Over a third of Indiana’s population lives in residences having private waste disposal systems. In many cases, the septic tank and soil absorption field were already in place when they moved in.
Most homeowners have little understanding of, or interest in, the operation of their waste disposal systems until problems arise. Sewage backed up in the house or a seeping, smelly area in the yard is an unpleasant and often expensive introduction to the ABCs of septic system maintenance.
This publication explains briefly how the common home sewage disposal system works and how to keep it working. It also contains a troubleshooting guide for determining the cause of system malfunction and its prevention or cure.
Waste water flows from the household sewer into an underground septic tank in a typical system (Figure 1). In the tank, the waste components separate; the heavier solids (sludge) settle to the bottom, the grease and fatty solids (scum) float to the top, and the more liquid portion (effluent) flows through an outlet to the soil absorption field.
The soil absorption field should consist of a distribution box to send effluent to a series of level trenches, each containing a distribution pipe embedded in coarse gravel. The effluent runs out through holes in the pipe, down through the gravel and into the soil. The soil filters out most remaining solids. Nutrients and microorganisms are also treated in the soil under the trench and are also diluted by groundwater.
A well-designed absorption field in the right kind of soil, properly constructed and maintained, should function trouble-free for a long time. However, many soils in Indiana are not well suited to conventional septic systems. Soil conditions such as slow permeability and high water table, especially where coupled with poor design, faulty construction, or lack of maintenance, significantly reduces the life of septic systems in Indiana.
The tank receives sewage from the house sewer system. An inlet baffle shows the sewage as it enters the tank, allowing the heavier solids to settle and scum, grease, and fats to float to the surface (see Figure 2). A slight opening between the top of the inlet baffle and the tank cover allows gases generated in the tank to vent back through the house sewer stack. Venting prevents an air lock from developing and permits smooth flow of waste water through the system.
Tank volume is typically designed to give 2-3 days retention before effluent overflows to the soil absorption field. A baffle at the outlet or an effluent filter prevents most solids and floating scum from leaving the tank.
Lines that transport sewage from house to tank and the effluent from tank to absorption field are typically 4-inch diameter sewer pipes. Watertight joints are necessary to prevent root penetration. Plastic pipe is generally used in construction, but vitrified clay and cast iron are common in older systems.
The sewer line from the house to tank should have a uniform slope of 1-2 inches vertical drop per 8 feet of horizontal distance (1-2%), with no high or low spots to keep solids from settling out in the line. Slope greater than 2 percent can cause the liquids to flow too fast to carry the solids. If steeper slopes are required, a vertical drop is normally used with the lateral pipe laid at 2 percent.
Slope of the pipe from tank outlet to soil absorption field should be at least 1 percent. A maximum slope is not important since solids are removed in the tank.
Late spring or summer is the best time to begin septic system operation. It is not necessary to fill the tank with water before use, although some hot water should be added to the tank if you will begin using it in winter.
As solids accumulate in the tank, the natural bacterial digestion process begins. Commercial bacteria “seeding” products are not necessary for successful operation.
All septic systems will fail unless the sludge and floating scum are periodically removed from the tank. Otherwise, solids overflow and clog the absorption field.
Septic tanks should be cleaned out every 3-5 years, depending on size and the amount of solids entering them. Estimate clean-out interval on the basis of 100 gallons of tank capacity per person per year. For example, a 1,000 gallon tank used by a family of two should be cleaned after 5 years [1000÷(100×2)].
(Note: Use of garbage disposals increases solids loading by about 50 percent!)
Commercial additives do not eliminate the need for periodic clean-out.
An effluent filter, if present, will plug when solids build up. When this happens, the filter can be removed and cleaned by hosing the solids off into the tank, and the tank pumped. Homeowners with older systems without an effluent filter can determine when it’s time to pump by checking sludge and scum build-up. Measurement of both sludge and scum depths should be taken at the outlet, starting the third or fourth year after a cleaning. Clean the tank when the sludge layer is within 12 inches from the bottom of the outlet baffle, or when the bottom of the scum layer is within 3 inches of the bottom of the baffle.
Poisonous hydrogen sulfide and explosive methane gases can be generated in a septic tank. Never enter a septic tank, during or after pumping. Hatches or manholes should be secured with childproof locks or nylon safety netting under the manhole lid to allow visual observation, but prevent entry.
The most common type of soil disposal system is a series of parallel absorption trenches, each consisting of a distribution pipe running through a bed of gravel (see Figure 3). This helps distribute the sewage effluent over the entire trench area and stores it until it can filter into the bottom and sidewalls of the trenches. This forms a biomass that helps treat the sewage effluent, but also slows its percolation into the soil.
There are no simple solutions for many problems where a system was installed in unsuitable soil or was improperly designed and constructed. But other problems, if correctly diagnosed, can be solved with a minimum of expense. Following is a guide for troubleshooting and correcting septic system problems.
If you are planning to repair a septic system, you should seek help from county Health Board personnel. They can make location, design, and construction recommendations that will minimize the chances of failure.
Purdue Publications on dealing with septics:
These publications are available through the Media Distribution Center at 1-888-EXT-INFO (398-4636). Publications and additional information are available at the following Web sites:
Purdue Extension Publications Onlinehttp://www.agcom.purdue.edu/AgCom/Pubs/menu.htm
Purdue On-Site Wastewater Disposalhttp://www.ces.purdue.edu/henv/Septicsystems.htm
Residential On-Site Wastewater Disposalhttp://cobweb.ecn.purdue.edu/~frankenb/NU-prowd/articles.htm
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