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Relative importance of the phenomena responsible for chlorine decay in drinking water distribution systems
~ Pergamon
Vol. 38. No.6, pp. 219-221. 1998. IAWQ «> 1998 Published by Elsevier Science LId. Printed In Oteat Britain. All righll reserved 0273-1223198 S19'00 + 0'00
Wa,. Sci.
pn: S0273-1223(98)00583-6
T~cla.
RELATIVE IMPORTANCE OF THE
PHENOMENA RESPONSIBLE FOR CHLORINE DECAY IN DRINKING WATER DISTRIBUTION SYSTEMS L. Kiene, W. Lu and Y. Levi Suez Lyonnaise Des Eaux, CIRSEE, 38. rue du President Wilson, 78230 Lt Pecq, France
ABSTRACT Free chlorine consumption in distribution systems is due both to chemical reactions occurring in the bulk phase and at the pipe walls. Knowledge of the relative importance of these various reactions ia needed in order to improve chlorine decay modeling. Experimental results carried out in this study make it possible to propose a hierarchical classification of the main parameters involved in the free chlorine decay observed in distribution systems. Corrosion of metallic pipe appears to be a major parameter. while synthetic materials are of little influence. The rate of chlorine decay in bulk phase can be estimated according to the TOC and the temperature. Influence of biofilms depends on the BDOC content of water. and on the pipe diameter. Chlorine decay due to corrosion phenomena must be modeled according to a zero order kinetics. while chlorine decay due to other parameters can be modeled according to a first order kinetics with respect to chlorine. @ 1998 Published by Elsevier Science Ltd. All rights reserved
KEYWORDS Biofilms; chlorine; corrosion; drinking water; modeling.
INTRODUcnON The free chlorine disappearance in drinking water during distribution is due both to chemical reactions in the aqueous phase (i), and to reactions occurring at the interfaces between pipes or reservoir walls (ii). (i) - Fast reactions with easily oxidizable compounds such as ammonia, ferrous ions... (Wei et al., 1974); these reactions are usually completed during disinfection treatment and should not be observed in the distribution network. - Slower reactions with less oxidizable compounds (organic matter...) (Dotson and Heitz, 1976). (ii) The reactivity of the pipe wall will be a function of pipe diameter, hydrodynamic conditions, nature of
the pipe material, amount and nature of deposits... (Gotho, 1989; Zhang et al., 1992).
The aim of this work is to propose, on the basis of experiments conducted at bench and pilot scales, a hierarchical classification of the main parameters which influence the free chlorine consumption in 219
220
L. KIENE tt aI.
distribution systems: water, materials, fixed biomass, deposits, corrosion of ferrous material. Results of this study have been used in order to improve the chlorine decay modeling, and for this purpose have been integrated into the modeling software PICCOLO. STUDY OF PARAMETERS INFLUENCING CHLORINE DECAY Influence of the water Quality The influence of TOC and temperature on the chlorine decay rate in the aqueous phase has been studied. For this purpose, kinetics of chlorine decay have been established. Experimental results showed a rapid chlorine depletion during a first phase of about I or 2 hours. This phase corresponds to reactions with easily oxidizable compounds and is usually completed during the disinfection step at the treatment plant. Therefore this phase should not be observed within the distribution network. During a second phase, chlorine decayed mOre slowly, and it was always possible to describe this phase with a first order kinetic rate with respect to chlorine: (1)
Then, all the experimental results have been expressed by the value of the first order kinetic constant k'e (min-I), related to the long term chlorine consumption. The study of 21 water samples with temperatures between S and 2SoC, and TOC concentrations between I and 3 mgIJ, has made it possible to establish the following empirical relationship:
k~ = a x [TOC] x exp( ~)
with, a=I,8.106 and b=60S0 and TO : temperature in Kelvin
(2)
Then under our experimental conditions, the kinetic constant k'e varies between 1.10-3 and 9.10- 3 min-I. These results must be taken into account for the modeling of chlorine decay in distribution systems. Values of constants a and b in equation [2], are probably a function of characteristics of organic matter in each location. So, as a result, it becomes necessary to determine the specific values of these constants for each location within the range of validity of TOC and temperature. Influence of synthetic materials The objective of this work was to quantify the influence of synthetic materials on chlorine decay. For this purpose, sections of pipes corresponding to five materials, among the most widely employed in distribution systems, were tested: High Density PolyEthylene (HOPE), Unplasticized PVC (U-PVC), Molecular Oriented PVC (MO-PVC), PolyPropylene (PP), Glass Reinforced Polyester (GRP). Each section of pipe was previously flushed for a few minutes with tap water. Then the pipe was filled with a chlorine solution (initial free chlorine concentration: l±O.2 mgl1; temperature: 18°C; pH 6.6), and thereafter the chlorine residual concentration was measured at regular intervals for about 12 hours. During this period, stirring was maintained in order to ensure a continuous renewal of chlorine solution at the pipe wall. Curve~ of chlorine decay were interpreted according to the assumption of a first order reaction r.ate (equatIon I), and the result (Figure 1) is expressed as the value of the global kinetic constant corresponding to the consumption of chlorine by both the water and the material: (k').
First we noted that for the material studied. k' decreased as a function of the number of rinsing operations. After rinsing, values of k' are in the same order of magnitude for all the tested materials. Chlorine
Phenomena responsible for the chlorine decay
221
consumption during the first contact period could be due to oxidizable compounds initially present at the pipe wall surface, but that are rapidly eliminated by rinsing (Anselme et al., 1992). For the succeeding periods, leachin~ of chemica~ compounds from the studied polymeric matrix does not appear to induce a significant chlonne consumptIOn.
k' (min· t l.8E-3
I.SE-3 1.2E-3
r;::::;c;:;::ls=tn =n=s.=ng=:::;-----r==;-----------, • 2nd nnslOg C 3rd rinslOg •
.
9.0E-4 6.0E-4 3.0E-4 O.OE+O
PEUD
diam.6"
PVC-CM
diam.4"
GRP
dlam.IO"
PP
dlam.4"
V-PVC
dlam.6"
V-PVC
dlam.4"
Figure I. Kinetic constant (k') of chlorine decay for potable waler in conlact with syntheUc pipes.
These results show that after rinsing, the influence of synthetic material on the total consumption of distribution networks must be very small. Therefore this aspect of chlorine decay will not be a key parameter for chlorine decay modeling. Influence of biofilms In order to study the influence of biofilms on chlorine decay, and avoid influence of the substratum, inert material which does not consume chlorine had previously been selected. Finally, 4 millimeter diameter polystyrene beads manufactured by Atochem (Lacqrene) were chosen for supporting the growth of the biofilm.
Colonization of biofilm. In order to colonize the beads with biofilm, they were incubated with three qualities f water produced by a plant using Seine river downstream of Paris. The main characteristics of these waters ~e reported in Table 1. The beads were colonized in open reactors and were continuously fed with the tested waters (temperature: 18°C). Table I. physical-chemical and biological quality of tested waters during the colonization period
sand filtered water ozonated water final water
TOC (mgIL)
(mgIL)
(O.D.lm)
UV
pH
1.69 1.60 1.29
0.40 0.60 0.35
2.6 1.5
7.67 7.72 7.59
BDOC
1.3
IIPC at 22°C HPC at 22°C R2Amedium PCAmedium (CFU/ml) (CFU/ml\ 64370 1499 2603 190 1076 9
Determination of the biofilm chlorine demand. After being sampled, beads were rinsed and immersed in a flask containing a solution of s~ium hypochl~rite. Then, the chlorine disappearance was followed up as a function of time (initial chlonne concen~attons: I or 2 mgll; surface to volume ratio (SN): 150 to 1500 cm2n; pH 6.6±O.2; temperature: 18±1 C). Chlorine decay profiles generally follow a ra~id first phase for 1 or 2 hours followed by a second slower hase. The first phase does not concern ~hlonne decay modeling in distribution systems; it corresponds to ihe accumulation, in the absence of chlonne residual, of easily oxidizable substances during the colonization phase. Therefore this phase would not be observed in a distribution system, where chlorine is continuously
222
L. KIENE et al.
present. The second phase, corresponding to long term chlorine consumption, can be described by a first order kinetic reaction rate with respect to chlorine. Results. Experimental results show that for each tested feed water, the fixed biomass is characterized by a higher kinetic constant (k'b) than non colonized beads. As shown on Figure 2. the kinetic constant increases with the colonization time, but a steady state is observed during the experimental period. Furthermore, the time needed to reach this steady state varies according to the quality of the test water where the biofilms are developed. Biomass colonized in ozonated water has the highest kinetic constant (k'b=5.5xlO- 3 min-I), and the steady state is obtained as from the 16th week. For the biomass colonized in sand filtered water. k'b leveled off around the 12th week. For the biomass colonized in final water, the profile of k'b is stable as from the 61h week. Results show that k'b depends on the quality of the feeding water: BDOC, UV absorbance. bacterial count (HPC). This conclusion is in agreement with previous works (Mathieu et al., 1992; Servais et al., 1995) which show a direct influence of these same parameters on biological growth in distribution systems. The most significant relation between k'b and parameters characterizing the quality of the feed water was observed with the BDOC. This relationship was obtained from the 16th week:
k~ = 7.95 X to- 3 x BDOC + 2.83 x to.....
(3)
r=0.92
From experimental data, it has also been possible to establish an empirical relationship (equation 4) to describe the evolution of the kinetic constant according to the water quality (BOOC) and the surface to volume ratio. Within the studied experimental field, equation 4 allows the prediction of k'b with a precision of 35%.
k~
= [5,3.10- 6 x (BDOC)] x ~
with BOaC in mg/L and SN in cm2/L
(4)
From equation 4, it appears that fixed biomass constitutes one of the parameters that can in some cases influence the chlorine decay in distribution systems. Nevertheless, for a 100 mm diameter pipe, and a BDOC of 0.2 mg/I, the value of k'b remains very small: approximately 4x to-4 min-I. The influence of fixed biom~s would become an important parameter only for very small diameters (smaller than 75 mm) and for high BOaC (greater than 0.6 mgll): k'b greater or equal to 2.10- 3 min-I. k'b.(mln") 7.0E-3
r------------:---r::=====:==Ji -
6.0E-3
i'~oZa",ted Watel
:
::~: ~i~'
2.0E-3
L_~_-€.~'~ =w:::lt::e'=::::'-'_--i [k'• .±.1"~~=~f-------':I=Fi="I=1 Standard aev..!'" _
I.OE.3
O.OE-+{)+-----+-----f------f----+-----l
4
8
12 . 16 Colonization Time (weeki
20
24
Figure 2. Evolution of the kinetic constant (k'b ) for beads colonized with three water qualities from a surface water treatment plant.
Influence of the corrosion Metallic materials in contact with water are generally damaged because they are subject to corrosion, the intensity of which is controlled by the physical-ehemical conditions (e.g., temperature, pH, carbonate
Phenomena responsible for the chlorine decay
223
balance. chloride, biofilm...) (Barbier et al.. 1990). Corrosion of ferrous material can be expressed as the oxidation of metallic iron (FeO) into ferrous iron (Fe2+) which can react with oxidizing compounds such as oxygen or chlorine (Kiene and Levi. 1996). If chlorine is the main oxidant, one can assume that the following reaction will occur:
2Fe2++ HOCI +W -+ 2Fe'+ + Cl" + Hp
(S)
On the other hand, HOCl could also be consumed because it plays a role in Feo oxidation. Nevertheless other results (Frateur, 1997 and Frateur et al., 1998) have shown that the reduction of dissolved oxygen remains the single cathodic process coupled with iron dissolution. Then chlorine decay due to a corrosion phenomenon should be only under the control of the production of felTous ions which are directly linked to the corrosion rate and could be described according to a zero order reaction rate: d rCl2 1 4 x 10-3 M n Cr :J.::::J= x - - x PFe x dt 't MFe D
[CIJ
chlorine concentration in mgIL
t
time in acc
t
3.16xlO' Icc/year atomic weight of a (35.5 g/mol) atomic weight of Fe (55.85 almol) iron density (7,860 kg/ml ) corrosion rate (IIm!year) pipe diameter (m)
Ma
M,. r.. C,
o
(6)
After calculation of the constant terms, equation [6] becomes:
~=KxCr dt
with K=6.32xlO·7
(7)
D
In order to verify these hypotheses, bench scale and pilot scale experiments have been carried out. Slectrochemical bench scale study The corrosion process was simulated by inducing dissolution of ferrous iron under the control of an electric current. the intensitr of which ~as imposed and controlled w~th a ~tentiostat. Under our experimental conditions, ferrous Iron p~oductlon de~nded solely upon the m~nslty of the imposed current, and was independent of other expenmental condItions (e.g., SN, water quality, oxygen and chlorine concentrations). Various current intensities were used between SO and SOO !lA. Therefore, it was possible to verify the linearity of the relationship between corrosion rate and the kinetics of chlorine decay. For all experimental results, a linear decre~ of chlorine was observed as a function of time. This result Suggests that the rat~ o~ .chlorine consumptIon was independent of chlorine residual levels, and that ferrous dissolution was the InDltmg factor. As shown in Figure 3, chlo~ine decay rates (d[CI 2]/dt) increase according to imposed current (i). Furthermore, oxygen concentration does not appear to influence chlorine decay due to a corrosion process. A linear relationship was found between K o and the dissolution current as well in the presence as in the absence of oxygen. The chlo~ne decay is therefore directly proportional to the production flux of Fe 2+, which is equivalent to a corrOSIon rate.
224
L. KffiNE et al.
dlOJl/dt (mg/L.see) 3.50&04r---;:==========::;--------••; - - , 3.00&04 • oxygen: 0 mg/L
•
• QIllson' 44 mslI
2.50E-04 2.ooE.Q4 1.50E.Q4 t.ooE.Q4 5.ooE-05
O.ooE+oo-I------~>--------------~
o
100
200
300
400
500
600
I (pA) Figure 3. Influence of the production flux of Fe2+ (proportional 10 the current intensity) on the chlorine decay rate (initial free chlorine I mgll; oxygen concentration 44 mgll).
Pilot scale study A pilot-scale PropelIer Loop Reactor (PLR) was used in this study (Figure 4) (Blenk, 1978). The internal tube was made of stainless steel, and the studied pipe, made of gray cast iron, constituted the external wall of the PLR. A propeller ensured the recirculation of water inside the pilot. The water corrosivity was monitored using a CORRATER SCAI (Rohrback Cosasco), which measures corrosion rate on the basis of polarization resistance. The free chlorine residual was monitored using a CHLORSCAN® (Seres), which uses amperometric measurement of the hypochloric acid (H0Cl) (Van den Berg et 01., 1993; Wable et 01., 1993). Modulation of water corrosivity was obtained by injection of a sodium chloride solution inside the pilot. During experiments (pH: 6.S; Ryznar index: 8), with chloride concentrations from 50 to 9000 mgll, corrosivity measured with the CORRATER SCAI was in the range 20 to 200 ~.Lrnlyear. Depletion of free chlorine concentration inside the PLR was followed as a function of time (initial free chlorine concentration: I mgll).
Fisure 4. Diagrammatic representation of the Propeller Loop Reactor (PLR) used for the pilot scale study.
225
Phenomena responsible for the chlorine decay
Experimental results show that chlorine decay curves present a linear shape, and can be interpreted according to a zero order reaction rate, limited by the corrosion rate. Then, the kinetic constant (K) was calculated from the graph 'free chlorine vs time' (slope equal to: K.C/D). According to these pilot experiments, the chlorine decay due to the corrosion process could be modeled as folloWS: with K=1.3610" Co and Co: chlorine concentration in mg/L t: Time in seconds C,: water corrosivity in 11m/year
(8)
D: pipe diameter in meters
As shown on Figure 5, the zero order reaction rate constant (K) which characterized the chlorine decay rate was linearly correlated with the water corrosivity. chlorine decay rate (mglsec) 2 . 0 E - Q 3 - r - - - - - - - - - - - - - - - - - - - . y = 1.36806 x - 2.6 e-4 • ~=0.73 1.5E-Q3
•
•
1.0E-Q3 5.1E-Q4 1,OE-QS .J--.....:::::::....-~-.l~-__i--_-_-__I o 200 400 600 800 1000 1200 1400 ratio: corroslvlty/plpe diameter Figure S. Evolution of the chlorine decay rale as a function of the water corrosivity dClennined from the measure of the polarization resistance (CORRATER SCAI).
In conclusion, pilot scale experiments confirm that corrosion dramatically influences chlorine decay, and demonstrate that a zero order reaction rate can be accepted in the case of metallic distribution pipes subject to corrosion. Nevertheless experime?tal numeric values do not co~spond closely to ex~ted values (equation 7): during our study expen~ental values were roughly 2 times greater than theoretical values. Other pathways of chlo~ne ~onsulll:PtlOn mu.st be consi~ered; for instance the reduction of free chlorine could be a cathodic reaction directly Involved In the corrosion process.
SYNTHESIS AND EXPERIMENTAL RESULT DISCUSSION In order to study which parameter consumes most chlorine (water, synthetic material, metallic material, biofilm...), all experimental results have been extr~pol~ted to an SN ratio of 160 cm21l, corresponding to an internal pipe diameter of 25? mm (Table 2~. The kmetic constant related to deposits was obtained during the study of a specific distributlo~ sy~tem. This value i~ only given as an illustration, because the influence of deposits on chlorine consumption IS probably very different from a distribution system to another.
L. K.lhNh
et al.
Table 2. Contribution of various parameters to the chlorine decay kinetic constant (k' in min-I) of a distribution system water pipe materials deposits
fixed biomass
drinking water from Seine river Treatment plant synthetic materials gray cast iron sampled in a distribution network by opening fire hydrants (contain mainly iron oxides and calcium carbonate) colonization time 23 weeks, TOe 1.3 mg/L, BDOC 0.35 mg/L, T 18°C
4.72 x 10 6.64 x 10" 3 2.73 x 101.12x10
3.00 x 10
Comparison of chlorine demand for each parameter must be made in two cases: synthetic pipes and gray cast iron pipes (Figure 6). For synthetic pipes, most chlorine is consumed by deposits, water and then biomass. But in a cast iron pipe, chlorine is principally consumed by the material, deposit and then the water. Chlorine demand of fixed biomass in this case is negligible. (b) I"Y till I,." pipe:
CI)' nlllfllt'l.IfH':
rhJorhw n .... mptJo. I" 1 Hun 0.50 "'lit..
thlorln~ fo"umpllon I.. Z houn 0.11 milL.
water
---r-';l~_
26%
n\atenal
2% mllenal IM~
37%
Figure 6. Complirison of chlorine consumption in a network made of synthetic (lO-a) and gray cast iron pipes (l0• h); (diameter 250 mm, residence time =2h, Clo=l mg/l).
CONCLUSIONS Experimental results carried out in this study make it possible to propose a hierarchical classification of the main parameters involved in the chlorine deeay observed in distribution systems. • •
•
•
Chlorine consumption by synthetic materials is negligible and does not appear to be an important parameter for the modeling of chlorine decay. In contrast. chlorine consumption of old cast iron pipes whose internal surfaces are not protected by a coating. is a major parameter for chlorine decay modeling. For cast iron or steel pipes, the rate of chlorine consumption can be directly under the dependence of corrosion phenomena. In this case the chlorine disappearance can be described according to a zero order reaction rate. Therefore, for a given pipe section, chlorine decay depends only on the pipe diameter and the corrosion rate. Chlorine decay due to bulk reactions is very variable according to the temperature and organic matter concentration. Variation of the kinetics can be predicted by a simple model taking into account the TOC and the temperature. Chlorine decay due to fixed biomass varies according to the colonization time, but k'b finally reaches a stationary state after a time depending on the water quality. Influence of biomass should become an important parameter, only for small diameter synthetic pipes fed with water characterized by high BDOC.
Phenomena responsible for the chlorine decay
227
From these results, it has been possible to improve chlorine decay modeling in distribution systems. This modeling takes into account the main parameters governing the disappearance of free chlorine: temperature, organic matter concentration in water, and corrosion rate for metallic materials. In addition to these parameters, the model factors in the hydrodynamic conditions that control the transfer of chlorine from the aqueous phase to the pipe walls. This enhanced modeling of free chlorine decay has been integrated into the hydraulic modeling software PICCOLO (Jarrige, 1989) and is currently tested on full-scale distribution systems. REFERENCES Anselme. C.• N'Guyen. K.• Bruchel, A. and Mallevialle. J. (1992). Can polyethylene pipes impart odors in drinking water? Environm Technol Len.• 6. 477-488. Barbier. J. P.• Cordonnier. J. and Gabriel. J. M. (1990). Corrosion ml!tallique et phl!nom~nes d'eaux rouges - utilisation d'inhibiteurs. T.S.M.-L'EAU. 183-188. Blenk. H. (1978). Mixing and dispersing in a loop reactor. Int Symposium on Mixing. Mons. Christman. R. F. and Ghassimi. M. J. (1976). Chemical nature of organic color in water. J.A. W. W.A.• 58. 773. Dotson. D. and Heitz, G. R. (1985). Chlorine decay chemistry in natural walers. In Water Chlorination: Environmental Impact and Health Effects. Jolley. R. L., Brungs. W. A. and Cumming. R. B. (Eds). Ann Arbor Science Publishers Inc.• 5. 713722. Fraleur. I. (1997). Incidence de la corrosion des ml!taux ferreux sur la demande en chlore lib", en rl!seaux de distribution d'eau . potable. Thesis. University of ~aris 6. Fraleur. I.. Deslouis. C.• Kil!nl!. L.• UVI. Y. and Tnbolet. B. (1998). Free chlorine consumption induced by cast iron corrosion in drinking water distribution systems. Wat. Res. (submined). Gotho. K. (1989). Chlorine residual concentration decreasing rate coefficient for various pipe materials. Wat. SuppL, 7(2·3).164172. Jarrlge. P. A. and Bos. M. (1989). Mathematical modelling of water distribution networks under steady-state conditions-Recent development and future projects. Aqua. 38. 352-357. Kil!nl!. L. and Uvi. Y. (1986). Influence des matl!riaux ferreux sur la consommation du chlore en rl!seau de distribution. Proc HYDROTOP 96. Marseille. 143-152. Mathieu. L.• Paquin. 1. L.• Block, J. C.• ~~do~. G.• Maill~ and Reasoner. D. (1992). Param~tres gouvernant la prolifl!ration bact4!rienne dans les rl!seauxde dlstnbution. Rev. SCI. Eou, 5. 91·112. Servais. P.• Laurent, P. and Randon. G. (1995). Comparison of the bacterial dynamics in various French distribution systems. J. WaterSRT.Aqua. 44(1).10-17. Van den Berg. A.• Grisel. A.• Verney·Norberg. E.• Van der School, B. H.• Koudelka·Hep. M. and de Rooij. N. F. (1993). On·wafer fabricated free-chlorine sensor with ppb detection limit for drinking monitoring. Sensors and Actuators 8ul.• 13·14. 396399. Wable. 0 .• Levi. Y.• Monti~l. ,,:,.• ~.hen~ult, M. and Veyrie. A. (1993). Miniature c~lorine microsensor for on-line control of drinking water quality 10 dlslJibution system. Proc AWWA WQTC, 209·216. Miami. Wei. I. W. T. and Morris: J. C. (1974). Dynamics o~ break.po~nt chlorination. In Chemistry of Water Supply, Treatment and Distribution. RubIO. A. J. (Ed.). Ann Arbor SCience Publishers. Inc.• 297·332. Zhang. G.• Kil!nl!. L.• Wable. 0 .• Chan. U. S. and Duguel, J. P. (1992). Modelling of chlorine decay in water distribution network of Macao. Environm. Technol., 13.937-946.
Vol. 38. No.6, pp. 219-221. 1998. IAWQ «> 1998 Published by Elsevier Science LId. Printed In Oteat Britain. All righll reserved 0273-1223198 S19'00 + 0'00
Wa,. Sci.
pn: S0273-1223(98)00583-6
T~cla.
RELATIVE IMPORTANCE OF THE
PHENOMENA RESPONSIBLE FOR CHLORINE DECAY IN DRINKING WATER DISTRIBUTION SYSTEMS L. Kiene, W. Lu and Y. Levi Suez Lyonnaise Des Eaux, CIRSEE, 38. rue du President Wilson, 78230 Lt Pecq, France
ABSTRACT Free chlorine consumption in distribution systems is due both to chemical reactions occurring in the bulk phase and at the pipe walls. Knowledge of the relative importance of these various reactions ia needed in order to improve chlorine decay modeling. Experimental results carried out in this study make it possible to propose a hierarchical classification of the main parameters involved in the free chlorine decay observed in distribution systems. Corrosion of metallic pipe appears to be a major parameter. while synthetic materials are of little influence. The rate of chlorine decay in bulk phase can be estimated according to the TOC and the temperature. Influence of biofilms depends on the BDOC content of water. and on the pipe diameter. Chlorine decay due to corrosion phenomena must be modeled according to a zero order kinetics. while chlorine decay due to other parameters can be modeled according to a first order kinetics with respect to chlorine. @ 1998 Published by Elsevier Science Ltd. All rights reserved
KEYWORDS Biofilms; chlorine; corrosion; drinking water; modeling.
INTRODUcnON The free chlorine disappearance in drinking water during distribution is due both to chemical reactions in the aqueous phase (i), and to reactions occurring at the interfaces between pipes or reservoir walls (ii). (i) - Fast reactions with easily oxidizable compounds such as ammonia, ferrous ions... (Wei et al., 1974); these reactions are usually completed during disinfection treatment and should not be observed in the distribution network. - Slower reactions with less oxidizable compounds (organic matter...) (Dotson and Heitz, 1976). (ii) The reactivity of the pipe wall will be a function of pipe diameter, hydrodynamic conditions, nature of
the pipe material, amount and nature of deposits... (Gotho, 1989; Zhang et al., 1992).
The aim of this work is to propose, on the basis of experiments conducted at bench and pilot scales, a hierarchical classification of the main parameters which influence the free chlorine consumption in 219
220
L. KIENE tt aI.
distribution systems: water, materials, fixed biomass, deposits, corrosion of ferrous material. Results of this study have been used in order to improve the chlorine decay modeling, and for this purpose have been integrated into the modeling software PICCOLO. STUDY OF PARAMETERS INFLUENCING CHLORINE DECAY Influence of the water Quality The influence of TOC and temperature on the chlorine decay rate in the aqueous phase has been studied. For this purpose, kinetics of chlorine decay have been established. Experimental results showed a rapid chlorine depletion during a first phase of about I or 2 hours. This phase corresponds to reactions with easily oxidizable compounds and is usually completed during the disinfection step at the treatment plant. Therefore this phase should not be observed within the distribution network. During a second phase, chlorine decayed mOre slowly, and it was always possible to describe this phase with a first order kinetic rate with respect to chlorine: (1)
Then, all the experimental results have been expressed by the value of the first order kinetic constant k'e (min-I), related to the long term chlorine consumption. The study of 21 water samples with temperatures between S and 2SoC, and TOC concentrations between I and 3 mgIJ, has made it possible to establish the following empirical relationship:
k~ = a x [TOC] x exp( ~)
with, a=I,8.106 and b=60S0 and TO : temperature in Kelvin
(2)
Then under our experimental conditions, the kinetic constant k'e varies between 1.10-3 and 9.10- 3 min-I. These results must be taken into account for the modeling of chlorine decay in distribution systems. Values of constants a and b in equation [2], are probably a function of characteristics of organic matter in each location. So, as a result, it becomes necessary to determine the specific values of these constants for each location within the range of validity of TOC and temperature. Influence of synthetic materials The objective of this work was to quantify the influence of synthetic materials on chlorine decay. For this purpose, sections of pipes corresponding to five materials, among the most widely employed in distribution systems, were tested: High Density PolyEthylene (HOPE), Unplasticized PVC (U-PVC), Molecular Oriented PVC (MO-PVC), PolyPropylene (PP), Glass Reinforced Polyester (GRP). Each section of pipe was previously flushed for a few minutes with tap water. Then the pipe was filled with a chlorine solution (initial free chlorine concentration: l±O.2 mgl1; temperature: 18°C; pH 6.6), and thereafter the chlorine residual concentration was measured at regular intervals for about 12 hours. During this period, stirring was maintained in order to ensure a continuous renewal of chlorine solution at the pipe wall. Curve~ of chlorine decay were interpreted according to the assumption of a first order reaction r.ate (equatIon I), and the result (Figure 1) is expressed as the value of the global kinetic constant corresponding to the consumption of chlorine by both the water and the material: (k').
First we noted that for the material studied. k' decreased as a function of the number of rinsing operations. After rinsing, values of k' are in the same order of magnitude for all the tested materials. Chlorine
Phenomena responsible for the chlorine decay
221
consumption during the first contact period could be due to oxidizable compounds initially present at the pipe wall surface, but that are rapidly eliminated by rinsing (Anselme et al., 1992). For the succeeding periods, leachin~ of chemica~ compounds from the studied polymeric matrix does not appear to induce a significant chlonne consumptIOn.
k' (min· t l.8E-3
I.SE-3 1.2E-3
r;::::;c;:;::ls=tn =n=s.=ng=:::;-----r==;-----------, • 2nd nnslOg C 3rd rinslOg •
.
9.0E-4 6.0E-4 3.0E-4 O.OE+O
PEUD
diam.6"
PVC-CM
diam.4"
GRP
dlam.IO"
PP
dlam.4"
V-PVC
dlam.6"
V-PVC
dlam.4"
Figure I. Kinetic constant (k') of chlorine decay for potable waler in conlact with syntheUc pipes.
These results show that after rinsing, the influence of synthetic material on the total consumption of distribution networks must be very small. Therefore this aspect of chlorine decay will not be a key parameter for chlorine decay modeling. Influence of biofilms In order to study the influence of biofilms on chlorine decay, and avoid influence of the substratum, inert material which does not consume chlorine had previously been selected. Finally, 4 millimeter diameter polystyrene beads manufactured by Atochem (Lacqrene) were chosen for supporting the growth of the biofilm.
Colonization of biofilm. In order to colonize the beads with biofilm, they were incubated with three qualities f water produced by a plant using Seine river downstream of Paris. The main characteristics of these waters ~e reported in Table 1. The beads were colonized in open reactors and were continuously fed with the tested waters (temperature: 18°C). Table I. physical-chemical and biological quality of tested waters during the colonization period
sand filtered water ozonated water final water
TOC (mgIL)
(mgIL)
(O.D.lm)
UV
pH
1.69 1.60 1.29
0.40 0.60 0.35
2.6 1.5
7.67 7.72 7.59
BDOC
1.3
IIPC at 22°C HPC at 22°C R2Amedium PCAmedium (CFU/ml) (CFU/ml\ 64370 1499 2603 190 1076 9
Determination of the biofilm chlorine demand. After being sampled, beads were rinsed and immersed in a flask containing a solution of s~ium hypochl~rite. Then, the chlorine disappearance was followed up as a function of time (initial chlonne concen~attons: I or 2 mgll; surface to volume ratio (SN): 150 to 1500 cm2n; pH 6.6±O.2; temperature: 18±1 C). Chlorine decay profiles generally follow a ra~id first phase for 1 or 2 hours followed by a second slower hase. The first phase does not concern ~hlonne decay modeling in distribution systems; it corresponds to ihe accumulation, in the absence of chlonne residual, of easily oxidizable substances during the colonization phase. Therefore this phase would not be observed in a distribution system, where chlorine is continuously
222
L. KIENE et al.
present. The second phase, corresponding to long term chlorine consumption, can be described by a first order kinetic reaction rate with respect to chlorine. Results. Experimental results show that for each tested feed water, the fixed biomass is characterized by a higher kinetic constant (k'b) than non colonized beads. As shown on Figure 2. the kinetic constant increases with the colonization time, but a steady state is observed during the experimental period. Furthermore, the time needed to reach this steady state varies according to the quality of the test water where the biofilms are developed. Biomass colonized in ozonated water has the highest kinetic constant (k'b=5.5xlO- 3 min-I), and the steady state is obtained as from the 16th week. For the biomass colonized in sand filtered water. k'b leveled off around the 12th week. For the biomass colonized in final water, the profile of k'b is stable as from the 61h week. Results show that k'b depends on the quality of the feeding water: BDOC, UV absorbance. bacterial count (HPC). This conclusion is in agreement with previous works (Mathieu et al., 1992; Servais et al., 1995) which show a direct influence of these same parameters on biological growth in distribution systems. The most significant relation between k'b and parameters characterizing the quality of the feed water was observed with the BDOC. This relationship was obtained from the 16th week:
k~ = 7.95 X to- 3 x BDOC + 2.83 x to.....
(3)
r=0.92
From experimental data, it has also been possible to establish an empirical relationship (equation 4) to describe the evolution of the kinetic constant according to the water quality (BOOC) and the surface to volume ratio. Within the studied experimental field, equation 4 allows the prediction of k'b with a precision of 35%.
k~
= [5,3.10- 6 x (BDOC)] x ~
with BOaC in mg/L and SN in cm2/L
(4)
From equation 4, it appears that fixed biomass constitutes one of the parameters that can in some cases influence the chlorine decay in distribution systems. Nevertheless, for a 100 mm diameter pipe, and a BDOC of 0.2 mg/I, the value of k'b remains very small: approximately 4x to-4 min-I. The influence of fixed biom~s would become an important parameter only for very small diameters (smaller than 75 mm) and for high BOaC (greater than 0.6 mgll): k'b greater or equal to 2.10- 3 min-I. k'b.(mln") 7.0E-3
r------------:---r::=====:==Ji -
6.0E-3
i'~oZa",ted Watel
:
::~: ~i~'
2.0E-3
L_~_-€.~'~ =w:::lt::e'=::::'-'_--i [k'• .±.1"~~=~f-------':I=Fi="I=1 Standard aev..!'" _
I.OE.3
O.OE-+{)+-----+-----f------f----+-----l
4
8
12 . 16 Colonization Time (weeki
20
24
Figure 2. Evolution of the kinetic constant (k'b ) for beads colonized with three water qualities from a surface water treatment plant.
Influence of the corrosion Metallic materials in contact with water are generally damaged because they are subject to corrosion, the intensity of which is controlled by the physical-ehemical conditions (e.g., temperature, pH, carbonate
Phenomena responsible for the chlorine decay
223
balance. chloride, biofilm...) (Barbier et al.. 1990). Corrosion of ferrous material can be expressed as the oxidation of metallic iron (FeO) into ferrous iron (Fe2+) which can react with oxidizing compounds such as oxygen or chlorine (Kiene and Levi. 1996). If chlorine is the main oxidant, one can assume that the following reaction will occur:
2Fe2++ HOCI +W -+ 2Fe'+ + Cl" + Hp
(S)
On the other hand, HOCl could also be consumed because it plays a role in Feo oxidation. Nevertheless other results (Frateur, 1997 and Frateur et al., 1998) have shown that the reduction of dissolved oxygen remains the single cathodic process coupled with iron dissolution. Then chlorine decay due to a corrosion phenomenon should be only under the control of the production of felTous ions which are directly linked to the corrosion rate and could be described according to a zero order reaction rate: d rCl2 1 4 x 10-3 M n Cr :J.::::J= x - - x PFe x dt 't MFe D
[CIJ
chlorine concentration in mgIL
t
time in acc
t
3.16xlO' Icc/year atomic weight of a (35.5 g/mol) atomic weight of Fe (55.85 almol) iron density (7,860 kg/ml ) corrosion rate (IIm!year) pipe diameter (m)
Ma
M,. r.. C,
o
(6)
After calculation of the constant terms, equation [6] becomes:
~=KxCr dt
with K=6.32xlO·7
(7)
D
In order to verify these hypotheses, bench scale and pilot scale experiments have been carried out. Slectrochemical bench scale study The corrosion process was simulated by inducing dissolution of ferrous iron under the control of an electric current. the intensitr of which ~as imposed and controlled w~th a ~tentiostat. Under our experimental conditions, ferrous Iron p~oductlon de~nded solely upon the m~nslty of the imposed current, and was independent of other expenmental condItions (e.g., SN, water quality, oxygen and chlorine concentrations). Various current intensities were used between SO and SOO !lA. Therefore, it was possible to verify the linearity of the relationship between corrosion rate and the kinetics of chlorine decay. For all experimental results, a linear decre~ of chlorine was observed as a function of time. This result Suggests that the rat~ o~ .chlorine consumptIon was independent of chlorine residual levels, and that ferrous dissolution was the InDltmg factor. As shown in Figure 3, chlo~ine decay rates (d[CI 2]/dt) increase according to imposed current (i). Furthermore, oxygen concentration does not appear to influence chlorine decay due to a corrosion process. A linear relationship was found between K o and the dissolution current as well in the presence as in the absence of oxygen. The chlo~ne decay is therefore directly proportional to the production flux of Fe 2+, which is equivalent to a corrOSIon rate.
224
L. KffiNE et al.
dlOJl/dt (mg/L.see) 3.50&04r---;:==========::;--------••; - - , 3.00&04 • oxygen: 0 mg/L
•
• QIllson' 44 mslI
2.50E-04 2.ooE.Q4 1.50E.Q4 t.ooE.Q4 5.ooE-05
O.ooE+oo-I------~>--------------~
o
100
200
300
400
500
600
I (pA) Figure 3. Influence of the production flux of Fe2+ (proportional 10 the current intensity) on the chlorine decay rate (initial free chlorine I mgll; oxygen concentration 44 mgll).
Pilot scale study A pilot-scale PropelIer Loop Reactor (PLR) was used in this study (Figure 4) (Blenk, 1978). The internal tube was made of stainless steel, and the studied pipe, made of gray cast iron, constituted the external wall of the PLR. A propeller ensured the recirculation of water inside the pilot. The water corrosivity was monitored using a CORRATER SCAI (Rohrback Cosasco), which measures corrosion rate on the basis of polarization resistance. The free chlorine residual was monitored using a CHLORSCAN® (Seres), which uses amperometric measurement of the hypochloric acid (H0Cl) (Van den Berg et 01., 1993; Wable et 01., 1993). Modulation of water corrosivity was obtained by injection of a sodium chloride solution inside the pilot. During experiments (pH: 6.S; Ryznar index: 8), with chloride concentrations from 50 to 9000 mgll, corrosivity measured with the CORRATER SCAI was in the range 20 to 200 ~.Lrnlyear. Depletion of free chlorine concentration inside the PLR was followed as a function of time (initial free chlorine concentration: I mgll).
Fisure 4. Diagrammatic representation of the Propeller Loop Reactor (PLR) used for the pilot scale study.
225
Phenomena responsible for the chlorine decay
Experimental results show that chlorine decay curves present a linear shape, and can be interpreted according to a zero order reaction rate, limited by the corrosion rate. Then, the kinetic constant (K) was calculated from the graph 'free chlorine vs time' (slope equal to: K.C/D). According to these pilot experiments, the chlorine decay due to the corrosion process could be modeled as folloWS: with K=1.3610" Co and Co: chlorine concentration in mg/L t: Time in seconds C,: water corrosivity in 11m/year
(8)
D: pipe diameter in meters
As shown on Figure 5, the zero order reaction rate constant (K) which characterized the chlorine decay rate was linearly correlated with the water corrosivity. chlorine decay rate (mglsec) 2 . 0 E - Q 3 - r - - - - - - - - - - - - - - - - - - - . y = 1.36806 x - 2.6 e-4 • ~=0.73 1.5E-Q3
•
•
1.0E-Q3 5.1E-Q4 1,OE-QS .J--.....:::::::....-~-.l~-__i--_-_-__I o 200 400 600 800 1000 1200 1400 ratio: corroslvlty/plpe diameter Figure S. Evolution of the chlorine decay rale as a function of the water corrosivity dClennined from the measure of the polarization resistance (CORRATER SCAI).
In conclusion, pilot scale experiments confirm that corrosion dramatically influences chlorine decay, and demonstrate that a zero order reaction rate can be accepted in the case of metallic distribution pipes subject to corrosion. Nevertheless experime?tal numeric values do not co~spond closely to ex~ted values (equation 7): during our study expen~ental values were roughly 2 times greater than theoretical values. Other pathways of chlo~ne ~onsulll:PtlOn mu.st be consi~ered; for instance the reduction of free chlorine could be a cathodic reaction directly Involved In the corrosion process.
SYNTHESIS AND EXPERIMENTAL RESULT DISCUSSION In order to study which parameter consumes most chlorine (water, synthetic material, metallic material, biofilm...), all experimental results have been extr~pol~ted to an SN ratio of 160 cm21l, corresponding to an internal pipe diameter of 25? mm (Table 2~. The kmetic constant related to deposits was obtained during the study of a specific distributlo~ sy~tem. This value i~ only given as an illustration, because the influence of deposits on chlorine consumption IS probably very different from a distribution system to another.
L. K.lhNh
et al.
Table 2. Contribution of various parameters to the chlorine decay kinetic constant (k' in min-I) of a distribution system water pipe materials deposits
fixed biomass
drinking water from Seine river Treatment plant synthetic materials gray cast iron sampled in a distribution network by opening fire hydrants (contain mainly iron oxides and calcium carbonate) colonization time 23 weeks, TOe 1.3 mg/L, BDOC 0.35 mg/L, T 18°C
4.72 x 10 6.64 x 10" 3 2.73 x 101.12x10
3.00 x 10
Comparison of chlorine demand for each parameter must be made in two cases: synthetic pipes and gray cast iron pipes (Figure 6). For synthetic pipes, most chlorine is consumed by deposits, water and then biomass. But in a cast iron pipe, chlorine is principally consumed by the material, deposit and then the water. Chlorine demand of fixed biomass in this case is negligible. (b) I"Y till I,." pipe:
CI)' nlllfllt'l.IfH':
rhJorhw n .... mptJo. I" 1 Hun 0.50 "'lit..
thlorln~ fo"umpllon I.. Z houn 0.11 milL.
water
---r-';l~_
26%
n\atenal
2% mllenal IM~
37%
Figure 6. Complirison of chlorine consumption in a network made of synthetic (lO-a) and gray cast iron pipes (l0• h); (diameter 250 mm, residence time =2h, Clo=l mg/l).
CONCLUSIONS Experimental results carried out in this study make it possible to propose a hierarchical classification of the main parameters involved in the chlorine deeay observed in distribution systems. • •
•
•
Chlorine consumption by synthetic materials is negligible and does not appear to be an important parameter for the modeling of chlorine decay. In contrast. chlorine consumption of old cast iron pipes whose internal surfaces are not protected by a coating. is a major parameter for chlorine decay modeling. For cast iron or steel pipes, the rate of chlorine consumption can be directly under the dependence of corrosion phenomena. In this case the chlorine disappearance can be described according to a zero order reaction rate. Therefore, for a given pipe section, chlorine decay depends only on the pipe diameter and the corrosion rate. Chlorine decay due to bulk reactions is very variable according to the temperature and organic matter concentration. Variation of the kinetics can be predicted by a simple model taking into account the TOC and the temperature. Chlorine decay due to fixed biomass varies according to the colonization time, but k'b finally reaches a stationary state after a time depending on the water quality. Influence of biomass should become an important parameter, only for small diameter synthetic pipes fed with water characterized by high BDOC.
Phenomena responsible for the chlorine decay
227
From these results, it has been possible to improve chlorine decay modeling in distribution systems. This modeling takes into account the main parameters governing the disappearance of free chlorine: temperature, organic matter concentration in water, and corrosion rate for metallic materials. In addition to these parameters, the model factors in the hydrodynamic conditions that control the transfer of chlorine from the aqueous phase to the pipe walls. This enhanced modeling of free chlorine decay has been integrated into the hydraulic modeling software PICCOLO (Jarrige, 1989) and is currently tested on full-scale distribution systems. REFERENCES Anselme. C.• N'Guyen. K.• Bruchel, A. and Mallevialle. J. (1992). Can polyethylene pipes impart odors in drinking water? Environm Technol Len.• 6. 477-488. Barbier. J. P.• Cordonnier. J. and Gabriel. J. M. (1990). Corrosion ml!tallique et phl!nom~nes d'eaux rouges - utilisation d'inhibiteurs. T.S.M.-L'EAU. 183-188. Blenk. H. (1978). Mixing and dispersing in a loop reactor. Int Symposium on Mixing. Mons. Christman. R. F. and Ghassimi. M. J. (1976). Chemical nature of organic color in water. J.A. W. W.A.• 58. 773. Dotson. D. and Heitz, G. R. (1985). Chlorine decay chemistry in natural walers. In Water Chlorination: Environmental Impact and Health Effects. Jolley. R. L., Brungs. W. A. and Cumming. R. B. (Eds). Ann Arbor Science Publishers Inc.• 5. 713722. Fraleur. I. (1997). Incidence de la corrosion des ml!taux ferreux sur la demande en chlore lib", en rl!seaux de distribution d'eau . potable. Thesis. University of ~aris 6. Fraleur. I.. Deslouis. C.• Kil!nl!. L.• UVI. Y. and Tnbolet. B. (1998). Free chlorine consumption induced by cast iron corrosion in drinking water distribution systems. Wat. Res. (submined). Gotho. K. (1989). Chlorine residual concentration decreasing rate coefficient for various pipe materials. Wat. SuppL, 7(2·3).164172. Jarrlge. P. A. and Bos. M. (1989). Mathematical modelling of water distribution networks under steady-state conditions-Recent development and future projects. Aqua. 38. 352-357. Kil!nl!. L. and Uvi. Y. (1986). Influence des matl!riaux ferreux sur la consommation du chlore en rl!seau de distribution. Proc HYDROTOP 96. Marseille. 143-152. Mathieu. L.• Paquin. 1. L.• Block, J. C.• ~~do~. G.• Maill~ and Reasoner. D. (1992). Param~tres gouvernant la prolifl!ration bact4!rienne dans les rl!seauxde dlstnbution. Rev. SCI. Eou, 5. 91·112. Servais. P.• Laurent, P. and Randon. G. (1995). Comparison of the bacterial dynamics in various French distribution systems. J. WaterSRT.Aqua. 44(1).10-17. Van den Berg. A.• Grisel. A.• Verney·Norberg. E.• Van der School, B. H.• Koudelka·Hep. M. and de Rooij. N. F. (1993). On·wafer fabricated free-chlorine sensor with ppb detection limit for drinking monitoring. Sensors and Actuators 8ul.• 13·14. 396399. Wable. 0 .• Levi. Y.• Monti~l. ,,:,.• ~.hen~ult, M. and Veyrie. A. (1993). Miniature c~lorine microsensor for on-line control of drinking water quality 10 dlslJibution system. Proc AWWA WQTC, 209·216. Miami. Wei. I. W. T. and Morris: J. C. (1974). Dynamics o~ break.po~nt chlorination. In Chemistry of Water Supply, Treatment and Distribution. RubIO. A. J. (Ed.). Ann Arbor SCience Publishers. Inc.• 297·332. Zhang. G.• Kil!nl!. L.• Wable. 0 .• Chan. U. S. and Duguel, J. P. (1992). Modelling of chlorine decay in water distribution network of Macao. Environm. Technol., 13.937-946.