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The role of powdered activated carbon in enhancing the performance of membrane systems for water treatment
Desalination 225 (2008) 288–300
The role of powdered activated carbon in enhancing the performance of membrane systems for water treatment Xiang-Juan Gai, Han-Seung Kim* Department of Environmental Engineering and Biotechnology, Myongji University, San 38-2 Namdong, Yongin, Kyonggido 449-728, South Korea Tel. +82 (31) 330-6695; Fax +82 (31) 336-6336; email: [email protected]
Received 12 February 2007; accepted 12 July 2007
Abstract This study was conducted to investigate the effects of powdered activated carbon (PAC) with immersed flat sheet membrane on the efficiencies of operation and treatment and to evaluate the performance of the system. The experiments were carried out under operating conditions of an average filtration rate of 0.65 m/d, water temperature of 22–26°C, and PAC dose of 0 g/L (System A) and 20 g/L (System B). The regular influent concentrations of total organic carbon (TOC) and UV absorbance at 254 nm (UV254) were 3.186 mg/L and 1.7 1/m, respectively. TOC removal of 66.2 and 69.8%, and UV254 removal of 70.6 and 82.4% were obtained from System A and System B, respectively. During an experimental period of 65 days, the value of TMP (trans-membrane pressure) in System B increased by 16 kPa, but the value of TMP in System B reached 61 kPa after 48 days. As a result, continuous filtration time was extended by using PAC. Bacteria count in the reactor of System B showed one hundred times higher than that of System A and most of the bacteria existed in the adsorbed state on the PAC. Higher biological activity can be expected by holding PAC in the filtration reactor. This study revealed that the filtrate quality and the performance efficiency were enhanced when PAC was introduced into the filtration system. Keywords: Microfiltration; Flat sheet membrane; PAC; Bacteria; Water treatment; Organic removal
1. Introduction Membrane separation technology, which is increasingly adopted in the field of water and wastewater treatment, has shown a good performance in removing particulates or dissolved matters [1– 11]. Nevertheless, the membrane process has some *Corresponding author.
limitations, for instance, raw water needs to be pretreated and the membrane has to be cleaned periodically to avoid the decrease of flux due to membrane fouling. Membrane processes such as reverse osmosis and nanofiltration can remove most of the pollutants, including dissolved organics, but their operational costs are high because of the high energy requirements and membrane
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fouling. Microfiltration (MF) or ultrafiltration (UF) is a cost-effective option, but they cannot remove dissolved organic matter due to their relatively larger pores [4,12,13]. Therefore, a membrane process requires a combination with proper treatment processes for successful application to water reclamation and reuse. As membranes are resistant to most oxidants and tolerant to high concentrations of suspended solids, they can also be coupled with a conventional process such as oxidation, coagulation, absorption, or biological treatment for the removal of both dissolved and particulate contaminants [7,13]. A typical example is coupling with oxidation for iron and manganese removal, as described below for the Rothesay case [7], or immersion of the membranes in powdered activated slurry for the removal of natural organic matter and synthetic organic chemicals, as reported by Lebeau et al. [4]. The hybrid membrane system coupled with powdered activated carbon has been increasingly studied as an advanced treatment process due to the activated carbon’s nature to remove soluble organic contaminants by adsorption [14,15]. Adham et al. [16] have incorporated powdered activated carbon into a hollow fiber ultrafiltration system (PAC–UF) to remove total organic carbon and a representative synthetic organic chemical, 2,4,6-trichlorophenol, from natural water. Pirbazari et al. [17] combined MF and PAC adsorption to treat water contaminated with organic compounds. Seo et al. [18] conducted an experimental study on the biological activated carbon MF system for removing refractory organic matter (or persistent organic pollutants). The results showed that the system could remove 83% of total organic carbon with 20 g/L PAC dose for 64 d. Furthermore, Kim et al. [19] found that the system could consistently remove more than 95% TOC with a PAC dose of 40 g/L for 40 d from a synthetic wastewater. The previous investigations showed that the addition of PAC could provide better physical
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removal of natural organic matter (NOM) and synthetic organic compounds (SOCs), reduce the direct loading of dissolved organic pollutants onto the membrane, and prevent membrane fouling [19–21]. Immersed membranes are flexible tools that can be used for direct filtration or can be coupled with a conventional process such as oxidation, coagulation, absorption, or biological treatment for the removal of both dissolved and particulate contaminants. The level of treatment can easily be upgraded as the source water quality or the drinking water standards evolve. Immersed membranes are also very versatile; they can be used to build new plants or to upgrade existing plants by immersing the membranes directly in a clarifier or a converted sand filter [7]. This submerged membrane adsorption hybrid system (SMAHS) has many advantages. The PAC can be used for a long period. As adsorbed organics undergo biodegradation, more adsorption sites are created on the PAC surface. The submerged membranes do not become clogged as almost all organics are removed by PAC and the role of the membrane is only to retain the PAC and other suspended solids. The energy requirement is very low (as low as 0.2 kWh/m3) and there is no major sludge problem [13]. A hybrid system of a hollow fiber membrane and adsorption (PAC was used as the sorbent) was studied previously [19]. However, compared with the flat-sheet type membrane, hollow fiber membranes are more expensive, and membrane replacement costs are higher. Moreover, small tube diameters make the fibers somewhat susceptible to plug the cartridge inlet [22]. The purpose of this study is to investigate the effects of the addition of powdered activated carbon on the performance of the submerged flat-sheet membrane system. 2. Materials and method 2.1. Concept of hybrid system This hybrid system, as shown in Fig. 1, is a
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bon in the reactor. The PAC input for System B was 80 g for a reactor volume of 4 L. At the beginning of this experiment, the high strength (100 times higher than regular concentration) influent was used and reacted in the reactors for 24 h with aeration before filtration. Then, the regular concentration raw water was used as soon as filtration was started. As the special disposal (used 100 times higher concentration for 24 h before filtration) at the beginning of the experiment, there was a dilution course in the reactors of these parameters, such as TOC, UV254, turbidity and so on. Fig. 1. Schematic of the bench-scale submerged membrane system.
membrane process which consists of a combination of powdered activated carbon and microfiltration (PAC–MF). The synthetic water was prepared by diluting and mixing the stock solutions in a raw water tank. This was then fed into the reactor by a peristaltic pump, reacting and cycling in the reactor by the blower. The water was filtered off and on by a suction pump at a flux of 16–18 LMH (Lm–2h–1). The water level of the filtration tank was maintained using a level sensor which controlled the influent pump and suction pump to keep the water volume constant in the reactor. Moreover, a pressure gauge was used to measure the value of TMP. 2.2. Experiment description Aeration was carried out at the rate of 3 L/min by an air blower through a diffuser attached to the bottom of the reactor, in order to fluidize PAC and to prevent the accumulation of PAC onto the membrane. The temperature range during the operation duration was 22–26°C. The PAC was placed into the filtration tank as soon as the start of the experiment and no addition of PAC was done. In this experiment, System A was without powdered activated carbon in the reactor and System B was with 20 g/L powdered activated car-
2.3. Synthetic water The compounds and concentration of the regular concentration raw water used in this study are listed in Table 1 [19]. Five stock solutions (concentrated 1000 times) were made for five compounds, respectively, and were stored in a refrigerator before use. Each stock solution of adequate quantity (20 mL) was introduced into a raw water tank (20 L) for dilution and mixing in order to obtain the final concentration listed in Table 1. At the beginning of this experiment, high strength raw water (100 times higher in concentration than the regular synthetic raw water) was fed into both reactors, and aerated without filtration for 24 h before the start. This was done in order to investigate more clearly the role of PAC even when the PAC was exhausted by pre-loading of organic matter. Both systems were then
Table 1 Components of raw water
Organic/inorganic matter
Concentration (mg/L)
Polypeptone Yeast extract Glucose MgSO4˙7H2O MnSO4˙4H2O
10.00 5.00 1.50 0.20 0.05
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operated in a continuous mode using the synthetic raw water with regular concentration. 2.4. Powdered activated carbon (PAC) and membrane PAC (Darco KB-B, Norit) used as adsorbent in the experiments was obtained from Osung Envitech. The particle size range of the PAC was between 100 and 325 mesh, and the surface area was approximately 1500 m2/g. The MF membrane, flat-sheet polyethersulfone (PES) membrane, was obtained from Kored. The effective area was 0.04 m2 with a corresponding pore size of 0.2 μm. The dimensions (mm) were 150 w × 300 h × 6 t, with the operating method of a suction type, and the operating pressure was –65~0 kPa.
2.5. Analysis 2.5.1. Sample Samples from the influent, reactors and effluents were taken once a day during the operation period. Since samples taken from the filtration tank included some suspended solids and/or PAC, they were filtered using a 0.45 μm filter and then stored in a 100 ml glass bottle, capped and placed inside a refrigerator prior to measuring.
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2.5.3. Microfiltration resistance analysis Membrane resistance was evaluated by the resistance-in-series model as follows: RT = RM + RP + RC + G =
TMP μ⋅J
(1)
where J is a filtration flux [m3/m2/s], TMP is a trans-membrane pressure [kg/m/s2] and μ is a dynamic viscosity of water [kg/m/s]. R is a filtration resistance of membrane [m–1]. RT is total resistance, RM is membrane resistance, RP is pore blocking resistance, and RC+G is cake/gel resistance. The experimental procedure to get each resistance value was as follows [23]: (1) RT was evaluated by the final flux of wastewater microfiltration; (2) the membrane surface was then flushed with water and cleaned with a sponge to remove the fouling cake layer. After that, the DI water flux was measured again to get the resistance of RM + RP; (3) RM was estimated by measuring the water flux of de-ionized (DI) water after chemical washing (the membrane was immersed in 0.2 N NaOH and 0.4% NaOCl solution for 12 h). RC+G obtained from (1 and 2) and RP was calculated from steps (2 and 3). 3. Results and discussion 3.1. Filtration performance
2.5.2. Measure method Total organic carbon with Shimadzu TOC 5000A, UV absorbance at 254 nm (UV254) with HACH DR/4000U spectrophotometer, turbidity with HACH 2100N Turbidimeter, ammonia nitrogen (NH4+–N) with pH ISE meter and nitrite (NO2–) and nitrate (NO3–) with Alltech Model 650 Conductivity Detector (Ion Chromatography) and automatic analyzer 3 (Bran Luebbe Company) were measured for each sample. Transmembrane pressure (TMP) was checked for each run at a scheduled time interval.
One of the important parameters in membrane systems is the transmembrane pressure (TMP) which increases with the operating time. TMP control is strongly related to the efficiency of the systems. Management cost can be lowered through efficient operation such as the elongation of the filtration time reaching a certain TMP [24]. The actual duration of the experiment was 65 d and almost 1280 L of raw water was treated. The organic substances in the reactor of System B (with PAC) adsorbed onto the powdered activated carbon enlarged the particle size, aiding membrane filtration. For System A (without PAC), a cake
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fouling layer was formed on the surface, and shortened the continuous filtration time. Fig. 2. shows the profiles of TMP variation in the two systems. As the special disposal at the beginning of the experiment, the value of TMP increased fast during the first three days, and then decreased a little, because of the dilution in the reactor (without PAC). After that, it increased gradually, and the duration of the filtration was 48 d until the TMP of System A rose to 61 kPa. Then, chemical washing was performed using 0.2 N NaClO and 40% NaOH solution (submerged for 12 h). For System B, the value of TMP did not yield an obvious increase during the whole experiment and rose to 16 kPa at the end. In other words, it can run more than two months without any problem under this condition. Moreover, the TMP increased gently without any protuberance. It means that PAC worked as a buffer against the high loading. However, the membrane in the reactor without PAC needs more frequent chemical washing during the operation. Using the method reported by Meng et al. [23], filtration resistance analysis was done with further modification, as shown in Fig. 3. Of the total resistance, around 96.6% came from the cake/gel resistance (RC+G) and about 0.6% came from the membrane resistance (RM). The pore resistance membrane (Rp) in System A was 3.3%, almost twice as much as the Rp in System B (1.9%). The
results suggest that PAC decreased the pore resistance.
Fig. 2. TMP variation in the two systems.
Fig. 3. Comparison of the resistance values.
3.2. Raw water condition The average concentrations of the regular raw water were as follows: TOC 3.19 mg/L; turbidity 1.042 NTU; NH 4+–N 1.117 mg/L; and UV254 1.70 1/m. Given that the objective of this study was for water reclamation and reuse, the raw water was supposed to be made with the same water quality with that of a secondary effluent. The effluent of the MBR is one kind of secondary effluent (for membrane bioreactors (MBR), the effluent is one kind of secondary effluent). Since the water qualities of the effluent for the MBR plant [25] and the studies of a submerged nanofiltration membrane bioreactor (NF MBR) by Dockko et al. [26] are close to that of the synthetic water treated in this study, this system can be used to treat the secondary effluent from the aforementioned MBR plants. 3.3. Turbidity Water clarity is important in products destined for human consumption and in many manufacturing operations. Turbidity in water is caused by suspended and colloidal matter such as clay, slit, finely divided organic and inorganic matter, and plankton and other microscopic organisms [27].
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some bacteria grew in the reactor. Most of the bacteria were suspended in the bulk of System A, while those in System B were adsorbed on the surface of powdered activated carbon. There was some loss of suspended solids in the whole operation. The suspended solids adhered on the membrane and the reactor. Moreover, there was much pond scum (any of various freshwater algae that form a usually greenish film on the surface of stagnant water) in the reactor of System A. Some suspended solids adhered on the pond scum. 3.4. Organic matters removal Except for the unstable period, the influent concentration of TOC ranged from as low as 1.76 mg/L to as high as 5.99 mg/L, having an average influent concentration of TOC of 3.19 mg/L. The changes in the daily samples and the average values of the five samples are shown in Fig. 5. The influent concentration is not stable due to the method of making raw water (dilution and mixing) as described in 2.3. Moreover, the addition of raw water every other day may have caused particulate accumulation in the raw water tank during the two days and lead to fluctuation. At
Turbidity (NTU)
Fig. 4 shows the daily change of the turbidity in the samples, yielding a similar removal rate for the two systems. From Fig. 4a, the dilution phase for the reactor of System A lasted almost more than 40 d, but System B was not evident. According to the figure of log turbidity vs. time (figure is not shown here), the unsteady state of System B lasted to the 10th day. The average concentration of turbidity in the influent was 1.042 NTU (from 11th to 64th day). The effluent concentration for System A was 0.065 NTU and the removal rate was 93.8%, the same result was obtained in System B. They almost had the same removal capability of turbidity. In the reactor of System B, the PAC adsorbed suspended and colloidal matter. For System A, a cake fouling layer was formed on the surface and improved the removal rate of organic matter. According to the condition of the reactors, as the suspended and colloidal matters were adsorbed on PAC, the turbidity of the bulk in System B was lower than influent. On the other hand, the increase in turbidity in the reactor of System A was similar to the accumulation of TOC in the same reactor. The organic matter was supposed to be rejected into the bulk rather than to be attached onto the membrane surface. Moreover,
1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 Raw w ater
Fig. 4a. The variation of turbidity.
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Effluent (System-A)
Fig. 4b. The average turbidity.
Effluent (System-B)
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Fig. 5a. The variation of TOC concentration.
Fig. 5b. The average value of TOC.
the beginning of this experiment, the high concentration influent was used and reacted in the reactors for 24 h with aeration before filtration. Then, the regular concentration raw water was used as soon as filtration started. As a result, there was an unsteady state from the 1st to the 18th day for TOC daily change. From the 19th day, the operation became steady. The average influent concentration of TOC was 3.19 mg/L during the steady state, from the 19th to the 64th day. The average concentration of the effluent was 0.96 mg/L and the removal rate was 69.8% for System B. On the other hand, System A showed the values of 1.08 mg/L and 66.2% for each one. The removal rate of System B was slightly higher than that of System A due to the existence of PAC that adsorbed the organic substances in the reactor. For System A, a cake fouling layer was formed on the surface and improved the removal rate of organic matter; meanwhile, it was thought to shorten the continuous filtration time. This result was corresponding to the report that the removal rates were 52.0% for the membrane system without PAC and 74.4% for the membrane system with 10 g/L PAC using hollow fibers in the experiments by Kim et al. [24]. Comparison of the results from the former and this study revealed that better re-
moval in TOC was obtained by the hollow fiber membrane, but the longer continuous filtration time was obtained by the flat sheet membrane system. The different characteristics in the performance between the two systems came from the difference in the pore size of the two membranes: 0.1 μm and 0.2 μm for the hollow fiber and flat sheet, respectively. The average concentration in the reactor of System A was 4.62 mg/L, and in System B it was 1.98 mg/L. Compared with the data of effluents, it was found there was some loss of TOC concentration in the two systems. Some of them were oxygenated in the reactors, and some of them were adhered on the membranes and the reactors. In System A, TOC accumulated in the reactor during filtration. A possible reason is that the TOC was rejected into the bulk rather than being attached to the membrane surface to form a fouling layer. On the other hand, most of the TOC was adsorbed onto the surface of the PAC in System B. The capability to withstand high loading of organic matter can be promoted by integrating PAC into the system, and that more stable and safer treatment performances can be obtained. The difference between the reactor and effluent in System A was 3.54 mg/L, with 76.7% of organic car-
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bon left in the reactor. On the other hand, the difference in System B was 1.02 mg/L, with 51.6% of organic carbon left in the reactor. This difference between the two systems revealed the fouling on the membrane in the two reactors. Fig. 6 shows the daily change of UV254 in the samples during the whole operation and the average concentrations for 39 d (from the 16th to the 64th day). There was also an unsteady state for UV254. The result is not obvious in Fig. 6a, but as shown in Fig. 6b, the removal in System B was better than in System-A, showing a clearer trend than TOC removal. The average value of the influent was 1.70 1/m. The average value of the effluent for System B was 0.30 1/m and the removal rate was 82.4%. On the other hand, System A resulted to 0.50 1/m and 70.6%. The function of PAC was detected. On the aspect of the hollow fiber, values obtained were 64.0% and 89.6% for the membrane system without PAC and the membrane system with 10 g/L PAC, respectively [24]. The two types of membranes had similar removal effects of UV254. The specific ultraviolet absorbance (SUVA) value, the ratio of UV absorbance (UVA) to dissolved organic carbon (DOC), were reported to illustrate the characteristics of organic matter in
the water. There was a strong relationship between UVA and trihalomethane formation potential (THMFP). A positive correlation (0.91) between humic content and the SUVA value and high SUVA values of the samples indicated a high reactivity to form THMs. This information makes it possible to consider the SUVA value as an index of THM precursor existence [24]. Basu et al. [28] speculated that the reason for the decrease in SUVA is the physical removal of the layer humic molecules or biological degradation by the microorganisms present in the biofilter. Amy et al. [29] reported that the preferential rejection of UVA254 over DOC suggested the effective removal of hydrophobic (fulvic and humic) acids, as measured by SUVA. In this study, the TOC values of all the samples can be regarded as DOC since the effluent samples were filtered through the membrane filter of 0.2 μm while the influent and bulk samples were filtered through 0.45 μm for analysis. Water quality and removal rates for organic matter are summarized in Table 2. The average SUVA values (L/ mg/m) were calculated to be 0.53 for the influent, and 0.31 and 0.46 for the effluents of System B and System A, respectively. In comparison to the decrease of the SUVA value of 41.5% in System B,
Fig. 6a. The variation of UV254 concentration.
Fig. 6b. The average values of UV254.
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Table 2 The average concentrations and removal rates of organic matter
TOC, mg/L UV254, 1/m SUVA, L/mg/m
Raw water
Reactor (System A)
Reactor (System B)
Effluent (System A)
Effluent (System B)
Removal (System A)
Removal (System B)
3.19 1.70 0.53
4.62 1.60 0.35
1.98 0.50 0.25
1.08 0.50 0.46
0.96 0.30 0.31
66.2% 70.6% 13.2%
69.8% 82.4% 41.5%
the decrease in System A was much lower (13.2%). This implies that the formation potential of THM decreased upon addition of PAC. For the hollow fiber system, the reductions were very high in the membrane system with 10 g/L PAC and in the membrane system with 40 g/L PAC, respectively [24]. The calculated data in this experiment confirmed the viewpoint above.
Nitrogen concentrations of ammonia, nitrite and nitrate were measured for the samples from the filtration tank as well as from the influent and the effluent. It was found that the concentration of nitrogen compounds in the filtration tank was equal to that in the effluent for all samples throughout the operation which implied that all the nitro-
gen compounds were not removed by membrane filtration [24]. Fig. 7 shows the daily change of ammonia– nitrogen in the samples during the filtration period of 64 d. There was also a dilution phase for ammonia because of the high concentration influent in the first day of this experiment. After the unsteady phase (from the 1st to the 10th day), the average concentration of NH4+–N in the influent was 1.117 mg/L during the steady phase (from the 11th to the 64th day) for ammonia. The effluent concentration for System A was 1.348 mg/L, the value for the effluent of System B was 1.343 mg/L. The quantity of NH4+–N changed with the influent. Although a small amount of NO2––N was observed (data was not shown here), the effluent concentration of NO3––N did not show an obvious
Fig. 7a. The variation of NH4+–N.
Fig. 7b. The average values of NH4+–N.
3.5. Nitrogen
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Fig. 8a. The variation of NO3––N.
Fig. 8b. The average values of NO3––N.
change, compared with the influent. Fig 8 shows daily variation and average values ofNO3––N. The average concentrations of NO3––N were 3.188 mg/L for the influent, 3.136 mg/L for the effluent of System A and 3.003 mg/L for the effluent value of System B. As there was no evident increase of NO3––N, the ammonia oxidation was not proven. Firstly, this may be due to the fact that the concentration of NH4+–N in the influent was too low for the reaction. Secondly, there were only organic nitrogen sources (polypeptone and yeast extract) and not any inorganic nitrogen sources. The oxidation reaction proceeds with difficulty in these conditions. Moreover, the aerated quantity was not sufficient, thus, the aerobic reaction was not completed. In the system using a hollow fiber with 40 g/L PAC [24], the ammonia oxidation was evident, but no differences in ammonia concentration were observed between the influent and effluents for both membrane system without PAC and membrane system with 10 g/L PAC. This means that the dosing of powdered activated carbon in System B (20 g/L) was not enough to lead to the augmentation of biological activity, which conduced to the ammonia oxidation in a comparatively short operating period.
3.6. pH
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As shown in Fig.9, there was no evident effect during the dilution phase (at the beginning of the filtration); the pH values of effluents were almost not changing with influent during the whole operation. The pH value increased after treatment. The average pH value of the influent was 7.20, the values of the two reactors were 7.47 (System B) and 7.45 (System A), respectively. The effluents were 7.52 (System B) and 7.53 (System A). The effluents were a little higher than reactors, and the reactors were a little higher than the influent. However, there was no significant difference between the effluent values of the two systems. It implies that the PAC does not have any effect on the pH value in this hybrid system during the operation period. 3.7. Bacteria counting From one month bacteria counting, the following data were obtained as seen in Table 3. The data in Table 3 show that the bacteria in the reactor of System B were around one hundred times higher than those in the reactor System A. The number of bacteria, counted from the sample after sedimentation of PAC, was less than that from
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Fig. 9a. Variation of the pH value.
Fig. 9b. The average values of pH.
Table 3 Bacteria counting (CFU/mL)
Date (day) Raw water Reactor (System A) Reactor (System B) Reactor (System B) Sedimentation
43 — 1.20×104 2.10×106 —
46 — 2.00×104 7.90×105 —
52*
54 2
9.10×10 — 1.97×105 —
58 2
7.83×10 3.87×104 7.15×105 —
61 2
6.88×10 1.47×104 7.10×105 1.83×103
65 2
7.36×10 1.27×104 1.19×106 1.75×103
5.67×102 5.17×103 7.60×105 1.67×103
*The membrane of the system without PAC was chemically washed
the mixture with PAC in the reactor of System B. Therefore, most of the bacteria were thought to exist in adsorbed state on the PAC (immobilized state). This information can lead us to a conclusion that the PAC provides a good habitat for bacteria which can be expected to further remove organic substances by biological activity. 4. Conclusions This study proved that PAC is a good candidate to be combined with the membrane processes. It contributed important roles in order to enhance the performance of the submerged membrane system for water treatment process. The continuous
filtration experiments were successfully carried out with a PAC dosage of as high as 20 g/L for an experimental period of 64 d without any difficulty, while the TMP of the membrane system without PAC rose to 61 kPa after 48 d. According to the increase of TMP in this experiment, the components of the raw water and the mass balance of TOC, UV254 and turbidity above-mentioned, it is concluded that the primary component of the fouling in the System A were organic matters. Based on the results of this study, the role of PAC in the membrane system was summarizes as follows: • PAC prolonged the continuous filtration time by mitigating membrane fouling that constituted mainly of organic substances;
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• PAC worked as a buffer against high loading of organics by its adsorbing ability; • PAC provided a good habitat for bacteria, which could lead to biodegradation of the organic substances. • Integrated PAC facilitated removal of turbidity and organic matters resulting in the enhancement of the effluent quality.
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Acknowledgement This work was supported by 2005 Research Fund of Myongji University. Authors also thank Kored Inc. (Korea) for providing the membrane and Osung Envitech for PAC. References [1] A.R. Gere, Micofiltration operation costs, J. AWWA, 89(10) (1997) 40–49. [2] R.S. Yoo, D.R. Brown, R.J. Pardini and G.D. Bentson, Microfiltration: a case study, J. AWWA, 87(3) (1995) 38–49. [3] P. Côté, Application of membranes in the water industry, in Proc. Workshop on Approval of Membrane Systems for Drinking Water Treatment, NSF International, Brussels, 1997. [4] T. Lebeau, C. Lelièvre, H. Buisson, D. C1éret, L.W.V. de Venter and P. Côté, Immersed membrane filtration for the production of drinking water: combination with PAC for NOM and SOCs removal, Desalination, 117 (1998) 219–231. [5] K.-H. Ahn and K.-G. Song, Treatment of domestic wastewater using microfiltration for reuse of wastewater, Desalination, 126 (1999) 7–14. [6] G. Tchobanoglous, J. Darby, K. Bourgeous, J. McArdle, P. Genest and M. Tylla, Ultrafiltration as an advanced tertiary treatment process for municipal wastewater, Desalination, 119 (1998) 315–322. [7] P. Côté, D. Mourato, C. Güngerich, J. Russell and E. Houghton, Immersed membrane filtration for the production of drinking water: case studies, Desalination, 117 (1998) 181–188. [8] S. Panglisch, W. Dautzenberg, O. Kiepke, R. Gimbel, J. Gebel, A. Kirsch and M. Exner, Ultra- and microfiltration pilot plant investigations to treat reservoir water, Desalination, 119 (1998) 277–288. [9] S.S. Madaeni, The application of membrane tech-
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Water Sci. Technol., 35 (1997) 163–170. [21] S. Vigneswaran, B. Vigneswaran and R. Ben Aim, Application of microfiltration for water and wastewater treatment, Environmental Sanitation Reviews, 31, June, Asian Institute of Technology, Bangkok, 1991. [22] M. Cheryan, Ultrafiltration and Microfiltration Handbook. Technomic Publishing Company, USA, 1998. [23] F.G. Meng, H.M. Zhang, F.L. Yang, Y.S. Li, J.N. Xiao and X.W. Zhang, Effect of filamentous bacteria on membrane fouling in submerged membrane bioreactor, J. Membr. Sci., 272 (2006) 161–168. [24] H.-S. Kim, H. Katayama, S. Takizawa and S. Ohgaki, Development of a microfilter separation system coupled with high dose of powdered activated carbon for advanced water treatment, Desalination, 186 (2005) 215–226.
[25] G. Tao, K. Kekre, Z. Wei, T. C. Lee, B. Viswanath and H. Seah, Membrane bioreactors for water reclamation, Wat. Sci. Technol., 51(6–7) (2005) 431– 440. [26] S. Dockko and K. Yamamoto, Wastewater treatment using directly submerged nanofiltration membrane bio-reactor (NF MBR), Proc. Membrane Technology for Wastewater Reclamation and Reuse, TelAviv, Israel, 9–13 September 2001, pp. 21–31. [27] L.S. Clescerl, A.E. Greenberg and A.D. Eaton, Standard Methods for the Examination of Water and Wastewater. 20th ed., USA, 1998. [28] O.D. Basu and P.M. Huck, Integrated biofilter-immersed membrane system for the treatment of humic waters, Water Res., 38 (2004) 655–662. [29] G. Amy and J. Cho, Interactions between natural organic matter (NOM) and membranes: rejection and fouling, Water Sci. Tech., 40(9) (1999) 131–139.
The role of powdered activated carbon in enhancing the performance of membrane systems for water treatment Xiang-Juan Gai, Han-Seung Kim* Department of Environmental Engineering and Biotechnology, Myongji University, San 38-2 Namdong, Yongin, Kyonggido 449-728, South Korea Tel. +82 (31) 330-6695; Fax +82 (31) 336-6336; email: [email protected]
Received 12 February 2007; accepted 12 July 2007
Abstract This study was conducted to investigate the effects of powdered activated carbon (PAC) with immersed flat sheet membrane on the efficiencies of operation and treatment and to evaluate the performance of the system. The experiments were carried out under operating conditions of an average filtration rate of 0.65 m/d, water temperature of 22–26°C, and PAC dose of 0 g/L (System A) and 20 g/L (System B). The regular influent concentrations of total organic carbon (TOC) and UV absorbance at 254 nm (UV254) were 3.186 mg/L and 1.7 1/m, respectively. TOC removal of 66.2 and 69.8%, and UV254 removal of 70.6 and 82.4% were obtained from System A and System B, respectively. During an experimental period of 65 days, the value of TMP (trans-membrane pressure) in System B increased by 16 kPa, but the value of TMP in System B reached 61 kPa after 48 days. As a result, continuous filtration time was extended by using PAC. Bacteria count in the reactor of System B showed one hundred times higher than that of System A and most of the bacteria existed in the adsorbed state on the PAC. Higher biological activity can be expected by holding PAC in the filtration reactor. This study revealed that the filtrate quality and the performance efficiency were enhanced when PAC was introduced into the filtration system. Keywords: Microfiltration; Flat sheet membrane; PAC; Bacteria; Water treatment; Organic removal
1. Introduction Membrane separation technology, which is increasingly adopted in the field of water and wastewater treatment, has shown a good performance in removing particulates or dissolved matters [1– 11]. Nevertheless, the membrane process has some *Corresponding author.
limitations, for instance, raw water needs to be pretreated and the membrane has to be cleaned periodically to avoid the decrease of flux due to membrane fouling. Membrane processes such as reverse osmosis and nanofiltration can remove most of the pollutants, including dissolved organics, but their operational costs are high because of the high energy requirements and membrane
0011-9164/08/$– See front matter © 2008 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2007.07.009
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fouling. Microfiltration (MF) or ultrafiltration (UF) is a cost-effective option, but they cannot remove dissolved organic matter due to their relatively larger pores [4,12,13]. Therefore, a membrane process requires a combination with proper treatment processes for successful application to water reclamation and reuse. As membranes are resistant to most oxidants and tolerant to high concentrations of suspended solids, they can also be coupled with a conventional process such as oxidation, coagulation, absorption, or biological treatment for the removal of both dissolved and particulate contaminants [7,13]. A typical example is coupling with oxidation for iron and manganese removal, as described below for the Rothesay case [7], or immersion of the membranes in powdered activated slurry for the removal of natural organic matter and synthetic organic chemicals, as reported by Lebeau et al. [4]. The hybrid membrane system coupled with powdered activated carbon has been increasingly studied as an advanced treatment process due to the activated carbon’s nature to remove soluble organic contaminants by adsorption [14,15]. Adham et al. [16] have incorporated powdered activated carbon into a hollow fiber ultrafiltration system (PAC–UF) to remove total organic carbon and a representative synthetic organic chemical, 2,4,6-trichlorophenol, from natural water. Pirbazari et al. [17] combined MF and PAC adsorption to treat water contaminated with organic compounds. Seo et al. [18] conducted an experimental study on the biological activated carbon MF system for removing refractory organic matter (or persistent organic pollutants). The results showed that the system could remove 83% of total organic carbon with 20 g/L PAC dose for 64 d. Furthermore, Kim et al. [19] found that the system could consistently remove more than 95% TOC with a PAC dose of 40 g/L for 40 d from a synthetic wastewater. The previous investigations showed that the addition of PAC could provide better physical
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removal of natural organic matter (NOM) and synthetic organic compounds (SOCs), reduce the direct loading of dissolved organic pollutants onto the membrane, and prevent membrane fouling [19–21]. Immersed membranes are flexible tools that can be used for direct filtration or can be coupled with a conventional process such as oxidation, coagulation, absorption, or biological treatment for the removal of both dissolved and particulate contaminants. The level of treatment can easily be upgraded as the source water quality or the drinking water standards evolve. Immersed membranes are also very versatile; they can be used to build new plants or to upgrade existing plants by immersing the membranes directly in a clarifier or a converted sand filter [7]. This submerged membrane adsorption hybrid system (SMAHS) has many advantages. The PAC can be used for a long period. As adsorbed organics undergo biodegradation, more adsorption sites are created on the PAC surface. The submerged membranes do not become clogged as almost all organics are removed by PAC and the role of the membrane is only to retain the PAC and other suspended solids. The energy requirement is very low (as low as 0.2 kWh/m3) and there is no major sludge problem [13]. A hybrid system of a hollow fiber membrane and adsorption (PAC was used as the sorbent) was studied previously [19]. However, compared with the flat-sheet type membrane, hollow fiber membranes are more expensive, and membrane replacement costs are higher. Moreover, small tube diameters make the fibers somewhat susceptible to plug the cartridge inlet [22]. The purpose of this study is to investigate the effects of the addition of powdered activated carbon on the performance of the submerged flat-sheet membrane system. 2. Materials and method 2.1. Concept of hybrid system This hybrid system, as shown in Fig. 1, is a
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bon in the reactor. The PAC input for System B was 80 g for a reactor volume of 4 L. At the beginning of this experiment, the high strength (100 times higher than regular concentration) influent was used and reacted in the reactors for 24 h with aeration before filtration. Then, the regular concentration raw water was used as soon as filtration was started. As the special disposal (used 100 times higher concentration for 24 h before filtration) at the beginning of the experiment, there was a dilution course in the reactors of these parameters, such as TOC, UV254, turbidity and so on. Fig. 1. Schematic of the bench-scale submerged membrane system.
membrane process which consists of a combination of powdered activated carbon and microfiltration (PAC–MF). The synthetic water was prepared by diluting and mixing the stock solutions in a raw water tank. This was then fed into the reactor by a peristaltic pump, reacting and cycling in the reactor by the blower. The water was filtered off and on by a suction pump at a flux of 16–18 LMH (Lm–2h–1). The water level of the filtration tank was maintained using a level sensor which controlled the influent pump and suction pump to keep the water volume constant in the reactor. Moreover, a pressure gauge was used to measure the value of TMP. 2.2. Experiment description Aeration was carried out at the rate of 3 L/min by an air blower through a diffuser attached to the bottom of the reactor, in order to fluidize PAC and to prevent the accumulation of PAC onto the membrane. The temperature range during the operation duration was 22–26°C. The PAC was placed into the filtration tank as soon as the start of the experiment and no addition of PAC was done. In this experiment, System A was without powdered activated carbon in the reactor and System B was with 20 g/L powdered activated car-
2.3. Synthetic water The compounds and concentration of the regular concentration raw water used in this study are listed in Table 1 [19]. Five stock solutions (concentrated 1000 times) were made for five compounds, respectively, and were stored in a refrigerator before use. Each stock solution of adequate quantity (20 mL) was introduced into a raw water tank (20 L) for dilution and mixing in order to obtain the final concentration listed in Table 1. At the beginning of this experiment, high strength raw water (100 times higher in concentration than the regular synthetic raw water) was fed into both reactors, and aerated without filtration for 24 h before the start. This was done in order to investigate more clearly the role of PAC even when the PAC was exhausted by pre-loading of organic matter. Both systems were then
Table 1 Components of raw water
Organic/inorganic matter
Concentration (mg/L)
Polypeptone Yeast extract Glucose MgSO4˙7H2O MnSO4˙4H2O
10.00 5.00 1.50 0.20 0.05
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operated in a continuous mode using the synthetic raw water with regular concentration. 2.4. Powdered activated carbon (PAC) and membrane PAC (Darco KB-B, Norit) used as adsorbent in the experiments was obtained from Osung Envitech. The particle size range of the PAC was between 100 and 325 mesh, and the surface area was approximately 1500 m2/g. The MF membrane, flat-sheet polyethersulfone (PES) membrane, was obtained from Kored. The effective area was 0.04 m2 with a corresponding pore size of 0.2 μm. The dimensions (mm) were 150 w × 300 h × 6 t, with the operating method of a suction type, and the operating pressure was –65~0 kPa.
2.5. Analysis 2.5.1. Sample Samples from the influent, reactors and effluents were taken once a day during the operation period. Since samples taken from the filtration tank included some suspended solids and/or PAC, they were filtered using a 0.45 μm filter and then stored in a 100 ml glass bottle, capped and placed inside a refrigerator prior to measuring.
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2.5.3. Microfiltration resistance analysis Membrane resistance was evaluated by the resistance-in-series model as follows: RT = RM + RP + RC + G =
TMP μ⋅J
(1)
where J is a filtration flux [m3/m2/s], TMP is a trans-membrane pressure [kg/m/s2] and μ is a dynamic viscosity of water [kg/m/s]. R is a filtration resistance of membrane [m–1]. RT is total resistance, RM is membrane resistance, RP is pore blocking resistance, and RC+G is cake/gel resistance. The experimental procedure to get each resistance value was as follows [23]: (1) RT was evaluated by the final flux of wastewater microfiltration; (2) the membrane surface was then flushed with water and cleaned with a sponge to remove the fouling cake layer. After that, the DI water flux was measured again to get the resistance of RM + RP; (3) RM was estimated by measuring the water flux of de-ionized (DI) water after chemical washing (the membrane was immersed in 0.2 N NaOH and 0.4% NaOCl solution for 12 h). RC+G obtained from (1 and 2) and RP was calculated from steps (2 and 3). 3. Results and discussion 3.1. Filtration performance
2.5.2. Measure method Total organic carbon with Shimadzu TOC 5000A, UV absorbance at 254 nm (UV254) with HACH DR/4000U spectrophotometer, turbidity with HACH 2100N Turbidimeter, ammonia nitrogen (NH4+–N) with pH ISE meter and nitrite (NO2–) and nitrate (NO3–) with Alltech Model 650 Conductivity Detector (Ion Chromatography) and automatic analyzer 3 (Bran Luebbe Company) were measured for each sample. Transmembrane pressure (TMP) was checked for each run at a scheduled time interval.
One of the important parameters in membrane systems is the transmembrane pressure (TMP) which increases with the operating time. TMP control is strongly related to the efficiency of the systems. Management cost can be lowered through efficient operation such as the elongation of the filtration time reaching a certain TMP [24]. The actual duration of the experiment was 65 d and almost 1280 L of raw water was treated. The organic substances in the reactor of System B (with PAC) adsorbed onto the powdered activated carbon enlarged the particle size, aiding membrane filtration. For System A (without PAC), a cake
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fouling layer was formed on the surface, and shortened the continuous filtration time. Fig. 2. shows the profiles of TMP variation in the two systems. As the special disposal at the beginning of the experiment, the value of TMP increased fast during the first three days, and then decreased a little, because of the dilution in the reactor (without PAC). After that, it increased gradually, and the duration of the filtration was 48 d until the TMP of System A rose to 61 kPa. Then, chemical washing was performed using 0.2 N NaClO and 40% NaOH solution (submerged for 12 h). For System B, the value of TMP did not yield an obvious increase during the whole experiment and rose to 16 kPa at the end. In other words, it can run more than two months without any problem under this condition. Moreover, the TMP increased gently without any protuberance. It means that PAC worked as a buffer against the high loading. However, the membrane in the reactor without PAC needs more frequent chemical washing during the operation. Using the method reported by Meng et al. [23], filtration resistance analysis was done with further modification, as shown in Fig. 3. Of the total resistance, around 96.6% came from the cake/gel resistance (RC+G) and about 0.6% came from the membrane resistance (RM). The pore resistance membrane (Rp) in System A was 3.3%, almost twice as much as the Rp in System B (1.9%). The
results suggest that PAC decreased the pore resistance.
Fig. 2. TMP variation in the two systems.
Fig. 3. Comparison of the resistance values.
3.2. Raw water condition The average concentrations of the regular raw water were as follows: TOC 3.19 mg/L; turbidity 1.042 NTU; NH 4+–N 1.117 mg/L; and UV254 1.70 1/m. Given that the objective of this study was for water reclamation and reuse, the raw water was supposed to be made with the same water quality with that of a secondary effluent. The effluent of the MBR is one kind of secondary effluent (for membrane bioreactors (MBR), the effluent is one kind of secondary effluent). Since the water qualities of the effluent for the MBR plant [25] and the studies of a submerged nanofiltration membrane bioreactor (NF MBR) by Dockko et al. [26] are close to that of the synthetic water treated in this study, this system can be used to treat the secondary effluent from the aforementioned MBR plants. 3.3. Turbidity Water clarity is important in products destined for human consumption and in many manufacturing operations. Turbidity in water is caused by suspended and colloidal matter such as clay, slit, finely divided organic and inorganic matter, and plankton and other microscopic organisms [27].
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some bacteria grew in the reactor. Most of the bacteria were suspended in the bulk of System A, while those in System B were adsorbed on the surface of powdered activated carbon. There was some loss of suspended solids in the whole operation. The suspended solids adhered on the membrane and the reactor. Moreover, there was much pond scum (any of various freshwater algae that form a usually greenish film on the surface of stagnant water) in the reactor of System A. Some suspended solids adhered on the pond scum. 3.4. Organic matters removal Except for the unstable period, the influent concentration of TOC ranged from as low as 1.76 mg/L to as high as 5.99 mg/L, having an average influent concentration of TOC of 3.19 mg/L. The changes in the daily samples and the average values of the five samples are shown in Fig. 5. The influent concentration is not stable due to the method of making raw water (dilution and mixing) as described in 2.3. Moreover, the addition of raw water every other day may have caused particulate accumulation in the raw water tank during the two days and lead to fluctuation. At
Turbidity (NTU)
Fig. 4 shows the daily change of the turbidity in the samples, yielding a similar removal rate for the two systems. From Fig. 4a, the dilution phase for the reactor of System A lasted almost more than 40 d, but System B was not evident. According to the figure of log turbidity vs. time (figure is not shown here), the unsteady state of System B lasted to the 10th day. The average concentration of turbidity in the influent was 1.042 NTU (from 11th to 64th day). The effluent concentration for System A was 0.065 NTU and the removal rate was 93.8%, the same result was obtained in System B. They almost had the same removal capability of turbidity. In the reactor of System B, the PAC adsorbed suspended and colloidal matter. For System A, a cake fouling layer was formed on the surface and improved the removal rate of organic matter. According to the condition of the reactors, as the suspended and colloidal matters were adsorbed on PAC, the turbidity of the bulk in System B was lower than influent. On the other hand, the increase in turbidity in the reactor of System A was similar to the accumulation of TOC in the same reactor. The organic matter was supposed to be rejected into the bulk rather than to be attached onto the membrane surface. Moreover,
1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 Raw w ater
Fig. 4a. The variation of turbidity.
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Effluent (System-A)
Fig. 4b. The average turbidity.
Effluent (System-B)
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Fig. 5a. The variation of TOC concentration.
Fig. 5b. The average value of TOC.
the beginning of this experiment, the high concentration influent was used and reacted in the reactors for 24 h with aeration before filtration. Then, the regular concentration raw water was used as soon as filtration started. As a result, there was an unsteady state from the 1st to the 18th day for TOC daily change. From the 19th day, the operation became steady. The average influent concentration of TOC was 3.19 mg/L during the steady state, from the 19th to the 64th day. The average concentration of the effluent was 0.96 mg/L and the removal rate was 69.8% for System B. On the other hand, System A showed the values of 1.08 mg/L and 66.2% for each one. The removal rate of System B was slightly higher than that of System A due to the existence of PAC that adsorbed the organic substances in the reactor. For System A, a cake fouling layer was formed on the surface and improved the removal rate of organic matter; meanwhile, it was thought to shorten the continuous filtration time. This result was corresponding to the report that the removal rates were 52.0% for the membrane system without PAC and 74.4% for the membrane system with 10 g/L PAC using hollow fibers in the experiments by Kim et al. [24]. Comparison of the results from the former and this study revealed that better re-
moval in TOC was obtained by the hollow fiber membrane, but the longer continuous filtration time was obtained by the flat sheet membrane system. The different characteristics in the performance between the two systems came from the difference in the pore size of the two membranes: 0.1 μm and 0.2 μm for the hollow fiber and flat sheet, respectively. The average concentration in the reactor of System A was 4.62 mg/L, and in System B it was 1.98 mg/L. Compared with the data of effluents, it was found there was some loss of TOC concentration in the two systems. Some of them were oxygenated in the reactors, and some of them were adhered on the membranes and the reactors. In System A, TOC accumulated in the reactor during filtration. A possible reason is that the TOC was rejected into the bulk rather than being attached to the membrane surface to form a fouling layer. On the other hand, most of the TOC was adsorbed onto the surface of the PAC in System B. The capability to withstand high loading of organic matter can be promoted by integrating PAC into the system, and that more stable and safer treatment performances can be obtained. The difference between the reactor and effluent in System A was 3.54 mg/L, with 76.7% of organic car-
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bon left in the reactor. On the other hand, the difference in System B was 1.02 mg/L, with 51.6% of organic carbon left in the reactor. This difference between the two systems revealed the fouling on the membrane in the two reactors. Fig. 6 shows the daily change of UV254 in the samples during the whole operation and the average concentrations for 39 d (from the 16th to the 64th day). There was also an unsteady state for UV254. The result is not obvious in Fig. 6a, but as shown in Fig. 6b, the removal in System B was better than in System-A, showing a clearer trend than TOC removal. The average value of the influent was 1.70 1/m. The average value of the effluent for System B was 0.30 1/m and the removal rate was 82.4%. On the other hand, System A resulted to 0.50 1/m and 70.6%. The function of PAC was detected. On the aspect of the hollow fiber, values obtained were 64.0% and 89.6% for the membrane system without PAC and the membrane system with 10 g/L PAC, respectively [24]. The two types of membranes had similar removal effects of UV254. The specific ultraviolet absorbance (SUVA) value, the ratio of UV absorbance (UVA) to dissolved organic carbon (DOC), were reported to illustrate the characteristics of organic matter in
the water. There was a strong relationship between UVA and trihalomethane formation potential (THMFP). A positive correlation (0.91) between humic content and the SUVA value and high SUVA values of the samples indicated a high reactivity to form THMs. This information makes it possible to consider the SUVA value as an index of THM precursor existence [24]. Basu et al. [28] speculated that the reason for the decrease in SUVA is the physical removal of the layer humic molecules or biological degradation by the microorganisms present in the biofilter. Amy et al. [29] reported that the preferential rejection of UVA254 over DOC suggested the effective removal of hydrophobic (fulvic and humic) acids, as measured by SUVA. In this study, the TOC values of all the samples can be regarded as DOC since the effluent samples were filtered through the membrane filter of 0.2 μm while the influent and bulk samples were filtered through 0.45 μm for analysis. Water quality and removal rates for organic matter are summarized in Table 2. The average SUVA values (L/ mg/m) were calculated to be 0.53 for the influent, and 0.31 and 0.46 for the effluents of System B and System A, respectively. In comparison to the decrease of the SUVA value of 41.5% in System B,
Fig. 6a. The variation of UV254 concentration.
Fig. 6b. The average values of UV254.
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Table 2 The average concentrations and removal rates of organic matter
TOC, mg/L UV254, 1/m SUVA, L/mg/m
Raw water
Reactor (System A)
Reactor (System B)
Effluent (System A)
Effluent (System B)
Removal (System A)
Removal (System B)
3.19 1.70 0.53
4.62 1.60 0.35
1.98 0.50 0.25
1.08 0.50 0.46
0.96 0.30 0.31
66.2% 70.6% 13.2%
69.8% 82.4% 41.5%
the decrease in System A was much lower (13.2%). This implies that the formation potential of THM decreased upon addition of PAC. For the hollow fiber system, the reductions were very high in the membrane system with 10 g/L PAC and in the membrane system with 40 g/L PAC, respectively [24]. The calculated data in this experiment confirmed the viewpoint above.
Nitrogen concentrations of ammonia, nitrite and nitrate were measured for the samples from the filtration tank as well as from the influent and the effluent. It was found that the concentration of nitrogen compounds in the filtration tank was equal to that in the effluent for all samples throughout the operation which implied that all the nitro-
gen compounds were not removed by membrane filtration [24]. Fig. 7 shows the daily change of ammonia– nitrogen in the samples during the filtration period of 64 d. There was also a dilution phase for ammonia because of the high concentration influent in the first day of this experiment. After the unsteady phase (from the 1st to the 10th day), the average concentration of NH4+–N in the influent was 1.117 mg/L during the steady phase (from the 11th to the 64th day) for ammonia. The effluent concentration for System A was 1.348 mg/L, the value for the effluent of System B was 1.343 mg/L. The quantity of NH4+–N changed with the influent. Although a small amount of NO2––N was observed (data was not shown here), the effluent concentration of NO3––N did not show an obvious
Fig. 7a. The variation of NH4+–N.
Fig. 7b. The average values of NH4+–N.
3.5. Nitrogen
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Fig. 8a. The variation of NO3––N.
Fig. 8b. The average values of NO3––N.
change, compared with the influent. Fig 8 shows daily variation and average values ofNO3––N. The average concentrations of NO3––N were 3.188 mg/L for the influent, 3.136 mg/L for the effluent of System A and 3.003 mg/L for the effluent value of System B. As there was no evident increase of NO3––N, the ammonia oxidation was not proven. Firstly, this may be due to the fact that the concentration of NH4+–N in the influent was too low for the reaction. Secondly, there were only organic nitrogen sources (polypeptone and yeast extract) and not any inorganic nitrogen sources. The oxidation reaction proceeds with difficulty in these conditions. Moreover, the aerated quantity was not sufficient, thus, the aerobic reaction was not completed. In the system using a hollow fiber with 40 g/L PAC [24], the ammonia oxidation was evident, but no differences in ammonia concentration were observed between the influent and effluents for both membrane system without PAC and membrane system with 10 g/L PAC. This means that the dosing of powdered activated carbon in System B (20 g/L) was not enough to lead to the augmentation of biological activity, which conduced to the ammonia oxidation in a comparatively short operating period.
3.6. pH
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As shown in Fig.9, there was no evident effect during the dilution phase (at the beginning of the filtration); the pH values of effluents were almost not changing with influent during the whole operation. The pH value increased after treatment. The average pH value of the influent was 7.20, the values of the two reactors were 7.47 (System B) and 7.45 (System A), respectively. The effluents were 7.52 (System B) and 7.53 (System A). The effluents were a little higher than reactors, and the reactors were a little higher than the influent. However, there was no significant difference between the effluent values of the two systems. It implies that the PAC does not have any effect on the pH value in this hybrid system during the operation period. 3.7. Bacteria counting From one month bacteria counting, the following data were obtained as seen in Table 3. The data in Table 3 show that the bacteria in the reactor of System B were around one hundred times higher than those in the reactor System A. The number of bacteria, counted from the sample after sedimentation of PAC, was less than that from
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Fig. 9a. Variation of the pH value.
Fig. 9b. The average values of pH.
Table 3 Bacteria counting (CFU/mL)
Date (day) Raw water Reactor (System A) Reactor (System B) Reactor (System B) Sedimentation
43 — 1.20×104 2.10×106 —
46 — 2.00×104 7.90×105 —
52*
54 2
9.10×10 — 1.97×105 —
58 2
7.83×10 3.87×104 7.15×105 —
61 2
6.88×10 1.47×104 7.10×105 1.83×103
65 2
7.36×10 1.27×104 1.19×106 1.75×103
5.67×102 5.17×103 7.60×105 1.67×103
*The membrane of the system without PAC was chemically washed
the mixture with PAC in the reactor of System B. Therefore, most of the bacteria were thought to exist in adsorbed state on the PAC (immobilized state). This information can lead us to a conclusion that the PAC provides a good habitat for bacteria which can be expected to further remove organic substances by biological activity. 4. Conclusions This study proved that PAC is a good candidate to be combined with the membrane processes. It contributed important roles in order to enhance the performance of the submerged membrane system for water treatment process. The continuous
filtration experiments were successfully carried out with a PAC dosage of as high as 20 g/L for an experimental period of 64 d without any difficulty, while the TMP of the membrane system without PAC rose to 61 kPa after 48 d. According to the increase of TMP in this experiment, the components of the raw water and the mass balance of TOC, UV254 and turbidity above-mentioned, it is concluded that the primary component of the fouling in the System A were organic matters. Based on the results of this study, the role of PAC in the membrane system was summarizes as follows: • PAC prolonged the continuous filtration time by mitigating membrane fouling that constituted mainly of organic substances;
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• PAC worked as a buffer against high loading of organics by its adsorbing ability; • PAC provided a good habitat for bacteria, which could lead to biodegradation of the organic substances. • Integrated PAC facilitated removal of turbidity and organic matters resulting in the enhancement of the effluent quality.
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