For a long time, Tehran still used a traditional sewage system using leaching pits. As the population grew, there was more sewage; the transfer of water to Tehran resulted in a surplus of water and the surfacing of subsurface water, which was likely to be polluted.
Thus the plan for developing a sewage system was given priority. The initial studies were conducted by the United Nations Developing Program and World Health Organization in 1971.
The final report of the first stage study was presented in 1974 by the consultant engineers Alexander Gabe and John Taylor, collaborated by Tehran consultant engineers. The plan aimed at collecting the city’s sewage and surface water.
In the advent of the revolution, the plan was suspended for a while. To update the previous studies and based on the new conditions, the plan was revised and the second stage of studies started in 1985. Tehran sewage system was finally completed in 1983. So far, a great part of the city has been connected to the system.