Antiscalant Technical Guide
As RO recovery increases, the corresponding salt concentrations increase in the RO reject stream and exceed the saturated limits of the sparingly soluble salts. Over time, hard scale deposits will form on membrane surfaces and within the feed channel spacers. Scaling within RO systems is a serious matter, for not only does scaling drastically reduce system performance, but also the irreversible damage on the membranes can come from more than just scale deposition. If we are talking about strictly scaling, then the damage to the membrane can include telescoping, mechanical damage to the components, and also abrasion. Examples of some common sparingly soluble salts are calcium carbonate and the sulfates of calcium, barium, and strontium. Less common salts include calcium phosphate and calcium fluoride. Filtration (MMF) is the most economical means of removing colloids from RO feed water. However, to be effective, in-line coagulation is required upstream of the filters. With coagulation, colloid removal rates of 95 to 99 percent are typical.
From 1970 to the mid 1980’s, the only scale inhibitor available on the market was sodium hexametaphosphate (SHMP). Although primarily used for sulfate scale, SHMP has a number of drawbacks including lack of effectiveness against calcium carbonate, its solutions were unstable, eventually reverting to orthophosphate. Orthophosphate not only lacks scale inhibition properties, but it may precipitate with calcium to form calcium phosphate scale. In the late 1960’s Monsanto and Pfizer developed the first organic scale inhibitors. Initially, these were limited to several organic phosphonates and polyacrylic acid. The current generation of scale inhibitors include a variety of different phosphonates and polymers, which are used either alone or in combination.
SCALE CONTROL METHODS
Over the years, the RO industry has developed methods of scale prevention. These methods fall into three categories:
• Ion exchange softening
• Scale inhibitor addition
Acidification shifts bicarbonate and carbonate alkalinity needed to produce calcium carbonate scale. While acidification prevents calcium carbonate scaling from occurring, it is not effective against the sulfate scales of calcium, barium, and strontium. Additional disadvantages of acidification include corrosivity of the acid, the cost of storage tanks, and the monitoring equipment required for proper acid dosing. Acidification also lowers permeate pH, requiring post-treatment pH adjustment or degassers to remove excess carbon dioxide.
The selection of the acid type for acidification is dependent on the feed water quality. The two most common acids for feed pH adjustment are HCl (hydrochloric acid) and H2SO4 (sulfuric acid). For waters containing elevated levels of barium, sulfuric acid would add additional sulfates that increase the potential for barium sulfate precipitation. Hydrochloric acid, on the other hand, off-gases and is not operator-friendly in terms of handling.