Septic Tank Sizes

[MasterPlumbersKeymaster]

You had a question on how to figure the size of a septic tank with some answers but I can’t open the answers. So please
tell me how to figure the size. Thanks.

[hjParticipant]

Your regulatory agency will be the determining factor for a septic tank. They will use their own minimum tank size and modify it by the size of the house and the number of residents.

[John Aldrich1Participant]

Jim, hj is correct, the local ISDS Regulations for your area dictate the required volume of a septic tank.

In Colorado the minimum volume is based on 30 hours of hydraulic detention time of the estimated maximum daily flow volume of sewage. For a 3 bedroom home the estimated maximum daily flow is 675 gallons, while a 4 bedroom home is estimated to produce a maximum daily flow of 900 gallons of sewage. The flow estimate is determined by the number of people who could possibly live in the home at 2 people per bedroom, each generating 112.5 gallons of sewage per day. In Idaho the required hydraulic detention time of the tank is 48 hours, but the estimated maximum daily flow varies depending upon the district in which the system is located.

Most ISDS Regulations were written by committees, and most assumed that the sludge would not be removed until the septic system failed. The regs. allow septic tank access holes to be completely buried, which produces systems that resist regular maintenance.

The exact location of most septic tanks in the USA is unknown, and undocumented. The ISDS permit normally is issued with the condition that the septage be removed “every 3 to 4 years”. The accumulation of sludge reduces the hydraulic detention time of the sewage flowing through the tank.

I prefer to use the 1/3 rule. When the measured sludge depth in the primary chamber is 1/3 the liquid depth of the tank, the sludge should be removed. In order to easily measure the sludge depth in a septic tank, risers to the ground surface must be provided so that the tank can be located. It is difficult to inspect a septic tank if you have to excavate the soil above the access hole, and you have only a general idea as to where to put the shovel. As a result, regular maintenance is usually ignored. This results in what I call “MAINTENANCE BY CRISES”.

I suggest that all septic tank access holes should have risers to the ground surface so that regular maintenance can be performed, and so that the tank can be easily located. I recommend that septic tank effluent filters be installed in the outlet tees of every septic tank, and that alternating leach fields be installed on every soil absorption system. Operation manuals should be prepared for every on-site system, so that the homeowner (system operator)will have some idea of what maintenance must be performed. JWA

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