Get to Know Your Septic Tank
By Roger Machmeier, Ph.D., P.E., Professor Emeritus, University of Minnesota
Unfortunately, too many homeowners don't know their septic tank. The tank is buried and forgotten, along with the rest of the system, until sewage is ankle deep in the basement or bubbles up in the yard. Then another septic tank system is unfairly criticized for failing to do its job.
How many people would buy an expensive car and never change the oil? How many would never remove the ashes from a wood stove or fireplace? When the car breaks down or the stove is so full of ashes that no more wood can be put in, we don't blame the car or the stove. But when a sewage system fails for lack of proper maintenance, the blame is often incorrectly placed on that "no-good" septic tank.
The septic tank is the essential first part of an onsite sewage treatment system; and it's both very simple and very complex. Raw sewage flows into the tank from the house sewer. The solids separate from the liquid and stay in the tank. The liquid flowing out is called septic tank effluent. A septic tank is watertight so that when 5 gallons of sewage enter from a toilet flush, 5 gallons of effluent must flow out of the tank. Bacteria that do not use oxygen from the air grow in the tank. These bacteria are called anaerobic and the by-products of their activity are methane and hydrogen sulfide gas, plus other substances having odor. Hence, the word "septic" has been applied to this tank. But the septic tank might also be described as a settling tank where the sewage solids are stored while the bacteria decompose them and reduce their volume. The volume is never reduced to zero, however, so a residue remains. It is this residue that must be cleaned out of the tank when the volume becomes too great.
Sewage flows to the septic tank through the house sewer. This pipe must have the proper slope; not too steep so that the liquids run away from the solids and not too flat so that the solids settle out in the sewer pipe. A grade of from one to two inches in eight feet is used. This is a slope of one to two percent. A one percent slope is a one-foot drop in a hundred feet of pipe.
The house sewer should not have any low spots where liquid can remain. In freezing climates these low spots are the places where sewer pipe freezing happens. A sagging sewer pipe and a dripping faucet are usually sure signs of a frozen house sewer in the northern climates.
The house sewer pipe should be smooth on the inside so that sewage won't catch and start a blockage. Toilet paper might hang up on a rough spot at a pipe joint causing a continuing problem of sewer pipe plugging every so often. If the homeowner notices that the toilet isn't flushing as fast as it used to, or the floor drain is backing up when the clothes washer discharges, the problem may be a partially plugged house sewer pipe. There are also many other causes but this is one place to start looking.
The house sewer becomes the inlet pipe to the septic tank. The bottom (invert) of this inlet pipe should be two to three inches higher than the invert of the outlet pipe of the septic tank. As the sewage reaches the tank it drops into the liquid in the tank with a downward flow direction. This drop tends to move the sewage into the depth of the tank. Most states specify an inlet device, either a baffle or a sanitary tee. The purpose of the inlet device is to prevent the floating solids, called the scum layer, from building up and plugging the end of the sewer pipe. Some states don't require inlet devices and many devices get knocked off if the house sewer is improperly rodded with a plumbing snake to open up a plugged pipe. These tanks seem to operate O.K. and as long as the house sewer pipe doesn't plug at the tank end; it's difficult to insist that an inlet device is absolutely essential.
Reprinted courtesy of the author and Pumper Magazine.