have been observed in surface water (Kolpin et al., 2002; Anderson et al., 2004; Rabiet et al., 2006), ground water (Rabiet et al., 2006), sewage effluent (Carballa et al., 2004; Nikolaou et al., 2007) and even in drinking water (Stackelberg et al., 2004). Pharmaceutical compounds can reach the aquatic environment though various sources such as, pharmaceutical industry, hospital effluent and excretion from humans and livestock (Ikehata et al., 2006; Nikolaou et al., 2007; Yang et al., 2008). Before discharging antibiotic wastewater from pharmaceutical industry to the environment, antibiotic degradation should take place. Problem that may be created by the presence of antibiotics at low concentrations in the environment is the development of antibiotic resistant bacteria (Walter and Vennes, 1985). In fact, bacteria have been observed to transfer their resistance in laboratory settings as well as in the natural environment (Kanay, 1983).

Advanced Oxidation Processes (AOPs) have proved to be highly effective for the removal of many of the pollutants in wastewaters (Pera-Titus et al., 2004). Oxidation with Fenton’s reagent is based on ferrous ions, hydrogen peroxide and hydroxyl radicals produced by the catalytic decomposition of H2O2 in acidic solution (Chamarro et al., 2001). In the photo-Fenton process, additional reactions occur in the presence of light that produce hydroxyl radicals or increase the production rate of hydroxyl radicals (Pignatello et al., 1999), thus increasing the efficiency of the process.

Coupling AOPs and biological processes has received attention in recent years as a promising alternative treatment for recalcitrant wastewater. Using AOPs as pretreatment for recalcitrant wastewater is important to improve the biodegradability and produce an effluent that can be treated biologically (Sarria et al., 2002). Combined photo-Fenton-SBR process has been reported to be effective for the treatment of recalcitrant wastewater such as Cibacron Red FN-R and Procion Red H-E7B dyes (Garcia-Montano et al., 2006a, b), Diuron and Linuron herbicides (Farre et al., 2007), Laition, Metasystox, Sevnol and Ultracid pesticides (Martin et al., 2009) and sulfamethoxazole antibiotic aqueous solution (Gonzalez et al., 2009).

Sequencing Batch Reactor (SBR) is a wastewater treatment system based on the activated sludge process. The operation cycle is divided into five phases: filling, aeration-reaction, settling, decant and idle. With respect to application, SBR has been successfully employed in the biodegradation of both municipal and industrial wastewater (Mace and Mata-Alvarez, 2002).

In our previous work, degradation of amoxicillin, ampicillin and cloxacillin antibiotics in aqueous solution using Fenton (Elmolla and Chaudhuri, 2009a), photo-Fenton (Elmolla and Chaudhuri, 2009b), TiO2 photocatalysis (Elmolla and Chaudhuri, 2010a), ZnO photocatalysis (Elmolla and Chaudhuri, 2010b) were studied. In addition, technical and economic comparison among different AOPs as well as simulation of the Fenton process for treatment of an antibiotic aqueous solution were made (Elmolla and Chaudhuri, 2010c; Elmolla et al., 2010). The present study was undertaken to examine the effect of photo-Fenton operating condition H2O2/Fe2+ molar ratio on the performance of photo-Fenton-SBR process for antibiotic wastewater treatment.

MATERIALS AND METHODS

Chemicals and antibiotics: Hydrogen peroxide (30% w/w) and ferrous sulphate (FeSO4•7H2O) were purchased from R and M Marketing, Essex, UK. Analytical grade amoxicillin (AMX) was purchased from Sigma and cloxacillin (CLX) from Fluka to construct HPLC analytical curve for determination and quantification of the antibiotics. Sodium hydroxide and sulfuric acid were purchased from HACH Company, USA. Potassium dihydrogen phosphate (KH2PO4) was purchased from Fluka and acentonitrile HPLC grade from Sigma.

Analytical methods: Antibiotic concentration was determined by HPLC (Agilent 1100 Series), equipped with micro-vacuum degasser (Agilent 1100 Series), diode array and multiple wavelength detector (DAD) (Agilent 1100 Series), at wavelength 204 nm. The data was recorded by a chemistation software. The column was ZORBAX SB-C18 (4.6x150 mm, 5 μm) and the column temperature was set at 60°C. Mobile phase was made up of 55% buffer solution (0.025 M KH2PO4 in ultra purified water) and 45% acentonitrile and flow rate 0.5 mL min-1. Ions present in raw wastewater such as SO42¯ and Cl¯ were determined by an ion chromatograph (Metrohm). The eluent phase consisted of 3.2 mM Na2CO3 and 1.0 mM NaHCO3. The analytical column was METROSEP A SUPP 5-150 (4.0x150 mm, 5 μm). The flow rate was 0.7 mL min-1 and the temperature was 20°C.


Table 1: Antibiotic wastewater characteristics

Chemical Oxygen Demand (COD) was determined according to the Standard Methods (APHA, 1992). If the sample contained hydrogen peroxide (H2O2), to reduce interference in COD determination pH was increased to be above 10 to decompose hydrogen peroxide to oxygen and water (Talinli and Anderson, 1992). The pH was measured using a pH meter (HACH sension 4) and a pH probe (HACH platinum series pH electrode model 51910, HACH Company, USA). Biodegradability was measured by 5-day Biochemical Oxygen Demand (BOD5) test according to the Standard Methods (APHA, 1992). DO was measured using an YSI 5000 dissolved oxygen meter. The seed for BOD5 test was obtained from a municipal wastewater treatment plant. TOC analyzer (Model 1010; O and I analytical) was used for determining Dissolved Organic Carbon (DOC). Determination of Total Suspended Solids (TSS) and Volatile Suspended Solids (VSS) were carried out according to the Standard Methods (APHA, 1992).

Antibiotic wastewater: Antibiotic wastewater used in this study was obtained from a local antibiotic industry producing amoxicillin, ampicillin and cloxacillin. The antibiotic wastewater characteristics are summarized in Table 1.

Experimental setup and procedure: Figure 1 shows a schematic of the combined photo-Fenton-SBR batch treatment system. The treatment was accomplished in two stages, photo-Fenton process as stage 1 and aerobic Sequencing Batch Reactor (SBR) as stage 2.

Stage 1: Photo-fenton process: Batch experiments were conducted using a 2.2 L Pyrex reactor with 2000 mL of antibiotic wastewater. The required amount of iron (FeSO4•7H2O) was added to the wastewater and mixed by a magnetic stirrer to ensure complete homogeneity during the reaction. Thereafter, necessary amount of hydrogen peroxide was added to the mixture simultaneously with pH adjustment to the required value using H2SO4. The mixture was subjected to UV irradiation and the source of UV light was an UV lamp (Spectroline Model EA-160/FE, 230 volts, 0.17 amps, Spectronics Corporation, New York, USA) with nominal power of 6 W emitting radiations at wavelength ≈365 nm and it was placed above the reactor.


Fig. 1: Schematic of combined photo-Fenton-SBR treatment system

The time at which hydrogen peroxide was added to the mixture was considered the beginning of the experiment. The reaction was allowed to continue for the required time (30 min). Thereafter, pH was increased to above 10 for iron precipitation and decomposing residual H2O2 (Talinli and Anderson, 1992). Precipitated iron was separated from the reactor and the supernatant was used for SBR feeding after pH adjustment to 6.8-7.2. Samples were taken and filtered through a 0.45 μm membrane syringe filter for soluble Chemical Oxygen Demand (sCOD), Biochemical Oxygen Demand (BOD5) and Dissolved Organic Carbon (DOC) measurements and filtered through a 0.20 μm membrane syringe filter for antibiotic measurements by HPLC.

Stage 2: Aerobic Sequencing Batch Reactor (SBR): The operating liquid volume of the 2 L SBR was 1.5 L. The reactor was operated at room temperature (23±2) and equipped with an air pump and air diffuser to keep dissolved oxygen above 3 mg L-1 and stirring plate and stirrer bar for mixing purpose. Feeding and decanting were performed using two peristaltic pumps. The cycle period was 12 h and divided into five phases: filling (0.25 h), aeration (10 h), settling (1.25 h), decant (0.25 h) and idle (0.25 h). The cycle was repeated 6-9 times as necessary to allow cell acclimation and/or to obtain repetitive results. Daily analysis of soluble Chemical Oxygen Demand (sCOD) and dissolved organic carbon DOC for both influent and effluent were carried out. Concentration of Mixed Liquor Suspended Solids (MLSS) and Mixed Liquor Volatile Suspended Solids (MLVSS) were monitored throughout the operation.

Start up of SBR: The SBR was inoculated with 200 mL of aerobic sludge. The source of seed sludge was the aeration tank in the Sewage Treatment Plant (STP) at the Universiti Teknologi PETRONAS campus. Concentration of biomass in the reactor after inoculation was 2400 mg L-1. In order to acclimate the biomass, Hydraulic Retention Time (HRT) was chosen to be 2 days and photo-Fenton-treated antibiotic wastewater was mixed with domestic wastewater obtained from the STP. The feed wastewater was a mixture of photo-Fenton-treated antibiotic wastewater and domestic wastewater with mixing ratio 25%:75%, 50%:50%, 75%:25% and 100% and the acclimation period was extended to 8 days.

RESULTS AND DISCUSSION

Photo-fenton treatment: Effect of H2O2/Fe2+ molar ratio: In photo-Fenton process, iron and hydrogen peroxide are two major chemicals determining the operation cost as well as the efficiency. The effect of H2O2/Fe2+ molar ratio on sCOD and DOC removal and biodegradability (BOD5/COD ratio) are shown in Fig. 2. The operating conditions were pH 3, initial sCOD 575 mg L-1 (17.97 mM), DOC 165 mg L-1, reaction time 30 min and H2O2/COD molar ratio 2.5. To study the effect of H2O2/Fe2+ molar ratio on biodegradability improvement and mineralization, experiments were conducted at constant H2O2 concentration (44.9 mM) and varying Fe2+ concentration in the range 0.3-4.5 mM.


Fig. 2: Effect of H2O2/Fe2+ molar ratio on sCOD, BOD5 and BOD5/COD ratio for treatment of antibiotic wastewater by photo-Fenton process

The corresponding H2O2/Fe2+ and COD/H2O2/Fe2+ molar ratio were 10, 20, 50, 100 and 150 and 1.0/2.5/0.25, 1.0/2.5/0.125, 1.0/2.5/0.05, 1.0/2.5/0.025 and 1.0/2.5/0.017, respectively. The sCOD removal percent was 67±1, 67±1, 59±1, 46±1 and 30±1 at H2O2/Fe2+ molar ratio 10, 20, 50, 100 and 150, respectively. The BOD5/COD ratio was 0.48±0.01, 0.50±0.02, 0.45±0.01, 0.32±0.01 and 0.19±0.02 at H2O2/Fe2+ molar ratio 10, 20, 50, 100 and 150, respectively. The DOC removal percent was 48±2, 51±3, 45±2, 36±1 and 24±1 at H2O2/Fe2+ molar ratio 10, 20, 50, 100 and 150, respectively. It may be noted that a wastewater is considered biodegradable if the BOD5/COD ratio is 0.40 (Al-Momani et al., 2002).

The results show that sCOD and DOC removal and BOD5/COD ratio increased with decrease of H2O2/Fe2+ molar ratio up to 20. Further decrease of H2O2/Fe2+ molar ratio below 20 did not significantly improve sCOD and DOC removal and BOD5/COD ratio. This may be due to direct reaction of OH radical with metal ions at high concentration of Fe2+ as in shown in the reaction (Joseph et al., 2000).

(1)

Based on the results, it may be considered that optimal H2O2/Fe2+ molar ratio is 20 for biodegradability improvement, sCOD removal and mineralization of antibiotic wastewater.

To study the degradation of amoxicillin (AMX) and cloxacillin (CLX) in the antibiotic wastewater, an experiment was conducted under the following operating conditions (H2O2/COD molar ratio 2.5, H2O2/Fe+2 molar ratio 20 and pH 3). As shown in Fig. 3, complete degradation of amoxicillin and cloxacillin occurred in 1 min. This agrees well with the results reported by Trovo et al. (2008) on degradation of amoxicillin by the Fenton process.


Fig. 3: Degradation of AMX and CLX


Fig. 4: Performance of SBR in terms of sCOD and DOC in Case PF1-PF5

They observed 90 and 89% amoxicillin degradation in 1 min reaction in distilled water and in sewage treatment plant effluent, respectively.

Aerobic Sequencing Batch Reactor (SBR): The SBR was operated for 30 days and fed with photo-Fenton-treated antibiotic wastewater under different H2O2/Fe2+ molar ratios (Case PF1-PF5) and the performance is shown in Fig. 4. The cycle period was 24 h and divided into five phases: filling (0.25 h), aeration (22 h), settling (1.25 h), decant (0.25 h) and idle (0.25 h). Table 2 shows a summary of photo-Fenton-treated effluent and SBR effluent characteristics at different H2O2/Fe2+ molar ratios.

The H2O2/Fe2+ molar ratio 10 (Fe2+ 250 mg L-1) was considered as a starting point. The other operating conditions of the photo-Fenton process were fixed at H2O2/COD molar ratio 2.5, reaction time 30 min and pH 3 (based on preliminary experiments). The characteristics of the photo-Fenton-treated effluent (Case PF1) were sCOD 183±2, DOC 80±2 and BOD5/COD ratio 0.48±0.01. The SBR efficiency was 69±1 and 70±2% for sCOD and DOC removal, respectively. When Fe2+ concentration was reduced to 125 mg L-1 (Case PF2), the characteristics of the photo-Fenton-treated effluent were sCOD 179±2, DOC 76±3 and BOD5/COD ratio was 0.50±0.02 and SBR efficiency was 69±1 and 69±2% for sCOD and DOC removal, respectively.


Table 2: Summary of experimental results for photo-Fenton and SBR process under different H2O2/Fe2+ molar ratios


Fig. 5: Photo-Fenton, SBR and combined photo-Fenton-SBR efficiency in terms of sCOD removal under different H2O2/Fe2+ molar ratios

To continue the same trend of Fe2+ reduction, Fe2+ concentration was reduced 80% from initial value to be 50 mg L-1 (H2O2/Fe2+ molar ratio 50 Case PF3). The characteristics of the photo-Fenton-treated effluent were sCOD 225±3, DOC 85±1 and BOD5/COD ratio was 0.45±0.01 and SBR efficiency was 63±1 and 65±2% for sCOD and DOC, respectively. When H2O2/Fe2+ molar ratio increased (decreasing of Fe2+ concentration) further, the SBR efficiency decreased further as in Case PF4 and PF5 (Table 2). The results show that Fe2+ concentration or H2O2/Fe2+ molar ratio is an important parameter for the combined photo-Fenton-SBR system. Decreasing SBR efficiency with decreased Fe2+ concentration (increase H2O2/Fe2+ molar ratios) is presumably due to decrease of biodegradability below 0.4 and this indicates inhibition of the aerobic oxidation by the antibiotic intermediates (Raj and Anjaneyulu, 2005).

It is noteworthy that SBR efficiency in terms of sCOD and DOC is very sensitive to BOD5/COD ratio below 0.40. SBR efficiency in terms of sCOD removal decreased from 69±1% at BOD5/COD ratio 0.49±0.01 to 44±1% at 0.19±0.02. With regard to the combined photo-Fenton-SBR efficiency (overall efficiency) as shown in Fig. 5, the overall efficiency was 90, 90, 85, 77 and 61% at H2O2/ Fe2+ molar ratio 10, 20, 50, 100 and 150, respectively. It should be noted that the Malaysian Standards (B) set for the discharge of treated industrial wastewater into receiving water bodies (lakes and rivers) is 100 mg L-1 in terms of total COD (Malaysian Environmental Quality, 1979). Assuming that COD contribution by suspended solids is ~30 mg L-1, minimum sCOD in the final effluent should be around 70 mg L-1. It is obvious from Table 2 that discharge limit could be met by the treated antibiotic wastewater effluent subjected to combined photo-Fenton-biological treatment (Case PF1 and PF2). Based on the results, the best H2O2/Fe2+ molar ratio for the treatment of antibiotic wastewater in this study is 20 (Case PF2).

The combined efficiency achieved by photo-Fenton-SBR process was similar to those observed in the reported studies. Farré et al. (2007) reported 80% DOC removal for treatment of Diuron and Linuron pesticides by combined photo-Fenton-SBR system at H2O2/ Fe2+ molar ratio ~ 12.7, HRT 2 days and VSS 0.60±0.03 g L-1. Garcia-Montano et al. (2006a) reported 80% DOC removal for treatment of a synthetic textile effluent containing an hetero-bireactive dye (Cibacron Red FN-R, 250 mg L-1) by combined photo-Fenton-SBR system at H2O2/Fe2+ molar ratio 12.5 HRT 1 day, irradiation time 90 min and VSS 0.56±0.03 g L-1. Gonzalez et al. (2009) reported 75.7% TOC removal for treatment of a synthetic wastewater containing 200 mg L-1 sulfamethoxazole by photo-Fenton-sequencing Batch Biofilm Reactor (SBBR). The treatment conditions were 300 mg L-1 H2O2 and 10 mg L-1 Fe2+ for photo-Fenton process and HRT 8 h for SBBR.

CONCLUSIONS

The study was undertaken to examine the effect of H2O2/Fe2+ molar ratio on the performance of combined photo-Fenton-SBR process for antibiotic wastewater treatment. As H2O2/Fe2+ molar ratio decreases (increase of Fe2+), BOD5/COD ratio of the photo-Fenton-treated effluent increases and SBR and overall efficiency increases. The SBR efficiency in terms of sCOD removal was observed to be very sensitive to BOD5/COD ratio below 0.40. It decreased from 69±1% at BOD5/COD ratio 0.49±0.01 to 44±1% at 0.19±0.02. The best H2O2/Fe2+ molar ratio for treatment of the antibiotic wastewater in this study was observed to be 20. Under photo-Fenton-SBR operating conditions (H2O2/Fe2+ molar ratio 20, H2O2/COD molar ratio 2.5, pH 3 and 30 min photo-Fenton irradiation time and 24 h hydraulic retention time in SBR), the combined photo-Fenton-SBR efficiency was 90% as sCOD and the final sCOD was 56±2 mg L-1, which meets the requirements of the discharge standard (B).

ACKNOWLEDGMENT

The authors are thankful to the management and authorities of the Universiti Teknologi PETRONAS (UTP) for providing facilities for this research.

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