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© IWA Publishing 2013 Water Science & Technology: Water Supply
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13.4
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2013
Removal of natural organic matter and trihalomethane formation potential in a full-scale drinking water treatment plant Ekaterina Vasyukova, René Proft, Johanna Jousten, Irene Slavik and Wolfgang Uhl
ABSTRACT A multidisciplinary approach was applied in this work to characterise natural organic matter and evaluate the performance of a full-scale waterworks treating organic-rich surface water. It was shown that the combination of the treatment processes considered efficiently removed the dissolved organic matter, including its specific fractions. Most of the dissolved organic carbon and nitrogen (DOC and DON), biodegradable DOC and DON, as well as assimilable organic carbon were removed by coagulation/ sedimentation. However, the coagulation process was not likely to be optimised for the removal of all molecular weight compounds. The breakdown of high molecular weight compounds into others of low
Ekaterina Vasyukova (corresponding author) René Proft Johanna Jousten Irene Slavik Wolfgang Uhl Technische Universität Dresden, Institute of Urban Water Management, Chair of Water Supply Engineering 01062 Dresden, Germany E-mail:
[email protected]
molecular weight, as well as the production of biodegradable organic matter during ozonation, proved to enhance their removal efficiency by subsequent biological activated carbon filtration. The specific trihalomethane formation potential decreased during treatment, indicating a decrease in reactivity of DOC with chlorine across the treatment train. Fractionation experiments demonstrated that high and medium molecular weight organics were likely to be the main precursors for the formation of trihalomethanes. However, other disinfection by-products (such as haloacetic acids) should also be controlled, as the chlorine demand pattern did not necessarily follow that of trihalomethane formation. Key words
| biodegradable organic matter, dissolved organic carbon, dissolved organic nitrogen, fractionation, liquid chromatography with online organic carbon and organic nitrogen detection, trihalomethane
INTRODUCTION AND OBJECTIVES The removal of natural organic matter (NOM) from water is a
Frimmel et al. ; Steinberg ; Huber et al. ). How-
significant issue for the drinking water industry as it causes
ever, still little information is known about the fate of
adverse effects within treatment processes and distribution sys-
NOM fractions and their removal across full-scale drinking
tems. NOM is considered a precursor for the formation of
water treatment plants (Lee et al. ; Baghoth et al. ;
potentially carcinogenic disinfection by-products during
Liu et al. ). The objectives of this study were: (i) to find
chlorination, and can decrease the effectiveness of disinfec-
out how the composition and behaviour of NOM changes
).
the
within a real waterworks, (ii) to evaluate the performance
performance of membrane systems by contributing to mem-
of the treatment processes in terms of NOM removal, and
brane fouling (Zularisam et al. ), as well as favour
(iii) to determine the impact of treatment on the potential
microbial growth and regrowth in distribution systems.
for trihalomethane (THM) formation. The main originality
tants
(Singer
Additionally
it
may
affect
Much research has already been conducted on the
of this work was to combine, for the first time on the same
characterisation of NOM in natural waters (Thurman ;
object, different techniques to better understand the character
doi: 10.2166/ws.2013.095
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E. Vasyukova et al.
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NOM removal and trihalomethane formation potential
Water Science & Technology: Water Supply
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2013
and removability of NOM during full-scale water treatment. A
of Ca(OH)2 and CO2 into the sedimentation basin’s clear
waterworks treating organic-rich, low-turbid surface water
water run-off. A gravity granular bed filtration step, consist-
was chosen for these investigations. Liquid chromatography
ing of a dual-media arrangement with 0.6 m anthracite
with online organic carbon and organic nitrogen detection
(grain size: 1.4–2.5 mm) and 1.2 m fine quartz sand (grain
(LC–OCD–OND) was used for the characterisation and
size: 0.71–1.25 mm), removes the remaining flocs and par-
quantification of dissolved organic carbon (DOC) and dis-
ticles. Further treatment consists of an oxidation step with
solved organic nitrogen (DON) in water. Complementary
ozone (0.65 ± 0.05 mgO3/L in the period investigated), fol-
experiments were conducted to assess the change in
lowed by pressure BAC filtration over 2 m (empty bed
bioavailability of dissolved organic compounds, as well as
contact time ∼15 min). Treated water is disinfected by a
the reactivity of NOM with chlorine to form trihalomethanes.
combination of free chlorine and chlorine dioxide. Dissolved organic matter analysis
MATERIALS AND METHODS Total DOC was measured by the catalytic combustion Study site and sampling
method using a LiquiTOC II organic carbon analyser (Elementar Analysensysteme GmbH). Ultraviolet absorbance
A waterworks facility which treats organic-rich, moor-
at a wavelength of 254 nm (UVA254) was measured in a
influenced water from the Galgenteich Reservoir (Saxony,
5 cm quartz cell using a Genesys 10 UV spectrophotometer
Germany) was investigated in the period from August to
(Thermo
October 2011. The source water is characterised by a high
samples were pre-filtered through 0.45 μm polycarbonate
Spectronic).
Prior
to
these
measurements,
organic matter content (DOC ∼ 8 mg/L), low turbidity
filters. The specific UV absorbance (SUVA) was calculated
(∼2 NTU), slightly acidic pH (∼6.0), and low conductivity
by dividing the UVA254 by the corresponding DOC
(∼70 μS/cm). Samples were collected before and after each
concentration.
treatment stage (n ¼ 3) in pre-cleaned DOC-free vials and W
The characterisation and quantification of NOM frac-
stored untreated at 4 C until analysis, which followed
tions by size exclusion chromatography was performed
within a week.
with an LC-OCD-OND system (Model 8; DOC Labor,
The treatment train of the studied waterworks com-
Germany) based on a Gräntzel thin-film reactor (Huber
prises coagulation/flocculation followed by sedimentation,
et al. ). The method is used to fractionate DOC and
rapid
sand
filtration,
ozonation,
biological
activated
DON into five fractions according to their apparent molecu-
carbon (BAC) filtration, and chlorine disinfection. Before
lar weight: biopolymers, humic substances, building blocks,
the mixing tank, the pH required for coagulation is adjusted
low molecular weight (LMW) acids and LMW neutrals. Due
(6.7 ± 0.1 in the period investigated) and the hardness
to interferences with nitrate, only DON of the biopolymer
increased by use of calcium hydroxide Ca(OH)2 and
and humic fractions could be quantified. The total DON
carbon dioxide CO2. At the inlet of the coagulation basin,
concentration was thus calculated as a sum of the two
ferric chloride is added as a coagulant (at a dosage of
latter fractions. Accompanying software (ChromCALC)
4
3.7 ± 0.13 × 10
3þ
mmolFe /L in the period investigated)
was used for the interpretation of the results.
and dispersed by rapid mixing. Downstream of the rapid mix, a copolymer of acrylamide and sodium acrylate is
Biodegradable dissolved organic matter and assimilable
added as a flocculation aid (0.72 mg/L of polymer on aver-
organic carbon assays
age). Flocs and particles are separated from the process stream in a sedimentation tank equipped with incline-
Biodegradable DOC and DON (BDOC and BDON) assays
plates. Subsequently, iron and manganese are oxidised by
were performed for two sampling campaigns in September
the application of potassium permanganate just prior to fil-
and November 2011, respectively (n ¼ 2). According to the
tration. For this, the pH is increased through the addition
method of Joret et al. (), inoculation was performed
1101
E. Vasyukova et al.
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NOM removal and trihalomethane formation potential
Water Science & Technology: Water Supply
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13.4
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2013
with the natural assemblage of sessile bacteria that pre-
with pre-filtered raw water and placed in 10 L clean DOC-
viously grew in water of the same origin during three
free glass bottles filled with MilliQ. The MilliQ water outside
weeks of incubation. One litre of sample was incubated in
the membrane was exchanged every 24 h, in such a way that
a glass bottle containing 100 mL quartz sand during 14
the final volume ratio of MilliQ to the dialysis compartment
days in the dark at a temperature of 22 ± 1 C. After incu-
was >100 (Vasyukova et al. ).
W
bation, samples were taken for LC–OCD–OND analysis.
The determination of trihalomethane formation poten-
The BDOC and BDON were determined by subtracting
tial (THMFP) was performed according to a German
the DOC and DON concentrations after the incubation
standard operation procedure (DVGW W–295 ).
time from the initial corresponding concentrations of a
Samples were spiked with 10 mgCl2/L in 250 mL chlorine
sample.
demand-free brown glass bottles using a commercial grade
The assimilable organic carbon (AOC) determination
sodium hypochlorite solution. After 48 ± 2 h incubation at
was based on a two-species bioassay using two bacteria
room temperature (22 ± 1 C) the samples were transferred
strains, Pseudomonas fluorescens (P-17) and Spirillum
into 100 mL brown glass bottles, and chlorine reaction
(NOX), according to the procedure described by van der
was quenched by addition of sodium thiosulphate solution.
Kooij & Hijnen (). One AOC assay (n ¼ 1) was per-
THM concentrations were analysed by gas chromatography
W
formed in October 2011 for raw water and each treatment
according to German standard DIN EN ISO 15680 by an
step of the studied waterworks facility.
accredited laboratory (Dresden, Germany). Aliquots of chlorinated samples were used to determine the remaining
Fractionation experiments and trihalomethane
free chlorine concentration using the DPD (N,N-diethyl-
formation potential
1,4-phenylenediamine) colorimetric method according to German standard DIN EN ISO 7393–2.
Fractionation experiments were performed on 0.45 μm prefiltered
samples
using
ultrafiltration
and
dialysis
membranes. Frontal ultrafiltration was performed using
RESULTS AND DISCUSSION
single-use SpecraPor® polyethersulphone filters with a molecular weight cut-off (MWCO) of 100 and 10 kDa. The
Raw water composition
filters were flushed with 50 mL MilliQ and 50 mL sample prior to ultrafiltration. Glycerine-cleaned SpectraPor® poly-
The considered reservoir raw water exhibited relatively high
ethersulphone dialysis membranes with a MWCO of 20 kDa
organic matter content with mean DOC and DON concen-
were thoroughly washed in 0.1 M HCl and MilliQ water, so
trations of 7.9 mgC/L and 296 μgN/L, respectively, and a
that the DOC blank remained
