ID-142

Cooperative Extension Service
Purdue University
West Lafayette, IN 47907


Nearly half of Indiana's population lives in residences having private waste disposal systems. Many of these people moved to their present dwellings from urban areas or communities that provided water and sewer service. And in most cases, when they moved in (even to a newly built house), the septic tank and soil absorption field were already in place.

The result is that many Hoosier homeowners have little understanding of, or interest in, the operation of their waste disposal systems until problems arise. Sewage backed up in the drains 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. Presented are recommendations on the location, design, construction, operation and maintenance of the septic tank, filter field and sewer lines. Also provided is a troubleshooting guide for determining the cause of system malfunction and its prevention or cure.

How a Septic System Works

In a typical system (Figure 1), waste water flows from the household sewer into an underground septic tank. There the waste components separate-the heavier solids (sludge) settling to the bottom, the grease and fatty solids (scum) floating to the top, and the more liquid portion (effluent) flowing through an outlet to the soil absorption field.

Figure 1. Schematic of on-site waste disposal system

The absorption field usually consists of a series of parallel trenches (fingers), each containing a distribution pipe or tile embedded in coarse gravel. The effluent leaks out through holes in the pipe or seams between tile sections, then down through the gravel and into the soil. The soil filters out the remaining minute solids, and nutrients are taken up by the roots of growing plants or slowly percolate down to groundwater.

A well~designed absorption field in the right kind of soil, properly constructed and maintained, should function trouble-free for a long, long time. However, many soils in Indiana are not well suited to septic systems, at least not a conventional one. Soil conditions such as slow permeability and high water table-too often coupled with poor design, faulty construction and lack of maintenance-reduce the average life of septic systems in Indiana to only 7-10 years!

The Septic Tank and Sewer Lines

The septic tank receives sewage from the house sewer system. An inlet baffle slows the sewage as it enters the tank, allowing the heavier solids to settle as sludge and the greases and fats to float to the surface forming a scum layer (see Figure 2). A slight opening left between the top of the 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 the sewage effluent overflows into the pipe leading to the soil absorption field. Recommended tank sizes are shown in Table 1. A baffle located at the outlet prevents floating scum from leaving the tank.

The lines that transport the sewage from house to tank and the effluent from tank to absorption field are typically 4-inch diameter sewer pipes. Absolutely watertight joints are necessary to prevent root penetration. (Concrete masonry joints will not be watertight, because mortar contracts as it dries.) Plastic pipe is used in most new construction, but vitrified clay and cast iron are common in many older homes.

Table 1. Recommended Minimum Septic Tank Capacities.

  No. of bedrooms Normal liquid capacity
   in dwelling          of tank

     2 or less     750 gallons
       3           900 gallons
       4          1100 gallons
       5          1250 gallons
       6          1500 gallons

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 (i.e., 1-2 percent), with no high or low spots. Slope less than 1 percent is too flat, slowing liquid velocity enough that solids settle 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.

Figure 2. Sewage treatment in a residential septic tank

Slope of the tank outlet pipe should be at least 1 percent. A maximum slope is not important since solids should have been removed in the septic tank.

Septic Tank Start-Up

Late spring or summer is generally considered the best time to begin operation. To start, simply begin using the tank It is not necessary to fill the tank with water before use, although hot water should be added to the system initially as often as possible (particularly if starting up in winter).

As the solids begin to accumulate in the tank, the natural bacterial digestion processes will begin. Commercial products are available for "seeding" the system with desirable bacteria, but this is not necessary for successful operation.

Septic Tank Clean-Out

Even the best designed and operated septic system eventually fails unless the sludge and floating scum are periodically pumped from the tank. If not removed, solids will overflow into and clog the soil absorption field.

Generally septic tanks should be cleaned out every 3-5 years, depending on the size of the tank and the amount and quality of solids entering it. As a rule of thumb, clean-out interval can be determined on the basis of 100 gallons of tank capacity per person per year. For example, a 1000 gallon tank used by a family of two should be cleaned after 5 years [1000÷ (100 X 2)]. (Note: Use of garbage disposals increases solids loading by about 50 percent!) Commercial septic tank additives will not eliminate the need for periodic clean-out.

Homeowners can determine when it's time to pump down the septic tank by occasionally checking sludge and scum build-up. But because that's an unpleasant task, the best suggestion for most homeowners is simply to have the tank routinely pumped every 3-5 years.

For those who would rather delay septic tank cleaning until it has to be done (but before trouble starts), measurement of both sludge and scum depths should be taken at the outlet side of the tank at least once a year, starting with the third or fourth year after a cleaning. Following is a brief explanation of how to do it (see Figure 3).

Figure 3. Scum and sludge build-up in a septic tank determines clean-out frequency needed

Measuring sludge depth. Wrap a long measuring stick with a rough-textured (e.g., terry cloth) towel to a height of about 4 feet. After removing the manhole cover nearest the discharge end of the tank, look at the outlet baffle. If it is a Tee-type or a flat, vertical plate with an opening at the top, run the measuring stick down through the baffle opening to the bottom of the tank. If the baffle is an elbow type with no top opening, first make a hole in the scum mat near the baffle with another stick, then lower the measuring stick through the hole.

With downward pressure, slowly rotate the measuring stick a couple of times to make sure it reaches the bottom; then slowly remove it. The sludge layer can be distinguished from the liquid depth by the black particles clinging to the towel. Simply measure the depth of this black portion to determine sludge depth.

Measuring scum depth. To determine scum accumulation, use a 3-inch square piece of wood attached at the end of a long stick. Push the measuring device through the scum layer into the liquid layer. As the stick is carefully moved up and down, a change in resistance on the piece of wood indicates the bottom of the scum layer. When the bottom is found, mark the stick at a convenient reference point (such as the top of the tank opening). With the same device, locate the bottom of the outlet baffle and remark the stick using the same reference point. The distance between the marks is the distance between the bottom of the scum layer and the bottom of the outlet baffle.

It is time to clean a septic tank when the top of the sludge layer is within 12 inches of the bottom of the outlet baffle, or when the bottom of scum layer is within 3 inches of the bottom of the baffle.

Septic tank cleaning should be done by reputable septic service companies or otherwise experienced operators with appropriate equipment. Their service usually includes pumping, inspection of tank openings and baffles, and disposal of the sludge in an approved manner. They should leave 6-8 inches of liquid in the tank after pumping as seed" to speed up decomposition when operation resumes.

Reputable companies dispose of septic tank sludge in a number of ways. They may haul to municipal sewage treatment plants or to approved sanitary landfills or septage burial sites, or they might incorporate it into cropland soil for its fertilizer value.

Septic Tank Safety

Hydrogen sulfide and methane gases can be generated in a septic tank-and they are extremely dangerous! You could be poisoned by breathing hydrogen sulfide fumes; therefore, never lean into or enter a septic tank, particularly during pumping. Methane is an explosive; so don't use torches or other flames near the opening of a septic tank.

The Soil Absorption Field

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 4). This porous bed helps distribute the sewage effluent over the entire trench area and stores it until it can filter into the soil. Gravel fill also protects the pipe from the effects of freezing temperatures and soil pressure.

Absorption trenches are relatively inexpensive, simple to construct and the preferred method of effluent treatment where soil conditions permit, the major influencing soil condition being permeability. As the soil filters sewage effluent, an organic mat (biomass) develops on the bottom and sidewalls of the trenches. Although this biomass helps treat the sewage effluent, it can also slow down its percolation into the soil. If such soil is already slowly permeable, severe problems can result.

Figure 4. In an absorption trench, effluent trickles down through the gravel from holes in the distribution pipe and slowly infiltrates into the soil surrounding the trench.

Following are suggestions on the design, construction, operation and maintenance of a soil absorption field that should help minimize the chances of trouble developing.

Design and Construction Tips

Table 2. Recommended Trench Bottom Areas for Various Soils*.


	Soil texture		Area per bedroom
-----------------------------------------------------
Gravel, coarse or medium sand	  Not recommended
Fine sand, loamy sand		  200 square feet
Sandy loam, loam	          250 square feet
Loam, porous silt loam		  325 square feet
Silty clay loam, clay loam, clay  Not recommended
-----------------------------------------------------
*Based on 150 gallons/day per bedroom and gravity-loaded trenches

Operating and Maintenance Tips

Troubleshooting Septic System Problems

A malfunctioning septic system can turn the dream of country living into a nightmare. There are no simple solutions for many problems where a system was installed in unsuitable soil or was improperly designed and constructed. But other malfunctions, if correctly diagnosed, can be solved quickly and with a minimum of expense.

Following is a guide for troubleshooting and correcting septic system problems when they arise.* Outlined are (1) the two most common symptoms of system malfunction, (2) the potential causes of each and how to check them out, and (3) suggested solutions to the problems once identified. (To facilitate troubleshooting, you should have a map of your system, showing location of the absorption field and tank clean-outs. This simplifies repair and prevents the lawn from being torn up needlessly.)

Symptom A. House Drains Don't Work or Sewage Backs Up into Basement

First, the trouble must be pinpointed. To do this, measure the liquid level in the septic tank. If it's normal (i.e., a foot or so below the top of the tank), go to Cause A. 1 or A.2; if above normal, go to Cause A.3.

Cause A. 1. A blockage between house and septic tank. The blockage may be in the house sewer, or the scum layer could be plugging the inlet pipe at the tank. If scum is the problem, the tank should be pumped down. Have the inlet baffle checked at that time. If functioning properly, it should keep floating scum away from the tank inlet.

A house sewer blockage can be removed with a sewer routing tool from the clean-out at the house end of the line. Sewer cleaning is usually a job for a professional. If root penetration is the reason for the blockage, pipe joints should be resealed after routing to make them watertight.

If the blockage re-occurs in a new system, the problem may be improper sewer line slope (see earlier discussion concerning sewer slope); and the only lasting solution is to relay the line, using the correct slope. If the blockage re-occurs in a previously trouble-free system, the most likely cause is a broken sewer pipe section, which must be located and replaced.

Cause A.2. A plugged house sewer vent. In some cases, a plugged sewer vent (or soil stack) can slow the rate at which sewer lines drain to the point that solids settle out (Figure 5). A plugged or incorrectly installed vent sometimes results in a sewer gas smell around house drains; but more typically, it will just cause a gurgling sound as air is pulled through the trap into the house sewer when drains are used.

Figure 5. Household plumbing system.

Plugged-vent problems should not occur if the plumbing code was followed during construction. However, temporary blockage can occur in winter when undersized roof vent openings freeze shut. The soil stack should be 4 inches in diameter where it passes through the roof, and should either extend 6 inches above the roof or 2 feet higher than any point on the roof that is 10 feet away (as measured horizontally), whichever is greater (Figure 6). Needed vent corrections should be made by a qualified plumber.

Figure 6. Household soil stack vent.

Cause A.3. A blockage between septic tank and absorption field. If the liquid level in the septic tank is found to be above normal, either: (a) the tank outlet is plugged, (b) the line from the tank to the absorption field is obstructed, or (c) the absorption field is clogged. If the latter is the case, there will probably be evidence of seepage or general wetness in the absorption area.

a. Plugged tank outlet. In septic tanks which have been used for many years, the outlet baffle sometimes disintegrates or collapses. This allows scum and sludge solids to overflow and plug the outlet or the line to the absorption field. The "cure" is pumping the tank down, routing out the line and replacing the defective baffle.

b. Obstruction of tank-to-field line.Likely causes are solids overflowing the tank, tree roots getting into pipe joints or collapse of a pipe section. Again, the immediate remedy is pumping the tank and clearing the line, followed by repair of the leaky joint or broken section.

Sewer pipe can break if not uniformly supported on soil or gravel fill or if a shallow line is driven over by heavy vehicles. Tree roots can get into sewer lines through leaky pipe joints. Pipe plugging should not be a problem in the trench area, since tree roots seldom penetrate very far into the gravel fill around the pipe.

c. Clogged absorption field. See discussion under Cause B.2 below.

Symptom B. Seepage or Swampy Conditions in Absorption Field Area

Cause B.1. Too small a filter field. Many older homes have septic systems that are inadequate to handle the large amounts of water used in modern living, especially in cases where the original home has been enlarged without expansion of the septic system. If this describes your situation consult Table 2 or your county Board of Health personnel to see if the absorption field is properly sized for your home and soil conditions.

If found to be seriously undersized, the present field should be enlarged or a new one constructed. In fact, the best decision might be a completely new system, since the septic tank also is probably too small. Often the old field can be reused as an alternate absorption area after resting a year or two.

If capacity is inadequate but not severely so, one solution could be to install water conservation devices in the home. Low-flow toilets and shower heads, faucet aerators and other devices can cut water use significantly with very little change in lifestyle.

Cause B.2. Clogged soil absorption field. Soil absorption fields naturally clog somewhat over time, since effluent from the septic tank contains dissolved materials. However, if the field is undersized or improperly constructed, development of a "clogging mat" can reduce soil permeability to the point where tank effluent cannot be absorbed at the rate it is produced.

The best solution to a soil clogging problem is to rest the absorption field. This allows the organic matter to oxidize (decompose), thus restoring the permeability of the soil around the trenches to near its natural state. Resting the area, however requires that a second filter field be available to accept effluent for a year's time. Being able to switch from one absorption area to the other every year reduces soil clogging problems and significantly lengthens the total system's life. Switching fields should be done during the summer when soil temperatures are high to obtain the best soil treatment of sewage. Alternating fields is especially effective in slowly permeable (high clay) soils.

Cause B.3. High water table in spring. Septic system operation can become sluggish and even fail in the spring, because a seasonal high water table may saturate the soil around the trenches. Homesites and absorption fields in flat areas having poor surface drainage are especially susceptible. If this describes your situation, about the only thing you can do is to use subsurface tile drainage to lower the water table. The drainage tile must discharge to a surface ditch or to a larger tile drainage system.

If located on a sloping site, installation of drain tile upslope from the absorption field should be very effective in lowering the water table. Place the tile at least 25 feet away from the filter field and at least as deep as the bottom of the trench. Home water conservation devices and practices may also be needed to make it through wet springtimes and prolonged rainfall periods.

Cause B.4. Solids carryover and leaking faucets. The overflow of solids from a septic tank which has filled with sludge or the increased water load from leaky faucets can seriously affect absorption field operation. The cures, of course, are periodic tank clean-out and keeping the plumbing fixtures in good repair.

Getting Additional Help

If you are planning to repair a septic system, it's best to first seek advice and help from county Health Board personnel. They can make valuable location, design and construction recommendations that will minimize the chances of failure.

If building a home today, soils information on the proposed homesite is required before the local Board of Health will issue a permit for absorption field construction. Your county Soil Conservation Service or Cooperative Extension Service office can provide or help gather this information. Your Extension office also has related publications available.

A well designed and located private home waste disposal system, properly constructed and maintained, should provide years of trouble-free, dependable service.


*This section is adapted from a publication by James C. Converse, Agricultural Engineering Department, University of Wisconsin, 1977.

A publication of the Purdue University On-Site Waste Disposal Project (involving Agronomy, Agricultural Engineering and Civil Engineering) in cooperation with the Indiana State Board of Health.


RR 11/92
Cooperative Extension Work in Agriculture and Home Economics, State of Indiana, Purdue University and U.S. Department of Agriculture Cooperating. H.A. Wadsworth, Director, West Lafayette, IN. Issued in furtherance of the Acts of May 8 and June 30, 1914. It is the policy of the Cooperative Extension Service of Purdue University that all persons shall have equal opportun ity and access to our programs and facilities.