Natural organic matter and formation of trihalomethanes in two water treatment processes

The investigation involved the study of performance of two local plants’ slightly different treatment processes for the removal of natural organic matter (NOM) which comprises trihalomethane (THM) precursors. Ultrafiltration separated the contained NOM into various apparent molecular weight (AMW) fractions to reveal the two processes NOM removal efficiencies, in terms of dissolved organic carbon (DOC), UV absorbance (UV254) and THM formation potential (THMFP). DOC and UV254 levels of source water at both plants tested low and they concentrated in the portion with AMW less than 3,000 Daltons. The combined expression of UV254 with DOC as specific ultraviolet light absorbance (SUVA) indicated that the source water of Macau had a value less than 3 L/mg-m. Therefore the main DOC content was fulvic in character, which is difficult to remove and favors the formation of brominated THMs. Low DOC removal percentages recorded in both processes confirm this observation. However, both processes showed similar removal efficiency of around 50% on humic substances and THM precursors quantified by measurements of UV254 and THMFP, respectively.

The performance of the conventional chlorination water treatment process and the direct filtration water treatment process on organic matter removal hence on reduction of THM formation potential was studied. Both plants’ processes exhibited comparable removal percentages for humic substance (UV254) and for THM precursors (THMFP), but larger differences in DOC removal. The difference in DOC removal was considered to be a result of a change of characteristics of the raw water treated at the MSR Plant due to that water’s extra retention time in the reservoir prior to treatment. Meanwhile the absence of the sedimentation step used in the direct filtration process of that plant may have also contributed to the difference. Nonetheless, it is noted that the aquatic organic content (DOC) of the source water at both sites was actually quite low and it concentrated in the portion of AMW less than 3,000 Daltons. More importantly, the SUVA values of the water were found always below 3 L/mg-m before and after treatment. This means that the main organic content was more fulvic in character, which is known to be difficult to remove. Although the current study looked at THM formation in chlorination as a whole, the speciation char- acteristic of THMs is very important since trihalomethanes have four species (chloroform, bromodichloromethane, di-bromochloromethane and bromoform) each with different carcinogenicity. It is known that fulvic acid generally contains more of the aliphatic ketone groups than aqueous humic acid, and aliphatic ketones are more reactive with bromine than chlorine, hence favoring the formation of brominated THMs [10]. Therefore, the source water treated at the two plants favors the formation of brominated THMs if ambient conditions are met; i.e. when seawater intrudes into the source water, increasing the concentration of bromide ion. This is the case during the dry season in Macau [11].