The reverse osmosis option

Osmoflo's RO plant at Roma.

Osmoflo's RO plant at Roma.

As coal seam gas developments come under closer scrutiny than ever before, water management becomes a major concern for developers. However, the produced water stream can become a revenue stream through reverse osmosis treatment.

Corporate social responsibility, stakeholder issues and now Queensland legislation mean that water treatment options are a key consideration for coal seam gas (CSG) developers. Desalination of the brackish water extracted can be achieved through a number of methods. The most common processes fall into three categories; membrane, chemical or thermal.

Reverse osmosis (RO), a membrane technology, provides the finest level of filtration available, says RO plant developer and operator Osmoflo. It removes 95–99 per cent of dissolved salts, inorganic molecules and organic molecules with a molecular weight of greater than 100. It also removes 98 per cent of residual biological and colloidal matter from the feed water.

Salinity of CSG water varies greatly, from drinking quality – less than 500 milligrams per litre (mg/L) – to more than 10,000 mg/L. Typically, CSG water contains between 1,500–10,000 mg/L of dissolved solids.

How does it work?

Osmosis is the tendency of water to flow from a purer solution through a semi-permeable membrane into a more concentrated solution, equalising the solute concentration on either side of the membrane. Solute molecules do not naturally pass through the membrane. Pressure can be used to reverse the natural course of osmosis. As such, RO involves applying pressure to brackish water to make it pass through a membrane, removing solutes.

Depending on the quality of the water, which varies considerably across sites, pre-filtration by other technologies also may be used in CSG water desalination plants. This ensures that larger particles that could damage the RO membranes are filtered out.

Considerations

Key issues in the installation of RO plants relate to water quality and variability. Water contaminants, which include silica, iron and hydrocarbons can all affect the performance of the system, as can extracted water temperatures. The changing nature of water volumes over the life of the project – increasing then decreasing – also needs to be considered.

The remote location of CSG fields can also pose a challenge. The use of modularised plants can help minimise onsite construction and commissioning activities, says Osmoflo.

Compared to other desalination technologies, Osmoflo believes that the disadvantages of RO are negligible, a result of new innovations in energy recovery and brine minimisation.

The advantages, aside from the fine level of filtration, include its ability to treat very large volumes of water very effectively, its small footprint and its cost-effectiveness.

Existing plants

Testament to the flexibility of RO in the treatment of CSG water is the diversity of its existing applications in Australia.

Veolia installed water treatment systems at the Condamine and Braemar 2 CSG power stations in Queensland. The plants combine a number of purification processes with RO, including clarification and microfiltration at Condamine and ultrafiltration and continuous deionisation at Braemar 2. The water has a variety of uses, including cooling tower basins, potable water, process water and demineralised water for boiler feed.

Osmoflo built and operates the 5.5 mega litres per day (ML/d) RO plant at QGC’s Roma CSG fields in Queensland. The plant uses oxidisation of iron, microfiltration and RO. The treated water is used for irrigation.

In August 2009, Impulse Hydro was awarded a contract to provide RO to Eastern Star Gas’ Narrabri CSG project in New South Wales. Previously, produced water at Narrabri had been disposed of using evaporation ponds, which was common in early CSG projects. As such, the RO system was installed in a way that would accommodate water from existing evaporation ponds as well as water straight from the well.

Water reuse

When constructing RO plants, it is the desired end use for the water that determines the level of filtration. Water treatment can be tailored to client needs, be they municipal or industrial.

Treated CSG water can be used for a range of purposes, depending on the purity of water the client specifies. Given the often remote location of CSG projects, one of the key issues is the cost and infrastructure needed to transport water for use other than at the immediate location, says Osmoflo. As such, water produced at CSG sites will tend to be used for irrigation, for general use onsite, or fed into local streams.

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