Reprinted from Journal of Agronomic Education
Vol. 19, No. 2 (Fall 1990)-Copyright permission granted

Working, Scale Models of On-Site Septic Tank-Soil Absorption Systems for Extension and Resident Education

M.T. Hoover and J.L. Beeson


We developed four working, scale demonstration models of on-site septic tank-soil absorption systems for use with both extension and resident education audiences. These include models of the conventional septic tank system and three alternatives: the low pressure pipe, sand mound, and recirculating sand filter systems. This article describes each system, the associated model, and the ways we have used these devices in our educational program in a number of extension and classroom settings. Water added to the septic tank of the models can be observed as it flows through the system and into the underlying soil. The most appropriate uses of the models are for demonstrations to small groups of 25 to 30 people, for training sessions, or for staffed exhibits. They are less effective as stand-alone displays. In a classroom evaluation, students responded very positively to use of the models.

Septic tank-soil absorption systems are used extensively throughout the USA to treat and dispose of household wastewaters in rural and suburban areas. By 1977, 20 million housing units in the USA, serving approximately 29% of the population, were using individual on-site disposal systems (Scalf et al., 1977). These on-site soil treatment systems (also called septic tank systems) are even more prevalent in North Carolina where people in as many as 54% of the housing units depend on them (Grayson et al., 1982). Moreover, between 35,000 and 50,000 new septic tank systems are installed each year in North Carolina (Hoover et al., 1987).

The use of septic tank systems is likely to increase in coming years. Federal funding for public sewerage works has decreased significantly in recent years and is expected to decrease even further in the future (Dearth, 1982). For this reason the on-site soil treatment system will be increasingly important for household wastewater treatment and disposal.

As more people depend on on-site soil treatment systems, programs to educate the public must reach larger audiences and must be better able to capture the public's attention. Many people considering land purchases do not realize that the soil is an integral part of a septic tank system. Homeowners, realtors, and land developers typically do not understand the arrangement of components within various types of soil treatment systems, nor do they realize how these systems function.

A part of the educational approach used at North Carolina State University has been to integrate four working, scale models of septic tank systems into both education and resident education programs. This article will describe the use of the demonstration devices and encourage the use of similar devices where appropriate. The specific objectives are to (i) describe the working models and the systems that they represent, (ii) discuss how we integrate these devices into both extension and resident education programs, and (iii) test the effectiveness of the models as an educational tool.


Although many people have heard of "septic tank systems," very few people are able to visualize the components of the distribution system or understand how these systems function. Each working model is a miniature septic tank system that illustrates the fundamental components of the system. The models are designed to :

1. Illustrate components that are typically buried and unobservable

2. Compare sizes and configurations of these components for different types of systems

3. Emphasize the importance of the soil as the treatment and disposal medium

4. Demonstrate various wastewater distribution techniques.

Model height and width are approximately one-fifteenth scale, whereas the length scale is smaller. Each model is the size of a small desktop and 15 to 25 cm tall. Each model has a green-colored, removable covering that represents a residential lawn. Removing the cover reveals the buried components of the system. Watching the cover being removed and viewing the "buried" components of the model helps individuals visualize the septic tank system buried under their own lawns. In addition, each model contains side panels that can be removed to reveal cross-sectional views of the system. Water added to the septic tank of the convention, the low pressure (LPP), and the recirculating sand filter systems can be observed flowing through the systems. Hoover and Beeson (1986) describe in more detail the design and construction of the models.

Conventional System

The conventional system used in many areas of the USA consists of a septic tank and a soil absorption field. The design of this system has been presented elsewhere (USEPA, 1980). In North Carolina a conventional system usually includes a septic tank and 1 m wide subsurface trenches dug 0.6 to 1.0 m deep on 3-m centers. Gravity distribution is used. System size varies from 30 to 200 linear meters of trench for a single family home.

The conventional system model (Fig. 1) is designed to illustrate a typical conventional system in North Carolina. Models developed for use in other states should be adapted to reflect typical system designs used there. Our model is 60 by 120 by 15 cm tall and is made primarily of brown styrene foam "soil" mounted on a varnished, veneered plywood board. Placed within the styrene foam is a Plexiglas septic tank and distribution box. Three trenches are "excavated" within the styrene foam to represent the soil absorption area. Each trench contains a perforated acrylic distribution pipe placed within gravel aggregate (we used aquarium gravel). The outermost trench is enclosed within a Plexiglas box. This box contains a perforated distribution pipe, gravel aggregate, and a permeable floral foam material that represents the soil beneath the trench bottom.

Water added to the septic tank of the model exits from the first few holes in the distribution pipe and flows down through the floral foam soil material in that part of the trench closest to the septic tank. This illustrates the poor in-trench effluent distribution that occurs with the conventional gravity distribution system. The floral foam becomes darkened upon wetting and thereby simulates the movement of a wetting front of sewage effluent down through the soil profile. After each demonstration, the used (wetted) floral foam should be replaced. This is accomplished by detaching the distribution pipe in the Plexiglas box from the pipe network in the rest of the model (we use soft, plastic tubing as a sleeve) and lifting the box away from the model. Then, removing the gravel and pipe from the box accommodates replacement of the floral foam. The floral foam must be exposed to the sun for 1 to 3 d prior to use to change its color from green to a soil-like brown. The floral foam material complicates the design, construction, and use of these models; however, it is essential for success of the models as effective demonstration tools.

Fig. 1. Conventional system model

(a) before removal of simulated lawn cover,

(b) after removal of the top and side panels,

(c) illustrating three-dimesional cross-sectional views, and

(d) after application of water to septic tank.

Note the darkened, simulated soil below the portion of the trench closest to the septic tank.

Low Pressure Pipe (LPP) System

On the basis of low costs, simplicity of design, and limited maintenance the conventional system is the most desirable system to use. Many soils and site situations, however, are not suited to the use of a conventional system (Kleiss and Hoover, 1986; Small Scale Waste Management Project, 1978). On some of these marginally suitable soils alternative septic tank systems such as the LPP system have been successfully used in North Carolina (Cogger and Carlile, 1984; Cogger et al., 1982a; Kleiss, 1981).

The LPP system consists of a septic tank, a pump tank, and a soil absorption field. Wastewater is pumped intermittently from a pump tank to a network of small-diameter, perforated pipes places in shallow, narrow trenches.

The LPP system model (Fig. 2) includes a Plexiglas septic tank and pump tank, a small aquarium pump, a buried manifold pipe, gravel aggregate, and eight exposed, small-diameter acrylic lateral pipes installed in brown styrene foam. An L-shaped styrene foam panel is removed to reveal the cross-sectional views of the soil absorption field. The entire soil absorption field (including the floral foam soil) is contained in a removable Plexiglas box.

Water added to the septic tank of the LPP system model flows to the pump tank where it accumulates until a dosing event occurs. At that time a dose of water is pumped throughout the entire distribution system and flows out from each of the holes in the pipe. A wetting front can then be observed moving down through the soil profile (floral foam) beneath each hole; thus, illustrating the uniform distribution concept of the low pressure pipe system.

Fig. 2. Low pressure pipe system model

(a) before removal of simulated lawn cover (note riser cap over pump tank),

(b) after removal of lawn cover,

(c) showing the cross-sectional view after removal of the L-shaped side panel,

(d) after activation of pump.

Note the more uniform distribution of water compare with the conventional system model.

Sand Mound System

The sand mound system consists of a septic tank and a pump tank buried in the ground, and a mound of sandy fill material that is placed on the soil surface (Cogger et al., 1982b). A network of small-diameter pipes encased in gravel aggregate is placed on top of the sandy fill and the entire mound is covered with topsoil. Although these systems can be utilized for some difficult soil and site conditions (Converse et al., 1978), surface discharge of partially treated effluent occurs on the most difficult sites (Converse and Tyler, 1985; Hoover et al., 1981).

The sand mound model (Fig. 3) includes removable styrene foam top and side panels. The mound itself is also made from styrene. Removal of the panels reveals a cross-section of the mound including the gravel aggregate, the small-diameter pipes, the sandy fill, and the underlying soil. No tanks are shown, nor is water or floral foam used with this model. The pressure distribution system is not illustrated, because it is demonstrated on our LPP system model.

Fig. 3. The sand mound system model after removal of top and side panels.

The septic tank and pump tank are not shown in this model.

Recirculating Sand Filter

There are three types of intermittent sand filters commonly used: the free-access sand filter, the buried sand filter, and the recirculating sand filter (USEPA, 1980). The recirculating sand filter is a relatively new concept that has been described by Hines and Favreau (1975) and Loudon et al. (1985). Wastewater is recirculated three to five times through a sand filter before it ultimately is disposed in a nearby stream, a subsurface sewage absorption area, or a spray irrigation system.

The recirculating sand filter model (Fig. 4) includes a Plexiglas septic tank and recirculation tank, a small aquarium pump, and a Plexiglas sand filter box. In this box is the sand filter as well as an overlying pressure distribution system made from acrylic pipe. The collection pipe system under the filter is encased within aquarium gravel. Delivery and return-flow pipes from the filter to the recirculation tank are also made from acrylic pipe. All these components are buried within gray-colored styrene foam. This model also contains a removable lid with a simulated lawn cover.

The flow splitting device and the ultimate method of wastewater disposal are not shown. Water added to the model makes a continual loop from the recirculation tank to the filter, and during a demonstration the ultimate method of disposal of the partially treated wastewater must be explained.

Fig. 4. The recirculating sand filter system model after removal of the simulated grass cover.

The flow splitter device and the ultimate method of disposal are not shown.

Cost and Time Considerations

The greatest limitation in construction of these models is the time required to develop techniques for working with the appropriate materials. Also, there are significant costs associated with the equipment needed to build these models. A shop equipped with a table saw, band saw, and drill press is required. In contrast, the materials actually used in each model are not very costly ($30- $60 per model). The cost of building one model would be somewhat greater because some materials must be purchased in large quantities.

The use of floral foam in the conventional and LPP system models complicates both design and use of the models. This floral foam must be replaced (at a cost of $1.50- $2.00 for both models) after each demonstration. The floral foam, however, is an integral part of the models for these two purposes :(I) comparing gravity vs. pressure distribution systems, and (ii) making the point that these are soils-based systems.


The models have been successfully used in a number of extension settings and also in the classroom as a regular part of the resident education program. In fact, our use of the models represents an example of the dual utility of small-scale demonstration devices and their applicability to both the extension audience and the university class. Our use of the models, however, did identify some limitations of trying to address both audiences.

Extension settings where the models have been used include 13 training sessions, 20 extension and health department public education programs, six display booths, and as part of 10 staffed exhibits. We have utilized the models at training sessions for more than 900 in-state sanitarians, out-of-state health regulators, county extension agents, realtors, local elected and appointed officials, and administrative law judges. These models have also been used in demonstrations to more than 8000 concerned citizens, land developers, contractors, and consultants in a number of extension settings such as county extension programs, health department open houses, staffed state fair exhibits, and Farm-City Week Fairs. On the basis of these experiences, we believe the most effective utilization of the models has been as part of demonstrations to groups of no more than 25 to 30 people. When greater numbers of people are present it is difficult for the audience to be close enough to see the models function. We have also used the models as stand-alone exhibits in nine display booths at shopping malls and county fairs. It is our observation that the models are least effective when used as a stand-alone display because they are neither self-explanatory nor self-functioning.

The models have been successfully used in three soil science courses at North Carolina State University and for visiting groups of high school students. We have used the models as part of lecture presentations, in laboratory discussion groups, and in the field for demonstrations at sites of actual installed and covered septic tank systems. Two of the university courses were introductory soil science courses (one in the Agricultural Institute 2-yr program), and the third is a more advanced undergraduate course concerning soil resources and land use. In the soil resources course the class was taken to locations of installed septic tank systems and, once there, the models were used to help the students understand the physical layout of the systems. This technique has helped to strengthen student understanding and improve the insightfulness of their questions.


Students in the Agricultural Institute course were separated into two lecture groups. This arrangement facilitated a direct comparison of the effectiveness of the models in the classroom. For both groups a lecture was presented concerning two topics: (I) the soil factors important to sewage disposal, and (ii) the different kinds of septic tank systems. In Presentation I, slides were used to address both topics and the models were not used. In presentation II, slides were used for the soil factors segment, whereas the models were used for the different kinds of septic tank systems segment. Each group of students responded to a questionnaire.

Overall, the students responded positively to both presentations. Table 1 presents the results from questions that related to the "different kinds of septic tank systems" portion of the presentations. Group evaluations indicated use of the models did not improve the style of the presentation, but did improve the student's perception of the usefulness of the information. The students who received Presentation II were very positive in their evaluation of the utility of the models (Table 1).

A third presentation technique (Presentation III in Table 1) used with the baccalaureate introductory course consisted of slides for the "soil factors" segment and the models for the "different kinds of systems" segment. In this case the soil factors segment was expanded and presented to the entire class in a one-period lecture. The models were used to present information about the different kinds of systems in subsequent one-period recitation or discussion sessions for groups of 5 to 25 students. Using the models with small groups substantially improved the students' perceptions of the "different kinds of septic tank systems" segment (Table 1). Again, the students responded positively to use of the models as an effective teaching tool. We currently are using this third presentation technique in both introductory soil science courses.

Table 1. Student responses to selected questions in a classroom evaluation of three presentation techniques concerning the models.

Selected Questions Response n=26 48 86
3. The information presented about the "different kinds of septic tank systems" was: not useful 4 0 0 useful 77 60 55 very useful 19 40 45 4. The style of presentation concerning the "different kinds of septic tank sys- tems" was: below average 0 2 4 average 65 58 33 above average 35 40 64 6. Rate the overall presenta- tion on the basis of how much you learned. very poor 0 0 0 poor 8 0 2 average 31 29 33 above average 50 67 54 excellent 12 4 11 7. How useful were the models in demonstrating how different septic tank systems worked? not useful ND** 0 0 useful ND 38 11 very useful ND 62 89 8. Did use of the models in- crease your understanding of these septic tank sys- tems? no, not much ND 0 2 a little bit ND 25 14 yes, a lot ND 75 84
*I- slides used alone in one-period lecture; models not used. II- slides and models used in one-period lecture. III- slides in lecture followed by models in recitation groups **- Not determined.


As septic tank-soil absorption systems become even more commonplace we must make greater efforts to educate the public regarding the appropriate uses and limitations of these waste treatment and disposal systems. To this end, four working, scale models of soil treatment systems have been developed and used successfully in both extension and resident education programs. In fact, their use serves as an example of the appropriateness of using one set of demonstration devices for both audiences.


Cogger, C.G., and B.L. Carlile. 1984. Field performance of conventional and alternative septic systems in wet soils. J. Environ. Qual. 13:137-142. Cogger, C.G., B.L. Carlile, D. Osborne, and E. Holland. 1982a. Design and installation of low pressure pipe waste treatment systems. UNC Sea Grant College Publ. UNC-SG-82-03. North Carolina State Univ., Raleigh, NC.

Cogger, C.G., B.L. Carlile, D. Osborne, and E. Holland. 1982b. Design and installation of mound systems for waste treatment. UNC Sea Grant College Publ. UNC-SG-82-04. North Carolina State Univ., Raleigh, NC.

Converse, J.C., and E.J. Tyler. 1985. Wisconsin mounds for very difficult sites. p. 199-130. In On-site wastewater treatment. Proc. of the 4th Natl. Symp. on Individual and Small Community Sewage Systems, New Orleans, LA. 10-11 Dec. 1984. Am. Soc. Agric. Eng., St. Joseph, MI.

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Hoover, M.T., and J.L. Beeson. 1986. Septic tank system demonstration models. Soil Science Facts SS-86-1. Soil Sci. Dep., North Carolina State Univ., Raleigh, NC.

Hoover, M.T., G.W. Petersen, and D.D. Fritton. 1981. Utilization of mound systems for sewage disposal in Pennsylvania. p. 41-60. In Individual on-site wastewater systems. Proc. of the 7th Natl. Conf., Ann Arbor, MI. 23-25 Sept. 1980. National Sanitation Conference, Ann Arbor, MI.

Hoover, M.T., A. Slagle, S.J. Steinbeck, and R.L. Uebler. 1987. Recent trends in on-site sewage disposal. Soil Sci. Soc. of North Carolina Proc. Vol. 30:169-181. c/o Soil Sci. Dep., North Carolina State Univ., Raleigh, NC.

Kleiss, H.J. 1981. Soil ratings for ground absorption sewage disposal systems: To be used with the Durham Co. Soil Survey. Soil Sci. Inf. Ser., Soil Sci. Dep., North Carolina State Univ., Raleigh, NC.

Kleiss, H.J., and M.T. Hoover. 1986. Soil and site criteria for on-site systems. p. 111-128. In E.C.A. Runge et al. (ed.) Utilization, treatment, and disposal of waste on land. SSSA, Madison, WI.

Loudon, T.L., D.B. Thompson, L. Fay, and L.E. Reese. 1985. Cold climate performance of recirculating sand filters. p. 333-432. In On-site wastewater treatment. Proc. of the 4th Natl. Symp. on Individual and Small Community Sewage Systems, New Orleans, LA. 10-11 Dec. 1984. Am. Soc. Agric. Eng., St. Joseph, MI.

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Small Scale Waste Management Project. 1978. Management of small waste flows. USEPA Rep. 600/2-78-173 (NTIS PB-286-560). Natl. Tech. Inf. Service, Springfield, VA.

U.S. Environmental Protection Agency. 1980. Design manual for on-site wastewater treatment and disposal systems. USEPA Rep. 625/1-80-012. U.S. Gov. Print. Office, Washington, DC.

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