Title: Appendix 75-A.7 - Distribution devices

Section 75-A.7 Distribution devices.

(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.

(2) Serial Distribution. (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 or a duplex pumping system with audible or visual alarms shall be used.