Modeling Chlorine Decay in Surface Water

Abstract

ABSTRACT. Chlorination is the most widely practiced form of disinfection in the US. However, there is concern that the disinfection by-products (DBPs) formed during chlorination might be carcinogenic. Because of this increasing concern in water supply systems, there is a need for models that can be used to predict chlorine residuals and optimize the disinfection practices. This paper presents a chlorine decay model based on the possible chlorine decay mechanisms. To evaluate this model, four raw surface and alum treated waters (Chester, Garden City, Maysville, and Lake Vandalia) were used. The chlorine residual at the end of the study period was maintained at the same concentration to avoid effects of chlorine concentration difference. Results show that this model predicts the chlorine residual extremely well, consistently yielding correlation coefficients greater than 0.98. Alum treatment substantially increased the fraction of rapidly reacting functional groups by 24% and decreased the specific chlorine demand (SCD) by an average of 14.4%. Therefore, alum coagulation processes may preferentially remove natural organic matter (NOM) having a slower reaction rate (with chlorine), higher specific chlorine demand, and higher chlorinated DBPs production.

Laboratory and pilot plant scale study on water dechlorination by medium pressure ultraviolet (UV) radiation

“…Higher UV fluence dose generated higher total chlorine removal efficiencies. The highest UV fluence dose applied in the laboratory scale experiments, i.e., 513 mJ/cm 2 , resulted in about 25% of chlorine removal, whilst the maximum fluence dose utilized in the plant scale experiments, i.e., 30mJ/cm 2 , resulted in about 9% of removal efficiency. The fluence-based free and combined chlorine decay rates were found and the results show that under exposure of polychromatic UV light generated by MP UV lamp, free chlorine was degraded faster than combined chlorine.…”

“…As shown in Figure 4, the highest UV fluence utilized in the laboratory experiments, 513 mJ/cm 2 , results in about 25% of total chlorine removal efficiency. The pilot plant scale experiments with typical UV fluence dose resulted in only about 9% of maximum removal efficiency at the highest UV fluence of 180 mJ/cm 2 .…”

“…Some advantages of this process include: (1) relatively low cost, (2) high efficiency, and (3) ease of use [2]. In this process, chlorine is commonly introduced in the forms of chlorine gas (Cl 2 ), sodium hypochlorite (NaOCl), or calcium hypochlorite (Ca(OCl) 2 ).…”

“…Some advantages of this process include: (1) relatively low cost, (2) high efficiency, and (3) ease of use [2]. In this process, chlorine is commonly introduced in the forms of chlorine gas (Cl 2 ), sodium hypochlorite (NaOCl), or calcium hypochlorite (Ca(OCl) 2 ). When each of these chemical is added to water, rapid hydrolysis occurs to form hypochlorous acid, HOCl, which is further hydrolyzed to yield hypochlorite ion, OCl -, as shown in the reaction (1) through (4).…”

“…Reaction (4) is strongly dependent on pH of the solution with equilibrium constant pKa = 7.5 (at 25°C) [3]. Both hypochlorous acid and hypochlorite ions are commonly referred to as "free chlorine residual" [2]. …”