Disinfection calculations

In addition to the phenol coefficient tests, a Use-Dilution Confirmation test has been introduced in the United States.The purpose of this test is to confirm that the recommended use-dilution of the disinfectant (calculated from the phenol coefficient of the fluid to be equivalent to that of a 5 per cent phenol solution) is in fact satisfactory. If the response in the test is not satisfactory, then an adjustment must be made to the dilutions recommended for use. 

In a parallel test with water instead of disinfectant the colony forming units (cfu) of surviving bacteria are determined and the reduction in viable counts is calculated. 

Phenol, CeHsOH, is used as a disinfectant. An aqueous solution of phenol was found to have a pH of 4.72. Is phenol acidic, neutral, or basic Calculate [HaO" ], [OH ], and pOH of the solution. 

O O Phenol, CeHsOH, is an aromatic alcohol with weak basic properties. It is used as a disinfectant and cleanser. Calculate the molar concentration of OH ions in a 0.75 mol/L solution of phenolate, CeHsO , ions [Kb = 7.7 X 10 ). What is the pH of the solution ... 

In some instances, provisional guideline values have been set for constiments for which there is some evidence of a potential hazard but where the available information on health effects is limited. Provisional guideline values have also been set for substances for which the calculated guideline value would be below the practical quantification level, or below the level that can be achieved through practical treatment methods, as well as for certain substances when it is likely that guideline values will be exceeded as a result of disinfection procedures. 

Pure 1,4-dichlorobenzene (1,4-DCB) is still used as a disinfectant and airfreshener in some public toilets. As an employee of the health department of a large city you are asked to evaluate whether the 1,4-DCB present in the air in such toilets may pose a health problem to the toilet personnel who are exposed to this compound for several hours every day. In this context you are interested in the maximum possible 1,4-DCB concentration in the toilet air at 20°C. Calculate this concentration in g per m3 air assuming that... 

It is possible to use this OH° concentration to predict k for the oxidation of other compounds under the same conditions. Von Gunten et al. (1995) calculated the actual concentration of OH° using this general and easy way for the ozonation of surface water at neutral pH in a two-stage pilot plant. Atrazine was used as the model compound, ozone decay was assumed to be of first order and the reactors completely mixed. Based on this model they were able to precisely predict the formation of bromate (Br03 ) by oxidation of bromide (Br ) for a full-scale water treatment plant. Bromate is a disinfection byproduct (DBP) of the ozonation of bromide-containing waters, and of concern because of its carcinogenic effects in animal experiments (see also Chapter A 3). 

Phenol (C5H5OH) is a weak acid used as a general disinfectant and in the manufacture of plastics. Calculate the pH and the concentrations of all species present (H30+,C6H50, C6H50H, and OH-) in a 0.10 M solution of phenol (Ka = 1.3 X 10-10). Also calculate the percent dissociation. 

Many home pools are disinfected by adding calcium hypochlorite, Ca(OCl)2. Reaction of the hypochlorite ion with water yields hypochlorous acid. Given that Ka for HOC1 is 3.5 X 10-8, calculate the pH of a 0.100 M solution of Ca(OCl)2. 

Ability to detect residual for disinfection Good Poor cannot calculate CT value for disinfection credit... 

In calculating the log and percent reductions of the disinfectant against M. bovis, the estimated counts which... 

As with chemical disinfection, the performance of UV irradiation systems (especially with 254-nm lamps) is determined by the disinfectant UV dose. In the case of solar disinfection of water, dose can then be calculated from the average solar UV intensity and the residence time in the irradiated part of the reactor dose = / x tr, where dose is the solar UV dose (Whm-2), I is the average intensity (W m-2), and t is the residence time (h). Note that only the UV part of the solar spectra is taken into account. 

Cabaj A, Sommer R, Schoenen D (1996) Biodosimetry Model Calculations for U.V. Water Disinfection Devices with Regard to Dose Distributions, Wat. Res. 30, No. 4 1003-1009. 

Having obtained m and k, the time t can be solved using Equation (17.2) from a knowledge of the value of /. This time is called the contact time for disinfection, and the intensity / is called the lethal dose. From Equation (17.2) any reasonable amount of dose is lethal when administered in a sufficient amount of contact time as calculated from the equation. We call Equation (17.2) the Universal Law of Disinfection. 

Example 17.6 In order to provide residual disinfectant in the distribution system, chloramination is applied to the treated water. If two moles of HOCl have been added per mole of ammonia, calculate the moles of nitfogen gas produced. 

Example 17.11 A flow of 25,000 m /d of treated water is to be disinfected using chlorine in pressurized steel cylinders. The raw water comes from a reservoir where the water from the watershed has a very low alkalinity. With this low raw-water alkalinity, coupled with the use of alum in the coagulation process, the alkalinity of the treated water when it finally arrives at the chlorination tank is practically zero. Calculate the amount of alkalinity required to neutralize the acid produced during the addition of the chlorine gas. 

Example 17.15 A chlorine disinfection study was conducted to determine the constants of Equation (17.2). For a log 2 removal efficiency, the value of m is found to be 0.35 and the value of k is found to be 100. Calculate the contact time if the regulatory agency requires a chlorine dose of 20 mg/L. 

Example 17.16 A total flow of 1000 mVd is to be disinfected. What should be the cross-sectional area of the serpentine channel in order to maintain selfcleaning velocity What would be the total combined length of the channel if the contact time has been calculated to be 35 min ... 

An activated sludge plant of a small development is out of order, and a decision has been made following approval from a state agency to discharge raw sewage to a river. The effluent was found to contain 8 mg/L of ferrous and 4 mg/L hydrogen sulfide. Calculate the mg/L of HTH needed to be dosed before actual disinfection is realized. What is the chlorine concentration ... 

In order to provide residual disinfectant in the distribution system, chloram-ination is applied to the treated water. If three moles of HOCl have been added per mole of ammonia, calculate the moles of nitrogen gas produced. In order to provide residual disinfectant in the distribution system, chloram-ination is applied to the treated water. If three moles of HOCl have been added per mole of ammonia, calculate the moles of trichloramine produced. In order to provide residual disinfectant in the distribution system, chloram-ination is applied to the treated water. If three moles of HOCl have been added per mole of ammonia, calculate the moles of laughing gas produced. Show the half reaction that will exhibit the property of organic chloram-ides as disinfectants. 

UV dose is the most important operational parameter in the UV disinfection. It can be calculated by the UV intensity multiplied by the exposure time (3,11,19). If the UV intensity is independent of time, the UV dose can be calculated by the following equation ... 

UV attenuation through water is often measured in terms of absorbance for a fixed path length at a certain wavelength such as 254 nm. Using the above equations, we can easily calculate either absorbance or transmittance. For example, if water has a UV absorbance of 0.022 cm the transmittance is 95%. A water with transmittance of 95%, 85%, 75%, and 65% is respectively considered as excellent, good, fair, and pretreatment needed for UV disinfection (1). 

From Fig. 3, one can find that the UV dose is 145 mW s/cm in order to achieve a final effluent pathogenic content of 170 MPN/100 mL. However, the aging and the fouling of the UV lamps must be taken into consideration. A safety factor of 1.2 is included in the calculation. Thus, the UV dose applied in the UV disinfection is... 

The total volume of water to be disinfected is then calculated ... 

An UV transmission monitor can measure the UV transmittance [or UV absorbance to calculate UVT by Eqs. (20) and (21)], which is critical to ensure the success of UV disinfection. The UVT can be determined either through grab samples with a laboratory instrument or on-line. The function of a temperature sensor is to protect UV lamp from overheating, and if the surrounding temperature surpasses the recommended operation temperature, the UV system will shut down. 

Appendices B1-B6 of this chapter present the CT values required to attain required reductions of Giardia cysts for chlorine. As shown, lower temperatures require higher CT values with chlorine, an increase in pH also increases the necessary CT values. If more than one disinfectant is used, the percentage inactivation achieved by each is additive and can be included in calculating the total CT value. 

To begin the calculations for determining the adequacy of the inactivation achieved by the disinfection system, the total contact time must be determined. 

The contact time multiplied by the concentration (mg/L) of residual chlorine in the water is the calculated CT value for the system. Proven inactivation of Giardia and viruses are correlated to calculated CT values in US EPA s Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water Sources (28). Appendix B contains excerpts from the CT tables in the manual. 

Potable Water Chlorination and Chloramination chapter introduces the detailed engineering procedures for calculation of CT values for disinfection, and both conventional and innovative process equipment, including the on-site chlorine gas and hypochlorite generation facilities (2). 

Current prion disinfection assays are slow, laborious and costly —they employ contaminated test tissue homogenates (prion strain and concentration variable, possible disinfectant inactivation), test animals which serve to assess the viability of treated tissue, with the log decrease being calculated from incubation period assays rather than from end-point titrations. 

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