AWWA MTC61071

The 8-inch diameter x 40-inch long spiral wound reverse osmosis (RO) element is used widely today for municipal RO systems. However, this 8-inch element is clearly not the optimum for a large plant. It has long been recognized that the number of elements, pressure vessels, pipe connections and O-rings required for a large RO plant is unreasonable. Additionally, a system based on 8-inch diameter elements requires significantly more footprint than a system based on larger diameter elements. Elements that are larger than the 8-inch x 40-inch element have been produced for some specific projects over the years, but large elements have not been fully commercialized in the past due to concerns related to manufacturing, operations and maintenance. This paper discusses the issues related to the manufacture and use of large elements, and demonstrates how these challenges have been addressed for an element that is 18-inches in diameter x 61-inches long. This element has seven times the membrane area of a typical RO element and therefore allows for a significant reduction in the number of components and the size of the RO system footprint. Since the flow capacity and forces on the element are proportional to the square of the element diameter, certain aspects of a standard spiral element required review and selection of alternatives. The authors discuss the choice of number and attachment of the component leaf units, the choices for the central core tube and inter-element connection system, and how the elements are supported within a multi-element housing against flow induced differential pressure. The authors also show how loading and handling issues dictate departures from traditional spiral element design. Results of in-house and field testing show how this large scale element meets the challenges related to operation and maintenance of an RO system. Includes 4 references, table, figures.