New York State Septic System Design Regulations 75-A.6 Septic tank design specifications InspectAPedia® -
Design specifications for septic tanks
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Section 6 of the septic design regulations discusses the design specifications for septic tanks, including
septic tank size requirements, septic tank cover access, septic tank characteristics, steel septic tanks,
tanks for aerobic septic systems, and related details.
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.
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."
(1) Septic tank capacities shall be based upon the number of household bedrooms.
An expansion attic shall be considered as an additional bedroom.
Table 3 specifies the minimum septic tank capacities and minimum liquid surface
areas.
TABLE 3
MINIMUM SEPTIC TANK CAPACITIES
NUMBER OF MINIMUM TANK MINIMUM LIQUID
BEDROOMS CAPACITY (GALLONS) SURFACE AREA (SQ. FT.)
------------ ------------------- -----------------------
1, 2, 3 1,000 27
4 1,250 34
5 1,500 40
6 1,750 47
NOTE: Septic tank size requirements for more than six bedrooms shall be
calculated by adding 250 gallons and seven square feet of surface area
for each additional bedroom. A garbage grinder shall be considered
equivalent to an additional bedroom for determining tank size.
(2) Septic tank covers shall always be accessible. Where manholes are
more than 12 inches below final grade, an extension collar shall be
provided over each opening. Extension collars shall not be brought
flush with the ground surface unless the cover can be locked to prevent
tampering. Driveways or other facilities shall not be constructed above
septic tanks unless specially designed and reinforced to safely carry
the load imposed.
(b) Design and Installation of Septic Tanks
(1) General Requirements for Septic Tanks. The following
applies to all septic tanks regardless of material.
(i) A minimum liquid depth of 30 inches. The maximum depth for
determining the allowable design volume of a tank shall be 60 inches.
Deeper tanks provide extra sludge storage, but no credit shall be given
toward design volume.
(ii) The minimum distance between the inlet and outlet shall be six
feet. All tanks shall meet the minimum surface area requirement for the
specific design volume specified in Table 3. The effective length of
rectangular tanks should not be less than two nor greater than four
times the effective width.
(iii) Tanks must be watertight, constructed of durable material not
subject to corrosion, decay, frost damage, or cracking. After
installation, all septic tanks shall be able to support at least 300
pounds per square foot (psf).
(iv) Tanks with a liquid depth of 48 inches or more shall have a top
opening with a minimum of 20 inches in the shortest dimension to allow
entry into the tank. Tanks with a liquid depth less than 48 inches
shall have a top opening that is at least 12 inches in the shortest
dimension.
(v) Tanks shall have inlet and outlet baffles, sanitary tees or other
devices to prevent the passage of floating solids and to minimize
disturbance of settled sludge and floating scum by sewage entering and
leaving the tank. Outlet designs such as gas deflection baffles are
strongly recommended in all tanks. Inlet and outlet baffles shall
extend a minimum of 12 inches and 14 inches respectively, below the
liquid level in tanks with a liquid depth of less than 40 inches, and 16
and 18 inches respectively, in tanks with a liquid depth of 40 inches or
greater. The distance between the outlet baffle and the outlet shall
not exceed six inches. Baffles shall be constructed of a durable
material not subject to corrosion, decay or cracking.
(vi) There shall be a minimum of one inch clearance between the
underside of the top of the tank and the top of all baffles, partition
and/or tees to permit venting of tank gases. Multi-chamber and
multi-tank systems shall also be designed to permit the venting of tank
gases.
(vii) Tanks shall be placed on at least a three inch bed of sand or pea
gravel. This will provide for proper leveling and bearing. Additional
instructions provided by the manufacturer shall also be followed.
(viii) There shall be a minimum drop in elevation of two inches between
the inverts of the inlet and outlet pipes.
(ix) Garbage grinders. An additional 250 gallons of capacity and seven
square feet of surface area is required when a garbage grinder can
reasonably be expected at the time of construction or in the future. A
gas deflection baffle or other acceptable outlet modification, and a
dual compartment tank or two tanks in series must also be provided.
(2) Design Specifications for Multi-compartment septic tanks or tanks in series.
(i) Dual compartments are recommended for all tanks and shall be
required on all tanks with an interior length of ten feet or more.
(ii) The first compartment or tank (inlet side) shall account for 60 to 75% of the required total design volume. (iii) The baffle separating
the compartments shall extend from the bottom of the tank to at least
six inches above the invert of the outlet pipe.
(iv) Compartments shall be connected by a four inch vertical slot at
least 18 inches in width, a six inch elbow, or two 4-inch elbows located
at a distance below the liquid level equal to one-third the distance
between the invert of the outlet and the bottom of the tank. At least
one access manhole shall be provided into each compartment.
(v) Tanks in series should be connected by a single pipe with a minimum
diameter of four inches.
(vi) The volume and surface area for meeting the requirements of Table 3
shall be based upon the total volume and surface areas of all the tanks
and chambers.
(3) Design Specifications for Concrete septic tanks.
(i) Concrete shall have a minimum compressive
strength of 2,500 pounds per square inch (psi) at 28 days set; 3,000 psi
concrete is recommended as a minimum.
(ii) Wall thickness shall be a minimum of three inches unless the design
has been certified by a New York licensed professional engineer as
complying with all appropriate requirements for thin-wall construction.
All walls, bottom and top shall contain reinforcing to assure support
for 300 psi.
(iii) All joints shall be sealed such that the tank is watertight;
joints below the liquid level must be tested for water tightness prior to
backfilling.
(iv) The walls and floor of cast-in-place tanks shall be poured at the
same time (monolithic pour).
(4) Design Specifications for Fiberglass and Polyethylene Septic Tanks. These tanks must meet the
following additional requirements:
(i) These tanks shall not be installed in areas where the groundwater
level can rise to the level of the bottom of the septic tank.
(ii) Particular care must be taken during installation, bedding, and
backfilling of these units so as to prevent damage to tank walls. The
manufacturer's installation instructions shall be followed.
(iii) All tanks should be sold by the manufacturer completely assembled.
If, because of size, the tank is delivered to the site in sections, all
joints shall be sealed with watertight gaskets and shall be tested for
water tightness after installation, and prior to backfilling.
(5) Design Specifications for Steel Septic Tanks. Steel septic tanks must have a label indicating corrosion
protection complying with Underwriters Laboratories, Inc., Standard
UL-70 or equivalent.
(6) Design specifications for Aerobic Septic Units. A homeowner may choose to install an aerobic unit
instead of a septic tank under the following conditions:
(i) The unit shall have a label indicating compliance with the standards
for a Class I unit as described in the National Sanitation Foundation
(NSF) Standard 40 or equivalent.
(ii) The rated capacity of the unit shall be equal to or greater than
the design flow as determined from Table 1.
(iii) The absorption system that follows the unit shall be sized in the
exact same manner as it would for a septic tank.
(iv) Units which do not include as a standard feature a service contract
which provides for, as a minimum, semiannual inspections and annual
pumping for three years or more are prohibited.
(v) The surface discharge of aerobic unit effluent is strictly
prohibited.
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