Creating useful products from desalination waste

ENGINEERS at MIT, US have proposed a method that could use desalination waste to produce useful chemicals, including chemicals that can make desalination more efficient. This could potentially provide an economically and ecologically beneficial method for the disposal of desalination waste.

Desalination is a rapidly growing industry. It is used to produce water for drinking and for agricultural purposes in arid coastal regions. The waste product of desalination is highly concentrated brine. Currently, 100bn L/d of water is produced by desalination, leaving a similar amount of brine. The brine is usually disposed of by dumping it in the sea, which requires expensive pumping systems and has to be managed carefully to prevent damage to marine ecosystems.

The group at MIT proposes a direct electrosynthesis (DE) process to produce sodium hydroxide (NaOH) and hydrochloric acid (HCl) from desalination brine as an alternative to current technologies. In the proposed DE process, the water is split to produce hydrogen (H⁺) and hydroxide (OH^-) ions, which are combined with the brine stream to produce NaOH and HCl.

NaOH can be used to pretreat water going into the desalination plant. It changes the pH of the water, which helps to prevent fouling of reverse osmosis (RO) membranes. Fouling is a major cause of interruptions and failures in typical RO desalination plants. NaOH pretreatment also helps to prevent scaling, in which salts such as calcium carbonate become supersaturated and precipitate onto the membrane. According to Amit Kumar, Research Scientist at MIT, industry uses and spends a lot of money on NaOH, and the ability to make it on site would offer a “big advantage”.

Using established processing methods, HCl could be produced on site, from desalination waste. HCl is also used by desalination plants, as well as in many other industrial processes. In desalination plants HCl can be used for cleaning and it is widely used in chemical production and as a hydrogen source.

The proposed method for converting brine into useful products uses well-known and standard chemical processes. The processes are not new but, in the study, researchers analyse the potential of the overall method. And, they propose a specific combination of products and chemical processes that could be used in commercial operations to improve the economic viability of desalination and reduce the environmental impact.

John Lienhard, Professor of Mechanical Engineering at MIT, said: “Environmentally safe discharge of brine is manageable with current technology, but it’s much better to recover resources from the brine and reduce the amount of brine released.”

Jurg Keller, Professor of Water Management at the University of Queensland, Australia, and not involved in the study, told an MIT reporter: “This could have some major energy and cost benefits, since the up-concentration and transport of these chemicals often adds more cost and even higher energy demand than the actual production of these at the concentrations that are typically used.”

According to John Lienhard, the group’s industrial collaborator is currently exploring scaling up the method, which has several challenges because “in a lab-scale system, the process parameters and system dynamics are easy to control, and the ions are well distributed within the electrochemical system.”

“Additionally, operational robustness, cost-effectiveness, and economic feasibility of the process is being evaluated,” he added.

The group is also investigating the possibility of extracting other, lower-concentration materials from the brine stream, including various metals and other chemicals that could improve the economics of brine processing.

The work was supported by Cadagua, a subsidiary of Ferrovial.

Nature Catalysis: http://doi.org/c253