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 Escherichia coli 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
nlore 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%.
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