Effect of easily biodegradable organic compounds on bacterial growth in a bench-scale drinking water distribution system

Many engineered (e.g., disinfectant residual concentration) and environmental (e.g., temperature) factors influence bacterial regrowth in drinking water distribution systems. This paper examines the effect of nutrients, specifically biodegradable organic matter (BOM) composition, BOM concentration, and hydraulic retention time on bacterial growth in an annular reactor (AR). Drinking water that had an alkalinity of 300 mg/L as CaCO3 and a free chlorine residual of approximately 0.2 mg/L was used as process water in the ARs. Prior to entering the ARs, the water was filtered through granular activated carbon (GAC) to remove background chlorine and background organic matter. A cocktail of easily biodegradable organic compounds consisting of carboxylic acids, aldehydes, and free amino acids were spiked into the ARs as the primary carbon source. It was found that the influent BOM concentration (p value = 0.013) and the presence of free amino acids in the BOM cocktail (p value = 0.009) significantly increased the number of viable culturable cells in the biofilm, as measured by heterotrophic plate counts (HPCs). The interaction between the BOM concentration and the presence of amino acids also significantly increased the number of biofilm HPCs (p value = 0.021). Alternatively, the BOM concentration and the amino acid fraction did not affect the number of bulk (i.e., suspended) bacteria. The number of biofilm HPCs in the reactor was approximately 10 times greater than the number of bulk HPCs at high influent BOM concentrations and low retention times (i.e., high BOM loading rates). At low loading rates, the ratio of number of biofilm to bulk cells was less than 2. Consequently, it was deduced that the BOM was utilized predominately by the biofilm cells. This indicates that removal of easily biodegradable organic compounds is an important factor for controlling biofilm growth in distribution systems.