Two antibiotic resistant Escherichia coli strains and two host-specific coliphage strains were
evaluated in a shallow-placed low pressure distribution (LPD) system placed in a soil that was unsuitable for
a conventional system because of shallow depth to a restrictive soil horizon. Four independent LPD
subsystems were installed with loading rates of 4.5, 5.7, and 9.0, (loading rates based on Virginia regulations), and
17.5 L d-1 m-2; narrow trench design. The evaluation was conducted under different
moisture and temperature conditions (summer 1989, and the winter 1990) and focused on the fate and transport
of the biological tracers below each subsystem. All subsystems performed equally well with respect to hydraulic
performance and retention of the biological tracers. Results indicated that greater than 99.9% of the
bacterial and coliphage tracers were retained within the subsystems during the summer of 1989, and greater than
99% were retained during the winter of 1990.
Two antibiotic resistant Escherichia coli strains and two host-specific coliphages were used to
evaluate a shallow-placed low pressure distribution (LPD) system installed in a soil that was unsuitable
for a conventional system because of the shallow depth to shale bedrock. Three independent LPD subsystems
were installed with design hydraulic loading rates of 7.7, 14.3 (based on Virginia regulations), and 30.6 L
d-1 m-2 (North Carolina design). The three subsystems were evaluated under different
moisture and temperature conditions (summer 1989 and winter 1990) with actual loading rates of 4.1, 7.7, and 16.7
L d-1 m-2, respectively, and focused on the fate and transport of biological tracers
below each subsystem. The North Carolina design failed within 6 months of installation because the effluent
loading rate was too high to permit infiltration through the fine-textured soil at the site. The lower
Virginia loading rate functioned better in terms of hydraulic performance and retention of bacterial and coliphage
tracers, retaining greater than 99.9% of the tracers, while the higher Virginia loading rate retained greater than
99% of the tracers over all samplings.
Nitrate removal by biological denitrification was examined in two shallow-placed low pressure distribution (LPD)
systems located in soils that were unsuitable for conventional disposal systems because of shallow depth to
restrictive soil horizons in an Edom soil or to fractured shale in a Penn-Bucks soil. Four independent LPD subsystems
were installed in the Edom soil with actual loading rates of 2.3, 2.9, 4.6, and 8.9 L d-1 m-2
while three subsystems were installed in the Penn-Bucks soil with actual loading rates of 3.6, 7.2, and 15.3
L d-1 m-2. Maximal rates of nitrate loss through denitrification were determined in the laboratory
based on the acetylene block procedure, while actual field denitrification rates were estimated based on nitrate:chloride
ratios. In the Edom soil, all four LPD subsystems demonstrated the same potential rates of denitrification in
laboratory tests, while field estimates of nitrate loss ranged from 2% to 21% over the four subsystems.
These low field denitrification rated were attributed to lower-than-designed loading rates which maintained
aerobic conditions. In the Penn-Bucks soils, the subsystem dosed at 15.3 L d-1 m-2 failed within
six months of installation and was not used further. The subsystem loaded at the rate of 7.2 L m-2 yielded
higher rates of denitrification under laboratory studies as compared to the subsystem loaded at 3.6 L m-2, and
a similar trend was observed in field nitrate losses through denitrification which were 71% and 65%, respectively.
These high denitrification rates (compared to the Edom subsystems) indicated that anaerobic conditions were present
in the Penn-Bucks subsystems.