1 Al-Mutaz,I.S., M. A. Al-Ghunaimi and S. A. Al-Busaili, pH Increase In Water Distribution Pipes, IDA World Congress on Desalination and Water Re-Use, San Diego, USA, Aug 29 to Sept 3, 1999. pH Increase In Water Distribution Pipes Ibrahim S. Al-Mutaz 1 , Mohammad A. Al-Ghunaimi 2 and Suliman A. Al-Busaili 2 1) Chemical Eng. Dept., College of Engineering, King Saud University, P O Box 800, Riyadh 1 1421 2) Riyadh Water and Sewage Authority, Riyadh, Saudi Arabia Extended Abstract: Production of water from a water treatment plant normally satisfies certain pH requirements. According to most of the standards, pH of the product water should be between 6.5 to 8.5. Water corrosivity is not necessary well reflected only by its pH values. Total alkalinity, calcium hardness, total dissolved solids as well as the temperature affect the water stability, tendency of water to scale or corrode. Producing water within the pH ranges does not ensure water stability. Langlier Saturation Index, LSI, is one of the popular methods to determine the tendency of water to scale or corrode through water distribution pipes. This index depends on the normal water pH and the saturation pH s It can be expressed as LSI = pH - pH s . Saturation pH s is either calculated or measured. It depends on total alkalinity, calcium hardness, total dissolved solids and temperature. For zero saturation index, water tends to be stable. Every water has certain pH value where it neither deposits scale nor cause corrosion. This pH is not necessary the directly measured pH and its values are not necessary fall into the specified standards. If pH is not in this specified range water tends to form scale or corrode the pipe materials. Monitoring the pH will give an idea about the water stability. It has been observed that water from Salbukh Water Treatment Plant in Riyadh, Saudi Arabia shows a continuous increase in the pH along the main water pipes. Water is delivered from Salbukh plant to the city through asbestos cement pipes. Normally the pH value at the plant is about 7.7. It increased to almost 9.0 after about 34.6 km. This paper will study the pH increase phenomena starting from its production from the plant and during its travel through the asbestos cement pipes. It will focus on the parameters that affect the pH all the way from the water treatment plant to the distribution system in the city. Water analysis at various locations along the asbestos cement pipes will be presented and discussed.