Membrane Filtration / Reverse Osmosis

High Quality water with minimum chemical manipulation.

Principles of Operation

Reverse Osmosis is a process for removing dissolved mineral salts, organic molecules and certain other impurities by forcing water under pressure to pass through a semi-permeable membrane. HEI industrial Reverse Osmosis systems are appropriate for applications with flow rates up to a standard size of 500 gpm per module, with modules being combined to provide even larger flow rates. Single-Pass Reverse Osmosis units are ideal for most situations, but Double-Pass Reverse Osmosis units can be provided for those situations where ultra-pure water is necessary. The Reverse Osmosis process reverses the natural osmotic effect in which fluids with a low concentration of dissolved solids pass through a membrane into an area of higher concentration. With Reverse Osmosis, water is made to pass from a state of high concentration to a state of low concentration. Since Reverse Osmosis does not occur naturally, it must be created by applying pressure to the high solids water in order to force it through the membrane, with pressures from 200 to 400 psig in most applications, 1,000 or even 1,200 psig for sea water desalination and high solids conditions. The pressure applied to the feed side of the RO membrane must be much higher than the natural osmotic pressure of the water in order for the osmotic process to be reversed. High pressure pumps are used to create the pressure needed to produce economically acceptable flow rates.

Design Considerations

HEI Reverse Osmosis systems work on the Crossflow Filtration method. Using this method , which takes the feed water and uses a percentage of it as a wash or reject stream, the solids are removed during the filtration process. This extends the life of the filter membrane. The product flow of an RO is mainly a function of temperature and pressure. System recovery (product divided by feed) is limited by the characteristics of the feed water and can be controlled through the use of recycle stream. Product quality is based on a percentage of dissolved solids fed to the membrane. There should be an economic balance between product quality and system recovery. High recoveries increase concentration of dissolved solids in the system which degrades quality, but high recoveries make the system more efficient and decrease waste. RO units do not deliver to service all of the water that is fed to them. During operation, some of the incoming water is used to wash down the membrane, and only part becomes finished product water. Purified water is referred to as product and wastewater is referred to as concentrate, or reject. The percent of water delivered as product is called the recovery, and depends upon the membrane and on total RO unit design considerations. RO units are volume rated at 77°F (25°C) incoming water temperature. Adjustments must be made if the incoming water temperature varies.


HEI can provide pretreatment of water prior to the RO process if it is required. Chlorine removal is important but high hardness minerals should also be controlled by a softener or other suitable methods of treatment. Hard water scale build-up impairs RO unit performance. Turbidity, pH, iron, and other impurities must be controlled for optimum RO performance.

Post Treatment

HEI can provide ozone, ultra-violet and chlorination systems for potable water systems.

Systems Engineering

RO units are often used to provide low solids feed water to deionizers. This lengthens the deionize service cycle and lowers regeneration frequency. Considerable money can be saved through reduction of regenerant chemicals. Systems engineering of water treatment problems takes on added significance as RO and DI processes are designed and operated together as a system.

Reject Treatment

HEI builds physical, chemical and biological waste systems to address treatment of the reject (concentrate), a concern often overlooked in process design.

Case Study

This study involves a Double Pass RO system for high quality turbine engine NOx suppression water at an electricity generating plant. They saved $500,000 capital costs over a resin system, and approximately $100,000 yearly in operating costs. The system is supplying at 75% recovery. The following photograph shows the major components of the installed system. From the left can be seen a) the membranes, b) local control panel, c) 2 upright pumps, d) filter. Behind is seen the panel room. Not shown but included in this installation is a carbon filter.

© Hoffland Environmental Inc, 2012