Page 142 - ro membanes

6.4 ADDITION OF SCALE INHIBITORS 125
and may result in scaling, especially in brackish plants operating at high recoveries (85% and above).
Scales vary in texture and appearancedtypically calcium sulfate scales that form on RO membranes, which treat seawater, have orderly prismatic crystalline structure. Sulfate crys- tals could reach lengths of 20 mm and widths of 5 mm (El-Manharawy and Hafez, 2001). However, sulfate scale formed from high-salinity brackish waters can vary measurably from one water source to another. On the other hand, carbonate scales formed on brackish water membranes are typically fine amorphous white deposits.
Scaling control depends on the particular mineral salts that precipitate on the membrane surface. For example, calcium carbonate scales can be prevented from forming by source water acidification. Acids convert carbonate ions CO32 into soluble bicarbonate ions and carbonic acid and ultimately into carbon dioxide.
To prevent calcium carbonate and other scaling, often commercially available scale inhib- itor chemicals (antiscalants) are added to the source water or, alternatively, scaling foulants are removed by softening or nanofiltration pretreatment facilities located upstream of the RO system.
Some of the compounds naturally contained in seawater (such as humic acids) serve as natural chelating agents and scale inhibitors. Therefore, acidification of seawater prior to membrane salt separation is not usually needed and commonly practiced.
When a high level of boron removal is targeted, seawater acidification will have a negative impact on boron removal. In addition, an overdose of acid can cause piping and equipment corrosion and create iron-based colloidal fouling on the RO membranes. Therefore, the benefits associated with acid addition will need to be weighed against the potential problems that acidification could cause.
Often, SWRO systems have to be designed to remove boron to levels below 1 mg/L. In this case, a most common practice for enhanced boron removal is to increase the source seawater pH from ambient levels (7.8e8.3) to a range between 8.8 and 11. At this high pH range, RO membrane scaling is very likely to occur and, therefore, scale inhibitors are typically added to prevent it.
Some scale inhibitors prevent the formation of seed crystals, whereas others deform the seed crystals so that they cannot grow and cause problems in the membrane system. In some cases, dispersants are added to the scale inhibitor formulations to aid in preventing colloidal material deposition.
It is important to note that antiscalants are designed so that they do not pass through the membranes and are, therefore, contained in the concentrate. This is an important issue in terms of their potential environmental impacts and toxicity.
Routine operation of antiscalant systems involves feeding of the selected scale inhibitor product at a rate proportional to the plant flow. As indicated earlier, antiscalant dosage and type are determined based on the mineral makeup of the saline source water and can be determined using specialized software packages available from the antiscalant manufac- turers or with the direct assistance from these suppliers.
Most antiscalants are delivered to the desalination plant site in a liquid form at 99% con- centration (as neat solution) and are stored in plastic containers or storage tanks. Usually, antiscalant is added to the pretreated saline source water before it is fed to the RO system. Antiscalant is first diluted from its neat (99% concentration) to a working concentration of