Seawater Desalination RO Membranes
Below are the essential components of any reverse osmosis system:
A reverse osmosis system is dependent upon and built around its individual membranes. Each membrane consists of a spiral-wound sheet of semipermeable material, which varies dependent on application. Membranes are available in 2-inch, 4-inch and 8-inch diameter with the 4-inch and 8-inch diameter sizes most commonly used in the water treatment industry.
The water treatment industry, as a whole, has accepted a 40-inch length as a standard size so that membranes from different manufacturers are interchangeable in equipment systems. One of the primary measurements of a membrane is its square footage. Membranes are available in the range of 350-450 square feet of surface area.
Semipermeable membranes were first constructed using cellulose acetate (CA). However, the industry later switched to the use of a thin film composite (TFC) being placed on top of a stronger substrate. TFC membranes are primarily used today.
Individual membranes are loaded into a single housing, which can hold up to six membranes in series. The housings are critical for holding the reverse osmosis filters stably in place. They also connect to the check valves and drain lines that prevent backflow and enable the water to drain effectively.
Housings are then grouped into parallel flow streams called stages. A single reverse osmosis system can have up to three stages in series. The major benefit of using RO stages in series is the reduction of wastewater. In an efficient RO staged system, wastewater can be reduced to 15% of the total flow, whereas the rejection flow from a single membrane alone can be as high as 50%.
A complete RO system will also include a pretreatment cartridge filtration and a pressuring pump. The pretreatment cartridge filters remove larger solids from the water before it proceeds to the semipermeable membrane. This step is necessary to prevent fouling of the membrane and extend its life span.
Cartridge filters tend to be simple constructions and are often made from wound strands of polypropylene.
A comprehensive RO system also has a controls system. In a small RO system, the controls may be as simple as valves and rotameters. A larger system will contain the flow, temperature and pressure transmitters, as well as control valves operated from a human-machine interface (HMI) or programmable logic controller (PLC). Larger systems also incorporate variable frequency drives (VFDs) for the pressurizing pump and sometimes energy recovery devices.
The controls system allows users to monitor and operate the RO system from beginning to end. It also often contains alarms that can notify operators of any RO system issues that require immediate attention.