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

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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 × 104 and 1.7 × 105/100 ml, respectively.

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