Removal of Natural Organic Matter Fractions by Two Potable Water Treatment Systems:
Dual Membrane Filtration and Conventional Lime Soda Softening
Charles D. Goss* and Beata Gorczyca
Department of Civil Engineering, University of Manitoba
15 Gillson Street, Winnipeg, Manitoba, R3T 5V6 Canada
Submitted to:
4th
Annual IWA Specialty Conference on Natural Organic Matter: From Source to Tap
and Beyond
July 27-29, 2011
Costa Mesa, California, USA
Acknowledgments
This group would like to acknowledge the following for financial support and technical
assistance on this project.
Financial Support
Manitoba Water Stewardship – Manitoba Water Stewardship Fund
Technical Support
Pembina Valley Water Cooperative and the Town of Morris, Manitoba
Portage la Prairie Water Treatment Plant and the City of Portage la Prairie, Manitoba
Jeff O’Driscoll and Ken Anderson (Genivar)
Victor Wei (Manager-University of Manitoba Environmental Laboratory
Abstract
The objective of this study was to investigate the removal of dissolved organic carbon
(DOC) fractions by two water treatment plants, the Portage La Prairie Water Treatment Plant
(PPWTP), which uses lime/soda softening with granular activated carbon (GAC) filtration, and
the Morris Water Treatment Plant (MWTP), a dual micro/nano membrane facility, located in
Manitoba, Canada. The study aimed to determine the cause of reportedly high trihalomethane
(THM) concentrations in plant effluent. Both the PPWTP and MWTP use surface water sources,
the Assiniboine and Red River, respectively, which are reportedly high in DOC, fluctuating from
7mg/L to 18mg/L throughout the year. As a result of the high DOC in the source water both
plants have reported high THM concentrations that have, in the past, exceeded the 100ppb
maximum limit set by the Province of Manitoba. Solid phase extraction (SPE) was used to
fractionate DOC in water samples collected during this study. The SPE method fractionated
DOC into six fractions: hydrophobic acid (HPOA), hydrophobic base (HPOB), hydrophobic
neutral (HPON), hydrophilic acid (HPIA), hydrophilic base (HPIB), and hydrophilic neutral
(HPIN).
Samples were collected from the PPWTP and Assiniboine River on November 8, 2010,
January 20, 2011 and April 2, 2011 to evaluate the DOC and DOC fraction removal throughout
the plant. Results found that the GAC filter was ineffective at removing DOC, often with DOC
concentrations increasing after the GAC filter. The HPOA fraction, largely believed to contain
the greatest THM precursors, was unaffected by GAC filter showing the potential cause for
reported high THM levels at the plant. All hydrophilic fractions increased after the GAC filter
and only the HPOB fraction was reduced by GAC filtration at the PPWTP. The recommendation
to the PPWTP from this group was to improve the coagulation process to reduce organic loads
on the GAC filter.
Samples were collected from the Red River on September 25, 2010, and fractionated, to
establish the relative composition of the river. The results found that the late summer
composition of the Red River was 45% hydrophobic and 55% hydrophilic, with 40% of the total
organic component being HPIN. On November 25, 2010 samples from the Red River and
MWTP were fractionated to establish membrane removal efficiency. The results were
unexpected finding that DOC increased from 8.7mg/L to 10.2mg/L. The HPIA and HPIN
fractions increased after the nano filter from 0.35-1.41mg/L and 2.00-4.00mg/L, respectively.
The HPOA fraction was found to be unaffected by the nano filter while the HPON, HPOB, and
HPIB faction had small decreases in concentrations. However, for samples collected in
February, 2011 DOC concentrations were reduced to <0.5mg/L by the nano filter. The reason
for the high DOC found after the nano for the November sampling period is unclear however it is
believed that (1) the samples were taken just prior to a cleaning event where filter was not
removing DOC effectively or (2) that the use of citric acid to clean the nano membrane could
have added a carbon source to the nano effluent. It is recommended that a pre-treatment process
be implemented prior to the micro/nano membranes to reduce the DOC load on the membranes
preventing both high THM concentrations and membrane fouling.
Introduction
Potable water treatment facilities in Manitoba often suffer from sources waters with high
concentrations of natural organic matter (NOM). High NOM causes concern due to the
formation of harmful disinfection byproducts (DBPs), such as trihalomethanes (THMs), which
form when chlorine reacts with NOM, during disinfection treatment (Singer, 1999). In Manitoba
water chlorine demands often exceed 5 milligrams per liter (mg/L), resulting in residuals of 0.04-
2.0 mg/L. If NOM is not removed prior to chlorination treatment the unreacted chlorine can
react with the NOM and form THMs, and other halogenated byproducts, which are potential
carcinogens (Krasner, 2009). As a result, the province of Manitoba had adopted standards set by
The Drinking Water Quality Act which requires all public potable water suppliers meet a
quarterly average of <0.100 mg/L for total THMs (TTHMs) (Manitoba Water Stewardship,
2007). Therefore, water treatment facilities are faced with improving the removal of the organic
matter from the raw water prior to chlorination in order to reduce the concentrations of THMs
and other halogenated byproducts.
NOM in Potable Water Treatment
Natural organic matter can be removed by both chemical and physical treatments. The
use of chemical coagulants in combination with filtration (sand or membrane) and physical
adsorption (activated carbon) are processes which treatment plants apply in order to remove
organics prior to disinfection. Although all of these processes remove NOM to some degree
each comes with their own set of drawbacks. Separating NOM laden coagulation flocs by
physical filtration either through sand or membrane filtration, or adsorption of NOM to activated
carbon filters, creates several challenges to treatment facilities. Mainly, all filtration systems
require some backwash, flushing or cleaning process in order to remove organics and other
materials built up on the surface. These cleaning procedures are often laborious and costly as it
requires the use of chemicals, water for backwashing, or physical cleaning by operators. Also,
the build-up of organics on the surface of membranes can cause reversible and irreversible
fouling and reduce the overall life of the membrane resulting in costly replacement (Agenson,
2007). The applications of adsorptive filters in situations where the NOM concentrations in the
source water are high have been used with limited success. Often these activated carbon filters
quickly reach adsorptive capacity due to size exclusion of large molecules blocking pores
thereby reducing their effectiveness of NOM removal (Amy, 1992). Therefore, these filters
require increased cleaning or flushing events and more frequent media replacement.
NOM largely consists of humic and fulvic acids which are largely believed to contain the
greatest THM precursors. Fulvic acids with high charge density, due to high carboxylic acid
moieties, are minimally affected by charge neutralization during coagulation compared to humic
acids (Musikavong et al., 2005; Amy, 1992). Although lowering the pH (pH 4-6.5) will improve
coagulation by reducing the charge density of fulvic acids as well as reduce the solubility of the
coagulant, water treatment operation at low pH ranges are difficult and costly due to high
volumes of acids required to overcome the buffering capacity of waters with high alkalinity
(Sawyer, 2003; Amy, 1992). Therefore, NOM not removed in coagulation pre-treatment has
potential to form THMs.
Natural organic matter is unique to location and therefore it is important to establish the
composition of organic matter as well as concentration (Chow, 2008). Organic carbon
fractionation studies are conducted to gain better understanding of the chemical and physical
properties of local organic matter. Studies often focus on dissolved organic matter, or dissolved
organic carbon (DOC), which is the organic carbon that is able to pass through a 0.45
micrometer (µm) filter paper, as this is typically harder to remove than particulate NOM.
Fractionation of DOC can be based on physical size through the use of filtration and size
exclusion chromatography. Likewise, chemical properties, such as hydrophobicity and charge,
can also be used to fractionate DOC. Methods developed that separate DOC on hydrophobicity
and charge were largely based upon methods originally developed by Leenheer and Aiken
(Leenheer, 1981, Aiken, 1992). Recent methods separate DOC into hydrophobic acid (HPOA),
hydrophobic base (HPOB), hydrophobic neutral (HPON), hydrophilic acid (HPIA), hydrophilic
base (HPIB), and hydrophilic neutral fractions (HPIN). The original methods used ion exchange
resins (eg. XAD-4 and XAD-8) which were often laborious to prepare and required long run
times (~24h). New methods have been developed to reduce sample preparation and run times.
One method developed by Ratpukdi et al. uses solid phase extraction (SPE) to fractionate DOC
into these six fractions (Ratpukdi, 2009). This method reduces run times to roughly 12 hours as
well as preparation times as the SPE cartridges are pre-packaged and the sample requires only
filtration and pH adjustment.
Dissolved Organic Carbon Fractions and THMs
It is largely suggested that the HPOA fraction, which is largely humic matter, has the
greatest potential to form THMs, and other by-products, due to its high aromaticity and reactivity
(Singer, 1999; Leenheer, 2003; Chow 2005). However, a review study conducted by Chow in
2005 mentions several studies that found the HPI fractions had greater THMFP while others
suggest that both HPO and HPI form THMs when chlorinated (Chow, 2005). This review, and
others, suggests that the formation of THMs is dependent on local environment and that THMFP
and organic composition are unique to that location. Therefore, caution should be taken when
estimating the potential of organic fractions to form THMs.
Objectives and Significance of Research
This study focused on the characterization of DOC and its removal in two water
treatment plants located in Manitoba that suffer from high DOC concentrations in the source
water. The first plant is located in Portage la Prairie and is a conventional softening plant with
ballasted flocculation pretreatment and granular activated carbon (GAC) filtration. The second
plant is located in Morris and is a newly constructed dual membrane (micro/nano) facility that
replaced a conventional lime soda softening plant. Both plants use surface water sources, the
Assiniboine and Red Rivers, which experience high NOM concentrations between 8 and 18
mg/L. As a result both plants have, in the past, experienced elevated THM concentrations
occasionally exceeding the Manitoba guideline of 0.100 mg/L.
This study utilized the Ratpukdi et al. solid phase extraction method to fractionate water
DOC in the samples collected from the Red and Assiniboine Rivers as well as the Morris and
Portage la Prairie water treatment plants in order to establish the removal efficiency of DOC
fractions by each process. Understanding removal efficiency of DOC from each plant will
provide operators and design engineers with greater knowledge of which processes are effective
at removing DOC and which need optimizing in order to reduce the formation of THMs.
Water Treatment Plants Analyzed in this Study
Portage la Prairie Water Treatment Plant (PPWTP)
The City of Portage la Prairie has a population of roughly 13,000 people and has been
growing at a steady rate for the past several years due to the development of food processing
industries and agriculture in the region. To meet the long term water demands of the growing
community, along with the construction of a new potato processing facility in the area, the
PPWTP underwent several upgrades from 2000-2003. Upgrades to increase water production
from 18 mega liters per day (ML/d) to 34ML/d were implemented in order to meet growing
demands, as well as, improving and adding treatment processes to overcome challenges such as
high turbidity, hardness, and organic matter, along with occasional increased algal growth, often
seen in the Assiniboine River. Turbidity levels at the PPWTP were found to exceed 1500
nephelometric turbidity units (NTU) and peaks of 6000 NTU have been recorded (Table 1). The
high levels of DOC (15mg/L) found in the river resulted in THM levels that often exceeded the
0.100 mg/L guideline set by the Province of Manitoba.
Table 1: Water quality for the Assiniboine River and PPWTP for samples collected in May
1999. Guidelines presented here are according to the Canadian Drinking Water
Quality Guidelines for 2001 (Table adapted from Anderson, 2003)
Parameters Raw Water Treated Water Guideline
Hardness (mg/L CaCO3) 349 133 200*
Turbidity (NTU) 140 0.3 1
TOC (mg/L) 15 7 10**
Chloroform (mg/L) 0.0003 0.100 -
Bromoform (mg/L) 0.0010 0.001 -
BDCM (mg/L) 0.0002 0.021 -
DBCM (mg/L) 0.0002 0.031 -
Total THMs (mg/L) 0.0017 0.153 0.1
Note: * = Operational objective
**= Aesthetic guideline
To improve the removal of turbidity a John Meunier ACTIFLO® Ballasted Flocculation
Clarification system was implemented as a preclarification step (Figure 1). Pilot studies found
that raw waters with turbidity levels of 2500 NTU could be reduced to 3 NTU overcoming the
occasional high spikes in turbidity seen in the Assiniboine River (Anderson, 2003 and 2004).
Figure 1: ACTIFLO® Ballasted Flocculation system (Anderson, 2003)
Along with the ACTIFLO® system, an additional clarifier and new chemical dosing systems
were installed to increase plant flow. Improvements to the backwash system for the four dual
media sand filters were made to improve plant performance and promote organics removal.
Ozone was applied to softened and clarified water to promote biologically stable water, to
minimize chlorine demand, reduce the formation of DBPs, and to improve taste and odor. In
addition to sand filtration granular activated carbon adsorption was introduced to reduce
organics. Upgrades were made to the storage reservoirs to increase disinfectant contact time and
plant capacity. Lastly, a state of the art control system replaced the original system in order to
provide operators with complete control and monitoring of the system.
Although preliminary plant performance tests indicate that the new system improved the
removal of turbidity and hardness, tests in 2007-2008 suggest the plant may not be effectively
removing organics as THM levels were found to occasionally exceed guideline limits (Table 2).
Table 2: THM results for Portage la Prairie Water Treatment Plant from 2007-2008
Date THM concentration (µg/L)
Chloroform BDCM DBCM Bromoform Total THM
January 9, 2007 20 16 12 <1 48
May 9, 2007 17 11 8 <1 36
August 23, 2007 79 30 13 1 123
December 11, 2007 42 23 6.7 0.3 72
January 7, 2008 57 26 8.9 0.6 92.5
April 22, 2008 50 < 9.5 < 59.5
*BDCM = Bromodichloromethane
DBCM = Dibromochloromethane
< = less than detection limits
Pembina Valley Water Cooperative (Morris, MB)
The Pembina Valley Water Cooperative owns and operates the water treatment plant in
Morris, Manitoba. The original Morris water treatment plant (MWTP) was constructed in 1998
and was a typical lime soda softening plant with a flow capacity of 32 liters per second (L/s)
however it was determined that the plant would need to be expanded to meet growth in
population and industry in the area. The source for the plant is the Red River and, like the
Assiniboine, is prone to high DOC concentrations often exceeding 12 mg/L, as well as turbidity
levels ranging from 37-455 NTU. Due to the high DOC concentrations in the Red River THM
levels were also found to, at times, exceed the Manitoba guidelines (Table 3).
Table 3: THM concentrations for two storage reservoirs supplied by the Morris water
treatment plant. Samples were collected on November 4, 2009
Parameter
THM concentration (µg/L)
Miami
(Influent)
Rosenort
(Influent)
Bromodichloromethane 21 17
Bromoform < <
Chloroform 100 44
Dibromochloromethane 2 3.8
In 2008, construction began at the MWTP to renovate the existing lime soda softening
plant into a state of the art dual membrane facility. The upgrade included Pall ARIA™ micro
membrane filtration and Pall Ultipleat High Flow nano membrane filters. According to Pall the
micro filters would remove turbidity to 0.1 NTU along with three log reduction of Giardia and
Cryptosporidium. The nano filters would remove hardness and reduce organic matter
concentrations to <0.5 mg/L. The implementation of the membrane system would expand
capacity from 32 L/s to 66 L/s with room to increase flow to 100 L/s if required. A 1,000,000 m3
retention pond was also constructed to provide the plant with more stable source water and to
ensure availability during drought. Figure 2 outlines the flow for the membrane facility in
Morris. Construction of the facility was completed in late 2009 and went online in early 2010.
Figure 2: Plant flow diagram for Pembina Valley Water Cooperative in Morris,
Manitoba (Figure supplied by Anderson, 2009)
Table 4: Water quality parameters for the Morris water treatment plant after the installation of
the membrane system. Samples were collected in March 26, 2010.
Sample
Identification Unit Raw Water Post Micro Post Nano Tap
DOC mg/L 13.5 10.5 <1.0 3.1
True Color TCU 15 15 <5.0 <5.0
TDS Calculated mg/L 743 735 <5.0 186
Turbidity NTU 0.82 0.1 <0.10 <0.01
Alkalinity mg/L CaCO₃ 316 315 3.5 74.2
The water quality parameters for the MWTP, taken after the installation of the micro and
nano membrane (Table 4), suggest that the system is capable of meeting water quality guidelines.
However, the presence of high organics found in the source water could cause significant
problems to membranes over long term use. DOC characterization can provide insight into the
potential for these organics to cause fouling preventing unnecessary replacement costs.
Research Methodology
Sample collection Portage la Prairie Water Treatment Plant and Assiniboine River
1 liter (L) water samples were collected from the Assiniboine River, via an intake in the
plant, as well as throughout the water treatment plant three times during this study; November 8,
2010, January 20, 2011 and April 2, 2011. These three sampling dates represent river water
conditions during fall (prior to snow fall), winter and spring. Figure 3 diagrams the sampling
locations at the PPWTP. November 8th
samples only tested the overall removal of DOC
throughout the plant. Samples were collected from before and after the GAC filter on January 20,
2011 to establish the DOC fraction removal efficiency of the filter. All samples collected at
Portage la Prairie and the Assiniboine on April 2, 2011 were fractionated using SPE method
(described below).
Figure 3: Sampling locations for Portage la Prairie water treatment plant. (1) Assiniboine
River (2) after ACTIFLO ballasted flocculation (3) after lime softening (4) after
recarbonation (5) after ozonation (6) after sand filtration (7) after sand filter reservoir
(8) after GAC (9) Finished water
Sample collection Morris Water Treatment Plant and Red River
1-4L water samples were collected at various times during the study from the Red River
as well as from the retention pond, post micro filter, and post nano filter effluent, prior to
blending. Samples were collected on August 11, 2010 to establish an estimate of the summer
THM and THM formation potential (THMFP) from the Red River and treated MWTP effluent.
THM concentration and THMFP analysis was conducted by ALS laboratories (ALS Laboratories
Winnipeg, Manitoba). Samples collected from the Red River on September 25, 2010 were
fractionated to establish the relative composition of the river DOC during late summer. On
November 23, 2010 and February 28, 2011 samples were collected from the river as well as the
retention pond, post micro filter and post nano filter for full fractionation study. Note that
February sampling did not include the Red River due to ice cover.
All DOC measurements were made using Standard Method 5310 with a Tekmar
Dohrmann Phoenix 8000 total organic carbon analyzer (Tekmar Dohrmann, Ohio) which was
calibrated according to the instrument manual. All DOC measurements were made in triplicate.
All samples collected during this study were filtered through 0.45µm nitro cellulose filter paper
prior to analysis except for THM and THMFP analysis conducted by ALS laboratories.
Fractionation using Solid Phase Extraction (SPE)
Prior to fractionation all samples were filtered through 0.45µm nitrocellulose filter paper
and were brought to room temperature. The fractionation method used in this study was
developed by Ratpukdi et al. (2009). The method fractionates DOC into six fractions:
hydrophobic acid (HPOA), hydrophobic base (HPOB), hydrophobic neutral (HPON),
hydrophilic acid (HPIA), hydrophilic base (HPIB), and hydrophilic neutral fractions and is as
follows. The fractionation procedure uses three nonpolar Bond Elute ENV cartridges (Varian
Inc, Lake Forest California), one Phenomenex Strata XC strong cation exchange cartridge, and
one Phenomenex Strata X-AW weak anion exchange cartridge (Phenomenex, Torrance,
California). All SPE cartridges contained 1gram of sorbent. The fractionation procedure begins
with all cartridges being conditioned with 10mL of HPLC grade methanol (MeOH). The Strata
XC and X-AW cartridges are then conditioned with 10mL of 1.0M hydrochloric acid (HCl). All
five cartridges are then rinsed with deionized water until the effluent DOC measured <0.100
mg/L. 1L of sample was adjusted to pH 7.0 using either concentrated sulfuric acid (H2SO4) or
1M sodium hydroxide (NaOH) and was drawn through the first ENV cartridge labeled ENV-1.
The fraction collected on the ENV-1 SPE cartridge is defined as the HPON fraction. The same
sample water was then adjusted to pH of 10 using 1.0M NaOH and drawn through the second
ENV cartridge (ENV-2). The fraction retained on this cartridge is the HPOB fraction. Next, the
sample water was adjusted to pH of 2 with H2SO4 and drawn through the third ENV cartridge
(ENV-3) capturing the HPOA fraction. Following the ENV-3 cartridge the sample water was
drawn through the Strata XC cartridge without pH adjustment. The HPIB fraction was retained
on the Strata XC cartridge. Lastly, the sample water was adjusted to pH of 7 with NaOH and
drawn through the Strata X-AW cartridge which captured the HPIA fraction. The fraction of
organic matter that was not retained by any of the five cartridges is defined as the HPIN fraction.
All samples were drawn though the SPE cartridges at 10-15 mmHg vacuum pressure. After the
sample was drawn through each SPE cartridge a 40mL sample was collected to measure the
DOC concentration. Figure 4 outlines the Ratpukdi et al. method.
Figure 4: DOC SPE fractionation setup (Ratpukdi et al., 2009)
Results and Analysis
Characterization of DOC in Assiniboine River and Portage la Prairie Water Treatment Plant
On November 23, 2010 samples were collected form the PPWTP and the Assiniboine
River to establish the general removal of organics during the treatment process. As shown in
Figure 5 the incoming DOC of 16.1mg/L is significantly reduced to 5.0 mg/L after coagulation
and clarification. The coagulant that is used at the PPWTP is Alufer S25. The DOC is further
reduced to 3.8mg/L after the sand filtration; however the concentration increases after the GAC
filter to 6.7mg/L.
Figure 5: DOC removal at the Portage la Prairie Water Treatment Plant for samples
collected on November 23, 2010 (Personal communication Hooshyar, 2010)
It was suspected that the increase in DOC may be due to the GAC media being over
capacity as a result of constant high DOC source water. Organics could leach into the water
from the filter increasing the concentration. This increase could potentially provide enough
organic content to form levels of THMs that would exceed guideline limits. It must be noted that
the effluent DOC was 1.5mg/L showing a removal after the GAC filter. However, there is no
treatment process after the GAC filter that would reduce the DOC; it is unclear how the removal
occurred. More data is currently being collected at this plant to evaluate DOC removal by the
GAC filter.
Water samples were collected on January 20, 2011, from before and after the GAC filter,
to determine the dissolved organic fraction removal efficiency/ability of the filter (Table 5). It
was found that there was no DOC removed by the GAC filter, on the contrary, the DOC was
found to increase by 0.2326mg/L or 3.2% following GAC filtration.
The water samples DOC were also fractionated (Table 5) to establish which fractions
were affected by the filter. HPIB and HPIA fractions showed the greatest increase in
concentration from 0.2950mg/L to 1.1228mg/L and 0.0438mg/L to 0.3229mg/L, while HPOA
and HPIN fractions were largely unaffected by the GAC filter. The HPON and HPOB fractions
experienced the greatest reduction by the GAC filter from 0.5711mg/L to 0.1075mg/L and
0.2879mg/L to 0.0441mg/L, respectively. The mechanisms that caused the overall change in
DOC composition occurring on/near the filter was not a focus of this study therefore reasoning
for the increase/decrease of fractions cannot be fully evaluated. However, the ineffective DOC
removal by the GAC filter suggests that the filter has exceeded adsorptive capacity.
0.02.04.06.08.0
10.012.014.016.018.0
DO
C (
mg/L
)
Table 5: DOC concentration changes occurring before and after the GAC filter at the
Portage la Prairie Water Treatment Plant for samples collected on January 20,
2011.
Fraction
DOC concentration (mg/L)
Before GAC After GAC
HPON 0.51±0.1 0.1075*
HPOB 0.288±0.003 0.0441*
HPOA 2.3±0.1 2.4442*
HPIB 0.2950±0.0006 1.12±0.01
HPIA 0.04±0.05 0.32±0.03
HPIN 3.7688±0.0008 3.4585*
Total 7.2674 7.5000
*No error value could be established due to instrument carryover.
Figure 6: Overall DOC removal at the Portage la Prairie Water Treatment plant. Samples were
collected on April 2, 2011(Hooshyar, 2011)
Figure 6 shows the overall removal of organics at the PPWTP for samples collected on
April 2, 2011. The removal of organics is largely done by the lime softening and recarbonation
processes removing 10mg/L, combined. Note that the ACTIFLO system was not active during
this sampling period. An increase in DOC from 7.0mg/L to 9.5 mg/L is seen after ozonation.
Sand filtration reduces the concentration of DOC down to 6.9mg/L however the GAC filter is
again ineffective at removing any organics showing an increase in DOC concentration to
7.2mg/L.
Fractionation of all samples collected on April 2, 2011 was conducted to determine how
each process within the plant affects DOC fraction removal (Figures 6 and 7). The fractionation
results for samples collected from the river and the PPWTP show that nearly all fractions were
reduced by lime softening and clarification with the HPOB fraction having the greatest reduction
of nearly 92%. After the ozonation process all fractions increased, except the HPIB fraction, by
15-50%. Similar results were seen in a study conducted by Śweitlik et al. (2004) where it was
found that after the application of ozone there was an increase in HPIA, HPIN and HPON, with a
small increase in HPIB (Śweitlik, 2004). Śweitlik et al. suggests that the application of ozone
will also increase the biodegradable organic carbon (BDOC) fraction (Śweitlik, 2004). Higher
0.02.04.06.08.0
10.012.014.016.018.0
DO
C (
mg
/L)
concentrations of BDOC could cause an increase in microbial populations in the system
potentially causing another source for the increase in organics seen in the PPWTP. Further
research on the effect of ozone and the increase in microbial populations is currently being
conducted to better understand if the presence of microbes influences the formation of THMs.
Sand filtration reduced all fractions by 21-42% except for HPIB which increased 97% after sand
filtration. The HPON fraction was unaffected by the sand filtration process. The GAC filter was
found to have similar results to the January 20, 2011 results which found the HPIN fraction to be
largely unaffected with only a small decrease in concentration of 0.5mg/L. The HPOA fraction
increased by 24% from1.6mg/L to 2.1mg/L, likewise the HPIB and HPIA fractions increased by
54% and 45%, respectively. Only the HPON and HPOB fractions were reduced by the GAC
filter with a removal of 64% for HPON and 100% for HPOB. These results clearly show that the
GAC filter is ineffective at removing all organic fractions. The GAC filter can be seen as an
unfavourable process as organics, especially the HPOA fraction, increase after the filter. In turn,
there is a large presence of HPOA in the finished water for this sampling period, and throughout
this study, suggesting there is potential to form THMs based on the high reactivity of the these
compounds noted in literature. However, there is some controversy as to the fraction of DOC
that contains the largest THMFP. Therefore, further analysis into the THMFP of organics found
in the Assiniboine River and PPWTP effluent is required. Currently, THMFP analysis for the
river and treatment plant is being conducted by this group. The results lastly show an increase in
HPIN, HPOA, HPOB and HPON fractions in the finished water while the HPIB and HPIA
fractions decreased. Although it is unclear why there is an increase in DOC in the system from
the GAC to finished water, this increase poses an issue for THM control. Further study into the
increase seen after the GAC to the finished water is ongoing.
Figure 7: Removal of DOC fractions from the Assiniboine River and Portage la Prairie Water
Treatment plant. Samples were collected on April 2, 2011.
0.000
1.000
2.000
3.000
4.000
5.000
6.000
7.000
DO
C (
mg
/L)
HPON HPOB HPOA HPIB HPIA HPIN
One suggestion to address the problem with high THMs at the PPWTP would be to
optimize the coagulation process prior to the GAC filter. If the coagulation process is optimized
for DOC removal the GAC filter will experience low concentrations allowing greater time before
the filter reaches capacity and no longer removes organics. The use of a more effective type of
GAC media is another possible solution to improve the removal of DOC. Cheng et al.
demonstrated that modified activated carbon such as iron impregnated or modification with
helium or ammonia significantly improved the removal of DOC over virgin activated carbon
(Cheng, 2005). Further investigation into a more effective type of carbon media is being
conducted by this group.
Characterization of DOC in Red River and Pembina Valley Water Cooperative (Morris,
Manitoba)
On August 11, 2010 samples were collected from the Red River and the Morris water
treatment plant to establish the THM concentrations as well as THMFP of the Red River and the
treated effluent (Tables 6 and 7). The results show that the THM levels for plant effluent are
below the guideline limits of 100ppb (Table 6), however the THMFP of the pond is significantly
higher than the guideline suggesting there is potential to for THM levels that exceed the 100ppb
limit. Likewise, the THMFP of the nano effluent also shows potential to form THM levels that
are greater than 100ppb. Although the Morris plant is seemingly controlling THMs there is
evidence that high DOC feed water can negatively impact the performance and lifetime of
membrane filters (Cho, 1999).
Table 6: THM concentrations for samples collected from the Red River and Morris water
treatment plant on August 11, 2010. Samples were analysed by ALS Laboratories
(Winnipeg, MB).
Sample Location
THM concentrations (mg/L)
Chloroform Bromoform Dibromochloromethane Bromodichloromethane
Red River <0.00050 <0.00050 <0.00050 <0.00050
Retention Pond <0.00050 <0.00050 <0.00050 <0.00050
Post Micro <0.00050 <0.00050 <0.00050 <0.00050
Post Nano 0.0280 <0.00050 0.00097 0.0062
Table 7: THMFP for samples collected on August 11, 2010 from the retention pond and post
nano effluent at the Morris water treatment plant. Samples were analysed by ALS
Laboratories (Winnipeg, MB).
Sample Location THMFP concentrations (mg/L)
Chloroform Bromoform Dibromochloromethane Bromodichloromethane Total
Retention Pond 0.495 <0.00050 0.00711 0.0627 0.565
Post Nano 0.1400 <0.00050 0.00153 0.0145 0.156
Samples collected from the Red River on September 25, 2010 were fractionated to
establish the relative composition of the river during late summer (Table 8). The results show
that the Red River is 45% hydrophobic and 55% hydrophilic, with 40% of the total organic
composition being HPIN. Although the concentrations of THMs at Morris are low (Table 6)
there is a large HPOA fraction of nearly 22% suggesting the potential to form THM upon
chlorination.
Table 8: Fractionation results for the Red River collected on September 25, 2010
Fraction DOC (mg/L) % DOC
HPON 2.47 22
HPOB 0.21 <2
HPOA 2.47 22
HPIB 0.21 <2
HPIA 1.46 13
HPIN 4.49 40
Total 11.3 100
Samples collected from the Red River and Morris water treatment plant on November 23,
2010 were fractionated to determine the removal efficiency of the micro and nano filter
membranes (Figure 8). The results obtained from this sample set deviated from what was
expected. It was suggested by the Pall Corporation that the nano membranes would be able to
reduce DOC concentrations to <0.5mg/L, however the results from this sample set found that
the DOC concentration increased after the nano filter from 8.7mg/L to 10.2mg/L. The HPIA and
HPIN fractions increased after the nano filter from 0.35-1.41mg/L and 2.00-4.00mg/L,
respectively. The HPOA fraction was unaffected by the nano filter while the HPON, HPOB, and
HPIB faction had small decreases in concentrations. It must be noted that shortly after this
sampling period the Morris plant reported unexpected levels of THMs in the distribution system
ranging from 75-86ppb (Fehr, 2010). Although the levels are still below the required 100ppb
guideline the increase is of concern since the maximum allowable THMs concentration are
rumoured to be reduced to 80ppb in the near future. The increase in THMs may have resulted
from the poor removal of organics seen during the November 23rd
sampling period. The reason
for the increase in DOC is not fully understood however there it is suggested that the sampling
event may have taken place just prior to a cleaning event where DOC rejection was not
efficiently occurring. Another potential cause is the use of citric acid as a cleaning agent for the
nano membranes. If the citric acid was not fully rinsed after the cleaning cycle it could have
caused the increase in HPIA. Further investigation into the relationship between membrane
cleaning cycles, citric acid as a possible organic source, and organics removal is suggested.
Figure 8: Fractionation results for Red River and throughout the Morris water treatment
plant. Samples collected on November 23, 2010
0
1
2
3
4
5
Red River Pond Post Micro Post Nano
DO
C (
mg
/L)
HPON
HPOB
HPOA
HPIB
HPIA
HPIN
Samples were collected on February 28, 2011 from the retention pond and at the Morris
water treatment plant for DOC and fractionation analysis. Note samples could not be collected
from the Red River due to ice cover. The DOC results found during this sampling period
resembled the expected results with the overall DOC being reduced from 9.00mg/L to 0.42mg/L
after nano filtration (Figure 9). Due to the low DOC in the nano filter effluent the sample could
not be fractionated. Pond water and post micro filter water fractionation results (Figure 10)
found that the change in overall DOC concentration was not significant after micro filter,
however there were noticeable changes in the overall composition of DOC following micro
filtration. There was a roughly 50% decrease in HPON (1.1mg/L to 0.53mg/L) and about a 50%
increase in HPIA (1.37mg/L to 2.46mg/L). This could be related to microbial growth on the
clean side of the nano-membrane surface or membrane surface-DOC interactions changing the
chemical properties of the compounds. The fractionation results also show there is a large
HPOA component in the post micro membrane filter effluent. This effluent is blended with nano
treated water at a 20-30% blend rate. This is done to increase hardness and alkalinity in the final
effluent that is removed by the nano filter. However, with the HPOA fraction being 35% of the
total DOC in the micro effluent caution should be taken when increasing this blend rate to ensure
organic concentrations do not increase to where THM levels are exceeding guidelines.
It is suggested by Fan et al. that hydrophobic polyvinylidene fluoride (PVDF)
membranes, such as those at the Morris plant, are fouled largely by HPIN and HPOA fractions
(Fan, 2001). These two fractions constitute nearly 52% of the total DOC entering the plant
suggesting there is a potential for fouling of the membranes.
Figure 9: Overall DOC removal at Morris water treatment plant. Samples collected February
28, 2011
Figure 10: DOC fraction removal at Morris water treatment plant. Samples collected
February 28, 2011
0
2
4
6
8
10
Pond Post Micro Post Nano
DO
C (
mg
/L)
0
1
2
3
4
Pond Post Micro
DO
C (
mg
/L)
HPON
HPOB
HPOA
HPIB
HPIA
HPIN
The evaluation of the organics removal at the Morris water treatment plant found that
although the membranes were effective at reducing the DOC concentrations and in turn control
the formation of THMs, there are points in time that the membrane is not effectively removing
DOC. The reasons for the increase in organics seen in the November samples is inconclusive
although there is evidence to suggest there may be a relation to cleaning events and/or the use of
citric acid in the cleaning of the membranes. It is recommended that the relationship between
cleaning events and increases in DOC should be investigated.
High concentrations of HPIN and HPOA, suggested by Fan et al., could cause organic
fouling to PVDF membranes. Therefore it is recommended that Morris re-implement a
pretreatment that will remove organics before entering the membranes. Initially the engineers
that designed the membrane upgrade used an existing clarifier from the original lime softening
plant as a coagulation tank to reduce the organic load seen by the membranes. However, this
process was not optimized and in turn unreacted coagulant (alum) was able to pass the micro
filters causing the nano filters pressure to dramatically increase to risky levels in a short time
(<24h). It was recommended by the engineers that the pretreatment be stopped due to the
potential damage of the nano membranes. If this pretreatment step were optimized (mixing
times, coagulant types, coagulant dose) the overall organic load would be reduced without the
risk of damaging the membranes. This could extend the life of the membranes, as well as reduce
the number of cleaning cycles, reducing the operation costs for the plant due to costly membrane
replacements. Optimization of the coagulation process for removal of targeted DOC fractions is
being currently investigated by Water Research Group at the University of Manitoba.
Conclusions and Recommendations
The objective of this research was to characterize the dissolved organic carbon and its
removal efficiency in two potable water treatment plants located in Portage La Prairie and Morris
(Manitoba, Canada). This study also aimed to establish the organic composition of the two river
sources for each plant, the Assiniboine and Red Rivers, and to evaluate the concentration of
fractions suspected to form THMs and foul membranes.
The first plant located in Portage la Prairie, which uses the Assiniboine River as a source,
is a lime softening plant with ballasted flocculation and granular activated carbon filtration. DOC
removal results from all sampling at the Portage la Prairie water treatment plant during this study
found that the granular activated carbon filter was ineffective at removing DOC, often with
concentrations increasing post GAC filter. The HPOA fraction, suspected to largely contain
THM precursors, was unaffected by the GAC filter and nearly all fractions, especially
hydrophilic compounds, increased after the GAC filtration.
The Morris plant is a newly designed dual membrane (micro/nano) facility that, like the
Portage plant, experiences source water high in DOC (Red River). Two sampling periods were
conducted on the Red River and at the Morris plant to evaluate DOC faction removal:
November, 2010 and February 2011. Results from the November, 2010 sampling found that the
nano membrane was not removing DOC effectively. Specifically the HPOA fraction and all
hydrophilic fractions were not removed by the nano filter. The results obtained in February,
2011 were very different than the November results as the nano membranes were found to
reduce DOC levels to <0.5mg/L. The reason for the high DOC found after the nano for the
November sampling period is unclear however it is believed that (1) the samples were taken just
prior to a cleaning event where filter was not removing DOC effectively or (2) that the use of
citric acid to clean the nano membrane could have added a carbon source to the nano effluent.
Further analyses are currently being conducted at the Morris plant.
Results obtained at both plants indicate that the major DOC fractions present in both raw water
supplies (Red and Assiniboine Rivers), ie. HPOA, HPIN, HPIA and HPIB are not effectively
removed by the treatment processes utilized. It is believed that the high concentration of the
HPOA fraction could lead to increased THM concentrations after chlorination.
The following recommendations can be made from this study:
Due to the uncertainty as to which fraction contains the greatest potential to form
THMs, a THMFP study of fractions collected from the Assiniboine River water
intake be conducted to establish which fractions in the local environment from
most THMs.
Optimize coagulation process and GAC filtration for removal of the targeted
DOC fractions.
Identify DOC fraction with the highest nano-filter fouling potential.
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