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Chapter 8: Septic Tank Design and Construction

This chapter is also included in the field manual, Ferrocement Tank Construction.    

These design graphics illustrate a ferrocement septic tank which was built in 1975. It has been used continuously and remains in perfect condition. Contrary to the experience of many well respected sewage plant engineers and operators, the cement has not deteriorated. Perhaps this is due to the hardness of the mortar, described in chapters one and nine; 550 to 850 kgf/cm2 for the ferrocement plaster mix versus 175 kgf/cm2 for standard concrete mix (8,000 to 12,000 psi vs. 2,500 pounds per square inch). The oldest ferrocement septic system known at ferrocement.com was built in 1968.

Wall thickness varies between 3.8 to 7.5 cm (1 1/2 to 3"). The light grey area represents floating organic matter. Sludge is mostly composed of microbial remains heavier than water; it is a darker grey color at the bottom. Large arrows indicate water flow direction. Small arrows indicate flow direction for gasses of decomposition.

This septic tank did not receive grey water from laundry, sink, or bath drains. It was emptied in the year 2000; estimated remaining capacity was approximately fifty percent after 25 years of use. The bottom sludge was used for garden fertilizer and the top mat of organic matter was left in the tank. Biodegradable toilet tissue was always used and no chemical or bacterial products were introduced.

The first chamber is 55 percent of the empty volume. The second chamber is 30% and the third is 15%. When the first chamber is half full of sludge and floating organic matter, the relatively clear water in between is approximately the same volume as the clear water volume in the second chamber.

Most septic tank designs include only two chambers. The third chamber adds to protection of the drainage area, which is an expensive and difficult part of septic system construction.

Sewage water remains in this tank for an average of 45 days; it is held in the first chamber for an average of two weeks when the chamber is half full. Most codes require only one day retention time of clear volume in the first chamber (at 50% full). This insufficient time limit has caused contaminated ground water in many urban areas of the United States. Home owners are required to install municipal sewer systems after this occurs. A new source of domestic water is also often necessary.

Septic systems are not practical in areas of dense human habitation unless the discharge is directed to community treatment facilities rather than individual drainage areas. There is almost always enough area to use grey water for garden and landscape irrigation where septic systems are successfully utilized.

Local codes do not usually require mixing of grey water and sewage water within the plumbing system before it leaves the structure. Plumbing systems that do not pollute grey water with sewage are environmentally wise. Grey water is a larger quantity than actual sewage water. The additional plumbing cost to keep them separate is not large. Check local building codes for grey water utilization.

Discharge from individual or community septic tanks can be directed to a municipal system at much less cost than sewer systems which carry all grey water and organic matter. Sewage collection accounts for 70 to 90 percent of municipal sewer system installation when all waste water and raw organic sewage are combined and then sent directly to a central location. Toxic chemical dumping through the sewage system is also easier to locate when sewage sludge is harvested from individuals.

Individual septic tanks overflow to a drain field as illustrated below. The width (X) times the length (Y) provides an area of percolation, which is determined by soil tests for absorption rate. The distance (Z) indicates the gravel bed beneath the drain pipe, 10 - 15 cm is common. The trench is filled with gravel to a similar distance from the surface. Connected arches are often used to create a cave at the bottom of the trench. This technique replaces both pipe and gravel. Plastic arch pieces sufficient for a residential septic system easily fit in the back of a small truck.

Depth to the drain pipe (P) must be sufficient to position the drain pipe below roots which will clog it. The leach field cannot be in an area of deep rooted trees. The depth (P) is also related to the slope of the terrain, a hillside leach field requires greater depth so that water does not surface on the downhill side (Ferrocement House Construction, p. 2).

Percolation tests for soil absorption rates follow procedures which may vary in detail from one locale to the next. The basic idea is to determine an absorption area that will be sufficient for the planned usage. This is usually accomplished by digging a preliminary hole large enough to work in. The preliminary hole is located near point (L) on the drawing. It is dug to full depth and is large enough to work in. A smaller test hole is then dug in the bottom of the work area. This hole is filled with water and the absorption rate is measured. If there is variation of soil type in the computed length (Y), additional absorption test pits are dug at either end.

The primary reason for varying methods of calculating a total absorption area is that anaerobic organisms grow on the bottom and walls of the leach area as it matures to equilibrium with the area soil life. These anaerobic organisms die and are consumed by aerobic organisms in the soil. The growth of this living layer is on the trench bottom and walls, where gravel meets soil. It slows water flow out of the leach field and also accomplishes final purification of the waste water. Soil temperature and biology have a large effect on this layer and explain much of the regional differences in formulas employed for calculating the total required leach area.

If leach area calculations are based on a usage factor which includes all grey water and raw sewage, the leach area will be larger than necessary (should grey water be subsequently utilized for irrigation). A method of dividing the water flow so that half the leach area is "rested" while the other half is used can be installed then if desired. Install an outflow pipe from the septic tank so that water can be directed to drop pipes for either separated half at the leach field mid point (Y). A clay barrier in the gravel between the two halves improves separation efficiency.

Ferrocement.com advises using grey water for irrigation in the surface soil and in sunlight whenever possible. Surface soil life and sunlight rapidly convert grey water to biological usefulness. As long as this water does not pool and breed mosquitos it is relatively safe. Local building codes will provide detailed information for each locality.

Construction of a septic tank in the ground is similar to the open top tank except there is no need for an outer layer of fine wire. Corner pieces are made as shown on page 28. Having a supply of these ready will greatly speed up fabrication of the armature.

Number three bar on a 30 - 45 cm grid between two layers of 15 by 15 cm square welded wire is an adequate steel schedule. The reinforcing steel bars in the chamber separation walls need not be bent so they overlap into the outer walls, the welded wire corner pieces are adequate attachment for inner chamber walls.

Two centimeter thick support blocks, with wires in them for attaching to the armature, are sufficient to keep the steel from being in contact with the soil. Ferrocement objects with straight walls are easiest to build using welded wire sold in flat sections rather than rolls; this eliminates the need to straighten pieces cut from a roll.

A two coat cementiscious water seal product that cures in one day was used inside the septic tank illustrated in this chapter.

Sanitary tee can be inspected and cleaned through small hatch from above.

The small ledge deflects strong currents so the bottom sludge is not disturbed.

Looking down on the entrance plumbing and the dispersion ledge below it.

The square opening at the top is the hatch, it can be wood or ferrocement.

Hatch covers are very simple, they only keep dirt from falling inside.

The upper white pipe is for return flow of gasses to the venting system.

A secondary small hatch for manual clean out of the entry fitting is recommended.

A turned down ell for water flow to the second chamber helps keep downward drifting solids from passing through.

Steel is omitted on the near side for photography.