臭氧应用英文文章20

Water Research 36(2002)4053–4063
Inactivation of Cryptosporidium parvum oocysts with ozone
and free chlorine
Benito Corona-Vasquez,Amy Samuelson,Jason L.Rennecker,Benito J.Mari *nas*
Department of Civil and Environmental Engineering,University of Illinois at Urbana-Champaign,205North Mathews Avenue,
Urbana,IL 61801,USA
Abstract
The objective of this study is to investigate the synergy involved in the sequential inactivation of C.parvum oocysts with ozone followed by free chlorine at 1–201C.Primary ozone and free chlorine inactivation curves are characterized by an initial lag-phase,followed by one or two post-lag-phase segments,the first segment at a faster rate than the second,of pseudo-first-order inactivation.The kinetics of primary inactivation with ozone and free chlorine has a relatively strong temperature dependence,and vary both with oocyst lot and oocyst age.Synergy is observed for the sequential inactivation of C.parvum oocysts with ozone/free chlorine.Ozone pre-treatment results in the disappearance of the lag-phase and the occurrence of a secondary free chlorine inactivation curve with generally two pseudo-first-order segments,the first segment at a faster rate than the second.The kinetics of both secondary segments is significantly faster than the post-lag-phase rate of inactivation with free chlorine alone.The temperature dependence for both phases of the secondary free chlorine inactivation kinetics is weaker compared to that for primary inactivation with ozone or free chlorine.As a result,the level of synergy in sequential disinfection with ozone/free chlorine increases with decreasing temperature within the range relevant to drinking water utilities.Good agreement is found between the kinetics determined using the modified in-vitro excystation method of viability assessment and animal infectivity data recently reported in the literature for both primary inactivation with ozone,and sequential disinfection with ozone/free chlorine.r 2002Elsevier Science Ltd.All rights reserved.
Keywords:Cryptosporidium parvum ;Free chlorine;Inactivation kinetics;Inactivation kinetics variability;Ozone;Sequential disinfection;Synergy;Temperature effect
1.Introduction
The control of Cryptosporidium parvum (C.parvum )oocysts remains a major challenge for drinking water utilities due to the fact that the common chemical disinfectants free and combined chlorine,when used singly,are practically ineffective for inactivating this protozoan under the conditions encountered in most treatment facilities [1–6].In contrast,ozone and chlorine dioxide have been shown to be more effective than free chlorine and monochloramine in inactivating C.parvum oocysts [1–3,5–11].However,CT (i.e.,product of
disinfectant concentration and contact time)require-ments for these stronger disinfectants are considered to
be exceedingly high at relatively low temperatures due to a strong temperature effect on the inactivation kinetics.For example,the CT required to achieve a C.parvum inactivation efficiency of approximately 99percent with ozone at 11C has been reported to be approximately 40mg min/L,or approximately 12times greater than that required at 201C [3,8].
Many drinking water systems using ozone as primary disinfectant also apply free or combined chlorine to provide protection in the distribution system.Surpris-ingly,the inactivation kinetics of C.parvum oocysts with free chlorine and monochloramine after ozone pre-treatment have been shown to be significantly faster compared to the kinetics with free chlorine and
*Corresponding author.Tel.:+1-217-333-6961;fax:+1-217-333-6968.
E-mail address:marinas@ (B.J.Mari *nas).0043-1354/02/$-see front matter r 2002Elsevier Science Ltd.All rights reserved.PII:S 0043-1354(02)00092-1
monochloramine alone[2,3,5,6,12].Li et al.[12] reported the occurrence of synergy for the sequential inactivation of C.parvum oocysts with ozone/free chlorine at pH6,and temperatures of11C and221C with oocyst viability determined by animal infectivity.A relatively greater level of synergy was reported at the lower temperature investigated but the actual reduction in CT was not quantified.Rennecker et al.[5] characterized the synergistic effects for both ozone/free chlorine and ozone/monochloramine at10–301C,and also found that the synergy was more pronounced at lower temperatures within this range.Determination of oocyst viability in split samples by in-vitro excystation and animal infectivity for selected ozone/monochlor-amine tests revealed consistency between the two viability methods.Driedger et al.[3]investigated the synergy of sequential disinfection with ozone/mono-chloramine at temperatures in the range of1–201C,and reported that the CT required for99percent inactiva-tion with monochloramine at11C could be reduced from approximately70,000mg min/L,when using this disin-fectant alone,to2000mg min/L,when using mono-chloramine after ozone pre-treatment at a CT of 22.5mg min/L.
The objective of this study was to investigate the synergy involved in the sequential inactivation of C. parvum oocysts with ozone followed by free chlorine at temperatures in the range of1–201C.Additional goals were to investigate the variability in inactivation kinetics among different lots of oocysts,and to assess the consistency between the in-vitro excystation and animal infectivity methods used to quantify oocyst viability.
2.Materials and methods
2.1.Oocyst preparation
Disinfection experiments were performed with three different lots of Iowa strain C.parvum oocysts propagated in bovine hosts at the University of Arizona. Each oocyst lot was shipped to the University of Illinois in a1.5-mL microcentrifuge tube containing1.0mL of an antibiotic solution consisting of0.01%(v/v)Tween 20s solution,100U penicillin and100m g of gentamicin. Upon receipt,the oocyst suspension was transferred to a 15-mL centrifuge tube and centrifuged at1120Âg for10min.After aspiration of the supernatant,the oocyst pellet was re-suspended in0.01M phosphate buffer solution(PBS),pH7.0,and stored at41C until used.The times elapsed from oocyst shedding to shipment,cleaning,and testing,for each disinfection experiment performed in this study,are presented in Table1.2.2.Experimental matrix
All disinfection experiments were performed in0.01M PBS.Experiments designed to characterize the inactiva-tion kinetics of C.parvum oocysts using ozone alone (Tests O-1À9,Table1)were performed at pH7.0and temperatures in the range of1–201C.The concentration of dissolved ozone ranged from0.45to 4.83mg/L. Primary disinfection with ozone in sequential disinfec-tion experiments was also performed at pH7.0and at the same temperature as that used for the secondary treatment step.Ozone concentration in sequential disinfection experiments ranged from0.45to3.78mg/L. Single-step disinfection experiments with free chlorine (Tests C-1À7,Table1)were run at pH 6.0and temperatures in the range of1–201C.The pH of6.0 was selected to ensure that most of the free chlorine was in the form of hypochlorous acid,the species responsible for inactivation of C.parvum oocysts in the pH range of 6.0–8.5[2].The initial concentration of free chlorine ranged from 4.96to8.48mg/L as Cl2.Secondary disinfection with free chlorine(Tests OC-1À16,Table 1),after ozone pre-treatment,was also performed at pH 6.0.The initial concentration of free chlorine in these experiments ranged from3.50to8.17mg/L as Cl2.
2.3.Experimental procedures
Ozone disinfection experiments were performed with a semi-batch reactor apparatus.The kinetics of primary and secondary inactivation with free chlorine was investigated with a batch reactor system.Experimental components used and methods followed were described previously[2,5,10].Each test was designed to obtain an inactivation curve with at least8data points,plus a control sample and a blank.A separate semi-batch reactor run was done to obtain each data point of an ozone inactivation curve,while a single batch reactor run with serial sampling was generally used to obtain all data points for a primary or secondary inactivation curve with free chlorine.
2.4.Viability assessment
Oocyst viability of control and disinfected samples was assessed by the modified in-vitro excystation method[10]for all experimental sets presented in Table 1.The various steps comprising the method have been described previously[10,11].The modified in-vitro excystation method allowed the determination of inactivation efficiencies up to99.5percent with coeffi-cients of variation within50percent.Experimental results obtained with the modified in-vitro excystation method have been shown to be generally consistent with infectivity data for primary disinfection with ozone [5,10]and chlorine dioxide[11,13],and secondary
B.Corona-Vasquez et al./Water Research36(2002)4053–4063 4054
disinfection with monochloramine after ozone pre-treatment[5].
3.Results and discussion
3.1.Single-step disinfection with ozone
The inactivation kinetics of C.parvum oocysts(Lot A) with ozone at pH7.0and1–201C(Tests O-1À4,Table1) are presented in Fig.1.Similar to the observation by Rennecker et al.[6]at201C for oocysts also from Lot A, all four curves were characterized by a lag-phase and two segments of pseudo-first-order kinetics.The length of the lag-phase increased and the post-shoulder rate of inactivation for each segment decreased with decreasing temperature within the range investigated.The following expression can be used to represent this type of inactivation curve[6]:
N
N0
¼
N
N0
c
if CT p CT lag¼
1
k1
ln
N1
N0
N
N
c
N1
N0
expðÀk1CTÞif CT lag p CT p CT12
N2
expðÀk2CTÞif CT X CT12¼
1
21
ln
N2
1
8
>>>
>>>
><
>>>
>>>
>:
ð1Þ
Table1
Summary of single-step and sequential disinfection tests and experimental conditions
Test Lot Time(days)from oocyst
shedding to T
(1C)
Control Ozone(pH7.0)Free chlorine(pH6.0)
Shipping Cleaning Testing S c/E O c(N/N0)c C(mg/L)t(min)N/N0C(mg/L)t(min)
O-1A29314220 3.410.7260.798 3.75–5.00———
4320 3.490.7220.454 1.10–11.00———
O-2A29314410 2.340.899 1.41 1.41–11.32———
O-3A2931455 2.710.775 3.33 1.00–7.92———
O-4A2931431 2.690.744 4.83 1.13–9.12———
O-5A293315320 2.830.6920.7580.65–4.62———
O-6B15161620 2.370.4650.5390.93–11.13———
O-7B1516231 2.450.746 2.270 2.50–19.96———
O-8C15222220 3.490.7120.497 1.00–12.1———
O-9C154010320 2.970.6890.485 1.03–12.4———
C-1A29314820 2.570.792———8.16–8.1661–366
C-2A29314810 3.730.734———8.20–8.08120–480
4910 3.730.734———8.08–8.08600–960
C-3A2931495 2.500.746——— 4.96–4.95480–3360 C-4A2931491 2.800.664———8.47–8.32480–3360 C-5B15161920 1.880.536———8.10–8.0660–478
C-6B1516321 2.290.514———7.67–7.58660–2640
1 2.290.514———7.70–7.583300–5280 C-7C15406720 2.400.781———7.80–7.7537–296
7520 2.680.710———8.48–8.48321–472 OC-1A29315520 3.430.7190.633 3.000.07618.12–7.825–20
OC-2A29315620 2.780.7590.624 2.720.196 3.50–3.474–32
OC-3A29317320 2.570.767 1.310 1.220.277 4.06–4.065–20
7320 2.570.767 1.310 1.220.2778.16–8.1612–60
OC-4A29315910 2.550.776 2.140 3.980.0880 4.92–4.864–32
OC-5A293314410 3.490.751 1.720 3.200.3488.00–7.9615–165 OC-6A2931585 2.530.727 2.320 6.450.144 4.98–4.904–24
OC-7A2931785 2.600.6950.95414.670.1678.17–8.172–60
OC-8A29331505 2.970.673 1.3409.310.1207.96–7.908–60
OC-9A2931521 2.960.761 3.6307.980.08768.06–8.066–30
OC-10A2931541 3.620.724 3.7807.280.1308.04–8.044–30
OC-11A2931751 2.810.714 1.49017.750.1318.14–8.106–42
OC-12A2933971 2.910.709 2.03011.320.2977.94–7.908–125
OC-13B15162120 1.810.6380.5279.860.06648.02–8.0010–80
OC-14B1516271 1.990.760 2.4707.280.2177.76–7.746–48
OC-15C15405220 3.780.6600.450 3.120.1937.81–7.6612–92
OC-16C15401094 2.970.6540.84514.20.2127.84–7.84104–801
B.Corona-Vasquez et al./Water Research36(2002)4053–40634055
where k 1and k 2are the rate constants or slopes of the faster and slower pseudo-first-order lines in L/(mg min),N 1=N 0and N 2=N 0are the respective intercepts with the ordinate axis resulting from extrapolation of the faster and slower pseudo-first-order lines,CT lag is the lag-phase CT in mg min/L,CT 12is the CT value corre-sponding to the intercept of the two pseudo first-order lines in mg min/L,and ðN =N 0Þc is the viability of the control given in Table 1.Because of the limited number of data points in each segment of the 201C curve,it was decided to use the fitting parameters obtained by Rennecker et al.[6]when fitting a more comprehensive data set at 201C with Eq.(1):k 1¼2:1070:17L/(mg min),k 2¼0:79770:049L/(mg min),N 1=N 0¼8:7473:14;and N 2=N 0¼0:34270:064:Next,the rate constants at lower temperatures were obtained from the 201C values using the expression:
k i ;T ¼k i ;201C exp E a R 1293À1
T ð2Þ
in which E a is the activation energy in J/mol,R is the ideal gas constant (8.314J/mol K),T is absolute temperature in K ;and subscript i can be 1or 2.The activation energy value E a ¼81;200J/mol reported by Rennecker et al.[10]for ozone inactivation in the temperature range of 4–301C was used to determine k 1and k 2values at 1,5and 101C.The data sets in Fig.1were then fitted with Eqs.(1)and (2).The resulting N 1=N 0values are presented in Table 2.As depicted in Fig.1,the temperature effect on the inactivation kinetics of Lot A oocysts was represented well with the activation energy by Rennecker et al.[10].
Previous studies [5,6]have shown that oocysts from different lots,or even from the same lot but of different age,can have different resistance to ozone inactivation.Experimental results obtained with C.parvum oocysts
from Lot A (Test O-5)that were 108–111days older than those from the same lot used for the temperature experiments described in the previous paragraph (Tests O-1–4)are shown in Fig.2together with the data for Test O-1.As depicted in the figure,there was good agreement between the curves for Tests O-1and O-5,and both were represented well by the predicted line corresponding to data reported by Rennecker et al.[6]for oocysts from the same lot that were 58–90days old.The inactivation curve previously reported by Driedger et al.[2]for oocysts from a different lot (Lot C,Test O-9in Table 1)is also shown in Fig.2.These authors interpreted the shape of the curve as the result of experimental variability.However,as depicted in Fig.2,the data were fitted with Eq.(1)using the same rate constants that Rennecker et al.[6]obtained for Lot A oocysts,i.e.,k 1¼2:1070:17L/(mg min),and k 2¼0:79770:049L/(mg min).The resulting N 1=N 0values are presented in Table 2.As shown in Fig.2,even if the curves for Lots A and C were different,the slopes of the two first-order segments were consistent.Also shown in Fig.2is an earlier inactivation curve obtained with Lot C oocysts (Test O-8).In contrast with the observation for Lot A,the resistance of Lot C oocysts to ozone changed significantly with time.Furthermore,the inactivation curve for the younger oocysts from Lot C was characterized by a lag-phase,similar in extent to that for the older Lot C oocysts,followed by what appears to be a single segment of pseudo-first-order kinetics within the CT range investi-gated.These results are consistent with the delayed Chick–Watson model [10]:
N
N 0¼
N N 0 c if CT p CT lag ¼1k ln N 1N 0 N 0N c
;N 1N 0exp ÀkCT ðÞif CT >CT lag ¼1k ln N 1N 0 N 0N c
;8>>><>>>:
ð3Þ
where k is the post-shoulder inactivation rate constant,
N 1=N 0is the intercept with the ordinate axis resulting from extrapolation of the pseudo-first-order line,ðN =N 0Þc is the viability of the control,and CT lag is the lag-phase CT :Fitting the data in Fig.2for the younger Lot C oocysts with Eq.3resulted in the inactivation parameters presented in Table 2.It is interesting to notice that the rate constant k ¼0:76270:096L/(mg min)is within 5percent of k 2¼0:79770:049L/(mg min)for Lot A oocysts and the older oocysts from Lot C.
Results obtained for Test O-6with oocysts from a third lot (Lot B)are also presented in Fig.2.As depicted in the figure,Lot B oocysts were more resistant to ozone than those from Lots A and C.Furthermore,similar to the observation for the younger oocysts from Lot C,the inactivation curve for Lot B oocysts was characterized by a lag-phase,longer than those for all other
oocysts
Fig.1.Effect of temperature on the kinetics of primary inactivation of C.parvum oocysts with ozone at pH 7.0.
B.Corona-Vasquez et al./Water Research 36(2002)4053–4063
4056
investigated,followed by pseudo-first-order kinetics at a different rate within the CT range investigated.Fitting the data in Fig.2for Lot B oocysts with Eq.(3)resulted in the parameters presented in Table2.It is interesting to notice once again that the rate constant k¼0:76270:029L/(mg min)was the same as that obtained for the younger oocysts from Lot C,and within5percent of the k2value for Lot A oocysts and the older oocysts from Lot C.Consistent with the argument made by Rennecker et al.[6],it appears that oocyst lots can include‘‘weak’’oocysts,‘‘strong’’oocysts,or a certain mixture of‘‘weak’’and‘‘strong’’oocysts.The inactivation rate constants for the‘‘weak’’and‘‘strong’’oocysts at201C,including those deter-mined in the present study are k1¼1:6922:10L/ (mg min)[3,6],and k or k2¼0:76221:04L/(mg min) [5,6,10],respectively.
Experimental results corresponding to Tests O-4and O-7at11C with oocysts from Lots A and B,respectively, are shown in Fig.3.It is interesting to observe that the curve for Lot B oocysts did not have a lag-phase. Gradual loss of the lag-phase is an aging effect consistent with the observation for oocysts from Lot C at201C discussed in the preceding paragraph,however, a complete loss of the lag-phase at such early oocyst age was not expected,a reminder of the complex and mostly not understood factors that affect the dynamics of oocyst viability.Nevertheless,it is interesting to observe that the inactivation rate estimated for Lot B oocysts by fitting the corresponding data in Fig.3to Eq.(3)(with CT lag¼0),k¼0:092770:0044L/(mg min)(see Table2) is within17percent of the value k=0.0792L/(mg min) estimated for the second pseudo-first-order segment for Lot A oocysts at11C based on the activation energy
Table2
Inactivation parameters estimated fromfitting the experimental data with Eqs.(1)or(3)
Test Lot Time elapsed(days)from
oocyst shedding to T(1C)First pseudo-first-order
inactivation segment
Second pseudo-first-order
inactivation segment
Shipping Cleaning Testing N1/N0k or k1(L/(mg min))N2/N0k2(L/(mg min)) O-1A293142–43208.7473.14a 2.1070.17a0.34270.064a0.79770.049a
O-2A2931441026.773.30.647b0.48070.0120.246b
O-3A293145528.871.10.348b0.34670.0140.132b
O-4A293143159.376.00.209b 1.1670.090.0792b
O-6B151616207.3871.420.76270.029N/O N/O
O-7B15162310.72470.0830.092770.0044N/O N/O
O-8C15222220 1.2370.510.76270.096N/O N/O
O-9C154010320 2.6570.32 2.1070.17a0.078670.00320.79770.049a
C-1A29314820 2.5671.620.0016970.00030N/O N/O
C-2A293148–4910 1.5870.510.00055170.000069N/O N/O
C-3A2931495 1.2670.190.00028270.000015N/O N/O
C-4A2931491 3.1471.380.00026670.000026N/O N/O
C-5B15161920 1.7670.850.00096070.000171N/O N/O
C-6B15163210.58370.1870.00016070.000020N/O N/O
C-7C154067–7520 1.4970.240.0013470.00006N/O N/O
OC-1A293155200.062270.01360.023470.0029N/O N/O
OC-2A293156200.14870.0180.022470.0019N/O N/O
OC-3A293173200.26070.0230.016270.00240.095870.00580.0053870.00020 OC-4A293159100.077370.01160.017770.0018N/O N/O
OC-5A2933144100.063870.02620.0038470.000800.078670.00320.79770.049
OC-6A29315850.10870.0140.019670.0018N/O N/O
OC-7A29317850.16270.0100.017270.00120.027470.01290.0032570.00147 OC-9A29315210.058470.01320.0099670.00154N/O N/O
OC-10A29315410.095570.02480.013470.0019N/O N/O
OC-11A29317510.098070.02630.012070.0019N/O N/O
OC-12A29339710.28270.0220.0077870.000660.11470.0110.0034270.00015 OC-13B151621200.060670.00710.0047170.00041N/O N/O
OC-14B15162710.20770.0320.0066170.00069N/O N/O
OC-15C154052200.1937N/M0.01427N/M0.038470.00210.0019870.00011 OC-16C154010940.21870.0130.0080870.000470.064370.01110.0024970.00030 N/O:not observed;N/M:not measurable because only two data points available.
a Inactivation parameters at201C reported by Rennecker et al.[6]for oocysts from Lot A.
b k
1and k2values estimated with Eq.(2)from corresponding values at201C and E a¼81;200J/mol[10].
B.Corona-Vasquez et al./Water Research36(2002)4053–40634057
reported by Rennecker et al.[10]for ‘‘strong’’oocysts.It appeared that Lot B oocysts changed by losing the lag-phase in a period of less than a week,but without much change in the pseudo-first-order inactivation rate con-stant.No explanation could be provided for these currently unpredictable and inconsistent dynamic effects in oocyst resistance to disinfectants.3.2.Single-step disinfection with free chlorine
Experimental results corresponding to the inactiva-tion kinetics of Lot A C.parvum oocysts with free
chlorine at pH 6.0and 1–201C (Tests C-1À4)are shown in Fig.4.Each inactivation curve was characterized by an initial lag-phase and,in contrast with the observation with ozone for the same oocyst lot,a single pseudo-first-order inactivation segment.Fitting each data set with Eq.(3)resulted in the curves shown with continuous lines in Fig.4.The corresponding fitting parameters are presented in Table 2.The inactivation rate constants were plotted in Fig.5according to the Arrhenius expression:
k ¼A exp ÀE a
RT
ð4Þ
in which A is the frequency factor in L/(mg min).The parameters resulting from fitting the rate constants with Eq.(4)were A ¼10ð9:48871:725ÞL/(mg min),and E a ¼68;98079310J/mol.The activation energy was within 7percent of the value E a ¼64;65077710J/mol reported by Corona-Vasquez et al.[1]for oocysts from the same lot,and E a ¼71;61077950J/mole reported by Rennecker et al.[5]for oocysts from a different lot,for experiments performed in the temperature range of 4–301C in both studies.
Similar to the ozone inactivation curve comparisons for different oocyst lots in Fig.2,a comparison of free chlorine inactivation curves for oocyst Lots A,B and C,also at 201C,is presented in Fig.6.The parameters resulting from fitting the curves for Lots B and C with Eq.(3)are presented in Table 2.In general,the differences between lots were relatively smaller com-pared to those observed for ozone (see Fig.2).Oocysts from Lot C were about the same or slightly more resistant to free chlorine than Lot A oocysts,an observation consistent with the age of Lot C oocysts (67–75days old)at the time of the free chlorine
test
Fig.3.Kinetics of primary inactivation of C.parvum oocysts with ozone at 11C and pH
7.0.
Fig.4.Effect of temperature on the kinetics of primary inactivation of C.parvum oocysts with free chlorine at pH
6.0.
Fig.2.Kinetics of primary inactivation of C.parvum oocysts with ozone at 201C and pH 7.0.
B.Corona-Vasquez et al./Water Research 36(2002)4053–4063
4058
being between those of the oocysts used in Tests O-8and O-9with ozone (22and 103days old,respectively),which were found to be stronger and weaker than the
more stable oocysts from Lot A.Similar to the observations with ozone,Lot B oocysts were the most resistant.However,the shape of the inactivation curve was erratic suggesting that the loss of the lag-phase for Lot B oocysts might have occurred while performing Test C-5.
Experimental results corresponding to Tests C-4and C-6at 11C with oocysts from Lots A and B,respectively,are shown in Fig.7.Consistent with the observation for ozone in Fig.3,the curve for Lot B oocysts did not have a lag-phase.The inactivation parameters estimated for oocysts from Lot B by fitting the corresponding data in Fig.7to Eq.(3)(with CT lag ¼0)are presented in Table 2.Despite the loss of lag-phase,the inactivation rate constant for Lot B oocysts was approximately 40percent lower than that for Lot A oocysts at the same temperature (see Table 2).In contrast to the observation that oocysts can be classified as ‘‘strong’’,‘‘weak’’or as a mixture of ‘‘strong’’and ‘‘weak’’,there does not appear to be a similar pattern of oocyst resistance to free chlorine.
3.3.Sequential disinfection with ozone/free chlorine Results corresponding to the inactivation kinetics of Lot A C.parvum oocysts with free chlorine (pH 6.0)after ozone pre-treatment (pH 7.0)at 20,10,5and 11C are shown in Figs.8–11,respectively.As depicted in the figures,ozone pre-treatment resulted in the disappear-ance of the lag-phase and the occurrence of secondary free chlorine inactivation curves which generally had two pseudo-first-order segments.An initial segment with relatively faster kinetics was followed by a phase with a slower rate of inactivation.Greater levels of ozone pre-treatment appeared to result in a somewhat longer
initial
Fig.6.Kinetics of primary inactivation of C.parvum oocysts with free chlorine at 201C and pH
6.0.
Fig.7.Kinetics of primary inactivation of C.parvum oocysts with free chlorine at 11C and pH
6.0.
Fig.5.Arrhenius plot of second-order rate constants for primary inactivation with ozone at pH 7.0and free chlorine at pH 6.0,and for secondary inactivation with free chlorine at pH 6.0after ozone pre-treatment at pH 7.0.
B.Corona-Vasquez et al./Water Research 36(2002)4053–40634059
segment of inactivation.For example,N =N 0decreased from 0.277to 0.0594,or approximately two-thirds of a log unit,during the initial secondary inactivation phase for Test OC-3.In contrast,N =N 0decreases correspond-ing to at least 1.10–1.27log units,nearly twice the drop observed for Test OC-3,were observed during the initial phase for oocysts from the same lot receiving greater level of ozone pre-treatment (Tests OC-1and OC-2).The actual extent of the initial segment could not be assessed at the highest ozone pre-treatment levels because the modified in-vitro excystation method used can only measure oocyst survival ratios down to N =N 0¼0:00320:005:The inactivation curves in Figs.8–11were fitted with Eq.(3).The resulting fitting parameters are summarized in Table 2.The rate constants corresponding to the first and second segment are plotted in Fig.5.Fitting of the parameters with Eq.(4)resulted in the linear plots shown in the figure.The corresponding Arrhenius parameters were A ¼10ð1:94871:679ÞL/(mg min),and E a ¼19;98079060J/mole for the initial
phase,and A ¼10ð0:830470:7917Þ
L/(mg min),and E a ¼17;49074270J/mole for the second phase.These activation energies are within 18percent of the value E a ¼21;130J/mole reported by Driedger et al.[3]for the secondary inactivation kinetics of C.parvum
oocysts
Fig.8.Kinetics of secondary inactivation of C.parvum oocysts with free chlorine at 201C and pH 6.0after ozone pre-treatment at 201C and pH
7.0.Fig.9.Kinetics of secondary inactivation of C.parvum oocysts with free chlorine at 101C and pH 6.0after ozone pre-treatment at 101C and pH
7.0.Fig.10.Kinetics of secondary inactivation of C.parvum oocysts with free chlorine at 4or 51C and pH 6.0after ozone pre-treatment at 4or 51C and pH
7.0.
Fig.11.Kinetics of secondary inactivation of C.parvum oocysts with free chlorine at 11C and pH 6.0after ozone pre-treatment at 11C and pH 7.0.
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with monochloramine after ozone pre-treatment at the same temperature range of1–201C.In contrast, the activation energy for both phases of the secondary free chlorine inactivation kinetics is significantly lower than the values observed for primary inactivation with ozone and free chlorine as depicted in Fig.5.This observation,consistent with previous reports for sequential disinfection with ozone/free chlorine[5] and ozone/monochloramine[3,5,12],confirms that the level of synergy in sequential disinfection with ozone/ free chlorine increases with decreasing temperature within the range relevant to most drinking water utilities.
Secondary inactivation curves obtained with oocysts from Lot C at201C and41C are also shown in Figs.8 and10,respectively.As depicted in Fig.8,the rate of inactivation for the second segment of the secon-dary curve at201C was63%slower for oocysts from Lot C oocysts compared to that for Lot A oocysts (see k2values for Tests OC-15and OC-3in Table2). In contrast,the rate of inactivation for the second segment of the secondary curve at41C for oocysts
from Lot C(Fig.10)was only23percent lower compared to that for Lot A oocysts at51C(see k2 values for Tests OC-16and OC-7in Table2),and part of this difference was due to the temperature difference of11C between the two experiments.Nevertheless,it appears that the kinetics of secondary free chlorine inactivation with Lot C oocysts had a tendency to approach that for Lot A oocysts as the former oocysts weakened with time.
The results for Lot B oocysts at201C shown in Fig.8 (Tests OC-13)revealed the occurrence of a secondary inactivation curve with a single pseudo-first-order inactivation segment for a loss of viability of approxi-mately one log.The corresponding rate constant k¼0:0047170:00041L/(mg min)was71percent slower than thefirst segment but within13percent of that for the second segment of Test OC-3even though the same level of ozone pre-treatment was applied for both tests. Interestingly,the rate constant for the test with Lot B oocysts was within22percent of the value k¼0:0038670:00029L/(mg min)reported by Renneck-er et al.[5]for experiments performed at201C with a different lot of oocysts having a secondary free chlorine inactivation characterized by a single pseudo-first-order segment.In contrast,the data for Lot B oocysts at11C shown in Fig.11(Test OC-14)was consistent with the first segment of the secondary inactivation curve observed for Lot A oocysts after the same level of ozone pre-treatment.These changes in relative resistance to secondary inactivation with free chlorine indicated, consistent with the observations for primary inactivation with ozone(see Fig.2)and free chlorine(see Fig.6),that the oocysts from Lot B deteriorated with time faster than those from Lot parison to animal infectivity data
The data reported by Li et al.[8]for the primary inactivation of C.parvum oocysts with ozone at1–371C with viability determined by animal infectivity are compared in Fig.12to the inactivation kinetics(con-tinuous lines)obtained in this study based on the in-vitro excystation method of viability determination.The linear inactivation curves shown in thefigure were obtained by a similar approach to that followed for the curves in Fig.1.The more comprehensive data set at 11C wasfirstfitted with Eq.(1)using the inactivation rate constants k1¼0:209and k2¼0:0792L/(mg min) obtained from those reported by Rennecker et al.[6]at 201C(k1¼2:10and k2¼0:797L/(mg min))after cor-recting for the temperature difference with Eq.(2)using the activation energy value E a¼81;200J/mole reported by Rennecker et al.[10].The resultingfitting parameters were N1=N0¼2:2270:30;and N2=N0¼0:27170:022: Because of the more limited number of data points,the linear inactivation curves at the other three temperatures (51C,131C and371C)were predicted by assuming that the values N1=N0¼2:22;and N2=N0¼0:271applied to all temperatures.Once again the rate constants k1and k2 (0.348and0.132L/(mg min)at51C,0.929and0.353L/ (mg min)at131C,and13.0and4.95L/(mg min)at371C) were obtained from the201C values(2.10and0.797L/ (mg min))reported by Rennecker et al.[6],using Eq.(2) with E a¼81;200J/mole[10].As depicted in Fig.12, despite the inherent variability of the animal infectivity data,and the uncertainty of the N1=N0and N2=N0 values used,there was good general agreement between the animal infectivity data of Li et al.[8],and the in-vitro excystation kinetics of this
study.
parison of C.parvum oocyst inactivation with ozone based on kinetics developed with the modified in-vitro excystation method(this study and[10])of viability assessment and animal infectivity data reported by Li et al.[8].
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