When should septic tanks (onsite sewage disposal system holding tanks) be pumped? The most common domestic wastewater treatment system used in rural areas is the septic tanksoil absorption system.
The septic tank removes settle-able and floatable solids from the waste water. The soil absorption field then filters and treats the clarified septic tank effluent and distributes it through the soil. Removing the solids from the wastewater protects the soil absorption system from clogging and failure.
In addition to removing solids, the septic tank also promotes biological digestion of a portion of the solids and stores the remaining undigested portion.
The first stage of the treatment system, the septic tank, removes solids by holding wastewater in the tank. This allows the heavier solids to settle as sludge and the lighter particles to form scum at the top. To accomplish this, wastewater should be held in the tank for at least 24 hours. Up to 50 percent of the solids retained in the tank decompose; the remaining solids accumulate in the bottom of the tank. Biological and chemical additives are not needed to aid or accelerate decomposition.
As the system is used, sludge continues to accumulate in the bottom of the septic tank. Properly designed tanks have enough space for up to three years safe accumulation of sludge.
When the sludge level increases beyond this point, sewage has less time to settle before leaving the tank and more solids escape into the absorption area. If too much sludge accumulates, no settling occurs before the sewage flows to the soil absorption field. Infiltration of sludge into the soil absorption field can cause system failure. To prevent this, the tank must be pumped periodically.
The material pumped is known as septage.
The frequency of pumping depends on several factors:
1. Capacity of the septic tank
2. Volume of wastewater (related to size of household)
3. Amount of solids in wastewater (e.g. garbage disposals produce more solids)
For example, a 900-gallon septic tank that is being used, and if six people reside in a three bedroom house, the tank should be pumped every 1.3 years. If the same system serves a family of two, the tank would be ready for pumping every 5.2 years.
If you have just moved into a home, you may not know the size of the tank. In this case, you should have the tank pumped and inspected. The company pumping the tank will tell you its size, age, and condition.
Septic tanks will not fail immediately if they are not pumped. However, an unmaintained septic tank is no longer protecting the soil absorption field from solids. Continued neglect may result in system failure and even replacement of the soil absorption field. In some cases, site limitations may make replacement of the absorption field impossible.
Cleaning Septic Tanks
Septic tank pump and haul contractors can clean your tank. It is a good idea to supervise cleaning to assure that it is done properly. To extract all the material from the tank, the scum layer must be broken up and the sludge layer mixed with the liquid portion of the tank. This is usually done by alternately pumping liquid from the tank and reinjecting it into the bottom of the tank. The septic tank should be pumped through the large central manhole, not the baffle inspection ports. Pumping a tank through the baffle inspection ports can damage the baffles, resulting in a destroyed leach field.
The use of additives in septic tanks to reduce the sludge volume or substitute for pumping is not recommended. In fact, relying on additives rather than conventional tank pumping may result in failure of the septic system.
Before closing the tank, check the condition of the baffles. If they are missing or deteriorated, replace them with appropriate sanitary tee baffles. It should never be necessary to enter a septic tank. Any work to replace the baffles or repair the tank should be done from the outside. Decomposing wastes in the septic tank produce toxic gases which can kill a human in a matter of minutes.
When working on a tank, be sure the area is well ventilated and that someone is standing nearby. Never go into a septic tank to retrieve someone who has fallen in and was overcome by toxic gases without a self contained breathing apparatus (SCBA). If a SCBA is not available, call for emergency services and put a fan at the top of the tank to blow in fresh air.
For more information of septic systems go to: https://www.epa.gov/septic/how-care-your-septic-system