How often you pump septic tanks depends...
 How often should I pump a septic tank? Do additives do any good? Is yeast as good as the other expensive
products. Lanney, Sandia Park, NM
A septic tank, drawing courtesy of Purdue's Environmental Education Software
Series
You pump out your septic tank to get rid of the solid sludge that accumulates at the bottom of the tank and the
grease that accumulates as scum floating near the top. Both sludge and grease can foul the leaching field. How often
to pump out depends on your tank size, family size, and how much of your wastewater is solid waste.
For example: suppose your tank is 1,500 gallons, you've got four people in the family, and your solid waste is the
average amount. Then you should pump your tank every 4 years. See Guidelines in Further
Reading below. On the
other hand, suppose there's only one person using a 1,500-gallon tank. Then pumping it every 19 years is fine. The
number of people dumping wastewater down the drain makes a big difference in how often the tank needs pumping.
So does the size of the tank. Going back to our family of four, suppose the tank is a dinky 900 gallons. Then you better pump it about every 2 years-i.e.,
twice as often as for a 1,500-gallon tank.
The volume of solids in the wastewater varies with such things as garbage disposals, dirty clothes, where you live. A family with a garbage disposal (compared
with one that scrapes the dishes into a trash can) dumps up to 50 % more solid wastes down the drain, and, therefore, needs to pump the septic tank twice as
often.
Additives don't do any good-or, at least, no controlled scientific study has shown them to be effective. So, save your money.
The reason why they don't work is simple. Some of the waste solids are sand, grit, bits of plastic and other non-organic junk. This stuff has to be pumped
out. No enzyme or bacteria can digest them. Other organic solids don't digest well so they accumulate.
When you add bacteria to the tank, the new bacteria have to compete with the established bacteria that are adapted to the tank. The adapted ones have the
home field advantage and, in general, eat the new ones.
Pump your tank, instead, every 3 to 5 years, or, better, determine your individual schedule from Table 1 in the Montana's State University Guidelines given
below in Further Reading.
(Answered Sep. 19, 2001, Updated June 12, 2009)
Further Reading:
Virginia Department of Health: How a septic tank fills
Montana State U: Guidelines for septic tanks
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