Bitterns from solar salt production explained

Key points:

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.

Heavier than seawater

One challenge the team needed to overcome was the issue of bitterns being heavier than seawater.

“If the bitterns were simply piped back into the ocean, they would likely pool at the bottom and accumulate there,” Mr Gödecke said.

“We needed a solution that will quickly and effectively enable the bitterns to dilute so that the seawater not too far from the discharge point would stay completely unaffected.”

To determine the best solution, several concepts were tested for diffuser design and Cormix modelling was undertaken to study mixing behaviour between bitterns and seawater under different conditions – such as orientation of jets, different tidal conditions, flow rates and even jetty configuration.

Thanks to the Research Partnership with the University of Western Australia the modelling could rely on real water quality data gathered over 18 months.

“When it comes to modelling to predict future outcomes, it is crucial that the data the model is based on is valid,” Mr Gödecke said.

The final proposed model, a diffuser, with hundreds of nozzles, discharging bitterns along the jetty aligned to the deepest water possible, was the solution to achieve the best bitterns dilution.

Choosing the best place to discharge bitterns

Choosing the best possible site to discharge the bitterns was also a high priority for the Ashburton Salt team. The ideal location would have ocean currents which could quickly mix and dilute the bitterns and the ocean bed would have minimal coral and sea grass.

The site selected to the north of the project into the Indian Ocean was selected as the best possible site, after a Geo Oceans survey and assessment of satellite imagery and sounder recordings was conducted and found the ocean floor was predominantly (93%) soft sediment.

The remaining 7% of seabed contained some macroalgae (5%) along reef along the coast and 2% macroalgae and sparse coral.

The site was also selected after detailed near and far-field modelling was conducted to define mixing rates at different parts of the ocean.

The proposed bitterns discharge point:

Is at the end of the proposed jetty to take advantage of deeper water and greater tidal movement to facilitate mixing.
Will be positioned at the berthing pocket which will be dredged for shipping purposes and that is unsuitable for seagrass habitat.

In addition, the ports of the discharge diffuser will be pointed upwards to force the bitterns to the surface, forcing the enhanced mixing and diffusion with faster moving surface waters.

Scale of impact

“Ultimately, our proposal will only be acceptable if the impact zone of bitterns discharge is very, very small,” Mr Gödecke said.

Researchers have spent significant amount of time modelling the area of impact – where the concentration of bitterns can be detected in ocean water.

Under EPA guidelines, seawater outside a defined area cannot contain any evidence of elevated bitterns elements at all. Bitterns dilution modelling undertaken by Water Technology has predicted the bitterns discharge influenced areas.

These areas are the areas where there will be some impact on the seabed and ocean concentrations. On average throughout the year, the affected area stretches 50m from the discharge point.

This distance fluctuates during the year, with the natural change of ocean currents. The way the diffuser is designed ensures that the area of low water quality is minimised at all times.

Importantly, only soft sediment outside the berthing pocket will be potentially impacted by bitterns and the mixing zone is not predicted to impinge on the macroalgal and sparse coral communities fringing the shoreline at the base of the jetty or offshore from Locker Point.