AWWA MTC95040

The Corkscrew Water Treatment Plant was designed to reduce the high iron levels, high hardness, and excessive TDS in the raw water. The feed, containing 4.4 ppm of iron, needed a reduction to less than 0.3 ppm. It was determined that the best way to meet these demands was to utilize membrane softening (i.e., nanofiltration). Each membrane softening train operates with three concentrate stages. The system is designed to take a portion of the second stage and all of the third stage permeate and recycle it back to the feed for better iron rejection. Each train is capable of producing 614,000 GPD, however, due to the recycle, it only produces 500,000 GPD net to use. In the plant's recent expansion to 1 MGD from 0.5 MGD, there were some changes made to the process design of the system. The original train utilized Dow FILMTEC's NF70 spiral membranes. However, the new train operates with NF90 membranes that have tighter salt and iron rejection. This paper presents a discussion on the history of this membrane softening facility, the plant's unique features, and a comparison between the old and new membranes.