Septic Tanks

Septic tanks (Fig. 9) are a conventional and widely applied facility for treatment and disposal of wastewater in areas which lack a sewer system. The tank has the function to separate and retain the particulate material by gravity. The accumulated sludge and scum undergo anaerobic biological degradation, which leads to the release of different gases, mainly methane and carbon dioxide. About 50-60% of the organic part of the solids in the sludge is converted to mineral end products, which makes it stable and reduces considerably its volume. Methane (biogas) is released in relatively low quantity; hydrogen sulphide is responsible for bed smells. The biological degradation of the sludge allows for the reduction of the volume of the sludge and its accumulation for 3-4 years, under normal operating conditions. But also, it leads to the release of organic compounds in the effluent. For this reason, it is more polluted then the influent and could be considered as a public health and environmental hazard. As it is seen from Figure 9, the conventional solution is to dispose it in the ground by means of an infiltration trench, also known as "soak away" or "drain field". Based on the infiltration capacity of the soil, the length of the trench or the area necessary for infiltration may vary; but normally the infiltration requires a considerable infiltration area to function properly.

An alternative to the soak away in cases of high ground water level is the evapotranspiration field (Fig.10), where the disposal of the wastewater from the septic tank is trough evapotranspiration. The size of the field is determined based on the type of vegetation and the climatic conditions. The functioning of this type of disposal system is similar to the constructed wetlands.