Expanded
Summary
Pretreatment impacts on biopolymers in adjacent
ultrafiltration plants
B ARBARA SIEMBID A - L Ö SC H , WIL L IA M B . A ND ER S O N , J AN E BO N S T EEL , AN D P ET ER M. H U C K
http://dx.doi.org/10.5942/jawwa.2014.106.0080
Full-scale studies were conducted to determine the effects
of the configuration or absence of pretreatment on
membrane performance at two full-scale ultrafiltration
(UF) drinking water treatment plants sharing a common
raw water source. One plant had no pretreatment before
the membranes and the other had ozone and biologically
active carbon contactor (BACC) pretreatment. At one
point during this study, ozonation was off, and this
presented an opportunity to compare ozone on/off
scenarios. Ozone addition was observed to reduce
biopolymer retention by UF membranes up to 23%.
Although lower normalized flux (20°C) was associated
with higher biopolymer retention by UF membranes,
higher retention was found to not affect membrane
permeability. Assimilable organic carbon (AOC) was not
a good surrogate for biopolymer removal by biofiltration.
Recently, a particular component of natural organic
matter (NOM)—i.e., biopolymers, which are composed
of polysaccharides, proteins, and protein-like substances—has been identified as a key low-pressure membrane (LPM) foulant (Hallé et al, 2009).
Those authors have shown that direct biofiltration (i.e.,
no prior coagulation or ozonation) can effectively remove
biopolymers and reduce fouling of UF membranes in
drinking water applications. Ozonation before biofiltration can also be used as a pretreatment step for LPMs.
This study evaluated, at full scale, the effect of the
ozonation/biofiltration sequence as pretreatment for UF
membrane performance.
oxidation and granular activated carbon contactors.
Hollow-fiber membranes1 were operated at an average
normalized flux of 56.5±7.4 LMH and 41.1±5.9 LMH
at Lakeview and Lorne Park, respectively.
Parameters of interest. Flux, transmembrane pressure,
temperature, and turbidity were monitored on-line.
Biweekly measurements of raw water and effluents of
each treatment step included pH, total organic carbon
(TOC), dissolved organic carbon (DOC), AOC, ultraviolet absorbance at 254 nm, specific UV absorbance, and
total and free chlorine residuals. In addition, biopolymers
were characterized by liquid chromatography/organic
carbon detection (LC-OCD) analyses.
MATERIALS AND METHODS
RESULTS AND DISCUSSION
Feedwater. The two adjacent full-scale drinking water
membrane plants—Lakeview and Lorne Park in Mississauga, Ont., Canada—are separately fed with water from
Lake Ontario that is prechlorinated for zebra mussel control. For all parameters, values for the two plants were
extremely close during the study period. The observed range
of temperature (9–23°C) is a result of seasonal changes.
Biopolymer concentrations in the raw water averaged
350±83 µg C/L and 338±70 µg C/L for the Lakeview and
Lorne Park water treatment plants (WTPs), respectively.
Plant configuration and operating conditions. An
advanced train of the Lakeview WTP operates ozonation
before BACCs as pretreatment for UF membranes. Under
normal operating conditions, chlorine is quenched with
sodium bisulfite before the water enters the ozone contactors; residual ozone following the contactors is also
quenched using the same agent.
The Lorne Park WTP operates without pretreatment,
and the membrane step is followed by UV disinfection/
Raw water quality. A strong correspondence was
observed between temperature and the biopolymer fraction in raw water as a result of higher microbial activity
in surface water at higher temperatures. It was observed
that the biopolymer fraction constituted up to 20% of
DOC in the raw water for both plants and that this ratio
increased when temperature increased.
Effect of pretreatment on organics at Lakeview. It is
expected that ozonation will transform biopolymers and
humics to lower-molecular-weight compounds rather
than completely mineralizing them as a result of the low
TOC/DOC concentrations and ozone-to-TOC ratios (<
1 mg O3/mg C) in this water. Ozonation accounted for
only minor DOC removal, up to 10% at an ozone dosage
of 1.8 mg O3/L.
Unexpectedly, the BACCs at Lakeview removed on
average only 6% (< 30 µg C/L) of the biopolymers regardless of the presence or absence of ozone. This occurred
even though AOC removals were up to 46%. This lack
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of correspondence between AOC and biopolymer removal
suggests that AOC removal may not be a reliable surrogate for biopolymer removal. The low removals are
believed to be related to the presence of free/total chlorine
residuals reaching the BACCs that might have affected
the biological activity and/or the short empty-bed contact
time of 7.6 ± 1.6 min, which may have been insufficient
for effective biopolymer removal.
Evaluation of membrane performance at both plants. Comparing periods when ozonation was on and off, it can be
clearly seen that the biopolymer retention by the UF
membranes was higher when ozonation was off (Figure
1, part A). At the highest biopolymer concentration in the
UF membrane influent, the difference was 23%.
Although the investigation was not able to partition
retention between biopolymer levels remaining on the
membrane and those in the concentrate, it is reasonable
to infer that higher retention as defined herein led to
greater deposition of biopolymers on the membrane.
Higher foulant retention by UF membranes may affect
membrane performance. Hallé et al (2009) demonstrated
that hydraulically reversible fouling was related to membrane-influent biopolymer concentration. In the present
study, it is reasonable to infer that ozonation altered the
structure of at least some biopolymers, facilitating their
passage through the membrane. UF membranes at Lorne
Park, operated without any pretreatment, retained on
average 220 ± 49 µg/L of the biopolymers, an amount
similar to the retention at Lakeview when ozonation was
off (232 ± 60 µg/L) (Figure 1, part B).
treating the same source water but differing in the presence or absence of pretreatment. The pretreatment consisted of ozonation followed by BACCs. For operational
reasons, ozonation was off during part of the study. The
use of LC-OCD allowed quantitation of the TOC’s biopolymer fraction, which has been previously shown to be
important for UF membrane fouling.
Ozonation prior to the BACCs resulted in lower biopolymer retention by downstream UF membranes of up
to 23% than when ozonation was not operating. Ozonation before biofiltration may therefore reduce downstream membrane fouling.
Biopolymer retention by UF membranes at Lorne Park
without pretreatment was similar to that observed at
Lakeview when ozonation was off.
There was no correspondence between AOC and biopolymer removal in the BACCs, despite the fact that the
AOC concentration was reduced by almost 50%. This
suggests that AOC removal cannot be used as a surrogate
to predict biopolymer removal through biofiltration.
FOOTNOTE
1ZeeWeed
1000®, GE Process Technologies, Oakville, Ont., Canada
REFERENCE
Hallé, C.; Huck, P.M.; Peldszus, S.; Haberkamp, J.; & Jekel, M., 2009.
Assessing the Performance of Biological Filtration as Pretreatment
to Low Pressure Membranes for Drinking Water. Environmental
Science & Technology, 43:10:3878. http://dx.doi.10.1021/es803615g.
Corresponding author: Barbara Siembida-Lösch is a
postdoctoral fellow at the NSERC Chair in Water
Treatment, Department of Civil & Environmental
Engineering, University of Waterloo, Waterloo, ON,
N2L 3G1, Canada; bksiembi@uwaterloo.ca.
CONCLUSIONS
The following conclusions can be drawn from this fullscale study conducted over a range of temperatures at
two adjacent ultrafiltration membrane drinking WTPs
FIGURE 1 Biopolymer retention* by UF membranes versus biopolymer concentration in the influent to UF membranes
for (A) Lakeview† and (B) both WTPs‡
B
Ozone on
Ozone off
400
350
y = 0.7137x + 5.5995
R2 = 0.9823
300
250
200
150
y = 0.4599x + 40.521
R2 = 0.8109
100
50
0
0
100
200
300
400
500
Biopolymer Retention—µg/L
Biopolymer Retention—µg/L
A
600
Lakeview WTP
Lorne Park WTP
400
350
300
y = 0.7137x + 5.5995
R2 = 0.9823
250
200
y = 0.6286x + 16.698
R2 = 0.9451
150
100
50
0
0
100
Biopolymers—µg/L
UF—ultrafiltration, WTP—water treatment plant
*Difference between influent and effluent concentrations
†Ozone on and off
‡No ozone
11 6
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200
300
400
Biopolymers—µg/L
500
600