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Acknowledgements
The authors thank ÅForsk for funding this study via project 12-130 “Ny
processkombination för att skydda bad- och dricksvatten från föroreningar från
bräddvatten” as well as The Swedish Water & Wastewater Association (Svenskt Vatten)
and VA-teknik Södra. The personnel at NSVA and Öresundsverket wastewater treatment
plant are acknowledged for their help in making this study possible. Dr. Kai Bester and
Ulla Bollman at the Department of Environmental Science - Environmental chemistry &
toxicology, Aarhus University, Roskilde, Denmark are acknowledged for analyzing the
samples for biocides as well as providing valuable discussions over the results.
Summary
A pilot-scale pretreatment (micro-screening) of incoming wastewater was set up at
Öresundsverket wastewater treatment plant consisting of flocculation, coagulation and
screening. The pre-treated wastewater was then subjected to chlorine dioxide in
laboratory-scale as well as to ozone in pilot-scale. The disinfection efficiency of the
pretreatment as well as the two disinfection agents was thereafter evaluated by analysis of
E. Coli, coliform bacteria and intestinal Enterococci. The treatments were further
evaluated by analysis of COD-, total phosphorus-, SS-, turbidity and biocide reduction.
Furthermore, biometahne potential of the cumulated sludge on the micro-sieves was
evaluated and compared with those of conventional primary and excess sludges from the
plant.
To reach the limits of bacterial content stipulated in the EU Bathing Water Directive
(2006/7 EC) 10 mg ClO2/l was needed. For ozone, the same results were only achieved
when 20 mg O3/l was applied. However, CD, TB, and IP biocides were significantly
reduced by ozone while no substantial effect on the biocide content could be discerned
when chlorine dioxide was applied except for TB. This study concludes that chlorine
dioxide was the most efficient disinfection agent in reducing the number of bacteria to
below the limits set by the EU Bathing Water Directive and that the pretreatment used
was highly efficient in reduction of suspended solids, organic matter and phosphorus.
Table of abbreviations
AT
BAM
BIT
BMP
BOD
CD
COD
CSO
DBP
DCOIT
DR
DWF
EPS
IP
IPBC
IRG
MCPP
MI
OIT
PAX XL 100
PPZ
SS
TB
TBU
TCD
THM
TP
TS
VFA
VS
WWF
WWTP
Atrazine
Dichlobenzamide
Benzylisothiazolinone
Bio-Methane Potential
Biochemical Oxygen Demand
Carbendazim
Chemical Oxygen Demand
Combined Sewer Overflow
Disinfection By-Product
Dichloro-N-Octylisothiazolinone
Diuron
Dry Weather Flow
Extracellular Polymeric Substances
Isoproturon
Iodocarb
Cybutryn
Mecoprop
Methylisothiazolinone
N-Octylisothiazolinone
Polyaluminium chloride
Propiconazole
Suspended Solids
Terbutryn
Tebuconazole
Thermal Conductivity Detector
Trihalomethanes
Total phosphorus
Total solids
Volatile Fatty Acid
Volatile Solids
Wet Weather Flow
Wastewater Treatmernt Plant
Contents
Background .......................................................................................................................... 1
Chlorine dioxide (ClO2): .................................................................................................. 2
Ozone (O3): ...................................................................................................................... 3
Materials and methods ......................................................................................................... 5
Chemicals ......................................................................................................................... 5
Chlorine dioxide ........................................................................................................... 5
Sodium sulfite ............................................................................................................... 5
Pilot plant experiments ..................................................................................................... 6
Chlorine dioxide experiments (lab-scale) ........................................................................ 8
Biocide reduction experiments ......................................................................................... 9
Bio-methane potential experiments ................................................................................ 10
Results and discussion ....................................................................................................... 13
Conclusions ........................................................................................................................ 19
References .......................................................................................................................... 21
Appendix – supplementary data......................................................................................... 23
Background
Access to clean water has been the initiating point for all civilizations. However,
providing clean water and protecting water resources are some of today’s essential
challenges. Availability of clean water in Sweden has not been a big question for
authorities; nevertheless the situation is changing nowadays. Deterioration of water
quality can be expected due to increased amounts of untreated overflow wastewater
considering the elevated levels of precipitation in the climate change scenario.
The capacity of wastewater treatment plants (WWTP) may be exceeded during wet
weather flow (WWF) leading to a portion of flow discharged into the recipient either
untreated (directly from the piping network) or only partially treated (from the WWTP).
This flow, combined sewer overflow (CSO) (Figure 1) contains storm water and partly
domestic wastewater. It is found to contain considerable amounts of pathogenic
microorganisms as well as other pollutants e.g. nutrients, organic matter (chemical
oxygen demand) and suspended solids (SS) (Steets and Holden, 2003; Ohlsson et al.,
2011). Furthermore it is reported that over 350 000 m3 of wastewater per year has been
released as CSO in Stockholm and its suburbs during the last decade (Stockholm Vatten,
2012). Studies have shown that about 10% of CSO is in form of domestic wastewater
(Stockholm Vatten, 2012).
Figure 1. Combined sewer system layout for both wet weather and dry weather flows.
The concentrations of pollutants/pathogens in the CSOs might be relatively high if it
occurs during the first flush flow, while rather diluted concentrations can be anticipated
during the later overflows in comparison with those of dry weather flow (DWF) (Kim et
al., 2009).
Reportedly, release of untreated wastewater not only may cause eutrophication in the
receiving waters but can also lead to considerable increase in pathogen content of the
recipient (Rechenburg et al., 2006). Considering that about 5% of the annual inflow to
WWTPs bypasses the treatment process as CSO, this is also in conflict with Sweden’s
national environmental objectives such as the––– objective of live lakes and
watercourses.
1
In order to increase the bathing water quality, EU passed a new directive (Directive
2006/7 EC) - substituting the older one from 1975 - by which imposed limitations on
concentrations of Escherichia Coli and intestinal Enterococci in coastal and transitional
waters at 500 and 185 cfu1/100 ml respectively for achieving sufficient hygiene level
(Table 1). Introduction of an efficient treatment process for overflow wastewater with
regards to nutrient removal and pathogen killing, especially in cases when the receiving
water is used for recreational purposes like bathing, is extraordinarily necessary.
Table 1. Limitations suggested by Directive 2006/7 EC on the pathogen content of coastal and
transitional waters.
Pathogenic agent
Maximum concentration
allowed (cfu/100 ml)
Escherichia coli
500*
500**
250**
Enterococci
185*
200**
100**
Hygiene level
Sufficient
Good
Excellent
* Based upon a 90-percentile evaluation
** Based upon a 95-percentile evaluation.
There have been successful examples of separate treatment of CSO in Sweden, for
instance in Karlskoga, in order to protect the recipient and improve the bathing water
quality (Winnfors, 2012). However, development of new techniques and methods and
their combination for achieving a higher efficiency needs further investigations in order to
face the growing challenges of the water quality.
It seems inevitable to use strong disinfecting agents in order to achieve the target
pathogenic populations mentioned above. In this study, filtrated CSO is treated with
ozone (both in continuous and batch systems) and chlorine dioxide (laboratory-scale
batch system) for evaluation of their pathogen removing efficiency. Lots of information
regarding disinfecting characteristics of chlorine dioxide and ozone can be found in the
literature but no reliable specific data is available comparing their effectiveness on
destruction of E. Coli and intestinal Enterococci in filtrated wastewater. Following, brief
introductions of chlorine dioxide and ozone as disinfectants are presented:
Chlorine dioxide (ClO2):
Chlorine dioxide is an extremely reactive oxidant, with 0.95V oxidation potential, which
reacts with a wide range of organics normally available in wastewaters. Chlorine dioxide
has a better disinfecting capacity in comparison to chloride. It is specifically more
effective against viruses, perhaps due to its strong affinity to proteins and that viruses are
usually coated by protein layers. Chlorine dioxide is highly toxic but since its residuals
are believed to decompose rapidly it cannot harm the aquatic environment seriously.
Disinfection by-products (DBP) associated with chlorine such as trihalomethanes (THM)
are least likely to occur in case of chlorine dioxide. However it is recommended to dechlorinate the water treated by chlorine dioxide with sulfur dioxide or other sulfite or
thiosulfate salts (Metcalf & Eddy, 2003).
1
Colony-forming units
2
Ozone (O3):
Ozone has an extremely high oxidation potential (2.07V) which is known to be more
effective than free chlorine against viruses and bacteria. Contact times as short as 10-30
minutes are found to be sufficient for appropriate disinfection of wastewater. On the other
hand it is shown that lower doses of ozone do not kill viruses, spores and cysts (Solomon
et al., 1998).
Ozone is an unstable molecule that converts to oxygen very rapidly. DBPs such as
aldehydes, acids and aldo- and ketoacids may be formed in the absence of bromide while
brominated byproducts are most likely to be formed in presence of bromide. Hydrogen
peroxide (H2O2) is another DBP of ozone which is also a strong oxidant. Although ozone
itself is a very toxic and corrosive agent; it is not believed to be harmful to aquatic
environmental due to its extreme decomposability. It is however crucial to use corrosion
resistant material (e.g. stainless steel) in the ozone treatment facilities. Ozone treatment is
found to be favorable for receiving water bodies since it increases the dissolved oxygen
(DO) content of water. One of the serious problems associated with ozone is that it is not
easy to measure its residual concentration in water (Solomon et al., 1998; Metcalf &
Eddy, 2003).
Suggested doses of chlorine dioxide and ozone for different log-reductions in pathogen
population are presented in Table 2. As noticed, the required CRt values for ozone in
order to obtain a certain log-reduction is the lowest compared to chlorine dioxide and free
chlorine in pure water samples.
Table 2. Required CRt values (residual concentration x time) for different log-reductions in pathogen
population (Adopted from Metcalf & Eddy, 2003).
Disinfectant
Unit
Chlorine (free)
Chlorine dioxide
Ozone
mg x min/L
mg x min/L
mg x min/L
Chlorine (free)
Chlorine dioxide
Ozone
mg x min/L
mg x min/L
mg x min/L
Chlorine (free)
Chlorine dioxide
Ozone
mg x min/L
mg x min/L
mg x min/L
Inactivation
1-log
Bacteria
0.1-0.2
2-4
2-log
3-log
4-log
0.4-0.8
8-10
3-4
1.5-3
20-30
10-12
50-70
2.5-3.5
2-4
0.3-0.5
Protozoan cysts
20-30
35-45
7-9
14-16
0.2-0.4
0.5-0.9
4-5
6-12
0.5-0.9
6-7
12-20
0.6-1.0
Virus
3
70-80
20-25
0.7-1.4
4
Materials and methods
Wastewater flowing into the wastewater treatment plant in Helsingborg, southern Sweden
(Öresundsverket, NSVA) was used in this study. The reason for using the incoming
wastewater flow as CSO was to test the pretreatment and disinfection methods for the
worst-case scenario where the flow is not subjected to any dilution due to storm water
flow. The statistics of the inflow to the wastewater treatment plant are presented in Table
B1 in the appendix. All the samplings and experimental work were done in December
2012 to February 2013.
Öresundsverket WWTP is a 220 000 dimensioned PE (actual load 2012 was 150 700 PE)
plant operating a totally biological nitrogen and phosphorus removal divided into four
separate lines. Incoming wastewater first passes through screens and aerated grit
chambers before entering the pre-sedimentation stage and subsequent biological
treatment. A more detailed description of the treatment train at Öresundsverket WWTP
has been published by Jönsson et al. (1996).
Chemicals
Chlorine dioxide
Chlorine dioxide was synthesized by addition of 25 mL of 9% HCl and 25 mL of 7.5%
NaClO2 into 400 mL of distilled water. The mixed solution was then diluted up to 1 L
which resulted in a concentration of about 1 g ClO2/L. The prepared stock solution was
then covered with aluminum foil to avoid any photosynthesis and left overnight in a +4°C
fridge in order to react (Hey et al., 2012).
Chlorine dioxide concentration in the stock solution was measured and controlled by
Wallace & Tiernan® Analyzers/Controllers P15 plus Photometer, Siemens (see Figure 2).
Sodium sulfite
Figure 2. Wallace & Tiernan® Analyzers/Controllers P15 plus Photometer,
Siemens.
A sodium sulfite solution at about 50 g/L of Na2SO3 was prepared by dissolution of 25 g
sodium sulfite in 500 mL of distilled water. Sodium sulfite solution was used to remove
5
the residual chlorine dioxide in the reactors after the desired retention time was fulfilled
(Hey et al., 2012). The required concentration of the sodium sulfite was calculated
according to the following reaction stoichiometry:
5Na2SO3 + 2ClO2 +H2O  5Na2SO4 + 2HCl
Pilot plant experiments
A container based coagulation/flocculation and micro-sieve pilot test unit was used and
supplied by Hydrotech AB (Figure 3).
Figure 3. Hydrotech container-based coagulation/flocculation/microsieve pilot unit
used in these experiments.
The test filter was a Hydrotech HSF1702/2F disc filter with a 100 micron woven media.
Coagulation and flocculation tanks, filter (except media) and mixers were made of
stainless steel. The mixer for the coagulation tank was a turbomix® with a SCABA
30V25 drive motor and a 3SHP1 impeller giving a mixing intensity of approximately
G~300 s-1 in coagulation. In case of flocculation, a Nord SK63S/4 frequency controlled
driver motor and 2 blade impeller with variable pitch (Hydrotech, G~150 s-1) was used.
The pilot plant was operated with a flow of 14.6 m3/h (min 12.6 and max 16.4 m3/h)
giving a hydraulic retention time for coagulation of 2.4 minutes (2.1-2.7) and for
flocculation 8.6 minutes (7.5-9.8 minutes). The coagulant dosing was 18.3 mgAl3+/L
(17.4-19.5) and the polymer dosage was 3.8 mg active material/L (3.6-4.1) during the
experiments. A schematic picture of the pilot plant as well as the sampling points is
displayed in Figure 4.
Polymer was prepared in a TOMAL Polyrex 0.6 polymer station. Influent/effluent
turbidity was monitored by HACH Lange Ultraturb plus SC turbidity meters. The influent
6
and effluent flows were measured with Siemens Sitrans FM Magflo MAG 5000 flow
meters, and for coagulant and flocculant dosing, Grundfos ALLDOS DME 2 and DME
60 dosing pumps were used. Polyaluminum chloride (PAX XL 100) was applied as the
coagulant together with the cationic polymer (5060) supplied by Kemira®.
Ozone was supplied by a container production unit (Primozone SM900) which housed a
Primozone GM6 ozone generator along with the necessary compressor and oxygen
production units. In order to measure the amount of ozone produced by the container a
BMT 964 ozone concentration meter sampled the ozone. Ozone was added to the microsieved wastewater (10 m3/h) through a venturi injector and the following pressurized
volume provided a hydraulic retention time of 8 minutes. By keeping the flow of
wastewater constant through the venturi injector and altering the production of ozone,
different ozone doses could be applied with the same hydraulic retention time.
Influent and effluent from disc filter and effluent after ozonation was collected during 1.5
hour sampling periods. Grab samples were taken to produce a composite sample which
was analyzed for phosphorous (TP and Ortho-P) with Hach Lange cuvette test tubes
(LCK 350/349) on-site directly. All the samples analyzed ortho-P were filtered through
0.45 µm filter papers while TP was measured in the unfiltered samples. The composite
samples were also analyzed for turbidity with Hach Lange 2100Qis turbidity meter
directly on-site. Suspended solids content was analyzed according to EN 872:2005, BOD7
according to SS 02 81 43 /EN 1899-1:1998 and COD with LCK-114 Hach Lange cuvette
tests.
Figure 4. Schematic layout of the pilot plant illustrating the sampling points for incoming water
(untreated), filtrated, and ozonated water.
7
All the samples were kept at about 4ºC for maximum 24 hours after the experiments and
were delivered for analysis for their pathogen content i.e. Coliform bacteria, Enterococci
and E. Coli. All the analyses for pathogen content were performed by ALcontrol
laboratories in Malmö, Sweden.
Chlorine dioxide experiments (lab-scale)
Lab-scale disinfection experiments with chlorine dioxide were performed on the
wastewater samples delivered from the pilot plant at the WWTP in Helsingborg. The
wastewater samples used for this experiment as well as the ozonation experiment had
already passed through a screen, sand trap as well as the coagulation/flocculation step and
micro screening as previously described. The experiments were done on the same day as
the wastewater was sampled.
All the reactors and lab equipment concerning the experiment were primarily washed in
the washing machine using DR. WASHER, LaboClean UW washing powder. Later on all
the washed items were autoclaved (Figure 5) at about 121°C in accordance with ANNEX
V of the Directive 2006/7 EC.
Initial tests revealed that the delivered wastewater sample with an approximate COD
content of 180 mg/L consumes 5 mg/L of chlorine dioxide in about 8 minutes of
reaction/contact time, thus 5 mg/L was selected to be the minimum dose. Accordingly,
three different doses of chlorine dioxide were applied in this experiment, as 5, 10, and 15
mg/L to study the disinfection efficiency of chlorine dioxide.
Figure 5. All the lab equipment was washed and autoclaved at about 1.2 (gbar) prior to the
experiment.
8
Three reactors containing 1 L of wastewater (filtrated) were covered with aluminum foil
to avoid light exposure. Adequate amounts of stock chlorine dioxide were added in each
reactor adjusting the concentrations to 5, 10, and 15 mg ClO2/L, respectively.
Ozone experiment in lab-scale
Lab-scale disinfection with ozone was performed using the equipment setup depicted in
Figure 6. Four liters per ozone dose of flocculated/coagulated/filtrated wastewater was
subjected to a flow of 1.25 NL/min of ozone containing gas (average 160 g O3/Nm3). The
gas coming from the ozone generator was allowed to flow into the ozone concentration
meter and subsequent destructor for 10 minutes. After that time, the flow was
reconfigured using a series of valves (depicted as a three-way valve for simplicity) to
allow for introduction into the batch reactor via a diffuser stone. The flow of ozone was
kept constant throughout the experiment while the time of ozonation was varied (2, 4, 6
and 8 minutes) to achieve different ozone doses.
Figure 6. Overview of the lab-scale disinfection system with ozone. 1: Oxygen (99%), 2: Pressure
regulator, 3: Ozone generator (Primozone GM2), 4: Three-way valve, 5: Ozone concentration meter
(BMT 963), 6: Ozone destructor, 7: Batch reactor (used volume: 4 l).
Biocide reduction experiments
It was hypothesized that both ozone and chlorine dioxide would reduce the biocide
content of the wastewater. Sampling of wastewater was performed on two rainy days
when the flow into the treatment plant was about 74000 m3/day (see days marked as “g”
and “h” in Table B1, appendix) compared to the average yearly flow of about 48000
m3/day. Duplicate samples were taken from the incoming wastewater, pre-treated
wastewater and ozonated effluent (with 20 mg O3/L) as well as single samples with 10
and 15 mg ClO2/L. The biocides chosen for analysis, along with the corresponding
abbreviations used throughout this article are presented in Table 3.
9
The analyses of biocides were performed by high performance liquid chromatography
with tandem mass spectrometry (HPLC-MS/MS) using electro spray ionization in
positive mode (ESI(+)) on an Ultimate 3000 HPLC-system (Dionex, Germeringen,
Germany) coupled to an API 4000 triple-quadrupole-MS (AB Sciex, Framingham, MA,
USA) according to Styszko et al. (2013). The separation was performed at 5°C using
Synergy polar-RP column (L=150 mm, ID=2 mm, particles=4 µm, Phenomenex,
Torrance, CA, USA). A multi-step gradient of water (A) and methanol (B) was used: 03 min 0% B, 3-5 min 0 to 50% B, 5-15 min 50 to 80% B, 15-15.5 min 80 to 100% B,
15.5-19 min 100% B, 19-20 min 100 to 0% B, 20-25 min 0% B. In order to change to
acidic conditions for the ionization, 0.03 mL min-1 of 0.2% formic acid in water was
added post column prior to introduction to the ion source of the mass spectrometer.
Table 3. Analyzed biocides and the corresponding abbreviations.
Name
Methylisothiazolinone
Benzylisothiazolinone
Mecoprop
Dichlobenzamide
Carbendazim
Isoproturon
Diuron
Iodocarb
Abbreviation
MI
BIT
MCPP
BAM
CD
IP
DR
IPBC
Name
Atrazine
Terbutryn
Cybutryn
N-Octylisothiazolinone
Tebuconazole
Dichloro-N-Octylisothiazolinone
Propiconazole
Abbreviation
AT
TB
IRG
OIT
TBU
DCOIT
PPZ
The sample extraction was performed following Bester and Lamani (2010) with minor
modifications. In a volumetric flask, a 100 ml sample was spiked with 50 µL of internal
standard solution, containing a mix of deuterated biocides (1 µg mL-1 in methanol
(gradient grade (lichrosolv), Merck, Darmstadt, Germany): isoproturon-D6, terbutryn-D5,
cybutryn-D9, tebuconazole-D6 and carbendazim-D4). In addition 3 mL of a 0.2M
phosphate buffer was added to adjust to pH = 7. A Bakerbond SDB-2 (6 mL, 200 mg)
SPE-cartridge (Mallinckrodt Baker, Deventer, The Netherlands) was conditioned with
12 ml acetonitrile (gradient grade (lichrosolv), Merck, Darmstadt, Germany) and 12 ml
Millipore-water successively. After extracting the 100 mL sample (using a velocity of
~ 2 mL min-1) the cartridge was washed with 12 mL Millipore-water and slightly dried
with vacuum. The combined eluates of 12 mL acetonitrile and 12 mL methanol were
condensed to 1 mL in a BÜCHI Syncore® multiport condenser (Büchi, Flawil,
Switzerland) at 50°C, 280 rpm and 100 mbar for about 90 minutes. The extracts were
then transferred to 1.5 mL auto sampler vials.
Bio-methane potential experiments
Bio-methane potential (BMP) estimation of the sludge samples (i.e. primary and excess
sludge from the plant as well as the sludge accumulated on the disc filters from the pilotscale pretreatment process) was done according to Hansen et al. (2004). The primary
sludge at Öresundsverket is subjected to hydrolysis in order to provide volatile fatty acids
(VFA) for the bio-P process in the activated sludge. The excess sludge is taken from the
settling tanks following the activated sludge basin in which nitrification, denitrification
and biological phosphorus removal is practiced in addition to organics removal.
10
Thickened excess sludge (3.6% TS, 2.4% VS), thickened primary sludge (5.4% TS, 4.1
VS), and overflow sludge (accumulated on the disc filters with 1% TS, 0.7% VS) were
used as main substrates. Cellulose was used as the reference substrate in order to verify
the quality/efficiency of the inoculum.
2-liter air-tight glass bottles where used as batch reactors and were inoculated using the
digestate from full-scale mesophilic reactors at Öresundsverket, Helsingborg. Inoculum in
the batch reactors accounted for about 60-70% of the total content on VS basis while the
rest was contributed by the applied substrates. The inoculated-fed bottles were then
flushed with nitrogen gas in order to create anaerobic conditions. All reactors were
insulated with resin septum and were kept in 35°C incubators for 40 days. Triplicate
bottles were provided for each substrate type and the methane content in the headspaces
of the reactors was measured twice a week (less frequent during later phases) by means of
Gas-Chromatography. Methane content of the biogas produced in the batch reactors were
measured with gas-chromatograph, Varian 3800 Gas Chromatograph, equipped with TCD
(thermal conductivity detector) and a column with the dimensions: 2.0 m x 0.3 mm x 2.0
mm.
11
12
Results and discussion
According to the results from the microbial analyses it is found that the incoming
wastewater contains over 105 cfu/100 mL for all the analyzed bacterial species i.e.
E. Coli, Fecal Coliforms and Enterococci (Table 4). This is in agreement with the results
from a recent study carried out by U.S. Geological Survey Scientific Investigations which
quantifies E. Coli, Fecal Coliforms, and Enterococci generally between 106 and 107
cfu/100 mL in primarily-settled wastewater (Francy et al., 2011).
Analysis of samples from screened wastewater revealed that the number of the pathogenic
agents decreased during the screening process especially regarding E. Coli and
Enterococci. The reduction of Coliform bacteria 35°C after screening was determined not
as efficient as those of E. Coli and Enterococci. In this study, the obtained results for
microbial quantification of screened wastewater samples have a relatively high standard
deviation, probably due to large variations in the incoming bacteria levels over 105
cfu/100 mL. Therefore it is not possible to determine the occurred log-reduction after the
screening step since the exact initial number of the bacteria could only be determined up
to 105 cfu/100 mL according to the adopted analysis method. Furthermore, severe
fluctuations in the reduced number of bacteria after screening can therefore be accredited
to the fluctuations in the bacterial content of the influent to the pilot plant.
Considering the pore size of the applied micro-sieve, 100 µm, no reduction of the bacteria
should have been expected in the screening step since the size of bacteria ranges between
0.5-5 µm (Hammer and Hammer, 2011). Thus, the observed reduction in the quantity of
the bacteria can be attributed to their flocculation/aggregation properties in the
wastewater. It is known that the extracellular polymeric substances (EPS) formed around
the bacteria, either loosely-bound or tightly-bound, are mainly in the form of
carbohydrates and proteins and have a net negative charge so that the bacteria can be
considered as negatively charged colloidal particles (Eboigbodin and Biggs, 2008).
Consequently, addition of positively charged coagulants such as PAX XL 100, as used in
this study, would lead to aggregation of the bacteria as well as other suspended material
and thus formation of larger flocs and hence better screening efficiency.
Based on the anticipation discussed above, it seems that the EPS formed by E. Coli and
Enterococcus play important roles in flocculation-coagulation while coliform bacteria at
35°C might have a relatively poor EPS content. However, only few studies concerning
aggregation properties of the bacteria with regards to their EPS-forming activities is
found in the literature.
The efficiency of pathogen removal through the precipitation and micro screening step is
not large enough to meet the requirements proposed by Directive 2006/7 EC as shown in
Table 1, thus further disinfection is required. As mentioned before, ozone in continuous
pilot operation and chlorine dioxide in laboratory-scale batch experiments were tested as
two disinfection methods for coagulated/filtrated overflow water.
13
Table 4. Summary of results from pathogen inactivation experiments including micro screening, ozone
treatment and chlorine dioxide for target pathogenic agents E. Coli, intestinal Enterococci, and coliform
bacteria.
Filtrated
Date *
Inflow
3
mg/L
6
mg/L
Ozone (O3)
9
12
mg/L
mg/L
20
mg/L
Chlorine dioxide (ClO2)
5
10
15
mg/L
mg/L
mg/L
E. Coli (cfu/100 mL)
a
b
c
d
e
f
g
h
>100000
>100000
>100000
>100000
>100000
>100000
>100000
>100000
3500
700
930
7300
610
940
650
7000
5100
a
b
c
d
e
f
g
h
>100000
>100000
>100000
>100000
>100000
>100000
>100000
>100000
3200
920
1400
540
2000
3500
360
3100
4500
a
b
c
d
e
f
g
h
>100000
>100000
>100000
25000
21000
27000
35000
53000
9600
8300
28000
24000
>100000
>100000
>100000
>100000
43
290
9
2000
10
10
10
10
10
10
10
10
10
10
27
9
10
10
10
10
18
10
45
85000
10
10
10
10
1500
680
380
10
10
Intestinal Enterococci (cfu/100 mL)
54
150
9
1800
260
410
560
18
10
Coliform bacteria (cfu/100 mL)
110
150
54
6200
3500
1900
600
10
10
*The column “Date” corresponds to the sampling dates as presented in Table B1 in the appendix.
The results show that there were no significant difference in the water quality regarding
TP, SS, BOD7 and COD as well as turbidity when comparing effluent from filter and after
ozonation (relevant figures are presented in Appendix - supplementary data, Figure A1A5). The incoming wastewater contained 7.4 mg P/L of phosphorus, which after the
pretreatment was reduced to 0.21 mg P/L corresponding to a reduction of 97.2%. No
effect on the level of phosphorus could be detected after the addition of ozone (Figure A1,
appendix). Suspended solids was measured at an average of 225 mg SS/L in the incoming
wastewater, pretreatment reduced this level to 4.4 mg SS/L and no further reduction was
observed after the addition of ClO2 and O3 (Figure A2, appendix).
For BOD7 and COD a marginal reduction improvement was observed after ozonation. On
average BOD7 and COD was reduced by 73-74% in the pretreatment step (i.e.
coagulation/flocculation and micro-screening) and an additional 3-4% improvement in
overall reduction could be attributed to the ozone. On average the filter effluent contained
125 mg/L COD and 39 mg/L BOD7. After ozonation the corresponding results where 105
and 34 mg/L, respectively (Figure A3 and A4), these findings are in accordance with
14
other results in literature where ozone is found to reduce the BOD and COD (Gesuale
et al., 2010).
Studying the results of disinfection experiments suggest that chlorine dioxide in batch
experiment is much more effective than ozone regarding pathogen removal (compare
Figures 7 and 8). Ozone treatment with doses over 6 mg/L resulted in some pathogen
reduction while none of the doses up to 12 mg/L could fulfill the required criteria as
indicated in Directive 2006/7 EC. It should be noted that the criteria was only met at high
ozone dosages at about 20 mg/L (Figure 7). Such a high ozone dose for disinfection has
also been observed in previously published studies where Gehr et al. (2003) needed doses
of ozone over 30 mg/L for destruction of fecal coliforms down to target level of 9000
cfu/100mL. Absi et al. (1993) also found that 17-20 mg/L of ozone in continuous pilot
run would satisfy the disinfection requirements. The reason for such high doses of ozone
was suggested to be the content of the organics in the tested water by which ozone is
chemically consumed before it could be utilized for disinfection (Absi et al., 1993).
100000
cfu/100mL
10000
1000
A
B
100
10
1
Incoming
Filtrated
3 mg/L
6 mg/L
(untreated) 100 µm
E. Coli
9 mg/L
12 mg/L
20 mg/L
treated with ozone
Coliform bacteria 35°C
Intestinal enterococci
Figure 7. Analysis results for ozonation experiment in pilot-scale continuous experiment
illustrating the pathogen poopulation at different steps of the process and different ozone
concentrations. A= EU directive limit of E. Coli, B = EU directive limit of intestinal Enterococci.
100000
cfu/100mL
10000
1000
A
B
100
10
1
Incoming
Filtrated
5 mg/L
(untreated)
100 µm
E. Coli
Coliform bacteria 35°C
10 mg/L
15 mg/L
treated with chlorine dioxide
Intestinal enterococci
Figure 8. Analysis results for disinfection with chlorine dioxide experiment in laboratory-scale batch
experiment illustrating the pathogen poopulation at different steps of the process and different chlorine
dioxide concentrations. A= EU directive limit of E. Coli, B = EU directive limit of intestinal Enterococci.
15
Disinfection efficiency of ozone was also tested in batch reactors under continuous
ozonation over different durations (Figure 9). Relatively better disinfection was observed
in the batch experiments especially regarding E. Coli which reached under target limit
after 2 minutes of ozonation. This could be explained by the initial low presence of E.
Coli population in the filtrated wastewater. No reasonable explanation was found for the
observed increase in the populations of Coliform bacteria and intestinal Enterococci after
ozone treatment for 8 minutes in the laboratory-scale batch reactors.
100000
cfu/100mL
10000
1000
A
B
100
10
1
0
(Filtrated 100μm)
E-Coli
2
4
6
Ozonation duration (minutes)
Coliform bacteria 35°C
8
Intestinal enterococci
Figure 9. Results of ozone treatment for pathogen inactivation at laboratory scale batch reactors with different
exposure times. A= EU directive limit of E. Coli, B = EU directive limit of intestinal Enterococci.
Application of chlorine dioxide as disinfection agent was found to be more effective
compared to ozone (both in pilot and batch systems) so that the criteria (Directive 2006/7
EC) was achieved just about at 5 mg/L of chlorine dioxide (minimum concentration
tested). Consequently complete fulfillment of the hygiene criteria with safer margin was
met at 10 mg/L of chlorine dioxide leaving about 4 mg/L residual after the detention time
(Appendix, Figure A6).
Although both chlorine dioxide and ozone are known as extremely reactive oxidants, they
have shown different efficiencies in disinfection of the selected pathogens. Short
retention/contact time for ozone (<10 min) could also be a reason for its lower
disinfection efficiency since contact times over 10 minutes are recommended for ozone as
disinfectant (Solomon et al., 1998). The difference in effectiveness can also be explained
by comparing the oxidation potential of the two chemicals, ozone has an oxidation
potential of 2.07V while chlorine dioxide has a potential of 0.95V. The higher oxidation
potential of ozone means that ozone oxidizes most compounds it interacts with while
chlorine dioxide does not. Since even the pre-treated wastewater, as used in this study,
contains a large amount of organic compounds in addition to the microorganisms
analyzed, ozone reacts as soon as it is dissolved. This is also in agreement with the better
disinfection efficiency observed through the batch experiment with ozone treatment in
which a certain volume of pretreated wastewater was exposed to continuous ozonation for
a certain duration. This probably resulted in oxidation of organics by ozone during the
16
early phases and consequently further availability of ozone was achieved for disinfection
purposes.
Chlorine dioxide on the other hand, with its lower oxidation potential can be expected to
be present in the wastewater for a longer time, thus increasing the potential of
encountering the microorganisms to inactivate. It also could be because of the
composition of the pathogens’ cell walls making them more susceptible to chlorine
dioxide than ozone. Also, the presence of different substances in the filtrated wastewater
may have disturbed the functionality of the disinfectants. This cannot be confirmed due to
very limited literature available on the subject and hence further studies needs to be done
in order to specify the oxidizability of the substances present in filtered wastewater
against chlorine dioxide and ozone.
Effect of micro screening, ozone and chlorine dioxide treatment on biocide content of the
overflow water was also examined through 2 samples. The biocides selected for the
experiment were MI, BIT, MCPP, BAM, CD, IP, DR, IPBC, AT, TB, IRG, OIT, TBU,
DCOIT and PA.
The results from biocide analysis are presented in Table 5. As it is noticed there were no
traces of MI, BIT, AT and IRG detected in the samples. High concentrations of MI
detected after chlorine dioxide treatment (not shown in the presented data) suggest likely
contamination of samples during the experiment procedure. Analysis results concerning
BAM, IPBC, OIT and PA suggest that contamination of chlorine dioxide treated samples
has most likely occurred. Biocides IP and CD are substantially reduced by ozone while no
effect is observed for treatment with chlorine dioxide. IPBC, TBU, DCOI, and PA are not
affected by any of the treatment processes while relatively high concentrations of IPBC,
TBU and DCOIT and minor amounts of PA were observed in the blank sample. TB have
been the only substance affected by both ozone and chlorine dioxide so that no traces of
TB were detected in the treated samples.
Table 5. Average results from the analysis of the biocide content in ng/L. Values in parenthesis are the standard
deviation. A graphical illustration of the data is presented in the appendix as Figure A7.
Sample
Incoming
(untreated)
MCPP
33.3
(1.0)
BAM
27.0
(1.7)
CD
35.6
(27.4)
IP
3.2
(0.5)
DR
6.4
(2.1)
IPBC
1.7
(0.5)
TB
6.1
(0.9)
OIT
0.0
(0)
TBU
0.7
(0.5)
DCOIT
12.7
(0.70)
PPZ
4.9
(0.70)
Pre-Treated
38.4
(3.1)
32.1
(3.8)
45.4
(33.6)
4.7
(1.3)
7.6
(2.3)
0.7
(0.2)
6.5
(0.5)
2.3
(3.3)
1.4
(0.1)
11.2
(2.0)
4.8
(0.20)
20 mg O3/l
50.9
(1.5)
24.8
(8.3)
4.3
(6.1)
0
(0)
3.1
(4.5)
3.6
(1.9)
0
(0)
0.9
(1.3)
1.8
(0.5)
11.7
(1.1)
5.8
(0.5)
10 mg ClO2/l
36.7
39.4
50.9
2.9
7.7
11.8
0
11.2
2.2
8.8
7.1
15 mg ClO2/l
Blank
32.0
0
42.6
0
48.9
1.3
2.2
0
7.9
0
12.4
2.6
0
0
9.3
0
1.6
1.7
10.8
9.3
7.5
0.6
17
The results from the biogas experiment shows that the bio-methane potential of the
overflow sludge (400-450 NmL CH4/gVS) is higher than that of both excess and primary
sludges accounting for 350 and 275 NmL CH4/gVS, respectively; while inoculum showed
to be of acceptable quality producing about 300 NmL CH4/gVS (Figure 10). The reason
for relatively high BMP of the overflow sludge can be linked to its hydrolysis rate noticed
in the dramatic gas production rate booming to about 350 NmL CH4/gVS in about 5 days.
500
450
Nml CH4/ g VS
400
350
300
250
200
150
100
50
0
0
10
20
30
40
50
Day
Excess sludge
Cellulose
Primary sludge
Overflow sludge
Figure 10. Results from the BMP test performed for cellulose (reference substrate), excess sludge,
primary sludge and overflow sludge according to the method suggested by Hansen et al. (2004).
Fairly fast initial increase in methane production from overflow sludge reveals that it has
a higher hydrolysis rate constant compared to the others. It could be discussed that micro
screening is capable of trapping micro-scale easily hydrolysable organics, mostly lipids
(fat), while these material are most probably consumed by microorganisms through
conventional treatment processes due to the specific configuration of the plant with prehydrolysis and biological phosphorus removal (bio-P) processes (Appendix - Figure A8).
In contrast to overflow sludge which might have high fat content, primary sludge at
Öresundsverket probably does not contain considerable concentrations of fat due to prehydrolysis step installed in the primary clarifier. At this step primary sludge is hydrolyzed
into volatile fatty acids (VFA) to be used in the bio-P process. Therefore a major
proportion of the lipids in the primary sludge leaves the primary clarifier as VFA towards
biological treatment step. This could also be the reason for lower BMP of the primary
sludge compared to that of excess sludge.
18
Conclusions
The first aim of this study was to evaluate the performance of
flocculation/coagulation/screening as a pretreatment to CSO disinfection. The results
show that the pretreatment removes a substantial amount of bacteria along with COD,
total phosphorus, SS and turbidity, resulting in a wastewater that contains fewer bacteria
for the disinfection agents to inactivate as well as less interference from COD and
particles. Since CSO, in most cases, is only partially treated before discharge the 97%
reduction in total phosphorus is highly beneficial when considering the eutrophication
issue in general. The second aim of this study was to evaluate ozone and chlorine dioxide
to investigate their differences in disinfection of CSO. The results show that chlorine
dioxide was clearly the most effective agent; achieving disinfection to within a clear
margin of the EU bathing water directive with a dose of 10 mg ClO2/L. Ozone on the
other hand did not achieve the same level of disinfection, unless 20 mg O3/L was applied.
Such a high dose of ozone does not seem to be feasible to practice in full-scale due to
high costs resulting from large volumes of overflow water.
It certainly would be beneficial to further study the reduction of biocides in wastewater by
ozone and other agents, as at least ozone did reduce the amount of the biocides CD, TB,
and IP quite substantially while only TB was removed by chlorine dioxide.
Biogas potential of the sludge accumulated on the micro-sieves during the pretreatment
step (overflow sludge) was shown to be higher than the primary and biological sludge at
Öresundsverket. This could possibly be due to higher fat content in the overflow sludge
since the fat in the primary sludge is hydrolyzed in the primary clarifier at
Öresundsverket.
19
20
References
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Ozone Disinfection of Physico-Chemically Treated Municipal Wastewater. In Ozone in
Water and Wastewater Treatment, Proceedings of the 11th Ozone Congress, San
Francisco, California; International Ozone Association: Stamford, Connecticut; pp S733–
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Bester, K.; Lamani, X., 2010 Determination of biocides as well as some biocide
metabolites from facade run-off waters by solid phase extraction and high performance
liquid chromatographic separation and tandem mass spectrometry detection. Journal of
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Francy, D.S., Stelzer, E.A., Bushon, R.N., Brady, A.M.G., Mailot, B.E., Spencer, S.K.,
Borchardt, M.A., Elber, A.G., Riddell, K.R., and Gellner, T.M., 2011, Quantifying
viruses and bacteria in wastewater—Results, interpretation methods, and quality control:
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Gehr, R., Wagner, M., Veerasubramanian, P. and Payment, P. 2003 Disinfection
efficiency of peracetic acid, UV and ozone after enhanced primary treatment of municipal
wastewater. Water Research, 37 (19), 4573-4586.
Gesuale G, Bellemare F, Liechti P A, Fournier M, Payment P Gagnon C, Hausler R.,
2010. Ozone: A wastewater disinfectant of the future. ). WEFTEC 2010, New Orleans
Louisiana, U.S.A.
Hammer, M.J., Hammer, Jr., M.J., 2011, Water and Wastewater Technology, 7th edition,
International edition, PEARSON publishing, ISBN 0-13-271988-6.
Hansen, T.L., Schmidt, J.E., Angelidaki, I., Marca, E., Jansen, J.la C., Mosbæk, H.,
Christensen, T.H., (2004). Method for determination of methane potentials of
solid organic waste. Waste Management 24, 393–400.
Hey, G., Ledin, A., la Cour Jansen, J., Andersen, H.R., 2012, Removal of
pharmaceuticals in biologically treated wastewater by chlorine dioxide or peracetic acid.
Environmental Technology, 33(9), 1041-1047.
Jönsson, K., Magnusson, P., Jönsson, L.-E., Hellström, B. G. and Jansen, J. la C. 1996
Identifying and Fighting Inhibition of Nitrification at Öresundsverket. Water Science and
Technology, 33 (12), 29-38.
21
Kim, W. J., Managaki, S., Furumai, H., Nakajima, F., 2009, Diurnal fluctuation of
indicator microorganisms and intestinal viruses in combined sewer systems. Water Sci.
and Tech. 60(11), 2791-2801.
Metcalf and Eddy, 2003. Wastewater Engineering, treatment and reuse. 4th edition.
McGraw Hill.
Ohlsson, Karlsson, and Gustafsson, 2011, Tätorters inverkan på recipienters bakteriella
status. SVU projekt 2011-08.
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treatment plants and combined sewer overflow basins on the microbiological quality of
surface water. Water Sci. and Tech. 54(3), 95-99.
Solomon, C., Casey, P., Mackne, C., Lake, A., 1998, Fact sheet: Ozone disinfection, a
general overview. The National Small Flows Clearinghouse. West Virginia University.
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Styszko, K., Bollmann, U.E., Wangler, T.P., Bester, K. 2013 Desorption of biocides from
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Winnfors, E., 2009, Bräddvattenreningen fungerar utmärkt. Cirkulation 5/09.
22
Total phosphorus (mg PO4-P/L)
Appendix – supplementary data
10
9
8
7
6
5
4
3
2
1
0
7,395
0,212
0,158
0,15
0,274
0,294
Incoming
Filtrated
3 mg/L
6 mg/L
9 mg/L
12 mg/L
(untreated)
100 µm
treated with Ozone
Figure A1. Total phosphorus concentration detected in different steps of treatment process through
continuous pilot-scale experiment.
Suspended solids concentration
(mg SS/L)
1000
224,8
100
10
4,44
5
Incoming
Filtrated
3 mg/L
(untreated)
100 µm
3,2
3,6
6 mg/L
9 mg/L
1
treated with Ozone
Figure A2. Suspended solids concentration measured in different steps of treatment process through
continuous pilot-scale experiment.
23
700
600
COD (mg/L)
500
400
300
200
100
0
Incoming Filtrated 5 mg/L
10 mg/L 15 mg/L
3 mg/L
(untreated) 100 µm treated with chlorine dioxide
6 mg/L
9 mg/L
12 mg/L
treated with ozone
Figure A3. COD content of samples measured in different steps of treatment process through continuous
pilot-scale experiment and laboratory-scale experiment with chlorine dioxide.
300
BOD7 (mg/L)
250
200
150
100
50
0
Incoming
Filtrated
(untreated)
100 µm
3 mg/L
6 mg/L
9 mg/L
12 mg/L
treated with ozone
Figure A4. BOD7 content of samples measured in different steps of treatment process through
continuous pilot-scale experiment.
24
Turbidity (FNU)
1000
100
10
1
Incoming
Filtrated
(untreated)
100 µm
3 mg/L
6 mg/L
9 mg/L
treated with Ozone
Figure A5. Turbidity of samples measured in different steps of treatment process through continuous
pilot-scale experiment.
Residual concentration of
chlorine dioxide
15
10
5
0
5 mg/L
10 mg/L
15 mg/L
Initial dose of chlorine dioxide
Consumed chlorine dioxide
Residual chlorine dioxide
Figure A6. Total added concentration, consumed concentration as well as residual concentration of
chlorine dioxide in laboratory-scale batch experiment of chlorine dioxide treatment.
25
90,0
Biocide concentration (ng/L)
80,0
70,0
60,0
50,0
40,0
30,0
20,0
10,0
0,0
-10,0
MI
BIT
MCPP
BAM
CD
IP
DR
IPBC
AT
TB
IRG
OIT
TBU
DCOIT
PA
Incoming (untreated)
0,0
0,0
33,3
27,0
35,6
3,2
6,4
1,7
0,0
6,1
0,0
0,0
0,7
12,7
4,9
Filtrated (100μm)
0,0
0,0
38,4
32,1
45,4
4,7
7,6
0,7
0,0
6,5
0,0
2,3
1,4
11,2
4,8
Treated with ozone
0,0
0,0
50,9
24,8
4,3
0,0
3,1
3,6
0,0
0,0
0,0
0,9
1,8
11,7
5,8
Treated with chlorine dioxide (10 mg/L)
0,0
0,0
36,7
39,4
50,9
2,9
7,7
11,8
0,0
0,0
0,0
11,2
2,2
8,8
7,1
Treated with chlorine dioxide (15 mg/L)
0,0
0,0
32,0
42,6
48,9
2,2
7,9
12,4
0,0
0,0
0,0
9,3
1,6
10,8
7,5
Blank
0,0
0,0
0,0
0,0
1,3
0,0
0,0
2,6
0,0
0,0
0,0
0,0
1,7
9,3
0,6
Figure A7. Analysis results for biocide removal experiment through filtration, ozonation, and chloride dioxide treatment for the selected substances. Note that all the results in this figure
are subjected to about 10% uncertainty interval.
26
Figure A8. Schematic layout of the wastewater treatment plant in Helsingborg (Öresundsverket) where pre-hydrolysis of primary sludge is implemented in the primary
clarifier followed by an activated sludge process with enhanced biological phosphorus removal.
27
Tabel B1. Flow data into the wastewater treatment on the sampling days over a period of 11 weeks. The average inflow
during 2012 is estimated about 48012 m3/day.
Inflow
Inflow
Date
(m3/day)
Sampling for experiment
Date
(m3/day)
2012-12-08 43396
2013-01-19
66669
2012-12-09 42091
2013-01-20
54205
2013-01-21
52272
a, O3 & ClO2 – pathogen 2012-12-10 34584
2013-01-22
54321
b, O3 & ClO2 - pathogen 2012-12-11 42658
2013-01-23
50363
c, O3 & ClO2 - pathogen 2012-12-12 43486
2012-12-13
40723
2013-01-24
49638
d, O3 – pathogen
2012-12-14 37727
2013-01-25
48475
2012-12-15 55700
2013-01-26
47333
2012-12-16 64894
2013-01-27
48707
2012-12-17
68520
2013-01-28
60805
e, O3 - pathogen
2012-12-18 60602
2013-01-29
71389
f, O3 - pathogen
2012-12-19 55969
2013-01-30
90622
2012-12-20 49382
81941
g, O3 & ClO2 – pathogen/biocide 2013-01-31
2012-12-21 51167
2013-02-01
65949
h, O3 – pathogen/biocide
2012-12-22 50418
2013-02-02
51847
2012-12-23 50060
2013-02-03
58139
2012-12-24 64770
2013-02-04
82846
2012-12-25 71763
2013-02-05
77530
2012-12-26 61580
2013-02-06
73841
2012-12-27 59201
2013-02-07
49917
2012-12-28 47465
2013-02-08
55497
2012-12-29 64670
2013-02-09
53200
2012-12-30 65677
2013-02-10
53076
2012-12-31 76697
2013-02-11
47969
2013-01-01 70890
2013-02-12
49733
2013-01-02 60290
2013-02-13
62070
2013-01-03 75033
2013-02-14
64003
2013-01-04 64140
2013-02-15
71848
2013-01-05 55763
2013-02-16
64406
2013-01-06 67969
2013-02-17
62608
2013-01-07 64860
2013-02-18
85593
2013-01-08 72354
2013-02-19
83312
2013-01-09 76109
2013-02-20
53915
2013-01-10 81390
2013-02-21
51568
2013-01-11 83320
2013-02-22
59281
2013-01-12 79499
2013-02-23
55615
2013-01-13 72926
2013-02-24
57253
2013-01-14 72525
2013-02-25
56609
2013-01-15 71318
2013-02-26
48319
2013-01-16 69643
2013-02-27
56275
2013-01-17 63383
2013-02-28
74300
O3 (lab-scale batch) – pathogen
2013-01-18 68873
*The information given in this column defines the chemicals used for different purposes/experiments as described in
the report. For example the note O3 & ClO2 – pathogen/biocide means that both ozone and chlorine dioxide were used
in pilot plant and laboratory scale experiments, respectively in order to study their effect on pathogen and biocide
reduction. Ozone treatment was mainly performed in pilot-scale continuous flow except for the test done on 2013/02/28
on which laboratory-scale batch experiment was done. 28
Sampling for experiment*