Modeling of Ozone as a Disinfectant of Indicators Bacteria in the Drinking Water

Background: Ozone is considered one of the most effective disinfectants that can inactivate resistant pathogenic microorganisms in which conventional disinfectant such as chlorine and chlorine dioxide fail due to its strong biocidal oxidizing property. Objective: The study aimed at modeling of ozone as a disinfectant of indicators bacteria in the drinking water. Methods: This work involved a series of batch experiments with raw water, taken from the intake of El-Nozha Water Purification Plant, Alexandria governorate. The ozone doses applied in this study were 1.2, 2.2, 3.4, and 4.3 mg/l. The disinfected effluent was collected at 5, 10, 20, and 30 min. The indicator microorganisms HPC bacteria, total coliforms, faecal coliforms and Streptococcus faecalis were examined before and after the ozonation. Results: The optimum ozone dosage applied in raw water to achieve 90% reduction of the indicator microorganisms was 2.5 mg/l. The optimum contact time to achieve 90% reduction of the indicator microorganisms in raw water was 8 min. By applying of ozone as post disinfection on filtered water, the reduction percent of the indicator microorganisms were 100%. This study estimated that the ozone cost was 1.76 piasters/m. Conclusion and Recommendations: Ozone as a primary disinfection must be applied on raw water to reduce the formation of THMs due to pre-chlorination of raw water at present, in addition to effective killing power of ozone onto microorganisms that will improve water quality.


INTRODUCTION
that remove many microorganisms from the water along with the suspended solids.
But these processes are not sufficient to ensure the complete removal of pathogenic bacteria or viruses.To accomplish this, the final treatment process in water treatment plant is disinfection. (1)e primary purpose of disinfecting raw water is to destroy and eliminate pathogenic organisms responsible for water borne-diseases.Monitoring of microorganisms in full-scale systems enables to assess contaminations in the raw water and the improvement through water treatment. (2)ere are several chemical oxidants used as disinfectants.Chlorine is the most commonly used disinfectant, but it reacts with organic matter present in most water sources to form chlorinated compounds, primarily trihalomethans (THMs).Most trihalomethans are of public health concern that (THMs) may cause cancer to humans. (3)onsequently, substantial Departures from Chick's law are common.
Rates of kill may increase or decrease with time rather than remain constant with time. (5)To formulate a valid relationship for the kill of organisms under a variety of conditions an assumption made is that: So, the purpose of this study was to set up modeling of ozone as a disinfectant of indicators bacteria in the drinking water.

MATERIAL AND METHODS
The present study was performed at the High Institute of Public Health laboratory, Alexandria University.

Study setting:
Ozonation experiments were performed with raw water taken from EL-Nozha Water Purification Plant in Alexandria governorate.

Sampling and analyses:
Grab samples of raw water (undisinfected water) and disinfected water were collected for microbiological analysis.The second opening tube allow the water sample withdrawal in which the end of the tube reached the middle of the reactor, while the third one allow the excess gases to be vented out.

HPC bacteria:
Table These results had major consequence for the design of ozone contactor for raw water disinfection demonstrating that no long contact time chamber is necessary. (10)ank et al results was that the reduction in faecal coliforms achieved in the ozone contactor were reported 99% -99.99% by applying ozone dose of 6 mg/L and contact time 10 min. (11) the present study, the failure to achieve high levels of disinfection e.g.

99.
9% can be attributed to some protection afforded by turbidity present in raw water; the average value of turbidity of raw water was 10 nephelometric turbidity units.This explanation is consistent with the fact that protection by solids may results in the diminution of microbial activation.
Foster et al found that turbidity levels of 5 nephelometric turbidity units provided protection of faecal coliforms when the ozone residual was ≤ 0.1.However no protection from disinfection was observed when the turbidity was ≤ 1 nephelometric turbidity units. (12)dde et al determined that turbidity was the most significant factor influencing faecal coliforms inactivation by ozone in waste water.The increased turbidity levels could have afforded the microorganism physical protection from disinfection. (13)e results indicated that that the HPC bacteria was more resistant to ozone Bull High Inst Public Health Vol.39 No.4 [2009]   disinfection than faecal coliforms.These results agree with Wolfe et al results.The resistance of fecal streptococci to ozonation was greater than that of faecal coliforms. (14)i) Kinetics of disinfection The    (16)  The study estimated that the ozone cost was 1.76 piasters/m 3 .Previous study carried out by Dyksen et al showed that ozone disinfection is the most costeffective disinfection treatment. (17)

CONCLUSIONS
The ozone dosages had significant effect on the reduction of HPC bacteria, total coliforms, faecal coliforms, and Ozone as a primary disinfection must be applied on raw water to reduce the formation of THMs due to pre-chlorination of raw water at present, in addition to effective killing power of ozone onto microorganisms that will improve water quality.
Drinking water contaminated with pathogenic microorganisms may be a major source of infectious diseases.The conventional water treatment processes including pre-chlorination, coagulation, flocculation, sedimentation and filtration Bull High Inst Public Health Vol.39 No.4 [2009] Cryptosporidium parvum oocyst) in which conventional disinfectant such as chlorine and chlorine dioxide fail due to its strong biocidal oxidizing property.Ozone is very effective against bacteria.Studies have shown the effect of small concentrations of dissolved ozone on E. coli and Legionella pneumophila. (4)It has been observed that for a given concentration of a disinfectant, the longer the contact time, the greater the kill.The kinetics of disinfection follows the time rate of kill described by Chick's law of disinfection dt dN = -kNt This law states that number of organisms destroyed per unit time is proportional to, the number of organisms remaining, where; Nt is number of organisms at time t, k is rate constant.

Figure ( 2 )
Figure (2) shows the relationship between the reduction percentage of HPC

Figure ( 7 )
Figure (6): Optimum Contact Time for Death of HPC Bacteria 2. Total Coliforms: Figure (7) shows the relationship between the reduction percentage of total coliforms and contact time.It is obvious from the figures, the reduction percentage reached its maximum ( over 90%) rapidly at time 8 following figures (10-13) illustrate the kinetics of disinfection of HPC bacteria, total coliform, faecal coliforms and streptococcus fecalis by applying various ozone dosages (1.2, 2.2, 3.4 and 4.3 mg/l) and contact time of (5, 10, 20 and 30 min).The following equation represents the kinetics of disinfection: Ln (N/N0) = kt m Where: (N/N0) is survival ratio of microorganisms, (N) is number of organisms at contact time (t) min, (N0) is number of organisms before disinfection, (k) is kinetic rate constant min -1 , and (m) is kinetic parameter.

Figure ( 10 ):
Figure (10): Kinetics of Death of HPC Bacteria by Applying Various Ozone Dosages prices 7.04 L.E.Ozone dosage applied = 2.5 mg/L = 2.5 × 10 -6 kg/ L Ozone costs = price × ozone dosage applied = 7.04 ×2.5 × 10 -6 L.E / L = 17.6 × 10 -6 L.E / L= 1.76 piaster/ m 3 Streptococcus fecalis.Greater reduction levels of all indicator organisms were achieved by increasing the applied ozone dosage.When the concentration of ozone is increased, the time required to obtain a certain level of disinfection (% reduction) decreased.The optimum ozone dosage applied in raw water to achieve 90% reduction of HPC bacteria, total coliforms, faecal coliforms and streptococcus fecalis was 2.5 mg/l.The optimum contact time to achieve 90% reduction of HPC bacteria, total coliforms, faecal coliforms and streptococcus fecalis in raw water was 8 min.