To enable coal seam gas (CSG) to be extracted, groundwater must be removed from the seam.
Inevitably, this groundwater contains dissolved solids and is of poor quality. Before any of this groundwater can be re-used or released into the environment, it requires purification or desalination.
However, in Australia evaporation ponds are being phased out as there are concerns about the potential long-term environmental consequences associated with the salt and minerals released from untreated CSG water. This ranges from brine permeation into ground water with unlined ponds to ponds overflowing into streams and rivers with the ensuing damage to flora and fauna. So a more technological solution is required.
Desalination technologies are generally classified as either thermal or membrane technologies. However, specific processes can incorporate mixtures of both technologies.
Thermal technologies can be simply defined as those that use heat to vaporise water at the inherent purification stage.
Membrane technologies use some form of interface between feed and product water that causes purification to occur. Typically, this is a reverse osmosis (RO) membrane.
Conventional thermal technologies
Conventional thermal technologies are based on the fact that when water boils into steam it leaves behind most of the dissolved solids and becomes very pure with usually less than 20 parts per million of total dissolved solids (TDS). Sea water would have approximately 35,000 TDS and domestic tap water would have less than 350 TDS.
Treatment options for CSG produced water generally involve sterilisation, desalination or filtration.
Multi-stage flash
This process involves pre-heating the saline feed water by using it as a cooling fluid for condensing the vapour generated in flash tanks, where water converts to steam. Here, final heating of the feed water utilises steam and the vapour is flashed from the heated saline water. A vacuum is then applied to the last vessel in the series. Water boils at 100 degrees Celsius at atmospheric pressure, if the pressure on the water is reduced the boiling temperature reduces, which in turn reduces the amount of energy required for the boiling process.
Multiple effect evaporation
Saline feed water is pre-heated by condensing the water vapour from the last effect. This recycles most of the latent energy contained in the steam, and greatly reduces the energy required for desalination. Typically this process is repeated a number of times. Vapour and saline water both flow from the effect to the next, with the vapour being used as a heat source. Condensate from each effect after the first is collected as clean water.
Vapour compression
These plants are based around either evaporator or flash units with a compressor unit that compresses the low energy content vapour into a high energy content steam that can be used for heating purposes in the plant.
Crystallisation
This is usually the final stage in desalination as it results in the production of mostly solid salts and distilled water.
The solids constitute a more readily disposable waste than brine and are more compact for transportation. A typical system consists of a separation vessel for crystals, water and vapour, and a recirculation loop through a heat exchanger for the crystal slurry.
Alternatives to thermal technologies for water desalination
Membrane distillation
This method uses the properties of a teflon membrane to pass vapour but not liquid. It is a hybrid thermal and membrane process that uses heat to improve the efficiency of the membrane. The technology is still relatively immature and is typically used on a small scale only.
Humidification-dehumidification distillation
Experimental studies have proposed types of this system, which relies on the evaporation of water into air by the use of sprays or wetted surfaces.
The advantage of this system is that it uses low energy inputs and pressure. However concentrations of magnesium and calcium in mine water significantly reduce evaporation rates and increases the amount of energy required for any given output.
Current mine water related plants
The major source of practical experience in coal mine waters has been the trialling of desalination technologies including membrane, thermal and hybrid techniques for mine waters in Poland.
Several mines there have mine water treatment plants that range from accelerated evaporation to full crystallisation with by-product recovery.
Seeding of the water with calcium sulphate crystals is used to encourage scale to form on the crystals rather than metal surfaces. When boiling water, once the temperature goes over approximately 65 degrees Celsius the calcines become solid and drop out of the water. This is both good and bad: good in the fact that the TDS of the water is reduced; bad from the tendency of the calcines to stick to heat exchange surfaces, which reduces heat exchange efficiency and increases the amount of energy required by the process.
A hybrid membrane-thermal desalination plant at the Debiensko coal mines initially used a chemical pre-treatment, but now uses nanofiltration prior to RO production of drinking water in conjunction with seeded brine concentration and crystallisation using thermal processes to produce distilled water and salt. This produces 4.5 mega litres per day (ML/d) of distilled water and 9.8 ML/d of drinking water. In addition the plant produces approximately 400 tonnes per day of salt and assorted by-products.
A range of saleable products are extracted from the salt, however, for the major salt product it has proven difficult to maintain magnesium levels below the maximum permitted for sale.
The Australian experience
The level of treatment of CSG water that is required is determined by the quality of the CSG water and the function for which it is to be re-used.
Experience in plant design and construction within Australia demonstrates that there are pitfalls in design that need to be considered. The main issue involves the brine pre-treatment necessary before the desalination process. This is due to the variability of the dissolved solids in the coal seam water and also the formation of algae on brine supply ponds. Changes in water quality over the life of the plant can have a serious impact on performance and plant maintenance costs.
In addition, whether treatment can be performed onsite or if the water requires transportation needs to be taken into account when considering the applicability of CSG water treatment. However, if technologies enable cost effective treatment of CSG water, it could become a valuable resource for environment, agricultural, industrial and possibly even domestic benefit rather than being wasted as a by-product.