Processing Salt Water
Energy Requirements
There are several options for removing salt from natural sources like ocean water, as well as from some industrial waste streams like oil and gas mining. These options have been discussed in the third part of this series.
Major common concerns about desalination don’t typically include availability of the raw resource. Instead, sustainability questions center around energy usage.
Thermal processes require substantial energy to heat, cool, or pressurize the brine and force it to crystallize at various points, while membrane processes require strong pumps to force brines through many layers of fine membrane to separate out salts, minerals, and other contaminants.
Is That Sustainable?
Currently, most desalination plants use energy ultimately sourced from fossil fuels. Transportation of materials (ocean water, membranes and other chemicals, waste disposal) also typically depend on fossil fuels. In response to those concerns, some companies are developing new concepts for integrated desalination plants that combine new technology in solar energy collection with more efficient equipment and improved processes to both reduce the overall consumption of energy and to make the operation more sustainable.
Handling Waste
Waste brines are essentially never 100% pure. Pure water and pure sodium chloride typically contain other types of salts, such as chlorides and sulfates of magnesium and potassium. Trace minerals such as silicates and nitrates from natural ocean water as well as various heavy metals and radioactive materials from mining wastewater are common. Chemical residue from mineral extraction includes coagulants and flocculants, while descalers and pH adjusters are commonly used in the desalination process itself. Organic materials are commonly found in ocean water or collected from agricultural and industrial sources.
All these materials are separated during the filtration, desalination and purification processes and must be dealt with appropriately. Some can be mechanically removed - organics are one example. Others can be removed using flocculants and similar chemicals, but then of course, the additional chemicals themselves must also be removed, through additional chemical or physical treatments. After the desalination process is complete, whatever is left over is considered waste, typically a highly concentrated solution of brine, minerals and chemicals. This solution is caustic and often highly toxic, and further treatment or disposal can be both expensive and environmentally risky. This is a critical concern in evaluating the sustainability of desalination technology.
Ocean Dumping
Several methods for handling brine waste are discussed in parts 9 and 10 of this series, but an additional practice is worth addressing here. Ocean dumping is by far the most common practice for disposing of reject brine, particularly for those plants located along coastal areas. The ocean is easily accessible, the practice is not monitored or regulated in some countries, and it’s very cheap. However, it presents serious problems from a sustainability perspective.
Concentrated brine doesn’t magically disperse and mix evenly with ocean water. In fact, it is denser than ocean water and often will simply sink to the bottom and remain there, essentially an invisible poison bubble threatening nearby marine life.
Brine released into the ocean is often warmer than the surrounding water, which increases the average temperature within that zone. In conjunction with increased salinity, this creates a condition known as hypoxia, where the water’s ability to hold dissolved oxygen is decreased. This lack of oxygen can severely affect marine life within that zone, creating ripple effects throughout the food chain.
While practices such as dispersed return or pressurized flows can help with some of these problems, ocean dumping is still an environmentally damaging disposal practice. Some alternatives for managing desalination waste mentioned in sections 8 and 9 focus on minimization, conversion, and zero liquid waste. These are realistically the only options for environmental sustainability.
Conclusion
There are several more-or-less economically viable options for disposal of reject brine from desalination plants. A few of them are approaching environmental sustainability, and options should improve as technology moves forward. In regions with high evaporation rates and relatively low-cost land, evaporative ponds are an excellent step in reduction and preparation for disposal of the elements in waste brine. The best, most environmentally and economically sustainable results are likely to come in full-process plants designed to take in salt water directly from the source and process it fully, including the use of evaporative ponds, to extract the maximum value in fresh water, minerals, chemicals and other valuable resources before it is completely dry and designated ready for disposal in a safe, monitored location. The advantages of handling the full cycle from withdrawal to disposal in a single location are obvious.
BTL Liners has decades of geotextile experience and is a leader in the highest quality reinforced polyethylene liners. If you’re considering how to manage brine waste in your industry, whether it’s from desalination or elsewhere, give us a call and we’ll help you walk through questions about durability, chemical resistance, ease of installation, and more.
