Service, Operations and Maintenance

Introduction
When a new set of reverse osmosis elements is installed, performance begins to decline immediately. Membrane manufacturers tell us that normalized flux declines by roughly 10% in the first 30 days of operation. This is truly a best case scenario, as it takes into account only the properties of the reverse osmosis membrane itself and does not take into account the many physical, chemical and biological processes that may combine to further reduce membrane performance.

Feedwater quality is a major determinant in the rate of performance decline a membrane system experiences. Suspended materials present in the water source foul elements beginning with the first stage. This can result in increased pressure differential and reduced flux. Dissolved minerals are concentrated as the feed passes through the brine-side of the membranes. As the mineral concentrations increase beyond saturation, salts precipitate. These salts tend to form in the tail elements first. The result is reduced flux and increased pressure drops in the final array. Bacteria can grow in any water, but they grow best in warm waters, and their optimal growth range overlaps with the typical feedwater temperature of reverse osmosis systems. Also, bacterial slimes can form in any part of the system. In practical experience, no one type of fouling occurs in isolation. In fact, each can influence the others, and it is common to have more than one problem foulant.