Approaches to the development and evaluation
of low-phosphorus composite scale inhibitors for reverse osmosis systems

Summary

The aspects of the selection and optimization of scale inhibitors for reverse osmosis and nanofiltration systems are considered. The relevance stems from the need to reduce phosphorus loading on natural water bodies while maintaining the high efficiency of inhibiting calcium carbonate and iron (III) hydroxide scale formation; these two being the most common types of deposits in the concentrate tray of spiral-wound membrane elements. The results of comparative tests of seven autoinhibitors based on phosphonates and polymers, as well as fourteen composite formulas based on these inhibitors are presented. The experiments were conducted under static conditions using model solutions simulating the composition of reverse osmosis concentrate in terms of carbonate hardness and iron ion concentration. Phosphorus-containing inhibitors were found to be the most effective against calcium carbonate. However, composite formulas with a high AA/MA copolymer concentration and small amounts of NTP additives (formulas No. 9 and No. 10) demonstrated significant synergism providing for reducing the phosphonate dosage by 4–7 times without any loss of effectiveness. With respect to iron hydroxide, polymeric inhibitors were slightly inferior to phosphonates, while composition No. 1 (80% AA/MA, 20% AA/AMPS) was the most effective among phosphonate-free formulas and comparable to the formula containing 30% NTP. Possible partial or complete replacing of phosphonates with polymer components while maintaining the required performance characteristics was demonstrated, which is consistent with the principles of developing environmentally friendly («green») antiscalants.

Key words

water treatment , reverse osmosis , nanofiltration , antiscalant , scale inhibitor , spiral-wound membrane element , inhibition efficiency