A Note on the Variation of the Rate of Disinfection with Change in the Concentration of the Disinfectant

Watson, H E

doi:10.1017/s0022172400015928

Cited by 291 publications

(159 citation statements)

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“…From the values of k at different concentrations and at different temperatures, the concentration exponent n' (Watson, 1908) and the temperature coefficient 0 (Chick, 1908(Chick, , 1910Phelps, 1911) can be estimated; and from n the true reaction velocity (constant at all concentrations) can be calculated from the formula -B KCn't = log1o b' where K is the true reaction velocity, C is the concentration of disinfectant, n' the concentration exponent, t the time necessary for disinfection, B the initial number of organisms, and b the final number of organisms. These characteristics K, n', and 0, Phelps suggests, should be deterinined for each disinfectant and would form a much more accurate and complete estimation of disinfectant activity than any tests based on end-point methods or the calculation of a coefficient from one estimation at one concentration.…”

Section: Techniquementioning

confidence: 99%

“…Phelps had assumed that all organisms died in every solution at a constant rate, and if this assumption had been true, his suggestion would have been useful; but the existence of the lag phase complicates matters, for the lag phase is likely to vary in extent, and consequently affect the relative reaction rates. This means that it is difficult to use relative mean k's for comparing bactericidal solutions, but this fact does not affect Phelps 's (1911) and Watson's (1908) (Withell, 1942) that the lag phase, observed when the logarithm of survivors is plotted against time, is a manifestation of the distribution of resistances of the organisms. The distribution of resistances of bacteria has been shown to be approximately normal when measured by the logarithm of the survival times.…”

Section: Techniquementioning

confidence: 99%

“…The phenol coefficient is, however, applied to the pure test bacteticide and the implied suggestion is that this value is a constant for this substance irrespective of its concentration. The effect of increase of concentration, or dilution of a bactericide, is determined by its concentration exponent n' (Watson, 1908;Tilley, 1939). Chick (1930 , Table III) has tabulated values of n' for different bactericides.…”

Section: Techniquementioning

confidence: 99%

“…When n' for each substance is found, the relative efficiency of the bactericides can be calculated at any concentration (Watson, 1908;Tilley, 1939). Watson (1908) showed that concentration and time of disinfection of any bactericide are related by the general formula Cn't=constant, where C is the concentration, n' is the concentration exponent, and t the time of disinfection. If t is transferred to the value of the time for 50 percentage response, then the difficulty of end-points is avoided and the sense of the equation preserved.…”

Section: E R Withell 349mentioning

confidence: 99%

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The evaluation of bactericides

Withell

1942

Epidemiol. Infect.

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The Rideal-Walker and Chick-Martin tests depend on end-point methods for comparing bactericides. End-point methods have a number of disadvantages: (1) It is difficult, if not impossible, to estimate end-points accurately. (2) The nature of the test demands a standard time. The results at any standard time will be arbitrary. If the sensitivity of the organisms varies in the two solutions tested, the coefficients obtained will vary with the time chosen as standard. It is impossible to estimate the manner in which the survivor times of the organism are distributed, by end-point methods, and therefore the standard tests in current use do not ascertain whether the result is arbitrary or not. This can only be appreciated if the tests are repeated using different times.Apart from the difficulties associated with end-points, the coefficients obtained in the present tests are calculated from one concentration of phenol and one concentration of test bactericide. This is also an arbitrary value, for the relative activity of two bactericides will depend on relative values of the concentration exponent,n, for each bactericide; if both phenol and the test bactericide have identical values ofnthe coefficients will be the same at all concentrations which are compared, but if values ofnare different then different coefficients will be obtained for every different concentration compared. As a corollary, unless the value ofnfor the test bactericide is the same as that of phenol, it is illogical to transfer the phenol coefficient obtained at one concentration to the pure substance, for this procedure implies that the phenol coefficient applies to any concentration of the test bactericide. This is only true when the concentration coefficients of phenol and test bactericide are the same. The standard methods offer no value forn.All these difficulties can be avoided if the rate of death is followed in both phenol and test bactericide, by viable counts. Two methods of interpreting the results are discussed:(1) The comparison of reaction rates, calculated by the formulaThis method is complicated by the occurrence of lag period at the start of the reaction between organism and bactericide. This lag period varies in extent in different experiments (often extending up to 50% of the deaths) and makes the interpretation of the results difficult. The lag period has been shown to be a manifestation of, the distribution of logarithmic survival times (Withell, 1942), and is a function of the standard deviation of the logarithmic survivor times (λ). This fact is taken into account in method (2).(2) The use of the time required for 50% response as a method of comparing bactericides. This time has been symbolized L.t. 50 and is analogous to the lethal dose for the 50% response (l.d. 50) widely used in biological assay. If a logarithmic time scale is used, the logarithmic survival times are found to be approximately normally distributed and consequently probit-logarithm of time lines are straight and in the case of phenol 0·5 % and para-chlor-meta-cresol 0·05% these lines are parallel. In this case a logarithmic time scale means that thecomparativetimes for all percentage responses are equal. In cases where the probit-logarithm-of-time lines are not parallel this would indicate that the standard is unsuitable for the test, forany methodsof assay based on a use of a standard to which the organism gives a different response-time relation from that which they give to the test substance or bactericide, are likely to give arbitrary results.L.T. 50 has been chosen because of the simplicity of the idea underlying it, and because a similar function, L.D. 50, has been used with success in biological assay work. There is also a solid mathematical backing for the use of L.D. 50 in assays involving biological response.It is suggested that by means of l.t. 50, phenol and the test bactericide can be compared in the following way:(a) determine the relative speeds of action of the two solutions at any convenient concentrations;(b) determine the concentration coefficient (ή) of both phenol and test bactericide by means of comparative l.t. 50 values;(c) determine the temperature coefficient for each substance by similar methods.From these observations the relative efficiency of the two bactericides can be calculated at any temperature and concentration. If two sets of figures are obtained, one with a vegetative organism, and the other with a sporing organism, a great deal of information can be summarized. It has been shown that an 0·05% solution of parachlor-meta-cresol is approximately one and a half times as efficient as an 0·5% phenol solution (against aMicrococcus). The comparative figure is about 1·0 whenBact. coliis used.

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Section: Techniquementioning

confidence: 99%

Section: Techniquementioning

confidence: 99%

Section: Techniquementioning

confidence: 99%

Section: E R Withell 349mentioning

confidence: 99%

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The evaluation of bactericides

Withell

1942

Epidemiol. Infect.

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“…Disinfection phenomena have often been considered as chemical reactions [9][10][11][12]. Consequently, species transport and subsequent residence time in a disinfection tank is of critical importance.…”

Section: Introductionmentioning

confidence: 99%

Spiral-shaped reactor for water disinfection

Soukane

Ait-Djoudi

Naceur

et al. 2016

Desalination and Water Treatment

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Chlorine based processes are still widely used for water disinfection. The disinfection process for municipal water consumption is usually carried out in large tanks, specifically designed to verify several hydraulic and disinfection criteria. The hydrodynamic behavior of contact tanks of different shapes, each with an approximate total volume of 50,000 m 3 , was analyzed by solving turbulent momentum transport equations with a computational fluid dynamics (CFD) code, namely ANSYS fluent. Numerical experiments of a tracer pulse were performed for each design to generate flow through curves and investigate species residence time distribution for different inlet flow rates, ranging from 3 m 3 ·s -1 to 12 m 3 ·s -1 . A new nature inspired Conch tank design of which shape follows an Archimedean spiral was then developed. The spiral design is shown to strongly outperform the other tanks designs for all the selected plug flow criteria with an enhancement in efficiency, less short-circuiting and an order of magnitude improvement in mixing and dispersion. Moreover, following the intensification philosophy, after 50% reduction of its size, the new design retains its properties and still gives far better results than the classical shapes.

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