Septic tank design specifications

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Septic tank design specifications:

These model 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.

We discuss septic tank materials, sizes, compartments, strengths, installation details, and special requirements for fiberglass, polyethylene septic tanks, concrete septic tanks, and for aerobic treatment unit construction.

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Regulations Governing Septic Tank Design

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. Effective Date: 12/01/90 Title: Appendix 75-A.6 -

(a) General information about Septic Tank Design

(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 given below specifies the minimum septic tank capacities and minimum liquid surface areas

Table 3: Minimum Septic Tank Capacities & Properties Based on Number of Bedrooms

Number of Bedrooms Septic Tank Size (Gallons) Minimum Liquid Surface Area (Sq.Ft.)
1, 2, or 3 1000 27
4 1,250 34
5 1,500 40
6 1,750 47


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.

A thorough table of septic tank size requirements and methods for calculating septic tank size are found separately at SEPTIC 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 used for septic tank construction 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 for concrete septic tanks 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 concrete septic tank 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 septic tanks shall be poured at the same time (monolithic pour).

(4) Design Specifications for Fiberglass and Polyethylene Septic Tanks

Fiberglass or polyethylene (plastic) septic tanks must meet the following additional requirements:

(i) Fiberglass or polyethylene septic 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 Fiberglass or polyethylene septic tanks so as to prevent damage to tank walls. The manufacturer's installation instructions shall be followed.

(iii) All Fiberglass or polyethylene septic 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 Septic Tanks or Aerobic Treatment Units (ATUs)

A homeowner may choose to install an aerobic unit instead of a septic tank under the following conditions:

(i) The aerobic treatment 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 aerobic treatment unit shall be equal to or greater than the design flow as determined from Table 1.

(iii) The absorption system that follows the aerobic treatment unit shall be sized in the exact same manner as it would for a septic tank.

(iv) aerobic treatment unit 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 treatment unit effluent is strictly prohibited [in some states such as New York, but permitted in other U.S. states such as areas of Texas].

Septic Tank Water-Tightness Design Specifications

See SEPTIC TANK LEAKS for a table of septic tank water tightness test procedure & critera for concrete septic tanks


Continue reading at SEPTIC TANK DESIGN STRENGTH SPECS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.




Or see DICTIONARY of SEPTIC SYSTEM & WASTEWATER TERMS for a more complete dictionary of septic system and wastewater terminology




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