Comparison Between the Reverse Osmosis and Multi-stage Flash Distillation Methods

The Desalination business is set to increase ten fold by 2015. Clients have the choice of two technologies to choose from. Multi Stage Flash Distillation (MSF) with two decades of proven technology or Revese Osmosis (RO) that has considerably advanced in recent years.

Following are some of the areas that require consideration while making a choice.

Energy required to produce one litre of water together with the cost of energy makes or breaks the need for desalination and the choice of the process.

RO has a much higher yield compared to MSF. This means for the same output the intake of sea water is only one third in RO systems. This considerably decreases the power and other costs required for pumping sea water to the plant and disposing the spent brine.

RO systems require considerable energy for pressurizing the water to the membrane filters. Whereas MSF require heat energy for heating the brine before vaporisation.

The technology of RO especially the membranes have drastically improved in the last decade resulting in lesser power consumption. The MSF requires around 17 kwhr /M³ of water (heating and pumping power) against 5 kwhr /M³ for RO process.

In today’s technology level MSF may be practical only when steam energy is freely available as in co-generation plants.

MSF systems produce water with less than 100 mg / L total dissolved solids as against 400 mg /L in the RO system. This sometimes necessitates the need of second set of membrane filters for higher water qualities.

In RO systems the output depends on the sea water feed quality. MSF being a vaporisation system performance does not depend on feed quality.

What works against the RO system is the amount of pre –treatment required for the process. Elaborate arrangements are required to remove the biological organisms, suspended solids and other debris. Also the pH and other chemical regimes of the feed water have to be controlled.

Since MSF is a vaporisation process such elaborate pre-treatment facilities are not required.

MSF plants are considerably larger than RO plants with special materials. Consequentially the construction and land costs are high. Since steam is used piping, condensate and other associated systems are involved. All these result in increased maintenance costs. Scaling also adds to maintenance issues.

Improved membrane technology, use of low cost materials and the modular system in RO have reduces maintenance costs drastically. Also thes have improved the reliability of RO systems.

Technological advances in the RO system have reduced the unit cost of water making it easier to choose between the systems.