On-site wastewater treatment systems

Septic tank/soil absorption fieldBruce Lesikar

Extension Agricultural Engineering SpecialistThe Texas A&M University System

Figure 1: A septic tank and soil absorption field system.

T he septic tank and soil absorption system is the most cost-efficient method available to treat residential wastewater. Butfor it to work properly, you need to choose the right kind of

septic system for your household size and soil type, and you need tomaintain it regularly.

This type of waste-treatmentsystem has two components: a septictank and a soil absorption system.

Septic tankA septic tank is an enclosed

watertight container that collects andprovides primary treatment ofwastewater by separating solids fromthe wastewater. It removes the solids

by holding wastewater in the tank andallowing the settleable solids to settleto the bottom of the tank while thefloatable solids (oil and greases) riseto the top. To provide time for thesolids to settle, the tank should holdthe wastewater for at least 24 hours.

Some of the solids are removedfrom the water, some are digested,and some are stored in the tank. Up to

50 percent of the solids retained in thetank decompose; the rest accumulateas sludge at the tank bottom and needto be removed periodically bypumping the tank.

There are three main types ofseptic tanks for on-site wastewatertreatment:

✓ Concrete septic tanks, the mostcommon;

✓ Fiberglass tanks, which are beingused more often because they areeasy to carry to “hard-to-get-to”locations; and

L-52272-99

Soil absorption fieldTwo-compartmentseptic tank

Perforated pipefor effluent disposal

Sand/loam soil

Gravel

Geotextile fabric

A watertightseptic tank prevents

rainwater fromentering the tank and

flooding the soilabsorption field

✓ Polyethylene/plastic tanks, whichcome in many different sizes andshapes. Like fiberglass tanks,these are light, one-piece tanksthat can be carried to “hard-to-get-to” locations.

All tanks must be watertight toprevent water from entering as well asleaving the system. Water entering thesystem can saturate the soil absorptionfield, resulting in a failed system.

From the septic tank, the waste-water passes through the outlet of thetank and enters the soil absorptionfield. The most common outlet is a teefitting connected to the pipe going tothe soil absorption field. However, aneffluent filter can be placed in theoutlet tee for additional filtering of thewastewater. The effluent filterremoves additional solids from thewastewater and keeps them fromclogging the absorption field andcausing it to fail prematurely.

Soil absorption fieldThe soil absorption field provides

final treatment and distribution of thewastewater. A conventional systemconsists of perforated pipes surround-ed by such media as gravel andchipped tires, covered with geotextilefabric and loamy soil. To treatwastewater, this system relies heavilyon the soil, where microorganismshelp remove the organic matter, solidsand nutrients left in the water.

As effluent continually flows intothe soil, the microbes eating thecomponents of the wastewater form abiological mat. The mat slows thewater’s movement through the soiland helps keep the area below the matfrom becoming saturated. The watermust travel into unsaturated soil sothat microbes there and in the mat canfeed on the waste and nutrients in theeffluent. The grass covering the soilabsorption system also uses thenutrients and water to grow.

TreatmentUsed properly, the septic tank and

soil absorption system works well. Itreduces two ratios commonly used tomeasure pollution: biological oxygendemand, which is lowered by morethan 65 percent; and total suspendedsolids, which are cut by more than 70percent. Oil and grease are typicallyreduced by 70 to 80 percent.

Using a septic tank to pretreatsewage also makes other secondarytreatment systems more effective. Theeffluent from the septic tank is mild,consistent, easy to convey and easilytreated by either aerobic (with freeoxygen) or anaerobic (without freeoxygen) processes.

DesignFor a septic tank to perform

successfully, it must be the proper sizeand construction and have a watertightdesign and stable structure.

Tank size: The size of septic tankyou need depends on the number ofbedrooms in the home, number ofpeople living there, the home’s squarefootage and whether or not water-saving fixtures are used. For example,a three-bedroom house, assuming fourpeople live there and it has no water-saving fixtures, would require a1,000-gallon tank (see Table 1).

Tank construction: A key factorin the septic tank’s design is therelationships between how muchsurface area it has, how much sewagethe tank can store, how much waste-water is discharged and how fast itexits. All affect the tank’s efficiencyand the amount of sludge it retains.

The greater the liquid surfacearea, the more sewage the tank cancollect. As more solids collect in thetank, the water there becomes shal-lower, which requires that the dis-charge be slower to allow more timeto separate the sludge and scum.

A key to maintaining a septic tankis placing risers on the tank openings.

If a septic tank is buried more than 12inches below the soil surface, a risermust be used on the openings to bringthe lid to within 6 inches of the soilsurface. Generally, the riser can beextended to the ground surface andprotected with a good lid. These risersreally make it easy to perform mainte-nance on the tank.

Soil texture: There are threetextures of soil: sand, silt and clay.Soil texture affects how fast thewastewater filters into the soil (calledhydraulic conductivity) and how bigan absorption field you need. Sandtransmits water faster than silt, whichis faster than clay. Texas regulationsdivide these three soil textures intofive soil types ( Ia, Ib, II, III, IV).Sandy soils are in soil type I and claysoils are in soil type IV. A standarddrain field cannot be used in a claysoil.

Hydraulic loading: Also impor-tant to the design is the hydraulicloading, which is the amount ofeffluent applied per square foot oftrench surface. Because water filtersthrough clay soils more slowly thanthrough sand or silt, the hydraulicloading rate is lower for clay than forsilt, and lower for silt than for sand.Because clay soils have a very lowconductivity, only nonstandard drainfields can be used in clay.

Absorption field size: The size ofthe absorption field needed is alsodetermined by how much wastewatergoes into the system each day. Dividethe wastewater flow by the hydraulicloading for the soil type in which thefield will be built.

How to keep it workingTo keep your septic system treating

sewage efficiently, you need to have thetank pumped periodically. As the septicsystem is used, sludge accumulates inthe bottom of the septic tank.

As the sludge level increases,wastewater spends less time in thetank, and solids are more likely to

escape into the absorption area. Ifsludge accumulates too long, nosettling occurs, the sewage goesdirectly to the soil absorption area,and little is treated.

Properly sized tanks generallyhave enough space to accumulatesludge for at least 3 years.

How often you need to pump itout depends on:

✓ The septic tank’s capacity;

✓ The amount of wastewaterflowing into the tank (related tosize of household); and

✓ The amount of solids in thewastewater (for example, it has

Table 1. Minimum septic tank capacities for residential houses.

Bedrooms House size Tank capacity Tank capacity(number) (square feet) [without water-saving [with water-saving

devices] (gallons) devices] (gallons)

1 or 2 less than 1,500 750 750

3 less than 2,500 1,000 750

4 less than 3,500 1,250 1,000

5 less than 4,500 1,250 1,250

6 less than 5,500 1,315 1,250

Figure 2. A two-compartment septic tank.

depth may vary, but not exceed 12 inchesunless a riser is installed on each cleanout

and extends to within 6 inches of the ground surface

12 inches clean out

Depth of 30 inches (minimum)

Sand/gravel cushion

Alternate tee fitting

3 inches

Tee fitting

Riser

Ground surface

Inlet

Tee fitting

Tee fitting

1/2 to 2/3of total tank length

Outlet

Scum

Liquid

more solids if you use a garbagedisposal).

In Texas, a 1,000-gallon septictank is used for a home with three-bedrooms without water savingdevices. If four people live in thatthree-bedroom house, the tank shouldbe pumped every 2.6 years (see Table2). If the same system serves a familyof two in a three-bedroom house, thetank should be pumped every 5.9years.

It is important to know that thesoil absorption field will not failimmediately if you don’t pump yourtank. However, the septic tank is nolonger protecting the soil absorption

field from solids. If you neglect thetank for long, you may have toreplace the soil absorption field.

Another maintenance task youneed to do periodically to keep thesystem from backing up is to clean theeffluent filter. Clean it periodically byspraying it with a hose directly intothe septic tank, or have your mainte-nance provider clean the filter.

Soil absorption fields need to beprotected from solids and rainfall. If

you don’t pump the tank, solids canenter the field. Rainfall running offroofs or concrete areas should bedrained around the soil absorptionfield to prevent the field from fillingwith water. Fields that are saturatedwith rainwater are unable to acceptwastewater. Planting cool-seasongrasses over the soil absorption fieldin winter can help remove water fromthe soil and help keep the systemworking properly.

Table 2. Recommended number of years between pumpings of septic tanks according to size of tank and household.

Tank Size Household Size (Number of People)———— —————————————————————————————————————————————

(gals) 1 2 3 4 5 6 7 8 9 10

500 5.8 2.6 1.5 1.0 0.7 0.4 0.3 0.2 0.1 —

750 9.1 4.2 2.6 1.8 1.3 1.0 0.7 0.6 0.4 0.3

1000 12.4 5.9 3.7 2.6 2.0 1.5 1.2 1.0 0.8 0.7

1250 7.5 4.8 3.4 2.6 2.0 1.7 1.4 1.2 1.0

1500 9.1 5.9 4.2 3.3 2.6 2.1 1.8 1.5 1.3

1750 6.9 5.0 3.9 3.1 2.6 2.2 1.9 1.6

2000 8.0 5.9 4.5 3.7 3.1 2.6 2.2 2.0

2250 6.7 5.2 4.2 3.5 3.0 2.6 2.3

2500 5.9 4.8 4.0 4.0 3.0 2.6

Note: More frequent pumping needed if a garbage disposal is used.

Soil absorption fieldsneed to be protectedfrom solids and rain

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All publications in the On-site Wastewater Treatment Systems series can be downloaded free from the World Wide Web at:

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Issued in fur therance of Cooperative Extension Work in Agriculture and Home Economics, Acts of Congress of May 8, 1914, as amended, and June 30, 1914, in cooperation with the United StatesDepar tment of Agriculture. Chester P. Fehlis, Deputy Director, The Texas Agricultural Exension Service, The Texas A&M Universit y System.30,000 copies, New ENG

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The On-Site Wastewater Treatment Systems series of publications is a result of collaborative efforts of various agencies,organizations and funding sources. We would like to acknowledge the following collaborators:

Texas State Soil and Water Conservation Board USEPA 319(h) ProgramTexas On-Site Wastewater Treatment Research Council Texas Agricultural Extension ServiceTexas Natural Resource Conservation Commission Texas Agricultural Experiment StationUSDA Water Quality Demonstration Projects Texas On-Site Wastewater AssociationConsortium of Institutes for Decentralized Wastewater Treatment USDA Natural Resources Conservation Service