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Designing Septic Tank Filter Fields Based on Effluent Storage during Climatic Stress

Dep. of Agronomy, Univ. of Arkansas, Fayetteville, AR 72701;
Agric. Engineering Dep., P.O. Box 1071, Univ. of Tennessee, Knoxville, TN 37901.

^* Corresponding author.

ABSTRACT

Septic tank filter fields (FFs) in many soils undergo seasonal hydraulic failure due to climatic stress. We investigated the hydraulic behavior of an experimental FT with a 61 cm wide, 9.0 m long, and 76 cm deep bed constructed in a loamy-skeletal, siliceous, mesic, Typic Fragiudult with a perched seasonal water table. The true water table was estimated to be deeper than 30 m. The FF was loaded with 15 L m^–2 d^–1 of septic tank effluent and monitored for 5 yr. At times each year, in response to rainfall, effluent rose within the bed to near the soil surface and spread laterally into the adjoining soil. This effluent was "stored" until climatic stress ceased. The experimental FF stored an estimated 20 d of effluent input; 220 L (14%) could be stored in the bed and 1410 L (86%) in the soil adjacent to the bed. Hydraulic stress events were the design-limiting periods for FF loading. The experimental FF operated without detection of surfaced effluent for 5 yr, so 20 d of storage was chosen as the design goal for evaluating FF designs in this soil. The loading rate for noninteracting beds was inversely proportional to bed width, so the most efficient FFs should consist of narrow beds. When 61 cm wide beds were placed closer together than 5.9 m center-to-center, their storage volumes began to interact and loading rates had to be reduced accordingly. This approach to FF design for soils with perched seasonal water tables suggests that loading rates be determined either by effluent storage or by the rate of effluent movement out of the beds, whichever is most limiting.

Contribution of the Dep. of Agronomy, Dep. of Agric. Engineering, Arkansas Agric. Exp. Stn., and the Arkansas Water Resour. Res. Center. Published with approval of the Director of the Arkansas Agric. Exp. Stn.

Received for publication November 20, 1991.