Sample storage impact on the assimilable organic carbon (AOC) bioassay

The effects of sample storage on the assimilable organic carbon (AOC) bioassay using Pseudomonas fluorescens strain P17 and Spirillum strain NOX have not been fully quantified to date, and in the current Standard Method, it is stated that samples can 'probably be held for several days' (Standard Methods for the Examination of Water and Wastewater, ed. A. D. Eaton, L. S. Clesceri, A. E. Greenberg, 19th Edn., (1995)). Experiments were performed by splitting 22 samples after chlorine residual neutralization and pasteurization at 70°C for 30 min, and holding one half of the replicate samples at 4°C for one week prior to analysis. The majority of the samples were taken from a local water treatment plant and distribution system with source water from the deep Floridan aquifer. The others were taken from the laboratory tap water, whose source was also the Floridan aquifer. All collected samples were tested for effects due to storage, with each sample tested for AOC as soon as possible while an identical replicate was stored for one week. After one week, the AOC of the held samples was also determined. By comparing the AOC of samples that were not stored with samples that were stored, it was observed that after one week of storage, the AOC of the stored identical sample replicates increased by approximately 65%. This was determined to result from BOM (biodegradable organic matter) fermentation to AOC by a yeast, Cryptococcus neoformans. Of the 22 samples tested, only four displayed no significant change in AOC and none displayed a significant decrease in AOC. It was then determined that samples heat treated at 70°C for 30 min could be stored for less than 2 days, but a modified pasteurization of 72°C for 30 min immediately followed by an ice bath for 30 min allowed storage for at least 7 days without significant changes in AOC. (C) 2000 Elsevier Science Ltd.