Don’t be so caustic!

Magnesium hydroxide is much better than caustic soda or lime for treating wastewater. Why aren’t its advantages better known?

Audrey Barucchi \ October 19, 2020
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There are over one thousand wastewater treatment plants in Australia. Most are owned by water utilities and local government authorities, but they also operated by mining companies, factories, industrial concerns and even jails and defence establishments. Many smaller operators like restaurants also do their best to purify the water they have used before discharging it in to the sewer system.

Treating wastewater is a major industry. Australia is a dry continent and returning clean water to the environment is a priority. One of the main reasons to treat wastewater is to reduce its acidity, or pH value, which is often caused by nitrification, often from effluent.

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 best way to reduce acidity is to dose the water with an alkaline substance. Alkalis are the opposite of acids and the right dose will result in a neutral pH level.

But there is a problem. Over 90 per cent of Australia’s treatment of wastewater is done using caustic soda, otherwise known as sodium hydroxide. Other widely used alkalis are soda ash (sodium carbonate) or hydrated lime (calcium hydroxide). But all these materials are hazardous and require high dose rates to be effective. It is called ‘caustic’ soda for a good reason.

Caustic soda is often used instead of powdered alkalis like lime or soda ash because it is easier to handle and requires less maintenance. The actual dosing of water with caustic soda is quite simple, but the substance must be handled with extreme care, with containment and OH&S measures, because it is very dangerous. Caustic soda is extremely corrosive and can cause severe burns on human skin, or even death if ingested.

Caustic soda is also bad for the environment. While it reduces the acidity of the water, it also raises its sodium level, which raises the salinity of any body of water it is discharged into. Soil salinity is a major problem in Australia.

A solution to the environmental problems of caustic soda

Fortunately, there is an alternative. An increasing number of water treatment plants have switched to magnesium hydroxide to reduce acidity in wastewater.

Magnesium hydroxide has similar effects to caustic soda in reducing pH levels, but is much more friendly to the environment. In fact, it is a nutrient, and can actually be beneficial.

Magnesium hydroxide has been around for a long time. It used to be called milk of magnesia, and was widely used as an antacid and a laxative. It is still often used as an active ingredient in both. In recent years it has been developed as a safer and more environmentally beneficial alternative to caustic soda and lime for reducing 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,” he 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 almost twice as many moles of hydroxide as does 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. Not so magnesium hydroxide. And then of course there are the many environmental benefits, which are becoming increasingly important.”

Calix, an Australian manufacturer of patented industrial sustainable solutions, has become a champion of magnesium hydroxide, which it manufactures and markets under its ACTI-Mag brand, for lowering the acidity of wastewater in Australia.

“It’s a no-brainer,” Lloyd-Smith says, “but 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.

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

Calix’s efforts are having some success. An increasing number of municipal water treatment plants have switched to magnesium hydroxide and the company has been very successful in having it adopted in areas where environmental concerns are paramount, such as vineyards.

“It’s a case of getting the message out there,” Lloyd-Smith says. “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.”

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.

Calix was founded in Australia in 2005 to commercialise the calcination process, which involves grinding minerals to tiny particles and heating them at temperatures up to a thousand degrees. The process is ideally suited to the manufacture of magnesium hydroxide, which now constitutes the bulk of Calix’s business.

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 experiment using 200 ml of 5 per cent vinegar, which is very acidic, with addition of alternative alkalis to neutralise the pH level to around 6.1 to 6.2 shows relative required doses of 10 ml 50 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 six millilitres to achieve a pH of 6.2. This makes magnesium hydroxide significantly more effective by weight compared with caustic soda, which can mean significant savings.

Caustic soda also has other problems. If it is used as a shock dose, it can immediately kill odour-causing micro-organisms in a sewer line, but it 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 pH 8.5 – 9 to hold sulphide in solution. It 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 floating to the bottom, but we’ve done a bit of tricky chemistry to ensure that doesn’t happen with our product.”

Because magnesium hydroxide is not soluble, but is delivered as a slurry, means that it actually assists with solids settling in wastewater treatment.

“It simultaneously provides coagulation with an alkalinity boost, which delivers increased effluent pH and alkalinity for secondary treatment,” he explains.

“Magnesium hydroxide increases sludge pH and alkalinity for improved digestion and odour reduction, and it reduces polymer dose requirements for dewatering, all of which are highly beneficial for the environment.

“And magnesium hydroxide creates the perfect pH balance for good bacteria to thrive. Caustic soda will either kill microorganisms or reduce them into periods of poor activity, and calcium from lime creates scaling. But it is almost impossible to cause pH overshoot with magnesium hydroxide. It’s good for the environment.”

Magnesium hydroxide is much safer to handle and is a nutrient for the environment rather than a detriment. It is less expensive in most applications, making it the ideal solution to treat wastewater and industrial discharge in an environmentally responsible way.

Such has been the success of ACTI-Mag, Calix expanded into Europe and North America where it also operates manufacturing facilities. It listed on the ASX in 2018.

ACTI-Mag Savings Calculator
  • Feel you are paying too much for your alkali?
  • Looking at a safer alternative to caustic soda for pH control in your nitrification process?
  • Want a greener alkaline additive for your wastewater treatment?

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A safer, greener and more cost effective alternative to caustic soda

ACTI-Mag has a higher neutralising value per kilo when compared with caustic whilst being significantly safer to handle than other traditional alkalis, making it a very safe and cost competitive option.

Find out more

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ACTI-Mag vs caustic soda

  • Feel you are paying too much for your alkali?
  • Looking at a safer alternative to caustic soda for pH control in your nitrification process?
  • Want a greener alkaline additive for your wastewater treatment?

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