PRETREATMENT
Sulfuric acid is added to condition the feed and inter-stage feed water for the membrane softening process. The 93% sulfuric acid is added to both the feed and inter-stage feed water to convert bicarbonate alkalinity to carbon dioxide in solution. The sulfuric acid lowers the pH to approximately 6.0. Lowering the pH reduces the potential for calcium carbonate scaling on the membrane elements. If the sulfuric acid is not added to the water, the efficiency or percent recovery of the membrane elements will not meet the design criteria.
Anti-scalant is not being used at this time. However, its purpose is to inhibit the formation of carbonate and sulfate scales.
MEMBRANE CHARACTERISTICS
We are currently using Hydranautics membrane elements 8540-LSY-PVD1 and 8540-LSY-NCM1 and 8540-USY-ESPA. In the first stage we are using both the PVD1 and NCM1 membranes. In the second stage and third stage all elements are ESPA. Our newest elements are the ESPA element which are the first in a new generation of membrane elements. The (E) stands for energy and the (S) for savings that is in response to a demand for increased energy savings and water purity. There are 7 elements per vessel and a total of 2,016 elements in the process.
A spiral membrane element is a method of packaging flat sheets of membrane. Two or more sheets of membrane are sealed around a perforated permeate collector tube with the active salt rejection surface of the membrane on the outer surface of the assembly. A permeate carrier is included between the membrane sheets to carry permeated water to the central collector. A feed spacer is placed over between the membrane surfaces to provide a flow path for feedwater in the rolled assembly. This assembly is then rolled around the central permeate collector.
A brine seal is affixed to the upstream end of the assembly to direct water flow through the active portion of the element. An anti-telescoping device is affixed to the downstream end of the assembly to support the outer layers of the element against the forces of differential pressure. The finished assembly is covered with an outer shell of fiberglass.