Dealing with salinity in Wheatbelt Valleys - Department of Water

Table 6: Salinity and market for various algae (Modified from Borowitzka 1997).
Alga Product or Market
Salinity
(mg/L)
Status
Microalga
Dunaliella salina Beta-carotene >200,000 Commercial
Aphanothece halophytica Polysaccharides, pigments >200,000 R&D
Isochrysis, Tetraselmis, etc. Aquaculture feed ~30,000 Produced in hatcheries
Spirulina platensis Health food ~30,000 Commercial overseas
Porphyridium cruentum Polysaccharides, pigments ~30,000 R&D
George and Coleman
Macroalga
Gracilaria spp. Aquaculture feed ~30,000 Some overseas culture
Ulva spp. Abalone feed ~30,000 Some overseas culture
Caulerpa spp. Luxury food (Japan) ~30,000 Commercial overseas (Philippines)
Aquaculture may provide a useful cash flow and a
means to use the water while it evaporates and the
salts concentrate, but it does not remove salt from
the environment. By contrast mineral extraction is
the most effective method of removing salts. The
term 'salts' is used deliberately because it is often
overlooked that the saline water is made up of salts
other than sodium chloride (halite) or common salt.
Halite makes up over 50% of the salts in most brines
but not all of this is recoverable. Other minerals
include magnesium and potassium salts. Halite or
common salt is best extracted by allowing the brine
to evaporate to saturation (at near ten times
seawater concentration). The saturated brine is then
pumped or gravitates into specially prepared ponds.
After the crystals of salt have formed they are then
mechanically harvested and processed. Processing
may include washing, drying and sorting before being
bagged ready for sale. The remaining minerals such
as the magnesium salts can then be extracted using a
patented process such as SAL-PROC (Ahmed et al.
2000) or sold as a road base additive or dust
suppressant. Additional products might include
calcium chloride, magnesium hydroxide, gypsum and
derivatives. Several small halite-based ventures have
already been established in the Wheatbelt (e.g.
Kalannie, Corrigin) and more are proposed.
Other aspects of using brine being researched
include the generation of electricity from solar ponds
(Ahmed et al. 2000) and desalinisation. Neither of
these two concepts is new but methods of applying
them are constantly been reviewed. For example
several new desalinisation companies have recently
been established, and cost of desalinisation of
wheatbelt valley water has reduced to about
$1 kilolitre (e.g. Merredin scheme) and $0.4 has been
mooted as realistic given new processes.
– 13 –
In order for solar energy generation to be a
successful option the following condition need to be
satisfied (from PPK 2001):
• Local reliance on expensive fuels such as
electricity, fuel oil or liquefied petroleum gas
(LPG)
• Demand for heat in the 40–80 °C temperature
range
• A sufficient supply of saline water and salt,
preferably available locally
• Availability of relatively flat land on which to
construct the solar pond
• Relatively high annual average solar radiation and
evaporation
• Low wind (or investments in reducing
turbulence).
The generation of heat requires a large pond with a
salinity gradient from the top (ambient temperature;
20ºC) to bottom (60–90ºC). The salinity gradient is
maintained by adding salt (brines; 300,000 mg/L) to
the bottom layer and freshwater (normal salinity
groundwater; say 30,000 mg/L) to the surface as it
evaporates. It is difficult to maintain the salinity
gradient under high wind and subsequent wave
action. The brine must also remain clear and
chemicals are needed to kill algae. The more saline
bottom layer retains energy as heat during the day.
The hot brine can be used to directly replace other
heat sources or can be converted to electricity.
Low-grade heat can be used in a number of
applications to heat or chill items. One of the main