DESIGN EXAMPLE OF AN INDIVIDUAL OSST SYSTEM

Design of a Septic Tank/Absorption Field

An individual OSST system must be designed to have a capacity capable of disposing of the sewage produced by the building or facility being served. Design is usually based on the maximum daily wastewater flow rate and average percolation rate.

Maximum daily wastewater flow rate (Q) is typically expressed in gallons per day or liters per day. In new construction, the anticipated maximum sewage flow rate is based on specific criteria and parameters outlined by the local public health authority, including number of bedrooms, bathrooms, occupants, and/or drainage fixture units. Tables 1 and 2 pro vide information on typical maximum daily wastewater flow rates. In existing buildings, the maximum daily sewage flow rate can be determined by measurement of existing conditions.

The septic tank is typically sized for detention of incoming sewage for a minimum of 2 days based on maximum daily flow. Typical allowable design capacities of septic tanks serving a residence are provided in tbl.3. These capacities vary by governmental jurisdiction. Check with the health authority having jurisdiction at the building site.

Minimum drainage area requirements for septic tank effluent are based on type of system and average percolation rate (t_perc) of the soil in min/in (min/cm). The percolation rate is determined in a test at the site that is performed under the supervision of a professional engineer or professional geologist adhering to procedures established by the governmental health entity, commonly a municipal, county, or state health office (discussed previously in this section). tbl.4 provides typical computation information.

Usually, the bottom of an absorption drainage field must be a minimum of 4 ft above the seasonal high water table and be a minimum of 4 ft above bedrock. The field must be covered with a 12 in minimum depth of 1/2 to 2 in gravel/rock. An absorption drainage field must be sloped in the direction of flow.

Code typically limits the maximum slope to 6 in (150 mm) in 100 ft (30 m) so that the effluent won’t simply flow to the end of the line and then back up. Code also limits any individual line to a length of 100 ft (30 m) and sets the minimum separation between lines at 6 ft (2 m).

TBL.3 TYPICAL ALLOWABLE DESIGN CAPACITY OF A SEPTIC TANK. VARIES BY GOVERNMENTAL JURISDICTION. CHECK WITH HEALTH AUTHORITY HAVING JURISDICTION AT THE BUILDING SITE.

Multifamily Maximum Daily Minimum Capacity Single Family Dwelling; Dwelling Units Sewage Flow Rate of Septic Tank(s)

TBL. 4 MINIMUM DRAINAGE AREA REQUIREMENTS FOR SEPTIC TANK EFFLUENT. VARIES BY GOVERNMENTAL JURISDICTION.

CHECK WITH HEALTH AUTHORITY HAVING JURISDICTION AT THE BUILDING SITE.

Minimum Drainage Area Requirement

[Multiply by 0.0245 to convert from area (ft^2) per gallon per day to area (m^2) per liter per day] All OSST Systems, Except Elevated Sand Elevated Sand Mound and Subsurface; Average Percolation Rate (t_perc) Mounds and Subsurface Sand Filters Sand Filter OSST Systems

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EX.1 An OSST system consisting of a conventional septic tank and an absorption bed drainage field will serve a single-family residence with 4 bedrooms and 4 bathrooms. The maximum daily sewage flow rate is anticipated to be 720 gal/day.

a. Determine the required septic tank size.

From tbl.3, a 1500-gal septic tank is required.

b. Determine the area required for an absorption bed drainage field based on a percolation rate of 12 min/in.

c. Determine the minimum trench length. Assume a trench width of 3 ft.

d. Determine the area required for an absorption bed drainage field based on a percolation rate of 50 min/in.

From tbl.4, the required drainage area must be determined based on a specified formula:

e. Determine the minimum trench length. Assume a trench width of 3 ft.

f. Assume an elevated sand mound system will be used in lieu of an absorption drain field based on the same percolation rate of 50 min/in.

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Design of a Seepage Pit

TBL. 5 ABSORPTION AREAS IN FT^2 AND M^2 OF A SELECTED SEEPAGE PIT DESIGN. 6 IN (150 MM) TO DEPTH PROVIDED TO ACCOMMODATE THE COVER.

TBL. 6 TYPICAL REQUIRED ABSORPTION AREAS FOR A SEEPAGE PIT. THE EFFECTIVE ABSORPTION AREA LISTED IS FOR EACH GALLON (OR LITER) OF WASTEWATER PER DAY BASED ON PERCOLATION RATE OF SOIL.

Required Absorption Area Percolation Rate of Wastewater per Day min/in (min/25 mm) ft^2 per gal m^2 per L

FGR.8 A seepage pit.

The size of the seepage pit is typically based on the outside area of the walls of the seepage pit. Some designers exclude the open bottom area of the pit from the absorption area required to allow for a safety factor, while others calculate the total area available and then size the system to allow some safety factor.

Wall areas for seepage pits of various diameters are pro vided in tbl.5. Seepage pits are sized from tbl.6, which lists percolation rates and the minimum required absorption area required for 100 gal (378.5 L) of wastewater. (See fgr.8.)

When more than one seepage pit is used, the pipe from the septic tank must be laid out so that the effluent will be spread uniformly to the pits. To provide equal distribution, a distribution box with separate laterals (each lateral feeding no more than two pits) provides the best results. The distance between the outside walls of the pits should be a minimum of 3 pit diameters and not less than 10 ft (3 m).

FGR.9 A septic tank to seepage pit system distributes clarified effluent flows through a distribution box to multiple seepage pits.

Avoiding Potential Design Problems---The uncertainty of exactly how much the soil will actually absorb is reflected in the values given in the various tables pro vided in OSST regulations. Even so, many designers prefer to oversize the OSST system slightly to allow for poor soil absorption and for future increased amounts of effluent, either because more people are using the facility than anticipated or because of an addition to the individual residence or an addition of various water-using fixtures that may not have been included in the original design.

Recent environmental regulations have led to increased land area requirements for the drainage field size of OSST systems. In some jurisdictions, additional land must be re served for a future a replacement drainage field. With limited land available for development in many areas, the amount of land occupied by individual OSST systems, including mandatory spacing and setbacks, must be considered at early stages of development.

One of the most wasteful uses of a residential OSST system is the connection of a clothes washing machine.

Many times, connecting a washing machine to an older system has resulted in more water flow than the soil can handle through the existing system. A clothes washer generates a significant amount of water, especially in large families. One of the simplest solutions is to spread washing out over several days, giving the soil a chance to absorb the water. Other solutions are to increase the size of the disposal field or add a seepage pit.

The connection of storm drainage (e.g., gutters, down spouts, and roof drains) to a drainage field may cause periodic overloads. When these are connected, the designer must in crease the size of the system to accommodate the periodic additional flow. Most designers prefer to run such connections into drywells if no storm drain system is available.

TBL. 7 MINIMUM CLEARANCE AND SETBACK REQUIREMENTS FOR OSST SYSTEM COMPONENTS. VARIES BY GOVERNMENTAL JURISDICTION CHECK WITH HEALTH AUTHORITY HAVING JURISDICTION AT THE BUILDING SITE.

Minimum Horizontal Distance | (Clearance) Required From Building Sewer Septic Tank Drain Field Seepage Pit or Cesspool

Buildings or structures; a Disposal field; Distribution box; On-site domestic water service line; Pressure public water main; Property line adjoining private property; Seepage pits or cesspools; Streams Trees Water supply wells

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