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The septic system consists of two parts, the septic tank and the drainfield. Together, they function to process plumbing wastes in a safe and sanitary manner. To do this, the septic system depends on friendly bacteria and a number of other conditions to operate efficiently. The purpose of this publication is to explain the basic principles involved in your septic systems.
The Septic Tank
The septic tank receives all of the plumbing drainage and begins the digestive process in much the same way that our stomachs do. Organic solids are dissolved into tiny particles, liquids, and gases. The particles, mostly carbon and minerals, settle to the bottom of the septic tank to form a heavy black layer called “sludge.” This is periodically removed by a pump truck. The liquid portion of the septic tank is a nutrient-rich solution of partially digested organic material called “gray water.” Gases such as carbon dioxide, carbon monoxide, methane and sulfides are emitted from the septic tank through plumbing vents on the roof of the building.
The drainfield is a network of underground perforated pipes surrounded by a bed of gravel. Its purpose is to receive the gray water from the septic tank and disperse it into the soil. It is here at the interface of the gravel and the soil that final digestion takes place. If conditions are correct, micro-flora and micro-fauna in the soil sanitize the gray water and allow it to safely enter our environment. As drainfields age, their ability to disperse water into the soil diminishes. Eventually, the process shuts down completely and the drainfield must be abandoned. A new drainfield network is attached to the septic tank and the cycle begins again. In good environmental conditions, a drainfield can be expected to last anywhere from 10 to 30 years.
Our primary concern is the proper size of the drainfield. The total area of the gravel bed must be large enough to accommodate the gallons/hour put into the system against the absorption rate of the soil. If the drainfield is too small or reduced in capacity by age, it is subject to flooding by gray water. This causes a backup of the entire system and can permanently damage the gravel bed with slimy biomass buildup.
Drainfields work best in sandy soil. Clay soils or lands that flood after a heavy rain are poor choices for drainfield locations. In these situations, larger drainfields are required or above-ground, “mound-type” drainfields may be considered.
Bacteria and other microorganisms are a septic system’s best friends. They do the work of tearing down organic matter in the sewage into basic elements such as carbon and calcium. Without a healthy stock of these organisms, the septic system would quickly clog up and stop working.
Oxygen is essential for our bacteria to live and work inside of the septic system. Gaseous byproducts such as methane and carbon monoxide must be removed from the system in order for it to remain healthy and functional. To accommodate these needs, engineers have designed an airway from the drainfield through the septic tank and up to the plumbing vent pipes on the roof of the building. The septic system literally breathes through these narrow passageways. When the system backs up, the air supply is cut off and the bacteria die.
Commonly Asked Questions
“How often should I have the septic tank pumped?”
The purpose of pumping out a septic tank is to remove the carbon and mineral buildup on the bottom of the tank. This buildup accumulates very slowly, usually about an inch per year and should be removed every three to five years.
Be cautious of those that recommend pumping out your tank every year or those that say a backed-up system can be cured by pumping out the septic tank.
In almost every case of septic tank backup, the problem is not that you need to have your tank pumped. Think about it – when you get sick, do you immediately go to the doctor to have your stomach pumped? No, you don’t – you find out exactly what is wrong and you take care of it.
Rely on your plumber to be the sanitary physician. Let him determine what is wrong and trust his professional judgment.
“What causes drainfields to stop working?”
In most cases, age is the culprit. Like most everything in our lives, aging causes a decrease in function. Older drainfields are less capable of processing large quantities of liquid per hour. One solution might be to stagger the usage of large quantities of water, such as multiple laundry loads.
Other common drainfield problems might be:
“How can I tell if my drainfield is working properly?”
The drainfield must be able to process the gray water without allowing it to rise above ground or leak out to lower elevations, such as a stream or pond nearby.
Typically one indication of an inadequate drainfield is a dark green, abnormally tall growth of grass above the drainfield or a persistent wet spot in the area of the drainfield.
“How many gallons of gray water should my drainfield be able to process in one hour?”
For a three-bedroom home, a good rule of thumb for maximum water output would be approximately 150 gallons per hour.
A reasonable test for the drainfield would be to remove the cleanout cap on the house sewer and run a garden hose (three to five gallons per minute) into the sewer for 30 to 45 minutes. At the end of this time, water should not be standing in the sewer pipe.
“If I have problems, do I have to replace the entire drainfield?”
Not necessarily. Your present drainfield still has some value. Remember, drainfields – like car tires – wear down by degrees. As a matter of fact, once a new drainfield is put on line, it relieves the burden on your old field lines and they may recover as much as 50 percent of their original value. Always ask your installer to make sure to leave the old field lines connected.
“How large is my drainfield?”
Drainfields are rated by the square foot of gravel bed that they lay in. The gravel bed is two feet wide and as long as needed to achieve the necessary square footage. (Example: A 50-foot long trench represents 100 square feet of drainfield)
Normally, a three-bedroom home is designed with 300 square feet of drainfield. A rule of thumb is 100 square feet per bedroom in normal soil conditions.
For detailed information about soil conditions or code requirements in your area, call your local county health department.
Grable Plumbing Co., Inc. has produced this publication as a public service. For additional copies, please call (813) 321-0444.