Basics and Principle of Reverse Osmosis
Overview of RO application
Reverse Osmosis (RO) is a separation technique that is suitable for a wide range of applications, especially when salt and/or dissolved solids need to be removed from a solution. Accordingly, RO can be used for seawater and brackish water desalination, to produce both water for industrial application, and drinking water. It can also be applied for the production of ultrapure water (e.g. semi- conductor, pharmaceutical industries) and boiler feed water. In addition, RO membrane systems are used for wastewater and water reuse treatments.
RO is currently considered one of the most economic and effective process for water desalination. Accordingly, it is often the appropriate technique to treat solutions having salt concentrations from 100 to over 50,000 mg/ liter. Solutions with salinity from surface water to sea water, and even brines, can be treated by RO membrane.
Cross flow is the configuration applied for membrane separation using RO membrane. As shown in Figure 1.1 the feed water stream flows tangentially to the membrane surface. A fraction of the water in this feed stream passes through the membrane, whereas the majority of the feed flow travels along the surface. Thus, two streams are collected:
- permeate, almost pure water containing low concentration of ions
- concentrate, having high concentration of small particles and dissolved ions
In operation, the RO membrane system is continuously supplied with feedwater which produces a constant water movement from feed to concentrate. When in cross-flow operation, there is little accumulation of the rejected solutes and fouling or scaling can be minimized.
Principle of Reverse Osmosis
Osmosis is a natural phenomenon which can be defined as the movement of pure water through a semi permeable membrane from a low to a high concentration solution (see Figure 1.2). The membrane is permeable to water and some ions but rejects almost all ions and dissolved solids. This process (movement of water) occurs until the osmotic equilibrium is reached, or until the chemical potential is equal on both sides of the membrane.
A difference of height is observed between both compartments when the chemical potential is equalized. The difference in height expresses the osmotic pressure difference between the two solutions.
Reverse osmosis is a process which occurs when pressure, greater than the osmotic pressure, is applied to the concentrated solution. Water is forced to flow from the concentrated to the diluted side, and solutes are retained by the membrane
RO membrane description
RO membranes can be supplied in both flat sheet and HFF (Hollow Fine Fiber) structural formats. The flat sheet RO membrane is composed of three layers.
As shown in Figure 1.3, there is a non- woven polyester support layer, a polysulfone layer, and on top the polyamide barrier layer. The barrier layer is formed by the polyamide of which the molecular structure is shown in Figure 1.3.
RO membrane performance
The performance of an RO membrane is defined by various parameters. The important parameters are defined below.
In RO device there are three streams. The feed stream is separated by RO membrane into permeate and concentrate streams. Flow rates of these streams are usually expressed in cubic meters per hour (m³/h) or in gallons per minute (gpm). Feed flow rate is defined as the rate of water entering the RO system. Permeate flow rate is defined as the rate of water passing through the RO membrane, and concentrate flow rate is defined as the rate of flow which has not passed through the RO membrane, and comes out from the RO system with rejected ions.
Several designs are available for making RO membrane and elements. These membrane devices are available in plate and frame, tubular and hollow fiber membrane module configuration.
The most common element device for RO membrane application is assembled according to spiral-wound configuration.
This format provides the highest degree of packing density. The spiral-wound module uses flat sheets wound around a centre pipe.
The membranes are glued along three sides to form membrane leaves attached to a permeate channel (centre pipe) placed along the unsealed edge of the membrane leaf. The internal side of the leaf contains a permeate spacer designed to support the membrane sheet without collapsing under pressure.
This permeate spacer is porous and conducts permeate to the centre pipe. A feed channel spacer (a net-like sheet) is placed between the leaves to define the feed channel height (typically round 1 mm) and provide mass transfer benefits. The membrane leaves are wound around the centre pipe and given an outer casing (Figure 1.5). This design provides a high packing density (300-1000 m2/m3).