Assimilable organic carbon and biodegradable dissolved organic carbon in Norwegian raw and drinking waters

Abstract

Assimilable organic carbon (AOC) and biodegradable dissolved organic carbon (BDOC) were measured in Norwegian raw and drinking waters. Several of the water-works investigated produce a drinking water which is, or approximates to, biological stability according to the AOC analysis. The reduction of AOC by water treatment was shown to be a function of the raw water quality and the particular treatment process. Levels of AOC were generally low in Norwegian raw and drinking waters, and thus the situation appears promising with respect to controlling microbial aftergrowth during distribution. Coagulation-filtration (mainly direct filtration) trains and anionic exchange achieved significant reductions in AOC, whereas membrane filtration had little effect. Treatment plants using limited or no specific measures for removal of dissolved organic carbon (DOC) and postchlorination, increased AOC levels. Disinfection with UV light may produce less AOC than chlorination. BDOC was effectively reduced by all water treatments. Membrane filtration performed at least as well as other treatments in removing BDOC. This suggests that bulk BDOC is typically of higher molecular weight than AOC. Correlations between AOC and BDOC in raw and drinking waters were not significant. This finding supports the contention that these parameters are independent measures of the labile organic fraction of natural waters. It seems likely that the AOC and BDOC analyses target different fractions of the biodegradable organic material (BOM). When AOC and BDOC were used to evaluate the effectiveness of treatment processes the results were more promising. Correlation factors showed that these parameters generally provided complementary information about the fate of BOM during water treatment. However, because BDOC decreased during water treatment at all but one plant, whereas AOC often increased, necessitates that the raw data and the correlation factors be presented together in studies of this nature. The addition of nutrient salts in the AOC assay usually gave moderate increases in the values obtained. Consequently, nutrients other than organic carbon may limit aftergrowth in Norwegian drinking water. Use of an alternative bioassay organism gave lower AOC values, thus confirming the suitability of the standard assay organism.