Canary in a membrane plant: A sentinel against membrane scaling

Abstract

A two-element membrane pressure vessel was installed as a "canary" unit on a full-scale reverse osmosis (RO) membrane process to monitor scale formation while changes in the plant's pretreatment processes were implemented. Sulfuric acid pretreatment ahead of an RO process was to be progressively reduced and discontinued per the findings of a pilot study. To safeguard the fullscale membrane process against unforeseen failure resulting from scaling when sulfuric acid was eliminated from the RO feedwater, the canary unit was introduced. Concentrate from the second stage of the RO process train served as the feed stream to the canary unit, thereby allowing the unit to provide an earlier indication of scaling in lieu of scaling the full-scale RO process. The canary unit was intended to act as an early warning to operators who could intervene to protect the RO plant from unforeseen loss of membrane productivity resulting from scaling. Expanded SummaryThe increasing availability of proprietary scale inhibitors that can inhibit and control carbonate, sulfate, and fluoride scales of calcium, strontium, and barium, in addition to inorganic foulants composed of iron, aluminum, and reactive silica, has resulted in a re-evaluation of the need to suppress the pH of feedwater to RO membranes (Abdulrazaq, 2011;Ning & Netwig, 2002;Butt et al, 1995). With this in mind, a Florida utility operating a 4.5-mgd, two-stage brackish water RO process, operating at 75% recovery rate, conducted pilot-testing to evaluate the elimination of sulfuric acid in its pretreatment process (Tharamapalan et al, 2013). The pilot-scale evaluation was carried out over a fourmonth period. Upon successful operation of the RO pilot without acid pretreatment, the researchers, in cooperation with the utility, instituted a robust monitoring system as the acid use was progressively eliminated from its process. A two-membrane element pressure vessel was proposed and developed as a sentinel against unforeseen failure during the staged elimination of sulfuric acid from the full-scale plant.The two-membrane element pressure vessel was called a canary unit. It was designed to serve as a sacrificial membrane so that if scaling occurred, the membranes in the unit would fail before the full-scale process. To accomplish the goal of the canary unit serving as a scaling-potential indicator, the unit was connected downstream of the second stage of one of three 1.5mgd RO process trains that made up the RO plant. With only high-pressure feed pumps before the first stage and no intermediate boosting in the process system, the feed pressure to the canary unit was the residual concentrate pressure from the second stage. METHODSPilot-testing of the acid-elimination plan was carried out first under the same conditions as the full-scale plant with a 2-mg/L dose of a scale inhibitor. The staged elimination program during the pilot study involved three incremental pH steps to bring the RO feedwater pH up to ambient water pH levels-5.8 (prevailing pH suppressed condition), 6.3, 6.6, ...