Sulphate is widely distributed in nature and may be present in natural waters at a range of concentrations. Sulphate is only considered toxic at very high levels, but may affect water taste and colour at concentrations higher than 600mg/L. Levels higher than 1,000mg/L have been known to provide a laxative effect, although more sensitive populations may be affected at levels of 500mg/L. The reduction of sulphates can also result in hydrogen sulphide production, posing a toxic threat to the environment and killing fish, plants and plankton.
Along with these issues, water rich in sulphates also has a high scaling potential, which can damage equipment in water treatment. mining or industrial processes. For these reasons, it is important to lower the water’s sulphate concentration before its use or disposal.
An acceptable level of sulphate concentration depends on what the water is being used for or where it is being disposed of, with the level varying widely between countries. For use as drinking water or discharge into surface water such as rivers, most countries have moved towards 250mg/L, a level which is generally low enough to prevent taste and colour changes, and preventing diarrheal symptoms.
Many suphate treatment technologies are widely used to treat sulphates, with some of the most common being chemical precipitation and membrane filtration.
Treating water with hydrated lime is probably the most common chemical precipitation method, and can be used on water with sulphate concentrations up to 8,000mg/L. The process is cheap, produces gypsum waste, and doesn’t require pre-treatment to perform. The gypsum has a high scaling potential as it precipitates out, easily forming a hard layer of scale on pipes and equipment. Up to 1,800mg/L of sulphates can remain in the solution also, requiring further treatment to reach acceptable concentration levels.
Reverse osmosis is a technology also used for sulphate treatment, but it typically only used for water with low calcium and sulphate concentrations (100-700mg/L) to prevent scaling, with pre-treatment required outside of this range. The high volume of brine produced can also result in significant costs, as a typical first pass achieving 60-70% recovery, with the remainder going to the waste stream.
Clean TeQ Water’s DESALX® technology is a 2-stage CIF® (continuous ionic filtration) process, which provides concurrent physical filtration and ion exchange. This allows for maximum water recovery with limited production of gypsum as the waste product, in a robust system where pre-treatment usually isn’t required. It is highly resistant to scaling and is generally cheaper then using reverse osmosis. The DESALX technology can also be combined with a high density slude (HDS) system, allowing the gypsum byproduct to be recycled, achieving zero liquid discharge (ZLD).
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