Phosphate limitation in reverse osmosis: An option to control biofouling?

The focus Of this Study was to develop and evaluate a method to measure phosphate down to a level of 1 mu g P/L or lower, and to make an inventory of the concentration of phosphate in reverse osmosis (RO) feed water. In addition, the phosphate concentration was measured before and after various pre-treatment steps in three integrated membrane systems (IMS), and the pre-treatment steps were evaluated and compared in terms of their ability to remove phosphate. An existing method was modified to allow phosphate to be measured to levels below 1 mu g P/L. After modification, the limit of detection was approximately 0.2 mu g P/L. The phosphate concentration was monitored in three IMS. The feed water to all four systems was Surface water, and the pre-treatment scheme comprised the following steps: (in-line) coagulation, UF/MF followed by antiscalant and/or acid addition prior to the RO units. The level of phosphate in the feed water (surface water) varied from 7-115 mu g P/L. In pre-treatment systems without in-line coagulation, no phosphate removal was observed. The combination of in-line coagulation and UF reduced the phosphate level by 75-98%, to 0.2-2.8 mu g P/L. However, after the addition of phosphonate antiscalant and/or acid, the phosphate concentration increased by 48 to 680% to 0.6-1.4 mu g P/L in three IMS.