Septic System Owner's Guide

Maintenance and CareSeptic System Features

The design and installation of a septic system is controlled by local and state rules through the permit process. The permit takes into consideration all specific site characteristics including the type of soil, size of house, and wastewater-contributing fixtures and appliances. The system must be installed by licensed contractors and inspected by qualified officials to ensure proper installation. Operation and maintenance of the system is the owner's responsibility. Contact the local responsible agency (planning and zoning, environmental services, etc.) with questions about local requirements. SepticSystem-Maint-Care6583f0i.gif

The complete septic system is made up of three primary components:

  • Plumbing: wastewater collection
  • Septic tank: primary treatment
  • Soil treatment system: final treatment

Because systems are individually designed and have been installed over many years, there are many variations in the features and descriptions that follow.

Plumbing: Wastewater Collection

All wastewater containing human wastes, nutrients, dirt, and other contaminants must be collected and delivered to the septic tank and drainfield for treatment and disposal. All water used in bathing, toilets, laundry, and dishwashing must be treated by the system. Drains allowing wastewater to enter the system should be equipped with strainers and other filtration devices to reduce the amount of food particles, hair, and lint entering the system.

Some older homes may have been plumbed to bypass the septic tank with wash water but this has proven unsuccessful and damaging to drainfields. Minnesota's rules require all wastewater to be treated. However, water from roof drains, basement drainage sump pumps, hot tubs, and swimming pools should not be put into the septic system. These large volumes of clean water will overload the system.

Original and remodeled plumbing systems must be correctly designed and installed to allow trouble-free operation. Before remodeling, consider the impact of changes on the septic system.

Septic Tank: Primary Treatment

How the Tank Works
The contents of the septic tank separate into three layers:

  • Floating scum layer - soaps, greases, toilet paper, etc.
  • Liquid layer - water, liquid, and suspended solids
  • Sludge - heavy organic and inorganic materials at the bottom of the tank.

Naturally-occurring bacteria in the sewage begin to break down organic materials in the tank. This is often referred to as primary treatment. Pathogens in the waste are NOT destroyed in the septic tank. Bacteria in the septic tank prepare the wastewater for final treatment in the drainfield.

Components of the Tank
The septic tank is the first step of the wastewater treatment process. The septic tank is a solid tank designed specifically to accept all wastewater from the home. Some installations may have two tanks in a row or one large tank with two compartments. Several tank designs are available but all tanks should have inlet and outlet baffles, inspection pipes, and a manhole for cleaning (Figure 1). A few homes on small lots or in poor soil treatment situations may have a large holding tank to store wastewater until the entire contents are hauled away for treatment at another location.

The inlet baffle forces wastewater entering the tank to be mixed with the liquid contents to begin bacterial breakdown of organic materials and separation of solids. The inlet baffle also prevents the floating scum layer from floating back and clogging the inlet pipe.

The outlet baffle prevents scum from leaving the tank. If the scum layer reaches the outlet pipe, the pipe will become plugged. Scum in the drainfield will clog soil pores and destroy its ability to treat wastewater. Filtering devices can be installed at the outlet of new or existing tanks to prevent solids from reaching the drainfield. Regular maintenance is required to keep the filters from clogging and causing backups. Filters are not a substitute for proper operating or maintenance practices!

Inspection pipes of 4- or 6-inch PVC (plastic) material should be located above the inlet and outlet baffles to allow for inspection of pipes and baffles. Clogs in the inlet or outlet pipes can be unplugged through the inspection pipes. When operating properly, the septic tank is always "full" to the level of the bottom of the outlet pipe.

Inspection pipes must always be capped. They may be cut off flush with the ground to ease lawn care; however, the pipes should be left "long" until the final grade on a new site is determined. Metal covers can help in locating the inspection pipes when the ground is covered with snow.

! Do NOT use inspection pipes
to clean or pump a tank!

The manhole in the cover of the septic tank is the large entrance (20"-24") through which the tank should be cleaned. The manhole is often buried below ground level. It may be raised from the cover of the tank with concrete or plastic rings for easier access. It is usually located in the center of the tank; however, some manufacturers locate it closer to the inlet end of the tank. There may be more than one manhole, in which case they are usually located at the ends of the tank. The manufacturer or installer may be able to tell you where it is.

! Be careful when removing
the manhole cover! It is
heavy and creates a large,
dangerous opening!!
A variety of manhole configurations exist.

The manhole allows proper cleaning and inspection of the tank (see Maintenance and Care). The manhole cover must be kept securely in place. If the septic tank cover does not have a manhole or inspection openings, sometimes a new cover with these features can be installed on an existing tank.

Soil Treatment System: Final Treatment

How the Soil Treatment System Works
All septic systems include the same basic plumbing and septic tank components. Final treatment of wastewater occurs in the soil. Uncompacted, unsaturated, undisturbed soil must surround the soil treatment system. This system may be a series of trenches or a mound (Figures 2 and 3). Soil treatment kills disease-causing organisms in the sewage and removes nutrients. There are millions of naturally-occurring beneficial microscopic organisms in every tablespoon of soil. These complete the sewage treatment process.

The beneficial bacteria in the soil need air to live. Therefore, a zone of unsaturated soil must be present below the drainfield for complete treatment. In Minnesota, a minimum of three feet of unsaturated soil below the drainfield is the recognized standard. Some local units of government have established more strict requirements, such as four feet of separation from saturated soil.

The biomat is a thin layer of fine solids, dead bacteria, and soil bacteria that forms where the sewage meets the soil. This biomat layer regulates how fast liquid passes out of the trench or bed into the soil so the soil beneath the trench remains unsaturated. Once the wastewater is through the biomat layer and three feet of unsaturated soil, harmful pathogens have been destroyed.

Saturated soil is determined by its color and mottling. Mottling is the change in color of the soil due to water saturation. It is detected by soil borings done by professionals when the system is designed.

Site conditions and local requirements determine the soil treatment system for each site. If there is three feet of separation from the bottom of the drainfield trench to saturated soil, the least expensive distribution and soil treatment system is gravity flow to a simple trench system, as illustrated in Figure 2. If there is not the required separation for a trench, a mound is required, as illustrated in Figure 3. A mound system is an elevated drainfield built with clean sand. There are many small variations in design, but all trenches and mounds accomplish the same treatment function. New mound systems require pumps and/or lift stations and a pressurized wastewater distribution system.

Soil Treatment System (Drainfield)
Common terms for the soil treatment system are: drainfield, mound, seepage bed, leach bed, and soil absorption field. The soil treatment unit is where the final treatment and disposal of the septic tank effluent takes place. A properly designed and installed soil treatment system will destroy all disease-causing pathogens and filter out the fine solids contained in the septic tank effluent. Phosphorus will be adsorbed by (attached to) soil particles, and nitrate-nitrogen may move through the soil with the water.

In the summer, a shallow drainfield trench supplies water and nutrients to grass and trees. The nutrients that remain in the downward percolating water will be either changed to gas by soil bacteria or diluted. Nitrates in drinking water are rarely a problem with a soil treatment system when the nearby well is deeper than 50 feet and has a sealed casing.

The two most commonly used types of soil treatment (drainfield) units are trenches and mounds.

Trench: Drainfield trenches effectively treat liquid flowing from the septic tank. They are the most economical to install and are preferred when possible. A drainfield trench is a level excavation 18 to 36 inches wide and up to 100 feet long. The trench contains a perforated pipe in a bed of 3/4-inch to 2-inch diameter rock covered by natural or synthetic permeable fibers. Some soil treatment systems use large plastic tubing or some other chamber wrapped with fabric in the trench in place of rock. A 6- to 12-inch deep layer of topsoil covers the trench. Sewage flows through the holes in the distribution pipe, to the rock (or tube), through the biomat, and into the soil. Bacteria and fine sewage solids are removed or destroyed in this process.

The trench system may be laid out in one of many configurations to allow for the necessary square feet of surface. There are often inspection pipes on one or both ends of the pipes. These can be cut off at ground level and capped for easier lawn maintenance. The ground surface level of the soil treatment area should always be level or slightly raised above the surrounding ground to avoid excess rainfall flooding the system.

Mound (Elevated Seepage Bed): A sewage treatment mound is a seepage bed raised with clean sand to provide adequate separation between the wastewater in the mound and the saturated soil or an impermeable hardpan of soil or bedrock beneath. The mound is carefully constructed to provide adequate treatment of sewage. It is equally as effective in treating sewage as a trench system as long as it is properly constructed and operated, and the septic tank is maintained correctly. The mound system, as illustrated in Figure 3, has a pressurized distribution system of 1-1/2 or 2-inch perforated pipe in a layer of small rock. A layer of sand covers the rock. The mound is covered with topsoil and planted to grass. The grass should be mowed regularly.

Distribution System
Each site has a unique shape and slope. The soil type, percolation rate, water volume to be treated, and other factors determine how large an area is needed to properly treat sewage. To provide the necessary area, the design may be a series of many pipes, or "stepped" down a slope.

The distribution of effluent into the soil treatment system is accomplished using drop boxes and distribution boxes (Figure 4). The covers of either kind of box can be removed for inspection and cleaning. All pipes to the drainfield trench or seepage bed are solid with sealed connections.

Pumps and Lift Stations
If the liquid flowing out of the septic tank cannot flow to the trench or mound soil treatment system by gravity, a lift station, a small concrete tank containing an ejection pump, will be installed to move the liquid (Figure 5). The pump operates on a float-controlled switch. When the storage area in the tank is full it will pump the contents to the soil treatment bed. The pump has an emergency alarm indicator circuit to warn the homeowner when the pump has failed to remove the contents. If this happens, the problem needs immediate attention. Be sure to know where this alarm is, what it means, and what to do when it is activated.

SepticSystem-Maint-Care-fig1.gif Fig-1
SepticSystem-Maint-Care-fig2.gif  Fig-2
SepticSystem-Maint-Care-fig3.gif  Fig-3
SepticSystem-Maint-Care-fig4.gif  Fig-4
SepticSystem-Maint-Care-fig5.gif  Fig-5
Septic System Costs

The only way to get a true estimate is to circulate an approved design to several licensed system installers.  The following costs should be considered "ballpark" only.

The cost of a standard gravity system for a three bedroom house on a level site in sandy soil can vary widely from place to place but it should be roughly between $1,800 to $4,000 complete, to a county licensed excavator plus state or local taxes. If plastic vaults are used, the cost will be on the higher side of this range. Vault systems will usually be smaller than gravel designs, but the cost of the vaults is more than the relatively inexpensive drainrock.

Availability and prices for tanks, drainrock, vaults and pipe doesn't vary a whole lot from place to place, but poor soils and extra bedrooms will simply cost you more.   Fine silty soils require more drainfield and are more risky to build due to slightly higher failure rates.  The drainfield vaults these days are around $25 to $35 each and good quality clean drainrock is around $8 - $12 per ton if the pit is nearby.   Concrete tanks are the only practical choice, and these run around $500 for a 1000 gallon tank delivered within 50 miles of the yard.  A 1250 gallon tank  for an extra $90 is preferred by some homeowners, and minimum tank size is determined by state and local rules.  I recommend a 1000 gal tank up to 4 bedrooms if allowed because pumping trucks are usually 1000 gallons and can pump your system in one trip.  The larger tank for an average house offers little advantage except for homeowners who put off maintenance.

The number of excavators licensed in the area and how busy they are will have a huge affect on costs.  The strictness of rules of local health in the design of systems and the toughness of the installer's test can encourage or discourage the number of contractors competing for the construction of systems.  Excavators are already mobile and they can enter a market area looking for better prospects.  Hungry new excavators in an area can drive down prices and put pressure on established contractors to lower rates.
Equipment for evaluating the site:
Each designer has slightly different preferences as to equipment. Besides the usual clipboard and assortment of pens, pencils and maps and a few special tools, I rely on 6 pieces of equipment to study the site: 1) a handpick, 2) a 25 ft tape (Lufkin® only), 3) a Rolatape® measuring wheel for distances (much superior to a 200 ft tape measure), 4) a Sunco® clinometer, indispensable for measuring and estimating slopes, banks and levels, 5) a spray bottle of ammonia based window cleaner used to moisten soil samples for checking texture as well as for general hand cleaning and, 6) a digital camera.

When you stop exploring soil and begin construction, there is often no going back.  The site at this point may be ruined forever by getting it wrong.

Drainfield location is generally downslope from the tank.  In our example, there is little extra space on the site so the drainfield location is more or less fixed.  Whatever soil is in this location, will have to be accepted as a given. The designer may have to negotiate with the contractor and the owner if there is not enough space for everything. You will generally not be able to clip a little off one of the trenches to fit in the corner of the driveway.

On a larger property, if the test holes showed a poor soil type (usually silty or restrictive soil, solid rock or water), another pit or two or three could be dug to get a better location for the drainfield. Fill in the poor test pits before local health sees them. In most counties this is customary and not considered deceptive.  Local health will only evaluate what you show them and your job is to put the best appearance on your property for the evaluation in the location where you want the drainfield. Finding a favorable location for the drainfield and then sizing it to match the ability of the soil to absorb water is the job of the soil expert. In most counties this expert is a consultant to the property owner and is known as a designer, an engineer or a soil scientist. In some counties, the health inspector or "sanitarian" will perform this duty. In some increasingly rare cases the local health inspector will both design the septic system, and perform the final inspection of the job too.

Soil classification is next. 
The soil classification system used in most places in the USA (and in some other countries), is called the US Department of Agriculture Soil Conservation Classification System. Be aware that the ability to determine such qualities as "soil texture" and "soil structure" comes with experience. If you blow the soil classification, you may wind up with less or more drainfield than you need. More drainfield means needless expense. Less drainfield means early failure, usually with guests over.

The most restrictive soil type found in any of the pits in the area of the drainfield should guide your choice in the case of mixed or confusing soils anywhere within the drainfield area. Most test pits involve four or more soil types as you look down through the soil profile within each pit so study the soil directly below where the drainfield will be (between 36 inches and 48 inches below grade usually).

By the way, a square foot of gravel in a drainfield is measured like a carpet, covering a 12 inch by 12 inch piece of ground, except the gravel is one foot thick. Therefore, a drainfield trench "covering" 300 square feet, is a pit, three feet wide and one hundred feet long, with a foot deep of gravel in the bottom. (In reality, you will use two trenches at 50 feet long each.) The gravel is covered with a tough but thin fabric to keep the dirt out called filter fabric. The whole thing is buried with a cover (backfill) of one to two feet of native soil. The thickness of backfill depends on the desired trench depth. The "gravel" is not really gravel at all, but a uniform clean washed rock with granules one and a half inches in diameter known to a gravel pit operator as "inch-and-a-half drainrock".

PVC Vault Technology:
To confuse things further, you may in some areas substitute a couple of rows of plastic vaults usually known as Infiltrators® (a trade name) instead of the gravel. These vaults hook together like a freight train and are very easy to build. Excavators and septic contractors and installers usually prefer the vaults over drainrock once they have tried them. Most health jurisdictions are recognizing the value and efficiency of the vault technology and the use of vaults is becoming widespread across the USA. Also, some health inspectors give "credit" for some of the sidewall of the trench for vault or gravel systems. You may get 400 Sq Ft of "credit" for our trench that actually contains only 300 Sq Ft of ground. This may save vital space on a tight site.

Vaults now come in several widths and lengths. However, the standard width is a little less than three feet wide (but generally 3 feet is the width used for design and space planning) and two standard lengths 

To determine the retail cost of your drainfield, figure that 6 foot vaults (6.25 ft) will cost $95 each in place with 4 foot vaults costing $65 in place ($20 - $30 at the store), and drainrock type drainfields in place will cost roughly $4 per square foot (drainrock is $15-20 / ton delivered). The tank is about $500 delivered. Some jobs require a D-Box with fittings and seals for about $130 total, and perhaps filter-fabric to keep dirt out of the drainrock at $30 to $50 depending on trench length (fabric not needed with vaults). Hiring an excavator costs another $7-900 for pipe laying, sewer line excavation final backfilling and other tasks, or a lot more depending on job layout and slopes. Click here for further cost estimates.

In most counties, in most states, the new plastic vault technology has replaced the traditional gravel drainfield.  Although, the Septic Design CD available on this site provides for both gravel and vault drainfields. Eventually the vaults will become the standard drainfield type. Notice that the vaults provide treatment with less area depending on the soil type. Usually local health will allow a quarter to a third less drainfield with the vaults. This is because of the superior storage volume provided by the vaults compared to drainrock.

Are you now ready to determine the area of trench needed for your home. Remember, dig a hole or two and check the soil type.

How to use the chart:
In spite of what you think, the drainfield size is not dependent on the number of bathrooms or fixtures in the home. Almost all health departments use the number of bedrooms in a home as a way to size drainfields. The number of people in the house (usually two to a bedroom), and their usual daily water use (usually sixty gallons per person per day), is how the flow rate is established. This flow rate of about three hundred sixty gallons of water use per day inside a three bedroom house determines the amount of sewage that must be sent to the drainfield. Once you have decided what type of soil is found under your future drainfield, look up the drainfield area required for your house size in the chart below and you have the required drainfield size.
Note: Your county may have slightly different sizes.

Soil Class Soil Type: take a soil sample 3 to 4 feet below grade in the drainfield area by digging a pit 2 Bedroom House 3 Bedroom House 4 Bedroom House
# 1 Coarse Sand

200 sq ft gravel or

10 six ft vaults or

16 four ft vaults

300 sq ft gravel or 14 six ft vaults or

22 four ft vaults

400 sq ft gravel or

18 six ft vaults or

28 four ft vaults

# 2 Medium Sand

240 sq ft gravel or 12 six ft vaults or

20 four ft vaults

360 sq ft gravel or 16 six ft vaults or

25 four ft vaults

480 sq ft gravel or 21 six ft vaults or

33 four ft vaults

# 3 Fine Sand - Loamy Coarse Sand - Loamy Med Sand

300 sq ft gravel or 10 six ft vaults or

16 four ft vaults

450 sq ft gravel or 15 six ft vaults or

24 four ft vaults

600 sq ft gravel or 20 six ft vaults or

32 four ft vaults

# 4 Very Fine Sand - Loamy Fine Sand - All Loams

400 sq ft gravel or 14 six ft vaults or

22 four ft vaults

600 sq ft gravel or 20 six ft vaults or

32 four ft vaults

800 sq ft gravel or 26 six ft vaults or

42 four ft vaults

# 5 All Silt Loams of Good Structure

540 sq ft gravel or 18 six ft vaults or

28 four ft vaults

800 sq ft gravel or 26 six ft vaults or

42 four ft vaults

1070 sq ft gravel or 35 six ft vaults or

55 four ft vaults

# 6 Other Silt Loams - All Clay Loams - All Clays

1200 sq ft gravel or 39 six ft vaults or

62 four ft vaults

1800 sq ft gravel or 60 six ft vaults or

94 four ft vaults

2400 sq ft gravel or 78 six ft vaults or

122 four ft vaults

Drainfield layout requires equal sized trenches. From the septic tank, at least two equal sized trenches or lines must be designed.  A single drain line is no longer advised.  The separation of flow into two, three or more lines is accomplished with a distribution box or "D-box" to split the flow.  In the D-box pipes are equipped with simple flow control valves in the form of eccentric plugs that evenly split the flow between lines.  The effluent (sewage that has gone through the septic tank)  flows downhill from the tank outlet, through the D-box and down to the individual trenches where it spills out onto the floor of each trench where treatment starts in the soil at that location. Don't forget, the individual trenches are NOT sloped, but are dead level from one end to the other.

Your local health department rules. Your county health department has rules and guidelines to follow. Sometimes rules are the same as state guidelines, but sometimes more restrictive rules special to your county must be followed.  These rules include depths and setbacks and construction details.   Such things as how far you can place the drainfield from a water well (usually 100 feet), a building (usually 10 feet), a water line (usually 10 feet), a stream, pond or lake (75 to 100 feet), the septic tank (generally 5 feet), or even a tree (5 to 50 feet depending on species) cuts and banks (varies state to state, and county to county) are spelled out. They will specify how deep the trenches can be (usually no deeper than three feet max from final grade down to the floor of the trench), and even whether you may use the plastic vault technology shown here.  The key to getting the correct results from your health department people, is to present your ideas clearly and completely in your drawings. There is often some back and forth with the health department. You may be asked several times to return with fresh drawings to meet all of the site requirements and rules that the department has on its books.

Before you begin drawing your project, you must have all the rules from local health. This may be in the form of a two-page handout or a thick ordinance of dozens of pages.