Cleaning Spiral Wound Membranes
CLEANER TECHNICAL GUIDE
All reverse osmosis (RO) systems foul over time. Symptoms of fouling include a decrease in normalized permeate flow, an increase in salt passage or an increase in differential pressure. Severe fouling may also cause irreversible damage to membranes, making effective cleanings difficult or impossible.
Industry guidelines suggest cleaning when one or more of the following events occur:
• Normalized flux decreases by 10 to 15 percent
• Salt passage increases by 10 percent
• Differential pressure increases by 10 to 15 percent
Figure 1 illustrates the impact of effective and ineffective cleanings on normalized flow rates. The blue line illustrates effective cleanings, which restore flux to the original baseline value each time. The gray line represents ineffective cleanings that over time restore less and less of the original flow,
resulting in the need to clean more frequently.
Increased cleaning frequencies result in greater downtime, reduced element life, and higher operating costs. Ineffective cleanings are often attributable to low-quality cleaners, poor cleaning procedures, and waiting too long to clean fouled elements.
This bulletin describes procedures and products that can help plant operators conduct more effective cleanings and achieve more cost-effective system operation.
There are five main foulant categories:
Metal foulants include iron, manganese, and aluminum. Less frequently encountered metals include zinc, copper, and nickel. Iron and manganese commonly occur in ground waters as soluble divalent ions. However, the introduction of air or chlorine may oxidize, followed by precipitation to their trivalent hydrous metal oxides. Iron corrosion products from steel well liners and feed piping may also cause membrane fouling. Carryover of aluminum hydroxide floc from clarifiers and filters is the major causes of aluminum fouling.
Supersaturation of sparingly soluble salts can result in fouling of membrane surfaces and element feed spacers. The most common scale formers are calcium carbonate and the sulfates of calcium, barium and strontium. Less common scales are silica and calcium fluoride. Silica scale is often very difficult to remove, requiring the use of corrosive and dangerous ammonium bifluoride containing cleaners.