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. 

Table 3. 1987 World Chlorine Demand by Consuming Seetor" ...

Chlorine Capacity. In 1982 U.S. chlorine capacity reached a record high of 36,864 t/day (13.2 million tons per year) as shown in Eigure 1. After 1982, decreased chlorine demand lowered market prices and some producers discontinued production, so that capacity dropped to 31,888 t/day in 1987 (5). Expansion to meet demand led to an increased capacity in March of 1989 of 32,550 t/day (11.1 x 10 t/yr). 

In reahty the chemistry of breakpoint chlorination is much more complex and has been modeled by computer (21). Conversion of NH/ to monochloramine is rapid and causes an essentially linear increase in CAC with chlorine dosage. Further addition of chlorine results in formation of unstable dichloramine which decomposes to N2 thereby causing a reduction in CAC (22). At breakpoint, the process is essentially complete, and further addition of chlorine causes an equivalent linear increase in free available chlorine. Small concentrations of combined chlorine remaining beyond breakpoint are due primarily to organic chloramines. Breakpoint occurs slightly above the theoretical C1 N ratio (1.75 vs 1.5) because of competitive oxidation of NH/ to nitrate ion. Organic matter consumes chlorine and its oxidation also increases the breakpoint chlorine demand. Cyanuric acid does not interfere with breakpoint chlorination (23). 

There are three basic terms used in the chlorination process chlorine demand, chlorine dosage and chlorine residual. Chlorine demand is the amount of chlorine which will reduced or consumed in the process of oxidizing impurities in the water. Chlorine dosage is the amount of chlorine fed into the water. Chlorine residual is the amount of chlorine still remaining in water after oxidation takes place. For example, if a water has 2.0 ppm chlorine demand and is fed into the water in a chlorine dosage of 5.0 ppm, the chlorine residual would be 3.0 ppm. 

In order to ensure the destruction of pathogens, the process of chlorination must achieve certain control of at least one factor and, preferably two, to compensate for fluctuations that occur. For this reason, some authorities on the subject stress the fact that the type and concentration of the chlorine residual must be controlled to ensure adequate disinfection. Only this way, they claim, can chlorination adequately take into account variations in temperature, pH, chlorine demand and types of organisms in the water. While possible to increase minimum contact times, it is difficult to do so. Five to ten minutes is normally all the time available with the type of pressure systems normally used for small water supplies. Many experts feel that satisfactory chlorine residual alone can provide adequate control for disinfection. In their opinion, superchlorination-dechlorination does the best job. Briefly, what is this technique and how does it operate ... 

Residuals of chloramine decline to a minimum value that is referred to as the breakpoint. When dosages exceed the breakpoint, free chloride residuals result. Breakpoint curves are unique for different water samples since the chlorine demand... 

Consider the steam supply line and associated control instrumentation. The designer s intention is that steam shall be supplied at a pressure and flow rate to match the required chlorine demand. 

Environmental issues are discussed below. It is worth pointing out now that the industry has had to adapt to chlorine demand changes in this sector such as CFCs, pulp and paper and solvents. In Europe no chlorine is used in the pulp industry and it is being run-down elsewhere. Many of the plants which supplied chlorine into the pulp sector are situated a long way from other chlorine end-users and there have been closures of chlorine units, mainly in Scandinavia, Canada and the West Coast of the USA. 

With PVC being the main driver for chlorine demand there is increasing pressure on the need to obtain cheap power, salt and ethylene feedstocks. Export-driven EDC and caustic plants have some place in meeting world demand though with the need to export both chlorine, in the form of EDC, and caustic soda there can be occasions when these plants are vulnerable to low returns if demand is much lower than supply. Traditional locations for these plants have been in the US Gulf and the Middle East, but there are plans to site such plants in Venezuela and Australia with both these countries having a large caustic soda demand in alumina. 

In contrast to this direct chlorination there is the oxychlorination of ethylene using hydrogen chloride and oxygen, the other major method now used. Since the chlorine supply is sometimes short and it is difficult to balance the caustic soda and chlorine demand (both are made by the electrolysis of brine), hydrogen chloride provides a cheap alternate source for the chlorine atom. Most of the ethylene dichloride manufactured is converted into vinyl chloride by eliminating a mole of HCl, which can then be recycled and used to make more EDC by oxychlorination. EDC and vinyl chloride plants usually are physically linked. Most plants are 50 50 direct chlorinationroxychlorination to balance the output of HCl. 

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. 

Most commonly, gaseous chlorine is added daily to large cooling systems, whereupon it combines with all possible reactants, as measured by the chlorine demand. When this demand has been satisfied, the breakpoint is reached, and a continuous free residual of chlorine is then permitted to... 

Chlorine dioxide does not hydrolyze in water and, as a result, C102 remains an effective biocide over a wide pH range. It also does not react with ammonia, so it can be useful in cooling systems where there is a high ammoniacal chlorine demand. It is a good slimicide and algaecide, has a fast reaction time, and does not lead to the same degree of corrosion risk as with chlorine. 

In practice, the required ratio can be different from theoretical, as quite often additional bleach is required to provide HOC1 as an oxidant for algal slimes and other forms of chlorine demand. Also, it is necessary to have a permanent source of oxidant available to effect the promotion of HOBr. However, not all the available bromine generated is lost by biocidal reaction or by (limited) volatility. There is, in fact, some degree of recycling of the bromide ion (Br ) back into HOBr, so monitoring of bromine plus the combined free and total chlorine is necessary to strike the correct halogen balance. 

An increase in chlorine demand, higher than normal water pressure differentials, or observation of rapid biofilm formation is often a good indicator of process contamination. Also, the system pH is often thrown off course. Process operators tend to notice reductions in heat-transfer efficiency or greater pressure differentials when process contamination occurs. 

The standard disinfectant for many of the world s potable drinking water supply systems (ozone and others are also widely used) and the product of choice for large cooling systems, usually available as a gas for lowest cost, but can be provided by liquids such as sodium hypochlorite (bleach) or solids such as calcium hypochlorite or isocyanurates. Any process contaminant leak tends to increase the chlorine demand, requiring additional chlorine to maintain disinfection rate. Poor penetrant of biomass and significantly reduced efficiencies over pH 8.0. 

The residual portion of the total chlorine in a cooling system after the chlorine demand has been satisfied by the addition of chlorine in some form. Usually a combination of hypochlorite ions (OC1 ) and hypochlorous acid (HOC1) and measured by diethyl-p-phenylene diamine(DPD) method. Operators frequently add chlorine once per day for several hours to provide a free chlorine reserve of 0.1 to 0.3 ppm. 

Fig. 26.6. U.S. chlorine demand in 2004. (With permission from Chemical Market Associates, Inc.)...

Ammonia is not of direct health concern but can compromise disinfection efficiency because it exerts a significant chlorine demand, reacting rapidly with chlorine. 

From this discussion, we can outline a plot of the total available (or residual) chlorine as a function of added chlorine. As chlorine is added, it attacks microorganisms and reacts with all the readily oxi-dizable substances present (i.e.,DOM and inorganic substances such as sulfides, and low-valent metal ions such as Mn2+ and Fe2+) until completion. This region of the plot is labeled immediate chlorine demand . See Figure 10.2. Adding more chlorine... 

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