The fact that many municipal wastewater treatment plants in the USA are overloaded when it comes to BOD capacity has only compounded the problem. Only a portion of the water found in the feed stock passes through the membrane, while the sugar becomes increasingly concentrated in the remaining stream. Using this approach, the volume of waste for disposal can often be cut by a factor of 5 to 10, depending on the original sugar concentration. Specific osmotic pressures generated by the different sugars are relatively easy to calculate and they are very repeatable empirically. All solutes exhibit osmotic pressure, the extent of which is dependent on their molecular size and concentration.
Osmotic pressure is greater for smaller molecules and increases as concentration increases.
This varies to quite some degree depending on the products being produced and the way water is used in the plant. Typically, most of the sugar comes from the rinse of blending containers and spillage in the canning or bottling production lines. As soft drinks have moved more toward using fructose as a sweetener, the resulting osmotic pressure levels found in a typical wastewater stream are, in fact, doubled versus when cane or beet sugar was used. This has affected the application of membranes somewhat, nevertheless RO continues to be one of the more economical methods of concentrating sugars. PA membrane has the advantage of providing high flow and excellent rejection of fructose.