The effects of UV disinfection on drinking water quality in distribution systems.
ABSTRACT UV treatment is a cost-effective disinfection process for drinking water, but concerned to have negative effects on water quality in distribution system by changed DOM structure. In the study, the authors evaluated the effects of UV disinfection on the water quality in the distribution system by investigating structure of DOM, concentration of AOC, chlorine demand and DBP formation before and after UV disinfection process. Although UV treatment did not affect concentration of AOC and characteristics of DOM (e.g., DOC, UV(254,) SUVA(254), the ratio of hydrophilic/hydrophobic fractions, and distribution of molecular weight) significantly, the increase of low molecular fraction was observed after UV treatment, in dry season. Chlorine demand and THMFP are also increased with chlorination of UV treated water. This implies that UV irradiation can cleave DOM, but molecular weights of broken DOM are not low enough to be used directly by microorganisms in distribution system. Nonetheless, modification of DOM structure can affect water quality of distribution system as it can increase chlorine demands and DBPs formation by post-chlorination.
- Biophysical Journal 01/2011; 100(3). · 3.67 Impact Factor
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ABSTRACT: This study assessed the usability of effluent water discharged from a secondary municipal wastewater treatment plant for mass cultivation of microalgae for biofuel production. It was observed that bacteria and protozoa in the effluent water exerted a negative impact on the growth of Chlorella sp. 227. To reduce the effect, filtration or UV-radiation were applied on the effluent water as pre-treatment methods. Of all the pretreatment options tested, the filtration (by 0.2 μm) resulted in the highest biomass and lipid productivity. To be comparable with the growth in the autoclaved effluent water, the filtration with a proper pore size filter (less than 0.45 μm) or UV-B radiation of a proper dose (over 1620 mJ cm(-2)) are proposed. These findings led us to conclude that the utilization can be realized only when bacteria and other microorganisms are greatly reduced or eliminated from the effluent prior to its use.Bioresource Technology 03/2011; 102(18):8639-45. · 5.04 Impact Factor
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ABSTRACT: The impact of orthophosphate addition on biofilm formation and water quality was studied in corrosion-resistant stainless steel (STS) pipe and corrosion-susceptible ductile cast iron (DCI) pipe using cultivation and culture-independent approaches. Sample coupons of DCI pipe and STS pipe were installed in annular reactors, which were operated for 9 months under hydraulic conditions similar to a domestic plumbing system. Addition of 5 mg/L of phosphate to the plumbing systems, under low residual chlorine conditions, promoted a more significant growth of biofilm and led to a greater rate reduction of disinfection by-products in DCI pipe than in STS pipe. While the level of THMs (trihalomethanes) increased under conditions of low biofilm concentration, the levels of HAAs (halo acetic acids) and CH (chloral hydrate) decreased in all cases in proportion to the amount of biofilm. It was also observed that chloroform, the main species of THM, was not readily decomposed biologically and decomposition was not proportional to the biofilm concentration; however, it was easily biodegraded after the addition of phosphate. Analysis of the 16S rDNA sequences of 102 biofilm isolates revealed that Proteobacteria (50%) was the most frequently detected phylum, followed by Firmicutes (10%) and Actinobacteria (2%), with 37% of the bacteria unclassified. Bradyrhizobium was the dominant genus on corroded DCI pipe, while Sphingomonas was predominant on non-corroded STS pipe. Methylobacterium and Afipia were detected only in the reactor without added phosphate. PCR-DGGE analysis showed that the diversity of species in biofilm tended to increase when phosphate was added regardless of the pipe material, indicating that phosphate addition upset the biological stability in the plumbing systems.The Journal of Microbiology 02/2012; 50(1):17-28. · 1.28 Impact Factor
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
a r t i c l e i n f o
Received 27 May 2009
Received in revised form
30 August 2009
Accepted 2 September 2009
Published online 16 September 2009
a b s t r a c t
UV treatment is a cost-effective disinfection process for drinking water, but concerned to
have negative effects on water quality in distribution system by changed DOM structure. In
the study, the authors evaluated the effects of UV disinfection on the water quality in the
distribution system by investigating structure of DOM, concentration of AOC, chlorine
demand and DBP formation before and after UV disinfection process. Although UV treat-
ment did not affect concentration of AOC and characteristics of DOM (e.g., DOC, UV254,
SUVA254, the ratio of hydrophilic/hydrophobic fractions, and distribution of molecular
weight) significantly, the increase of low molecular fraction was observed after UV treat-
ment, in dry season. Chlorine demand and THMFP are also increased with chlorination of
UV treated water. This implies that UV irradiation can cleave DOM, but molecular weights
of broken DOM are not low enough to be used directly by microorganisms in distribution
system. Nonetheless, modification of DOM structure can affect water quality of distribution
system as it can increase chlorine demands and DBPs formation by post-chlorination.
ª 2009 Elsevier Ltd. All rights reserved.
Disinfection by ultraviolet light (UV) is considered as a cost-
effective and easily implementable system for drinking water
disinfection. Interest in UV disinfection process has been
increased sharplyin drinking
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 UV-
C 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 (here-
after 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 ultra-
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@example.com (Y.-j. Choi).
0043-1354/$ – see front matter ª 2009 Elsevier Ltd. All rights reserved.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
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 irradi-
ation can modify DOM structure and increase biodegrad-
ability (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 re-
growth 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 distribu-
tion 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 organicwater with
(5w17.4 mg/L) and high UV dosage of 14w1,000 J/cm2were
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
In this study, the authors used pilot-scale continuous flow
UV systems with LPUV, LPUV for TOC, and MPUV, and inves-
tigated change of DOM structure, probability of microbial re-
growth, chlorine demand and THMs formation before and
after UV treatment to evaluate the effects of UV disinfection
on water quality in distribution system.
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).
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 wave-
lengths at 185 and 254 nmis installed to evaluate TOCremoval
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 inten-
sity 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 systemwerelisted in Table 1. The sandfilteredwaterand
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 (1þ1) was added instantly to the samples (40 mL)
to quench residual chlorine. For THMFP analyses, the samples
were chlorinated (TOC : chlorine¼1: 3) and incubated at 25?C
for 48 h. After incubation, residual chlorine was quenched with
ascorbic acid and HCl (1þ 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 UV254were analyzed with TOC analyzer
3C), respectively. SUVA254was calculated from DOC and UV254.
2.3.Separation of hydrophilic and hydrophobic carbon
DOM was separated into hydrophobic and hydrophilic frac-
tions with resin (Amberitic XAD-7HP, Rohm & Haas Co.,
Table 1 – The characteristics of the UV system in the pilot
System Lamp type Wavelength of UV
90 W Low
M1300 1.3 kW
85 W Low
water research 44 (2010) 115–122
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)glasscolumnand0.5 NNaOHwasintroduced
into the column to clean the resin. The resin was extracted
sequentially with methanol, acetonitrile, and methanol for
12 h.Finally,thecolumnwasrinsedwithultrapurewater,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.
passed through clean glass column with flow rate of
15w20 mL/min. The hydrophobic carbon was the fraction that
out through the column was determined as hydrophilic frac-
tion. After hydrophilic and hydrophobic fractions were
adjusted pH 7?0.2 with 0.1 N H3PO4and 0.1 N NaOH, DOC was
analyzed with TOC Analyzer (Ionics, Sievers 820).
2.4.Apparent molecular weight
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 con-
sisting of UV/Visble (SPD-20AD, Shimadzu) and DOC (Modified
Sievers Total Organic Carbon Analyzer 820 Turbo). Mobile
phase solution(pH6.8and ionicstrength 0.1 M)was madewith
4 mM phosphate buffer and 25 mM sodium sulfate. Poly-
ethylene glycols (PEGs, 200 600, 2000, 4000, 8000 dalton)
were used for molecular weight (MW) calibration of chro-
matograms. The pH and ionic strength of each sample were
also adjusted with phosphate buffer and sodium sulfate solu-
tions as similar to the mobile phase as possible before analysis
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. hiand Miare the
height of HPLC-SEC chromatogram and molecular weight.
2.5.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
Table 2 – DOC, UV254, and SUVA254of pre- and post-UV treated water.
SystemDOC (mg/L)UV254(cm?1) SUVA254(L/mg$m)
SFL90-4 L90-15 M1300L85 M350
SF L90-4 L90-15 M1300L85M350
SF L90-4 L90-15 M1300
Fig. 1 – The ratio of hydrophilic and hydrophobic fractions in pre- and post-UV treated water.
water research 44 (2010) 115–122
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.
P17 AOCðmg=LÞ ¼
4:1 ? 106ðCFU=mgCÞ? 1000 mL=L
NOX AOCðmg=LÞ ¼
1:2 ? 107ðCFU=mgCÞ? 1000 mL=L
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
reactor. Chlorine decay rate was measuredwith the procedure
proposed by Powell et al. (2000). The freshly cleaned glassware
was filled with distilled water and sodium hypochlorite solu-
tion 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 homoge-
neity. 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 chlo-
rine 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
Table 3 – Percentage of each fraction of molecular weight in pre- and post-UV treated water of each system.
>2 K1–2 K 0.5–1 K
Normal times SF
Mn: Number-average molecular weight Mw: Weight-average molecular weight
BOD, TOC (mg/L)
Turbidity (NTU), Rain fall (mm), Chl.a (µg/L)
Oct. Nov. Dec. Jan. Feb. Mar. Apr. May Jun Jul. Aug. Sep. Oct. Nov. Dec.
Fig. 2 – Hydraulic characteristics and the raw water quality change by season.
water research 44 (2010) 115–122
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.
There were little change in DOC, UV254, and SUVA254after
UV treatment throughout all seasons (Table 2). Only in the L90
system, with low pressure lamp for TOC reduction, a little
decrease of UV254and SUVA254were 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 coag-
ulation 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 hydro-
phobic fractions before and after UV treatment throughout all
seasons. Shaw et al. (2000)also reportedthat therewas 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 molec-
ular 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 weight-
averagedmolecular 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
Normal timesDry season Rainy season
Fig. 3 – Seasonal AOC concentration before and after UV
treatment in each system.
Pre-UV P17 AOC (µg/L)
equal value line
Pre-UV NOX AOC (µg/L)
Post-UV P17 AOC (µg/L)
Post-UV NOX AOC (µg/L)
Fig. 4 – P17 AOC and NOX AOC before and after UV
water research 44 (2010) 115–122
composed of more simple compounds and less complex
Decrease in molecular weight of DOM was shownwith L90-
15, 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.Effects of UV treatment on AOC
Concentration of AOC, indicator of potential biological re-
growth, 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
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 AOCofall UV systemswereplottedagainsta lineofequal
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 sepa-
rate 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 ¼0.557, 0.964,
0.545) and MPUV (p ¼0.234, 0.053, 0.386) at 95 percent confi-
dence 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
value¼0.021) while there was little statistical evidence that
UV treatment affected NOX AOC (p value ¼0.381).
Effects of UV treatment on chlorine decay and DBPs
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
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).
Chlorine demands (mg/L)Decay rate (h?1) Chlorine demands (mg/L)Decay rate (h?1)
ID 24 h48 hK1
ID24 h48 hK1
Residual Chlorine (mg/L)
020406080 100 120 140 160 180 200
020406080100 120 140 160 180 200
Fig. 5 – Chlorine decay trends before and after UV treatment.
water research 44 (2010) 115–122
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 chlo-
rine decay rate (K2) does not change significantly. This
suggests that UV disinfection increases the initial rapid chlo-
rine consumption within the clearwell, but it can not affect
significantly the slow chlorine decay rate in the distribution
Chlorine consumption increased after UV irradiation can
induce increase of DPBs formation. THMs and THMFP
concentrations were investigated seasonally before and after
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 statisti-
cally different from those after UV treatment at 95 percent
confidence level (LPUV p ¼0.065, MPUV p ¼0.039, All UV
p ¼0.009). 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 wave-
length. 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 chloroformthan LPUV. The authors attributed
the observation to lower molecular weight organic acids
generatedby the broader bandofUV lightemittedfromMPUV.
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 haloge-
nation of low molecular weight compounds produced by
breakdownoflargeNOMcompounds. But THMFPwasreduced
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. Unfor-
tunately, 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,
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)
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 irradia-
tion might stimulate reaction with chlorine, and result in
increase of DBP formation.
SFL90-4L90-15 M1300L85 M350
Fig. 6 – THMs and THMFP of pre- and post-UV treated water.
water research 44 (2010) 115–122
UV disinfection with low dosage of 40 mJ/cm2can not
mineralize DOM,but mightsplitchemicalbondsor changethe
characteristics of functional groups of DOM. This modification
of DOM structure by UV is likely not to stimulate biological re-
growth 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-chlorina-
tion, 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.
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