COOPERATIVE EXTENSION
EB0707


CARE AND FEEDING
OF SEPTIC TANKS

Ronald E. Hermanson, P.E.

The most popular method of sewage disposal for dwellings in areas without community sewage facilities is a septic tank with an underground drainfield. If you live in or plan to buy a home with a septic tank system, you should be aware of the system's design and operation.

A well-designed, properly constructed, and adequately maintained septic tank system can serve a home as efficiently as a city sewer. On the other hand, a system that does not receive proper care and attention can become a burdensome expense to a homeowner and a health menace.

Before buying a home, ask the local health department if the septic tank and drainfield's design capacity will adequately serve the anticipated number of occupants of the home. Also, ask the department if the system's location endangers the water supply, and if the drainfield and soil system can dispose of the liquid wastes throughout the year under all weather conditions. See A Septic Tank System for Your Home, Bulletin DSHS 22-45A, for further information.

Septic System Function

Septic tanks settle and treat such household wastes as soapy water from the laundry and bath, discarded food scraps, and body wastes. Solids settle to the bottom of the tank, bacteria destroy some organic matter by changing it into gases, and clarified liquid flows into the drainfield where it seeps into the soil. Filtration by the soil and soil microorganism activity remove most of the impurities in the liquid wastes. By the time treated wastewater reaches groundwater, disease-causing organisms will be reduced to safe levels in a properly designed, operated, and maintained system.

Maintenance

When septic tank systems are improperly located, designed, installed, or maintained, liquid wastes can flow to the ground's surface and can also pollute groundwater. Surface flow creates offensive odors and can be a health hazard, often containing organisms that cause typhoid fever, dysentery, and other diseases. Surface sewage flow into ponds provides a breeding ground for mosquitos and other disease-carrying insects. It is particularly hazardous for children and pets playing in the area.

To maintain a septic tank properly, inspect it at least once every two years and pump it out if necessary. Tank size, sewage volume from the home, and the sewage's solids content determine how often this must be done. Sewage volume depends upon the number of people the system serves and the amount of waste they create. The number and types of household appliances also influence sewage volume. Using a household garbage grinder, for example, requires more frequent pump-outs unless the tank is large enough to accommodate the additional wastes. A better solution is to not use a garbage disposal, since they can increase solids by 50 percent.

Cleanout

Clean out septic tanks every 3-5 years, depending on tank size and the amount and quality of solids entering it. Determine the cleanout interval on the basis of 100 gallons of tank capacity per person per year. For example, clean out a 1000-gallon tank used by a family of two after 5 years [1000 ÷ (100 X 2)]. Commercial septic tank additives do not eliminate the need for periodic cleanout.

You may routinely pump out your tank every 3-5 years. Or, to determine when to pump the septic tank, measure both sludge and scum depths at the outlet side of the tank at least once a year, starting with the third or fourth year after a cleaning. In a two-compartment tank, measure at the outlet of the first compartment to the second compartment, if possible.

Measuring Sludge Depth. Wrap a long measuring stick with a terry cloth towel to a height of about 4 feet. Remove the observation hole cover nearest the discharge end of the tank, and 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 tank bottom. If the baffle is an elbow type with no top opening, make a hole in the scum mat near the baffle with another stick, and then lower the measuring stick through the hole. Next, press down and slowly rotate the measuring stick a couple of times to make sure it reaches the bottom. Finally, slowly remove it. Black particles clinging to the towel distinguish the sludge layer from the liquid depth. Measure the depth of this black portion to determine sludge depth.

Measuring Scum Depth. Use a 3-inch square piece of wood attached to the end of a long stick. Push the measuring device through the scum layer into the liquid layer. As you move the stick carefully up and down, a change in resistance on the wood indicates the bottom of the scum layer. When you find the bottom, 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. Use the same reference point to re-mark the stick. The distance between the marks equals the distance between the scum layer bottom and the outlet baffle bottom.

Clean a septic tank when the top of the sludge is within 12 inches of the outlet baffle bottom, or when the bottom of the scum layer is within 3 inches of the baffle bottom. Hire a reputable septic service company or otherwise experienced operator with appropriate equipment to clean your septic tank. Their service usually includes pumping, inspection of tank openings and baffles, and disposal of the sludge in an approved manner. Pump the tank as empty as possible to remove most solids and restore tank capacity.

Fit septic tanks with concrete or fiberglass risers over the observation hole, so that the tank can be located and inspected without digging up your lawn. Make sure the risers have secure covers to prevent accidents or mischief.

Additives and Household Products

No known additives can reduce the solids in a septic tank sufficiently to make cleaning unnecessary. However, household wastewater contains an abundant supply of microorganisms that degrade the organics so that additives are not needed.

Soaps, detergents, bleaches, or other water-based household cleaning compounds used in normal quantities won't interfere with septic tank action. Don't discard solvents, paint thinners, pesticides, automotive oil, or petroleum-based products in septic tank systems. For a period of time, large quantities of these materials can affect septic tank function. More importantly, even small quantities may leach into and contaminate groundwater.



Scum and sludge building determines cleanout frequency.

Call the Washington State Department of Ecology Recycling Hot Line (1-800-RECYCLE), and refer to WSU publication EB 1522, Disposal of Hazardous Waste, for information on the proper recycling and disposal of these materials. Do not place paper towels, disposable diapers, newspapers, wrapping papers, rags, sanitary napkins, tampons, cat litter, and other items not normally considered sewage into your septic tank. They may plug the plumbing, they do not decompose readily, and they fill the tank more rapidly. This can lead to failure of the drainfield if the tank is not pumped out.

Heavy trucks or equipment moving over an area where septic tanks and drainfields are located can damage them. Make a diagram of the system to guide vehicles away from the critical area. If there is no way to avoid crossing the sewer line, install cast iron pipe under the crossing. The local health department may recommend other alternatives.

Tree and shrub roots may enter the drain lines and clog them, forcing the liquid wastes to the ground surface or back into the house. When this occurs, consider relocating the drainfield, digging up and cleaning the drain lines, or treating the drainfield with copper sulfate. Your local health department may permit flushing 3 pounds of copper sulfate crystals down a toilet to temporarily solve the problem. The crystals damage chrome, iron (but not cast iron), and brass, so keep the crystals away from those metals. Consult your health department if the problem continues.

Drainfields suffer most frequently from septic tank neglect. When the tank is neglected, solids fill it and flow into the drainfield, where they stop the flow of liquid wastes. The system then must be uncovered, cleaned, and relocated. This costly process underscores the need for periodic inspection and pump-out of the septic tank. Preventive maintenance saves this needless expense and trouble.

Recent research has demonstrated that softened water and the salt brine from water softener regeneration do not damage the drainfield if both are put into the system. For hard water, a water softener regenerates once a week and discharges 50-100 gallons of water into the septic tank per hour. Common household uses discharge this much water and does not overload the drainfield. More than this amount of water use can overload a drainfield that borders on failure. Avoid this overload in a new drainfield, or remedy it in existing ones, by sizing the drainfield for one more person than will actually be in the home.

Alternative Systems

Pressure distribution periodically pumps a fixed volume of effluent into the drainfields of many newer septic systems. Other systems have pressure distribution into a sand mound or sand filter. These systems usually use electrical pumps and controls, and require additional inspection and maintenance. Find out if your system employs one of these alternative designs. Washington Department of Health brochures describing special maintenance for these systems are available at local health departments and Cooperative Extension offices.

Other Considerations


The best treatment method results from careful consideration of such factors as economics, water quality characteristics, water end-use, water temperature variances, and the inherent limitations of treatment technology. Consult local water treatment representatives before purchase and installation of any water treatment equipment.

Caution: never enter an empty or partially filled septic tank. Explosive, poisonous gases produced in the tank can cause death.


By

Ronald E. Hermanson, Ph.D., P.E., Washington State University Extension Agricultural Engineer, Pullman.

Acknowledgement: The staff of the Wastewater Management Section of the Washington Department of Health reviewed this publication and
concurred with the contents.

Issued by Washington State University Cooperative Extension and the U.S. Department of Agriculture in furtherance of the Acts of May 8 and June 30, 1914. Cooperative Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, color, gender, national origin, religion, age, disability, and sexual orientation. Evidence of noncompliance may be reported through your local Cooperative Extension office. Trade names have been used to simplify information; no endorsement is intended. Revised May 1991. Reprinted June 1994. Subject code 376. A. EB0707