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. (where cm = centimeter and kgf = kilograms of force).
Wall thickness varies between 3.8 to 7.5 cm. 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.
