Evaluation of shallow-placed low pressure distribution systems in soils marginally suited for on-site waste treatment

Abstract

Two shallow-placed low pressure distribution (LPD) systems were evaluated in soils that were marginally suited for a conventional on-site wastewater disposal system (OSWDS) because of low hydraulic conductivity and shallow depth of soil to bedrock. The soils used for this study were Edom (fine, illitic, mesic, Typic Hapludult) and Penn-Bucks soil (fine-loamy, mixed, mesic, ultic Hapludult). In the Edam soil, the LPD system was installed with four subsystem designs operating: a narrow trench design with a design loading rate of 17.5 Lpd/m², and three designs based on Virginia regulations with design loading rates of 9.0 Lpd/m², 4.5 Lpd/m², and 5.7 Lpd/m². In the Penn-Bucks soil, the LPD system was installed with three subsystem designs operating: a narrow trench design with a design loading rate of 30.6 Lpd/m², and two designs based on Virginia regulations with design loading rates of 14.3 Lpd/m², and 7.3 Lpd/m².

The evaluation was conducted under different moisture and temperature conditions (summer of 1989, and the winter of 1990), and focused on the fate and transport below each system of two antibiotic resistant <i>Escherichia coli</i> strains and two host-specific bacteriophage strains. The potential loss of N0₃"-N through the biological process of denitrification was also examined.

In the Edom soil, a narrow trench design, and designs based on the Virginia regulation all removed >99.9% of the bacterial and viral tracers during the summer of 1989, and >99% during the winter of 1990 throughout a 152 cm depth. The potential loss of N0₃"-N in the Edom soil by denitrification was estimated to be 38%.

In the Penn-Bucks soil, the narrow trench design failed within six months of installation because the effluent loading rate was too high to permit infiltration through the silty clay loam soil, once biological clogging developed with the subsequent decrease in infiltrative capacity. The lower Virginia loading rate was mlore effective at microbial retention with >99.9% removal throughout a 114 cm depth in both the summer of 1989 and the winter of 1990. The normal Virginia loading rate removed> 99% of the bacterial and viral tracers throughout a 102 cm depth in both the summer of 1989 and the winter of 1990. The overall loss of N0₃"-N in the Penn-Bucks soil through denitrification was estimated at 67%.