United StatesEnvironmental ProtectionAgency

Office of WaterWashington, D.C.

EPA 832-F-00-079September 2000

Decentralized SystemsTechnology Fact SheetSeptic Tank Systems for Large Flow Applications

DESCRIPTION

A septic tank system is a traditional wastewatertreatment technology utilizing treatment in a tanksystem followed by soil absorption. The systemoperates on gravity and has been used in residentialareas for decades. A modification to the traditionalsystem is an enlargement to accommodate manyhomes and/or commercial discharges. This isaccomplished with individual septic tanks followedby a community collection and subsurface disposalsystem, or a community collection system followedby a single treatment system. Commercialestablishments, such as restaurants, nursing homes,hospitals and other public use areas do not generallyuse septic tank systems due to oil & grease, odor,and flow issues.

The primary device in treatment is a septic tankenclosed in a watertight container that collects andprovides primary treatment of wastewater byseparating solids from the wastewater. The tankremoves solids by holding wastewater in the tankand allowing settleable solids to settle to the bottomof the tank while floatable solids (oil and grease)rise to the top. In large commercial systems, aseparate oil/grease removal system is applied to thecommercial waste before introduction to the septictank. The tank should hold the wastewater for atleast 24 hours to allow enough time for the solids tosettle.

Some solids are removed from the water and storedin the tank while some are digested. Up to 50percent of solids retained in the tank decomposewhile the remainder accumulate as sludge at thetank bottom and must be removed periodically bypumping the tank.

Three main types of septic tanks are used forwastewater treatment:

• Concrete.

• Fiberglass.

• Polyethylene/plastic.

All tanks must be watertight because groundwaterentering the system can saturate the soil absorptionfield, resulting in a failed system. Furthermore, ininstances where septic tanks precede a secondarytreatment process, excess groundwater mayinundate the downstream process, causing it toperform poorly.

From the septic tank, the clarified wastewaterpasses through the tank outlet and enters the soilabsorption field. The most common outlet is a teefitting connected to the pipe leading to the soilabsorption field. The top of the tee retains floatablesolids (scum, oil, and grease) that might otherwiseclog the absorption field. An effluent filter can beplaced in the outlet tee for additional filtering ofwastewater. The effluent filter removes additionalsolids, keeping them from clogging the absorptionfield and causing premature failure. Effluent filtersmust be cleaned regularly.

Soil Absorption Field

The soil absorption field provides final treatmentand distribution of the wastewater. A conventionalsystem consists of perforated pipes surrounded bymedia such as gravel, chipped tires, or othermaterial, covered with geotextile fabric and loamysoil. This system relies heavily on the soil to treatwastewater, where microorganisms help removeorganic matter, solids, and nutrients from the water.

As effluent continually flows into the soil, themicrobes eating the components of the wastewaterform a biological mat. The mat slows themovement of the water through the soil and helpskeep the area below the mat from becomingsaturated. The water must travel into unsaturatedsoil so microbes there and in the mat can feed onthe waste and nutrients in the effluent. The grasscovering the soil absorption system also uses thenutrients and water to grow.

Treatment

Used properly, the septic tank and soil absorptionsystem works well, reducing two parameterscommonly used to measure pollution: (1)biochemical oxygen demand, which is lowered bymore than 65 percent; and (2) total suspendedsolids, which are cut by more than 70 percent. Oiland grease are typically reduced by 70 to 80 percent(EPA 1980).

Using a septic tank to pretreat sewage fromcommercial sources also makes other secondarytreatment systems more effective. The effluentfrom the septic tank is consistent, easy to convey,and easily treated by either aerobic (with freeoxygen) or anaerobic (without free oxygen)processes.

Common Modifications

Septic tanks for large flow systems may befollowed by traditional soil absorption systems orby one of several alternate technologies such asconstructed wetlands or slow sand filtration.Pressure sewers and small diameter gravity sewersmay also be used as alternate collection systems fortransport of effluent to central treatment facilities.These systems are discussed in other fact sheets(see Reference section). This fact sheet focuses onthe traditional septic tank system applied tocommercial waste and multiple sources, usingsubsurface infiltration for wastewater disposal.

Subsurface Infiltration

Subsurface wastewater infiltration systems (SWISs)are subgrade land application systems mostcommonly applied in unsewered areas by individualresidences, commercial establishments, mobilehome parks, and campgrounds (EPA, 1992). Thesoil infiltration surfaces are exposed in buriedexcavations that are generally filled with porousmedia. The media maintain the structure of theexcavation, allows the free flow of pretreatedwastewater to the infiltrative surfaces, and providesstorage of wastewater during times of higher flows.The wastewater enters the soil where treatment isprovided by filtration, adsorption, and biologicallymediated reactions which consume or transformvarious pollutants. Ultimately, the wastewatertreated in the SWIS enters and flows with the localgroundwater.

Various SWIS designs have been developed forvarious site and soil conditions encountered. Thedesigns differ primarily in where the filter surfaceis placed. The surface may be exposed within thenatural soil profile (conventional or alternativetechnology) or at or above the surface of the naturalsoil (at-grade or mound systems) (see related FactSheets). The elevation of the filter surface iscritical to provide an adequate depth of unsaturatedsoil between the filter surface and a limitingcondition (e.g. bedrock or groundwater) to treatwastewater applied.

The geometry of the filter surface also varies, withlong, narrow filter surfaces (trenches) much

Backfill 1-3’

4’ minimum

Barrier Material

6-12” gravel

Perforated Distribution Pipe

Source: Robillard and Martin, 2000

FIGURE 1 SECTION OF TRENCH SOILABSORPTION SYSTEM

preferred. Wide filter surfaces (beds) and deepfilter surfaces (pits and deep trenches) do notperform as well, although they require less area.

Subsurface infiltration systems are capable of highlevels of treatment for most domestic wastewaterpollutants. Under suitable site conditions, theyprovide nearly total removal of biodegradableorganics, suspended solids, phosphorus, heavymetals, and virus and fecal indicators.

The fate of toxic organics and metals is not as welldocumented, but limited studies suggest that manyof these constituents do not travel far from thesystem. Nitrogen is the most significant wastewaterparameter not readily removed by the soil. Nitrateconcentrations above the drinking water standard of10 mg-N/L are commonly found in groundwaterimmediately below SWISs (EPA 1992), but theseconcentrations fall with distance down-gradient ofthe SWIS.

APPLICABILITY

Community Establishments

Septic tanks are usually the first component of anon-site system and are the most widely used on-sitewastewater treatment option in the United States.Currently, about 25 percent of new homes in theUnited States use septic tanks for treatment prior todisposal of home wastewater.

Septic tanks for single family homes are generallypurchased as “off the shelf” items, which meansthat they are ready for installation and based on astandard flow. The wastewater characteristics usedto design septic tanks are generally those for atypical residence.

Commercial Establishments

For many commercial establishments, thewastewater-generating sources are sufficientlysimilar to the wastewater-generating sources in aresidential dwelling. For other establishments,however, the wastewater characteristics may beconsiderably different from those of typicalresidential wastewater.

Commercial establishments can take advantage ofa centralized system if the flows and capacities aresufficient and adequate pretreatment is available.Wastewater must be pretreated prior to beingdischarged to a soil absorption system. Wastewateris most commonly pretreated by an on-site septictank when a soil absorption system is used fortreatment/disposal. In areas where soil andgroundwater conditions are favorable forwastewater disposal and land costs are low, acommunity soil absorption system is usually themost cost effective wastewater treatment/disposaloption for flows below 35,000 gallons per day.Careful application of the effluent to the soilabsorption system ensures uniform application ofeffluent over the filtration surface. Distributionlaterals should be provided with cleanouts foraccess and flushing. Ponding monitors should beinstalled in trench areas to allow observation ofliquid level in trenches.

Subsurface Infiltration

In some instances, it is desirable to bury theabsorption system. Buried systems, known assubsurface wastewater infiltration systems (SWISs),are advantageous because the land above a SWISmay be used as green space or park land, andbecause they provide groundwater recharge.Subsurface infiltration systems are well suited fortreatment of small wastewater flows. Small SWISs,commonly called septic tank systems, aretraditionally used in unsewered areas by individualresidences, commercial establishments, mobilehome parks, and campgrounds. Since the late1970s, larger SWISs have been increasingly used byclusters of homes and small communities wherewastewater flows are less than 25,000 gpd. Theyare a proven technology, but require specific siteconditions to be successfully implemented. SWISsare often preferred over on-site mechanicaltreatment facilities because of their consistentperformance with few operation and maintenancerequirements, lower life cycle costs, and less visualimpact on the community.

DESIGN CRITERIA

Pretreatment of Wastewater for CommercialSeptic Tank Systems

The most serious operational problem encounteredwith commercial septic tank systems has been thecarry-over of solids, oil and grease due to poordesign and lack of proper maintenance. Thecarryover of suspended material is most seriouswhere a disposal field is to be used to dispose ofseptic tank effluent without further treatment.Recognizing that poor septic tank maintenance iscommon, some regulatory agencies require theaddition of a large septic or other solids separationunit before collected septic tank effluent can bedisposed of in subsurface disposal fields. The useof oil and grease traps reduces the discharge of TSSand oil and grease significantly. The presence of oiland grease in effluents from septic tanks servicingrestaurants has led to the failure of downstreamtreatment processes such as intermittent andrecirculating sand filters. As a consequence ofthese problems, pretreatment is recommended.

Pretreatment in centralized treatment systemsinvolves coarse screening, comminution, gritremoval, oil and grease removal, flow equalization,and TSS removal.

Pretreatment for Oil and Grease Removal

Wastewater from restaurants, laundromats, andother commercial establishments may containsignificant amounts of oils and grease which maybe discharged to the soil absorption system whenthey enter a septic tank. Oils and greases tend toaccumulate on the surface of the soil absorptionsystem, reducing the infiltration capacity. Oils andgreases are especially troublesome because of theirpersistence and low rate of biodegradation. Toavoid problems in decentralized wastewatertreatment and disposal systems, the effluent oil andgrease concentration should be reduced to less thanabout 30 mg/L before it is introduced to the soilabsorption system (Crites and Tchobanoglous1998).

The problems associated with the removal of oilsand greases become more complex with the

increase in different types of oils and greasesavailable for cooking. The problem is furthercomplicated because many of these oils are solubleat relatively low temperatures, making theirremoval more difficult. Typically, skimming orinterceptor tanks (grease traps) are used to trapgreases and oils. Figure 2 shows a schematic of anoil and grease trap with an external samplingchamber.

Several commercial oil and grease traps areavailable. Most commercial units are rated byaverage flow rather than instantaneous peak flowsobserved in the field from restaurants and laundries.The use of conventional septic tanks as interceptortanks has also proven to be effective in removingoil and grease. Depending on the tankconfiguration, some replumbing may be necessarywhen septic tanks are used to trap grease.Typically, the inlet is located below the watersurface while the outlet is placed closer to thetank’s bottom. The larger volume provided by theseptic tank helps achieve the maximum possibleseparation of oils and greasy wastes. Forrestaurants, the use of a series of three interceptortanks is effective to separate oil and grease. Highconcentrations of oil and grease associated withrestaurants make the use of three interceptor tanksin series necessary to reduce this concentration toacceptable levels.

Note: 1 in = 2.54 cm

Source: Crites and Tchobanoglous, 1998

FIGURE 2 SCHEMATIC OF OIL ANDGREASE TRAP WITH EXTERNAL

SAMPLING CHAMBER

Volumes for grease interceptor tanks typically varyfrom one to three times the average daily flowrate.For example, if a restaurant serves 100customers/day at an average flow of 38liters/day/customer (10 gallons/day/customer), thesize of the grease interceptor tank should bebetween 3,800 and 11,400 liters (1,000 and 3,000gallons). Depending on the activities at a givenfacility, accumulated sludge and scum should beremoved every three to six months (Crites andTchobanoglous 1998).

Septic Tanks

A septic tank must be the proper size andconstruction and have a watertight design and stablestructure to perform successfully.

• Tank size. The required size of a septictank for a commercial establishmentdepends on anticipated flows from thefacility, coupled with additional flow fromresidences or other inputs, if on acommunity system.

• Tank construction. A key factor in septictank design is the relationship between theamount of surface area, its sewage storagecapacity, and the amount and speed ofwastewater discharge. These factors affectthe tank’s efficiency and the amount ofsludge retention. Tank construction mustalso assure a watertight structure.

A key to maintaining a septic tank is placing riserson the tank openings. If a septic tank is buriedbelow the soil surface, a riser must be used on theopenings to bring the lid to the soil surface. Theserisers make it easier to locate and maintain the tank.

Septic tank effluent may be applied to the soilabsorption field by intermittent gravity flow or viaa pump or dosing siphon. Periodic applicationusing a dosing siphon maintains an aerobicenvironment in the disposal field, allowingbiological treatment of the effluent to occur morerapidly. Dosing siphons are particularly desirablefor fields composed of highly permeable soilsbecause they help maintain the unsaturated flow

conditions necessary to achieve effective biologicaltreatment of effluent.

Subsurface Infiltration

Important considerations in designing subsurfaceinfiltration systems include:

• Soil texture. There are three sizes of soilparticles: sand, silt and clay. Texturereflects the relative percentage of each ofthese soil particles at a particular site. Soiltexture affects the rate at which wastewaterinfiltrates into and percolates through thesoil (called hydraulic conductivity). Thesefactors determine how large an absorptionfield is needed. Sand transmits water fasterthan silt, which is faster than clay.

• Hydraulic loading. This is the amount ofeffluent applied per square foot of trenchsurface or field, an important factor inseptic tank design. Because water filtersthrough clay soils more slowly than throughsand or silt, the hydraulic loading rate islower for clay than for silt, and lower forsilt than for sand. Because clay soils have avery low conductivity, they may easilysmear and compact during construction,reducing their infiltration rate to half theexpected rate.

Site Selection

Selection criteria for a site on which wastewatertreatment and renovation is to occur must considertwo fundamental design factors. These are theability of a site to assimilate the desired hydraulicload and the ability of the site to assimilate theprocess load. The process load consists of theorganic matter, nutrients, and other solids containedin the wastewater. The hydraulic assimilativecapacity of a site is often determined by the textureof the soil material on a site. Sites with sandytextured soils generally are assigned high hydraulicloadings while sites with fine textured clay are oftenassigned low hydraulic assimilative capacities.This typical hydraulic loading scenario often resultsin excessive loadings of the process constituents ona sandy site.

Sandy textured soils generally exhibit rapidpermeability. This suggests that these soils willdrain rapidly and reaerate quickly. Thesecharacteristics allow moderately high organicloadings onto these soils, but limit the potential forthese soils to attenuate soluble pollutants such asnitrogen and phosphorus. The fine textured soils -those that contain clays - exhibit high potential toattenuate soluble pollutants, but exhibit very limitedcapacity to transmit liquid; consequently thehydraulic loadings applied to these soils must bevery conservative. No soil provides the optimumcharacteristics to assimilate all constituents appliedand the challenge to the onsite wastewaterprofessional is to balance the loadings applied withthe total assimilative capacity of the designedreceiver site. Treatment objectives must be utilizedto optimize system design.

When large volumes of wastewater are designatedfor application onto a site, then a groundwatermounding analysis may be required. This analysisis required to assure that the separation distancebetween the bottom of the trench and the shallowgroundwater is adequate to provide necessarytreatment. Large systems should be designed sothat the longest dimension of the trench is along sitecontour lines and the shortest dimension crossesfield contours. This generally results in systemsdesigned with hydraulic gradients that facilitatetreatment.

Soil and site conditions on which wastewater willbe treated will vary from location to location. Sitesselected as receivers for wastewater must exhibitcharacteristics that facilitate treatment andrenovation of wastewater. Sites for wastewatertreatment and renovation must be selected based oncriteria established by local regulatory agencies asacceptable

Trench bottom application rates range from 0.2 to1.2 gpd/ft2 depending on soil conditions. Table 1contains suggested rates of wastewater applicationsfor trench and bed bottom areas.

Hydraulic Loading Rate

The design hydraulic loading rate is determined bysoil characteristics, groundwater moundingpotential, and applied wastewater quality. Cloggingof the infiltrative surface will occur in response toprolonged wastewater loading, which will reducethe capacity of the soil to accept the wastewater.However, if loading is controlled, biologicalactivity at the infiltrative surface will maintainwaste accumulations in relative equilibrium soreasonable infiltration rates can be sustained.

Selection of the design hydraulic loading rate mustconsider both soil and system design factors.Typically, design rates for larger SWISs are basedon detailed soil analyses and experience, rather thanmeasured hydraulic conductivities.

TABLE 1 SUGGESTED RATES OFWASTEWATER APPLICATION

Soil TexturePercolationRate (min/in/

min/cm)

ApplicationRate (gpd/ft2/

Lpd/m2)

Gravel, coarsesand

<1/<0.4

not suitable

Coarse tomedium sand

1 - 5/0.4 - 2.0

1.2/0.049

Fine to loamysand

6 - 15/2.4 - 5.9

0.8/0.033

Sandy loam toloam

16 - 30/ 6.3 - 11.8

0.6/0.024

Loam, poroussilt

31 - 60/12.2 - 23.6

0.45/0.018

Silty clay loam,clay loam

61 - 120/24.0 - 47.2

0.2/0.008

Clay, colloidalclay

>120 / >47.2 not suitable

Notes: 1) min/in x 0.4 = min/cm 2) gpd/ft2 x 40.8 = Lpd/m2

Source: Crites & Tchobanoglous, 1998.

Wastewater Pretreatment

At a minimum, wastewater treatment in a septictank is required before application to a SWIS.Figure 3 presents a schematic of a dual soilabsorption system. Higher levels of treatment suchas achieved with an aerobic treatment unit (ATU)can reduce SWIS size or prolong system life, butthis must be weighed against the increased costs ofpretreatment and potential damage from poormaintenance of the system.

ADVANTAGES AND DISADVANTAGES

Advantages

Subsurface infiltration systems are ideally suited fordecentralized treatment of wastewater because theyare buried. They are often the only method ofwastewater treatment available for rural homes andbusiness establishments. Some communitieschoose subsurface infiltration systems to avoidcostly sewer construction. Where individual lotsare not suited for their use, remote sites may beused to cluster homes onto a single SWIS, limitingthe need for sewers. Alternatively, wastewater fromentire communities may be treated by a SWIS.Because the system is buried, the land area can beused as green space or park land. In addition,SWISs provide groundwater recharge.

Disadvantages

Use of SWISs is limited by site and soil conditions.Because the infiltrative surface is buried, it can bemanaged only by taking it out of service every 6 to12 months to “rest”, requiring the construction ofstandby cells with alternating loading cycles.Therefore, larger SWISs are usually restricted towell-drained sandy soils to reduce land arearequirements. Because nitrogen is not effectivelyremoved by SWISs, pretreatment may be necessaryto prevent nitrate contamination above drinkingwater standards in underlying groundwater.

Flows from commercial establishments greater thanthe design capacity of the system may overwhelmthe SWIS and produce overflow conditions andobjectionable odors.

PERFORMANCE

Septic tanks and other pretreatment units must beproperly maintained to keep a SWIS system treatingsewage efficiently. As the septic tank or ATU isused, sludge accumulates in the bottom of thetreatment unit. As the sludge level increases,wastewater spends less time in the tank and solidsmay escape into the absorption area. Properly sizedseptic tanks generally have enough space toaccumulate sludge for at least three years. ATUsrequire aggressive sludge management.

The frequency of tank pumping depends on:

• The capacity.

• The amount of wastewater flowing into thetank (related to size of household).

• The amount of solids in the wastewater (forexample, more solids are generated ifgarbage disposals are used).

The soil absorption field will not immediately failif the tank is not pumped, but the septic tank will nolonger protect the soil absorption field from solids.If the tank or ATU is neglected for long, it may benecessary to replace the soil absorption field.

Source: Barrett and Malina, 1991

FIGURE 3 PLAN VIEW OF DUAL SOILABSORPTION BED SYSTEM

One example of septic tank/absorption field systemfailure is found in Missouri. Several statewidesurveys have shown that 70 percent (150,000) ofsystems are not functioning properly, causing nearly60 million gallons of untreated or semi-treatedsewage per day to reach groundwater supplies(Schultheis and Hubble). Based on the general soilsmap of Missouri, 60 to 99 percent of counties in theOzarks region show severe limitations in the use ofabsorption field systems.

Many studies of failing septic tank systems havebeen conducted. The Lower Colorado RiverAuthority (LCRA) received a grant from the TexasWater Commission (TWC) to identify clusteredsites of on-site wastewater treatment and disposalfacilities in the Lavaca and Colorado coastal basinsthat may be failing. Information from this studywill identify areas which may qualify for fundingunder Section 319 of the Clean Water Act.

A study was conducted by the Texas WaterCommission (TWC) to gauge whether septic tankswere polluting Lake Granbury in Hood County,Texas (TWRI Spring 1993). Because so manyseptic tanks were in use near the lake, there wasadditional concern of fecal coliform contamination.Analysis of samples taken in coves along the lakeshowed that 10 percent of tested areas had morethan 200 colony-forming units per 100 milliliters,indicating that the lake is highly contaminated withfecal coliform bacteria.

Increasingly stringent discharge regulations haveled many communities to turn to more effective on-site means to treat waste. One example is EagleMountain Lake near Fort Worth, Texas, where theTarrant County Water Control and ImprovementDistrict (WCID) is taking strides to improve theeffluent quality of the 2,500 local on-sitewastewater systems at Eagle Mountain Lake. Manyhomes in this area are weekend homes, with septictanks designed for limited use. WCID is designingthe on-site system to be large enough for full timeuse to improve effluent quality.

In the Texas Panhandle, the Texas NaturalResource Conservation Commission (TNRCC) hasused innovative on-site technologies to solvewastewater problems in the region associated with

failing septic tank systems due to rapid growth inthe region. In the 1980s, the town of Umbargerinstalled a 44,000-gallon septic tank and a 30,720-square foot drainfield to serve its 325 residents.This community system replaced the collection ofmany smaller septic tanks distributed throughoutthe town, many of which had previouslyexperienced failures.

OPERATION AND MAINTENANCE

Subsurface Infiltration

A well-designed SWIS requires limited operatorattention. Management functions primarily involvetracking system status, testing for solidsaccumulation, evaluating pump performance,monitoring system controls, and monitoringperformance of pretreatment units, mechanicalcomponents, and wastewater ponding levels abovethe filtration surface. Operator intervention may berequired if a change is noted. Routine servicing ofSWIS is generally limited to annual or semiannualalternating of infiltration cells.

Another maintenance task to prevent a system frombacking up is to clean the screen on the effluent endof the septic tank. This filter must be cleanedperiodically by removing the filter from the outletand spraying it with a hose directed back into theseptic tank.

Soil absorption fields must be protected from solidsand rainfall. If a tank is not pumped, solids canenter the field. Rainfall running off roofs orimpermeable surfaces such as concrete areas shouldbe diverted around the soil absorption field toprevent it from becoming saturated with rainwater.Fields saturated with rainwater cannot acceptwastewater. Planting cool-season grasses over thesoil absorption field in winter can help removewater from the soil and keep the system workingproperly.

COSTS

Subsurface Infiltration

Land and earthwork are the most significant capitalcosts. Where fill must be used to bed the primary

infiltrative surface, the cost of transporting thematerial also becomes significant. Other costsinclude pretreatment and transmission ofwastewater to the treatment site.

Other factors that affect septic tank costs includesubsurface site conditions, location of and access tothe site, and the type of tank used. Costs of tanks,including installation, typically range from $1.00 to$4.00 per gallon of tankage. Pumping septic tanksranges from $150 to $200 per 2,000 gallons. If atank is pumped once every 3 ½ years, themaintenance cost will be about $50 per year, with apump and haul cost of $175.

REFERENCES

Other Related Fact Sheets

Mound SystemsEPA 832-F-99-074September 1999

Pressure SewersEPA 832-F-00-070September 2000

Small Diameter Gravity SewersEPA 832-F-00-038September 2000

Other EPA Fact Sheets can be found at thefollowing web address:http://www.epa.gov/owmitnet/mtbfact.htm

1. Barret, Michael E. and J. F. Malina, Jr.Sep. 1, 1991. Technical Summary ofAppropriate Technologies for SmallCommunity Wastewater Treatment Systems.The University of Texas at Austin.

2. Corbitt, Robert A. 1990. StandardHandbook of Environmental Engineering.McGraw-Hill, Inc. New York.

3. Community Environmental Services, Inc.Septic Tank. Fact Sheet. City of Austin, TX.

4. Crites, R. and G. Tchobanoglous. 1998.Small and Decentralized WastewaterManagement Systems. WCB McGraw-Hill,Inc. Boston.

5. Robillard, Paul D. and Kelli S. Martin.Septic Tank Soil Absorption Systems.Agricultural and Biological EngineeringFact Sheet. Penn State, College ofAgriculture. Site accessed 2000.http://www.klinesservices.com/ps1.cfm.

6. Schultheis, Robert A. and Gwen Hubble. AH o m e o w n e r ’ s G u i d e : S e p t i cTank/Absorption Field Systems. ExtensionService, U.S. Department of Agriculture,Project Number 90-EWQI-1-9241,publication WQ0401.

7. Texas On-Site Insights, Volume 2, Number1: Spring 1993, Measuring the Impact ofSeptic Tanks on Lake Granbury. TexasWater Research Institute.

8. Texas On-Site Insights, Volume 2, Number2: Summer 1993, LCRA Receives Grant toStudy On-Site Systems in Colorado, LavacaCoastal Basins. Texas Water ResearchInstitute.

9. Texas On-Site Insights, Volume 4, Number2: Spring 1995, Managing On-SiteWastewater Programs at Eagle MountainLake. Texas Water Research Institute.

10. Texas On-Site Insights, Volume 6, Number2: June 1997, Texas Panhandle is Site ofMany Innovative On-Site Systems. TexasWater Research Institute.

11. U.S. Environmental Protection Agency.1980. Design Manual: Onsite WastewaterTreatment & Disposal Systems. EPA Officeof Water. EPA Office of Research &Development. Cincinnati. EPA 625/1-80/012.

For more information contact:

Municipal Technology BranchU.S. EPAMail Code 42041200 Pennsylvania Avenue, NWWashington, D.C. 20460

ADDITIONAL INFORMATION

Lower Colorado River AuthorityBurt CarterP.O. Box 220Austin, TX 78767

Tarrant County WCIDDavid Jensen10201 North Shore DriveFort Worth, TX 76135

Texas Natural Resource Conservation CommissionLezlie Cooper or Steve Green3918 Canyon DriveAmarillo, TX 79109

Texas Natural Resource Conservation CommissionWilson Snyder6801 Sanger Avenue, Suite 2500Waco, TX 76710

David Venhuizen, P.E.5803 Gateshead DriveAustin, TX 78745

The mention of trade names or commercialproducts does not constitute endorsement orrecommendations for use by the United StatesEnvironmental Protection Agency (EPA).