Journal of Environmental Sciences 26 (2014) 783–791
Available online at www.sciencedirect.com
Journal of Environmental Sciences www.jesc.ac.cn
Effect of ozone on the performance of a hybrid ceramic membrane-biological activated carbon process Jianning Guo1 , Jiangyong Hu2 , Yi Tao1 , Jia Zhu3 , Xihui Zhang1,∗ 1. Research Center for Environmental Engineering and Management, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China. E-mail: [email protected] 2. Department of Civil and Environmental Engineering, National University of Singapore, 119260, Singapore 3. School of Construction and Environmental Engineering, Shenzhen Polytechnic, Shenzhen 518055, China
article info
abstract
Article history: Received 13 May 2013 revised 14 August 2013 accepted 16 August 2013
Two hybrid processes including ozonation-ceramic membrane-biological activated carbon (BAC) (Process A) and ceramic membrane-BAC (Process B) were compared to treat polluted raw water. The performance of hybrid processes was evaluated with the removal efficiencies of turbidity, ammonia and organic matter. The results indicated that more than 99% of particle count was removed by both hybrid processes and ozonation had no significant effect on its removal. BAC filtration greatly improved the removal of ammonia. Increasing the dissolved oxygen to 30.0 mg/L could lead to a removal of ammonia with concentrations as high as 7.80 mg/L and 8.69 mg/L for Processes A and B, respectively. The average removal efficiencies of total organic carbon and ultraviolet absorbance at 254 nm (UV254 , a parameter indicating organic matter with aromatic structure) were 49% and 52% for Process A, 51% and 48% for Process B, respectively. Some organic matter was oxidized by ozone and this resulted in reduced membrane fouling and increased membrane flux by 25%–30%. However, pre-ozonation altered the components of the raw water and affected the microorganisms in the BAC, which may impact the removals of organic matter and nitrite negatively.
Keywords: biological activated carbon ceramic membrane hybrid process ozone DOI: 10.1016/S1001-0742(13)60477-5
Introduction Membrane filtration is an effective method to remove particles, microorganisms and organic matter from drinking water (Karnik et al., 2005a). Using membrane filtration technology, such as ultrafiltration or micro-filtration membrane, can therefore substitute the conventional particle separation processes. Most of the membranes used now are made of polymeric materials. However, polymeric membrane may be damaged by ozone, chlorine or potassium permanganate (Farahbakhsh et al., 2004; Zularisam et al., 2006), which are widely used as disinfectants in drinking water treatment or to improve the removals of odour and organic ∗ Corresponding
author. E-mail: [email protected]
matter. Compared with most organic membranes, ceramic membrane has enhanced thermal, chemical and structural stability (Pendergast and Hoek, 2011). It is resistant to the chemical corrosion during membrane chemical cleaning (Karnik et al., 2005b). Furthermore, ceramic membranes are ozone resistant and when combined with ozone, generate very high and stable permeate fluxes without causing membrane damage (Karnik et al., 2005b; Sartor et al., 2008; Kim et al., 2008, 2009). The use of ozone-ceramic membrane filtration process in drinking water treatment has been gaining acceptance (Laine et al., 2000) and there are pilot test (Schlichter et al., 2004) and demonstration plant (Sartor et al., 2008) running now. Various studies about ozone-ceramic membrane filtration focused on the effect of ozone on membrane flux elevation (Karnik et al., 2005a) and organics removal (Karnik et al., 2005b, 2007; Kim et al., 2009; Alpatova et al., 2013). Some
784
Journal of Environmental Sciences 26 (2014) 783–791
studies have reported that ozonation not only mineralized organic matter (Zhu et al., 2010) but also broke down large molecules into smaller ones or changed the molecular structure of natural organic matter (NOM) (Zhu et al., 2010). All of them result in reduced organic matter and particle loads of membrane and allow to achieve a high permeate flux. Although many studies have focused on the research of ozone-ceramic membrane filtration process, there are still some problems: (1) only 30%–40% of the organic matter in raw water was removed by micro-filtration or ultrafiltration alone (Sartor et al., 2008; Byun et al., 2011); (2) some micro-pollutants, such as ammonia, could not be removed by ozone-ceramic membrane process and (3) ozonation might increase the biodegradable dissolved organic carbon in the permeate (Chen, 2003), which led to the increased risk of biologically unstable drinking water. There are studies reported the new hybrid ozoneceramic membrane-activated carbon process (Schlichter et al., 2004; Sartor et al., 2008), which improved the removal percentage of organic matter to more than 80%. However, most of the organic matter removed was contributed to the adsorption effect of activated carbon. Since biological activated carbon (BAC) filtration is an effective method to remove organic matter (Reungoat et al., 2012) and ammonia (Zhang et al., 2011) from drinking water, the combination of ozone-ceramic membrane with BAC may remove the particulate matter, organic matter and ammonia, simultaneously. The biological degradation process can also greatly extend the service life of activated carbon in the filter, which can save the production cost of water treatment plant. In this work, ozone-ceramic membrane was combined with BAC filtration to form a hybrid system. The primary objective of the present study was to investigate the removal efficiencies of organic matter and ammonia of the hybrid system. To simulate the increased dissolved oxygen (DO) concentration in the membrane effluent in an ozone-
membrane-BAC process, DO in the membrane effluent was adjusted by pure oxygen aeration prior to the BAC column. The effect of DO on the removal efficiency of ammonia was also examined. In practice, the hybrid process tested in this article might be very helpful in upgrading of conventional water treatment plants. Ozone/ceramic membrane might replace coagulation, sedimentation and ozonation to remove turbidity particles and taste/odour matters, while the followed BAC unit might replace the sand filtration to remove dissolved organics and ammonia. This might be of significance in saving construction area and investment cost for upgrading of water treatment plants.
1 Materials and methods 1.1 Raw water The raw water was collected from Dongjiang River, an important water source for several large cities in Southern China. Polluted water from an urban river was added into the Dongjiang River water to simulate the seasonal pollution phenomenon. Water samples were prefiltered through a stainless sieve (150 mesh) to remove large impurities. The ammonia concentration was controlled by adjusting the ratio of the two kinds of water. The characteristics of the raw water are as following: total organic carbon (TOC) 2.3–4.9 mg/L; ultraviolet absorbance at 254 nm (UV254 ) 0.058–0.114 cm−1 , ammonia 3.7–14.6 mg/L, particle count (larger than 2 µm) 2.0 × 104 –2.6 × 104 cnt/mL, conductivity 200–310 µS/cm, turbidity 6–10 NTU, DO 4.0–5.0 mg/L, pH 7.0–8.0, temperature 22–25°C. 1.2 Hybrid ozone-ceramic membrane-BAC apparatus A schematic of the hybrid ozone-ceramic membrane-BAC system is shown in Fig. 1. Pure oxygen gas (99.99%) from a gas cylinder was fed into the ozone generator (Model-
Gas pipe
Ozone generator
Ozone absorption
Ozone contact tank
Oxygen cylinder
Valve
Pressure gauge
Flowmeter Membrane housing
Oxygen contact tank
Water pipe
Backwash water effluent
Ceramic membrane
Raw water Peristaltic pump
Centrifugal pump
Backwash water Peristaltic pump Treated water
Fig. 1
Schematic of the ozone-ceramic membrane-biological activated carbon (BAC) system.
BAC column
Journal of Environmental Sciences 26 (2014) 783–791
CFG3-10g, Guolin, China) to generate ozone. The gaseous ozone concentration was controlled by varying the voltage applied to the generator. The ozone was mixed with the raw water through a diffuser installed at the bottom of the ozone contact tank. The excess ozone was vented after passing through a 2% potassium iodide solution. A singlechannel tubular ceramic membrane (Pall, Germany) with nominal pore size of 100 nm was used in the experiment. A stainless steel centrifugal pump (SDL2-7, CNP, China) with a frequency converter was used to change the transmembrane pressure (TMP). The materials of all the pipes and valves in the system were 316L stainless steel or polytetrafluoroethylene. A chromatography column with an inner diameter of 15 mm was used as the BAC column. The BAC was collected from a pilot plant, which has been continuously run for about 1.5 years. The equivalent diameter of the crushed activated carbon was 1.1 mm. The ratio of column’s diameter to activated carbon’s was a little smaller than the suggested ratio of 30 (Letterman, 1999). Therefore, the wall effect was neglected considering that the water flow rate as low as 1.0 m/hr in BAC column. 1.3 Experimental procedure Ozone-ceramic membrane-BAC (Process A) and ceramic membrane-BAC (Process B) were tested in parallel to investigate the effect of ozone on the performance of the hybrid process. The typical characteristic of ceramic membrane and operating conditions of the hybrid system are given in Table 1. The membrane filtration was operated as a batch experiment. In Process A, the raw water was pre-ozonized in the ozone contact tank. The ozone dosage was defined as the total amount of ozone injected divided by the water volume. The ozone transfer efficiency was about 70%, which was calculated according to the amount of ozone generated and residual ozone in the off-gas. Raw water was pre-ozonized for 10 min after the injection of ozone (ozone contact time = 10 min) and then it was pumped to the membrane module. In Process B, raw water was directly pumped into the membrane without preozonation. The Table 1 Characteristic of ceramic membrane and operating conditions of the hybrid system Process
Parameters
Values
Ozone-membrane
Membrane filtering area Membrane pore size Trans-membrane pressure Filtration mode Initial membrane flux (pure water) Ozone dosage Gas flow rate Empty bed contact time Flow rate
44 cm2 100 nm 1×105 Pa Dead end 2045 L/(m2 ·hr)
BAC
2.5 mg/L 150 mL/min 25 min 2.5–3.0 mL/min
785
batch filtration was carried out with an interval of 3 days Ten liters of membrane effluent was prepared in each batch filtration with and without pre-ozonation, respectively. Once membrane flux decreased to about 200 L/(m2 ·hr), the membrane was replaced by a clean one. The contaminated membrane was cleaned with sodium hydroxide (0.5 mol/L) and nitric acid (0.3 mol/L) according to the manufacturer’s recommendation. The membrane effluent was stored in the oxygen contact tank after the batch experiment and then it was continuously pumped into the BAC column using a peristaltic pump (BT100, Longer, China) at a constant flow rate. The DO was adjusted by pure oxygen aeration. Water samples were collected from the oxygen contact tank (representing membrane effluent) and BAC effluent for the analyses of nitrogen (ammonia, nitrite and nitrate), organic matter (TOC, UV254 and molecular weight distribution (MWD)) and particulate matters. All of the samples were acidified to pH 2 and stored at 4°C except those for particulate matters. 1.4 Analytical methods An ozone monitor (Model-600, Ebara, Japan) calibrated with iodimetry method was used to measure the concentration of gaseous ozone online. Aqueous ozone was measured by spectrophotometer using indigo method (Bader and Hoign, 1981). The concentrations of ammonia, nitrite and nitrate were analyzed using an auto analyzer (TA-88, Sinsche, China). Prior to the TOC measurement, the water samples were purged with ultrapure nitrogen gas for 3 min to remove CO2 . TOC concentration was measured using a TOC analyzer (TOC-V CPH, Shimadzu, Japan). UV254 was measured by an ultraviolet-visible spectrophotometer (UV-1700, Shimadzu, Japan) using a 1-cm quartz cell. Turbidity and particulate matters were measured using a turbidity meter (2100P, HACH, USA) and a particle count analyzer (IBR, Hangzhou Grean, China), which can detect particles in the size range of 2–25 µm. The concentration of DO was measured using an Orion 3-Star DO Portable Meter (Thermo Scientific, USA). A LC-20A high performance liquid chromatograph system (Shimadzu, Japan) with a UV detector (SPD-20A, Shimadzu, Japan) was used to measure the MWD. A TSKGEL G3000SWXL gel chromatograph column (Tosoh, Japan) was used to separate the organic matter. The gel column was calibrated with sodium polystyrene sulphonate standards (17 kDa, 10 kDa, 6.8 kDa, 4.3 kDa and 210 Da, PSS Polymer Standards Service GmbH, Germany) and salicylic acid according to the method developed by Zhou et al. (2000).
2 Results and discussion 2.1 Effect of ozone on membrane fouling
786
Journal of Environmental Sciences 26 (2014) 783–791
10000 40 30 20 10 0
>2
>3
>5 >7 > 10 Particle size (μm)
> 15
Fig. 3 Effect of ozone on particulate matters in membrane effluent. Ozone dosage = 2.5 mg/L, turbidity = 14 NTU, TOC = 3.2 mg/L. All data are average of duplicates within experiments and duplicate experiments. The maximum standard deviation of the values is 10%.
factor for alleviated membrane fouling. Figure 4 shows the effects of ozone on the removals of TOC and UV254 . The TOC removed by pre-ozonation alone was 4% but it increased to 16% for UV254 . The presumed explanation is that ozonation only led to the structure transformation of organic matter, rather than mineralizing it into H2 O and CO2 . About 20% of TOC and 14% of UV254 were removed by membrane filtration alone in Process B. But in Process A, the removal efficiency decreased to 14% for TOC while it increased to 27% for UV254 . The decreased TOC removal suggested a reduced organic load of membrane and this was probably a result of the changed structure of organic matter. Ozonation can oxidize organic matter containing aromatic moieties and carbon-carbon double bonds and generate small molecule organic matter with high hydrophilicity (Kim et al., 2008, 2009; Zhu et al., 2011; Nguyen and Roddick, 2010). This led to the reduced membrane organic matter load and
Process A Process B
TOC 25 TOC and UV254 removal (%)
0.8 Normalized membrane flux
20000
30
1.0
0.6
0.2
0.1
0.0
Influent in Process A Influent in Process B Effluent in Process A Effluent in Process B
30000 Particle count (cnt/mL)
The initial membrane flux was 2045 L/(m2 ·hr) with the TMP of 0.1 MPa. As shown in Fig. 2, the membrane flux decreased sharply in the first 10 min. This was also observed in some other studies (Zhu et al., 2011; Byun et al., 2011). The sharply decrease in membrane flux was probably due to the pore blocking and fouling layer forming. From 10 to 60 min, the membrane flux decreased gradually with increasing time for both processes as the results of increasing thickness of fouling layers. At the end of the batch filtration experiment, the normalized membrane fluxes were 0.13 and 0.10 for Process A and Process B, respectively. It was clear that pre-ozonation alleviated membrane fouling. Both particulate matters (Sch¨afer et al., 2000) and NOM (Karnik et al., 2005b) are main factors affecting the membrane flux in water treatment. The effects of ozonation on particulate matters and organic matter were considered to find out the reason for alleviated membrane fouling in batch filtration experiment. Since the turbidity of membrane effluent was lower than 0.1 NTU, particle count was used to evaluate the effect of particulate matters on membrane fouling. Figure 3 shows the particle counts in the raw water and membrane effluents in two processes. Ozonation slightly increased the particles larger than 2 µm and decreased those larger than 3 µm in raw water. The increased count of small particles was a result of the decomposition of large ones, which was also reported by other study (Yan et al., 2007). Both membrane filtrations with or without pre-ozonation removed more than 99% of the particles with size larger than 2 µm. Although the particle counts in membrane effluent in Process A were a little higher than those in Process B, the difference was not statistically significant. It means that the membrane particle loads were nearly the same for Processes A and B. Therefore, the effect of ozonation on particulate matters was not the major
0
10
20
30 40 Time (min)
50
60
Fig. 2 Comparison of membrane flux in Process A (ozone dosage = 2.5 mg/L) and Process B (ozone dosage = 0 mg/L). TMP = 0.1 MPa. All data are average of triplicate experiments. The maximum standard deviation of the values is 5%.
UV254
20 15 10 5 0
Ozone
Membrane
Ozone+membrane
Fig. 4 Effect of ozone on TOC and UV254 removal by membrane. Ozone dosage = 2.5 mg/L, ozone contact time = 10 min. The maximum standard deviation of the values is 15% and 5% for TOC and UV254 , respectively.
787
Journal of Environmental Sciences 26 (2014) 783–791
Raw water Raw water+ozone
Relative intensity
2300
2000 Da
1700 380 Da 1100 500
1100 Da
-100 10
100
1000 10000 Molecular weight (dalton)
100000
Fig. 5 Effect of ozonation on molecular weigh distribution of the organic matter in raw water. Ozone dosage = 2.5 mg/L, ozone contact time = 10 min, TOC = 3.2 mg/L.
mitigated membrane fouling. The MWD of the organic matter before and after ozonation shown in Fig. 5 gives a direct explanation for the changes of molecular structure of organic matter. The proportion of organic matters with molecular weight of about 1100–2000 Da decreased while that with molecular weight of about 380 Da increased. The small molecule organic matter with high hydrophilicity generated by ozonation reduced fouling layer’s hydraulic resistance and mitigated the adsorption of NOM to membrane (Kim et al., 2009). Therefore, the altered molecular structure of organic matter was the dominant factor alleviating membrane organic fouling.
ages of ammonia were caused by (1) physical retention, which is the primary mechanism of ultrafiltration purification, is unable to remove ammonia in the form of NH+4 , and (2) the reaction rate of ozone with ammonia is very slow (Hoign et al., 1985; von Gunten, 2003). A small part of ammonia removed by membrane filtration was a result of ozonation and removal of particles with adsorbed ammonia (Shi et al., 2005). As shown in Fig. 6, after BAC filtration, the average removal percentages of ammonia were 58% and 62% for Processes A and B, respectively. About 75% of the ammonia removed by the system were contributed to BAC filtrations in both processes. But no statistically significant difference in ammonia removals was observed between the processes. The concentrations of DO and ammonia were adjusted to evaluate the removal ability of BAC. The DO concentration increased from 7.5 mg/L gradually to about 30.0 mg/L (Fig. 7) after pure oxygen aeration. Prior to oxygen aeration, the average removal percentages of ammonia were 45% and 49% for Processes A and B, respectively. The percentages increased to about 85% when DO concentrations were enhanced to 30.0 mg/L and the ammonia concentrations in the BAC effluents were 40
2.2 Removal of ammonia by membrane-BAC and ozone-membrane-BAC processes Figure 6 illustrates the variations of ammonia in Processes A and B. Membrane filtration with or without pre-ozonation could hardly remove the ammonia from raw water. The removal percentages were 3%–14% and 3%–9% for membrane filtration in Process A and B, respectively. The difference between the ammonia concentrations in the membrane effluents in tests was not significant, indicating that ozonation did not affect the ammonia removal of membrane. The low removal percent-
85% removal
67% removal
8
4
0
4
8
12 16 Time (day)
20
24
Ammonia (mg/L)
Ammonia (mg/L)
BAC
0
4
8
12 16 Time (day)
20
24
Fig. 7 Variations of DO in Process A and Process B. All data are average of triplicates within experiment. The maximum standard deviation of the values is 5%.
16
BAC
W/pure oxygen aeration
W/O aeration
0
Raw water Membrane effluent of Process A BAC effluent of Process A BAC 45% removal
0
20
10
16
12
BAC influent of Process A BAC influent of Process B BAC effluent of Process A BAC effluent of Process B
30
DO (mg/L)
2900
Raw water Membrane effluentof Process B BAC effluent of Process B
12
BAC 49% removal
8
BAC
BAC
83% removal
68% removal
4
0
0
4
8
12 16 Time (day)
20
24
Fig. 6 Variations of ammonia in Process A and Process B. All data are average of triplicates within experiment. The maximum standard deviation of the values is 5%.
788
Journal of Environmental Sciences 26 (2014) 783–791
lower than 1.00 mg/L (day 16–19). After the ammonia concentrations in raw water increased to 13.5–14.5 mg/L (day 20–25), the average ammonia concentrations in BAC effluents increased to 4.38 mg/L in Process A and 4.55 mg/L in Process B. The amounts of ammonia removed by BAC (day 20–25) were about 7.80 mg/L and 8.69 mg/L for Processes A and B, respectively. It was reasonable to presume that most of the ammonia was removed by organism because the activated carbon had been used for more than one year. The relationship between the DO consumed and the ammonia removed by BAC was also calculated. The theoretical mass ratio of N and O is 1:4.57 if O is the only electron acceptor in the transformation of ammonia to nitrate. The average mass ratios of N and O in Processes A and B were 1:4.1 and 1:3.8 during the experiment, respectively. The lower mass ratios of N and O probably means that part of the ammonia was not transformed to nitrate but nitrite, which needs less O during the transformation. This was consistent with the result reported by Yu et al. (2007). 2.3 Effect of pre-ozonation on the variations of nitrite and nitrate in membrane-BAC process The variations of nitrite and nitrate in Processes A and B are shown in Fig. 8. The average nitrite concentration in the raw water was lower than 0.38 mg/L. Membrane filtration
alone could not remove nitrite effectively. By using preozonation as the pre-treatment in Process A, about 20%– 50% of nitrite was removed because of oxidation by ozone. There was no nitrite accumulated in the BAC effluents on the premise of the ammonia concentration in raw water lower than 4.5 mg/L (day 0–12) without oxygen aeration (day 0–15). However, accumulations of nitrite in the BAC effluents in Processes A and B were observed after the ammonia concentration in raw water increased to 6.5 mg/L (day 12–15), and the corresponding nitrite concentrations in the BAC effluents increased to 0.95 mg/L and 1.26 mg/L, respectively. The accumulation of nitrite in BAC effluents means that the ammonia was not fully transformed to nitrate. As shown in Fig. 7, the DO concentration in membrane effluent increased to about 30 mg/L after oxygen aeration (day 16–25) and no nitrite was detected in the BAC effluent in Process B (day 16–19). However, the nitrite concentration in Process A continuously increased to 1.63 mg/L, although DO concentrations in both processes were nearly the same. The nitrite concentrations in BAC effluents increased in both process under the condition of high ammonia concentration (day 20–25). The highest concentration of nitrite in BAC effluent in Process A was 5.23 mg/L, which was about four times higher than that in Process B. The variations of nitrate concentrations also
6
6 Raw water Membrane effluent of Process A BAC effluent of Process A
4 3 2 1
0
4
8
12 16 Time (day)
20
3 2
0
24
12
0
4
8
12 16 Time (day)
20
24
12 Raw water Membrane effluent of Process A BAC effluent of Process A
8 W/O aeration
6 4
Raw water Membrane effluent of Process B BAC effluent of Process B
10
W/pure oxygen aeration
2
Nitrate (mg/L)
10 Nitrate (mg/L)
4
1
0
0
Raw water Membrane effluent of Process B BAC effluent of Process B
5 Nitrite (mg/L)
Nitrite (mg/L)
5
8 W/O aeration
6
W/pure oxygen aeration
4 2
0
4
8
12 16 Time (day)
20
24
0
0
4
8
12 16 Time (day)
20
24
Fig. 8 Variations of nitrite and nitrate in Process A and Process B. All data are average of triplicates within experiment. The maximum standard deviation of the values is 5%.
789
Journal of Environmental Sciences 26 (2014) 783–791
Table 2 Total nitrogen mass balance in membrane effluent and BAC effluent Total nitrogen
Membrane effluent (mg/L)
BAC effluent (mg/L)
ANOVA
Progress A Progress B
7.96 ± 2.68 7.79 ± 2.68
7.88 ± 2.88 7.94 ± 2.88
p > 0.1 p > 0.1
Total nitrogen is the sum of ammonia, nitrite and nitrate nitrogen; average value of total nitrogen for the 26 days experiment; ANOVA for the difference between membrane effluent and BAC effluent, n = 52.
8.40
8.08 7.90
7.81
7.80
7.60
7.57
BAC effluent of Process B
7.86
8.00
BAC effluent of Process A
8.20
pH 7.60 7.40
Membrane effluent of Process B
Membrane effluent of Process A
7.00
Ozonation alone
7.20 Raw water
illustrate that there was a significant difference between the nitrate concentration in BAC effluents in Process A and B (ANOVA, n = 50, p = 0.066). Compared with Process A, more nitrate was generated by BAC in Process B. Nitrification is the main mechanism for the removal of ammonia in drinking water treatment with BAC (Yapsakli et al., 2010). The mass balance of total nitrogen (sum of ammonia, nitrite and nitrate nitrogen) in the membrane effluent and the BAC effluent is given in Table 2. The results showed that there were no significant differences in the average concentrations of the total nitrogen in membrane effluents and BAC effluents in both processes. There was no nitrogen loss in the BAC system, indicating that only nitrification occurred in the BAC. The dissimilarities between nitrite and nitrate concentrations in the BAC effluents (day 16–25) were probably caused by the different activities of nitrifying bacteria in Process A and B. Ammonia oxidizing bacteria and nitrite oxidizing bacteria play important roles in nitrogen transformation (Yapsakli et al., 2010). The accumulation of nitrite in Process A indicated that the converting nitrite into nitrate was inhibited. The only difference between the two processes were the pre-ozonation in Process A. Sun et al. (2007) reported that pre-ozonation with an ozone dosage of 2 mg/L decreased the biomass in the BAC, even in the deeper layer of the BAC bed (100 and 160 cm). Generally, no residual ozone exists in the deeper layer of BAC bed. This means that the microorganisms in BAC were not affected by ozone directly, but by some other factors (such as NOM, pH and particulate matters) altered by ozonation. As shown in Figs. 4 and 5, pre-ozonation altered the molecular structure of NOM and increased the TOC concentration in membrane effluent in Process A. Yapsakli et al. (2010) found although no residual ozone existed in BAC influent, pre-ozonation with 2 mg O3 /mg TOC changed the composition of raw water and affected the ammonia oxidizing bacteria species in the BAC. It is reasonable to presume that pre-ozonation affected the nitrifying bacteria by changing the characteristics of raw water. As shown in Fig. 9, membrane filtration alone increased the pH of membrane effluent in Process B. Preozonation decreased the pH of raw water and this caused a lower pH of membrane effluent in Process A. Most of the nitrifying bacteria are sensitive to pH (Villaverde et al.,
Fig. 9 Variations of pH in Process A and Process B. All data are average of triplicates within experiment. The maximum standard deviation of the values is 5%.
1997) and nitrification consumed alkalinity and decreased the pH of BAC effluent. The higher pH of membrane effluent in Process B was beneficial for nitrifying bacteria. 2.4 Removals of TOC and UV254 by membrane-BAC and ozone-membrane-BAC processes The effect of pre-ozonation on the removals of TOC and UV254 is shown in Fig. 10. The TOC removed by ozonation alone was less than 5% and it was not discussed here. The average percentages of TOC removed by membrane filtration in Process A and B were 14% and 20%, respectively. Ozonation decreased the removal of TOC in Process A. This occurred as a result of oxidation of organic matter by ozone. Ozonation of organic matter decreased the molecular weight of the organic matter and increased its hydrophylicity, which caused the decreased membrane retention. However, the difference between the TOC concentrations in membrane effluents was not significant, indicating that ozonation with a dosage of 2.5 mg/L had no significant effect on the TOC removal of ceramic membrane, but only led to a slightly increase in TOC concentration in membrane effluent. Both the BAC in Process A and B improved the removals of TOC. As shown in Fig. 10, the average removal percentages of TOC were 49% and 51% for Process A and B, respectively. About 71% of the TOC removed by the system was contributed by BAC in Process A and about 63% in Process B. Since the BAC has been saturated by organic matter, most of the organic matter removed was degraded by microorganisms in BAC. Pre-ozonation had no significant effect on the removal of TOC during the experiment period without oxygen aeration (day 0–15). However, a statistically significant decrease (ANOVA, n
790
Journal of Environmental Sciences 26 (2014) 783–791
0.12
6 Raw water Membrane effluent of Process A Membrane effluent of Process B BAC effluent of Process A BAC effluent of Process B
TOC (mg/L)
4
0.10 0.08 UV254 (cm-1)
5
3 2 1
0.06 0.04 0.02
W/pure oxygen aeration
W/O aeration 0
Raw water Membrane effluent of Process A Membrane effluent of Process B BAC effluent of Process A BAC effluent of Process B
0
4
8
12 Time (day)
16
20
24
W/pure oxygen aeration
W/O aeration 0.00 0
4
8
12 16 Time (day)
20
24
Fig. 10 Variations of TOC and UV254 in Process A and Process B. The maximum standard deviation of the values for TOC is 15% and 5% for UV254 .
= 20, p = 0.086) in TOC concentration in Process B was found during the experiment period with oxygen aeration (day 16–25). Both of the processes have the same operating condition except pre-ozonation in Process A. Thus, the phenomenon illustrated that microorganisms in Process A were indirectly affected by pre-ozonation (Yapsakli et al., 2010). It appears that the effect was more obvious under high DO condition, under which the negative effect of ozonation on the microorganisms was magnified. The variations of UV254 in Process A and B. UV254 was removed from 0.077 to 0.065 cm−1 by membrane filtration alone in Process B, with the removal percentage of 15%. Process A removed UV254 to 0.056 cm−1 , with the removal percentage of 27%, nearly two times higher than that in Process B. The UV254 represents the organic matter containing aromatic component or carbon-carbon double bonds, which can react with ozone quickly and thus improves the UV254 removal by membrane in Process A. The following BAC increased the removal percentage of UV254 to 52% in Process A and 48% in Process B. It indicates pre-ozonation improved the overall removal percentage of UV254 in Process A. The UV254 concentration in BAC effluent in Process A was significantly lower (ANOVA, n = 32, p = 0.054) than that in Process B during the period without oxygen aeration (day 0–15). However, the difference between the UV254 in BAC effluents became insignificant during the period with oxygen aeration (day 16–25). One explanation was that the microorganisms in the BAC was affected by ozonation indirectly and this reduced the removal of UV254 in Process A. Alternatively, it was also explained by the lower UV254 concentration in the membrane effluent in Process A, which decreased the removal efficiency of UV254 by BAC.
3 Conclusions This study demonstrates that pre-ozonation in ceramic membrane-BAC system resulted in an improvement in
membrane flux by 30%. The alleviated membrane fouling is mainly due to the oxidation of organic matter by ozonation. Ceramic membrane-BAC processes with or without ozonation removed more than 99% of the particles in the size range of 2–15 µm. BAC filtration greatly improved the removal of ammonia. In addition, the increase of DO in the membrane effluent played an important role in the ammonia removal. However, pre-ozonation changed the composition of raw water and caused some negative effects on the activities of microorganisms in BAC. The activities of the nitrite oxidizing bacteria in Process A were inhibited and this led to the accumulation of nitrite in the BAC effluent. The overall removal percentages of TOC and UV254 increased greatly after BAC filtration, suggesting that BAC is a good option to compensate the low removal efficiency of organics by ceramic membrane. Pre-ozonation improved the overall removal of UV254 in Process A. This study gave some new insights into the ozone-ceramic membrane-BAC process. It may improve the application of ceramic membrane in drinking water treatment, especially in the cases that ammonia contaminated source water is considered. However, the indirect effect of ozonation on the microorganisms in BAC in the hybrid system need to be further investigated. Acknowledgments This work was supported by the National Grand Water Project (No. 2008ZX07423-002), the National Natural Science Foundation of China (No. 50978170) and the Guangdong Provincial Funding (No. 2012B030800001).
references Alpatova, A.L., Davies, S.H., Masten, S.J., 2013. Hybrid ozonationceramic membrane filtration of surface waters: The effect of water characteristics on permeate flux and the removal of DBP precursors, dicloxacillin and ceftazidime. Separ. Purif. Technol. 107, 179–186. Bader, H., Hoign, J., 1981. Determination of ozone in water by the indigo
Journal of Environmental Sciences 26 (2014) 783–791
method. Water Res. 15(4), 449–456. Byun, S., Davies, S.H., Alpatova, A.L., Corneal, L.M., Baumann, M.J., Tarabara, V.V. et al., 2011. Mn oxide coated catalytic membranes for a hybrid ozonation-membrane filtration: Comparison of Ti, Fe and Mn oxide coated membranes for water quality. Water Res. 45(1), 163–170. Chen K.C., 2003. Ozonation, Ultrafilration, and Biofiltration for the Control of NOM and DBP in Drink Water. East Lansing: Michigan State University. Farahbakhsh, K., Svrcek, C., Guest, R.K., Smith, D.W., 2004. A review of the impact of chemical pretreatment on low-pressure water treatment membranes. J. Environ. Eng. Sci. 3(4), 237–253. Hoign, J., Bader, H., Haag, W.R., Staehelin, J., 1985. Rate constants of reactions of ozone with organic and inorganic compounds in water–III. Inorganic compounds and radicals. Water Res. 19(8), 993–1004. Karnik, B.S., Davies, S.H., Baumann, M.J., Masten, S.J., 2005a. The effects of combined ozonation and filtration on disinfection byproduct formation. Water Res. 39(13), 2839–2850. Karnik, B.S., Davies, S.H., Baumann, M.J., Masten, S.J., 2007. Removal of Escherichia coli after treatment using ozonation-ultrafiltration with iron oxide-coated membranes. Ozone-Sci. Eng. 29(2), 75–84. Kim, J., Davies, S., Baumann, M.J., Tarabara, V.V., Masten, S.J., 2008. Effect of ozone dosage and hydrodynamic conditions on the permeate flux in a hybrid ozonation-ceramic ultrafiltration system treating natural waters. J. Membr. Sci. 311(1-2), 165–172. Karnik, B.S., Davies, S., Chen, K.C., Jaglowski, D.R., Baumann, M.J., Masten, S.J., 2005b. Effects of ozonation on the permeate flux of nanocrystalline ceramic membranes. Water Res. 39(4), 728–734. Kim, J., Shan, W.Q., Davies, S., Baumann, M.J., Masten, S.J., Tarabara, V.V., 2009. Interactions of aqueous NOM with nanoscale TiO2 : Implications for ceramic membrane filtration-ozonation hybrid process. Environ. Sci. Technol. 43(14), 5488–5494. Laˆın´e, J.M., Vial, D., Moulart, P., 2000. Status after 10 years of operation – Overview of UF technology today. Desalination 131(1-3), 17–25. Letterman R.D., 1999. Water Quality and Treatment: A Handbook of Community Water Supplies. McGraw-Hill, New York. Nguyen, S.T., Roddick, F.A., 2010. Effects of ozonation and biological activated carbon filtration on membrane fouling in ultrafiltration of an activated sludge effluent. J. Membr. Sci. 363(1-2), 271–277. Pendergast, M.M., Hoek, E., 2011. A review of water treatment membrane nanotechnologies. Energy Environ. Sci. 4(6), 1946–1971. Reungoat, J., Escher, B.I., Macova, M., Argaud, F.X., Gernjak, W., Keller, J., 2012. Ozonation and biological activated carbon filtration of wastewater treatment plant effluents. Water Res. 46(3), 863–872. Sartor, M., Schlichter, B., Gatjal, H., Mavrov, V., 2008. Demonstration
791
of a new hybrid process for the decentralised drinking and service water production from surface water in Thailand. Desalination 222(1-3), 528–540. Sch¨afer, A.I., Schwicker, U., Fischer, M.M., Fane, A.G., Waite, T.D., 2000. Microfiltration of colloids and natural organic matter. J. Membr. Sci. 171(2), 151–172. Schlichter, B., Mavrov, V., Chmiel, H., 2004. Study of a hybrid process combining ozonation and microfiltration/ultrafiltration for drinking water production from surface water. Desalination 168, 307–317. Shi, H.X., Liu, H.J., Qu, J.H., Dai, R.H., Wang, M.H., 2005. Property of adsorption about ammonium on suspended mineral matters in eutrophic water. Environ. Sci. 26(5), 72–76. Sun, G.F., Qiao, T.J., Liu, X.F., Luo, H.X., 2007. Variation and effect of biomass in biologically activated carbon process. Technol. Water Treat. 33(7), 44–47. Villaverde, S., GarciaEncina, P.A., FdzPolanco, F., 1997. Influence of pH over nitrifying biofilm activity in submerged biofilters. Water Res. 31(5), 1180–1186. von Gunten, U., 2003. Ozonation of drinking water: Part I. Oxidation kinetics and product formation. Water Res. 37(7), 1443–1467. Yan, M., Wang, D., Shi, B., Wang, M., Yan, Y., 2007. Effect of preozonation on optimized coagulation of a typical North China source water. Chemosphere 69(11), 1695–1702. Yapsakli, K., Mertoglu, B., Cecen, F., 2010. Identification of nitrifiers and nitrification performance in drinking water biological activated carbon (BAC) filtration. Proce. Biochem. 45(9), 1543–1549. Yu, X., Qi, Z.H., Zhang, X.J., Yu, P., Liu, B., Zhang, L.M. et al., 2007. Nitrogen loss and oxygen paradox in full-scale biofiltration for drinking water treatment. Water Res. 41(7), 1455–1464. Zhang, D., Li, W., Zhang, S., Liu, M., Zhao, X., Zhang, X., 2011. Bacterial community and function of biological activated carbon filter in drinking water treatment. Biom. Environ. Sci. 24(2), 122– 131. Zhou, Q.H., Cabaniss, S.E., Maurice, P.A., 2000. Considerations in the use of high-pressure size exclusion chromatography (HPSEC) for determining molecular weights of aquatic humic substances. Water Res. 34(14), 3505–3514. Zhu, B., Hu, Y.X., Kennedy, S., Milne, N., Morris, G., Jin, W.Q. et al., 2011. Dual function filtration and catalytic breakdown of organic pollutants in wastewater using ozonation with titania and alumina membranes. J. Membr. Sci. 378(1-2), 61–72. Zhu, H.T., Wen, X.H., Huang, X., 2010. Membrane organic fouling and the effect of pre-ozonation in microfiltration of secondary effluent organic matter. J. Membr. Sci. 352(1-2), 213–221. Zularisam, A.W., Ismail, A.F., Salim, R., 2006. Behaviours of natural organic matter in membrane filtration for surface water treatment-a review. Desalination 194(1-3): 211–231.
Available online at www.sciencedirect.com
Journal of Environmental Sciences www.jesc.ac.cn
Effect of ozone on the performance of a hybrid ceramic membrane-biological activated carbon process Jianning Guo1 , Jiangyong Hu2 , Yi Tao1 , Jia Zhu3 , Xihui Zhang1,∗ 1. Research Center for Environmental Engineering and Management, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China. E-mail: [email protected] 2. Department of Civil and Environmental Engineering, National University of Singapore, 119260, Singapore 3. School of Construction and Environmental Engineering, Shenzhen Polytechnic, Shenzhen 518055, China
article info
abstract
Article history: Received 13 May 2013 revised 14 August 2013 accepted 16 August 2013
Two hybrid processes including ozonation-ceramic membrane-biological activated carbon (BAC) (Process A) and ceramic membrane-BAC (Process B) were compared to treat polluted raw water. The performance of hybrid processes was evaluated with the removal efficiencies of turbidity, ammonia and organic matter. The results indicated that more than 99% of particle count was removed by both hybrid processes and ozonation had no significant effect on its removal. BAC filtration greatly improved the removal of ammonia. Increasing the dissolved oxygen to 30.0 mg/L could lead to a removal of ammonia with concentrations as high as 7.80 mg/L and 8.69 mg/L for Processes A and B, respectively. The average removal efficiencies of total organic carbon and ultraviolet absorbance at 254 nm (UV254 , a parameter indicating organic matter with aromatic structure) were 49% and 52% for Process A, 51% and 48% for Process B, respectively. Some organic matter was oxidized by ozone and this resulted in reduced membrane fouling and increased membrane flux by 25%–30%. However, pre-ozonation altered the components of the raw water and affected the microorganisms in the BAC, which may impact the removals of organic matter and nitrite negatively.
Keywords: biological activated carbon ceramic membrane hybrid process ozone DOI: 10.1016/S1001-0742(13)60477-5
Introduction Membrane filtration is an effective method to remove particles, microorganisms and organic matter from drinking water (Karnik et al., 2005a). Using membrane filtration technology, such as ultrafiltration or micro-filtration membrane, can therefore substitute the conventional particle separation processes. Most of the membranes used now are made of polymeric materials. However, polymeric membrane may be damaged by ozone, chlorine or potassium permanganate (Farahbakhsh et al., 2004; Zularisam et al., 2006), which are widely used as disinfectants in drinking water treatment or to improve the removals of odour and organic ∗ Corresponding
author. E-mail: [email protected]
matter. Compared with most organic membranes, ceramic membrane has enhanced thermal, chemical and structural stability (Pendergast and Hoek, 2011). It is resistant to the chemical corrosion during membrane chemical cleaning (Karnik et al., 2005b). Furthermore, ceramic membranes are ozone resistant and when combined with ozone, generate very high and stable permeate fluxes without causing membrane damage (Karnik et al., 2005b; Sartor et al., 2008; Kim et al., 2008, 2009). The use of ozone-ceramic membrane filtration process in drinking water treatment has been gaining acceptance (Laine et al., 2000) and there are pilot test (Schlichter et al., 2004) and demonstration plant (Sartor et al., 2008) running now. Various studies about ozone-ceramic membrane filtration focused on the effect of ozone on membrane flux elevation (Karnik et al., 2005a) and organics removal (Karnik et al., 2005b, 2007; Kim et al., 2009; Alpatova et al., 2013). Some
784
Journal of Environmental Sciences 26 (2014) 783–791
studies have reported that ozonation not only mineralized organic matter (Zhu et al., 2010) but also broke down large molecules into smaller ones or changed the molecular structure of natural organic matter (NOM) (Zhu et al., 2010). All of them result in reduced organic matter and particle loads of membrane and allow to achieve a high permeate flux. Although many studies have focused on the research of ozone-ceramic membrane filtration process, there are still some problems: (1) only 30%–40% of the organic matter in raw water was removed by micro-filtration or ultrafiltration alone (Sartor et al., 2008; Byun et al., 2011); (2) some micro-pollutants, such as ammonia, could not be removed by ozone-ceramic membrane process and (3) ozonation might increase the biodegradable dissolved organic carbon in the permeate (Chen, 2003), which led to the increased risk of biologically unstable drinking water. There are studies reported the new hybrid ozoneceramic membrane-activated carbon process (Schlichter et al., 2004; Sartor et al., 2008), which improved the removal percentage of organic matter to more than 80%. However, most of the organic matter removed was contributed to the adsorption effect of activated carbon. Since biological activated carbon (BAC) filtration is an effective method to remove organic matter (Reungoat et al., 2012) and ammonia (Zhang et al., 2011) from drinking water, the combination of ozone-ceramic membrane with BAC may remove the particulate matter, organic matter and ammonia, simultaneously. The biological degradation process can also greatly extend the service life of activated carbon in the filter, which can save the production cost of water treatment plant. In this work, ozone-ceramic membrane was combined with BAC filtration to form a hybrid system. The primary objective of the present study was to investigate the removal efficiencies of organic matter and ammonia of the hybrid system. To simulate the increased dissolved oxygen (DO) concentration in the membrane effluent in an ozone-
membrane-BAC process, DO in the membrane effluent was adjusted by pure oxygen aeration prior to the BAC column. The effect of DO on the removal efficiency of ammonia was also examined. In practice, the hybrid process tested in this article might be very helpful in upgrading of conventional water treatment plants. Ozone/ceramic membrane might replace coagulation, sedimentation and ozonation to remove turbidity particles and taste/odour matters, while the followed BAC unit might replace the sand filtration to remove dissolved organics and ammonia. This might be of significance in saving construction area and investment cost for upgrading of water treatment plants.
1 Materials and methods 1.1 Raw water The raw water was collected from Dongjiang River, an important water source for several large cities in Southern China. Polluted water from an urban river was added into the Dongjiang River water to simulate the seasonal pollution phenomenon. Water samples were prefiltered through a stainless sieve (150 mesh) to remove large impurities. The ammonia concentration was controlled by adjusting the ratio of the two kinds of water. The characteristics of the raw water are as following: total organic carbon (TOC) 2.3–4.9 mg/L; ultraviolet absorbance at 254 nm (UV254 ) 0.058–0.114 cm−1 , ammonia 3.7–14.6 mg/L, particle count (larger than 2 µm) 2.0 × 104 –2.6 × 104 cnt/mL, conductivity 200–310 µS/cm, turbidity 6–10 NTU, DO 4.0–5.0 mg/L, pH 7.0–8.0, temperature 22–25°C. 1.2 Hybrid ozone-ceramic membrane-BAC apparatus A schematic of the hybrid ozone-ceramic membrane-BAC system is shown in Fig. 1. Pure oxygen gas (99.99%) from a gas cylinder was fed into the ozone generator (Model-
Gas pipe
Ozone generator
Ozone absorption
Ozone contact tank
Oxygen cylinder
Valve
Pressure gauge
Flowmeter Membrane housing
Oxygen contact tank
Water pipe
Backwash water effluent
Ceramic membrane
Raw water Peristaltic pump
Centrifugal pump
Backwash water Peristaltic pump Treated water
Fig. 1
Schematic of the ozone-ceramic membrane-biological activated carbon (BAC) system.
BAC column
Journal of Environmental Sciences 26 (2014) 783–791
CFG3-10g, Guolin, China) to generate ozone. The gaseous ozone concentration was controlled by varying the voltage applied to the generator. The ozone was mixed with the raw water through a diffuser installed at the bottom of the ozone contact tank. The excess ozone was vented after passing through a 2% potassium iodide solution. A singlechannel tubular ceramic membrane (Pall, Germany) with nominal pore size of 100 nm was used in the experiment. A stainless steel centrifugal pump (SDL2-7, CNP, China) with a frequency converter was used to change the transmembrane pressure (TMP). The materials of all the pipes and valves in the system were 316L stainless steel or polytetrafluoroethylene. A chromatography column with an inner diameter of 15 mm was used as the BAC column. The BAC was collected from a pilot plant, which has been continuously run for about 1.5 years. The equivalent diameter of the crushed activated carbon was 1.1 mm. The ratio of column’s diameter to activated carbon’s was a little smaller than the suggested ratio of 30 (Letterman, 1999). Therefore, the wall effect was neglected considering that the water flow rate as low as 1.0 m/hr in BAC column. 1.3 Experimental procedure Ozone-ceramic membrane-BAC (Process A) and ceramic membrane-BAC (Process B) were tested in parallel to investigate the effect of ozone on the performance of the hybrid process. The typical characteristic of ceramic membrane and operating conditions of the hybrid system are given in Table 1. The membrane filtration was operated as a batch experiment. In Process A, the raw water was pre-ozonized in the ozone contact tank. The ozone dosage was defined as the total amount of ozone injected divided by the water volume. The ozone transfer efficiency was about 70%, which was calculated according to the amount of ozone generated and residual ozone in the off-gas. Raw water was pre-ozonized for 10 min after the injection of ozone (ozone contact time = 10 min) and then it was pumped to the membrane module. In Process B, raw water was directly pumped into the membrane without preozonation. The Table 1 Characteristic of ceramic membrane and operating conditions of the hybrid system Process
Parameters
Values
Ozone-membrane
Membrane filtering area Membrane pore size Trans-membrane pressure Filtration mode Initial membrane flux (pure water) Ozone dosage Gas flow rate Empty bed contact time Flow rate
44 cm2 100 nm 1×105 Pa Dead end 2045 L/(m2 ·hr)
BAC
2.5 mg/L 150 mL/min 25 min 2.5–3.0 mL/min
785
batch filtration was carried out with an interval of 3 days Ten liters of membrane effluent was prepared in each batch filtration with and without pre-ozonation, respectively. Once membrane flux decreased to about 200 L/(m2 ·hr), the membrane was replaced by a clean one. The contaminated membrane was cleaned with sodium hydroxide (0.5 mol/L) and nitric acid (0.3 mol/L) according to the manufacturer’s recommendation. The membrane effluent was stored in the oxygen contact tank after the batch experiment and then it was continuously pumped into the BAC column using a peristaltic pump (BT100, Longer, China) at a constant flow rate. The DO was adjusted by pure oxygen aeration. Water samples were collected from the oxygen contact tank (representing membrane effluent) and BAC effluent for the analyses of nitrogen (ammonia, nitrite and nitrate), organic matter (TOC, UV254 and molecular weight distribution (MWD)) and particulate matters. All of the samples were acidified to pH 2 and stored at 4°C except those for particulate matters. 1.4 Analytical methods An ozone monitor (Model-600, Ebara, Japan) calibrated with iodimetry method was used to measure the concentration of gaseous ozone online. Aqueous ozone was measured by spectrophotometer using indigo method (Bader and Hoign, 1981). The concentrations of ammonia, nitrite and nitrate were analyzed using an auto analyzer (TA-88, Sinsche, China). Prior to the TOC measurement, the water samples were purged with ultrapure nitrogen gas for 3 min to remove CO2 . TOC concentration was measured using a TOC analyzer (TOC-V CPH, Shimadzu, Japan). UV254 was measured by an ultraviolet-visible spectrophotometer (UV-1700, Shimadzu, Japan) using a 1-cm quartz cell. Turbidity and particulate matters were measured using a turbidity meter (2100P, HACH, USA) and a particle count analyzer (IBR, Hangzhou Grean, China), which can detect particles in the size range of 2–25 µm. The concentration of DO was measured using an Orion 3-Star DO Portable Meter (Thermo Scientific, USA). A LC-20A high performance liquid chromatograph system (Shimadzu, Japan) with a UV detector (SPD-20A, Shimadzu, Japan) was used to measure the MWD. A TSKGEL G3000SWXL gel chromatograph column (Tosoh, Japan) was used to separate the organic matter. The gel column was calibrated with sodium polystyrene sulphonate standards (17 kDa, 10 kDa, 6.8 kDa, 4.3 kDa and 210 Da, PSS Polymer Standards Service GmbH, Germany) and salicylic acid according to the method developed by Zhou et al. (2000).
2 Results and discussion 2.1 Effect of ozone on membrane fouling
786
Journal of Environmental Sciences 26 (2014) 783–791
10000 40 30 20 10 0
>2
>3
>5 >7 > 10 Particle size (μm)
> 15
Fig. 3 Effect of ozone on particulate matters in membrane effluent. Ozone dosage = 2.5 mg/L, turbidity = 14 NTU, TOC = 3.2 mg/L. All data are average of duplicates within experiments and duplicate experiments. The maximum standard deviation of the values is 10%.
factor for alleviated membrane fouling. Figure 4 shows the effects of ozone on the removals of TOC and UV254 . The TOC removed by pre-ozonation alone was 4% but it increased to 16% for UV254 . The presumed explanation is that ozonation only led to the structure transformation of organic matter, rather than mineralizing it into H2 O and CO2 . About 20% of TOC and 14% of UV254 were removed by membrane filtration alone in Process B. But in Process A, the removal efficiency decreased to 14% for TOC while it increased to 27% for UV254 . The decreased TOC removal suggested a reduced organic load of membrane and this was probably a result of the changed structure of organic matter. Ozonation can oxidize organic matter containing aromatic moieties and carbon-carbon double bonds and generate small molecule organic matter with high hydrophilicity (Kim et al., 2008, 2009; Zhu et al., 2011; Nguyen and Roddick, 2010). This led to the reduced membrane organic matter load and
Process A Process B
TOC 25 TOC and UV254 removal (%)
0.8 Normalized membrane flux
20000
30
1.0
0.6
0.2
0.1
0.0
Influent in Process A Influent in Process B Effluent in Process A Effluent in Process B
30000 Particle count (cnt/mL)
The initial membrane flux was 2045 L/(m2 ·hr) with the TMP of 0.1 MPa. As shown in Fig. 2, the membrane flux decreased sharply in the first 10 min. This was also observed in some other studies (Zhu et al., 2011; Byun et al., 2011). The sharply decrease in membrane flux was probably due to the pore blocking and fouling layer forming. From 10 to 60 min, the membrane flux decreased gradually with increasing time for both processes as the results of increasing thickness of fouling layers. At the end of the batch filtration experiment, the normalized membrane fluxes were 0.13 and 0.10 for Process A and Process B, respectively. It was clear that pre-ozonation alleviated membrane fouling. Both particulate matters (Sch¨afer et al., 2000) and NOM (Karnik et al., 2005b) are main factors affecting the membrane flux in water treatment. The effects of ozonation on particulate matters and organic matter were considered to find out the reason for alleviated membrane fouling in batch filtration experiment. Since the turbidity of membrane effluent was lower than 0.1 NTU, particle count was used to evaluate the effect of particulate matters on membrane fouling. Figure 3 shows the particle counts in the raw water and membrane effluents in two processes. Ozonation slightly increased the particles larger than 2 µm and decreased those larger than 3 µm in raw water. The increased count of small particles was a result of the decomposition of large ones, which was also reported by other study (Yan et al., 2007). Both membrane filtrations with or without pre-ozonation removed more than 99% of the particles with size larger than 2 µm. Although the particle counts in membrane effluent in Process A were a little higher than those in Process B, the difference was not statistically significant. It means that the membrane particle loads were nearly the same for Processes A and B. Therefore, the effect of ozonation on particulate matters was not the major
0
10
20
30 40 Time (min)
50
60
Fig. 2 Comparison of membrane flux in Process A (ozone dosage = 2.5 mg/L) and Process B (ozone dosage = 0 mg/L). TMP = 0.1 MPa. All data are average of triplicate experiments. The maximum standard deviation of the values is 5%.
UV254
20 15 10 5 0
Ozone
Membrane
Ozone+membrane
Fig. 4 Effect of ozone on TOC and UV254 removal by membrane. Ozone dosage = 2.5 mg/L, ozone contact time = 10 min. The maximum standard deviation of the values is 15% and 5% for TOC and UV254 , respectively.
787
Journal of Environmental Sciences 26 (2014) 783–791
Raw water Raw water+ozone
Relative intensity
2300
2000 Da
1700 380 Da 1100 500
1100 Da
-100 10
100
1000 10000 Molecular weight (dalton)
100000
Fig. 5 Effect of ozonation on molecular weigh distribution of the organic matter in raw water. Ozone dosage = 2.5 mg/L, ozone contact time = 10 min, TOC = 3.2 mg/L.
mitigated membrane fouling. The MWD of the organic matter before and after ozonation shown in Fig. 5 gives a direct explanation for the changes of molecular structure of organic matter. The proportion of organic matters with molecular weight of about 1100–2000 Da decreased while that with molecular weight of about 380 Da increased. The small molecule organic matter with high hydrophilicity generated by ozonation reduced fouling layer’s hydraulic resistance and mitigated the adsorption of NOM to membrane (Kim et al., 2009). Therefore, the altered molecular structure of organic matter was the dominant factor alleviating membrane organic fouling.
ages of ammonia were caused by (1) physical retention, which is the primary mechanism of ultrafiltration purification, is unable to remove ammonia in the form of NH+4 , and (2) the reaction rate of ozone with ammonia is very slow (Hoign et al., 1985; von Gunten, 2003). A small part of ammonia removed by membrane filtration was a result of ozonation and removal of particles with adsorbed ammonia (Shi et al., 2005). As shown in Fig. 6, after BAC filtration, the average removal percentages of ammonia were 58% and 62% for Processes A and B, respectively. About 75% of the ammonia removed by the system were contributed to BAC filtrations in both processes. But no statistically significant difference in ammonia removals was observed between the processes. The concentrations of DO and ammonia were adjusted to evaluate the removal ability of BAC. The DO concentration increased from 7.5 mg/L gradually to about 30.0 mg/L (Fig. 7) after pure oxygen aeration. Prior to oxygen aeration, the average removal percentages of ammonia were 45% and 49% for Processes A and B, respectively. The percentages increased to about 85% when DO concentrations were enhanced to 30.0 mg/L and the ammonia concentrations in the BAC effluents were 40
2.2 Removal of ammonia by membrane-BAC and ozone-membrane-BAC processes Figure 6 illustrates the variations of ammonia in Processes A and B. Membrane filtration with or without pre-ozonation could hardly remove the ammonia from raw water. The removal percentages were 3%–14% and 3%–9% for membrane filtration in Process A and B, respectively. The difference between the ammonia concentrations in the membrane effluents in tests was not significant, indicating that ozonation did not affect the ammonia removal of membrane. The low removal percent-
85% removal
67% removal
8
4
0
4
8
12 16 Time (day)
20
24
Ammonia (mg/L)
Ammonia (mg/L)
BAC
0
4
8
12 16 Time (day)
20
24
Fig. 7 Variations of DO in Process A and Process B. All data are average of triplicates within experiment. The maximum standard deviation of the values is 5%.
16
BAC
W/pure oxygen aeration
W/O aeration
0
Raw water Membrane effluent of Process A BAC effluent of Process A BAC 45% removal
0
20
10
16
12
BAC influent of Process A BAC influent of Process B BAC effluent of Process A BAC effluent of Process B
30
DO (mg/L)
2900
Raw water Membrane effluentof Process B BAC effluent of Process B
12
BAC 49% removal
8
BAC
BAC
83% removal
68% removal
4
0
0
4
8
12 16 Time (day)
20
24
Fig. 6 Variations of ammonia in Process A and Process B. All data are average of triplicates within experiment. The maximum standard deviation of the values is 5%.
788
Journal of Environmental Sciences 26 (2014) 783–791
lower than 1.00 mg/L (day 16–19). After the ammonia concentrations in raw water increased to 13.5–14.5 mg/L (day 20–25), the average ammonia concentrations in BAC effluents increased to 4.38 mg/L in Process A and 4.55 mg/L in Process B. The amounts of ammonia removed by BAC (day 20–25) were about 7.80 mg/L and 8.69 mg/L for Processes A and B, respectively. It was reasonable to presume that most of the ammonia was removed by organism because the activated carbon had been used for more than one year. The relationship between the DO consumed and the ammonia removed by BAC was also calculated. The theoretical mass ratio of N and O is 1:4.57 if O is the only electron acceptor in the transformation of ammonia to nitrate. The average mass ratios of N and O in Processes A and B were 1:4.1 and 1:3.8 during the experiment, respectively. The lower mass ratios of N and O probably means that part of the ammonia was not transformed to nitrate but nitrite, which needs less O during the transformation. This was consistent with the result reported by Yu et al. (2007). 2.3 Effect of pre-ozonation on the variations of nitrite and nitrate in membrane-BAC process The variations of nitrite and nitrate in Processes A and B are shown in Fig. 8. The average nitrite concentration in the raw water was lower than 0.38 mg/L. Membrane filtration
alone could not remove nitrite effectively. By using preozonation as the pre-treatment in Process A, about 20%– 50% of nitrite was removed because of oxidation by ozone. There was no nitrite accumulated in the BAC effluents on the premise of the ammonia concentration in raw water lower than 4.5 mg/L (day 0–12) without oxygen aeration (day 0–15). However, accumulations of nitrite in the BAC effluents in Processes A and B were observed after the ammonia concentration in raw water increased to 6.5 mg/L (day 12–15), and the corresponding nitrite concentrations in the BAC effluents increased to 0.95 mg/L and 1.26 mg/L, respectively. The accumulation of nitrite in BAC effluents means that the ammonia was not fully transformed to nitrate. As shown in Fig. 7, the DO concentration in membrane effluent increased to about 30 mg/L after oxygen aeration (day 16–25) and no nitrite was detected in the BAC effluent in Process B (day 16–19). However, the nitrite concentration in Process A continuously increased to 1.63 mg/L, although DO concentrations in both processes were nearly the same. The nitrite concentrations in BAC effluents increased in both process under the condition of high ammonia concentration (day 20–25). The highest concentration of nitrite in BAC effluent in Process A was 5.23 mg/L, which was about four times higher than that in Process B. The variations of nitrate concentrations also
6
6 Raw water Membrane effluent of Process A BAC effluent of Process A
4 3 2 1
0
4
8
12 16 Time (day)
20
3 2
0
24
12
0
4
8
12 16 Time (day)
20
24
12 Raw water Membrane effluent of Process A BAC effluent of Process A
8 W/O aeration
6 4
Raw water Membrane effluent of Process B BAC effluent of Process B
10
W/pure oxygen aeration
2
Nitrate (mg/L)
10 Nitrate (mg/L)
4
1
0
0
Raw water Membrane effluent of Process B BAC effluent of Process B
5 Nitrite (mg/L)
Nitrite (mg/L)
5
8 W/O aeration
6
W/pure oxygen aeration
4 2
0
4
8
12 16 Time (day)
20
24
0
0
4
8
12 16 Time (day)
20
24
Fig. 8 Variations of nitrite and nitrate in Process A and Process B. All data are average of triplicates within experiment. The maximum standard deviation of the values is 5%.
789
Journal of Environmental Sciences 26 (2014) 783–791
Table 2 Total nitrogen mass balance in membrane effluent and BAC effluent Total nitrogen
Membrane effluent (mg/L)
BAC effluent (mg/L)
ANOVA
Progress A Progress B
7.96 ± 2.68 7.79 ± 2.68
7.88 ± 2.88 7.94 ± 2.88
p > 0.1 p > 0.1
Total nitrogen is the sum of ammonia, nitrite and nitrate nitrogen; average value of total nitrogen for the 26 days experiment; ANOVA for the difference between membrane effluent and BAC effluent, n = 52.
8.40
8.08 7.90
7.81
7.80
7.60
7.57
BAC effluent of Process B
7.86
8.00
BAC effluent of Process A
8.20
pH 7.60 7.40
Membrane effluent of Process B
Membrane effluent of Process A
7.00
Ozonation alone
7.20 Raw water
illustrate that there was a significant difference between the nitrate concentration in BAC effluents in Process A and B (ANOVA, n = 50, p = 0.066). Compared with Process A, more nitrate was generated by BAC in Process B. Nitrification is the main mechanism for the removal of ammonia in drinking water treatment with BAC (Yapsakli et al., 2010). The mass balance of total nitrogen (sum of ammonia, nitrite and nitrate nitrogen) in the membrane effluent and the BAC effluent is given in Table 2. The results showed that there were no significant differences in the average concentrations of the total nitrogen in membrane effluents and BAC effluents in both processes. There was no nitrogen loss in the BAC system, indicating that only nitrification occurred in the BAC. The dissimilarities between nitrite and nitrate concentrations in the BAC effluents (day 16–25) were probably caused by the different activities of nitrifying bacteria in Process A and B. Ammonia oxidizing bacteria and nitrite oxidizing bacteria play important roles in nitrogen transformation (Yapsakli et al., 2010). The accumulation of nitrite in Process A indicated that the converting nitrite into nitrate was inhibited. The only difference between the two processes were the pre-ozonation in Process A. Sun et al. (2007) reported that pre-ozonation with an ozone dosage of 2 mg/L decreased the biomass in the BAC, even in the deeper layer of the BAC bed (100 and 160 cm). Generally, no residual ozone exists in the deeper layer of BAC bed. This means that the microorganisms in BAC were not affected by ozone directly, but by some other factors (such as NOM, pH and particulate matters) altered by ozonation. As shown in Figs. 4 and 5, pre-ozonation altered the molecular structure of NOM and increased the TOC concentration in membrane effluent in Process A. Yapsakli et al. (2010) found although no residual ozone existed in BAC influent, pre-ozonation with 2 mg O3 /mg TOC changed the composition of raw water and affected the ammonia oxidizing bacteria species in the BAC. It is reasonable to presume that pre-ozonation affected the nitrifying bacteria by changing the characteristics of raw water. As shown in Fig. 9, membrane filtration alone increased the pH of membrane effluent in Process B. Preozonation decreased the pH of raw water and this caused a lower pH of membrane effluent in Process A. Most of the nitrifying bacteria are sensitive to pH (Villaverde et al.,
Fig. 9 Variations of pH in Process A and Process B. All data are average of triplicates within experiment. The maximum standard deviation of the values is 5%.
1997) and nitrification consumed alkalinity and decreased the pH of BAC effluent. The higher pH of membrane effluent in Process B was beneficial for nitrifying bacteria. 2.4 Removals of TOC and UV254 by membrane-BAC and ozone-membrane-BAC processes The effect of pre-ozonation on the removals of TOC and UV254 is shown in Fig. 10. The TOC removed by ozonation alone was less than 5% and it was not discussed here. The average percentages of TOC removed by membrane filtration in Process A and B were 14% and 20%, respectively. Ozonation decreased the removal of TOC in Process A. This occurred as a result of oxidation of organic matter by ozone. Ozonation of organic matter decreased the molecular weight of the organic matter and increased its hydrophylicity, which caused the decreased membrane retention. However, the difference between the TOC concentrations in membrane effluents was not significant, indicating that ozonation with a dosage of 2.5 mg/L had no significant effect on the TOC removal of ceramic membrane, but only led to a slightly increase in TOC concentration in membrane effluent. Both the BAC in Process A and B improved the removals of TOC. As shown in Fig. 10, the average removal percentages of TOC were 49% and 51% for Process A and B, respectively. About 71% of the TOC removed by the system was contributed by BAC in Process A and about 63% in Process B. Since the BAC has been saturated by organic matter, most of the organic matter removed was degraded by microorganisms in BAC. Pre-ozonation had no significant effect on the removal of TOC during the experiment period without oxygen aeration (day 0–15). However, a statistically significant decrease (ANOVA, n
790
Journal of Environmental Sciences 26 (2014) 783–791
0.12
6 Raw water Membrane effluent of Process A Membrane effluent of Process B BAC effluent of Process A BAC effluent of Process B
TOC (mg/L)
4
0.10 0.08 UV254 (cm-1)
5
3 2 1
0.06 0.04 0.02
W/pure oxygen aeration
W/O aeration 0
Raw water Membrane effluent of Process A Membrane effluent of Process B BAC effluent of Process A BAC effluent of Process B
0
4
8
12 Time (day)
16
20
24
W/pure oxygen aeration
W/O aeration 0.00 0
4
8
12 16 Time (day)
20
24
Fig. 10 Variations of TOC and UV254 in Process A and Process B. The maximum standard deviation of the values for TOC is 15% and 5% for UV254 .
= 20, p = 0.086) in TOC concentration in Process B was found during the experiment period with oxygen aeration (day 16–25). Both of the processes have the same operating condition except pre-ozonation in Process A. Thus, the phenomenon illustrated that microorganisms in Process A were indirectly affected by pre-ozonation (Yapsakli et al., 2010). It appears that the effect was more obvious under high DO condition, under which the negative effect of ozonation on the microorganisms was magnified. The variations of UV254 in Process A and B. UV254 was removed from 0.077 to 0.065 cm−1 by membrane filtration alone in Process B, with the removal percentage of 15%. Process A removed UV254 to 0.056 cm−1 , with the removal percentage of 27%, nearly two times higher than that in Process B. The UV254 represents the organic matter containing aromatic component or carbon-carbon double bonds, which can react with ozone quickly and thus improves the UV254 removal by membrane in Process A. The following BAC increased the removal percentage of UV254 to 52% in Process A and 48% in Process B. It indicates pre-ozonation improved the overall removal percentage of UV254 in Process A. The UV254 concentration in BAC effluent in Process A was significantly lower (ANOVA, n = 32, p = 0.054) than that in Process B during the period without oxygen aeration (day 0–15). However, the difference between the UV254 in BAC effluents became insignificant during the period with oxygen aeration (day 16–25). One explanation was that the microorganisms in the BAC was affected by ozonation indirectly and this reduced the removal of UV254 in Process A. Alternatively, it was also explained by the lower UV254 concentration in the membrane effluent in Process A, which decreased the removal efficiency of UV254 by BAC.
3 Conclusions This study demonstrates that pre-ozonation in ceramic membrane-BAC system resulted in an improvement in
membrane flux by 30%. The alleviated membrane fouling is mainly due to the oxidation of organic matter by ozonation. Ceramic membrane-BAC processes with or without ozonation removed more than 99% of the particles in the size range of 2–15 µm. BAC filtration greatly improved the removal of ammonia. In addition, the increase of DO in the membrane effluent played an important role in the ammonia removal. However, pre-ozonation changed the composition of raw water and caused some negative effects on the activities of microorganisms in BAC. The activities of the nitrite oxidizing bacteria in Process A were inhibited and this led to the accumulation of nitrite in the BAC effluent. The overall removal percentages of TOC and UV254 increased greatly after BAC filtration, suggesting that BAC is a good option to compensate the low removal efficiency of organics by ceramic membrane. Pre-ozonation improved the overall removal of UV254 in Process A. This study gave some new insights into the ozone-ceramic membrane-BAC process. It may improve the application of ceramic membrane in drinking water treatment, especially in the cases that ammonia contaminated source water is considered. However, the indirect effect of ozonation on the microorganisms in BAC in the hybrid system need to be further investigated. Acknowledgments This work was supported by the National Grand Water Project (No. 2008ZX07423-002), the National Natural Science Foundation of China (No. 50978170) and the Guangdong Provincial Funding (No. 2012B030800001).
references Alpatova, A.L., Davies, S.H., Masten, S.J., 2013. Hybrid ozonationceramic membrane filtration of surface waters: The effect of water characteristics on permeate flux and the removal of DBP precursors, dicloxacillin and ceftazidime. Separ. Purif. Technol. 107, 179–186. Bader, H., Hoign, J., 1981. Determination of ozone in water by the indigo
Journal of Environmental Sciences 26 (2014) 783–791
method. Water Res. 15(4), 449–456. Byun, S., Davies, S.H., Alpatova, A.L., Corneal, L.M., Baumann, M.J., Tarabara, V.V. et al., 2011. Mn oxide coated catalytic membranes for a hybrid ozonation-membrane filtration: Comparison of Ti, Fe and Mn oxide coated membranes for water quality. Water Res. 45(1), 163–170. Chen K.C., 2003. Ozonation, Ultrafilration, and Biofiltration for the Control of NOM and DBP in Drink Water. East Lansing: Michigan State University. Farahbakhsh, K., Svrcek, C., Guest, R.K., Smith, D.W., 2004. A review of the impact of chemical pretreatment on low-pressure water treatment membranes. J. Environ. Eng. Sci. 3(4), 237–253. Hoign, J., Bader, H., Haag, W.R., Staehelin, J., 1985. Rate constants of reactions of ozone with organic and inorganic compounds in water–III. Inorganic compounds and radicals. Water Res. 19(8), 993–1004. Karnik, B.S., Davies, S.H., Baumann, M.J., Masten, S.J., 2005a. The effects of combined ozonation and filtration on disinfection byproduct formation. Water Res. 39(13), 2839–2850. Karnik, B.S., Davies, S.H., Baumann, M.J., Masten, S.J., 2007. Removal of Escherichia coli after treatment using ozonation-ultrafiltration with iron oxide-coated membranes. Ozone-Sci. Eng. 29(2), 75–84. Kim, J., Davies, S., Baumann, M.J., Tarabara, V.V., Masten, S.J., 2008. Effect of ozone dosage and hydrodynamic conditions on the permeate flux in a hybrid ozonation-ceramic ultrafiltration system treating natural waters. J. Membr. Sci. 311(1-2), 165–172. Karnik, B.S., Davies, S., Chen, K.C., Jaglowski, D.R., Baumann, M.J., Masten, S.J., 2005b. Effects of ozonation on the permeate flux of nanocrystalline ceramic membranes. Water Res. 39(4), 728–734. Kim, J., Shan, W.Q., Davies, S., Baumann, M.J., Masten, S.J., Tarabara, V.V., 2009. Interactions of aqueous NOM with nanoscale TiO2 : Implications for ceramic membrane filtration-ozonation hybrid process. Environ. Sci. Technol. 43(14), 5488–5494. Laˆın´e, J.M., Vial, D., Moulart, P., 2000. Status after 10 years of operation – Overview of UF technology today. Desalination 131(1-3), 17–25. Letterman R.D., 1999. Water Quality and Treatment: A Handbook of Community Water Supplies. McGraw-Hill, New York. Nguyen, S.T., Roddick, F.A., 2010. Effects of ozonation and biological activated carbon filtration on membrane fouling in ultrafiltration of an activated sludge effluent. J. Membr. Sci. 363(1-2), 271–277. Pendergast, M.M., Hoek, E., 2011. A review of water treatment membrane nanotechnologies. Energy Environ. Sci. 4(6), 1946–1971. Reungoat, J., Escher, B.I., Macova, M., Argaud, F.X., Gernjak, W., Keller, J., 2012. Ozonation and biological activated carbon filtration of wastewater treatment plant effluents. Water Res. 46(3), 863–872. Sartor, M., Schlichter, B., Gatjal, H., Mavrov, V., 2008. Demonstration
791
of a new hybrid process for the decentralised drinking and service water production from surface water in Thailand. Desalination 222(1-3), 528–540. Sch¨afer, A.I., Schwicker, U., Fischer, M.M., Fane, A.G., Waite, T.D., 2000. Microfiltration of colloids and natural organic matter. J. Membr. Sci. 171(2), 151–172. Schlichter, B., Mavrov, V., Chmiel, H., 2004. Study of a hybrid process combining ozonation and microfiltration/ultrafiltration for drinking water production from surface water. Desalination 168, 307–317. Shi, H.X., Liu, H.J., Qu, J.H., Dai, R.H., Wang, M.H., 2005. Property of adsorption about ammonium on suspended mineral matters in eutrophic water. Environ. Sci. 26(5), 72–76. Sun, G.F., Qiao, T.J., Liu, X.F., Luo, H.X., 2007. Variation and effect of biomass in biologically activated carbon process. Technol. Water Treat. 33(7), 44–47. Villaverde, S., GarciaEncina, P.A., FdzPolanco, F., 1997. Influence of pH over nitrifying biofilm activity in submerged biofilters. Water Res. 31(5), 1180–1186. von Gunten, U., 2003. Ozonation of drinking water: Part I. Oxidation kinetics and product formation. Water Res. 37(7), 1443–1467. Yan, M., Wang, D., Shi, B., Wang, M., Yan, Y., 2007. Effect of preozonation on optimized coagulation of a typical North China source water. Chemosphere 69(11), 1695–1702. Yapsakli, K., Mertoglu, B., Cecen, F., 2010. Identification of nitrifiers and nitrification performance in drinking water biological activated carbon (BAC) filtration. Proce. Biochem. 45(9), 1543–1549. Yu, X., Qi, Z.H., Zhang, X.J., Yu, P., Liu, B., Zhang, L.M. et al., 2007. Nitrogen loss and oxygen paradox in full-scale biofiltration for drinking water treatment. Water Res. 41(7), 1455–1464. Zhang, D., Li, W., Zhang, S., Liu, M., Zhao, X., Zhang, X., 2011. Bacterial community and function of biological activated carbon filter in drinking water treatment. Biom. Environ. Sci. 24(2), 122– 131. Zhou, Q.H., Cabaniss, S.E., Maurice, P.A., 2000. Considerations in the use of high-pressure size exclusion chromatography (HPSEC) for determining molecular weights of aquatic humic substances. Water Res. 34(14), 3505–3514. Zhu, B., Hu, Y.X., Kennedy, S., Milne, N., Morris, G., Jin, W.Q. et al., 2011. Dual function filtration and catalytic breakdown of organic pollutants in wastewater using ozonation with titania and alumina membranes. J. Membr. Sci. 378(1-2), 61–72. Zhu, H.T., Wen, X.H., Huang, X., 2010. Membrane organic fouling and the effect of pre-ozonation in microfiltration of secondary effluent organic matter. J. Membr. Sci. 352(1-2), 213–221. Zularisam, A.W., Ismail, A.F., Salim, R., 2006. Behaviours of natural organic matter in membrane filtration for surface water treatment-a review. Desalination 194(1-3): 211–231.
