Isabel C. Escobar

ASSIMILABLE ORGANIC CARBON (AOC) AND BIODEGRADABLE DISSOLVED ORGANIC CARBON (BDOC): COMPLEMENTARY MEASUREMENTS

The objective of this study was to evaluate the necessity of measuring both assimilable organic carbon (AOC) and biodegradable dissolved organic carbon (BDOC) as indicators of bacterial regrowth potential. AOC and BDOC have often been measured separately as indicators of bacterial regrowth, or together as indicators of bacterial regrowth and disinfection by-product formation potential, respectively. However, this study proposes that both AOC and BDOC should be used as complementary measurements of bacterial regrowth potential. In monitoring of full-scale membrane filtration, it was determined that nanofiltration (NF) removed over 90% of the BDOC while allowing the majority of the AOC through. Heterotrophic plate counts (HPC) remained low during the entire period of monitoring due to high additions of disinfectant residual. In a two-year monitoring of a water treatment plant that switched its treatment process from chlorination to chlorination and ozonation, it was observed that the plant effluent AOC increased by 127% while BDOC increased by 49% after the introduction of ozone. Even though AOC is a fraction of BDOC, measuring only one of these parameters can potentially under- or overestimate the bacterial regrowth potential of the water.

Removal of assimilable organic carbon and biodegradable dissolved organic carbon by reverse osmosis and nanofiltration membranes

Abstract The main objective of this study was to evaluate the effectiveness of reverse osmosis (RO) and nanofiltration (NF), under various solution chemistries, on bacterial regrowth potential as quantified by assimilable organic carbon (AOC) and biodegradable dissolved organic carbon (BDOC). The bench-scale experiments, using tap groundwater spiked with acetate as organic carbon, revealed that AOC removals by RO/NF membranes were strongly dependent on charge repulsion. AOC removals were greater at conditions of low ionic strength and low hardness, and were slightly higher at high pH values. BDOC removals by NF membrane also increased with decreasing hardness and ionic strength, and increasing pH. However, the RO membrane showed less dependence on feed solution chemistry for BDOC removal, suggesting that BDOC removal was determined by the combined effect of both size exclusion and charge repulsion. The bench-scale observations were compared to a full-scale drinking water treatment plant that used nanofiltration as a primary treatment process. From full-scale operation, it was observed that nanofiltration was a very effective means to reduce BDOC, but conversely, did not reject the bulk of raw water AOC. The high BDOC rejection by NF membranes at full scale can be explained by size exclusion, since a significant fraction of BDOC in raw surficial ground water consists of compounds, such as humic and fulvic acids, which are larger than the pores of NF membranes. The insignificant AOC rejection observed in the full-scale system was probably due to the low pH, high hardness, and high ionic strength (TDS) of the raw groundwater combined with acid addition during pretreatment. These solution environments repress the electrostatic interaction between charged organic compounds and membranes, allowing passage of small molecular weight compounds and thus reducing AOC rejection.

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

Abstract 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 30xa0min, 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 30xa0min could be stored for less than 2xa0days, but a modified pasteurization of 72°C for 30xa0min immediately followed by an ice bath for 30xa0min allowed storage for at least 7xa0days without significant changes in AOC.