Infiltrative Surface Clogging that Develops during Soil Treatment of Wastewater as Affected by the Interaction of Cations with Organic Matter.

Wastewater effluent infiltration into soil typically leads to genesis of a biozone at the infiltrative surface that results from biofilm formation, filtration of suspended solids, and accumulation of organic matter and microbial byproducts at the infiltrative surface and within the soil matrix. A set of experiments described in this paper enabled observations concerning the interactions of wastewater cations with the complex organic material that accumulates at the infiltrative surface. In the research, domestic septic tank effluent (STE) was applied to sand columns and in situ test cells in sandy loam soil. STE was applied for a period of about one year before chelating agents were introduced. As expected, STE application caused a dramatic loss in soil infiltrability (>90 to 99%). After this loss occurred, chelating agents (EDTA, EGTA, or citrate) were applied and observations were made concerning recovery of infiltrability and changes in soil pore water quality. In sand columns, application of chelating agents increased infiltration rates by an average factor of 12.2 for approximately three weeks, concurrent with increases in Al³⁺, Ca²⁺, Fe²⁺, and Mg²⁺ in the pore water exiting the columns. In the test cells, citrate application increased infiltration rates by a factor of 2.7, and released Al⁺³ and Fe⁺² cations into the soil pore water. EDTA application decreased infiltration rates by a factor of 0.94 within two weeks and mobilized Fe²⁺. The results suggest that the bridging of organic molecules by polyvalent cations at and near the infiltrative surface can occur and might be a mechanism that contributes to biozone genesis and changes in infiltrability resulting from effluent infiltration during soil treatment of wastewater.