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The effects of UV disinfection on drinking water quality in distribution systems
water research 44 (2010) 115–122
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The effects of UV disinfection on drinking water quality in distribution systems Yonkyu Choi, Young-june Choi* Division of R&D for Water, Waterworks Research Institute, Seoul Metropolitan Government, 552-1, Chunho Daero, Kwangjin-Ku, Seoul, Republic of Korea, 143-820
article info
abstract
Article history:
UV treatment is a cost-effective disinfection process for drinking water, but concerned to
Received 27 May 2009
have negative effects on water quality in distribution system by changed DOM structure. In
Received in revised form
the study, the authors evaluated the effects of UV disinfection on the water quality in the
30 August 2009
distribution system by investigating structure of DOM, concentration of AOC, chlorine
Accepted 2 September 2009
demand and DBP formation before and after UV disinfection process. Although UV treat-
Published online 16 September 2009
ment did not affect concentration of AOC and characteristics of DOM (e.g., DOC, UV254,
Keywords:
weight) significantly, the increase of low molecular fraction was observed after UV treat-
UV
ment, in dry season. Chlorine demand and THMFP are also increased with chlorination of
Distribution system
UV treated water. This implies that UV irradiation can cleave DOM, but molecular weights
Molecular weight
of broken DOM are not low enough to be used directly by microorganisms in distribution
AOC
system. Nonetheless, modification of DOM structure can affect water quality of distribution
Chlorine demand
system as it can increase chlorine demands and DBPs formation by post-chlorination.
SUVA254, the ratio of hydrophilic/hydrophobic fractions, and distribution of molecular
ª 2009 Elsevier Ltd. All rights reserved.
DBP
1.
Introduction
Disinfection by ultraviolet light (UV) is considered as a costeffective and easily implementable system for drinking water disinfection. Interest in UV disinfection process has been increased sharply in drinking water industry, since researchers demonstrated that even very low dosage of UV light could inactivate Cryptosporidium effectively in the late 1990s (Bukhari et al., 1999; Clancy et al., 2000). UV spectrum is divided into four regions; vacuum UV (100w200 nm, hereafter VUV), UV-C (200w280 nm), UV-B (280w315 nm), and UV-A (315w400 nm). UV disinfection primarily occurs due to the germicidal action of UV-B and UVC light on microorganisms. Although VUV can disinfect microorganisms, it is not efficient to use VUV for water disinfection because it rapidly dissipates through water in
very short distances (EPA, 2006). VUV is also known to breakdown bonds of organic carbons (Buchanan et al., 2004; Thomson et al., 2004). Two UV systems are generally applied for drinking water disinfection process. Monochromatic low pressure UV (hereafter LPUV) emits single wavelength at 254 nm which is close to the maximum microbial action spectrum. Polychromatic medium pressure UV (hereafter MPUV) emits a wide range of wavelength including UV-A, -B, -C and visible light. Special LPUV emitting two wavelengths at 185 and 254 nm (hereafter LPUV for TOC) is applied to remove TOC for producing ultrapure water. Although these UV systems are inactivate most of microorganisms effectively except for some viruses, they can not guarantee biological safety of tap water because the effect of UV irradiation can not be maintained throughout
* Corresponding author. Tel.: þ82 2 3146 1810; fax: þ82 2 3146 1811. E-mail address: [email protected] (Y.-j. Choi). 0043-1354/$ – see front matter ª 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2009.09.011
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water research 44 (2010) 115–122
distribution system. On the contrary, UV disinfection is concerned to have negative effects on water quality by UV photolysis. Many researchers have reported that UV irradiation can modify DOM structure and increase biodegradability (Frimmel, 1998; Thomson et al., 2004; Buchanan et al., 2005; Goslan et al., 2006). Especially, VUV irradiation is known to be more effective than UV-C irradiation in formation of biodegradable compounds and mineralization (Buchanan et al., 2004). UV-A and UV-B can also splits large NOM molecules into organic acids with lower molecular weight (Frimmel, 1998). This change of DOM structure can increase biodegradability, which stimulates microbial regrowth and biofilm formation in distribution system. Increase of biofilm can also cause taste and odor problems and reduction of hydraulic capacity (Shaw et al., 2000). Therefore, sequential disinfection process with additional chemical disinfectant such as chlorine or monochloramine was applied to prevent microbial re-growth in the distribution system. With chlorination as secondary disinfection process, UV treatment is often expected to reduce chlorine demand and DBPs formation. Liu et al. (2006), however, reported that the DBPs formation of four organic waters was increased by chlorination after UV irradiation. The effect of UV irradiation on water quality depends on many factors, such as characteristics of source water quality, UV wavelength and applied dosage. Previous studies have often been carried out under bench-scale conditions, and organic water with relatively high DOC level (5w17.4 mg/L) and high UV dosage of 14w1,000 J/cm2 were used, which were not the conditions used for drinking water disinfection process (Frimmel, 1998; Buchanan et al., 2006; Goslan et al., 2006; Liu et al., 2006). Under drinking water with low DOC level less than 2 mg/L and UV dosage less than 40 mJ/cm2, the impact of UV irradiation on water quality could be different from the results of the previous studies. Moreover, the results under lab scale bench test can hardly reflect the real reactions under full-scale continuous flow system. In this study, the authors used pilot-scale continuous flow UV systems with LPUV, LPUV for TOC, and MPUV, and investigated change of DOM structure, probability of microbial regrowth, chlorine demand and THMs formation before and after UV treatment to evaluate the effects of UV disinfection on water quality in distribution system.
2.1.
UV pilot plant
The UV pilot plant with four UV reactors, LPUV (L85), LPUV for TOC (L90), MPUV (M1300, M350), is installed at the end of sand filters in the WTP. Sand filtered water (SF) was introduced to the reactors, and total capacity of the system was 1080 m3/ day. The experiments were carried out with UV dose of 40 mJ/cm2, which was usually applied for drinking water disinfection process. LPUV for TOC (L90) emitting two wavelengths at 185 and 254 nm is installed to evaluate TOC removal efficiency of vacuum UV. As higher UV dosage is required for TOC mineralization, additional experiments were carried out with UV dose of 150 mJ/cm2. UV dosage of each reactor was calculated from UV intensity by online sensor and contact time at each flow rate. Online sensor of LPUV (L90 and L85) and MPUV (M1300 and M350) can measure at 254 nm and between 200w300 nm, respectively. L90 system emits UV light with 254 nm and 185 nm with the ratio of 3:1. The detailed characteristics of each system were listed in Table 1. The sand filtered water and the five UV treated waters were investigated. The samples were taken from both the inflow and outflow of each reactor.
2.2.
Analytical method
The samples taken from the pilot plant were brought to the laboratory in 2 h and stored in the refrigerator below 4 C. For analyses of THMs already formed by pre-chlorination, ascorbic acid and HCl (1 þ 1) was added instantly to the samples (40 mL) to quench residual chlorine. For THMFP analyses, the samples were chlorinated (TOC : chlorine ¼ 1: 3) and incubated at 25 C for 48 h. After incubation, residual chlorine was quenched with ascorbic acid and HCl (1 þ 1) not to form THMs any more. THMs were analyzed by purge and trap method with GC (Varian, CX3600) equipped with ECD detector according to the EPA 502.2 (EPA, 1995). DOC and UV254 were analyzed with TOC analyzer (Ionics, Sievers 820) and UV/VIS spectrophotometer (Varian Cary 3C), respectively. SUVA254 was calculated from DOC and UV254.
2.3.
Separation of hydrophilic and hydrophobic carbon
DOM was separated into hydrophobic and hydrophilic fractions with resin (Amberitic XAD-7HP, Rohm & Haas Co.,
Table 1 – The characteristics of the UV system in the pilot plant.
2.
Materials and methods
In this study, the characteristics of DOM, biological re-growth potential, chlorine demand, and formation potential of disinfection byproducts before and after UV irradiation were compared to evaluate the effects of UV disinfection on water quality in distribution system. A UV pilot plant was installed at a water treatment plant (WTP) in Seoul, Korea. The samples were taken three times in 2005 and 2006, considering seasonal variation of the raw water quality ; 1) dry season with high algal biomass and BOD from winter to spring, 2) rainy season with high turbidity due to heavy rainfall during summer, and 3) normal times (Fig. 2).
System Lamp type Wavelength of UV Capacity Dosage emission (nm) (m3/h) (mJ/cm2) L90-4 L90-15
90 W Low pressure for TOC
185, 254
180 50
40 150
M1300
1.3 kW Medium pressure 85 W Low pressure 350 W Medium pressure
185w400
650
40
254
120
200w400
260
L85 M350
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water research 44 (2010) 115–122
Table 2 – DOC, UV254, and SUVA254 of pre- and post-UV treated water. System
UV254 (cm1)
DOC (mg/L) Normal
Dry
Rainy
Normal
Dry
Rainy
Normal
Dry
Rainy
0.96 0.96 0.96 0.97 0.96 0.95
1.26 1.31 1.34 1.25 1.27 1.23
1.27 1.27 1.30 1.24 1.24 1.23
0.014 0.014 0.013 0.014 0.014 0.015
0.015 0.015 0.014 0.015 0.015 0.015
0.022 0.022 0.020 0.022 0.022 0.020
1.45 1.45 1.35 1.44 1.45 1.57
1.19 1.14 1.04 1.20 1.18 1.21
1.73 1.73 1.53 1.77 1.77 1.62
SF L90-4 L90-15 M1300 L85 M350
France). Resin was cleaned with sequential soxhlet extraction method (Ma et al., 2001). XAD-7HP resin was packed in 31 mm (ID) 230 mm (H) glass column and 0.5 N NaOH was introduced into the column to clean the resin. The resin was extracted sequentially with methanol, acetonitrile, and methanol for 12 h. Finally, the column was rinsed with ultrapure water, 0.1 N NaOH, 0.1 N HCl and ultrapure water in order, until the concentration of TOC of the effluent was less than 0.1 mg/L. Each sample was adjusted to pH < 2 by adding (1 þ 1) H3PO4 and passed through clean glass column with flow rate of 15w20 mL/min. The hydrophobic carbon was the fraction that adsorbed to the surface of the resin and the carbon that passed out through the column was determined as hydrophilic fraction. After hydrophilic and hydrophobic fractions were adjusted pH 7 0.2 with 0.1 N H3PO4 and 0.1 N NaOH, DOC was analyzed with TOC Analyzer (Ionics, Sievers 820).
to maintain constant pH and ionic strength for all samples and reduce undesirable interactions. Number-averaged MW (Mn), weight-averaged MW (Mw), and polydispersivity (r) were determined using the following equations. hi and Mi are the height of HPLC-SEC chromatogram and molecular weight. n P
hi Mn ¼ i¼1 n P hi Mi
i¼1
n P
Mw ¼ i¼1
hi Mi
n P
hi
i¼1
r¼
2.4.
SUVA254 (L/mg$m)
Mw Mn
Apparent molecular weight 2.5.
High performance liquid chromatography-size exclusion chromatography (HPLC-SEC) was used to fractionate apparent molecular weight of DOM (Her et al., 2003). Separation by size exclusion was performed using a TSK-50S (Toyopearl HW SOS, 30 mm resin) column prior to sequential on-line detectors consisting of UV/Visble (SPD-20AD, Shimadzu) and DOC (Modified Sievers Total Organic Carbon Analyzer 820 Turbo). Mobile phase solution (pH 6.8 and ionic strength 0.1 M) was made with 4 mM phosphate buffer and 25 mM sodium sulfate. Polyethylene glycols (PEGs, 200 600, 2000, 4000, 8000 dalton) were used for molecular weight (MW) calibration of chromatograms. The pH and ionic strength of each sample were also adjusted with phosphate buffer and sodium sulfate solutions as similar to the mobile phase as possible before analysis
Assimilable organic carbon(AOC)
AOC was analyzed with the method proposed by Kaplan et al. (1993). AOC is defined as the amount of carbon used as energy or converted into biomass by bacteria. Two pure-culture bacterial strains, Pseudomonas fluorescens strain P17 (hereafter, P17) and Spirillum strain NOX (hereafter, NOX) were used. The sample was taken in a glass vial baked at 550 C over 2 h and sodium thiosulfate was added to quench residual chlorine. The sample was pasteurized at 70 C for 30 min in water bath, and spiked with P17 and NOX, and incubated at 15 C for 7 days. The incubated sample was taken out, inoculated in R2A media and incubated at 25 C for 72 h. The colony counts of P17 and NOX in stationary phase were converted into bacterial biomass by multiplying each carbon conversion
Hydrophilic
Hydrophobic
100% 20
22
21
19
20
19
80%
30
29
28
70
71
72
31
30
29
69
70
71
L85
M350
35
34
34
35
33
38
65
66
66
65
67
62
L85
M350
60% 40%
80
78
79
81
80
81
20% 0% SF
L90-4 L90-15 M1300
Dry season
L85
M350
SF
L90-4 L90-15 M1300
Normal times
SF
L90-4 L90-15 M1300
Rainy season
Fig. 1 – The ratio of hydrophilic and hydrophobic fractions in pre- and post-UV treated water.
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Table 3 – Percentage of each fraction of molecular weight in pre- and post-UV treated water of each system. System
>2 K
1–2 K
0.5–1 K
<0.5 K
Mn
Mw
r
Normal times
SF L90-4 L90-15 M1300 L85 M350
4.3 5.2 4.9 4.5 5.1 4.7
17.0 17.0 17.1 16.6 17.3 16.0
32.1 31.3 31.6 31.7 31.6 31.4
46.6 46.5 46.4 47.2 46.0 47.9
750 766 771 740 773 739
1574 1558 1581 1427 1547 1446
2.10 2.04 2.05 1.93 2.00 1.96
Dry season
SF L90-4 L90-15 M1300 L85 M350
5.4 3.5 5.0 4.1 5.5 4.4
15.1 14.2 14.8 14.6 15.1 14.7
29.0 29.8 29.0 29.7 28.6 30.1
50.5 52.5 51.2 51.6 50.8 50.8
744 736 671 696 756 707
1627 1640 1246 1392 1709 1454
2.19 2.23 1.86 2.00 2.26 2.06
Rainy season
SF L90-4 L90-15 M1300 L85 M350
1.7 1.8 1.5 1.9 2.1 1.9
13.2 13.3 13.0 13.5 13.5 13.3
31.6 31.4 31.0 31.5 31.1 31.2
53.5 53.5 54.5 53.1 53.3 53.6
613 602 616 622 628 619
979 984 1002 1046 1078 1033
1.60 1.63 1.63 1.68 1.72 1.67
Mn: Number-average molecular weight Mw: Weight-average molecular weight
factor. P17 AOC and NOX AOC were calculated by the following equations, and AOC was calculated by the sum of P17 AOC and NOX AOC. P17ðCFU=mLÞ 1000 mL=L 4:1 106 ðCFU=mgCÞ
NOX AOCðmg=LÞ ¼
NOXðCFU=mLÞ 1000 mL=L 1:2 107 ðCFU=mgCÞ
AOCðmg=LÞ ¼ P17 AOC þ NOX AOC
2.6.
Chlorine demand and decay rate
Chlorine demand and decay rate were estimated for the sand filtered water and the UV treated water taken from each UV
Turbidity (NTU), Rain fall (mm), Chl.a (µg/L)
250
5 Rainy season
200 Dry seaon
Turbidity Rain fall Chl.a BOD TOC Sampling
4
Noraml times
150
3
100
2
50
1
0 '05
BOD, TOC (mg/L)
P17 AOCðmg=LÞ ¼
reactor. Chlorine decay rate was measured with the procedure proposed by Powell et al. (2000). The freshly cleaned glassware was filled with distilled water and sodium hypochlorite solution was added to make 10 mg/L of free chlorine solution and left for 24 h. It was then emptied, rinsed thoroughly with ultrapure water and left to dry. 2 L volumetric flask was filled with ultrapure water and the sample water. Chlorine was added to 1w 2 mg/L and left for 15 min to ensure homogeneity. The sample water was decanted into eleven 125 mL brown glass bottles without headspace and sealed with teflon lined caps. All the bottles were stored in the incubator, at 4 and 15 C. The chlorine concentration was measured with time. Initial chlorine concentration was defined as the chlorine concentration when the same amount of chlorine was added to 2 L of ultrapure water. Chlorine concentration was measured by the DPD colorimetric method using Hach pocket
0
Oct. Nov. Dec.'06 Jan. Feb. Mar. Apr. May Jun Jul. Aug. Sep. Oct. Nov. Dec.
Month Fig. 2 – Hydraulic characteristics and the raw water quality change by season.
water research 44 (2010) 115–122
450
Normal times
Dry season
Rainy season
400 350
AOC (µg/L)
300 250 200 150 100 50 0
SF
L90-4
L90-15
M1300
L85
M350
Fig. 3 – Seasonal AOC concentration before and after UV treatment in each system.
chlorine colorimeters (pocket Hachs). All samples were taken and analyzed in triplicate. The decay rate constants were estimated by the first-order chlorine decay model (Jadas-He´cart et al., 1992). In this study, two chlorine decay rates were used, i.e. rapid chlorine decay rate (K1) for the first 4 h and slow chlorine decay rate (K2) after 4 h considering the retention time in the clearwell of the WTP.
3.
Results and discussions
3.1.
Effects of UV treatment on DOM properties
DOC, UV254, SUVA254, hydrophilic/hydrophobic ratio and apparent molecular weight before and after UV treatment were investigated. The DOC concentration of the sand filtered water was less than 1 mg/L in normal times but increased to 1.5 mg/L in dry and rainy seasons.
0
20
Pre-UV NOX AOC (µg/L) 40 60 80
100
120
400
120
300
80 L90-4 (P17) L90-15 (P17) M1300 (P17) L85 (P17) M350 (P17)
200
60
equal value line L90-4 (NOX) L90-15 (NOX) M1300 (NOX) L85 (NOX) M350 (NOX)
100
0
0
100 200 300 Pre-UV P17 AOC (µg/L)
40
20
400
Fig. 4 – P17 AOC and NOX AOC before and after UV treatment.
0
Post-UV NOX AOC (µg/L)
Post-UV P17 AOC (µg/L)
100
119
There were little change in DOC, UV254, and SUVA254 after UV treatment throughout all seasons (Table 2). Only in the L90 system, with low pressure lamp for TOC reduction, a little decrease of UV254 and SUVA254 were observed with the UV dosage of 150 mJ/cm2. The reductions might be caused by high energy of short wavelength at 185 nm and high dosage of 150 mJ/cm2. With dosage of 40 mJ/cm2, which is usually applied for drinking water disinfection process in WTP, all UV systems had no effect on DOC, UV254, and SUVA254. The ratio of hydrophilic and hydrophobic fractions was calculated from DOC concentration of each fraction. The fraction of hydrophilic DOC was relatively high throughout all seasons with the range of 62w81 %, but hydrophobic fraction was increased in rainy season (Fig. 1). The source water from the Han river, has been known to have relatively higher concentration of hydrophilic organic fraction (Oh et al., 2003; Kim et al., 2007; Jeong et al., 2007). The ratio of hydrophilic and hydrophobic fractions can be changed in water treatment process. The hydrophilic fraction tends to be increased in treated water as humic material with high SUVA value and high hydrophobic organic carbon is removed easily by coagulation process (White et al., 1997). It was also reported, from the previous studies with the Han river as the source water, that hydrophilic fraction in the settled water was increased (Oh et al., 2003; Kim et al., 2007). However, there was not significant difference in the ratio of hydrophilic and hydrophobic fractions before and after UV treatment throughout all seasons. Shaw et al. (2000) also reported that there was little or no statistical evidence that hydrophilic and hydrophobic ratios were altered by UV treatment. Distribution of apparent molecular weight was measured by HPLC-SEC system with UV and TOC detectors. The molecular weight of most DOM (over 95 %) was less than 2 kDa (Table 3), and especially DOM fraction between 0.3 and 0.4 kDa was dominant throughout all seasons. While the distribution of apparent molecular weight was not changed before and after UV treatment in normal times and rainy season, there was increase in low molecular weight fraction around 0.3 kDa after UV treatment in dry season (data was not shown). Number-averaged molecular weights (Mn) and weightaveraged molecular weight (Mw) have also shown that average molecular weight in the post-UV treated water was decreased in dry season (Table 3). This suggested that DOM structure in dry season is broken down more easily by UV radiation than those in other seasons. DOM structure might be related with the origin of DOM of each season. In aquatic system, the origin of DOM can be categorized as allochthonous DOM entering from the terrestrial watershed, and autochthonous DOM derived from biota (e.g., algae, bacteria) growing in the water body (Aiken and Cotsaris, 1995). In Korea, during the rainy season in late summer with lots of heavy rain, DOC increases because heavy rain washes large amount of organic carbon from the watershed into river while in dry season, algal biomass and BOD increases (Fig. 2). This allochthonous DOM in rainy season is known to be relatively refractory DOM with high SUVA, high molecule weight, and hydrophobic properties. In contrast, the autochthonous DOM is relatively labile, and consists of low SUVA, low molecular weight, and hydrophilic DOM (Wetzel, 1983; Kitis et al., 2002). Ma et al. (2001) reported that hydrophilic fraction was
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water research 44 (2010) 115–122
Table 4 – Chlorine demands and chlorine decay rate before and after UV treatment, K1 : rapid decay rate(< 4 h), K2 : slow decay rate(> 4 h). 4 C Chlorine demands (mg/L)
SF L90-4 L90-15 M1300 L85 M350
15 C Decay rate (h1)
ID
24 h
48 h
K1
K2
0.10 0.09 0.22 0.12 0.10 0.11
0.50 0.54 0.71 0.56 0.51 0.48
0.60 0.63 0.79 0.68 0.61 0.60
0.52 0.57 0.73 0.67 0.62 0.57
0.005 0.006 0.004 0.005 0.005 0.005
composed of more simple compounds and less complex mixtures. Decrease in molecular weight of DOM was shown with L9015, M1300 and M350 systems in dry season. The observation suggested that short wavelength below 254 nm is more effective to break down the bonds of organic carbons, and various wavelength of light could be related to degradation of DOM. It has been reported that UVA (315–400 nm) and UVB (280–315 nm) splits large DOM molecules to generate lower molecular weight organic acids (Frimmel, 1998).
3.2.
Chlorine demands (mg/L) ID 0.06 0.21 0.22 – 0.16 0.16
Decay rate (h1)
24 h
48 h
K1
K2
0.32 0.50 0.55 0.49 0.46 0.49
0.44 0.61 0.66 0.55 0.58 0.61
0.025 0.073 0.079 0.060 0.060 0.074
0.004 0.005 0.006 0.006 0.006 0.006
affect AOC level, since there was not consistent trend of increase in each system. AOC after UV exposure was compared with AOC of the sand filtered water. P17 AOC and NOX AOC of all UV systems were plotted against a line of equal value. More P17 AOC data points fell on or above the line than below, while more NOX AOC data points fell on or below than above the line (Fig. 4). Paired t-tests were carried out in separate group, LPUV (L90-4, L85) and MPUV (M1300, M350). P17 AOC, NOX AOC and AOC of sand filtered water were not different statistically from those of LPUV (p ¼ 0.557, 0.964, 0.545) and MPUV (p ¼ 0.234, 0.053, 0.386) at 95 percent confidence level. Shaw et al. (2000) reported that UV treatment did not appear to affect the AOC concentration, but there were difference in the P17 and NOX data. Only the P17 AOC concentration substantially increased after UV treatment (p value ¼ 0.021) while there was little statistical evidence that UV treatment affected NOX AOC (p value ¼ 0.381).
Effects of UV treatment on AOC
Concentration of AOC, indicator of potential biological regrowth, was investigated before and after UV irradiation. AOC was measured from increased living biomass of P17 and NOX spiked in the samples. AOC of the sand filtered water was 121 mg/L in normal times when DOC was low. There was difference in AOC levels of dry and rainy seasons with similar DOC level. AOC in dry and rainy season were 341 mg/L and 149 mg/L, respectively. The results can be interpreted that DOM in the dry season was much more biodegradable than in the rainy season. Increase of AOC was observed in some cases with L90-15, M1300, and L85 systems after UV treatment (Fig. 3). However, it was not possible to determine if the UV irradiation could
3.3. Effects of UV treatment on chlorine decay and DBPs formation Chlorine demand, chlorine decay rate, THMs and THMFP concentrations were investigated for the samples before and after UV treatment to evaluate the effect of UV disinfection on chlorine demand and DBPs formation in distribution system with post-chlorination process. The chlorine decay rate was
1.6
Residual Chlorine (mg/L)
SF L90-4 L90-15 M1300 L85 M350
SF L90-4 L90-15 M1300 L85 M350
1.4 1.2 1.0 0.8 0.6 0.4
4 °C
15 °C
0.2 0.0 0
20
40
60
80
100 120 140 160 180 200
0
20
40
60
80
100 120 140 160 180 200
Time (hrs) Fig. 5 – Chlorine decay trends before and after UV treatment.
121
Fall
100 90 80 70 60 50 40 30 20 10 0
winter
spring
Summer
THMFP(ug/L)
THMs(ug/L)
water research 44 (2010) 115–122
SF
L90-4 L90-15 M1300
L85
M350
Fall
100 90 80 70 60 50 40 30 20 10 0 SF
winter
spring
L90-4 L90-15 M1300
L85
Summer
M350
Fig. 6 – THMs and THMFP of pre- and post-UV treated water.
very rapid just after addition of chlorine and became slower with time. The rapid and slow decay rates are likely due to different reactions such as oxidation of inorganic compounds (rapid) and substitution reactions with DOM (relatively slow). In this study, the chlorine consumption in 15 min, 24 h and 48 h were defined as instant demand (ID), 24-h demand, and 48-h demand, respectively. The experiments were conducted at 4 and 15 C considering seasonal variation of temperature. ID, 24-h and 48-h demands were increased in all UV systems at 15 C while there were not significantly different among the systems at 4 C except for L90-15 and M1300 systems (Table 4, Fig. 5). This observation suggested that high energy of UV modify DOM structure and stimulate to react with chlorine at higher water temperature. In this study, rapid chlorine decay rate (K1) and slow chlorine decay rate (K2) were compared before and after UV treatment (Table 4). Rapid chlorine decay rate (K1) was increased after UV irradiation while slow chlorine decay rate (K2) does not change significantly. This suggests that UV disinfection increases the initial rapid chlorine consumption within the clearwell, but it can not affect significantly the slow chlorine decay rate in the distribution system. Chlorine consumption increased after UV irradiation can induce increase of DPBs formation. THMs and THMFP concentrations were investigated seasonally before and after UV treatment. THMs, already formed by pre-chlorination process, were not removed by UV system. On the contrary, THMFP tended to increase after UV exposure up to 16.5 %. Especially, high increases of THMFP were observed in the L90-15 and M1300 systems in summer rainy season (Fig. 6). Paired t-tests were carried out in separate group, LPUV (L90-4, L85), MPUV (M1300, M350) and all UV (L90-4, L90-15, L85, M1300, M350). THMs were not significantly different in all cases (p > 0.072). THMFP of sand filtered water was statistically different from those after UV treatment at 95 percent confidence level (LPUV p ¼ 0.065, MPUV p ¼ 0.039, All UV p ¼ 0.009). This result suggested that UV disinfection process can increase concentration of THMs by post-chlorination to prevent bacterial re-growth in drinking water distribution system, especially in case of UV system with short wavelength. Liu et al. (2006) reported that statistically significant increase in the chloroform, DCAA, TCAA, CNCl formation from chlorination of four organic waters by UV irradiation. The impacts from UV exposure were found to be most
significant in chloroform formation, and MPUV formed slightly more of chloroform than LPUV. The authors attributed the observation to lower molecular weight organic acids generated by the broader band of UV light emitted from MPUV. Buchanan et al. (2006) reported reduction after initial increase of THMFP by UV irradiation. The initial increase of THMFP at relatively low dosage is presumably consequence of halogenation of low molecular weight compounds produced by breakdown of large NOM compounds. But THMFP was reduced at high dosage, which is thought to be primarily due to removal of NOM. VUV irradiation reduced THMFP much faster than UV irradiation, which may be resulted from the faster mineralization and decrease in precursor due to hydroxyl radical produced by VUV. This hydroxyl radicals (OH) formed via water photolysis at 185 nm can mineralize organic matters (Thomson et al., 2004; White, 1999). The destructive capacity of OH radical depends entirely upon the rate of reaction between the OH radicals and the organic substrates. Unfortunately, the reaction rate of OH radical with saturated organic compounds including chloroform is very slow, so THMs can not be removed effectively by OH radical (White, 1999).
4.
Conclusions
The effects of UV disinfection on the quality of drinking water in distribution system were evaluated in three aspects, 1) potential of biological re-growth, 2) chlorine demand and 3) DBPs formation. At 40 mJ/cm2, the dosage applied for drinking water disinfection, UV treatment can not significantly affect DOM characteristics and AOC concentration which is indicator of biological re-growth in distribution system. Although the increase of low molecular portion was observed in dry season in medium pressure and 185 nm emitting low pressure systems, it did not increase AOC concentration significantly. The broken DOM is not likely small enough to be used directly by microorganisms in the distribution system. The chlorine demands and THMFP were increased after UV exposure. This observation differs from general expectation that UV disinfection can reduce post-chlorine demand and DBP formation. Modification of DOM structure by UV irradiation might stimulate reaction with chlorine, and result in increase of DBP formation.
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UV disinfection with low dosage of 40 mJ/cm2 can not mineralize DOM, but might split chemical bonds or change the characteristics of functional groups of DOM. This modification of DOM structure by UV is likely not to stimulate biological regrowth and biofilm formation in distribution system, but can have negative effects on water quality by increase of chlorine demands and DBP formation with following post-chlorination, especially in medium pressure and vacuum UV systems. To guarantee the safety of drinking water from pathogenic microorganisms and harmful DBPs at the same time, the processes to reduce the precursors of DBP are required when considering UV installation.
references
Aiken, G., Cotsaris, E., 1995. Soil and hydrology: their effect on NOM. J. Am. Water Works Assoc. 87 (1), 36–45. Buchanan, W., Roddick, F., Porter, N., 2004. Enhanced biodegradability of UV and VUV pretreated natural organic matter. Water. Sci. Technol. 4 (4), 103–111. Buchanan, W., Roddick, F., Porter, N., Drikas, M., 2005. Fractionation of UV and VUV pretreated natural organic matter from drinking water. Environ. Sci. Technol. 39, 4647–4654. Buchanan, W., Roddick, F., Porter, N., 2006. Formation of hazardous by-products resulting from the irradiation of natural organic matter: comparison between UV and VUV irradiation. Chemosphere 63, 1130–1141. Bukhari, Z., Hargy, T.M., Bolton, J.R., Dussert, B., Clancy, J.L., 1999. Medium-pressure UV for Oocyst inactivation. J. Am. Water. Works. Assoc. 91 (3), 86–94. Clancy, J.L., Bukhari, Z., Hargy, T.M., Bolton, J.R., Dussert, B.W., Marshall, M.M., 2000. Using UV to inactivate Cryptosporidium. J. Am. Water Works Assoc. 92 (9), 97–104. EPA, 1995. Method 502.2 Volatile organic compounds in water by purge and trap capillary column gas chromatography with photoionization and electrolytic conductivity detectors in series. EPA, 2006. Ultraviolet disinfection guidance manual for the final long term 2 enhanced surface water treatment rule. Chapter 2, 1–20. Frimmel, F.H., 1998. Impact of light on the properties of aquatic organic matter. Envrion. Int 24 (5/6), 559–571. Goslan, E.H., Gurses, F., Banks, J., Parsons, S.A., 2006. An investigation into reservoir NOM reduction by UV photolysis and advanced oxidation processes. Chemosphere 65, 1113–1119.
Her, N., Amy, G., McKnight, D., Sohn, J., Yoon, Y., 2003. Characterization of DOM as a function of MW by fluorescence EEM and HPLV-SEC using UVA, DOC and fluorescence detection. Water. Res. 37, 4295–4303. Jadas-He´cart, A., El Morer, A., Stitou, M., Bouillot, P., Legube, B., 1992. The chlorine demand of a treated water. Water Res. 26 (8), 1073–1084. Jeong, Y., Kweon, J., Lee, S., 2007. Characteristics of natural organic matter (NOM) on Han river and criterion of enhanced coagulation. Journal of the Korean Society of Water and Wastewater 21 (6), 653–661. Kaplan, L.A., Bott, T.L., Reasoner, D.J., 1993. Evaluation and simplification of the assimilable organic carbon nutrient bioassay for bacterial growth in drinking water. Appl. Environ. Microbiol. 59 (5), 1532–1539. Kim, S.E., Gu, Y.H., Yu, M.J., Chang, H.S., Lee, S.W., Han, S.H., 2007. Characterization of NOM behavior and DBPs formation in water treatment processes. J. KSWW 21 (4), 395–407. Kitis, M., Karanfil, T., Wigton, A., Kilduff, J.E., 2002. Probing reactivity of dissolved organic matter for disinfection byproduct formation using XAD-8 resin adsorption and ultrafiltration fractionation. Water Res. 36, 3834–3848. Liu, W., Cheung, L.-M., Yang, X., Shang, C., 2006. THM, HAA and CNCl formation from UV irradiation and chlor(am)ination of selected organic waters. Water Res. 40, 2033–2043. Ma, H., Allen, H.E., Yin, Y., 2001. Characterization of isolated fractions of dissolved organic matter from natural waters and a wastewater effluent. Water Res. 35 (4), 985–996. Oh, H.K., Kim, H.C., Ku, Y.H., Yu, M.J., Park, H., Chang, H.S., 2003. Characterization and disinfection by-product formation potential of natural organic matter in drinking water treatment. J. of KSEE 25 (10), 1252–1257. Powell, J.C., Hallam, N.B., West, J.R., Forster, C.F., Simms, J., 2000. Factors which control bulk chlorine decay rates. Water Res. 34 (1), 117–126. Shaw, J.P., Malley Jr., J.P., Willoughby, S.A., 2000. Effects of UV irradiation on organic matter. J. Am. Water Works Assoc. 92 (4), 157–167. Thomson, J., Roddick, F., Drikas, M., 2004. Vacuum ultraviolet irradiation for natural organic matter removal. J. Water SRTAqua 53, 193–206. Wetzel, R.G., 1983. Limnology, second ed. Saunders College, Publishing. 487–518, 667–678. White, M.C., Thompson, J.D., Harrington, G.W., Singer, P.C., 1997. Evaluating criteria for enhanced coagulation compliance. J. Am. Water Works Assoc. 89 (5), 64–77. White, G.C., 1999. Handbook of Chlorination and Alternative Disinfectants, fourth ed. A Wiley-Interscience Publication. 1459–1467.
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The effects of UV disinfection on drinking water quality in distribution systems Yonkyu Choi, Young-june Choi* Division of R&D for Water, Waterworks Research Institute, Seoul Metropolitan Government, 552-1, Chunho Daero, Kwangjin-Ku, Seoul, Republic of Korea, 143-820
article info
abstract
Article history:
UV treatment is a cost-effective disinfection process for drinking water, but concerned to
Received 27 May 2009
have negative effects on water quality in distribution system by changed DOM structure. In
Received in revised form
the study, the authors evaluated the effects of UV disinfection on the water quality in the
30 August 2009
distribution system by investigating structure of DOM, concentration of AOC, chlorine
Accepted 2 September 2009
demand and DBP formation before and after UV disinfection process. Although UV treat-
Published online 16 September 2009
ment did not affect concentration of AOC and characteristics of DOM (e.g., DOC, UV254,
Keywords:
weight) significantly, the increase of low molecular fraction was observed after UV treat-
UV
ment, in dry season. Chlorine demand and THMFP are also increased with chlorination of
Distribution system
UV treated water. This implies that UV irradiation can cleave DOM, but molecular weights
Molecular weight
of broken DOM are not low enough to be used directly by microorganisms in distribution
AOC
system. Nonetheless, modification of DOM structure can affect water quality of distribution
Chlorine demand
system as it can increase chlorine demands and DBPs formation by post-chlorination.
SUVA254, the ratio of hydrophilic/hydrophobic fractions, and distribution of molecular
ª 2009 Elsevier Ltd. All rights reserved.
DBP
1.
Introduction
Disinfection by ultraviolet light (UV) is considered as a costeffective and easily implementable system for drinking water disinfection. Interest in UV disinfection process has been increased sharply in drinking water industry, since researchers demonstrated that even very low dosage of UV light could inactivate Cryptosporidium effectively in the late 1990s (Bukhari et al., 1999; Clancy et al., 2000). UV spectrum is divided into four regions; vacuum UV (100w200 nm, hereafter VUV), UV-C (200w280 nm), UV-B (280w315 nm), and UV-A (315w400 nm). UV disinfection primarily occurs due to the germicidal action of UV-B and UVC light on microorganisms. Although VUV can disinfect microorganisms, it is not efficient to use VUV for water disinfection because it rapidly dissipates through water in
very short distances (EPA, 2006). VUV is also known to breakdown bonds of organic carbons (Buchanan et al., 2004; Thomson et al., 2004). Two UV systems are generally applied for drinking water disinfection process. Monochromatic low pressure UV (hereafter LPUV) emits single wavelength at 254 nm which is close to the maximum microbial action spectrum. Polychromatic medium pressure UV (hereafter MPUV) emits a wide range of wavelength including UV-A, -B, -C and visible light. Special LPUV emitting two wavelengths at 185 and 254 nm (hereafter LPUV for TOC) is applied to remove TOC for producing ultrapure water. Although these UV systems are inactivate most of microorganisms effectively except for some viruses, they can not guarantee biological safety of tap water because the effect of UV irradiation can not be maintained throughout
* Corresponding author. Tel.: þ82 2 3146 1810; fax: þ82 2 3146 1811. E-mail address: [email protected] (Y.-j. Choi). 0043-1354/$ – see front matter ª 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2009.09.011
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water research 44 (2010) 115–122
distribution system. On the contrary, UV disinfection is concerned to have negative effects on water quality by UV photolysis. Many researchers have reported that UV irradiation can modify DOM structure and increase biodegradability (Frimmel, 1998; Thomson et al., 2004; Buchanan et al., 2005; Goslan et al., 2006). Especially, VUV irradiation is known to be more effective than UV-C irradiation in formation of biodegradable compounds and mineralization (Buchanan et al., 2004). UV-A and UV-B can also splits large NOM molecules into organic acids with lower molecular weight (Frimmel, 1998). This change of DOM structure can increase biodegradability, which stimulates microbial regrowth and biofilm formation in distribution system. Increase of biofilm can also cause taste and odor problems and reduction of hydraulic capacity (Shaw et al., 2000). Therefore, sequential disinfection process with additional chemical disinfectant such as chlorine or monochloramine was applied to prevent microbial re-growth in the distribution system. With chlorination as secondary disinfection process, UV treatment is often expected to reduce chlorine demand and DBPs formation. Liu et al. (2006), however, reported that the DBPs formation of four organic waters was increased by chlorination after UV irradiation. The effect of UV irradiation on water quality depends on many factors, such as characteristics of source water quality, UV wavelength and applied dosage. Previous studies have often been carried out under bench-scale conditions, and organic water with relatively high DOC level (5w17.4 mg/L) and high UV dosage of 14w1,000 J/cm2 were used, which were not the conditions used for drinking water disinfection process (Frimmel, 1998; Buchanan et al., 2006; Goslan et al., 2006; Liu et al., 2006). Under drinking water with low DOC level less than 2 mg/L and UV dosage less than 40 mJ/cm2, the impact of UV irradiation on water quality could be different from the results of the previous studies. Moreover, the results under lab scale bench test can hardly reflect the real reactions under full-scale continuous flow system. In this study, the authors used pilot-scale continuous flow UV systems with LPUV, LPUV for TOC, and MPUV, and investigated change of DOM structure, probability of microbial regrowth, chlorine demand and THMs formation before and after UV treatment to evaluate the effects of UV disinfection on water quality in distribution system.
2.1.
UV pilot plant
The UV pilot plant with four UV reactors, LPUV (L85), LPUV for TOC (L90), MPUV (M1300, M350), is installed at the end of sand filters in the WTP. Sand filtered water (SF) was introduced to the reactors, and total capacity of the system was 1080 m3/ day. The experiments were carried out with UV dose of 40 mJ/cm2, which was usually applied for drinking water disinfection process. LPUV for TOC (L90) emitting two wavelengths at 185 and 254 nm is installed to evaluate TOC removal efficiency of vacuum UV. As higher UV dosage is required for TOC mineralization, additional experiments were carried out with UV dose of 150 mJ/cm2. UV dosage of each reactor was calculated from UV intensity by online sensor and contact time at each flow rate. Online sensor of LPUV (L90 and L85) and MPUV (M1300 and M350) can measure at 254 nm and between 200w300 nm, respectively. L90 system emits UV light with 254 nm and 185 nm with the ratio of 3:1. The detailed characteristics of each system were listed in Table 1. The sand filtered water and the five UV treated waters were investigated. The samples were taken from both the inflow and outflow of each reactor.
2.2.
Analytical method
The samples taken from the pilot plant were brought to the laboratory in 2 h and stored in the refrigerator below 4 C. For analyses of THMs already formed by pre-chlorination, ascorbic acid and HCl (1 þ 1) was added instantly to the samples (40 mL) to quench residual chlorine. For THMFP analyses, the samples were chlorinated (TOC : chlorine ¼ 1: 3) and incubated at 25 C for 48 h. After incubation, residual chlorine was quenched with ascorbic acid and HCl (1 þ 1) not to form THMs any more. THMs were analyzed by purge and trap method with GC (Varian, CX3600) equipped with ECD detector according to the EPA 502.2 (EPA, 1995). DOC and UV254 were analyzed with TOC analyzer (Ionics, Sievers 820) and UV/VIS spectrophotometer (Varian Cary 3C), respectively. SUVA254 was calculated from DOC and UV254.
2.3.
Separation of hydrophilic and hydrophobic carbon
DOM was separated into hydrophobic and hydrophilic fractions with resin (Amberitic XAD-7HP, Rohm & Haas Co.,
Table 1 – The characteristics of the UV system in the pilot plant.
2.
Materials and methods
In this study, the characteristics of DOM, biological re-growth potential, chlorine demand, and formation potential of disinfection byproducts before and after UV irradiation were compared to evaluate the effects of UV disinfection on water quality in distribution system. A UV pilot plant was installed at a water treatment plant (WTP) in Seoul, Korea. The samples were taken three times in 2005 and 2006, considering seasonal variation of the raw water quality ; 1) dry season with high algal biomass and BOD from winter to spring, 2) rainy season with high turbidity due to heavy rainfall during summer, and 3) normal times (Fig. 2).
System Lamp type Wavelength of UV Capacity Dosage emission (nm) (m3/h) (mJ/cm2) L90-4 L90-15
90 W Low pressure for TOC
185, 254
180 50
40 150
M1300
1.3 kW Medium pressure 85 W Low pressure 350 W Medium pressure
185w400
650
40
254
120
200w400
260
L85 M350
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water research 44 (2010) 115–122
Table 2 – DOC, UV254, and SUVA254 of pre- and post-UV treated water. System
UV254 (cm1)
DOC (mg/L) Normal
Dry
Rainy
Normal
Dry
Rainy
Normal
Dry
Rainy
0.96 0.96 0.96 0.97 0.96 0.95
1.26 1.31 1.34 1.25 1.27 1.23
1.27 1.27 1.30 1.24 1.24 1.23
0.014 0.014 0.013 0.014 0.014 0.015
0.015 0.015 0.014 0.015 0.015 0.015
0.022 0.022 0.020 0.022 0.022 0.020
1.45 1.45 1.35 1.44 1.45 1.57
1.19 1.14 1.04 1.20 1.18 1.21
1.73 1.73 1.53 1.77 1.77 1.62
SF L90-4 L90-15 M1300 L85 M350
France). Resin was cleaned with sequential soxhlet extraction method (Ma et al., 2001). XAD-7HP resin was packed in 31 mm (ID) 230 mm (H) glass column and 0.5 N NaOH was introduced into the column to clean the resin. The resin was extracted sequentially with methanol, acetonitrile, and methanol for 12 h. Finally, the column was rinsed with ultrapure water, 0.1 N NaOH, 0.1 N HCl and ultrapure water in order, until the concentration of TOC of the effluent was less than 0.1 mg/L. Each sample was adjusted to pH < 2 by adding (1 þ 1) H3PO4 and passed through clean glass column with flow rate of 15w20 mL/min. The hydrophobic carbon was the fraction that adsorbed to the surface of the resin and the carbon that passed out through the column was determined as hydrophilic fraction. After hydrophilic and hydrophobic fractions were adjusted pH 7 0.2 with 0.1 N H3PO4 and 0.1 N NaOH, DOC was analyzed with TOC Analyzer (Ionics, Sievers 820).
to maintain constant pH and ionic strength for all samples and reduce undesirable interactions. Number-averaged MW (Mn), weight-averaged MW (Mw), and polydispersivity (r) were determined using the following equations. hi and Mi are the height of HPLC-SEC chromatogram and molecular weight. n P
hi Mn ¼ i¼1 n P hi Mi
i¼1
n P
Mw ¼ i¼1
hi Mi
n P
hi
i¼1
r¼
2.4.
SUVA254 (L/mg$m)
Mw Mn
Apparent molecular weight 2.5.
High performance liquid chromatography-size exclusion chromatography (HPLC-SEC) was used to fractionate apparent molecular weight of DOM (Her et al., 2003). Separation by size exclusion was performed using a TSK-50S (Toyopearl HW SOS, 30 mm resin) column prior to sequential on-line detectors consisting of UV/Visble (SPD-20AD, Shimadzu) and DOC (Modified Sievers Total Organic Carbon Analyzer 820 Turbo). Mobile phase solution (pH 6.8 and ionic strength 0.1 M) was made with 4 mM phosphate buffer and 25 mM sodium sulfate. Polyethylene glycols (PEGs, 200 600, 2000, 4000, 8000 dalton) were used for molecular weight (MW) calibration of chromatograms. The pH and ionic strength of each sample were also adjusted with phosphate buffer and sodium sulfate solutions as similar to the mobile phase as possible before analysis
Assimilable organic carbon(AOC)
AOC was analyzed with the method proposed by Kaplan et al. (1993). AOC is defined as the amount of carbon used as energy or converted into biomass by bacteria. Two pure-culture bacterial strains, Pseudomonas fluorescens strain P17 (hereafter, P17) and Spirillum strain NOX (hereafter, NOX) were used. The sample was taken in a glass vial baked at 550 C over 2 h and sodium thiosulfate was added to quench residual chlorine. The sample was pasteurized at 70 C for 30 min in water bath, and spiked with P17 and NOX, and incubated at 15 C for 7 days. The incubated sample was taken out, inoculated in R2A media and incubated at 25 C for 72 h. The colony counts of P17 and NOX in stationary phase were converted into bacterial biomass by multiplying each carbon conversion
Hydrophilic
Hydrophobic
100% 20
22
21
19
20
19
80%
30
29
28
70
71
72
31
30
29
69
70
71
L85
M350
35
34
34
35
33
38
65
66
66
65
67
62
L85
M350
60% 40%
80
78
79
81
80
81
20% 0% SF
L90-4 L90-15 M1300
Dry season
L85
M350
SF
L90-4 L90-15 M1300
Normal times
SF
L90-4 L90-15 M1300
Rainy season
Fig. 1 – The ratio of hydrophilic and hydrophobic fractions in pre- and post-UV treated water.
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water research 44 (2010) 115–122
Table 3 – Percentage of each fraction of molecular weight in pre- and post-UV treated water of each system. System
>2 K
1–2 K
0.5–1 K
<0.5 K
Mn
Mw
r
Normal times
SF L90-4 L90-15 M1300 L85 M350
4.3 5.2 4.9 4.5 5.1 4.7
17.0 17.0 17.1 16.6 17.3 16.0
32.1 31.3 31.6 31.7 31.6 31.4
46.6 46.5 46.4 47.2 46.0 47.9
750 766 771 740 773 739
1574 1558 1581 1427 1547 1446
2.10 2.04 2.05 1.93 2.00 1.96
Dry season
SF L90-4 L90-15 M1300 L85 M350
5.4 3.5 5.0 4.1 5.5 4.4
15.1 14.2 14.8 14.6 15.1 14.7
29.0 29.8 29.0 29.7 28.6 30.1
50.5 52.5 51.2 51.6 50.8 50.8
744 736 671 696 756 707
1627 1640 1246 1392 1709 1454
2.19 2.23 1.86 2.00 2.26 2.06
Rainy season
SF L90-4 L90-15 M1300 L85 M350
1.7 1.8 1.5 1.9 2.1 1.9
13.2 13.3 13.0 13.5 13.5 13.3
31.6 31.4 31.0 31.5 31.1 31.2
53.5 53.5 54.5 53.1 53.3 53.6
613 602 616 622 628 619
979 984 1002 1046 1078 1033
1.60 1.63 1.63 1.68 1.72 1.67
Mn: Number-average molecular weight Mw: Weight-average molecular weight
factor. P17 AOC and NOX AOC were calculated by the following equations, and AOC was calculated by the sum of P17 AOC and NOX AOC. P17ðCFU=mLÞ 1000 mL=L 4:1 106 ðCFU=mgCÞ
NOX AOCðmg=LÞ ¼
NOXðCFU=mLÞ 1000 mL=L 1:2 107 ðCFU=mgCÞ
AOCðmg=LÞ ¼ P17 AOC þ NOX AOC
2.6.
Chlorine demand and decay rate
Chlorine demand and decay rate were estimated for the sand filtered water and the UV treated water taken from each UV
Turbidity (NTU), Rain fall (mm), Chl.a (µg/L)
250
5 Rainy season
200 Dry seaon
Turbidity Rain fall Chl.a BOD TOC Sampling
4
Noraml times
150
3
100
2
50
1
0 '05
BOD, TOC (mg/L)
P17 AOCðmg=LÞ ¼
reactor. Chlorine decay rate was measured with the procedure proposed by Powell et al. (2000). The freshly cleaned glassware was filled with distilled water and sodium hypochlorite solution was added to make 10 mg/L of free chlorine solution and left for 24 h. It was then emptied, rinsed thoroughly with ultrapure water and left to dry. 2 L volumetric flask was filled with ultrapure water and the sample water. Chlorine was added to 1w 2 mg/L and left for 15 min to ensure homogeneity. The sample water was decanted into eleven 125 mL brown glass bottles without headspace and sealed with teflon lined caps. All the bottles were stored in the incubator, at 4 and 15 C. The chlorine concentration was measured with time. Initial chlorine concentration was defined as the chlorine concentration when the same amount of chlorine was added to 2 L of ultrapure water. Chlorine concentration was measured by the DPD colorimetric method using Hach pocket
0
Oct. Nov. Dec.'06 Jan. Feb. Mar. Apr. May Jun Jul. Aug. Sep. Oct. Nov. Dec.
Month Fig. 2 – Hydraulic characteristics and the raw water quality change by season.
water research 44 (2010) 115–122
450
Normal times
Dry season
Rainy season
400 350
AOC (µg/L)
300 250 200 150 100 50 0
SF
L90-4
L90-15
M1300
L85
M350
Fig. 3 – Seasonal AOC concentration before and after UV treatment in each system.
chlorine colorimeters (pocket Hachs). All samples were taken and analyzed in triplicate. The decay rate constants were estimated by the first-order chlorine decay model (Jadas-He´cart et al., 1992). In this study, two chlorine decay rates were used, i.e. rapid chlorine decay rate (K1) for the first 4 h and slow chlorine decay rate (K2) after 4 h considering the retention time in the clearwell of the WTP.
3.
Results and discussions
3.1.
Effects of UV treatment on DOM properties
DOC, UV254, SUVA254, hydrophilic/hydrophobic ratio and apparent molecular weight before and after UV treatment were investigated. The DOC concentration of the sand filtered water was less than 1 mg/L in normal times but increased to 1.5 mg/L in dry and rainy seasons.
0
20
Pre-UV NOX AOC (µg/L) 40 60 80
100
120
400
120
300
80 L90-4 (P17) L90-15 (P17) M1300 (P17) L85 (P17) M350 (P17)
200
60
equal value line L90-4 (NOX) L90-15 (NOX) M1300 (NOX) L85 (NOX) M350 (NOX)
100
0
0
100 200 300 Pre-UV P17 AOC (µg/L)
40
20
400
Fig. 4 – P17 AOC and NOX AOC before and after UV treatment.
0
Post-UV NOX AOC (µg/L)
Post-UV P17 AOC (µg/L)
100
119
There were little change in DOC, UV254, and SUVA254 after UV treatment throughout all seasons (Table 2). Only in the L90 system, with low pressure lamp for TOC reduction, a little decrease of UV254 and SUVA254 were observed with the UV dosage of 150 mJ/cm2. The reductions might be caused by high energy of short wavelength at 185 nm and high dosage of 150 mJ/cm2. With dosage of 40 mJ/cm2, which is usually applied for drinking water disinfection process in WTP, all UV systems had no effect on DOC, UV254, and SUVA254. The ratio of hydrophilic and hydrophobic fractions was calculated from DOC concentration of each fraction. The fraction of hydrophilic DOC was relatively high throughout all seasons with the range of 62w81 %, but hydrophobic fraction was increased in rainy season (Fig. 1). The source water from the Han river, has been known to have relatively higher concentration of hydrophilic organic fraction (Oh et al., 2003; Kim et al., 2007; Jeong et al., 2007). The ratio of hydrophilic and hydrophobic fractions can be changed in water treatment process. The hydrophilic fraction tends to be increased in treated water as humic material with high SUVA value and high hydrophobic organic carbon is removed easily by coagulation process (White et al., 1997). It was also reported, from the previous studies with the Han river as the source water, that hydrophilic fraction in the settled water was increased (Oh et al., 2003; Kim et al., 2007). However, there was not significant difference in the ratio of hydrophilic and hydrophobic fractions before and after UV treatment throughout all seasons. Shaw et al. (2000) also reported that there was little or no statistical evidence that hydrophilic and hydrophobic ratios were altered by UV treatment. Distribution of apparent molecular weight was measured by HPLC-SEC system with UV and TOC detectors. The molecular weight of most DOM (over 95 %) was less than 2 kDa (Table 3), and especially DOM fraction between 0.3 and 0.4 kDa was dominant throughout all seasons. While the distribution of apparent molecular weight was not changed before and after UV treatment in normal times and rainy season, there was increase in low molecular weight fraction around 0.3 kDa after UV treatment in dry season (data was not shown). Number-averaged molecular weights (Mn) and weightaveraged molecular weight (Mw) have also shown that average molecular weight in the post-UV treated water was decreased in dry season (Table 3). This suggested that DOM structure in dry season is broken down more easily by UV radiation than those in other seasons. DOM structure might be related with the origin of DOM of each season. In aquatic system, the origin of DOM can be categorized as allochthonous DOM entering from the terrestrial watershed, and autochthonous DOM derived from biota (e.g., algae, bacteria) growing in the water body (Aiken and Cotsaris, 1995). In Korea, during the rainy season in late summer with lots of heavy rain, DOC increases because heavy rain washes large amount of organic carbon from the watershed into river while in dry season, algal biomass and BOD increases (Fig. 2). This allochthonous DOM in rainy season is known to be relatively refractory DOM with high SUVA, high molecule weight, and hydrophobic properties. In contrast, the autochthonous DOM is relatively labile, and consists of low SUVA, low molecular weight, and hydrophilic DOM (Wetzel, 1983; Kitis et al., 2002). Ma et al. (2001) reported that hydrophilic fraction was
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water research 44 (2010) 115–122
Table 4 – Chlorine demands and chlorine decay rate before and after UV treatment, K1 : rapid decay rate(< 4 h), K2 : slow decay rate(> 4 h). 4 C Chlorine demands (mg/L)
SF L90-4 L90-15 M1300 L85 M350
15 C Decay rate (h1)
ID
24 h
48 h
K1
K2
0.10 0.09 0.22 0.12 0.10 0.11
0.50 0.54 0.71 0.56 0.51 0.48
0.60 0.63 0.79 0.68 0.61 0.60
0.52 0.57 0.73 0.67 0.62 0.57
0.005 0.006 0.004 0.005 0.005 0.005
composed of more simple compounds and less complex mixtures. Decrease in molecular weight of DOM was shown with L9015, M1300 and M350 systems in dry season. The observation suggested that short wavelength below 254 nm is more effective to break down the bonds of organic carbons, and various wavelength of light could be related to degradation of DOM. It has been reported that UVA (315–400 nm) and UVB (280–315 nm) splits large DOM molecules to generate lower molecular weight organic acids (Frimmel, 1998).
3.2.
Chlorine demands (mg/L) ID 0.06 0.21 0.22 – 0.16 0.16
Decay rate (h1)
24 h
48 h
K1
K2
0.32 0.50 0.55 0.49 0.46 0.49
0.44 0.61 0.66 0.55 0.58 0.61
0.025 0.073 0.079 0.060 0.060 0.074
0.004 0.005 0.006 0.006 0.006 0.006
affect AOC level, since there was not consistent trend of increase in each system. AOC after UV exposure was compared with AOC of the sand filtered water. P17 AOC and NOX AOC of all UV systems were plotted against a line of equal value. More P17 AOC data points fell on or above the line than below, while more NOX AOC data points fell on or below than above the line (Fig. 4). Paired t-tests were carried out in separate group, LPUV (L90-4, L85) and MPUV (M1300, M350). P17 AOC, NOX AOC and AOC of sand filtered water were not different statistically from those of LPUV (p ¼ 0.557, 0.964, 0.545) and MPUV (p ¼ 0.234, 0.053, 0.386) at 95 percent confidence level. Shaw et al. (2000) reported that UV treatment did not appear to affect the AOC concentration, but there were difference in the P17 and NOX data. Only the P17 AOC concentration substantially increased after UV treatment (p value ¼ 0.021) while there was little statistical evidence that UV treatment affected NOX AOC (p value ¼ 0.381).
Effects of UV treatment on AOC
Concentration of AOC, indicator of potential biological regrowth, was investigated before and after UV irradiation. AOC was measured from increased living biomass of P17 and NOX spiked in the samples. AOC of the sand filtered water was 121 mg/L in normal times when DOC was low. There was difference in AOC levels of dry and rainy seasons with similar DOC level. AOC in dry and rainy season were 341 mg/L and 149 mg/L, respectively. The results can be interpreted that DOM in the dry season was much more biodegradable than in the rainy season. Increase of AOC was observed in some cases with L90-15, M1300, and L85 systems after UV treatment (Fig. 3). However, it was not possible to determine if the UV irradiation could
3.3. Effects of UV treatment on chlorine decay and DBPs formation Chlorine demand, chlorine decay rate, THMs and THMFP concentrations were investigated for the samples before and after UV treatment to evaluate the effect of UV disinfection on chlorine demand and DBPs formation in distribution system with post-chlorination process. The chlorine decay rate was
1.6
Residual Chlorine (mg/L)
SF L90-4 L90-15 M1300 L85 M350
SF L90-4 L90-15 M1300 L85 M350
1.4 1.2 1.0 0.8 0.6 0.4
4 °C
15 °C
0.2 0.0 0
20
40
60
80
100 120 140 160 180 200
0
20
40
60
80
100 120 140 160 180 200
Time (hrs) Fig. 5 – Chlorine decay trends before and after UV treatment.
121
Fall
100 90 80 70 60 50 40 30 20 10 0
winter
spring
Summer
THMFP(ug/L)
THMs(ug/L)
water research 44 (2010) 115–122
SF
L90-4 L90-15 M1300
L85
M350
Fall
100 90 80 70 60 50 40 30 20 10 0 SF
winter
spring
L90-4 L90-15 M1300
L85
Summer
M350
Fig. 6 – THMs and THMFP of pre- and post-UV treated water.
very rapid just after addition of chlorine and became slower with time. The rapid and slow decay rates are likely due to different reactions such as oxidation of inorganic compounds (rapid) and substitution reactions with DOM (relatively slow). In this study, the chlorine consumption in 15 min, 24 h and 48 h were defined as instant demand (ID), 24-h demand, and 48-h demand, respectively. The experiments were conducted at 4 and 15 C considering seasonal variation of temperature. ID, 24-h and 48-h demands were increased in all UV systems at 15 C while there were not significantly different among the systems at 4 C except for L90-15 and M1300 systems (Table 4, Fig. 5). This observation suggested that high energy of UV modify DOM structure and stimulate to react with chlorine at higher water temperature. In this study, rapid chlorine decay rate (K1) and slow chlorine decay rate (K2) were compared before and after UV treatment (Table 4). Rapid chlorine decay rate (K1) was increased after UV irradiation while slow chlorine decay rate (K2) does not change significantly. This suggests that UV disinfection increases the initial rapid chlorine consumption within the clearwell, but it can not affect significantly the slow chlorine decay rate in the distribution system. Chlorine consumption increased after UV irradiation can induce increase of DPBs formation. THMs and THMFP concentrations were investigated seasonally before and after UV treatment. THMs, already formed by pre-chlorination process, were not removed by UV system. On the contrary, THMFP tended to increase after UV exposure up to 16.5 %. Especially, high increases of THMFP were observed in the L90-15 and M1300 systems in summer rainy season (Fig. 6). Paired t-tests were carried out in separate group, LPUV (L90-4, L85), MPUV (M1300, M350) and all UV (L90-4, L90-15, L85, M1300, M350). THMs were not significantly different in all cases (p > 0.072). THMFP of sand filtered water was statistically different from those after UV treatment at 95 percent confidence level (LPUV p ¼ 0.065, MPUV p ¼ 0.039, All UV p ¼ 0.009). This result suggested that UV disinfection process can increase concentration of THMs by post-chlorination to prevent bacterial re-growth in drinking water distribution system, especially in case of UV system with short wavelength. Liu et al. (2006) reported that statistically significant increase in the chloroform, DCAA, TCAA, CNCl formation from chlorination of four organic waters by UV irradiation. The impacts from UV exposure were found to be most
significant in chloroform formation, and MPUV formed slightly more of chloroform than LPUV. The authors attributed the observation to lower molecular weight organic acids generated by the broader band of UV light emitted from MPUV. Buchanan et al. (2006) reported reduction after initial increase of THMFP by UV irradiation. The initial increase of THMFP at relatively low dosage is presumably consequence of halogenation of low molecular weight compounds produced by breakdown of large NOM compounds. But THMFP was reduced at high dosage, which is thought to be primarily due to removal of NOM. VUV irradiation reduced THMFP much faster than UV irradiation, which may be resulted from the faster mineralization and decrease in precursor due to hydroxyl radical produced by VUV. This hydroxyl radicals (OH) formed via water photolysis at 185 nm can mineralize organic matters (Thomson et al., 2004; White, 1999). The destructive capacity of OH radical depends entirely upon the rate of reaction between the OH radicals and the organic substrates. Unfortunately, the reaction rate of OH radical with saturated organic compounds including chloroform is very slow, so THMs can not be removed effectively by OH radical (White, 1999).
4.
Conclusions
The effects of UV disinfection on the quality of drinking water in distribution system were evaluated in three aspects, 1) potential of biological re-growth, 2) chlorine demand and 3) DBPs formation. At 40 mJ/cm2, the dosage applied for drinking water disinfection, UV treatment can not significantly affect DOM characteristics and AOC concentration which is indicator of biological re-growth in distribution system. Although the increase of low molecular portion was observed in dry season in medium pressure and 185 nm emitting low pressure systems, it did not increase AOC concentration significantly. The broken DOM is not likely small enough to be used directly by microorganisms in the distribution system. The chlorine demands and THMFP were increased after UV exposure. This observation differs from general expectation that UV disinfection can reduce post-chlorine demand and DBP formation. Modification of DOM structure by UV irradiation might stimulate reaction with chlorine, and result in increase of DBP formation.
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UV disinfection with low dosage of 40 mJ/cm2 can not mineralize DOM, but might split chemical bonds or change the characteristics of functional groups of DOM. This modification of DOM structure by UV is likely not to stimulate biological regrowth and biofilm formation in distribution system, but can have negative effects on water quality by increase of chlorine demands and DBP formation with following post-chlorination, especially in medium pressure and vacuum UV systems. To guarantee the safety of drinking water from pathogenic microorganisms and harmful DBPs at the same time, the processes to reduce the precursors of DBP are required when considering UV installation.
references
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