Septic Systems

by Chet Boddy

This article was written for my monthly real estate column, "Back to the Land," which has appeared in the Mendocino Coast Real Estate Magazine since January, 1995.

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ONE FOURTH OF THE HOMES and about one third of the population in this country use some kind of on-site private sewage disposal system, more commonly called a septic system. Septic systems are the largest producers of wastewater and also the most common source of groundwater contamination in the U.S. However, when properly designed and maintained, septic systems are relatively trouble-free and have no harmful environmental effects.

The septic tank, introduced in England around 1900, is a watertight underground mini sewage treatment plant. Today they are made of pre-cast concrete, concrete blocks or fiberglass. Septic tanks range in size from about 750 to 1500 gallons and have two or three chambers separated by baffles.

Everything that goes down your toilet or drain ends up in your septic tank. Anaerobic bacteria digest this human and household waste and produce solid byproducts which sink to the bottom of the tank in the form of sludge. The bacteria also produce gases such as carbon dioxide, methane and hydrogen sulfide which exit through the household plumbing vents on your roof.

Anyone who has made wine or beer in a glass carboy has seen a similar process at work. A scum layer floats on top while a sludge layer forms on the bottom, leaving a clear liquid in the middle. In a septic tank this clear liquid, called effluent, drains into a septic leach field (also called a soil absorption field) where it undergoes a secondary aerobic treatment process and returns to the earth as groundwater.

A Brief History of Sewage Systems

Sewage systems are not new. Archeologists have found sewers and even indoor toilets among prehistoric ruins in Europe and the Middle East. The Romans engineered their sewer systems to carry storm water runoff along with refuse that accumulated in the streets. During the Middle Ages, city-dwellers continued the practice of dumping raw sewage and household wastes into the streets, where it rotted and was eaten by maggots, rats and domestic pigs. These deplorable conditions led to devastating epidemics of cholera, plague and other diseases.

Pit privies and cesspools became common toward the end of the Middle Ages. When they filled up, workers dumped the contents into rivers and lakes, on vacant land or in farmers’ fields to be used as fertilizer. Much later, the invention of the steam engine, water pumps and cast iron pipes allowed the development and widespread use of indoor plumbing and flush toilets. In the late 1800s an English plumber named Thomas Crapper, improving on earlier designs, developed the modern flush toilet which bears his immortal name.

During the 1900s, sewage treatment plants and private septic systems have replaced pit toilets and cesspools, along with the practice of dumping raw municipal sewage into rivers, oceans and lakes. Most cities now have separate storm sewers and sanitary sewers, replacing earlier systems which combined the two. Modern sewage treatment systems have contributed greatly to preventing disease, protecting the environment and conserving and recycling water.

Site Evaluations and Septic Permits

Before installing a private septic system in Mendocino County you must have the potential leach field and a reserve area of equal size evaluated by a qualified Civil Engineer, Registered Environmental Health Specialist or Certified Professional Soil Scientist. You can pick up a list of qualified site evaluators at your nearest County Health Department/Division of Environmental Health office. The County also has a booklet which describes the permit process and current design standards.

The site evaluator will arrange for at least six test pits to be dug by a backhoe or soil augur. He or she will examine the soil horizons, look for evidence of groundwater saturation levels and identify any impermeable layers. After taking soil samples from each test pit, the evaluator sends them to a laboratory for analysis. In some cases, the evaluator may also need to conduct wet-weather soil percolation tests, called “perc” tests.

Most farmers and gardeners are familiar with the “soil texture triangle” chart which describes soil types according to their percentage of sand, silt and clay. You might have even conducted the standard classroom experiment of shaking up a soil sample in a jar of water and letting it settle into layers. The thickness of these layers indicates the soil texture. Soil texture determines how fast your leach field will accept sewage effluent, which influences the design of the system.

The site evaluator also prepares a plot plan showing the slope of the land, the location of nearby wells, all existing and proposed structures, ponds, streams, cut banks, roadways and other features. These also influence the design of the septic system.

The County Environmental Health Division must review and approve the site evaluation report before you can apply for a sewage disposal system permit, which is valid for two years. If you are replacing or repairing a septic system you can apply for a septic system repair permit which may or may not require soil testing. If your septic system was installed after 1979, the County probably has a site evaluation report on file.

If you are constructing an addition or remodeling, the building permit will be routed to Environmental Health to determine if you need to expand your septic system.

Septic System Design

There is a septic system design that will work for almost every rural property. However, some properties have high groundwater, heavy soils, steep topography or other constraints which may require expensive design solutions.

Septic tanks and leach fields must be set back 5 to 8 feet from buildings, lot lines and roadways, and 25 to 50 feet from cliffs or cut banks. Potable water wells (including your neighbor’s well) must be 50 feet from septic tanks and 100 feet from the edge of any leach field. Leach fields must be 100 feet away from any year-round streams, lakes, ponds or the ocean mean high tide line. The County may grant exceptions under certain conditions.

In a conventional gravity system, the effluent flows from the septic tank to a distribution box, and then to perforated plastic pipes set in parallel gravel-filled trenches beneath a layer of soil. The size and length of the pipes and trenches is determined by the system capacity and how well the soil drains.

Current County design standards now require “inspection risers” teed into the end of each leach line. These risers extend vertically from the bottom of the gravel bed to just above natural grade. Inspection risers are useful for monitoring effluent levels and marking the end of the leach lines.

If your property has a high water table, you may need a “high line” system, where the leach lines are installed near the natural grade beneath a mound of fill dirt. A “shallow leach bed” system uses a shallow gravel distribution bed (instead of trenches) beneath a dirt mound to raise the system above the water table. A “Wisconsin mound” system raises the leach field completely above the natural grade by using a shallow distribution bed inside a mound.

If your property has steep slopes, you may need a “deep trench” system, where the leach lines are installed deeper than usual and run parallel to the topographic contour lines. If you have problems with lateral groundwater, you may require “French” or “curtain” drains to intercept the flow and divert it away from your leach field.

On some properties, the leach field has to be constructed at a higher elevation than the septic tank. The effluent must be pumped up to the leach field, where flows by gravity. Another system which requires pumping is a “pressure field,” where the effluent is pumped into engineered perforated pipes under a controlled rate and pressure which matches the capacity of the distribution beds.

Mendocino County allows composting toilets and pit privies in combination with “gray water” systems on parcels which are ten acres or larger. A gray water system is the same as a conventional septic tank and leach field system, except it does not accept toilet wastes. Since gray water accounts for about 60 percent of household wastewater, the County allows gray water septic systems to be half the size of conventional septic systems.

What Are the Warning Signs of a Failing Septic System?

The average household septic system lasts about 25 years, depending on how well it was designed and maintained. Some systems may last a lot longer. Problems associated with high groundwater may appear during the rainy season and disappear when the soil dries out. Septic systems can fail suddenly or can give you some early warning signs. These include the following.

  • lush growth over the leach field
  • excessive weed or algae growth in nearby ponds or streams
  • a strong odor of sewage near the septic tank or leach field
  • mushy ground around the leach field
  • sewage or effluent which breaks out and surfaces in the leach field
  • toilets and household drains which empty slowly, stop draining or back up

Dos and Don’ts for Septic System Owners

Here are some ways to make your septic system last longer and avoid the expense of replacing a failed system.

  • Have residential septic tanks inspected and pumped every 3 years by a dependable septic pumping contractor. Commercial septic systems may require pumping as often as every 3 months. Record the date when you have your tank pumped. Your contractor can help you establish a pumping schedule that best matches the needs of your system. Establish a regular pumping schedule and stick to it.
  • The next time you have your septic tank pumped, have an outlet filter installed. Now required for all new septic systems in Mendocino County, outlet filters add another line of defense against solids which can permanently clog your leach lines.
  • Conserve water. More water puts more strain on your septic system.
  • Mark the location of your septic tank, distribution box and leach field.
  • Install inspection risers at the ends of your leach lines. These are now required for new systems.
  • If your system has a flow diversion valve, mark its location and turn it once a year.
  • Don’t pour anything down the drain that will harm your septic system or the surrounding environment. This includes all household hazardous wastes, caustic drain cleaners and excessive chlorine bleach.
  • Don’t pour kitchen oils or grease down the drain. They can’t be digested by the bacteria in your septic tank and can clog the leach field. Commercial kitchens should have a grease trap, and it should be cleaned out on a regular basis.
  • Garbage disposals are acceptable, but they introduce a lot of undigested organic material into your septic tank which will require more frequent pumping. Start a compost pile instead.
  • Don’t use your toilet as a trash can. Don’t flush anything that you can more effectively dispose of in the trash. This includes sanitary napkins, cigarette butts and paper towels.
  • Don’t park or drive vehicles over any part of the septic system.
  • Don’t allow trees or shrubs to grow on or near the leach field. Their roots can clog and damage the leach lines. The best leach field cover is mowed un-irrigated grass.
  • If tree and shrub roots are a potential problem, use a commercial root killer once a year as recommended by your septic contractor.
  • Septic tank additives don’t improve the function of your septic tank and may actually harm it. Don’t waste your money.
  • Don’t allow roof drainage, driveway runoff or other excess water to enter your leach field.

Chet Boddy, Real Estate Appraisal, Sales and Consulting

43300 LR Airport Road, #59, Little River, CA 95456
707-937-4011, office
707-937-4818, fax

Copyright © 2002 Chet Boddy, All Rights Reserved

Chet Boddy is a Certified General Real Estate Appraiser, Realtor and real estate consultant who has lived on the Mendocino Coast since 1976. Look for this and other real estate columns on Chet’s web site at