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Columns Representing Mound-type Disposal Systems for Septic Tank Effluent: II. Nutrient Transformations and Bacterial Populations¹

ABSTRACT

Carbon, nitrogen, and phosphorus transformations and bacterial populations were studied in columns representing a mound disposal system. Total-N, total-P, and the chemical oxygen demand (COD) of the influent (septic tank effluent) averaged 42, 21, and 257 mg/liter, respectively. The reduced-N forms present in the influent (NH₄-N and organic-N) were oxidized to NO₃-N in the aerobic fill and about 32% of the total-N was lost by denitrification during water passage through the anaerobic silt loam at the bottom of the column. Phosphorus (mainly dissolved orthophosphate) concentrations in column effluents, increased gradually, stabilizing at about 60% of the influent P concentration. Phosphorus removal from the percolating water probably occurred by both sorption and precipitation. The COD of column effluents was very low, indicating essentially complete C removal.

After crusting developed and caused permanent ponding the fill became anaerobic, and the column effluent N was mainly NH₄-N. Phosphorus concentrations of the effluent increased gradually with time and reached a constant value of 2 to 6 ppm. The COD of the effluent from the totally anaerobic columns was higher than that from the uncrusted columns.

Perforating column walls to more closely simulate field conditions permitted aerobic conditions in the fill after crust development caused continuous ponding. The C, N, and P transformations in the perforated crusted columns were similar to those observed in the nonperforated uncrusted (i.e., aerobic) columns.

Fecal indicator bacteria were not detected in any column effluent even though the counts of fecal streptococci and fecal coliform in the influent averaged 3.8 x 10⁴ and 1.7 x 10⁵/100 ml, respectively.

Key Words: water pollution • soil crusting • nitrogen • phosphorus • carbon • pathogenic bacteria • denitrification • phosphorus sorption and precipitation

¹ Contribution from the Soil Science Dep., Univ. of Wisconsin, and the Geological and Natural History Survey, University-Extension, Madison, Wis. 53706. This research is part of the Small Scale Waste Management Project, funded by the State of Wisconsin and the Upper Great Lakes Regional Comm.

² Postdoctoral Fellow, Associate Professors of Soil Science, and Project Assistant, Dep. of Bacteriology, respectively, University of Wisconsin, Madison, 53706. Senior author is presently Assistant Professor of Plant and Soil Science, University of Vermont, Burlington, 05401.

Received for publication June 22, 1973.