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What Is RO system?

RO system is a membrane treatment system used to separate dissolved solutes from water, most often in the field of seawater desalination to remove salts and other impurities from seawater, but also in industrial boilers, drinking water systems, pharmaceutical production, and other fields.

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What Is Osmosis?

To understand the principles and process of reverse osmosis, first understand the osmosis process that would occur naturally. Osmosis is a naturally occurring phenomenon in which two solutions of different concentrations are separated by a semi-permeable membrane (a membrane that allows solvent molecules to pass through and does not allow solute molecules to pass through) and water molecules or other solvent molecules pass from the lower concentration solution through the semi-permeable membrane into the higher concentration solution. This process is found everywhere in nature, where plants use it to absorb water and nutrients from the soil, and in humans, where the kidneys use osmosis to absorb water from the blood.

Osmosis working principle

A simple experiment to help you understand the phenomenon of osmosis. Suppose a container of sugar water is divided into two parts by a membrane, like an animal cell membrane. This membrane can be permeable to water, but not sugar molecules. As shown in the figure, the circle is compared to sugar. The A side has more sugar molecules than the B side, with a higher concentration. This time, the membrane, in order to maintain the concentration balance between the two sides, should allow the A side of a part of the sugar molecules to move to the B side, but the membrane does not allow the sugar molecules to pass through, so in order to maintain the equilibrium phenomenon, B side of the water molecules move to the other side, dilute the A side of the concentration until the two parts of the solute concentration are equal, and finally, the experiment to obtain the B side of the solution volume decreased, while the A side of the volume increased.

Two containers describe the movement trend of water molecules at different concentration differences
What Is Reverse Osmosis?

Osmosis membranes allow only water molecules to pass through, molecules larger than water such as minerals, salts and bacteria cannot pass through. Osmosis is a natural occurrence that does not require energy. It has been found that water can flow from high to low concentrations by applying pressure greater than the natural osmotic pressure to push the water through the osmosis membrane, a phenomenon known as reverse osmosis.

RO working principle

For example, salt water and pure water are on both sides of a container separated by an osmosis membrane. If it is a natural osmosis phenomenon, pure water will flow into salt water until the concentration is equal on both sides. However, when a certain pressure is applied to the brine side, the water molecules in the brine will flow to the pure water side to achieve reverse osmosis. Through this phenomenon, the idea of seawater desalination was generated and applied in the field of seawater desalination to desalinate and purify seawater to extract fresh water. After much research and technological development, this technology is now used in industrial boilers, drinking water systems, seawater desalination, pharmaceutical production, cosmetics production, food and beverage processing, and many other applications.

What Is Reverse Osmosis Membrane?

RO membrane is also a semi-permeable membrane that allows only water molecules to pass through, not impurities such as organic matter, inorganic matter, microorganisms, colloids, and bacteria. This membrane works by using pressure to separate water from a highly concentrated solution containing solutes and filter out impurities such as ions, microorganisms, and organic matter from raw water, resulting in high purity water.

Filtration effect of RO membrane
RO membrane
Reverse Osmosis Common Impurities & Removal Methods
How Do Reverse Osmosis Systems Work?

A high pressure pump is used to increase the pressure on the inlet of the RO membrane, forcing the water substance through the RO membrane and retaining almost all (95% to 99%) of the impurities, resulting in pure water. The amount of pressure required depends on the impurity content of the feed water and the type of membrane selected, and the clean permeate from desalination is called pure water, leaving the concentrated produced water carrying contaminants and impurities as concentrated water.

Reverse osmosis system inlet water condition

The RO system includes raw water pre-filters system, membrane purification system, and after treatment system, which can be divided into the following components:

A complete reverse osmosis system

Basic components of reverse osmosis equipment

RO system work process

Complete reverse osmosis system

Role of Pretreatment

Due to the wide variety of raw water, complex composition, raw water quality, RO system recovery rate, and other major process parameters, selecting the appropriate pre-filters system, can reduce RO membrane fouling, and scaling phenomenon, to prevent RO membrane desalination rate and water production rate of the problem of too low.

The following is a brief description of the role of the pretreatment stages:

Is Water Quality Analysis Needed?

Water quality analysis is an essential step before any water treatment project. It includes hardness, alkalinity, TDS, ion concentration, and other data. Design engineers will use the water quality analysis report provided to develop water treatment plans, both to establish a reasonable pretreatment program to ensure the removal of impurities to reduce fouling of the RO membrane fouling phenomenon; to develop the best membrane arrangement to achieve the maximum recovery rate and desalination rate of the RO system; and to identify potential contaminants. It can also identify potential contaminants for timely prevention and removal. For example, the Langelier Stability Index is a measure of whether the water has a tendency to scale, which needs to be calculated based on data from the water quality analysis report to achieve the purpose of scale prevention.

Therefore, a detailed water quality analysis before entering the RO system is absolutely necessary for the effectiveness and safety of the entire system operation.

A water quality testing report
Difference Between 'Pass' and 'Stage' of Reverse Osmosis

'Pass' and 'stage' are two concepts that are often confused in RO systems, here is a brief distinction.

Two-Stage RO system

In a two-stage system, concentrated water from the first stage becomes feed water for the second stage, with the aim of increasing the recovery rate of the system.

Two-Pass RO system

In a two-pass process system, the water produced from the first pass process will become feed water from the second pass process with the aim of improving the water quality of the system produced (increasing the desalination rate).

For more detailed information on system 'pass' and 'stage' you can visit RO membrane arrangement

Desalination Rate and Recovery Rate

The desalination rate refers to the RO membrane's ability to remove salts and stop other dissolved solids. The RO membrane separates water molecules from dissolved salts and other contaminants, and the higher the desalination rate, the better the RO membrane performance, calculated as follows:

Desalination rate calculation formula

The recovery rate indicates the ratio of the permeate flow rate in the system to the feed water rate. The recovery rate is affected by many factors and is related to the quality of the incoming water, the type of membrane element, the connection method, etc. The higher the recovery rate means that the higher the water produced, the lower the concentrated emissions, and is calculated as follows:

Recovery rate calculation formula
Advantages of Reverse Osmosis Process

As a mature water treatment process, the RO process has many benefits:

Weaknesses and Challenges of RO System

Although reverse osmosis is one of the most effective filtration technologies on the market today, there are still some weaknesses that cannot be avoided and remain a challenge for us: