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Seepage Pit Sizing TableDrywell & Seepage Pit Design Specifications

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Specifications for drywells or seepage pits or soakpits:

This document discusses and describes design specifications for seepage pits and drywells. We describe sepage pit size, area requirements, depth, and other construction requirements if you plan to use a seepage pit for roof or site drainage, to receive graywater or to receive septic effluent.

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Site Requirements for septic seepage pits & drywells

(i) If soil and site conditions are adequate for absorption trenches, seepage pits shall not be used.

(ii) A minimum three foot vertical separation must exist between the bottom of any pit and the high groundwater level, bedrock, or other impervious layer.

Design Criteria for seepage pits used to dispose of septic effluent

(i) The required "effective seepage pit area" is obtained from Tables 6 and 7 which are shown below.

(ii) No allowance for infiltration area is made for the bottom area of a pit or the surface area of impervious soil layers (percolation rate slower than 60 minutes/inch).

(iii) The effective diameter of a seepage pit includes the diameter of the lining plus the added diameter provided by the annular ring of aggregate. Any area surrounding the liner with rock smaller than 2 1/2 inches in size shall not be included as part of the effective diameter.

(iv) Effective seepage pit depth is measured from the invert of the seepage pit inlet to the floor of the pit, with the thickness of impervious layers deducted.

Site Distance Requirements for Drywells

SKETCH of a typical aerobic treatment unit tank, aerator, chamberThis sketch (from New York's Wastewater Regulations) shows the recommended site clearances between seepage pit (used to receive septic tank effluent in this case) and other building and site features.

A more detailed list of site clearances between all site features and all types of septic system components is listed at "More Reading" just below.

Notice that absorption pits like this one not only need to be at a sufficient distance from the building, from wells, from property lines, but also if multiple absorption pits are installed, if you expect each pit to dispose of the graywater or effluent sent to it, they should be adequately separated from one another.

Effluent Soil Absorption Area Requirements for Seepage Pits

The table given above for historical purposes, shows and older copy of the required soil absorption area for seepage pits as a function of soil percolation rate and anticipated daily wastewater flow in gallons. A current seepage pit sizing table is given just below.

[Click to see an enlarged, detailed version of this or any other image or table found at InspectApedia.com]

TABLE 6 - SEEPAGE PITS - REQUIRED ABSORPTIVE AREA (IN SQUARE FEET) FOR HOUSEHOLD SYSTEMS

Given in more detail here:

NYS Table 6: Seepage Pits / Soak Pits: Required Absoptive Area for Household Systems

(Table body gives the required square feet of soakpit absorptive surface area)
Soil Percolation Rate
(Minutes per inch)		 Sewage Application Rate (Gallons per day per square foot)
2 Bedrooms
3 Bedrooms
4 Bedrooms
5 Bedrooms
6 Bedrooms
Septic Wastewater Effluent Input Flow Rate (Gallons per Day)  
220 gpd 260 gpd 300 gpd 330 gpd 390 gpd 450 gpd 440 gpd 520 gpd 600 gpd 550 gpd 650 gpd 750 gpd 660 gpd 780 gpd 900 gpd
1 - 5 1.20 183 217 250 275 325 375 367 433 500 458 542 625 550 650 750
6 - 7 1.00 220 260 300 330 390 450 440 520 600 550 650 750 660 780 900
8 - 10 0.90 244 289 333 367 433 500 489 578 667 611 722 833 733 867 1000
11 - 15 0.80 275 325 375 413 488 563 550 650 750 688 813 938 825 975 1125
16 - 20 0.70 314 371 429 471 557 643 629 743 857 786 929 1071 943 1114 1286
21 - 30 0.60 367 433 500 550 650 750 733 867 1000 917 1083 1250 1100 1300 1500
31 - 45 0.50 440 520 600 660 780 900 880 1040 1200 1100 1300 1500 1320 1560 1800
46 - 60 0.45 489 578 667 733 867 1000 978 1156 1333 1222 1444 1667 1467 1733 2000
Over 60 Unsuitable soils for conventional seepage pit design. Use a special design.

Notes

Original source: New York State NYS75-A.8 Table 4A. Updated 12 January 2015. The current standard document is provided as a PDF and can be seen in its entirety atReferences or Citations .

Watch out: OPINION: as we explain in this article series, because of the depth below ground surface of most seepage pits or soakpits or cesspools there is a lack of adequate aerobic bacteria for proper treatment of sewage wastewater effluient. As a result, even if the system successfully disposes of effluent it is probably not treating it adequately. The result is a risk of environmental and water aquifer contamination by pathogens.

Seepage Pit Size Requirements to Obtain Necessary Absorption Area

The table below shows the size of seepage pit necessary in order to provide the required soil absorption area for seepage pits. First use the preceding table to determine the necessary seepage pit absorption area, then use this table (below) to determine the necessary seepage pit size to provide that absorption area.

[Click to enlarge any image]

Original source: New York State NYS75-A.8 Table 4A. Updated 12 January 2015. The current standard document is provided as a PDF and can be seen in its entirety atReferences or Citations .

TABLE 7 - SEEPAGE PITS (CYLINDRICAL) - DIMENSIONS FOR REQUIRED ABSORPTIVE AREA (IN SQUARE FEET)

Older copy for historical reasons:

Seepage pit sizing to get necessary surface area - NYS

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Continue reading  at DRYWELL / SEEPAGE PIT CONSTRUCTION DETAILS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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