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An Evaluation of Alternative Simulated Treatments of Septic Tank Effluent¹

ABSTRACT

Six modified soil columns (180 by 15 cm) with an upper unsaturated zone and a lower saturated zone were dosed twice daily for 220 days with 1.65 cm of septic tank effluent via a cinderblock cube. Examined were: (i) the performance of the cinderblock material as a means of introducing effluent into two soils having different textures, (ii) the treatment capability of the two soils under unsaturated flow to purify septic tank effluent, and (iii) denitrification in a saturated zone containing either a Histic epipedon-sand mixture (9% OM) or gravel only.

Water as simulated rainfall totaling 4.3 cm was added in five increments to the soil overlying the cinderblock cubes during the first 45 days of dosing. Severe biological clogging occurred after 50 days in the cinderblock material embedded in the loamy sand columns. This resulted in continuously ponded effluent inside the cinderblock cubes. Continuous ponding did not develop in the cubes placed in the sand columns.

At a distance 95 cm below the cinderblock cube, conversion of total nitrogen to NO₃ in sand columns ranged from 83 to 100% and the PO₄-P content of the percolating effluent averaged < 1 mg/liter. Column effluent samples analyzed for fecal coliform were all negative. Soil samples revealed no penetration of fecal coliforms below 30 cm in either the loamy sand or sand columns. In the saturated zone containing the Histic epipedon-sand mixture, NO₃ was reduced by 93% at 42 days, but only 22% at 95 days. Chemical oxygen demand in the column effluent gradually declined from 188 mg/liter at 42 days to 85 mg/liter at 220 days. No significant denitrification occurred in the saturated zones containing gravel only.

Key Words: cinderblock distribution • denitrification • nitrification • biological clogging • soil texture

¹ Contribution from the Dep. of Soil Science, North Carolina State Univ. Paper no. 5816 of the Journal Series of the North Carolina Agric. Res. Serv., Raleigh, NC 27650.

² Research Assistant and Associate Professors of Soil Science, North Carolina State Univ., Raleigh, NC 27650.

Received for publication October 12, 1978.