Reverse osmosis membranes must be manufactured into specific configurations before they can be applied in water treatment systems. At present, the main membrane configurations include plate-and-frame, tubular, spiral wound, and hollow fiber types. Among these, spiral wound and hollow fiber configurations are the most commonly used in water treatment applications. For example, YIME RO membranes adopt a spiral wound structure, which is widely used in reverse osmosis systems.
For spiral wound configurations, commonly used membranes include cellulose acetate (CA) membranes and thin-film composite (TFC) membranes. These membranes are assembled into membrane elements, which are then placed inside a pressure vessel to form a complete membrane module.
Membranes used for spiral wound elements are typically first manufactured as flat sheet membranes. The structure of a cellulose acetate membrane is shown in Figure 1. The upper layer is a dense thin layer (0.1–1.0 μm), known as the desalination layer. Beneath the desalination layer is a slightly thicker porous support layer (100–200 μm). Water can easily pass through the dense layer and flow into the porous support layer. The dense layer acts as the semi-permeable membrane, effectively preventing salts from passing through and achieving desalination.
The structure of a composite membrane is shown in Figure 2. A composite membrane consists of three layers: an ultra-thin desalination layer on the top, a porous polysulfone intermediate support layer in the middle, and a polyester support fabric at the bottom. Because the polyester support layer is relatively uneven and porous, it cannot directly support the desalination layer. The surface pore size of the polysulfone layer can be controlled at about 0.015 μm. The desalination layer has a thickness of approximately 0.2 μm. Supported by the polysulfone layer, the membrane can withstand higher operating pressures and exhibits strong resistance to mechanical stress and chemical corrosion.
For hollow fiber membrane configurations, numerous hollow fibers made from aromatic polyester membranes are directly assembled inside a pressure vessel, forming the basic desalination unit for water treatment - the membrane module.
Whether it is a spiral wound configuration or a hollow fiber configuration, the design must generally meet the following requirements:
- Provide adequate mechanical support so the membrane can withstand feed water pressure.
- Ensure that feed water, concentrate, and permeate flow separately, with uniform distribution and proper flow conditions, thereby minimizing concentration polarization.
- Allow the membrane's inherent salt rejection and water permeability to be fully utilized within the configuration.
- Maximize the effective utilization of membrane surface area.
In the next section, we will focus on the spiral wound configuration commonly used in YIME membranes, and explain its structure as well as the differences between various membrane types.
