A septic tank is an underground chamber made of concrete, fibreglass or plastic through which domestic wastewater (sewage) flows for basic treatment. Settling and anaerobic processes reduce solids and organics, but the treatment efficiency is only moderate (referred to as “primary treatment”).
Septic tank systems are a type of simple onsite sewage facility (OSSF). They can be used in areas that are not connected to a sewerage system, such as rural areas. The treated liquid effluent is commonly disposed of in a septic drain field which provides further treatment. However, groundwater pollution may occur and can be a problem.
Wastewater Of Effective Capacity Needed (1)
Flow – Gallons Per
3 bedrooms 1000 gal.
4 bedrooms 1200 gal.
5-6 bedrooms 1500 gal.
Calculating Septic Tank Capacity in Gallons
(alternative method 1)
(alternative method 2)
Length x Width x Depth in feet / .1337 = gallons
Volume in Gallons
One gallon of water has a volume of .1337 cubic feet. For a rectangular septic tank, multiply depth (or inside
“height”) in feet times width times length. Divide this figure by .1337 to establish the number of gallons in
the septic tank.
dimensions were 4ft. x 5ft. x 8ft. = 160 cubic feet. Using the conversion factor to convert cubic feet to
gallons, 160 / .1337 = 1196 – or about a 1200-gallon tank.
One cubic foot of volume can contain 7.481 gallons of liquid. So a second approach to calculating septic
tank actual size or capacity in gallons is to multiply the septic tank volume in cubic feet by this constant,
which we round up to 7.5 gallons/cubic foot
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