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Don’t be so caustic! Use magnesium hydroxide

25-11-2020
by 
in 
Magnesium hydroxide is safe to use

Wastewater from food processing usually results in water having a high pH and becoming acidic.

Australia’s water authorities are becoming less inclined to provide secondary treatment to these companies and environmental laws will eventually mandate they need to treat their own wastewater to a high level.

As you would appreciate, balancing the pH is vital in preventing harm to the environment or to infrastructure such as pipes and storage facilities, and to minimise odour.

The great majority of Australia's treatment of acidic wastewater is done using caustic soda, otherwise known as sodium hydroxide. But this is hazardous and requires high dose rates to be effective.

Caustic soda is extremely corrosive and can cause severe burns on human skin or can even cause death if ingested. It is also bad for the environment by increasing the salinity of any body of water it is discharged into.

Fortunately, there is an alternative that is very safe, costs the same or less, and is readily available. Some companies who treat wastewater have switched to magnesium hydroxide to raise pH levels, which has similar effects to caustic soda in raising pH levels but is much more friendly to the environment. In fact, it is a nutrient and can actually be beneficial.

Magnesium hydroxide is going to be “the next big thing” in food manufacturing and processing. You may know it by another name: milk of magnesia. So it’s pretty safe.

In recent years it has been developed as a safer and more environmentally-beneficial alternative to caustic soda and lime for raising the pH levels of wastewater.

So, with all its benefits, why has it not been more widely adopted? We spoke to Ralph Lloyd-Smith, Technical Support Engineer with leading Australian wastewater treatment company, Calix.

“Many people think it is more expensive,” Lloyd-Smith and explains.

“When you look at it by volume or unit price, that might appear to be the case, but it is actually cheaper than caustic soda in most applications. The same volume of magnesium hydroxide provides 60 percent more alkalinity than concentrated caustic soda.”

There have been other things holding back usage of magnesium hydroxide, says Lloyd-Smith. “It’s a little more difficult to handle. Magnesium hydroxide is a slurry, with small particles of solid suspended in water, while caustic soda is a liquid.

“But that is more than counterbalanced by the fact that magnesium hydroxide presents no safety problems. Anybody who spills caustic soda has to report it immediately, which is not the case with a benign material like magnesium hydroxide. And then of course there are the many environmental benefits, which are becoming increasingly important.”

Calix is an Australian manufacturer of patented industrial sustainable solutions including magnesium hydroxide, which it markets as ACTI-Mag, for neutralising acidic wastewater in Australia.

“Many people are still using caustic soda because they don’t know there’s an alternative, or because they may be tied into a contract with a supplier,” said Lloyd-Smith.

“Some food processing plants use caustic soda to clean their equipment regularly and figure they may as well also use it to raise the pH of their wastewater. We encourage the use of magnesium hydroxide because it is so much safer and because of its many environmental benefits.”

Calix’s efforts are having success with an increasing number of industrial wastewater treatment plants having switched to magnesium hydroxide. It has also been adopted in areas where environmental concerns are paramount, such as vineyards.

“It’s an education process. When we speak to environmental managers and the hands-on people at the wastewater facilities, it’s not a difficult story to tell. The penny drops. A lot of them had never realised there was an environmentally friendly and safer alternative to caustic soda.

“We also work with a lot of customers where wastewater is not their core competency. Food manufacturers, for example, want to concentrate on what they do best. We can add value and expertise in areas like diagnostics and problem-solving,” he said.

The use of magnesium hydroxide as an alkali for wastewater treatment was introduced into Australia by major chemical company Orica around 20 years ago, but it is only since Calix took on the technology six years ago that its usage in Australia has become more popular.

In recent years there has been increased emphasis on environmental concerns also on OHS issues, which explains why more wastewater treatment plant operators are switching to magnesium hydroxide.

 

Counting the advantages

The advantages of magnesium hydroxide over caustic soda and other traditional alkalis becomes apparent when the chemistry of the various substances is examined. A simple neutralisation experiment using vinegar and various alkalis demonstrates that all alkalis are not created equal.

A simple test using 200 mL of cleaning vinegar to neutralise the pH level to around 6.1 to 6.2 shows relative required doses of 10 mL per cent caustic, 11 grams of soda ash, or 19 mL of 30 per cent lime.

By contrast, magnesium hydroxide requires relatively low doses to have the same effect. At a 60 per cent concentration, it requires only 6 mL to achieve a pH of 6.2. This makes magnesium hydroxide significantly more effective by weight, when compared with caustic soda, which can mean significant cost savings.

Caustic soda also has other problems. When used as a shock dose, it can kill odour-causing micro-organisms in a sewer line but this is environmentally hazardous and offers no long-term benefit.

As well as adding more sodium to the environment, caustic soda is dangerous to handle. Of the different alkaline additives, caustic soda is the most dangerous and has a pH of 14. Even diluted, caustic soda and lime can cause significant injuries to unprotected skin, especially with prolonged contact. Soda ash is also a strong irritant, making it only slightly less dangerous than caustic soda or lime.

As well as being perfectly safe for people to work with, magnesium hydroxide is a slow-release alkali which offers a stable buffering at a pH level of 8.5 to 9 to hold sulphide in solution. It automatically stabilises at this level, which makes it almost impossible to lose control of the treatment process, as can happen with caustic soda.

Magnesium hydroxide is also a soil nutrient, delivering long-term benefits compared with caustic soda. In fact, magnesium sits at the centre of the chlorophyll molecule.

Lloyd-Smith says there is another problem with caustic soda. “In colder climates, or in winter in much of Australia, it crystalises,” he says. “The only way to prevent that is to dilute it more, which makes it less effective. That doesn’t happen with magnesium hydroxide. People might remember the old milk of magnesia settling, the particles sinking to the bottom, but we’ve done a bit of tricky chemistry to ensure that doesn’t happen with our product.”

“Magnesium hydroxide also offers a range of other benefits such as sludge compaction and dewatering, which means a reduction in other chemical additions and thus further cost savings. It creates the perfect pH balance for good bacteria to thrive.

“Caustic soda, if overdosed, may either kill beneficial microorganisms or reduce their activity. Either way this will compromise the effectiveness of treatment. Because it is almost impossible to cause pH overshoot with magnesium hydroxide. It’s good for the environment.”

In short, magnesium hydroxide is much safer to handle and rather than being environmentally detrimental, it is a micronutrient. It is also less expensive in most applications, making it the ideal solution to treat wastewater and industrial discharge in an environmentally responsible way.

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