Étude expérimentale et modélisation de la désinfection par le chlore des eaux usées épurées Experimental study of waste water chlorination and application to the design of chlorine contact facilities

Résumé

Pour étudier la désinfection d'une eau usée épurée au stade secondaire, traitée à l'hypochlorite de sodium, des essais en réacteur fermé ont été effectués en utilisant des doses variant entre 1 et 10 mg de chlore par litre. Les résultats obtenus montrent que la cinétique de désinfection est loin d'être uniforme. L'utilisation du modèle de Chick et Watson n'est en effet possible que si on l'adapte pour tenir compte de la modification de la vitesse de désinfection au cours du processus. Le modèle de Collins et Selleck permet de rendre compte de façon satisfaisante de l'évolution de la vitesse d'élimination des germes au cours du temps. La faible valeur du paramètre t trouvée (0.26 min.mg/l pour les coliformes totaux et 0.58 min.mg/l pour les coliformes fécaux) semble cependant démontrer que la période de latence est relativement peut importante surtout lorsqu'on utilise des dose de chlore élevées. Il s'avère d'autre part que la demande en chlore de ce type d'eau est très importante. La concentration en chlore résiduel dans le réacteur décroît très rapidement pour atteindre environs 10 % de la dose de chlore injectée et cela quelle que soit la dose utilisée (de 1 à 10 mg/l). Le dimensionnement des réacteurs de désinfection fonctionnant en continu nécessite de prendre en compte le comportement hydrodynamique de l'eau dans le réacteur. Sachant qu'un abattement de 3 U-Log est nécessaire, dans le cas de la réutilisation de l'eau pour l'irrigation, un modèle intégrant l'expression de la cinétique de désinfection et l'hydrodynamique du contacteur a été proposé. Les résultats mettent en évidence l'intérêt de concevoir des réacteurs se rapprochant le plus possible de l'écoulement piston.

Mots-clés:

• Désinfection,
• chloration,
• eau usée urbaine,
• cinétique,
• réacteur de désinfection

Abstract

Secondary wastewater is considered as an important additional resource of water in a country, which has a semiarid climate as Tunisia. On quality basis, the use of water for irrigation is governed by chemical parameters of the water that affect plant, soils conditions and the underlying groundwater. Treated urban wastewater is susceptible to be used in irrigation without major risks (see table 1). However, the use of such a water can represent a risk of contamination of edible crops, pasture lands, and feed crops by direct contact with disease agent carried in reclaimed water or aerosols from spray irrigation. These sanitary risks can be considerably reduced with the practice of an efficient disinfection. Chlorinating is one of the simplest and less expensive disinfection processes. The objectives of this work are the study of the disinfection kinetics and the rate of exertion of chlorine demand of water, when the sodium hypochlorite is used for treating a secondary wastewater. for this, a batch reactor tests have been done. After applying a dose of chlorine (between 1 and 10 mg/l), we have determined the evolution with time of chlorine residual concentration and rate of inactivation of the total and fecal coliform.

Chlorine demand

To describe chlorine decay in the complex milieu of water, like wastewater, Haas and Karra (1984) have developed followed equation:

C=C₀[X_E^-k1t+(1-X)e]^-k2t

where:

C : concentration of residual chlorine, mg/l

C₀ : dose of chlorine, mg/l

X : empirical constant

t : contact time, min

k₁and k₂ : rate constants, min^-1

In the case of wastewater chlorinating, we can ascertain that the chlorine concentration in the batch reactor decrease rapidly, in the beginning of the reaction, and very moderately after satisfaction of the initial chlorine demand. This initial chlorine demand is very important. It represents near 90% of the dose injected in the reactor. The chlorine concentration decay can be described by Haas and Karra equation with X=0.9, k₁=3 min^-1 and k₂=0.001 min^-1.

After some minutes of contact time, we can notes, see figure 2, that the chlorine concentration decay is susceptible to be approximate by a first order equation:

t ≥ 2min ⇒ C ≅ 0.1e^{-0.001 t}

The concentration of residual chlorine becomes practically constant in the reactor, once initial chlorine demand has been satisfied, with C ≅ 0.1 C₀.

Disinfection kinetics

Using a pseudo first order Chick and Watson model reveals that the rate of inactivation of coliform bacteria is not uniform. We can employ this model to fit experimental data only when we subdivide the process in two stages characterised by different kinetics. for example, the logarithm of fecal coliform survival rate can be expressed by the relationships:

for C ∙ t ≤ 2.85 min ∙ mg/L Ln(N/N₀)= -1.65C ∙ t

for C ∙ t ≥ 2.85 min ∙ mg/L Ln(N/N₀)= -3.89-0.29C⋄t

Collins and Selleck (1972) developed a general kinetic expression for the effect of combined chlorine residual on both total and fecal coliform. Combining the work of chick and Gard (Gard, 1957) they developed the following formula describing the survival of these bacteria:

for C∙t<τ (N/N₀)=1

for C∙t>τ (N/N₀)=(τ/C∙t)ⁿ

where C : the combined chlorine residual, mg/l

t : contact time, minute

t : an environmental coefficient or induction time

n : Constant

In this formula, we have assumed that the concentration of chlorine remains constant. We can apply this model when we consider that C is the chlorine concentration after the immediate wastewater demand had been satisfied.

Using Collins and Selleck disinfection model to plot the survival rate of total and fecal coliform in chlorinated secondary effluent, we have obtained the coefficients below:

for total coliform : n=1.94 t=0.26 min∙mg/l

for fecal coliform : n=3.1 t=0.58 min∙mg/l

The constant τ represents the time required for the disinfectant to diffuse through the cell wall and begin its disinfectant activity. we can see that this initial lag time is very short. The disinfectant becomes rapidly efficacious to inactivate coliform bacteria.

Design of disinfectant contact facilities

So as to estimate the influence of the hydraulic efficiency of chlorine contact chambers on disinfection process performance, we have compared efficiency of two ideals reactors: the completely stirred tank reactor and plug flow reactor. The kinetic equations presented above had been established in a batch reactor. To obtain the mean conversion in the entire contactor, one has to apply the segregated flow model. Using the concept of segregated flow system, the survival ratio will be the sum of the batch reactions of all small aggregates, or:

           ∞

(N/N₀)=∫(N/N₀)_batchE(Ɵ)dƟ

           0

Ɵ : normalized contact time (contact time/hydraulic residence time)

E(Ɵ) : normalized frequency distribution of ages

In completely mixed regime, fluid particles are exponentially distributed throughout a tank until the fluid properties exhibited by the effluent leaving the unit are identical to those within the unit. Using Collins and Selleck model, the survival ratio corresponding to hydraulic residence time Ts and to the residual chlorine concentration C is expressed by equation:

           ∞                      (τ/C∙T_s)           ∞

(N/N₀)=∫(N/N₀)_batche^-ƟdƟ = ∫ e^-ƟdƟ + ∫ (τ/C∙t)^me^-ƟdƟ

           0                            0         (τ/C∙T_s)

The survival ratio for a hydraulic residence times between 5 min and 60 min and a residual chlorine concentration between 0.2 mg/l and 1 mg/l are reported in table 3 and table 4.

In a plug flow regime all particles entering in a basin have equal velocity values, travel on parallel flow paths, and remain in the unit for an identical period known as the hydraulic residence time. the performance of such a reactor is identical at the batch reactor performance. In table 5 and table 6 we have reported the survival ratio given by plug flow reactor with residence time until 60 min and residual chlorine concentration between 0.2 mg/l and 1 mg/l.

Knowing that the secondary wastewater reuse necessitates a 3 U-Log fecal coliform inactivation, we can see clearly the importance of reactor design on his performance. Indeed, to reach a 3 U-log inactivation, it is sufficient (see table 6) to use a dose of 2 mg/l of chlorine and a hydraulic residence time of 30 minutes, in plug flow reactor. Such dose and the same residence time given only a 1 U -Log inactivation (see table 4) when the reactor is completely mixed.

Keywords:

• Disinfection,
• chlorination,
• wastewater reuse,
• disinfection kinetics,
• chlorine contact basin