This publication contains the theoretical basis with which it is possible to design the elements involved in applying this technique: the sizing of the tank and the infiltration field (ditches or wells), according to the size of the population to be served, the customs of the people and the volumes of wastewater they produce. In addition, the concept of "improved septic tank" is presented, as a technique composed of two or more units placed in series for the treatment of wastewater, before the final disposal of the treated water, either by infiltration into the soil or poured into a permanent flow channel.