Getting the Tank Size Right

One sure way to avoid septic tank problems is to make sure the septic tank is the right size for the system.  In general, the tank is sized based on both expected and maximum usage and how to achieve an appropriate wastewater retention time.

According the EPA, hydraulic retention time is 6 to 24 hours, allowing time for material to settle, or sedimentation, to occur.  This general time is dependent on several factors, such as tank geometry, height of fluid within tank, the inlet (where the waste enters the tank) and outlet (where the waste exits the tanks) heights, estimated daily usage and what local recommendations are for pumping. Generally, it is recommended to have 2 to 3 times the estimated daily use (or estimated design flow) as tank capacity, allowing a minimum of 24 hour retention time.  In most jurisdictions daily usage for residential septic systems is based on the number of bedrooms.

Based on the International Code Council, one and two bedroom homes require 750 gallons, homes with three bedrooms require 1000 gallon septic tank, with four bedrooms a 1200 gallon tank and a 5 bedroom home requires a 1425 gallon tank.  However, many local codes require a minimum of 1000 gallons capacity, and some jurisdictions may require larger tank sizes.  These requirements are utilized in the US. In other countries, such as in Europe, homes generally use less water and have wastewater that is less diluted and, as a result, have different treatment needs.

The geometry of septic tanks favors elongated rectangular tanks, allowing separation and sedimentation to occur as waste travels from the inlet end to the outlet. When waste water enters the tank, solid material mostly remains suspended within the liquid and settles out over time. An elongated tank is preferred with a 3:1 ratio of length to width being ideal. The use of baffles on the tank inlet reduces the velocity of the liquid allowing for less turbulence in the tank and more rapid sedimentation of the wastewater. The height of the septic tank also is important; a tank with less than 36 inches at the outlet is considered to be too shallow, and requires more frequent pumping due to water inflow being more apt to disturb the bottom sludge layer.

Another feature available in some pre-manufactured septic tanks is compartments. The use of compartments, or in some cases using multiple small tanks placed in series, can decrease the level of suspended solids more than a single tank.  Past studies at the University of Wisconsin have shown that the highest rate of removal occurs when the first compartment is one-half to two-thirds of the total volume.

Undersizing a septic tank results in low hydraulic retention times and, in the long run, will lead to system problems. The guidelines set forth in this blog will help you ask questions and make sure you have the appropriately sized septic tank for your system.


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