Seawater Desalination
Extracting pure water from saline water is an energy driven process and can either be accomplished by a phase change (distillation) or the forced movement of water molecules through a selective membrane. The predominant processes are multi-stage flash distillation (MSF) and membrane-based reverse osmosis (RO), respectively.

Desalination plants range in size from heavy coastal industry to small local plants. While large facilities may produce several hundred thousand cubic meters of fresh water per day, small plants usually have a capacity of only a few hundred cubic meters per day.

Regional differences are also significant: the majority of worldwide plants are located in the Arabian Gulf, where the total installed seawater desalination capacity exceeds 10 million cubic meters per day. Desalination of seawater is a major industrial activity in this semi-enclosed sea and considered a major source of pollution. In comparison, desalination is so far only a minor industry in other regions like the Mediterranean, the Red Sea, or the Caribbean, but future growth of desalination activity must be anticipated.

The need for chemical pretreatment

By the technical process of desalination, salts and other contaminants are removed from seawater, thereby delivering a product of high quality for human benefit. At the same time, a highly saline waste is generated and discharged to the sea. Depending on the process, different pretreatment chemicals are added to the intake water to improve plant performance. Their residuals are present in the brine and are consequently discharged to the marine environment.

Irrespective of different modifications in the distillation process, i.e. multi-stage flash (MSF), multi-effect (ME) or vapor compression (VC) distillation, all thermal plants use heat exchanger surfaces to separate vapour/freshwater from seawater/brine. On the seawater side, these surfaces are prone to biological fouling and scaling, which reduce the plant's thermal efficiency and promote corrosion. Without proper seawater pretreatment, tube blockage or leakage can seriously impair the plant's performance and result in shutdown for cleaning or replacement of pipes.

The most vulnerable point in reverse osmosis plants is the membrane, which is permeable for water but not for most dissolved and suspended material. Comparable to a sieve, the membrane is easily clogged by suspended solids or biofilm formation if certain types of seawater constituents are not controlled by chemical pretreatment before the feedwater enters the RO unit.

Similar problems arise in both technologies due to the nature of seawater: conventional pretreatment steps in both applications include scale and biofouling control. However, a few basic differences can be attributed to process design: removal of suspended material and dechlorination is necessary to protect RO membranes, whereas distillation plants often extend their pretreatment scheme to corrosion and foam control.

Photos: Thomas Höpner

© 2005 Sabine Lattemann
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