Multi-Effect Distillation (MED)

Various methods of desalination are available to help purify water – in particular to remove dissolved solids from the feed water. Reverse osmosis (RO) does this through the use of pressure, whereas Multi-Effect Distillation (MED) does this through the use of heat (normally via steam), to “boil” off and re-capture the pure water (over a number of cycles or “effects”), leaving the unwanted solids behind.

As with any type of standard desalination, MED desalination is actually a process of extracting pure water from the impure feed water, rather than actually extracting the dissolved solids or contaminants from the impure feed water. As fresh water is extracted, the feed water becomes more highly concentrated with its contaminants, and therefore the extraction of the fresh water becomes increasingly difficult. Thus, most desalination methods face limits in the amount of fresh water which is able to be extracted from impure feed water. This value is usually between 20% and 50%, i.e. for every 1000L of feed water processed, around 200L to 500L of fresh water will be extracted. The remaining feed water (which is now more heavily concentrated with its contaminants) is left behind as a brine solution. MED desalination plants, if designed correctly, can generally achieve higher recovery rates than RO plants, up to around 75%.

A further benefit (and in fact the most significant benefit) of MED technology is its ability to utilise waste heat as its primary power input. For example, exhaust heat from existing on-site gas or diesel generator sets can be used to create the steam required to drive the MED plant. Of course if excess process steam already exists on site then this may be used as a direct input to power the desalination plant.

MED plants are suited to applications where large quantities of very pure water are required (typically less than 5ppm TDS). DEC Zhonghe's MED units are generally available with production capacities of between 3,000 T (3 ML) and 8,000 T (8 ML) per day of fresh water in a single unit, with larger plants being realised by multiple unit installations.

The typical mass gain output ratio (GOR) of DEC Zhonghe MED units is 9, which means that for every 1 T of steam consumed, 9 T of fresh water is produced by the system. In some cases the GOR may be as high as 12, however this is highly dependant upon the available heat (or steam) source.

DEC Zhonghe's MED plants generally consume less than 1kWh of electrical energy per 1 T (1kL) of fresh water produced. They operate at low temperature, and can be adapted to suit a number of different heat sources.

In comparison other desalination plant types, MED plants are very simple and cost-effective to operate, and generally require very little operator intervention.

 Further Information

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