2.0 SYSTEM DESIGN
© Presby Environmental, Inc., Design & Installation Manual, September 2020 Edition
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All systems with multiple bed distribution shall use flow equalizers in each D-box outlet with each bed or
section limited to a maximum of 15 gpm, due to the flow constraints of the equalizers.
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Example: pumping to a system using 3 D-box outlets, the maximum delivery rate is (3 x 15) = 45
gpm. Higher flow rates can be accommodated by connecting multiple D-box outlets to each line.
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The rate at which effluent enters the AES pipe shall be controlled. Excessive effluent velocity can disrupt
solids that settle in the pipes.
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Effluent shall never be pumped directly into AES pipes.
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A D-box or tank shall be installed between the pumping chamber and the AES pipe 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 AES system.
Two methods of velocity reduction:
2.7 Venting
An adequate air supply is essential to the proper functioning of AES systems. Venting is always required. All
systems shall utilize differential venting.
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.
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Vent openings shall be located to ensure the unobstructed flow of air through the entire system.
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The low vent inlet shall be a minimum of 1 ft above final grade or anticipated snow level. Vents extending
more than 3 ft above grade must be anchored.
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Sch. 40 or SDR 35 PVC (or equivalent) should be used for all vent stacks.
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One 4 in vent is required for every 1,000 ft of AES pipe.
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A single 6 in vent may be installed in place of up to three 4 in vents.
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If a vent manifold is used, it shall be at least the same diameter as the vent(s).
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Vent piping should slope downward toward the system to prevent moisture from collecting in the pipe
and blocking the passage of air.
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Remote venting may be utilized to minimize the visibility of vent stacks.
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When venting multiple beds, it is preferred that each bed be vented separately rather than connecting
bed vents together. Multiple vents can be remotely located to the same location if desired.