AWWA MTC57663

As populations continue to grow and adequate supplies of fresh water have become fewer and more difficult to utilize, many municipalities are considering the use of brackish and even seawater to meet growing demands for potable water. Reverse Osmosis (RO) can be used to desalt these supplies. However, the application of RO treatment poses many challenges associated with the stability and corrosivity of the finished water. RO permeate water is characteristically low in mineral content, alkalinity, and pH. The lack of carbonate alkalinity makes permeate water very unstable and prone to wide variations in pH due to the low buffering capacity. Lack of carbonate alkalinity and calcium may also contribute to increased corrosion, since protective calcium carbonate films cannot be deposited on pipe walls. Quite often, monovalent ions, such as chlorides as well as gases like hydrogen sulfide and carbon dioxide, may pass RO membranes to a larger degree than other ions or molecules, which can also contribute to corrosion potential. These issues must be considered when developing post-treatment concepts for RO processes. Post-treatment processes that contribute to increased stability and hardness, and provide pH adjustment are often implemented. Chlorides, hardness, alkalinity, carbon dioxide, and pH play a significant role in the assessment of stability and corrosion potential in RO treated water. These parameters are frequently used by design engineers to evaluate stability and corrosion potential, thereby helping them develop design criteria for post-treatment processes. However, there are a variety of tools to assess stability and corrosion. These tools include scaling and stability indices such as: the Langelier Saturation Index (LSI), Calcium Carbonate Precipitation Potential (CCPP), Agressivity Index (AI); as well as corrosion indices like the Larson Ratio. This paper addresses the use of stability, scaling, and corrosion indices to develop post-treatment concepts for brackish and seawater RO treatment processes. Practical examples are provided that cover the proper interpretation of these indices for development of post-treatment process design criteria. Commonly used terms for corrosion and stability control process evaluation are defined. Easy to understand examples that are widely applicable to a variety of municipalities considering brackish and seawater RO treatment are included. Includes 17 references, tables, figures.