Determination chlorine demand

The methods described earlier for determining the total, free, or combined chlorine residual also are used in establishing the chlorine demand of a water supply. The chlorine demand is defined as the quantity of chlorine that must be added to a water supply to completely react with any substance that can be oxidized by chlorine while also maintaining the desired chlorine residual. It is determined by adding progressively greater amounts of chlorine to a set of samples drawn from the water supply and determining the total, free, or combined chlorine residual. 

A detailed supply-chain model, which showed that two days inventory was required to supply the forecast market demand, determined chlorine storage tank number and size. The minimum number of drums and cylinders of liquid chlorine required to be held on-site was also calculated from the supply-chain model. This approach was also extended to calculation of the number and size of storage tanks for products such as caustic soda, sodium hypochlorite and hydrochloric acid. 

The concentrations of the solutions were selected to give a realistic level of total organic carbon (i.e., approximately 3 mg/L). The solutions were adjusted to pH 6.2 with phosphate buffer. They were then chlorinated for 24 h at room temperature in the dark with sodium hypochlorite to a residual of <1 mg/L of total available chlorine. The chlorine demand of the solutions was determined in preliminary experiments prior to chlorination of larger samples for concentration by XAD-2 resin adsorption and mutagenicity testing. Corresponding extracts of unchlorinated solutions of the model compounds were also prepared and tested. 

Other treatment units that affect the oxidation unit include raw sludge thickening and oxidized sludge dewatering. If the raw sludge thickener is out of service, the throughput rate will not be affected unless the maximum capacity is exceeded. If this occurs, the unit hours of operahon will have to be extended. The chlorine feed rate should be adjusted. The amount of adjustment is determined by the results of a chlorine demand test. 

The large difference between the optimum dose of CI2 determined in this study and that reported by Robeck et al. 21) may result from the amount of CI2 consumed in oxidizing formulating solvents, plus the chlorine demand of the river water which was characterized by a chemical oxygen demand of 5-35 mg/liter and a carbon-chloroform extract of 185-1320 /xg/liter. 

An FAC residual is regarded by the US Environmental Protection Agency as a sign of adequate disinfection. The problem encountered in ensuring that water leaving the treatment plant has been treated with enough chlorine to leave an FAC residual is illustrated by breakpoint chlorination. In breakpoint chlorination, dissolved chlorine is added to the water in a stepwise manner to determine the chlorine demand and to allow for the formation of chloramines. 

The chlorine demand is the difference between the chlorine added and the residual concentration after a designated reaction time of approximately 10 min. Total residual chlorine is determined by the oxidation of AA -diethyl-p-phenylenediamine (DPD) to produce a red-colored product Addition of iodine then catalyzes further reaction with chloramines, and it is possible to obtain concentration values for aU chloramines and free chlorine. 

The demand for the determination proved so great that the method has been placed on a routine basis. One man can make 40 to 50 chlorine determinations on dry resins in an 8-hour day with a precision approaching that of the almost prohibitively time-consuming potassium fusion method. The Parr-bomb method permits about 40 analyses by two men in an 8-hour day, but precision suffers considerably when the work is done at this speed. 

The chemical method for the determination of the chemical oxygen demand of non-saline waters involves oxidation of the organic matter with an excess of standard acidic potassium dichromate in the presence of silver sulfate catalyst followed by estimation of unused dichromate by titration with ferrous ammonium sulfate. Unfortunately, in this method, the high concentrations of sodium chloride present in sea water react with potassium dichromate producing chlorine ... 

The availability and preparation of standards Is a major problem In the environmental field. There are standards for many relatively stable constituents such as metals, nutrients or parameters such as turbidity or alkalinity to name but a few. However, there Is a demand for standards for dissolved oxygen, suspended solids, chlorine, biochemical oxygen demand, oils and fats. It would also be a great aid to have pH standards containing some of the organic substances commonly present In water and Introducing errors in the determinations. 

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