The characteristics of the minute clay particles in the water from the Vaal River were found to be similar to those of the Nile River in Egypt, and consequently the design of the treatment works was based on those used in Egypt.  It comprised treatment with aluminium sulphate and removal of approximately 95% of the particles in horizontal flow sedimentation tanks, followed by rapid gravity sand filtration and chlorination (from 1925).  The water was stored in clear water reservoirs and then pumped via the booster pumping station at Zwartkopjes to the Johannesburg mines.  Treatment with lime and activated silica followed by carbonation for pH correction came into use in the 1950’s and apart from the use of synthetic polyelectrolytes from the mid 1970’s, little has changed in the purification process.