Modalidad de investigación

Desalination of CSG waters has become increasingly important because of the potential for beneficial use of the desalted water as a recycled water resource and for aquifer recharge (DERM Queensland). Desalination technologies, both thermal and non- thermal, require pre-treatment to prevent fouling and to enhance the proportion of water recovered. For source water of poor quality such as CSG water, pre-treatment processes can form a significant proportion of the water treatment plant. The selection of a suitable desalination technique depending on primarily on a combination of influent salinity level and the content of scaling compounds, usually silica, and the product water quality required (GE, 2010).

Medium and high salinity CSG water in Australia undergoes RO desalination to reduce TDS of the water and remove some undesirable contaminants. Additionally the reject

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stream (or brine) generated by the RO system will require treatment via a set of brine concentrators and crystallisers (GE, 2010). The overall treatment train is site specific, depending on quality and quantity of CSG water produced, existing infrastructure and community needs, but generally a relatively big scale plant consists of (figure 2.7):

- Pre-treatment by coagulation, this is required to mitigate the upstream variability associated with suspended solids, DOC, metals and silica concentrations. It must be noted that coagulation is frequently used again prior to brine treatment via brine concentrators.

It is well understood principle that the operation of downstream membrane process depend on the efficiency and type of pre-treatment technologies. Figure 2.7 illustrates typical water treatment infrastructure used for CSG water treatment and residual management.

There are many factors that can influence the operating recovery of a membrane plant and cleaning of the membranes (AECOM 2012). Of particular note in the raw water design basis results are total calcium, barium, magnesium, silica, aluminium and dissolved organics which can be in concentrations that contribute significantly to solids loading and should be reduced before microfiltration. In absence of clarification process, consideration of settling time in holding ponds and the design of a floating intake structure may allow some reduction of solids and hardness concentrations, however, the dissolved organics concentrations need to be reduced to < 5 mg/L, preferably < 3mg/L after microfiltration. This can only practically occur through a combination of the following typical process steps:

- Direct coagulation & microfiltration.

- Enhanced coagulation & powder activated carbon & clarification. - Granular activated carbon.

- Dissolved air flotation or similar.

The use of the upstream clarification processes provides /more stable operation for the MF/UF processes that is designed to protect the upstream IX and RO operations. The use of anti-scalant is not typical for MF/UF, however, with the calcium carbonate

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precipitation potential and alkalinity buffering capacity of these feed waters; the use of any acid for reducing pH and therefore reducing carbonate scale potential is impractical.

A silica specific anti-scalant is a good option for the MF/UF feed allowing the MF/UF membrane technology to focus on insoluble organics and suspended solids removal with minimum carbonate scale that can be rectified during the daily and monthly cleans. A drawback of anti-scalant used to mitigate silica scaling can be increased of residual aluminium.

The use of IX has shown merit on CSG associated raw water, particularly with the legislative drivers around the phasing out the use of evaporation ponds for the purposes of RO reject/brine storage management. It should be noted, however, that the selection of resin and appropriate design parameters are critical to the success of the IX operation and more importantly the RO membrane plant. The affinity for individual ions varies for different resin types, a factor which relies on the type of acid and from which monomer the resin has been synthesised, as well as the feed pH and prevailing cations in the feedwater. Some silica removal by IX will be also an important consideration in the CSG plant. With a reliable and efficient ion exchange process upstream, it is possible to maintain between 92 to 94% RO recovery on a well-designed three stage RO system (CSG water treatment, AECOM 2013). However, monitoring of silica scale formation is key for successful high recovery operation.

Elevated silica concentrations (150 – 250 mg/L), in particular for salt recovery from RO reject streams, are undesirable and required pre-treatment prior to brine processing via brine concentrators (BC). Different technology provider companies like GEA, GE, Veolia – HDP use slightly different treatment processes for silica removal including coagulation and clarification of BC, chemical treatment, dilution of feed stream to control silica concentrations. The brine treatment process generally consists of extracting and purifying the salt and soda ash from the brine relying primary on cooling, addition of carbon dioxide, and evaporation to selectively crystallize sodium bicarbonate and sodium chloride (salt).

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47 The presence of undesirable minerals in the brine such as fluoride, potassium, boron, lead, dissolved organic material and silica requires additional processing steps to keep the sodium bicarbonate and salt within specification. The primary processing operations are:

- Brine treatment for the removal of suspended solids, organics, some silica, and heavy metals, including, powdered activated carbon (PAC), redissolving of mixed salts, redissolving of secondary sodium bicarbonate, coagulation, clarification and filtration.

- Primary carbonation for the extraction of sodium bicarbonate crystals including saturation with carbon dioxide and sodium bicarbonate washing & filtration. - Evaporation processing for extraction of clean water, sodium chloride crystals

and mixed salts including de-carbonation, BC, salt crystallization and mixed salt.

- Fluoride treatment for the precipitation of fluoride and purging of boron, potassium, nitrates and other soluble impurities including a secondary carbonation step.

Of course the treatment steps described above will depend on the purity of product (final commercial product) required from the concentrated brine. Elevated silica concentrations remain a problem for all thermal desalination technologies including MED, mechanical vapour compression (MVC), BC (vertical and horizontal). Standard equipment specification requires < 50mg/L silica concentration in the feed brine. The presence of silica in the feed brine can contaminate the product salts and can cause fouling in the evaporators which would decrease the overall system efficiency and may also accumulate to levels affecting product quality. In order to reduce the amount of silica that advances into the main section of the plant, MgO usually will be added to precipitate the silica (Veolia, 2010).