Sep 10, 2001 - Abstract Slow sand filtration experiments at pilot scale were conducted to determine removal of microorganisms. Using bacteriophage MS2 as ...
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Removal of microorganisms by slow sand filtration
Y. J. DUllemont1, J. F. Schijven 2 , W.A.M. Hijnen 3 , M. Colin1, A. Magic-Knezev4, W.A. Oorthuizen 5 1Amsterdam Water Supply, Amsterdam, The Netherlands 2Nationallnstitute of Public Health and the Environment, Bitlhoven, The Netherlands 3Kiwa Research, Nieuwegein, The Netherlands 4Het Waterlaboratorium, Haarlem, The Netherlands 5Dune Water Company South Holland, Voorburg, The Netherlands.
Abstract Slow sand filtration experiments at pilot scale were conducted to determine removal of microorganisms. Using bacteriophage MS2 as a model virus for enteroviruses, it was found that viruses were the most critical microorganisms for the removal efficiency of slow sand filtration because they were removed the least (1.7-2.2log lO ) and were affected the least by the presence of the Schmutzdecke at low temperatures « 1O°C). Removal of E. coli was about 2 log10 higher, probably due to straining in the Schmutzdecke. Removal of viruses and bacteria by slow sand filtration did not depend on the seeding concentration. E. coli was found to be a useful model bacterium for campylobacters that were removed at least as well or about one log10 more. At higher temperatures (15-20 0q, removal of microorganisms by slow sand filtration, including viruses, increased by about two log10. It was shown that a Schmutzdecke that was scraped off was restored within 53 days. Removal of oocysts of Cryptosporidium parvum that were seeded for 100 days was highly efficient (5.3 log10). Only a few percent of the retained oocysts could be recovered, implying degradation and/or predation of the oocysts. Removal of clostridium spores and centric diatoms was much less efficient than of the oocysts. Furthermore, it was shown that clostridium spores persisted very well in the slow sand filter. Given this difference in removal and persistence, it was concluded that spores and diatoms are not useful model organisms for oocysts. Keywords Slow sand filtration, Schmutzdecke, removal microorganisms, Campylobaeter Iari, E. coli, Cryptosporidium oocysts, MS2 bacteriophage, Clostridium perfringens, Stephanodiseus hantzehii.
Introduction The Dutch Drinking Water Act (2001) requires that concentrations of (entero)viruses, oocysts of Cryptosporidium and cysts of Giardia in drinking water are so low that an annual infection risk of 10-4 per person is not exceeded. To demonstrate compliance, drinking water companies need to conduct a quantitative microbiological risk assessment. As part ofthis © IWA Publishing 2006. Recent Progress in Slow Sand and Alternative Biofiltration Processes edited by Rolf Gimbel, Nigel J.D. Graham and M. Robin Collins. ISBN: 9781843391203. Published by IWA Publishing, London, UK.
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Removal of microorganisms by slow sand filtration
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risk assessment, removal of these pathogens or appropriate indicator or model organisms by drinking water treatment needs to be known. Slow sand filtration is a drinking water treatment step applied by the drinking water companies ofAmsterdam and the Hague. Sofar, monitoring ofthe removal offaecal indicator bacteria and clostridium spores did not provide sufficient information on the removal ofthese pathogens. The current study aimed to determine removal of viruses, bacterial spores, bacteria, oocysts of Cryptosporidium and centric diatoms by slow sand filtration. This encompasses a wide spectrum ofmicroorganism sizes (26 nm - 7 ~m). Bacteriophage MS2 was studied as a model virus for enteroviruses. E. coli was evaluated as model bacterium for campylobacter. Clostridium spores and small centric diatoms (Stephanodiscus hantzchii), were studied as model organisms for oocysts of Cryptosporidium because of their high persistence in the aqueous environment. Because of their persistence, oocysts, spores and diatoms may accumulate in a sand filter over time, which may confound the interpretation of their actual removal. Special emphasis was given to the role of the so-called Schmutzdecke or filter cake on top of slow sand filters. Periodically, the Schmutzdecke is scraped off to prevent clogging of the filter, but this may adversely affect the performance of the slow sand filter. Therefore, the contribution of the Schmutzdecke to the removal of microorganisms and the time to restore after scraping were investigated at a temperature range from 8 to 19°C. In addition, an effect of concentrations of microorganisms in the influent was studied because peak concentrations can be of overriding importance for risk analysis. When passing a slow sand filter, microorganisms are removed from the water by inactivation (degradation and/or predation), physical straining and attachment to the sand grains. Attached microorganisms may also detach and then pass the sand filter. Breakthrough curves were constructed in order to provide insight into these removal processes. Modelling of these breakthrough curves will be conducted for quantifying these processes, but is not the scope of the current paper (Schijven et ai., in preparation).
Materials and methods Microorganisms Bacteriophage MS2, as a conservative model virus. Due to its negative charge it attaches to sand as much as or less than other viruses. - Escherichia coli WRI, as model bacterium. Due to its negative charge it attaches poorly like MS2. Campylobacter lari, as model for pathogenic Campylobacter. Spores of Clostrium perjringens D I0 as model for Cryptosporidium parvum Centric diatoms Stephanodiscus hantzschii as model for Cryptosporidium parvum Oocysts of Cryptosporidium parvum Microorganisms were enumerated as described in Hijnen et al. (2004), Hijnen and Schijven (2003) and Schijven (2000).
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Recent Progress in Slow Sand and Alternative Biofiltration Processes
ff";J
Table 1 Slow sand filtration experiments
Exp.
Filter
Start date of experiment
Age of Schmutzdecke (days)
A B C D E F
F1 F1 F2 F2 F3 F1 F4 F4 F4 F1
Sep 10,2001 Feb 04, 2002 Feb 04,2002 Feb 18, 2002 Jan 17, 2005 Jan 31, 2005 Mar 21,2005 Apr4,2005 May 23,2005 Feb 15,2005
553 12 81 4 56 1105 137 4 53 1121
G H I J
I
Average grain size (mm) 0.27 0.27 0.27 0.27 0.53 0.27 0.53 0.53 0.53 0.27
Microorganisms 4 10 10 10 7.0 9.9 13 14 16 8.2-19
MS2 MS2, WR1 MS2, WR1 MS2, WR1 MS2, WR1, C. fari WR1, C. fari MS2, WR1 MS2, WR1 MS2, WR1 Oocysts , D10, diatoms
MS2 = bacteriophage MS2; WR1 = Escherichia coli WR1; C.fari = Campy/obacter fari; D1 0 = spores of Clostridium perfringens; occysts = oocysts of Cryptosporidium parvum; diatoms = Stephanodiscus hantzschii F1/F3: slow sand filters after rapid sand filtration, ozonation and active carbon filtration F2/F4: slow sand filters after rapid sand filtration.
Slow sand filters (pilot plant) Series of experiments were conducted in 2002 and in 2005 at pilot plant scale. Slow sand filters with a filter depth ofapproximate1y 1.5 m and a surface areaof2.56 m 2 were operated in parallel G~d under the same conditions as the plant scale slow sand filters. The flow rate was 0.3 mifhour. Table I lists the experiments. In experiments A-D, MS2 and WRI were seeded for 24 hours onto the slow sand filters at a low concentration (10 4 _10 5 II), followed by 24-hours seeding at a 1000 times higher concentration (10 7-10 8/1). In experiments E-I, microorganisms were seeded for 24 hours only at the high concentration. The effect of the presence of the Schmutzdecke was studied by comparing experiment A and B, and C and D, respectively. Filter I and 2 differed in the influent that was applied to these filters. The influent of Filter I was ozonated/active carbon filtrated, but that of Filter 2 not. Therefore, the Schmutzdecke of Filter 2 needed to be scraped off more frequently than that of Filter I. Experiments G, H and I were conducted in order to investigate the time that is needed for a Schmutzdecke to restore. Breakthrough of the microorganisms was monitored for about one week in each experiment. In experiment J, oocysts of C. parvum (260 11),spores of C.perfringens DIO (10 3 /1) and centric diatoms, Stephanodiscus hantzschii, (260 II) were seeded simultaneously onto Filter I during a period of approximately 100 days. Breakthrough of these microorganisms was monitored for a period 250 days.
Results and discussion Figure la-b shows the breakthrough curves ofMS2 and WRI from experiments B, C and D. Maximum breakthrough concentrations are determined by inactivation, attachment to the sand grains and straining in the pores in the sand. After seeding of the microorganisms was
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Removal of microorganisms by slow sand filtration (a) bacteriophage MS2
100000
-0-
influent B
-0-
influent C, D
--- effluent B -+- effluent C, D
1000
C (N/ml) 10
0.1
"
:
0.001 +---r--r----,,----.--,----r-.,---,---r--r----,,----.o 2 4 6 8 10 12 14 16 18 20 22 24 Time (days)
(b) E. coli WR1
100000
1000 C (N/ml)
10
0.1
0.001 -te--.---.------,----,---r--,--.,---,--.---.------,----,o 2 4 6 8 10 12 14 16 18 20 22 24 Time (days)
Figure 1 Breakthrough curves of experiments B, C (left) and D (right).
stopped, tailing of the breakthrough curves was observed, due to slow detachment of previously attached microorganisms. Removal ofthe microorganisms was calculated from the seeding concentrations Co and the maximum breakthrough concentrations Cmax (Table 2). Removal of bacteriophage MS2 in experiments B, C and D was very similar, indicating little, if any, effect of the presence of a Schmutzdecke, whereas removal of E. coli WRI was 2 10glO higher when the Schmutzdecke was present. Because E. coli is much larger (about 1.2 J.!m) than bacteriophage MS2 (26 nm), this indicates straining of E. coli by the Schmutzdecke but not ofMS2. In experiment A, removal ofMS2 was higher. In that case, temperature was higher, and, possibly, MS2 may have been removed more efficiently due to a more biologically active sand filter (predation).
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Recent Progress in Slow Sand and Alternative Biofiltration Processes Table 2 Removallo91Q (Co/Cmax ) WR1
MS2
>2.2 1.7 1.8 1.7
3.5 1.8 2.2 1.9
2.1 3.9 2.0
MS2
WR1
c.
0.57
3.9 4.8
3.4 2.8 3.9
2.5 4.8 4.9 3.1 5.6
Oocysts
D10
Diatoms
5.3
3.8
1.6
A
C D
E F G H I
J -
Co
Low
Low
B
Co
High Co
Experiment
High
Co
2.3 4.2 2.8
far;
= not done.
Regression analysis of removal of bacteriophage MS2 in experiments B, C and D, and of E. coli WRl removal in experiments Band D as a function of seeding concentrations showed that removal did not change significantly with the seeding concentration. The very low removal of bacteriophage MS2 in experiment E possibly reflects an operational failure of the slow sand filtration (Table 2). In the coagulation/sedimentation step prior to the sand filtration, accidentally too much caustic soda was added that temporarily may have increased pH of the sand filter, whereby MS2 was removed less, because it attaches less at high pH. Removal of E. coli WR1 in experiment E was similar to that of filters where the Schmutzdecke was scraped off (experiments B, D and H). This suggests that in experiment E, the Schmutzdecke was not effective. Note that removal of E. coli WRl in experiment F was much more efficient, even higher than in experiment B (same filter). The Schmutzdecke of this filter was not scraped off since then. Experiments E and F showed the same or more removal of CampyLobacter Lari compared to the removal of E. coli. This is supported by the study of Hijnen et aJ. (2004), where about one IOglO more removal was reported of environmental campylobacters than of environmental E. coLi at full plant scale. Note that removal ofMS2 in experiment G was as high as in experiment A, demonstrating that an effectively working Schmutzdecke appeared to affect removal ofMS2 too. After scraping of the Schmutzdecke (experiment H), removal ofWRI is again about 2 log 10 less efficient, as was demonstrated before in experiments B, C and D. This time also the removal of MS2 is reduced by 0.6 loglO. Dizer et aZ. (2004) also found a relatively small enhanced removal of bacteriophages in the presence of a Schmutzdecke. After 53 days (experiment I), the removal efficiency of Filter 4 was fully restored, i.e. removal was even higher than in experiment G, probably because operational temperature was higher as well. Figure 2 shows the breakthrough curves ofMS2 and WRl from experiments G, H and I. The shape of these curves differs from those of experiments B, C and D (Figure l). Because
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Removal of microorganisms by slow sand filtration 100000 ~~~;;;;::::::9~L
1000
f
CIN/mll 10
-0- influent G
(a) bacteriophage MS2
=::::---......
-0-
influent H
-ft-
influent I
...
..... effluent G effluent H
~~rnl
0.1
0.001 0
4
3
2 Time (days)
100000
(b) E. coli WR1
1000
C (N/ml)
10
0.1
0.001 0
2
3
4
Time (days)
Figure 2 Breakthrough curves of experiments G, H and I.
more microorganisms were retained in experiments G, H and I than in B, C and D, the maximum breakthrough concentrations are relatively lower and approximately at the same level as the tails of the breakthrough curves. In experiment J, high removal of oocysts (5.3 lOglO) and spores (3.8 lOglO) but relatively low removal of centric diatoms (1.6 lOglO) was found (Table 2 and Figure 3). In total 20 oocysts were observed in a total sample volume of 36000 litre of effluent. In total 336 spores were observed in a total sample volume of 2200 litre and 218 diatoms were counted in a total sample volume of 29 litre. The low removal of centric diatoms was most likely due to environmental centric diatoms of the same cell size as the seeded diatoms (4-7 !-Lm) that were present in the influent (median concentration 76 per litre; n = 22). Concentrations of environmental oocysts and spores of sulphite-reducing clostridia in the influent were below the detection limit.
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Recent Progress in Slow Sand and Alternative Biofiltration Processes 10000 1000
""-C.parvum
o
