Reverse osmosis elements and systems remove contaminants such as salts, pyrogens, and microorganisms from water to produce clean water as an output. Applications for reverse osmosis elements and systems include waste water treatment, marine vessel potable water creation, and manufacturing processes that need pure clean water such as pharmaceuticals and semiconductors. Complete systems are often used with other water purification equipment such as ion exchange equipment, and will often include mechanical pre-filters and strainers to remove larger contaminants before it can clog up the RO membrane.

Operation

The primary function of a reverse osmosis elements and systems is to remove contaminants from water. This is accomplished by using feed water at a high enough pressure, typically 150 to 800 psi, to overcome the natural process of osmosis and force feedwater with high contaminant concentration through a semi-permeable membrane, removing the contaminants and producing clean water on the other side. These membranes block contaminants by their molecular weight and ionic charge. Any particle with a molecular weight higher than a water molecule will be effectively removed. Ions such as Ca⁺⁺ that are divalent will be removed with higher efficiency than monovalent ions such as Na⁺. Contaminants are flushed out as the system operates in concentrated waste water but some will remain so that an RO system will require periodic cleaning.

Types of Membranes for Reverse Osmosis 

There are multiples types of membranes used in reverse osmosis systems.

• Aromatic polyamide membranes: Thin films bonded to layers of porous but supportive materials. Thin-film composite membranes are suitable over a pH range of 2 to 11 and can tolerate temperatures as high as 115° F.
• Cellulose acetate membranes: Membranes designed for a pH range of approximately 2.5 to 7 and can handle a maximum temperature of about 85° F.

Configurations

Reverse osmosis systems can be configured as multi-pass systems, multi-stage systems, or both. In a multi pass RO system the clean permeate water from one membrane will be passed through another membrane to remove any contaminants that passed through the previous membrane. In a multi-stage RO system the concentrated waste water that is rejected from the first stage will then go through another stage to be cleaned.

Typical stages of an RO system.

Specifications

The follow specifications determine the functionality of a reverse osmosis element or system. 

• Filtration product: The filtration product is the type of media the system or element is designed to take as feed.
• Maximum flow rate: The maximum flow of feed media into the system. This will be slightly higher than the output due to some of the feed is used to flush out the contaminants as concentrated waste.
• Filtration grade: Filtration grade (micron rating, Beta ratio) describes the ability of a fluid filter to remove contaminants by particle size.

Features

These features provide additional utility or ease of use.

• High differential pressure indication: High differential pressure can indicate a problem with the system or the need for cleaning.
• Integral bypass: An integral bypass can allow for bypassing the system in times of maintenance or repair without needing to pipe in a separate bypass route. The bypass can also act as a pressure relief if there is system failure.

Resources

Puretec Industrial Water—What is Reverse Osmosis?

Image credits:

AccuStream/Hypertherm, Inc. | Puretec Industrial Water