How to Use Reverse Osmosis in Practice:
In practice, reverse osmosis is applied as a cross-flow filtration process. The simplified process is shown below:
With a high pressure pump, feed water is continuously pumped at elevated pressure to the membrane system. Within the membrane system, the feed water will be split into a low-saline and/or purified product, called permeate, and a high saline or concentrated brine, called concentrate or reject. A flow regulating valve, called a concentrate valve, controls the percentage of feed-water that is going to the concentrate stream and the permeate which will be obtained from the feed.
The key terms used in the reverse osmosis / nanofiltration process are defined as follows.
Recovery - the percentage of membrane system feed-water that emerges from the system as product water or “permeate”. Membrane system design is based on expected feed-water quality and recovery is defined through initial adjustment of valves on the concentrate stream. Recovery is often fixed at the highest level that maximizes permeate flow while preventing precipitation of super-saturated salts within the membrane system.
Rejection - the percentage of solute concentration removed from system feed-water by the membrane. In reverse osmosis, a high rejection of total dissolved solids (TDS) is important, while in nanofiltration the solutes of interest are specific, e.g. low rejection for hardness and high rejection for organic matter.
Passage - the opposite of “rejection”, passage is the percentage of dissolved constituents (contaminants) in the feed-water allowed to pass through the membrane.
Permeate - the purified product water produced by a membrane system.
Flow - Feed flow is the rate of feed-water introduced to the membrane element or membrane system, usually measured in gallons per minute (gpm) or cubic meters per hour (m3/h). Concentrate flow is the rate of flow of non-permeated feed-water that exits the membrane element or membrane system. This concentrate contains most of the dissolved constituents originally carried into the element or into the system from the feed source. It is usually measured in gallons per minute (gpm) or cubic meters per hour (m3/h).
Flux - the rate of permeate transported per unit of membrane area, usually measured in gallons per square foot per day (gfd) or liters per square meter and hour (l/m2h).
Factors Affecting Reverse Osmosis Performance:
Permeate flux and salt rejection are the key performance parameters of a reverse osmosis or a nanofiltration process. Under specific reference conditions, flux and rejection are intrinsic properties of membrane performance. The flux and rejection of a membrane system are mainly influenced by variable parameters including:
- feed water salt concentration
The following graphs show the impact of each of those parameters when the other three parameters are kept constant. In practice, there is normally an overlap of two or more effects. Figure 1.6, Figure 1.7, Figure 1.8 and Figure 1.9 are qualitative examples of reverse osmosis performance. The functions can be understood with the Solution-Diffusion-Model, which is explained in more detail in Section 3.11.2. In nanofiltration, the salt rejection is less depending on the operating conditions.
Not to be neglected are several main factors which cannot be seen directly in membrane performance. These are maintenance and operation of the plant as well as proper pretreatment design. Consideration of these three ‘parameters’, which have very strong impact on the performance of a reverse osmosis system, is a must for each OEM (original equipment manufacturer) and end user of such a system.
With increasing effective feed pressure, the permeate TDS will decrease while the permeate flux will increase as shown in Figure 1.6.
If the temperature increases and all other parameters are kept constant, the permeate flux and the salt passage will increase (see Figure 1.7).
Recovery is the ratio of permeate flow to feed flow. In the case of increasing recovery, the permeate flux will decrease and stop if the salt concentration reaches a value where the osmotic pressure of the concentrate is as high as the applied feed pressure. The salt rejection will drop with increasing recovery (see Figure 1.8).
Feed water Salt Concentration
Figure 1.9 shows the impact of the feed water salt concentration on the permeate flux and the salt rejection.