When soil columns were operated aerobically on a flooding-drying schedule in a previous study, good removals were observed for several organic compounds at concentrations ranging from 1 to 1,000 ,ug per liter in primary wastewater. In this study, fractional breakthroughs of most compounds increased substantially once operating parameters were modified and the soil became anaerobic. These results imply that microbial removal of trace organic compounds can be inhibited if anaerobic conditions

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... lop during rapid infiltration of wastewater. Wastewater disposal by rapid infiltration generally requires a specific flooding-drying schedule to preserve the infiltration characteristics of the soil (4). Research has shown that nitrogen removal can be optimized if the schedule is modified to develop anaerobic conditions and thus promote denitrification (3, 10). However, the effects of anaerobic conditions on the fate of trace organic compounds are unknown. Results from a field study have shown that trace organic compound removal can decrease towards the end of the flooding period during rapid infiltration of primary wastewater, and it was hypothesized that this was due to the development of anaerobic conditions (8). To study whether microorganisms were involved in trace organic compound removal, soil was transferred from the field site to laboratory columns. After the required 16-day drying period, the soil columns were operated, using design parameters from the field. The column study demonstrated that microorganisms were actively involved in trace organic compound removal (9). In brief, eight soil columns were maintained on a 6-dayflooding-16-day-drying schedule for three inundation cycles. Each set of duplicate columns had received primary wastewater at an infiltration rate of 34 cm per day, spiked with p-dichlorobenzene (PDCB), 2-methylnaphthalene (2MN), o:phenylphenol (OPP), p-(1,1,3,3-tetramethylbutyl)phenol (TMBP), 2-(methylthio)benzothiazole (2MTBT), and benzophenone (BZPN) in concentrations ranging from 1 to 1,000 ,ug/liter. In contrast to the results of the field study, trace organic compound removal was consistent throughout the flooding period. However, inorganic analyses indicated that the soil remained aerobic during flooding. We therefore induced anaerobic conditions to develop in the soil columns to evaluate the effect on trace organic compound removal. Sixteen days after the final flooding of the aerobic column study, the columns were flooded at the same infiltration rate with unfiltered primary wastewater for 6 days before beginning the subsequent 6-day flooding period with filtered primary wastewater spiked with the selected compounds. During this extended flooding period, nitrogen was delivered to the column headspace instead of air. In addition, the feed reservoirs for the columns were * Corresponding author.