Removal of inhibitory phenolic compounds by biological activated
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Removal of inhibitory phenolic compounds by biological activated carbon coupled membrane bioreactor Quach Thi Thu Thuy* and C. Visvanathan** * Environmental Engineering Course, Department of Urban Engineering, University of Tokyo, Japan (Email: firstname.lastname@example.org) ** Environmental Engineering and Management Program, School of Environment, Resources and Development, Asian Institute of Technology, Thailand (Email: email@example.com) Abstract Phenolic compounds cause problem for conventional treatments due to their toxic and inhibitory properties. This work investigated the treatability of phenolic compounds by using two membrane-bioreactor systems namely: activated sludge coupled with MBR (AS-MBR) and biological granular activated carbon coupled with MBR (BAC-MBR). Initially, the system was fed with phenol (500 mg/L) followed by adding 2, 4- Dichlorophenol (2,4-DCP). Phenol, 2,4-DCP, TOC and COD removal were higher than 98.99% when the organic load ranged between 1.80 to 5.76 kg/m3.d COD. In addition to MBR system development, removal mechanisms were also investigated. Relatively low value of phenol adsorption of GAC and biomass, and high maximum substrate removal rate obtaining from biokinetic experiment proved that the removals would be mainly due to biodegradation. Analysis of sludge indicated significant difference in sludge characteristics of the two reactors. The high EPS content in BAC-MBR lead to higher viscosity and poor sludge settling properties. The relationship between sludge properties and EPS components revealed that settleability had no direct correlation with EPS, though it was better correlated to protein/carbohydrate ratios. Keywords Membrane bioreactor; biological granular activated carbon; phenol; sludge properties Introduction Phenol and 2, 4-Dichlorophenol are found in wastewater from oil refineries, petrochemical units, pharmaceutical, pesticide, petrochemical plants, etc. Treatment of these toxic compounds has been studied by using sequencing batch reactors (SBR) (Young and Lant 2001), activated carbon adsorption, and SBR coupled with granular activated carbon (Buitron et al. 2001; Vininthanthrat 1999). However, in order to meet discharge standards, conventional biological treatments have their own difficulties, such as sludge deflocculation, process instability (Galli et al, 1998) resulting from inhibitory properties to microorganism of phenols. In recent years, membrane bioreactor (MBR) process has been applied widely in wastewater treatment, i.e. municipal wastewater, high molecular weight compounds, and oily wastewater (Tellez et al, 2002). High quality effluent and stability, small size of treatment unit, less sludge production, and flexibility of operation are advantages of MBR (Visvanathan et al., 2000). However, MBR are unable to remove low molecular weigh cut-off (MWCO) organic matters. Modifications of MBR have been studied to improve permeate quality. Some researchers have proposed the addition of powdered activated carbon (PAC) in MBR (Pirbazari et al., 1996). The process is so-called biological activated carbon coupled MBR (BAC-MBR). Because activated carbon would absorb low molecular weigh cut-off (MWCO) organic matters and functions as media for attached bacterial growth, BAC-MBR has proven to increase permeate quality (Matsui et al. 2001). Though BAC-MBR has been applied in varied wastewater treatment, little has been studied on toxic and inhibitory phenolic wastewater. Apart from the treatment efficiency, membrane fouling is an important aspect. The main factor causing fouling is the accumulation of bacterial cell and/or bacterial product such as extracellular polymeric substances (EPS) or soluble microbial products (SMP). Due to its hydrophobicity and condensed gel characteristics, EPS would adsorb inorganic and organic compounds in the MBR system and adhere on the membrane. It is noted that the amount and characteristic of EPS depend on bacterial activities, which is linked to bacteria species (attached or suspended growth), feed water quality and solid retention time (SRT). Thus, there might have relations between EPS with physical characteristics of sludge such as: viscosity, settleability, and dewatering. When activated carbon added to MBR, it not only adsorbs organic matter but also act as supporting media for attached bacteria growth. On the other hand, activated carbon would influence bacterial population responsible for biodegradation. However, the effects of activated carbon addition on EPS in MBR are not well reported in literature. It is envisaged that, BAC-MBR would be able to treat inhibitory phenolic compounds. With the addition of activated carbon, BAC-MBR would be different from AS-MBR in terms of permeate quality, EPS, other sludge characteristic, and fouling. The objective of this work was to investigate the treatability of wastewater containing phenolic compounds (phenol and 2, 4-DCP) in BAC-MBR and comparing it with AS-MBR. GAC adsorption, biosorption and biokinetic of sludge were also studied to investigate the removal mechanism. In addition, transmembrane pressure and factors causing fouling and physical characteristic of sludge were investigated and their inter-relations were discussed. Materials and methods Material A synthetic wastewater containing 500 mg/L Phenol as carbon source was used as a feed, followed by addition of 50 and 100 mg/L 2,4-DCP, consequently. The feed wastewater nutrient composition as follows (all the parameters in mg/L): (NH4)2SO4- 270, KH2PO4- 300, K2HPO4- 200, NaHCO3- 330, MgSO4.7H20- 500, MnSO4.H2O- 5, CaCl2- 4, FeCl3.6H2O- 5, CuSO4- 0.2, ZnSO4 - 0.1, (NH4)6Mo24.4H2O- 0.1 and CoCl2- 0.1. Phosphate buffer was used to maintain the pH around 7. Coconut shell granular activated carbon Pure 160 A was supplied by Sigma Chemical Company. U-shaped hollow fiber microfiltration (0.1µm) membrane manufactured by Mitsubishi Rayon Company, Japan (Sterapore) was used for the study. Sludge taken from industrial wastewater treatment plan was initially acclimated by fill-and-draw process by step-wise increasing the phenol concentration from 30 to 500 mg/L. Adsorption, biosorption and biokinetic Phenol adsorption kinetic of GAC and biomass were done by conduct a series adsorption tests at 120 rpm and 25oC. In biomass adsorption test, sodium azide (1%) was added in order to prevent biodegradation (Ning et al., 1996). Langmuir and Freundlich model were applied to find the suitable adsorption isotherm. The biokinetic studies were carried out in a respirometer that continuously monitored using an online DO meter (Fig.1). Acclimated sludge was used for the experiment after dilution with mineral medium consisting of 500 mg/L NaHCO3, 15 mg/L N NH4Cl and 3 mg/L 10 KH2PO4. Concentrated substrate as phenol was injected into the cell through the expansion funnel with a syringe. The initial substrate concentration to initial biomass HUB/MAU NIC concentration taken for the study was around % UTILIZATION GD R E I F JA K B L C M7 N 8 O9 TAB PRINT ENTER RUN 8 0.01-0.2 (Mathieu and Etienne 2000). All GD GD GD HELP BNC 4Mb/s GD T 2 U 3 ALPHA V0 WX Y Z . SHIFT the experiments were conducted at DO meter temperature of 20 ± 1oC using water jacket 9 at pH 7.0 ± 0.2. 10 mg/L of Allylthiourea (ATU) was used to inhibit nitrification. The Computer endogenous respiration (OURx,e), total respiration rate (OURx,t) and net oxygen 1. Respirometer cell 6. Mixing speed controller consumption (OC) were recorded. Then, 2. Water jacket 7. Expansion funnel specific growth rate (µ) and yield coefficient 3. Air diffuser 8. DO meter (Y) half velocity constants (Ks) and 4. DO probe 9. PC for data record 5. Magnetic bar 10. Air supply observed specific growth rate (µobs) were Fig.1 Schematic of respirometer calculated. Experimental set-up The Fig 2. presents the experimental set- Timer up of the MBR system. There are two reactors: BAC-MBR with GAC 5g/L Air Backwash Solenoid and AS-MBR. Reactor working volume valve Hg F Manometer of 5 L was designed with two chambers Air pressure H I (left) and II (right). A perforated baffle regulator separated the two chambers to prevent Feed Pump GAC (chamber I) direct contact with Tank Water pump membrane (chamber II). Acclimated Chamber II sludge was used as microbial seed with Level control Chamber I tank Effluent initial mix-liquor suspended solids Tank Influent Air Membrane (MLSS) of 8000 mg/L. Air was supplied MEMBRANE Diffuser module at 12 L/m2.min at the bottom of chamber BIOREACTOR II to prevent membrane fouling and maintained dissolved oxygen (DO) at Fig. 2 Experimental set-up both chambers about 2 mg/L. The GAC and/or sludge were kept at suspension form by the turbulence force of water pump equipped in chamber I. Permeate quality (COD, TOC, phenol and 2,4-DCP) were measured under different hydraulic retention times (HRTs) of 16, 12, 8 and 5 hours. Membrane fouling was also monitored by transmembrane pressures (TMP) record. Sludge characteristics were analyzed in terms of mix- liquor suspended solid (MLSS); physical characteristic (viscosity, sludge volume index (SVI, settleability), capillary suction time (CST), EPS (soluble and bound), and their protein and carbohydrate components. Analytical methods Phenol and 2,4-DCP were analyzed using gas chromatography with flame ionization detector. While measuring the EPS, the bound and soluble EPS present in the bioreactor were separated by centrifuging mixed liquor at 3,200 rpm for 30 minutes. As the supernatant represented the soluble EPS, it was analyzed as TOC (mgC/L). The centrifuged sludge was washed and re- suspended with NaCl (0.9%), then heated at 100ºC for 1 hour. Bound EPS was separated by centrifugation at 3,200 rpm for 30 minutes (Chang and Lee 1998) and measured as TOC. The protein and carbohydrate component of both bound and soluble EPS were examined. Other analyses in the study were conducted according to the procedure given in Standard Methods (APHA et al. 1998). Results and discussion Adsorption, biosorption and biokinetic Experimental data on GAC adsorption and biosorption of phenol showed that the removal was very low. Here Kf-adsorption capacity of biosorption and GAC adsorption were 10-6 and 12.3 mg/g (Fig.3), respectively. The growth pattern of bioreactor sludge and growth kinetics is presented in Fig. 4. The maximum substrate removal rate of bioreactor sludge was found to be 27.7 g COD/ g VSS.d. On the other hand, the acclimatization of sludge to phenol was effective and the sludge used in the study was well adapted to treat toxic inhibitory wastewater. Therefore, the high removal of phenol and 2,4-DCP would be mainly due to biodegradation. 250 0.4 0.35 Specific growth rate (1/h) 200 0.3 µ= µ m ax s 0.25 Ks + s 150 Q (mg/g) 0.2 µ m ax = 0.369 (h -1 ) K s = 18 (m g/L) 0.15 R 2 = 0.991 100 0.4803 y = 12.292x 0.1 2 R = 0.9671 0.05 50 0 0 50 100 150 200 250 0 Substra te c onc entra tion as C O D (mg/L) 0 100 200 300 400 500 Ce (mg/L) Fig. 3 Adsorption isotherm of phenol according Fig 4. Growth of membrane bioreactor sludge to Freudlich model (500 mg/L phenol, temp at different substrate concentration 250C, 120 rpm, 12 hours) Removal efficiency Figure 5 summarizes the removal efficiency of MBRs at different HRTs (16, 12, 8 and 5hrs). Both AS-MBR and BAC-MBR gave the phenol, TOC and COD removal more than 98.99% with the organic loading range varied from 1.80 to 5.76 kg/m3.d as COD. The treatment efficiencies decreased when HRT reduced to 5hrs at 73th day and phenol effluent in AS-MBR was higher than BAC-MBR. However, after 3 days (BAC-MBR) and 7 days (AS-MBR), the effluents were both meet discharge standard. The results confirmed that high removal efficiency given mainly by biodegradation. Thus, it could be inferred that both MBR systems are suitable for treating wastewater with high concentration of phenol and BAC-MBR was more stable for shock loading. Organic loading (kg/m3.d COD) 1.8 2.4 3.6 5.76 100 2 1.75 99.95 1.5 Effluent Phenol (mg/L) % Removal as phenol Removal BAC-MBR (%) 1.25 Removal AS-MBR (%) 99.9 1 Effluent BAC-MBR Effluent AS-MBR 0.75 Effluent Standard 99.85 0.5 0.25 99.8 0 1 6 11 16 20 24 27 35 38 45 60 73 77 80 87 Days Fig. 5 Treatment efficiency (influent phenol 500mg/L) As the phenol wastewater could be treated effectively, the study was continued further with a more toxic compound, namely the 2,4-DCP. Two concentrations (50 and 100 mg/L) of 2,4-DCP were added to feed water with Phenol. HRT and SRT were 8h and 20 days, respectively. The results showed that about 95% of 2,4-DCP and 99% of phenol could be removed in both MBR systems. This confirms that the MBR systems were effective for treating toxic wastewater containing a mixture of phenolic compounds. Membrane fouling Figure 6 show the TMP patterns in two 16 1.8 systems. Interestingly, they are AS-MBR BAC-MBR 1.6 14 different. In the BAC-MBR, TMP Permeate flux 1.4 th Flux (L/m2.h) TMP (kPa) started to increase linearly since the 26 12 1.2 day, while AS-MBR suddenly raised 1 after 63 days. Theoretically, fouling is 10 0.8 formed by the adhesion of bacteria, 8 0.6 metal ions and GAC on membrane 0.4 surface and membrane pore. The 6 0.2 driving forces of this formation are 4 0 physical (electrostatic attraction) and 1 8 14 20 26 34 40 57 70 77 82 chemical (functional group and Days hydrophobic interaction). Fig. 6. Transmembrane pressure during optimum HRT run The adhesion of bacteria to membrane surface could be a function of experimental factors, i.e. ionic strength, pH, membrane properties, and sludge characteristic. Since operation conditions, system design and feed water are the same in both MBRs, the difference in fouling could only result from sludge properties. Sludge characteristics For better understanding the difference in fouling, the sludge of MBRs were taken on the 30th (MBR-BAC started increasing TMP), 60th (MBR-AS started increasing TMP) and 90th days of the operation (both MBRs are almost the same TMP). Bound and soluble EPS and their protein and carbohydrate, SVI, CST, and viscosity, are monitored. Table 1 summarizes the experimental data. Table 1 Characteristics of membrane bioreactor sludge Parameters Units AS-MBR BAC-MBR Operation time day 30 70 90 30 70 90 SVI mL/L 68 53 66 150 123 129 CST s 14.6 38.8 101.3 48.5 153.4 377.7 Viscosity mPa.s 14.6 25.6 30.8 33.5 40.1 45.9 Soluble EPS mgC/L 12.6 14.0 20.0 12.3 16.1 20.1 Bound EPS mgC/gVSS 37.6 47.1 39.9 83.6 68.1 59.1 mgC/L 274.4 430.1 557.9 555.9 591.1 640.9 Protein/Carbohydrate - 0.97 2.72 1.75 0.86 2.13 1.32 One would see that bound EPS production (mgC/gVSS) in BAC-MBR was higher than AS-MBR 1.7 times in average. F/M ratio has effect on bound EPS production rate. High bound EPS production was achieved at high F/M ratio. However, the F/M in both reactors were similar and between 0.23 to 0.36 d-1. In BAC-MBR, bacteria are both suspended and attached growth. The growth rate in attached growth is lower than suspended one. Thus, the higher bound EPS production in BAC-MBR would be the result of different suspended bacteria. GAC addition would adsorb organic matters and develop attached growth bacteria, and thus suspended sludge would have to compete for substrate and some of them might be inhibited. Figure 7 shows the microscope images of sludge in two reactors. The BAC-MBR sludge was more dispersed and thus their flocs were smaller than that of AS-MBR. a) b) Fig.7 Microscope images of MBR sludge at 40th day a) AS-MBR sludge, b) BAC-MBR sludge Higher CST, SVI and viscosity also found in BAC-MBR. In activated sludge process, EPS acts as the bridge to aggregate bacterial cells and create floc/biofilm formation. This role of EPS enhances the settleability of sludge. However if the EPS content is too high, microbial cell is dispersed (Jenkins et al, 1993). Thus, as EPS production increased with operational time (table 1) viscosity and sludge dewatering was also increased (Fig.8). Though the major focus is towards bound EPS, little importance has been given to the EPS components (protein, carbohydrate) and its soluble form that considered as SMP. The P/C and SMP would also affect membrane fouling. It is noted that protein/carbohydrate (P/C) ratio effects on hydrophobic and surface charge value. Carbohydrate was found to have a negative influence on the hydrophobicity, while protein had a positive influence. Whereas, total EPS content in sludge does not affect hydrophobicity and surface charge of sludge (Liao et al., 2000, Urbain et al., 1992). Experimental data on SVI showing settleability had no direct correlation with EPS, though it was correlated to P/C (Fig. 9). Thus, it confirmed the effect of P/C on hydrophobicity. As adhesion of bacterial cells on membrane surface is the function of hydrophobic interaction, P/C would be important parameter in fouling study. The soluble EPS was found to gradually increase with operating time in both systems. The soluble EPS, in the mixed liquor measured as TOC, is actually the soluble microbial product (SMP) of phenol degradation. SMP mainly consists of high-molecular weight organic compounds. Thus, the accumulation of soluble EPS within the reactor was caused by membrane interception performance would form gel layer in membrane pore. 60 500 4 200 Viscosity-BAC Viscosity-AS P/C-BAC P/C-AS 450 180 50 CST-BAC CST-AS 3.5 SVI-BAC SVI-AS 400 160 3 Viscosity (mPa.s) 40 350 140 SVI (mL/g) 2.5 CST (s) 300 P/C ratio 120 30 250 2 100 200 1.5 80 20 150 60 1 100 40 10 0.5 20 50 0 0 0 0 30 70 90 30 70 90 Days Days Fig.8. Viscosity and CST correlations Fig. 9. SVI and P/C ratio correlations The sludge characteristics in two systems were found to be significantly different in EPS and its P/C, SVI, CST, and viscosity. The correlations between P/C and SVI, and EPS and CST and viscosity were obtained (Fig.8 and 9). The results would explain the higher TMP value in BAC- MBR and emphasize the role of P/C in fouling study. However, the reasons for changing sludge characteristics due to GAC addition to BAC-MBR and different TMP patterns are still not clear. The presence of attached growth bacteria might compete suspended growth one and change the microbial activities and properties. It is noted fouling is also the function of floc size distribution, which is significantly different between the two MBR systems, as presented in Fig.7. Further detailed study linking the physical and biological properties of the flocs would bring better understanding of this issue Conclusions The treatability of inhibitory phenol and 2,4-DCP by GAC-MBR were successfully investigated. Phenol adsorption capacity of GAC and sludge found to be low: 12.3 mg/gGAC and 10-6 mg/gVSS, respectively. Biokinetic coefficients of the sludge were µmax of 0.369 h-1, Ks of 18 mg/L and Yobs of 0.32 d-1 and then the maximum substrate removal rate were calculated to be 27.7 g COD/ g VSS.d. This was confirmed by high treatment efficiency of both GAC-MBR and AS-MBR. The TMP patterns were different in GAC-MBR and AS-MBR, indicating different fouling mechanisms. Study in sludge characteristic also showed EPS, SVI, CST, and viscosity were higher in BAC-MBR. Further, when the relationship between sludge properties and EPS components was investigated, there was a correlation between EPS, CST and viscosity. 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