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Title: Appendix 75-A.6 - Septic tanks and Enhanced Treatment Units

Effective Date


Section 75-A.6 Septic tanks and Enhanced Treatment Units. (a) General information.

(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 minimum septic tank capacities and minimum liquid surface areas.

NOTE: 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. (1) General Requirements. 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) Multi-compartment 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 - 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) Concrete 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 watertightness prior to backfilling.

(iv) The walls and floor of cast-in-place tanks shall be poured at the same time (monolithic pour).

(4) Fiberglass and polyethylene 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 watertightness after installation, and prior to backfilling.

(5) Steel tanks. Steel tanks must have a label indicating corrosion protection complying with Underwriters Laboratories, Inc., Standard UL-70 or equivalent.

(6) Enhanced Treatment Units (ETU)

(i) General. ETUs shall have a label indicating compliance with the standards for a Class I unit as described in the National Sanitation Foundation (NSF) International Standard 40 or equivalent testing.

(ii) Design Criteria.

(a) The minimum rated daily capacity of these units shall be 400 gallons or the daily design flow as determined from Table 1, whichever is greater.

(b) ETUs shall have an effluent filtering mechanism as part of the manufactured product or an effluent filter with a label indicating compliance with NSF Standard 46 or equivalent installed on the system outlet prior to discharge to the absorption area.

(c) Unless otherwise specified, the absorption system that follows an ETU shall be designed in the same manner as it would for septic tank effluent.

(d) Absorption areas receiving ETU effluent may be designed with a 33% reduction in the total absorption trench length listed in Table 4A or as calculated from Table 4B, when one of the following situations exist:

(1) ETUs are subject to the jurisdiction of a Responsible Management Entity (RME), or

(2) Local sanitary codes or watershed rules or regulations incorporate the requirement to maintain and service the ETU in accordance with the manufacturer's recommendations.

(e) The trench length reduction, may only be used for conventional absorption trench systems and shallow absorption trench systems.

(f) The trench length reduction may not be further reduced by the trench length reduction allowed for gravelless systems as described in paragraph 75-A.8(c)(3).

(g) The trench length reduction specified above in clause 75A.6(b)(6)(ii)(d) is not applicable at properties located within the New York City Watershed.


VOLUME A-1a (Title 10)