AWWA MTC57663

As populations continue to grow and adequate supplies of fresh water have become fewer and more difficult toutilize, many municipalities are considering the use of brackish and even seawater to meet growing demands forpotable water. Reverse Osmosis (RO) can be used to desalt these supplies. However, the application of ROtreatment 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 alkalinitymakes permeate water very unstable and prone to wide variations in pH due to the low buffering capacity. Lack ofcarbonate alkalinity and calcium may also contribute to increased corrosion, since protective calcium carbonatefilms cannot be deposited on pipe walls. Quite often, monovalent ions, such as chlorides as well as gases likehydrogen 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-treatmentconcepts for RO processes. Post-treatment processes that contribute to increased stability and hardness, and providepH adjustment are often implemented.Chlorides, hardness, alkalinity, carbon dioxide, and pH play a significant role in the assessment of stability andcorrosion potential in RO treated water. These parameters are frequently used by design engineers to evaluatestability 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 stabilityindices 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 forbrackish and seawater RO treatment processes. Practical examples are provided that cover the proper interpretationof these indices for development of post-treatment process design criteria. Commonly used terms for corrosion andstability control process evaluation are defined.Easy to understand examples that are widely applicable to a variety of municipalitiesconsidering brackish and seawater RO treatment are included. Includes 17 references, tables, figures.

Product Details

Edition:

Vol. - No.

Published:

03/05/2003

Number of Pages:

14

File Size:

1 file , 1.5 MB