Key points The Ashburton Salt project proposes a series of salt-water ponds to be located on existing salt flats. These salt flats form part of a wider wetland ecosystem, supportingRead more
Bitterns from solar salt production explained
For the Ashburton Salt team, traditional ways of dealing with bitterns are not good enough for a modern solar salt development.
So what are bitterns and why should we care what happens to this salty by-product?
- Bitterns are what is left when most of the water and salt (NaCl) is removed from seawater through evaporation.
- K+S is proposing pre-diluting bitterns with seawater and returning them to the ocean using a multi-port-diffuser located on the proposed jetty.
- The area of discharge, in ocean north of the site, was chosen as the least likely to have environmental impact.
- The diffuser spans 400 m of the northern end of the jetty and will be placed at a depth below low tide levels. Diffuser design promotes effective dilution of the bitterns as it mixes with seawater.
- Coral and macroalgal communities along the nearby coast are not predicted to be impacted by the bitterns discharge.
What are bitterns?
Through the solar salt process, seawater is evaporated by sun and wind to the point where salt (sodium chloride) crystallises and can be removed. Some 70% of the sodium chloride is removed through the solar salt process.
The remaining product is called bitterns. Bitterns contain the other minerals of the original ocean water.
Bitterns are also produced in potash production and sea-water desalination (for drinking or industrial purposes).
No additives are used in salt production so the only elements that exist in the bitterns are the naturally occurring elements in seawater.
Even so, at highly concentrated levels, bitterns can have a negative impact on marine flora and fauna.
Returning bitterns to the ocean.
Traditionally, solar salt sites pump undiluted bitterns into tidal creeks and channels where passive mixing takes place before final discharge into the ocean.
But according to K+S Salt Managing Director Gerrit Gödecke, this un-controlled method of managing bitterns would not be acceptable today.
“The potential environmental impacts of this method would be too high for a new solar salt project,” Mr Gödecke said.
“We didn’t consider long-storing bitterns on site either, as has been suggested for other projects in the past,” Mr Gödecke added.
“This would have a high risk of soil and ocean contamination if storage ponds were breached during cyclone events.”
Instead, Mr Gödecke said, K+S Salt Australia planned to do things differently by carefully returning bitterns to the ocean in a controlled process using a diffuser system.
This would minimise the impact of returning the diluted bitterns to the ocean and would be done in a process with three key steps:.
Unlike other salt operations, the bitterns at Ashburton Salt will be diluted with seawater (at a rate of approximately one to one) to reduce the solution’s concentration. The diluted solution will flow from the crystalliser ponds into a bitterns dilution pond. The pond will be located north of the crystalliser ponds
Instead of using a naturally occurring creek into shallow and possibly still water, the solution will be transported into deeper water by using a pipeline along the proposed export jetty. A bitterns dilution pond and a pump station will assist in transportation for bitterns from the coast to deeper waters.
A diffuser with multiple discharge ports will be used to discharge the bitterns. The diffuser has been designed to minimise the area influenced by the bitterns.