Large septic tanks hold more waste, but tank size isn't the only consideration. The amount of solid waste and water you flush into the tank on a daily basis matters. For example, a 1,500-gallon tank for a two-person home may not need draining for nine years, but a 900-gallon tank would bring that time down to five years, according to North Carolina State University Extension. Most estimates use the number of people in a household instead of the number of gallons or pounds of waste because few homeowners know exactly how much they flush per day.
Adding more solid waste to a tank shortens the amount of time you can go without draining or pumping. Solid waste generally settles to the bottom of the tank and decomposes slowly or not at all. When solids build up enough, they block the openings that allow water to leave the tank. Garbage disposals are a major cause of excessive solid waste in a septic system, states the North Carolina Cooperative Extension Service. Flushed sanitary napkins and other paper or plastic hygiene products also contribute to this problem.
Using certain chemicals or solvents in your plumbing system leads to more frequent tank draining in most cases. Antibacterial products such as bleach, some soaps and toilet-bowl fresheners all kill the beneficial bacteria living in the septic system, according to the Arizona Cooperative Extension. These bacteria break down waste products. When a chemical additive kills a large percentage of the bacteria colony, solid waste builds up faster and tank pumping is required much sooner. Drain cleaners or clog removers also cause this bacterial damage.
Checking the Tank
If you're unsure when the tank was drained last, the charts given by most health departments won't help you determine when to have your septic system serviced. An inspection by a professional service company gives you a good idea of how high the solid waste level is in your tank. If you can access the port or opening to your tank without digging up your entire yard, wrap a towel around the end of an 8-foot-long pole and touch the bottom of the concrete tank with it. The towel will show the height of waste build up.