Septic system distribution line specs, distribution boxes, gravity flow, pressure distribution, dosing, siphons design specifications for septic systems

    - CONTENTS: Example design specifications for piping between the septic tank and the drainfield or absorption bed based on New York regulations.
    Design specifications for septic system distribution boxes. Description & design specs for the D-box - septic distribution box. Description of gravity distribution of septic effluent and of pressure dosing for septic effluent disposal. Specifications for septic effluent pump and lift pump capacity, storage, and pump types
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Design specifications & regulations for the D-box and septic effluent distribution/disposal:

These model 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 septic tank outlet lines, effluent distribution lines, distribution boxes, gravity flow, pressure distribution, dosing, and siphons.

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Regulations Governing Effluent Distribution Devices for Septic Systems

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. NYS Section 75-A.7

The FLOUT® effluent dosing system shown at page top is discussed in detail at GRAVITY/SIPHON DOSING SYSTEMS.

Effective Date: 12/01/90 Title: Appendix 75-A.7 - Distribution devices

(a) Gravity Distribution of Septic System Effluent - D-Box & Piping

The maximum length of absorption lines used in conjunction with the gravity distribution shall be 60 feet.

(1) Distribution Box Design Specifications - the D-Box Specs

(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 SEPTIC D-BOX INSTALL REPAIR a chapter of Inspecting, Testing, & Maintaining & Designing Residential Septic Systems, our online book.

(2) Serial Distribution [of Septic System Effluent]

(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 on Sloping Sites Control Effluent Distribution

(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 specifications for septic system effluent disposal

(1) Pressure distribution or pressure dosing methods permit the rapid distribution of septic effluent throughout the septic system absorption fields (drainfield, leach field, mound system, raised bed system, soakaway bed, drainage trench) 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 of septic effluent 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 systems for septic effluent 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 all septic systems as it promotes better treatment of wastewater and system longevity.

(3) In septic 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 for septic effluent distribution.

(5) Pipe used in pressure distribution of septic effluent 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) Septic effluent 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) Sewage (septic effluent or grinder) 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.

Also see SEPTIC CODES & REFERENCES and SEPTIC CONSULTANTS. Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved 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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