Monitoring the disinfection by products using differential UV spectroscopy in drinking water reservoirs

Abstract

Chlorination has been widely used as a disinfection method in drinking water treatment. Disinfection of surface water supplies containing natural organic matter (NOM) with chlorine leads to formation of disinfection by- products (DBPs) such as trihalomethane (THM) and haloasetic acids (HAA). These halogenic compounds have adverse health effects on human being. Epidemiological studies indicated that there is a possible link between DBPs and development of cancer. Concerns regarding the potential health effects of DBPs prompted several industrialized countries to develop a number of regulations. Subsequently, United States of America Environmental Protection Agency (USEPA) promulgated a regulatory standard for THMs as 80 µg/L. Further, USEPA also introduced a HAA standard of 60 µg/L for the sum of five spices of HAAs in drinking waters. On the other hand European Union (EU) regulated THM limit as 100 µg/L. Moreover, Turkish Government recently regulated 100 µg/L THM limit in drinking water to comply with EU regulations.

The relationship among chlorination conditions, pH, temperature, reaction time, chlorine dosage, NOM concentration and the formation of DBPs are highly complex. Developing formal kinetic or statistical models for DBPs formation require substantial cost and effort of analyzing for disinfection by-products as THMs. Several people have tried to relate water quality parameters to DBPs in an effort to find a useful surrogate  parameter to better understand the chemical nature of  DBPs formation process. Surrogate parameters that have been used to estimate the formation of disinfection by-products include ultraviole absorbance (UV), spesific UV absorbance (SUVA) which is UV absorbance divided by dissolved organic carbon (DOC) concentration. Some researches reported that there was a simple and reliable relationships between change in UV absorbance of NOM after chlorination and the formation of chlorinated by-products. Since the aromatic functional groups are thougt to be both the dominant chromopheres in NOM and the dominant sites are attacked by chlorine on NOM molecule, the UV absorbance at 254 nm (UV₂₅₄) has frequently proposed to predict DBPs. However, the use of UV spectroscopy to estimate the formation of DBPs such as THMs is thougt to be problematic by many researches. Therefore, in this study, the potential use of differential UV absorbance at a wavelength of 272 nm (∆UV₂₇₂) to monitor the formation of THMs in drinking water has been investagated.

This method is an excellent and practical technique for monitoring instantaneous and continuous the formation of THMs online. The magnitude of decrease in UV absorbance at 272 nm is an excellent technique of DBPs formation from resulting chlorination. Further, the ΔUV₂₇₂ technique is used not only detect chromophores destroyed by the chlorination reactions but also to monitor the amount of formation of chlorinated by-products like THM. During the study, three different raw waters of Terkos, B.çekmece and Ömerli has been studied for this purpose. Raw water samples are chlorinated at variable pH levels, contact times and Cl₂/ DOC ratios. The total THM (TTHMs) versus ∆UV₂₇₂ correlations are quantified by linear equations with regression coefficient (R²). The results of study show that the relationships between THMs and ∆UV₂₇₂ are very strong (R² >0.96) in chlorinated raw water samples at variable reaction conditions. The THM vs. ΔUV₂₇₂ correlations may have practical value since they provide an alternative approach for monitoring the formation of THM online, and further, ΔUV₂₇₂ can be determined in a short period of time, using a small volume of sample, and does not require sophisticated sample pretreatment and analytical equipment.