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Effect of Loading Rate and Soil Amendments on Inorganic Nitrogen and Phosphorus Leached from a Wastewater Soil Absorption System¹

ABSTRACT

Permissable wastewater loading rates for ground absorption sewage disposal systems in North Carolina are currently estimated from an on-site characterization of soil texture and structure; but, after the trenches of the system are dug, the actual ability of the soil to absorb the applied wastewater may be far less than the estimated loading rate due to smearing of the trench-soil interface. This study was designed to evaluate the effects of wastewater loading rate and soil amendments proposed to help overcome the detrimental effect of soil smearing on wastewater treatment by a soil absorption system.

Three loading rates, 7.5, 11.3, and 15.0 L m^–2 d^–1, were tested and two soil amendments, cement and lime, were compared with a control. The amendments were added to the soil on a 5% by-weight basis. Three absorption trenches, 3.0 m in length, were dug for each loading rate-soil amendment combination. Soil water samples were obtained monthly for 12 months from ceramic cups placed 15 and 30 cm below the trenches. Water samples were analyzed for NH₄⁺-N, NO₃⁺-N, soluble P, chloride and pH.

Concentrations of NH₄⁺-N, NO₃^–-N, and soluble P varied significantly with the time of sampling. Transformation of NH₄⁺-N to NO₃^–-N was enhanced by the soil amendments, indicating that the aeration status of the soil near the trenches was improved by the amendments. The effect of the amendments was most noticeable during the spring recovery period as aeration status was increasing from winter minimums. Higher NO₃^–-N levels for the cement as compared with the lime in spring suggest that cement may be a more effective amendment than lime. The lowest loading rate had significantly higher concentrations of NO₃^–-N during the spring months. Soluble P levels increased linearly from 0.1 to 0.9 mg L^–1 over the first year operation of the system.

Key Words: wastewater treatment • land treatment • soil stabilization • lime • cement • amendment

¹ Contribution from the Dep. of Soil Sci., North Carolina State Univ. Paper no. 8365 of the Journal Ser. of the North Carolina Agric. Res. Service, Raleigh, NC 27650.

² Formerly Assistant Professor of Soil Science, North Carolina State Univ., Raleigh, NC. Now Soil Specialist, North Carolina Div. of Health Services, Greenville, NC 27834.

Received for publication June 18, 1982.