How Often Are Septic Tanks Emptied, and Where Do the Contents Go?

Even if you have a well-functioning septic system, a good drain field and a healthy microbial ecosystem breaking down the septage, the sludge and scum layers in your tank will build up over time. Thus, they should be pumped out periodically — generally when the bottom of the floating scum layer is within 6 inches (15.2 centimeters) of the bottom of the outlet tee or the top of the sunken sludge layer is within 12 inches (30.4 centimeters) of it (septic tanks have an inlet tee, or pipe, that brings wastewater in from the house, and an outlet tee, which, in turn, takes the wastewater out of the tank). Without a well-developed power of extrasensory perception, it's impossible to tell when your waste has reached these levels.

This is why the U.S. Environmental Protection Agency (EPA) recommends that people with a septic tank have their system checked every one to three years. Having your system inspected includes getting your sludge and scum levels measured, checking the system's pipes and mechanisms, and inspecting the drain field to make sure it's percolating the effluent properly. The average septic tank system usually requires pumping every three to five years.

There are some products on the market that are meant to prolong periods between pumping. These products contain chemicals designed to hasten the process of breaking down the sludge in your tank by acting like tiny Pac-Men chomping magic pellets. At the least, products such as these may be superfluous: The additives are simply joining the microbial party already in full swing below your lawn. In the worst case, the chemicals can throw the primordial ecosystem that's developed over time in your septic tank thoroughly out of whack and disrupt the natural enzymes' ability to properly break down the waste sent to it. The EPA recommends that septic tank users play it safe and not substitute these products for regular septic tank inspections and pumping.

Without regular pumping of the septic tank, the system can overflow. Sometimes overflow can lead back to the house, where toilets and drains belch forth what's supposed to be in the septic tank. Overflow can also lead to a sudden deluge of unprocessed waste flooding the drain field. When this happens, water can seep above ground, which leads to a flooded yard and run-off into nearby water bodies like creeks and rivers. Below ground, this flood can cause further damage, tainting groundwater. Together, these intrusions of waste can contaminate the water people use and drink. The waste introduced into the water supply can carry harmful bacteria and diseases like hepatitis.

But even after it's taken from your backyard, your septic tank's contents still pose a hazard. So what happens to it?

Wherever there are a lot of people operating their homes' waste systems on septic tanks, you can bet you'll find plenty of local businesses that specialize in removing the scum and sludge that accumulate in the tank over time. This is an important service; if too much sludge builds up, it can lead to overflow, which is bad for everybody.

Generally, commercial septic pumping involves a pump truck removing the sludge, effluent and scum in the tank, then leaving the tank empty and ready to be filled again. Once the waste is removed, there are only so many things that can be done with it. Before federal laws that restrict septic sludge dumping, waste companies could simply bury it in dump sites. As it became clear that sites like these were a health hazard, they were outlawed. These sites remain, though many are in the process of remediation (cleanup).

These days, federal and state laws govern the final destination of the contents of your septic tank. In some cases, the septic contents are taken to waste treatment plants and added to the stew piped in from a municipal sewer system or delivered to independent, for-profit companies specializing in the treatment of septage. Septage may also be dumped in approved landfills. The guidelines concerning septage dumping are strict and sites can be few and far between, however.

Because of the dilemma posed by disposing of your septic tank's contents, septage is often used in another way: to grow your food. Unless there's a "USDA Organic" label on it, it's possible the food on your table was grown using the sludge that used to be in your septic tank as fertilizer.

This use of septage can be a controversial one. Proponents, including many commercial farmers, say it's a win-win situation, since municipalities don't have to worry what to do with the waste, and farmers get cheap fertilizer for their crops. If applied correctly to cropland with good soil and a low water table, the soil should act as a filter in the same way a drain field does in the backyard of a home with a septic tank. It is a contentious subject and the use of biosolids in agriculture continues to come under fire as a potential health and environmental threat. The practice can be seen as an effective way to recycle waste, but there are scientists and health professionals who believe that the use of biosolids in agriculture is dangerous for the nation’s farmland and is creating a number of health risks.

There's another, more cutting-edge way septage is being used these days — generating electricity to power homes. It's long been known that the methane produced as a waste product in the breakdown of sewage could be used to generate electricity. While methane is a simple fuel, it can also be broken down and used to generate electricity using fuel cells located on-site at treatment plants. The electricity produced doesn't combust, so there's little or no pollution generated.