New York State Septic System Design Regulations 75-A.7 Distribution lines, distribution boxes, gravity flow, pressure distribution, dosing, siphons design specifications for septic systems
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- Design specifications for piping between the septic tank and the drainfield or absorption bed
- Design specifications for septic system distribution boxes
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Section 7 of the septic design regulations discusses the means of distribution or movement of effluent
from the septic tank to the absorption system or leach field. Components reviewed include setic tank outlet lines, effluent distribution lines, distribution boxes, gravity flow, pressure distribution, dosing, and siphons.
This document uses the New York State wastewater treatment standard for individual household septic systems
(Appendix 75-A) to provide an example of state regulated design and installation of both conventional
tank and leach field septic systems and alternative septic system
designs, including raised septic systems, septic mound systems, intermittent sand filter septic systems,
and evaportion-transpiration septic systems.
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to the author. Technical review by industry experts has been performed and is ongoing - reviewers welcomed and are listed at "References."
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Effective Date: 12/01/90
Title: Appendix 75-A.7 - Distribution devices
Section 75-A.7 Distribution devices [for Septic System Effluent. ]
(a) Gravity Distribution.
The maximum length of absorption lines used in conjunction with the
gravity distribution shall be 60 feet.
(1) Distribution Box.
(i) For accessibility, it is necessary that the distribution box be
located and have a removable cover not more than 12 inches below grade.
Where, due to site conditions, a distribution box must be greater than
12 inches below the surface, an extension collar shall be installed to
within 12 inches of the surface.
(ii) All outlets from the distribution box shall be at the same level to
insure the even distribution of flow.
(iii) To minimize frost action and reduce the possibility of movement
once installed, distribution boxes must be set on a bed of sand or pea
gravel at least 12 inches thick.
(iv) The drop between inlet and outlet inverts shall be at least two
inches. A baffle is required at the inlet side of the box when the
slope from the septic tank to the box exceeds 1/2 inch per foot or when
siphon dosing is used.
(v) There shall be a minimum two inch clearance between the inverts of
the outlets and the bottom of the box to prevent short-circuiting and
reduce solids carry-over.
(vi) Distribution boxes may be constructed in place or purchased
prefabricated. When concrete is used to construct boxes, it shall have
a minimum compressive strength of 2,500 psi at 28 day set.
(vii) Prefabricated boxes may be constructed of concrete, fiberglass, or
plastic. The boxes shall be installed in conformance with the
manufacturer's instructions in addition to the requirements above.
Readers of this page section on distribution boxes should also see
The D-Box a chapter of Inspecting, Testing, & Maintaining & Designing
Residential Septic Systems, our online book.
(2) Serial Distribution [of Septic System Efffluent]
(i) In serial distribution, an upper distribution line is allowed to
fill before the effluent overflows into a lower line. This method is
acceptable for use with dosing systems only.
(ii) The connections between distribution lines is made with
non-perforated pipe placed in undisturbed soil.
(3) Drop Manholes.
(i) Drop manholes are used on sloping sites to reduce the velocity of
flow to lower distribution lines. This system may be used with gravity
distribution.
(ii) Baffles at the inlet end of the manhole and approximately four
inches from the inlet are required in drop manholes.
(iii) The inverts of all outlets in each manhole shall be at the same
level.
(b) Pressure distribution and dosing.
(1) These methods permit the rapid distribution of effluent throughout
the absorption system followed by a rest period during which no effluent
enters the system. The maximum length of absorption lines used in
conjunction with these methods shall be 100 feet.
(i) Pressure distribution utilizes a sewage effluent pump to move the
effluent through the pipe network and into the soil. The volume
discharged in each cycle will exceed the volume available in the pipe
network and will be discharged from the pipe under pressure.
(ii) Dosing involves the use of a pump or siphon to move the effluent
into the pipe network. Discharge from the pipe is by gravity. The
volume of effluent in each dose should be 75% to 85% of the volume
available in the pipe network.
(2) Dosing or pressure distribution is recommended for all systems as it
promotes better treatment of wastewater and system longevity.
(3) In absorption fields, single dosing units are required when the
total trench length exceeds 500 feet. Alternate dosing units are
required when the length exceeds 1,000 feet.
(4) The use of manually operated siphons or pumps is not acceptable.
(5) Pipe used in pressure distribution shall have a minimum diameter of
1.5 inches and a maximum diameter of three inches. Pipe for siphon
dosing is sized to conform with the volume of the dose and can range
from three to six inches in diameter based upon the volume of each dose.
The ends of all pipes shall be capped.
(6) Only pumps designated by the manufacturer for use as sewage effluent
pumps shall be used.
(7) Pump chambers shall be equipped with an alarm to indicate
malfunction. Siphon dosing systems normally include an overflow to the
distribution laterals. Pressure distribution systems shall not be
equipped with an overflow.
(8) Pump chambers shall be sized to provide a minimum of one day's
design flow storage above the alarm level. Siphon chambers shall have a
minimum total storage of one day's design flow below the overflow pipe.
Readers of this page should also see Pressure Dosing Septic Systems a chapter of Inspecting, Testing, & Maintaining & Designing Residential Septic Systems, our online book.
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