Free chlorine (hypochlorous acid and hypochlorite ion) photolysis using UV is a novel advanced oxidation process (AOP) for drinking water treatment. It is currently being assessed at both lab- and full-scale for the control of seasonal taste and odor episodes. One concern with using UV/chlorine is the potential formation of chlorine-based disinfection by-products (DBPs). The chlorine dose required to make UV/chlorine treatment practical (e.g. 5-10 mg/L) is higher than doses commonly used for disinfection. However, the chlorine contact time during UV/chlorine treatment may be in the order of less than a minute, provided that the excess chlorine is quenched immediately after UV/chlorine treatment, or that the photolysis of chlorine during exposure to UV is relatively complete and very little chlorine remains following UV exposure. This work explores chlorine-based DBP formation under these unique conditions. A series of full-scale experiments were conducted at the Cornwall Water Purification Plant (Ontario, Canada), which employs medium pressure UV reactors. Sulfuric acid or sodium hydroxide were added to adjust pH to 6.5, 7.5, and 8.5. Free chlorine was dosed simultaneously at the same position to achieve 2, 6, or 10 mg/L. Chlorine residual downstream of the UV reactor was quenched immediately, followed by the collection of DBP samples. Chlorine contact time between injection and sampling ports was approximately 0.5 to 1.5 min, close to the hydraulic retention time of the flow. Monitored DBPs included chlorinated/brominated trihalomethanes, haloacetic acids, haloacetonitriles, haloketones, chloropicrin, adsorbable organic halides, chlorate, chlorite, perchlorate, and bromate. Results showed that the formation of all organic DBPs during UV/chlorine treatment at any chlorine dose and at any pH during the 0.5 to 1.5 minute contact time was minor compared to 24 hour (chlorine) formation potential tests conducted as controls, and was well below common regulatory limits. The only inorganic DBP formed was chlorate. Chlorate formation was approximately 10% (by mass) of the photolysed chlorine. Work is currently ongoing to assess DBP formation under laboratory conditions similar to those experienced at Cornwall, using several different types of surface waters (e.g., high/low TOC, etc.). This work employs an 80 L pilot-scale medium pressure UV reactor with a 1 kW medium pressure lamp to simulate treatment. The experiments are expected to be completed by April 2014.

Disinfection by-product formation in the UV/chlorine advanced oxidation process for drinking water treatment

248th National Meeting (2014)

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