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Inactivation of indicator bacteria in wastewater by chlorine—a kinetics study
Bioresource Technology 72 (2000) 85±93
Inactivation of indicator bacteria in wastewater by chlorineÐa kinetics study Abdennaceur Hassen a,*, Abderrahim Heyouni b, Hedi Shayeb b, Mohamed Cherif c, Abdellatif Boudabous d a
Institut National de Recherche Scienti®que et Technique, Laboratoire Environnement, B.P. 24-1082, Cit e Mahraj ene, Tunis, Tunisia b Ecole Nationale des Ing enieurs de Tunis, Campus Universitaire, B.P. 37-1002, Tunis Belv ed eres Tunis, Tunisia c Institut National Agronomique de Tunisie, 43 Avenue Charles Nicole, 1082, Cit e Mahraj ene, Tunis, Tunisia d Facult e des Sciences de Tunis, Laboratoire de Microbiologie, Campus Universitaire, 1060 Tunis, Tunisia Received 28 February 1998; received in revised form 28 April 1999; accepted 25 May 1999
Abstract The aim of this study was to characterise the kinetics of chlorine consumption and of inactivation of indicator bacteria in secondary wastewater using a batch laboratory reactor. In this time-course study, dierent concentrations of chlorine, used as NaOCl, were injected into the reactor, the levels of the dierent forms of residual chlorine were measured, and the numbers of faecal coliforms and faecal streptococci were determined. The results of the kinetics of chlorine consumption showed that monochloramines and trichloramines were the more important forms of residual chlorine as compared to free chlorine and dichloramines. The high contents of trichloramines indicated that the reaction of chlorine with ammoniacal nitrogen was very fast and that the transformation of chlorine into trichloramines was carried out in a time shorter than 1 min. Experimental results showed that the application of the model of Chick-Watson in its original form was not representative of the kinetics of inactivation of faecal coliforms and faecal streptococci. Modi®cation of this model, in considering an initial reduction just at the contact of water with chlorine, improved the results of adjustment of the model. The same ®ndings are valid for the model of Collins-Selleck in considering a value m imposed to the concentration of residual chlorine, since it appeared clearly that the concentration of chlorine in¯uenced the output of disinfection more than did the time of contact. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Wastewater; Disinfection; Chlorine; Modelling; Residual chlorine; Indicator bacteria
1. Introduction Disinfection is often a stage for the reusability of treated wastewater. The main objective of disinfection is to reduce sanitary risks related to the presence of pathogens. Assessment of sanitary risks consists in indexing and identifying pathogens that may be present in such a water. Nevertheless, a serious analytical problem may be encountered, especially when the number of these pathogens is relatively low and their presence is random. A growing body of evidence has indicated that the use of indicator bacteria, which have the advantage of being abundant and easily countable, is very useful in the assessment of sanitary risks related to the presence of pathogens (Wyer et al., 1997). Water disinfection can be achieved via dierent means such as chlorination, ozonation and ultraviolet *
Corresponding author. Tel.: +216-1-788-436; fax: +216-1-430-934.
radiation (Moeller and Calkins, 1980; Come and Bariou, 1980, Anonymous, 1989). Disinfection by chlorine has gained wide acceptance commercially, probably because of its simplicity and its moderate cost; despite the major problem of secondary harmful products generated by this treatment (Pourmoghaddas and Stevens, 1995; Racaud and Rawzy, 1994; Sanchez, 1993). Disinfection achieved by such a process is often accomplished in two steps: a step of fast mixture and a step of contact between chlorine and water, during a suciently long time to allow the product to play its disinfection role. The success of the operation of disinfection depends greatly upon the conditions prevailing during the second step. Besides the physico-chemical quality of the treated water, many other factors may aect this operation, among which we ®nd the hydraulic functioning of the contactor, the kinetics of chlorine reaction with the components present in water and the kinetics of bacterial inactivation.
0960-8524/00/$ - see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 9 9 ) 0 0 0 8 6 - 3
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In order to characterize the functioning of such a process, we have undertaken an experimental study enabling modelling of the kinetics of chlorine consumption and bacterial inactivation in secondary wastewater. Once combined with an adequate hydrodynamic model, the obtained kinetics would allow the construction of a simulation model of a chlorination reactor. The goal of this study was, therefore, to characterize the kinetics of chlorine consumption and indicator bacterial inactivation in secondary wastewater.
2. Methods 2.1. Wastewater treatment Chlorination assays were performed on samples of secondary treated wastewater (trickling ®ltrate) previously sand ®ltered in a semi-industrial pilot plant. Sand ®ltration allowed an appreciable improvement of the quality of water and resulted in a considerable lowering of the initial demand for chlorine. 2.2. Experimental procedure Disinfection assays: Experiments were performed in 3 l pyrex beakers each containing a magnetic stirrer rotating at 500 rpm. Wastewater (2 l) was mixed with 50 ml of disinfectant (sodium hypochlorite) prepared at ´ 10 the required ®nal dose. Wastewater chlorination was tested with concentrations varying from 6.5 to 25 mg/l. Samples (50 ml) were taken at regular intervals varying from 2 to 40 min, and the oxidative reaction was immediately stopped by addition of sodium thiosulphate before residual chlorine measurement and faecal bacteria enumeration. Each assay was carried out at least 4 times. In the control, 50 ml of sterile distilled water were mixed with wastewater samples instead of the disinfectant. Residual chlorine measurement: The dierent forms of residual chlorine were determined by the sulphate diethyl-p-phenylenediamine (DPD) method according to Nicholson (1965). Chlorine and derived compounds react with DPD to give a red colour susceptible of titrimetric measuring with ammonium ferrous (II) sulphate solution. Addition of potassium iodide (KI) as an oxidiser, at dierent contact times, allowed dierentiation among free chlorine and dierent combined forms of chlorine such as mono-, di-, and trichloramines. Enumeration of indicator bacteria: The number of indicator bacteria was determined based on the French standard methods of the most probable number MPN (Rodier, 1978), and the corrected MPN tables proposed by Man (1983) were used.
3. Results and discussion The goal of the present investigation was to characterize the kinetics of chlorine consumption and bacterial inactivation. The rates of reduction of faecal indicator bacteria, as a function of time of contact, in the presence of dierent chlorine concentrations were determined. The rate of inactivation was associated with a concentration, C, of residual free chlorine and with a time of contact measured using as origin time t 0 corresponding to the moment of chlorine injection into the water. 3.1. Reaction of chlorine consumption The wastewater, even when puri®ed and treated by ®ltration, contained relatively large quantities of organic and mineral matters with a BOD5 content varying from 15 to 30 mg O2 /l and a COD ranging from 20 to 30 mg O2 /l. The content in ammoniacal nitrogen ¯uctuated between 8 and 20 NH3 ±N mg/l according to the experiments. After the initial mixture of chlorine and water, there was competition among two types of reactions: · Oxido-reduction with some reducing compounds that consume a part of the injected chlorine rendering it unavailable for the disinfection. · Substitution that forms some compounds of addition or of substitution mainly with ammonia to form chloramines, which have a low bactericidal power. Fig. 1 shows chlorine consumption where only the results obtained for the concentrations of 6.5 and 13.6 mg/l have been represented. From Fig. 1, it appears that for all the studied concentrations and regardless of the time of contact: · Monochloramines and trichloramines remained very important as compared to free chlorine and dichloramines. · Dichloramines appeared as the lowest chlorine form, with a concentration usually less than 10% of the injected concentration. · Trichloramines appeared as the most important form of residual chlorine. Their levels decreased with the time of contact, to reach their lowest levels by the end of the experiment (20±30 min). These results indicated that the reaction of chlorine with ammoniacal nitrogen of the water was very rapid and that evolution of chlorine as trichloramines took place in less than 1 min. This result appears to agree with those mentioned in the literature by several authors (Soulard, 1982; Saunier, 1979; Alouini et Seux, 1987; Adin et al., 1991). According to Adin et al. (1991), based on kinetic constants of these reactions, the formation of monochloramines is the most rapid. Reactions of chloramines destruction are relatively slow and trichloramines are only gradually hydrolysed following their appearance in the medium.
A. Hassen et al. / Bioresource Technology 72 (2000) 85±93
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Fig. 1. Evolution of the content of residual chlorine as a function of the time of contact. Clustred error bar graphs and bars represent standard error of mean with at least n 4.
· The quantity of free chlorine, the main cause of disinfection, was variable according to the injected concentration. In contrast, it showed, on average, a low variation in the course of time for the same concentration of total chlorine. For instance, with the high chlorine concentration of 20 mg/l, the level of free chlorine was nearly constant and of the order of 3.5 mg/l (Result not shown). This result clearly indicated that after the immediate demand
of water for chlorine, conditioned by the presence of reducing compounds as well as compounds of addition or substitution, the free residual chlorine remained more or less at the same level until the end of the experiment. It appears particularly that the content of residual free chlorine in the secondary wastewater always exceeded the value of 10% of the quantity of chlorine injected at the beginning of the experiment. This result was valid at any time
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of contact studied and regardless of the chlorine concentration applied. Based on all these results, it may be concluded that the consumption of chlorine during the process of wastewater disinfection undergoes two steps: · A ®rst rapid step corresponding to the consumption of chlorine following its reaction with the reducing compounds in water. This step, called `the immediate satisfaction demand', seemed to be instantaneous. · A second step, during which residual chlorine was transformed rapidly into the free and combined chlorine forms. This step concerns particularly the process of disinfection since residual chlorine will be available to exert its antiseptic power. It is clear that the modelling of the kinetics of chlorine consumption or disinfection would concern only the second step of the reaction, which is amenable to measurement. On the other hand, as has previously been mentioned, combined chlorine (mono-, di- and trichloramines) appeared as the most important form of residual chlorine. So, the role of this form of residual chlorine could not be disregarded compared with free chlorine. In fact, free chlorine is considered as the best disinfectant, but the role of combined chlorine in the bacterial reduction is not negligible (Anonymous, 1989).
and the number of faecal bacteria. For each chlorine concentration applied, tests were conducted at progressive contact times ranging from 2 to 40 min. The three retained chlorine concentrations (6.5, 8.6 and 13.6 mg/l) were tested on wastewater with MPN of 6.02 ´ 104 to 6.02 ´ 105 of faecal coliforms per 100 ml and 3.16 ´ 104 to 5.62 ´ 105 of faecal streptococci per 100 ml. All results are reported in Table 1. The initial contents of faecal bacteria (N0 ) in wastewater diered from one experiment to another (results not shown), which was related to the quality of water used in each experiment. The speed of faecal bacteria reduction varied from one concentration to the other. The higher the initial concentration of chlorine was, the higher was the reduction of bacteria. For instance, during the same contact time of 10 min, a reduction in faecal streptococci of 1.63 and 3.36 logarithmic units was obtained with the respective initial chlorine concentrations of 6.5 and 13.6 mg/l. In order to ensure a sucient safety during wastewater reuse or rejection, the number of faecal coliforms or streptococci must be less than 103 bacteria per 100 ml (White, 1976). In this study, this objective was reached after contact times of 30, 20 and 10 min following the injection of a concentration of sodium hypochlorite of, respectively, 6.5, 8.6 and 13.6 mg/l.
3.2. Kinetic study of water disinfection by chlorine
3.3. Modelling of the kinetics of disinfection
The experimental methodology adopted consisted in measuring simultaneously residual chlorine contents
It is important to note that the approach to the kinetics of microorganism inactivation is generally em-
Table 1 Log reductions of faecal coliforms and faecal streptococci as a function of time and chlorine concentration Chlorine (mg/l)
a
Time (min)
Faecal coliforms
Faecal streptococci
(N/N0 )
Reductiona
(N/N0 )
Reduction
Free chlorine (mg/l)
Monochloramines (mg/l)
Dichloramines (mg/l)
Trichloramines (mg/l)
6.5
2 5 10 20 30 40
0.0339 0.0324 0.0234 0.0100 0.0018 0.0009
1.47 1.49 1.63 2.00 2.75 3.04
0.1185 0.0372 0.0234 0.0135 0.0020 0.0005
0.92 1.43 1.63 1.87 2.69 3.27
1.62 0.37 1.22 0.26 1.25 0.15 1.35 0.15 1.40 0.12 1.40 0.12
3.45 0.38 1.77 0.32 2.52 0.24 2.47 0.85 1.92 0.42 ND
0.97 0.20 1.07 0.48 0.47 0.05 0.52 0.12 0.67 0.14 ND
4.10 0.88 1.90 0.54 2.10 0.40 2.40 1.20 0.93 0.22 ND
8.6
2 5 10 20 30 40
0.0170 0.0174 0.0022 0.0018 0.0005 0.0005
1.76 1.75 2.65 2.74 3.28 3.28
0.1632 0.0724 0.0100 0.0052 0.0007 0.0007
0.78 1.32 2.00 2.28 3.12 4.12
2.68 0.69 1.72 0.28 3.30 0.97 1.52 0.22 2.97 0.70 2.52 0.42
5.62 0.99 4.70 1.14 4.45 1.02 4.87 0.28 2.32 1.16 ND
1.55 0.60 1.78 0.58 0.95 0.52 0.72 0.23 0.60 0.18 ND
9.60 0.70 6.40 2.50 6.46 0.60 4.00 1.86 0.35 0.04 ND
13.6
2 5 10 20 30 40
0.01000 0.00120 0.00091 0.00038 0.00034 0.00019
2.00 2.92 3.04 3.41 3.45 3.70
0.0575 0.0087 0.0004 0.00004 0.00014 0.00004
1.24 2.06 3.36 4.36 3.84 4.36
2.48 0.34 3.15 0.25 4.62 1.04 3.50 0.28 1.22 0.05 1.72 0.24
10.15 0.30 7.65 1.04 5.80 1.20 6.35 0.28 7.48 0.58 ND
0.60 0.17 1.88 1.04 0.68 0.35 1.62 0.36 1.08 0.32 ND
12.00 3.20 18.00 1.97 8.60 1.80 8.20 2.30 10.30 1.00 ND
Logarithmic units ÿlog (N/N0 ); N: Number of micro-organisms at the instant t; N0 : Number of micro-organisms at the instant t 0; : Standard error.
A. Hassen et al. / Bioresource Technology 72 (2000) 85±93
pirical, based on laboratory studies, and consequently a model is only valid in conditions similar to those of its establishment. So, concentration-time (CT) values expressed in mg.min/l were calculated by mathematical integration of the concentration of residual free chlorine in water versus time. In this paper only results of the models of Chick-Watson and Collins-Selleck will be considered as a reference and the whole results obtained with the dierent concentrations of chlorine will be combined in order to establish an expression for the kinetics of disinfection. 3.4. Chick-Watson model The expression of the kinetics of disinfection according to the model of Chick-Watson is given as follows: dN =dt ÿ KC n N with N is the number of microorganisms, C the concentration of residual free chlorine, n the coecient of dilution, which is a function of the quality of water, and K is the coecient translating the disinfecting power. Parameters to identify are K and n. By using the integrated form of the model and by changing to the logarithm: N ln K n ln C ln T ; ln ÿ ln N0 and with the help of a linear regression, we can determine the values of K and n.
89
In this way, expressions obtained for the rate of inactivation were ÿ N for faecal coliforms : exp ÿ 0:1046 C 1:1 T N0 with a coecient of determination R2 of 0.12. ÿ N exp ÿ 0:1635 C 0:7 T for faecal streptococci : N0 with a coecient of determination R2 of 0.42. An illustration of these adjustments is given in Fig. 2. Fig. 2 shows an important variation between the measured and the calculated values. Consequently, the model of Chick-Watson was not representative of the kinetics of disinfection. Therefore, a second approach to modelling was adopted and an initial microbial reduction, just at the moment of contact of water with chlorine, was considered. The model becomes in this case: N =N0 A exp ÿKC n T with A is the initial reduction just at the moment of contact of water with chlorine, The expressions obtained for the rate of reduction were: ÿ N 0:011 exp ÿ 0:0369 C 1:1 T for faecal coliforms : N0 with a coecient of determination R2 of 0.63. for faecal streptococci :
N N0
ÿ 0:0727 exp ÿ 0:1065 C 0:7 T
Fig. 2. Determination of the kinetic of disinfection of faecal coliforms and faecal streptococci according to the model of Chick-Watson. N: Number of micro-organisms at the instant t; N0 : Number of micro-organisms at the instant t 0; C: Free chlorine concentration; n parameter n of the model; T: Time; x C n T ; Symbols, measured; lines, calculated.
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A. Hassen et al. / Bioresource Technology 72 (2000) 85±93
with a coecient of determination R2 of 0.78. An illustration of these adjustments are given in Fig. 3. Considering, v " u #2 uX N N ÿ t ; N0 cal N0 exp which is a parameter representative of the deviation among calculated and measured values, the couples of values of before (0 ) and after (m ) modi®cation of the model (0 1.2250; m 0:0393) and (0 0.9200; m 0:1319), obtained, respectively for faecal coliforms and faecal streptococci, indicated that the modi®ed model of Chick-Watson N =N0 A exp ÿKC n T better described the kinetics of bacterial disinfection than did the original form of the model N =N0 exp ÿKC n T . This ®nding is also con®rmed by Fig. 3 which indicates that the modi®ed model of Chick-Watson describes the kinetics of bacterial disinfection when an initial reduction was considered. 3.5. Model of Collins-Selleck The same procedure was applied for the model of Collins-Selleck. Parameters of identi®cation of this model are s and n with
N 1 for CT 6 s; N0 s n N for CT P s: N0 CT By exploiting all experimental points, and by changing to the logarithmic form (ln N =N0 n ln s ÿ n ln CT ), the values of s and n were determined using a linear adjustment. The obtained expressions were: For faecal coliforms: N 1 for CT 6 0:2028; N0 1:2664 N 0:2028 for CT P 0:2028 with r2 0:69: N0 CT For faecal streptococci: N 1 for CT 6 1:9068; N0 2:276 N 1:9068 for CT P 1:9068 with r2 0:80: N0 CT The model of Collins-Selleck has been used by Qualls and Johnson (1985) and Montgommery (1985) for different kinds of wastewater. In those studies, chlorine and dioxide of chlorine were used as disinfectants. These authors reported s 4:06 and n 2.82 for faecal coliforms. These values are very dierent from those obtained in our study (s 0:2028 and n 1.266), which
Fig. 3. Determination of the kinetic of inactivation of faecal coliforms and faecal streptococci according to the model of Chick-Watson with an initial reduction just at the moment of contact of water with chlorine. N: Number of micro-organisms at the instant t; N0 : Number of micro-organisms at the instant t 0; C: Free chlorine concentration; n parameter n of the model; T: Time; x C n T. Symbols, measured; lines, calculated.
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91
Fig. 4. Determination of the kinetic of disinfection of faecal coliforms and faecal streptococci according to the model of Collins-Selleck. N: Number of micro-organisms at the instant t; N0 : Number of micro-organisms at the instant t 0; C: Free chlorine concentration; m parameter m of the model; T: Time. Symbols, measured; lines, calculated.
may be explained by the quality of the water and the operating conditions. Fig. 4 shows rather large discrepancy between the measured and the theoretical values for faecal coliforms, indicating that the model of Collins-Selleck was not representative of the obtained experimental results. In order to improve the representativeness of this model, a value m was imposed on the concentration of residual chlorine, since it appeared clearly from the Figs. 2±4 that the concentration of chlorine in¯uenced the output of disinfection more than did the time of contact. Indeed, for a ®xed contact time, the reduction of faecal coliforms and faecal streptococci was as great as the concentration of applied chlorine was high. By contrast, for a ®xed chlorine concentration, bacterial reduction progressed relatively slowly as a function of the time of contact. The model becomes in this case: N 1 for CT 6 s; N0 s n N for CT P s: N0 CmT and the parameters to be identi®ed are s, n and m. By changing to the logarithm: N ln n ln s ÿ nm ln C ÿ n ln T ; N0 the values of n, m and s were determined using a linear regression. The expressions obtained were:
For faecal coliforms: N 1 for C 1:8 T 6 0:2484; N0 1:1775 N 0:2484 for C 1:8 T P 0:2484 with r2 0:74: N0 C 1:8 T For faecal streptococci: N 1 for CT 6 1:5783; N0 2:3091 N 1:5783 for CT P 1:5783 with r2 0:80: N0 C 0:68 T Fig. 5 indicates, particularly for faecal coliforms, that the model of Collins-Selleck in the modi®ed form: n N =N0 s=C m T allows a better description of the kinetics of decontamination than does its original form: N =N0 s=CT n : Indeed, when we calculated the deviation r i2 Xh N =N0 cal ÿ N =N0 exp for the two models, the values obtained according to the modi®ed form of the model of Collins-Selleck were lower than those corresponding to the original form of the same model (Table 2). Also, the dierent expressions obtained according to the kinetic approaches of both Chick-Watson and Collins-Selleck are reported in Table 2.
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Table 2 Expressions obtained according to the kinetic models studieda Model
Expression
R2
FC
Chick-Watson Chick-Watson modi®ed Collins-Selleck Collins-Selleck modi®ed
N N0 N N0 N N0 N N0
exp ÿ0:1046 C T ÿ0:011exp ÿ0:0269 C 1:1 T 1:2664 ÿ0:2028 CT 0:2484 1:1775 C1:8 T
0.12 0.63 0.69 0.74
1.2250 0.0393 0.0327 0.0248
FS
Chick-Watson Chick-Watson modi®ed Collins-Selleck Collins-Selleck modi®ed
N N0 N N0 N N0 N N0
exp ÿ0:1635 C 0:7 T ÿ0:0727 exp ÿ0:106C 0:7 T 2:276 ÿ1:9068 C T 2:3091 C1:5738 0:68 T
0.42 0.78 0.80 0.80
0.9200 0.1319 0.2227 0.1900
Bacteria
a FC: Faecal coliforms, FS: Faecal streptococci, R2 : Coecient of determination, : Deviation among calculated and measured values, N: Number of micro-organisms at the instant t; N0 : Number of micro-organisms at the instant t 0; C: Free chlorine concentration; T: Time.
Fig. 5. Determination of the kinetic of disinfection of faecal coliforms and faecal streptococci according to the modi®ed model of Collins-Selleck. N: Number of micro-organisms at the instant t; N0 : Number of micro-organisms at the instant t 0; C: Free chlorine concentration; m parameter m of the model; T: Time; x C m T . Symbols, measured; lines, calculated.
4. Conclusions The following conclusions may be drawn from the present kinetics study: · Monochloramines and trichloramines appeared as the most important residual chlorine forms as compared to free chlorine and dichloramines. · The high levels of trichloramines showed clearly that the reaction of chlorine with ammoniacal nitrogen is very fast and that evolution of chlorine into trichloramines is carried out in a time shorter than 1 min. · The original model of Chick-Watson ln N =N0 ÿKC n T was not able to describe the inactivation kinetics of indicator bacteria. Therefore, a modi®cation, based on the same model but after taking into consideration an initial inactivation during the con-
tact of water with chlorine ln N =N0 A exp ÿKC n T , described very well the kinetics of disinfection of faecal coliforms and faecal streptococci. · The same remarks were valid for the model of Collins-Selleck, which, in modi®ed form, N =N0 n s=C m T described the kinetics of disinfection better than did the original form. Acknowledgements This investigation was supported by grants from the International Foundation for Science (H-2327, IFS, Sweden) and from the CEE (Avicenne No. 93 AVI 054). We thank Professor Jean J. Damelincourt, Centre de
A. Hassen et al. / Bioresource Technology 72 (2000) 85±93
Physique des Plasmas et Applications de Toulouse, University Paul Sabatier, France, for his help. References Adin, A., Katzh, A., Endler, D., Alkaslassy, D., 1991. Trihalomethane formation in chlorinated drinking water: A kinetic model. Water Res. 25, 797±805. Alouini, Z., Seux, R., 1987. Cinetique et mecanisme de l'action oxydative sur les acides amines lors de la desinfection des eaux. Water Res. 21, 335±343. Anonymous, 1989. La chloration des eaux: principe, exploitation, desinstallation. Fondation de lÕeau, Edition Sode, Limoge, France, 35p. Come, L., Bariou, A., 1980. Etude comparative de cinq procedes de desinfection des eaux usees urbaines. Rapport du Centre de Recherches et dÕEssais Appliques aux Techniques de lÕEau, France, p. 127. Man, J.C., 1983. MPN tables corrected. J. Appl. Biotechnol. 17, 301± 305. Moeller, I.R., Calkins, S., 1980. Bactericidal agents in wastewater lagoons and lagoons design. J. Water Poll. Control Fed. 52, 2441± 2451. Montgommery, J.M. (Ed.), 1985. Disinfection. Water Treatment, Principles and Design, 262±279. Wiley, Washington, DC. Nicholson, N.G., 1965. An evaluation of the methods for determining residual chlorine in water. Analyst 90, 187±192.
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Pourmoghaddas, H., Stevens, A., 1995. Relationship between Trihalomethane and Halocetic acids with total organic halogen during chlorination. Water Res. 29, 63±66. Qualls, R.G., Johnson, J.D., 1985. Modelling and eciency of ultraviolet disinfection systems. Water Res. 19, 1039±1046. Racaud, P., Rawsy, S., 1994. Etude de la cinetique de formation des principaux sous-produits de chloration. Tech. Sci. Muni. 5, 243± 249. Rodier, J., 1978. Analyse de l'eau: eaux naturelles, eaux residuaires, eau de mer. Dunod technique, Edition Bordas, Paris, p. 1135. Sanchez, J.M.C., 1993. Reaction kinetics of humic acid with sodium hypochlorite. Water Res. 27, 815±820. Saunier, B.M., 1979. The kinetics of break point chlorination in continuous ¯ow system. J. Wat. Wks. Ass. 71, 164±172. Soulard, M., 1982. Systeme chlore ou brome-ammoniaque-eau. Determination, domaine dÕexistence et cinetique dÕevolution des especes. Thesis of doctorate, University of Haute Alsace, France, p. 180. White, G.C., 1976. Pratiques courantes de la chloration et de la dechloration des eaux usees, des eaux recyclees et des eaux de refroidissements. Conferences on the water chlorination organised by CIFEC at Neuilly-sur-Seine. L' Eau, Industrie, les Nuisances, 7, 1±23. Wyer, M.D., O'Neill, G., Kay, D., Crowther, J., 1997. Non outfall sources of faecal indicator organisms aecting the compliance of coastal waters with directive 76/160/EEC. Wat. Sci. Technol. 35, 141±156.
Inactivation of indicator bacteria in wastewater by chlorineÐa kinetics study Abdennaceur Hassen a,*, Abderrahim Heyouni b, Hedi Shayeb b, Mohamed Cherif c, Abdellatif Boudabous d a
Institut National de Recherche Scienti®que et Technique, Laboratoire Environnement, B.P. 24-1082, Cit e Mahraj ene, Tunis, Tunisia b Ecole Nationale des Ing enieurs de Tunis, Campus Universitaire, B.P. 37-1002, Tunis Belv ed eres Tunis, Tunisia c Institut National Agronomique de Tunisie, 43 Avenue Charles Nicole, 1082, Cit e Mahraj ene, Tunis, Tunisia d Facult e des Sciences de Tunis, Laboratoire de Microbiologie, Campus Universitaire, 1060 Tunis, Tunisia Received 28 February 1998; received in revised form 28 April 1999; accepted 25 May 1999
Abstract The aim of this study was to characterise the kinetics of chlorine consumption and of inactivation of indicator bacteria in secondary wastewater using a batch laboratory reactor. In this time-course study, dierent concentrations of chlorine, used as NaOCl, were injected into the reactor, the levels of the dierent forms of residual chlorine were measured, and the numbers of faecal coliforms and faecal streptococci were determined. The results of the kinetics of chlorine consumption showed that monochloramines and trichloramines were the more important forms of residual chlorine as compared to free chlorine and dichloramines. The high contents of trichloramines indicated that the reaction of chlorine with ammoniacal nitrogen was very fast and that the transformation of chlorine into trichloramines was carried out in a time shorter than 1 min. Experimental results showed that the application of the model of Chick-Watson in its original form was not representative of the kinetics of inactivation of faecal coliforms and faecal streptococci. Modi®cation of this model, in considering an initial reduction just at the contact of water with chlorine, improved the results of adjustment of the model. The same ®ndings are valid for the model of Collins-Selleck in considering a value m imposed to the concentration of residual chlorine, since it appeared clearly that the concentration of chlorine in¯uenced the output of disinfection more than did the time of contact. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Wastewater; Disinfection; Chlorine; Modelling; Residual chlorine; Indicator bacteria
1. Introduction Disinfection is often a stage for the reusability of treated wastewater. The main objective of disinfection is to reduce sanitary risks related to the presence of pathogens. Assessment of sanitary risks consists in indexing and identifying pathogens that may be present in such a water. Nevertheless, a serious analytical problem may be encountered, especially when the number of these pathogens is relatively low and their presence is random. A growing body of evidence has indicated that the use of indicator bacteria, which have the advantage of being abundant and easily countable, is very useful in the assessment of sanitary risks related to the presence of pathogens (Wyer et al., 1997). Water disinfection can be achieved via dierent means such as chlorination, ozonation and ultraviolet *
Corresponding author. Tel.: +216-1-788-436; fax: +216-1-430-934.
radiation (Moeller and Calkins, 1980; Come and Bariou, 1980, Anonymous, 1989). Disinfection by chlorine has gained wide acceptance commercially, probably because of its simplicity and its moderate cost; despite the major problem of secondary harmful products generated by this treatment (Pourmoghaddas and Stevens, 1995; Racaud and Rawzy, 1994; Sanchez, 1993). Disinfection achieved by such a process is often accomplished in two steps: a step of fast mixture and a step of contact between chlorine and water, during a suciently long time to allow the product to play its disinfection role. The success of the operation of disinfection depends greatly upon the conditions prevailing during the second step. Besides the physico-chemical quality of the treated water, many other factors may aect this operation, among which we ®nd the hydraulic functioning of the contactor, the kinetics of chlorine reaction with the components present in water and the kinetics of bacterial inactivation.
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In order to characterize the functioning of such a process, we have undertaken an experimental study enabling modelling of the kinetics of chlorine consumption and bacterial inactivation in secondary wastewater. Once combined with an adequate hydrodynamic model, the obtained kinetics would allow the construction of a simulation model of a chlorination reactor. The goal of this study was, therefore, to characterize the kinetics of chlorine consumption and indicator bacterial inactivation in secondary wastewater.
2. Methods 2.1. Wastewater treatment Chlorination assays were performed on samples of secondary treated wastewater (trickling ®ltrate) previously sand ®ltered in a semi-industrial pilot plant. Sand ®ltration allowed an appreciable improvement of the quality of water and resulted in a considerable lowering of the initial demand for chlorine. 2.2. Experimental procedure Disinfection assays: Experiments were performed in 3 l pyrex beakers each containing a magnetic stirrer rotating at 500 rpm. Wastewater (2 l) was mixed with 50 ml of disinfectant (sodium hypochlorite) prepared at ´ 10 the required ®nal dose. Wastewater chlorination was tested with concentrations varying from 6.5 to 25 mg/l. Samples (50 ml) were taken at regular intervals varying from 2 to 40 min, and the oxidative reaction was immediately stopped by addition of sodium thiosulphate before residual chlorine measurement and faecal bacteria enumeration. Each assay was carried out at least 4 times. In the control, 50 ml of sterile distilled water were mixed with wastewater samples instead of the disinfectant. Residual chlorine measurement: The dierent forms of residual chlorine were determined by the sulphate diethyl-p-phenylenediamine (DPD) method according to Nicholson (1965). Chlorine and derived compounds react with DPD to give a red colour susceptible of titrimetric measuring with ammonium ferrous (II) sulphate solution. Addition of potassium iodide (KI) as an oxidiser, at dierent contact times, allowed dierentiation among free chlorine and dierent combined forms of chlorine such as mono-, di-, and trichloramines. Enumeration of indicator bacteria: The number of indicator bacteria was determined based on the French standard methods of the most probable number MPN (Rodier, 1978), and the corrected MPN tables proposed by Man (1983) were used.
3. Results and discussion The goal of the present investigation was to characterize the kinetics of chlorine consumption and bacterial inactivation. The rates of reduction of faecal indicator bacteria, as a function of time of contact, in the presence of dierent chlorine concentrations were determined. The rate of inactivation was associated with a concentration, C, of residual free chlorine and with a time of contact measured using as origin time t 0 corresponding to the moment of chlorine injection into the water. 3.1. Reaction of chlorine consumption The wastewater, even when puri®ed and treated by ®ltration, contained relatively large quantities of organic and mineral matters with a BOD5 content varying from 15 to 30 mg O2 /l and a COD ranging from 20 to 30 mg O2 /l. The content in ammoniacal nitrogen ¯uctuated between 8 and 20 NH3 ±N mg/l according to the experiments. After the initial mixture of chlorine and water, there was competition among two types of reactions: · Oxido-reduction with some reducing compounds that consume a part of the injected chlorine rendering it unavailable for the disinfection. · Substitution that forms some compounds of addition or of substitution mainly with ammonia to form chloramines, which have a low bactericidal power. Fig. 1 shows chlorine consumption where only the results obtained for the concentrations of 6.5 and 13.6 mg/l have been represented. From Fig. 1, it appears that for all the studied concentrations and regardless of the time of contact: · Monochloramines and trichloramines remained very important as compared to free chlorine and dichloramines. · Dichloramines appeared as the lowest chlorine form, with a concentration usually less than 10% of the injected concentration. · Trichloramines appeared as the most important form of residual chlorine. Their levels decreased with the time of contact, to reach their lowest levels by the end of the experiment (20±30 min). These results indicated that the reaction of chlorine with ammoniacal nitrogen of the water was very rapid and that evolution of chlorine as trichloramines took place in less than 1 min. This result appears to agree with those mentioned in the literature by several authors (Soulard, 1982; Saunier, 1979; Alouini et Seux, 1987; Adin et al., 1991). According to Adin et al. (1991), based on kinetic constants of these reactions, the formation of monochloramines is the most rapid. Reactions of chloramines destruction are relatively slow and trichloramines are only gradually hydrolysed following their appearance in the medium.
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Fig. 1. Evolution of the content of residual chlorine as a function of the time of contact. Clustred error bar graphs and bars represent standard error of mean with at least n 4.
· The quantity of free chlorine, the main cause of disinfection, was variable according to the injected concentration. In contrast, it showed, on average, a low variation in the course of time for the same concentration of total chlorine. For instance, with the high chlorine concentration of 20 mg/l, the level of free chlorine was nearly constant and of the order of 3.5 mg/l (Result not shown). This result clearly indicated that after the immediate demand
of water for chlorine, conditioned by the presence of reducing compounds as well as compounds of addition or substitution, the free residual chlorine remained more or less at the same level until the end of the experiment. It appears particularly that the content of residual free chlorine in the secondary wastewater always exceeded the value of 10% of the quantity of chlorine injected at the beginning of the experiment. This result was valid at any time
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of contact studied and regardless of the chlorine concentration applied. Based on all these results, it may be concluded that the consumption of chlorine during the process of wastewater disinfection undergoes two steps: · A ®rst rapid step corresponding to the consumption of chlorine following its reaction with the reducing compounds in water. This step, called `the immediate satisfaction demand', seemed to be instantaneous. · A second step, during which residual chlorine was transformed rapidly into the free and combined chlorine forms. This step concerns particularly the process of disinfection since residual chlorine will be available to exert its antiseptic power. It is clear that the modelling of the kinetics of chlorine consumption or disinfection would concern only the second step of the reaction, which is amenable to measurement. On the other hand, as has previously been mentioned, combined chlorine (mono-, di- and trichloramines) appeared as the most important form of residual chlorine. So, the role of this form of residual chlorine could not be disregarded compared with free chlorine. In fact, free chlorine is considered as the best disinfectant, but the role of combined chlorine in the bacterial reduction is not negligible (Anonymous, 1989).
and the number of faecal bacteria. For each chlorine concentration applied, tests were conducted at progressive contact times ranging from 2 to 40 min. The three retained chlorine concentrations (6.5, 8.6 and 13.6 mg/l) were tested on wastewater with MPN of 6.02 ´ 104 to 6.02 ´ 105 of faecal coliforms per 100 ml and 3.16 ´ 104 to 5.62 ´ 105 of faecal streptococci per 100 ml. All results are reported in Table 1. The initial contents of faecal bacteria (N0 ) in wastewater diered from one experiment to another (results not shown), which was related to the quality of water used in each experiment. The speed of faecal bacteria reduction varied from one concentration to the other. The higher the initial concentration of chlorine was, the higher was the reduction of bacteria. For instance, during the same contact time of 10 min, a reduction in faecal streptococci of 1.63 and 3.36 logarithmic units was obtained with the respective initial chlorine concentrations of 6.5 and 13.6 mg/l. In order to ensure a sucient safety during wastewater reuse or rejection, the number of faecal coliforms or streptococci must be less than 103 bacteria per 100 ml (White, 1976). In this study, this objective was reached after contact times of 30, 20 and 10 min following the injection of a concentration of sodium hypochlorite of, respectively, 6.5, 8.6 and 13.6 mg/l.
3.2. Kinetic study of water disinfection by chlorine
3.3. Modelling of the kinetics of disinfection
The experimental methodology adopted consisted in measuring simultaneously residual chlorine contents
It is important to note that the approach to the kinetics of microorganism inactivation is generally em-
Table 1 Log reductions of faecal coliforms and faecal streptococci as a function of time and chlorine concentration Chlorine (mg/l)
a
Time (min)
Faecal coliforms
Faecal streptococci
(N/N0 )
Reductiona
(N/N0 )
Reduction
Free chlorine (mg/l)
Monochloramines (mg/l)
Dichloramines (mg/l)
Trichloramines (mg/l)
6.5
2 5 10 20 30 40
0.0339 0.0324 0.0234 0.0100 0.0018 0.0009
1.47 1.49 1.63 2.00 2.75 3.04
0.1185 0.0372 0.0234 0.0135 0.0020 0.0005
0.92 1.43 1.63 1.87 2.69 3.27
1.62 0.37 1.22 0.26 1.25 0.15 1.35 0.15 1.40 0.12 1.40 0.12
3.45 0.38 1.77 0.32 2.52 0.24 2.47 0.85 1.92 0.42 ND
0.97 0.20 1.07 0.48 0.47 0.05 0.52 0.12 0.67 0.14 ND
4.10 0.88 1.90 0.54 2.10 0.40 2.40 1.20 0.93 0.22 ND
8.6
2 5 10 20 30 40
0.0170 0.0174 0.0022 0.0018 0.0005 0.0005
1.76 1.75 2.65 2.74 3.28 3.28
0.1632 0.0724 0.0100 0.0052 0.0007 0.0007
0.78 1.32 2.00 2.28 3.12 4.12
2.68 0.69 1.72 0.28 3.30 0.97 1.52 0.22 2.97 0.70 2.52 0.42
5.62 0.99 4.70 1.14 4.45 1.02 4.87 0.28 2.32 1.16 ND
1.55 0.60 1.78 0.58 0.95 0.52 0.72 0.23 0.60 0.18 ND
9.60 0.70 6.40 2.50 6.46 0.60 4.00 1.86 0.35 0.04 ND
13.6
2 5 10 20 30 40
0.01000 0.00120 0.00091 0.00038 0.00034 0.00019
2.00 2.92 3.04 3.41 3.45 3.70
0.0575 0.0087 0.0004 0.00004 0.00014 0.00004
1.24 2.06 3.36 4.36 3.84 4.36
2.48 0.34 3.15 0.25 4.62 1.04 3.50 0.28 1.22 0.05 1.72 0.24
10.15 0.30 7.65 1.04 5.80 1.20 6.35 0.28 7.48 0.58 ND
0.60 0.17 1.88 1.04 0.68 0.35 1.62 0.36 1.08 0.32 ND
12.00 3.20 18.00 1.97 8.60 1.80 8.20 2.30 10.30 1.00 ND
Logarithmic units ÿlog (N/N0 ); N: Number of micro-organisms at the instant t; N0 : Number of micro-organisms at the instant t 0; : Standard error.
A. Hassen et al. / Bioresource Technology 72 (2000) 85±93
pirical, based on laboratory studies, and consequently a model is only valid in conditions similar to those of its establishment. So, concentration-time (CT) values expressed in mg.min/l were calculated by mathematical integration of the concentration of residual free chlorine in water versus time. In this paper only results of the models of Chick-Watson and Collins-Selleck will be considered as a reference and the whole results obtained with the dierent concentrations of chlorine will be combined in order to establish an expression for the kinetics of disinfection. 3.4. Chick-Watson model The expression of the kinetics of disinfection according to the model of Chick-Watson is given as follows: dN =dt ÿ KC n N with N is the number of microorganisms, C the concentration of residual free chlorine, n the coecient of dilution, which is a function of the quality of water, and K is the coecient translating the disinfecting power. Parameters to identify are K and n. By using the integrated form of the model and by changing to the logarithm: N ln K n ln C ln T ; ln ÿ ln N0 and with the help of a linear regression, we can determine the values of K and n.
89
In this way, expressions obtained for the rate of inactivation were ÿ N for faecal coliforms : exp ÿ 0:1046 C 1:1 T N0 with a coecient of determination R2 of 0.12. ÿ N exp ÿ 0:1635 C 0:7 T for faecal streptococci : N0 with a coecient of determination R2 of 0.42. An illustration of these adjustments is given in Fig. 2. Fig. 2 shows an important variation between the measured and the calculated values. Consequently, the model of Chick-Watson was not representative of the kinetics of disinfection. Therefore, a second approach to modelling was adopted and an initial microbial reduction, just at the moment of contact of water with chlorine, was considered. The model becomes in this case: N =N0 A exp ÿKC n T with A is the initial reduction just at the moment of contact of water with chlorine, The expressions obtained for the rate of reduction were: ÿ N 0:011 exp ÿ 0:0369 C 1:1 T for faecal coliforms : N0 with a coecient of determination R2 of 0.63. for faecal streptococci :
N N0
ÿ 0:0727 exp ÿ 0:1065 C 0:7 T
Fig. 2. Determination of the kinetic of disinfection of faecal coliforms and faecal streptococci according to the model of Chick-Watson. N: Number of micro-organisms at the instant t; N0 : Number of micro-organisms at the instant t 0; C: Free chlorine concentration; n parameter n of the model; T: Time; x C n T ; Symbols, measured; lines, calculated.
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with a coecient of determination R2 of 0.78. An illustration of these adjustments are given in Fig. 3. Considering, v " u #2 uX N N ÿ t ; N0 cal N0 exp which is a parameter representative of the deviation among calculated and measured values, the couples of values of before (0 ) and after (m ) modi®cation of the model (0 1.2250; m 0:0393) and (0 0.9200; m 0:1319), obtained, respectively for faecal coliforms and faecal streptococci, indicated that the modi®ed model of Chick-Watson N =N0 A exp ÿKC n T better described the kinetics of bacterial disinfection than did the original form of the model N =N0 exp ÿKC n T . This ®nding is also con®rmed by Fig. 3 which indicates that the modi®ed model of Chick-Watson describes the kinetics of bacterial disinfection when an initial reduction was considered. 3.5. Model of Collins-Selleck The same procedure was applied for the model of Collins-Selleck. Parameters of identi®cation of this model are s and n with
N 1 for CT 6 s; N0 s n N for CT P s: N0 CT By exploiting all experimental points, and by changing to the logarithmic form (ln N =N0 n ln s ÿ n ln CT ), the values of s and n were determined using a linear adjustment. The obtained expressions were: For faecal coliforms: N 1 for CT 6 0:2028; N0 1:2664 N 0:2028 for CT P 0:2028 with r2 0:69: N0 CT For faecal streptococci: N 1 for CT 6 1:9068; N0 2:276 N 1:9068 for CT P 1:9068 with r2 0:80: N0 CT The model of Collins-Selleck has been used by Qualls and Johnson (1985) and Montgommery (1985) for different kinds of wastewater. In those studies, chlorine and dioxide of chlorine were used as disinfectants. These authors reported s 4:06 and n 2.82 for faecal coliforms. These values are very dierent from those obtained in our study (s 0:2028 and n 1.266), which
Fig. 3. Determination of the kinetic of inactivation of faecal coliforms and faecal streptococci according to the model of Chick-Watson with an initial reduction just at the moment of contact of water with chlorine. N: Number of micro-organisms at the instant t; N0 : Number of micro-organisms at the instant t 0; C: Free chlorine concentration; n parameter n of the model; T: Time; x C n T. Symbols, measured; lines, calculated.
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91
Fig. 4. Determination of the kinetic of disinfection of faecal coliforms and faecal streptococci according to the model of Collins-Selleck. N: Number of micro-organisms at the instant t; N0 : Number of micro-organisms at the instant t 0; C: Free chlorine concentration; m parameter m of the model; T: Time. Symbols, measured; lines, calculated.
may be explained by the quality of the water and the operating conditions. Fig. 4 shows rather large discrepancy between the measured and the theoretical values for faecal coliforms, indicating that the model of Collins-Selleck was not representative of the obtained experimental results. In order to improve the representativeness of this model, a value m was imposed on the concentration of residual chlorine, since it appeared clearly from the Figs. 2±4 that the concentration of chlorine in¯uenced the output of disinfection more than did the time of contact. Indeed, for a ®xed contact time, the reduction of faecal coliforms and faecal streptococci was as great as the concentration of applied chlorine was high. By contrast, for a ®xed chlorine concentration, bacterial reduction progressed relatively slowly as a function of the time of contact. The model becomes in this case: N 1 for CT 6 s; N0 s n N for CT P s: N0 CmT and the parameters to be identi®ed are s, n and m. By changing to the logarithm: N ln n ln s ÿ nm ln C ÿ n ln T ; N0 the values of n, m and s were determined using a linear regression. The expressions obtained were:
For faecal coliforms: N 1 for C 1:8 T 6 0:2484; N0 1:1775 N 0:2484 for C 1:8 T P 0:2484 with r2 0:74: N0 C 1:8 T For faecal streptococci: N 1 for CT 6 1:5783; N0 2:3091 N 1:5783 for CT P 1:5783 with r2 0:80: N0 C 0:68 T Fig. 5 indicates, particularly for faecal coliforms, that the model of Collins-Selleck in the modi®ed form: n N =N0 s=C m T allows a better description of the kinetics of decontamination than does its original form: N =N0 s=CT n : Indeed, when we calculated the deviation r i2 Xh N =N0 cal ÿ N =N0 exp for the two models, the values obtained according to the modi®ed form of the model of Collins-Selleck were lower than those corresponding to the original form of the same model (Table 2). Also, the dierent expressions obtained according to the kinetic approaches of both Chick-Watson and Collins-Selleck are reported in Table 2.
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Table 2 Expressions obtained according to the kinetic models studieda Model
Expression
R2
FC
Chick-Watson Chick-Watson modi®ed Collins-Selleck Collins-Selleck modi®ed
N N0 N N0 N N0 N N0
exp ÿ0:1046 C T ÿ0:011exp ÿ0:0269 C 1:1 T 1:2664 ÿ0:2028 CT 0:2484 1:1775 C1:8 T
0.12 0.63 0.69 0.74
1.2250 0.0393 0.0327 0.0248
FS
Chick-Watson Chick-Watson modi®ed Collins-Selleck Collins-Selleck modi®ed
N N0 N N0 N N0 N N0
exp ÿ0:1635 C 0:7 T ÿ0:0727 exp ÿ0:106C 0:7 T 2:276 ÿ1:9068 C T 2:3091 C1:5738 0:68 T
0.42 0.78 0.80 0.80
0.9200 0.1319 0.2227 0.1900
Bacteria
a FC: Faecal coliforms, FS: Faecal streptococci, R2 : Coecient of determination, : Deviation among calculated and measured values, N: Number of micro-organisms at the instant t; N0 : Number of micro-organisms at the instant t 0; C: Free chlorine concentration; T: Time.
Fig. 5. Determination of the kinetic of disinfection of faecal coliforms and faecal streptococci according to the modi®ed model of Collins-Selleck. N: Number of micro-organisms at the instant t; N0 : Number of micro-organisms at the instant t 0; C: Free chlorine concentration; m parameter m of the model; T: Time; x C m T . Symbols, measured; lines, calculated.
4. Conclusions The following conclusions may be drawn from the present kinetics study: · Monochloramines and trichloramines appeared as the most important residual chlorine forms as compared to free chlorine and dichloramines. · The high levels of trichloramines showed clearly that the reaction of chlorine with ammoniacal nitrogen is very fast and that evolution of chlorine into trichloramines is carried out in a time shorter than 1 min. · The original model of Chick-Watson ln N =N0 ÿKC n T was not able to describe the inactivation kinetics of indicator bacteria. Therefore, a modi®cation, based on the same model but after taking into consideration an initial inactivation during the con-
tact of water with chlorine ln N =N0 A exp ÿKC n T , described very well the kinetics of disinfection of faecal coliforms and faecal streptococci. · The same remarks were valid for the model of Collins-Selleck, which, in modi®ed form, N =N0 n s=C m T described the kinetics of disinfection better than did the original form. Acknowledgements This investigation was supported by grants from the International Foundation for Science (H-2327, IFS, Sweden) and from the CEE (Avicenne No. 93 AVI 054). We thank Professor Jean J. Damelincourt, Centre de
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