Water Qualityin Georgia Septic Tank Design

and ConstructionThe University of Georgia College of Family & Consumer Sciences and College of Agricultural& Environmental Sciences • Cooperative Extension Service

Cecil Hammond and Tony Tyson, Extension Engineers

The first known installation of aseptic tank in the United States was in1876, although Louis Mouras of Ve-soul, France, was given a patent in1881 and credited with the invention.Baffles, which regulate the flow, wereadded in 1905 to make the septic tankmore efficient. The first baffles weremade of oak boards.

At the turn of the century, there weresome very large community septic tanks.In 1903, four community tanks wereconstructed in Saratoga, New York, witha total capacity of one million gallons.

By 1920, septic tanks began to be acommon feature. After World War II,septic tanks became important to hous-ing developments in unsewered areas.

Septic Systemsand Groundwater

A few rules of thumb tell us whenseptic systems are most likely to functionproperly and minimize groundwatercontamination:

Good soil facilitates treatment anddisposal of septic system wastewater.Soil profiles made of sand, silt andclay work best. If there is too muchclay in the soil, the waste may perco-late poorly. If the soil contains toomuch sand and large particles, waste-water may pass through to thegroundwater without being treated bysoil microbes.

Soil treatment occurs best whenabove the water table and the soil isrelatively dry with oxygen present.Water at greater depths allows waste-water to remain in the unsaturated soil,where it can be treated most effectivelybefore reaching groundwater.

Septic systems need space. Onlypart of the microorganisms and chemi-

cals are removed from wastewater as itmoves downward. Even properly oper-ating systems can discharge somephosphates, nitrates and bacteria orviruses into the groundwater. To re-duce loading of groundwater with ef-fluent, install systems on lots withadequate space.

Proper design and use is impor-tant. Septic systems are designed totreat and dispose of a specific volumeand type of wastewater in the condi-tions found at the site. The systemmust not be overloaded. Hazardouschemicals or large amounts of greaseshould not be disposed in septic sys-tems. Kitchen grease should be placedin the garbage, not the septic tank.Water conservation extends the life ofthe system.

Routine maintenance is critical.Septic tanks must eventually bepumped. Sludge and scum accumulateand, if allowed to remain, will eventu-

ally cause the tank to overflow andclog the drainfield.

Good judgment in planning and designand diligent maintenance are the mostimportant aspects of an effective septicsystem management program.

Septic Tank FunctionSewage or untreated household waste

will quickly clog all but the most porousgravel formations. The septic tank con-ditions sewage to allow percolation ofthe liquid portion into the subsoil. Themost important function of septic tanksis to protect the absorption ability of thesubsoil. In doing this, the septic tankdoes the following three things.

Removes solids from liquid. Assewage enters the tank, the rate of flowis reduced and heavy solids settle,forming sludge. Grease and other lightsolids rise to the surface, forming ascum. The sludge and scum are re-tained and break down while the clari-

Figure 1. Cross-section of a septic tank

fied effluent (liquid) is discharged tothe drainfield for soil absorption.

Provides biological treatment.Natural processes break down the sol-ids and liquids by bacterial action. Thebreakdown occurs in the absence ofoxygen (anaerobic conditions). Theanaerobic conditions are referred to as“septic,” giving the tank its name.

Stores scum and sludge. The solidsaccumulate in the bottom of the tank toform sludge. The scum is a partiallysubmerged mat of floating solids andgrease. Scum and sludge are digestedover time and compacted into a smallvolume. Areas with warm climates,such as the southern United States, al-low more complete breakdown of solidsand scum than in the cooler climates ofthe North. For this reason, tanks inwarm climates do not usually need to bepumped or cleaned out nearly as oftenas those in cold climates. Regardless ofclimate, a non-volatile residue of mate-rial remains in the tank. Sufficient vol-ume for the solids must be provided inthe tank between pumpings or clean-ings. If the solids fill the tank and enterthe drainfield, the solids can clog thesoil in the drainfield.

Grease from the kitchen is detrimentalto septic tank functions. Effluent fromgrease traps must go through septic tanksbefore being discharged to drainfields toprevent soil plugging. The best approach

is to put kitchen grease in old milk jugsand place in the garbage rather than intothe drain. Small amounts of kitchengrease can go into the septic tank with-out damaging the system.

Effluent — Bacteriaand Nutrients

The liquid faction that leaves the septictank and enters the drainfield is called theeffluent. The bacterial level of the effluentis quite high, contrary to popular belief.

The effluent also contains nitrates(among other nutrients), which movedownward. To reduce potential forgroundwater contamination by the efflu-ent, many areas restrict building lotsizes. Larger lots reduce loading ratesand help protect groundwater. Some ar-eas with porous or sandy soils are lo-cated in groundwater recharge areas.These areas may be unsuited for septictanks or require building lot sizes 50 to100 percent larger than lots not in therecharge areas. Pathogens break downwith soil contact and pathogen levels arereduced as the effluent percolatesthrough the soil. Bacteria eventually dieand are removed by the filtering effect ofthe soil, further purifying the effluent.

The drainfield pipe is placed on thecontour and perforated to allow the efflu-ent to percolate into the soil. For this rea-son, the percolation of the soil is a critical

factor when determining the amount ofdrainfield needed. A percolation test ofthe soil in the drainfield area is essential.If you are considering installing a septictank, contact your local health departmentand building inspector for local require-ments. The percolation test procedure isdescribed here for your information.

Percolation TestTo conduct a percolation test, dig sev-

eral straight-sided holes (with a holedigger or auger) at least 4 inches in di-ameter down to the drainfield level in thearea to be used for the drainfield.Roughen or scratch any slick clay orcompacted soil in the bottom or sides ofthe holes by scraping lightly. Removeloose material and add two inches of finegravel to each hole. Put water into theholes and saturate the soil, allowing timefor clay to swell. All soils except sandsmust be soaked at least four hours beforepercolation test results are analyzed.

After soaking, add six inches of waterover the gravel and select a fixed point atground level where repeated measure-ments can be made. Use the same timeinterval between measurements and rec-ord the settling distance over the timeinterval. Add water if the depth of thewater over the gravel falls below twoinches. Take measurements at approxi-mately the same time intervals until aconstant rate of percolation is found. Thetime in minutes required for the water todrop one inch is the percolation rate inminutes per inch (see Table 1).

Boring to determine ground water ele-vation in low areas may be required bythe county health department. In such

Figure 2. Septic systems can affect groundwater.

Figure 3. The percolation test

cases, bore to a depth of six feet andprovide sufficient time for the waterlevel to stabilize. Sufficient time maymean overnight for clay soils and noless than 30 minutes in sandy soil.

Drainfield SizeOnce the percolation rate is known,

the drainfield trench bottom area canbe found. Table 1 can be used forresidential areas.

Table 1: Residential drainfield area perbedroom in house

Averagepercolationrate at tile

depth(min/inch)

Trenchbottom perbedroom(square

feet)

Length of trenchin feet

18" 24" 36"wide wide wide

5+ 125 84 63 4210 165 110 83 5515 190 127 95 6420 215 144 108 7230 250 167 125 8445 300 200 150 10050 315 210 158 10560 340 227 170 11370 360 240 180 12080 380 254 190 12790* 400 267 200 134

+ Fastest rate allowed; * Slowest rate allowed

If more than 500 linear feet of drain-field is needed, a dosing siphon is re-quired to disperse liquid throughout thedrainfield. The siphon must have a ca-pacity equivalent to 60 to 75 percent ofthe interior volume of the lines to bedosed. A typical cross-section of a drain-field line is shown in Figure 4.

Location and DimensionsDrainfields should be at least 100 feet

from the closest well or spring, at least10 feet from water supply lines, and notcloser than 50 feet to a pond or stream.

Drainfield trenches should normallybe level and not less than 25 inches ormore than 36 inches in depth. In rarecases, trenches will be deeper and filledwith several feet of gravel to obtain ac-ceptable percolation. The tile drain musthave at least 12 inches of soil over thetile. The aggregate should be a minimumof six inches deep under the drain tile.The drainfield trenches should not ex-ceed 36 inches in width.

Septic Tank CapacitySeptic tanks must provide at least 24-

hour retention time or at least 750 gal-

lons for a one- or two- bedroom house,900 gallons for a three-bedroom houseand 1,000 gallons for a four-bedroomhouse. Add 250 gallons for each bed-room exceeding four. Septic tanks musthave access openings over inlet andoutlet baffles.

Access location should be marked andvisible for easy inspection.

Selecting A Site• Stay at least 100 feet from drinking

water sources, 50 feet from streams orponds and 10 feet from water lines.

• Slope drainfields away from houses,buildings and the water supply.

• Keep drainfields unshaded and freefrom trees and shrubbery.

• Allow sufficient space to enlarge thedrainfield if it becomes necessary.

• Keep septic tanks or drainfields un-covered by driveways or concrete.

• Locate septic tanks and drainfieldsaway from drainage areas and wa-terways.

• Never use an open flame or matches toinspect a septic tank. Sewer gases mayexplode violently.

Planning the DrainfieldDrainfields consist of two or more

trenches not more than 100 feet inlength. Each trench contains sections ofopen-jointed four-inch drain tile or per-forated plastic drainpipe laid with theholes down. The pipe is level to disperseeffluent evenly over the soil area. The

percolation rate indicates how fast waterwill penetrate the soil and sizes thedrainfield. Some counties require addi-tional soil analysis.

How to ConstructDrainfields

• Using the percolation rate and Table 1,find the trench bottom area requiredper bedroom. Multiply this by thenumber of bedrooms to get the totaltrench bottom area. Decide on thetrench bottom width (not to exceed 36inches) and then deter-mine the totaldrainfield length (no one trench canexceed 100 feet).

• Drive stakes to mark the position of thetrenches on the contour. A builder’slevel is helpful but not essential. Drivegrade stakes and attach a board by us-ing a good carpenter’s level.

• Use a distribution box to distributeeffluent to the drainfield. Drop boxesare also used in some states, but arenot commonly used in Georgia.

Figure 4. Cross-section of a typical drainfield line

Figure 5. Drainfield tile can beleveled using a grade board.

• Use serial distribution if the terrain ishilly. The drainfield lines should beat least eight feet apart.

• The distribution box method can beused on level or sloping terrain andis required in Georgia when dosingtanks are used.

A firm earthen or concrete foun-dation extending at least 12 inchesbeyond the box walls is required toensure against tilting of the box.Care is required when backfillingover and around the box. The top ofthe box must have a minimum of sixinches of soil over the top, but nomore than 24 inches unless ready ac-cess is provided. The sewer line fromthe septic tank or dosing tank entersthe distribution box and terminates ina downward turned elbow.

• Absorption lines or drainfield linesof equal lengths are connected to thedistribution box outlets by independ-ent watertight sewers.

The absorption or drainfieldtrenches may be installed at the sameelevation or at different elevations.All the watertight sewers leadingfrom the distribution box outlet tothe drainfield lines must be at thesame elevation at the distribution

box and the watertight sewers mustbe level for the first two feet as theylead away from the distribution box.

If drainfield trenches are at differentelevations, some special requirementsshould be met. After extending two feetfrom the box, the watertight sewersmust have a slope of at least l/8-inchper foot down to the individual drain-field trenches. The drainfield linesshould also be installed on a uniformgrade of not less than two inches normore than four inches per 100 feet (twoinches is preferred). An increased num-ber of shorter trenches is preferable tofewer longer trenches in this situation.

ConclusionsProperly designed and installed septic

tanks can function for many years. An-nual inspection to determine sludgedepth is desirable to prevent tank solidsfrom overflowing and sealing the soil inthe drainfield. Minimize the amount ofgrease from the kitchen and garbagedisposal solids going into septic tank.Water conservation reduces the loadingand saturation of the drainfield.

Check with your local health depart-ment for specific requirements in yourcounty before purchasing lots or begin-

ning construction. Septic tanks servinga central system to serve commercial orindustrial facilities, institutions, traveltrailer and mobile home parks, subdivi-sions or multiple family dwellings offive or more family units require a de-sign by a professional engineer.

To protect groundwater, many areasincrease lot size requirements to reduceseptic tank densities. Septic tank permitsmay be subject to additional restrictionsin groundwater recharge areas.

Contact your local health departmentor county Extension office for additionalinformation on septic tank maintenance.

Grateful appreciation is expressed to Commissioner Tommy Irving and the Georgia Department of Agriculture,and to the U.S. Environmental Protection Agency (Region IV) for financial support of this publication.

Circular 819-2 Reprinted May 1999

The University of Georgia and Ft. Valley State University, the U.S. Department of Agriculture and counties of the state cooperating. The Cooperative Extension Service, the Uni-versity of Georgia College of Agricultural and Environmental Sciences offers educational programs, assistance and materials to all people without regard to race, color, nationalorigin, age, sex or disability.

An Equal Opportunity Employer/Affirmative Action Organization Committed to a Diverse Work Force

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, The University of Georgia College of Agricultural and Environmental Sciences and theU.S. Department of Agriculture cooperating.

Gale A. Buchanan, Dean and Director

Figure 6. Sewage treatment system with distribution box Figure 7. Sewage treatment system with drop boxes

Figure 8. Serial distribution does notrequire a distribution box and can beused on sloping land. The individualdrainfield lines are on the contour. Thefirst trench receives effluent from thetank and when full overflows throughthe relief line to the next trench.