2.0 SYSTEM DESIGN
© Presby Environmental, Inc., Design & Installation Manual, July 2020 Edition
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2.5 Pump Systems
Pumped systems supply effluent to the system using a pump and D-box when site conditions do not allow for a
gravity system. Dosing siphons are also an acceptable means of delivering effluent to the system.
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Pump volume per dose shall be no greater than 40 gallons times the total number of EF units.
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Pump dosing should be designed for a minimum of 6 cycles per day; 6-8 cycles per day are recommended.
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If possible, the dosing cycle should provide one hour of drying time between doses.
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Pump systems must have a high-water alarm float or sensor installed inside the pump chamber. Follow
state, local and national code requirements.
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Pumped systems with equalized flow distribution are limited to a maximum dose rate of 40 gpm and do
not require the use of a flow equalizer on the D-box outlet. Never pump directly into the EF FDU.
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All pump systems require differential venting.
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All pumped systems require a D-box with baffles, a velocity reducing tee or other means to be used for
velocity reduction.
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All systems with combination distribution or multiple bed distribution shall use flow equalizers in each D-
box outlet with each bed or EQ distribution section or EQ combination system limited to a maximum of 15
gpm, due to the flow constraints of the equalizers.
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Example: pumping to a combination system with 3 sections (using 3 D-box outlets). The maximum
delivery rate is (3 x 15) = 45 gpm.
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The rate at which effluent enters the EF FDU shall be controlled. Excessive effluent velocity can disrupt
solids that settle in the FDU.
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Effluent shall never be pumped directly into the EF unit.
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A D-box or tank shall be installed between the pumping chamber and the unit to reduce effluent
velocity.
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Force mains shall discharge into a D-box (or equivalent) with velocity reducer such as a baffle, 90°
bend, tee or equivalent.
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Velocity reducers are also needed for gravity systems when there is excessive slope between the septic
tank and the system.
Two methods of velocity reduction:
2.6 Venting
An adequate air supply is essential to the proper functioning of EF systems. Venting is always required. All systems
shall utilize differential venting. Venting is established through suction (chimney effect) created by the draw of air
from the high vent, which pulls air from the low vent.
General Rules
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Differential venting is the use of high and low vents in a system.
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In a gravity system, the roof stack acts as the high vent.
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High and low vent openings shall be separated by a minimum of 10 vertical ft.
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If possible, the high and low vents should be of the same capacity.
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Roof vent diameter must be a minimum of 3 in, 4 in diameter is recommended. If the roof vent is less
than 3 in, an additional high vent is recommended.