CHLORINE-FREE DISINFECTION OF WATER CONTAMINATED WITH E. Coli B BY TREATMENT WITH AN ALTERNATING CURRENT

Mostafa Rais El Fenni,
Micro-Tracers, Inc , San Francisco, CA


CHLORINE-FREE DISINFECTION OF WATER CONTAMINATED WITH E. Coli B BY TREATMENT WITH AN ALTERNATING CURRENT

Introduction
Water, both in drinking and wastewater applications, contains microorganisms. Various water treatment systems are provided for disinfecting water to a level suitable for human and animal consumption. Various electrical processes have been proposed for disinfection and sterilization of contaminated waters and food. They have included ohmic heating via the passage of current at moderate voltage, high-voltage pulsing that disrupts and otherwise destroys bacteria, and electrolytic processes that produce oxidants at low voltage.
It is known that the mechanism of electrolytic disinfection depends on many parameters, including the nature of the electrolyte [1-3]. Recently the mechanism of electrochemical (EC) wastewater disinfection has been discussed by Li et al. [4] who investigated the behavior of E.coli in solutions of NaCl, NaNO3 and Na2SO4. The experimental results do not favor the hypotheses that the EC bactericidal action was due to cell destruction by the electric field and the production of persulfate [5-8]. In comparison to direct chlorination, the EC process displayed a much stronger disinfecting capability than that of electrochlorination assumed for EC disinfection. Study of the E.coli bacteria of wastewater treated by different means of disinfection suggested that the cells were likely killed during the EC treatment by chemical products with oxidizing and germicidal powers similar to that of ozone and much stronger that of chlorine.
All the findings [4] support the theory that the major killing function of EC disinfection is provided by short-lived and high-energy intermediate EC products, such as free radicals.
In the present study deionized water, highly contaminated with E.coli and containing ammonium sulfate as an electrolyte, was treated in a circulating system with use of electrochemical cell with 10 stainless steel electrodes and alternating current. The object was to verify the effectiveness of the process, to reveal the influence of some parameters such as treatment time, concentration of an electrolyte in solution and starting concentration of bacteria on the effectiveness of the method, to propose a kinetic model of bacteria destruction and to find quantitative method to describe the rate of interaction between hydroxyl radicals and bacteria. The concentration of hydroxyl radicals formed by water discharge during electrolysis was estimated using N,N-dimethyl-p-nitrosoaniline (RNO) as a spin trap.
Experimental Part
Test water and bacterial counts
Deionized , DI, water i.e. top water purified with Ultra Pure Water System, Model D8961, Barnstead/Thermolyne (Dubuque, IA) was used for the experiments before it has undergone any treatment. The preliminary bacterial load of the DI water was performed by using the Pour Plate Method for Heterotrophic Plate Count. Colonies were manually counted using a Leica Dark Field Colony Counter according to [8].
The same test was performed for samples of water prepared by addition of electrolyte – ammonium sulfate with concentration, 0.025, 0.05, 0.2 and 0.5% and N,N-dimethyl-p-nitrosoaniline, RNO used as a spin trap at concentration from 0.0027 to 0.003% and the progress of the hydroxyl radicals generation can be monitored by measuring the bleaching of the yellow color with maximum around 440 nm.
The initial concentration of bacteria in the contaminated water used in experiments was between 0.95 x 106 and 6.92 x 106 CFU/ml.
Electrochemical Treatment
The experimental set up employed is shown on Fig.1. This apparatus includes the pump, 1, Flow meter, 2, transparent glass tube 3, which can be irradiated by visible light from the luminescent lamp, 4 (in this part of the study in will be not applied), plastic electrochemical cell, 5, with 10 electrodes made of round shape stainless steel screen installed parallel to each other at distance of 3 mm apart, with diameter 5.5 cm. The choice of AC (60Hz) and an initial current of 0.21 A corresponding to a current densities 60 ma/cm2 was based on fact that there is no danger of corrosion of the stainless steel electrodes in chosen range of ammonium sulfate concentration in water.
RESULTS AND DISCUSSION
Preliminary measurements were carried out in order to see whether the method employed was capable to destroy bacteria. 2000 ml of 0.2% aqueous solution of ammonium sulfate with starting population of E. coli of 0.9 x 106 CFU/ml was treated with a current density of 60 ma/cm2 .The microbiological analysis showed the at least 3.9 log reduction in population of bacteria within 60 min of treatment.
The dependence of the logarithm of number of living bacteria (Log n) in the treated deionized water containing E. coli on treatment time (t) is depicted on Fig. 2 . The first conclusion which can be made from these data is that the electrochemical treatment (series 1,3) is much more effective sanitizing method than pumping the contaminated water through the screen electrodes without the voltage applied (series 2, 4 ). The second conclusion is that there is a is a linear relation between Log n and t in case of the electrochemical treatment for both types of electrolytes.
On next step we applied the modified kinetic model which was originally developed for sanitizing natural water, contaminated with coliforms, that was electrochemically treated in a stirred batch system with the use of two titanium electrodes and direct current, the polarity of which alternated automatically in half cycles of 1 min [6]. According to this modified model, the initial number of bacteria no, CFU/ml, and current number of bacteria n, CFU/ml, in a moment t is related as follows:
Log n = log no - k’ t (1)
where k’ is a constant depending on the value of no and on current density for a constant volume of treated water and a constant surface area of electrodes.
The values of the constant k’ and the times td in which the straight lines intersect the time axis, i.e. the minimum time needed for complete disinfection, for all four experiments with different concentration of electrolyte were calculated. Consequently the values constants k1 = k’/log no = 1/td were calculated (Table 1). From the values of the correlation coefficients (r) which are close to 1 the validity of the linear model is clear. It is obvious that the k1 and td values are independent of the no values.
In the same series of experiments, which are summarized in Table 1 and Fig. 2, the monitoring of the formation of hydroxyl radicals by reaction was performed. The generation of hydroxyl radical during electrochemical treatment of contaminated water has been successfully used [9] for combustion of organic pollutants, such as phenol.
Table 1 – Values of constants k’, k1, time needed for complete disinfection td, and correlation coefficient r,
extracted from fitting the experimental results
Exp. # Concentration of electrolyte, % Log no k’ r k1
min-1 td,
min
1 0.025 6.12 0.0599 0.9807 0.00972 102.8
2 0.05 6.10 0.0637 0.9897 0.01044 95.8
3 0.2 5.95 0.0665 0.9863 0.01119 89.4
4 0.2 6.19 0.0658 0.9804 0.01063 94.1
5 0.2 6.28 0.0632 0.9746 0.01007 99.3
6 0.2 6.92 0.0661 0.9860 0.0096 104.7
7 0.5 6.23 0.0698 0.9720 0.01120 89.3

The indirect technique for the detection and identification of low concentration of hydroxyl radicals which involves trapping of the .OH radical by an addition reaction (spin trap) to produce a more stable radical (spin adduct) was used:
.OH + Spin Trap  Spin adduct
The advantage of RNO for the detection of .OH radicals formed by water electrolysis is that RNO at electrochemically inactive [9]. Besides, the reaction of RNO with hydroxyl radicals is very selective and has the high rate (constant is 1.2 x 10-10 mol-1 s-1).
Fig.3 shows the absorbance spectra of RNO at concentration 2.0 x 10-6 mol/l in water containing 0.5% of ammonium sulfate before pumping through the electrochemical cell and after pumping through it for 5, 20, 40 and 60 min under electrochemical conditions in case of presence of E.coli.
If we neglect the possibility of other reactions with participation of hydroxyl radicals except interaction with RNO and destruction of bacteria, the quantitative consideration of these radicals consumption during electrochemical treatment can be described as simple competition reactions:
kOH
.OH + RNO  R-(HO)NO.
k’OH
.OH + E. Coli B  dead bacteria
Under this assumption it is possible to use the kinetic model proposed for description of two competitive reactions with participation of .OH radicals [9]:
1/Gt = 1/G o x { 1+(k’OH [B]) /(kOH [RNO]} (2)
where Gt is bleaching rate of RNO in the presence of bacteria; Go is bleaching rate of RNO in the absence of bacteria; [B] is bacteria concentration in the electrolyte; is RNO concentration in the electrolyte; k’OH and kOH are the rate constants of the corresponding reactions.
According to equation (2), a plot of 1/Gt vs [B]/ [RNO] should yield a straight line with slope 1/G ox( k’OH / kOH), from which the relative rate constant for the reaction of hydroxyl radicals with bacteria can be calculated . The concentration of RNO was taken for measurements after 40 min of electrochemical treatment for four experiments with different concentrations of bacteria: 0.9 x 106 , 1.55 x106 ,1.9 x 106 and 8.5 x106 CFU/ml (Fig. 4).
Data from Fig.4 have been used for graphical representation on the plot of 1/Gt vs [B]/ [RNO] which allowed to determine the value of k’OH = 6.01 x 106 CFU x s-1 (the rate constant between RNO and hydroxyl radical is equal to 1.2 x 1010 M-1s-1 [9]).
CONCLUSIONS
1. AC electrochemical disinfection is an effective method for sanitizing DI water contaminated with E. coli.under chlorine-free conditions with using ammonium sulfate as electrolyte.
2. The formation of hydroxyl radicals during AC electrolysis of aqueous solutions of ammonium sulfate, which has been experimentally confirmed by using N,N-dimethyl-p-nitrosoaniline (RNO) as a spin trap, seems to make a significant contribution in sanitizing action of electrochemical treatment.

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