Purifying Water With Prickly Pear Goo

Yet another plant can save the world, and this time it's a cactus. Just add grubby water to the slimy insides of a Prickly Pear and guess what, clean water.

Yes, the cactus Australians love to hate, may help some of the 1.1 billion people in the world who don't have access to clean drinking water.

This may all sound familiar. It was only a few weeks ago that I was raving about the virtues of the Moringa which include grinding up its seeds to cheaply purify water. It seems this is a hot research topic and the moment, and so it should be.

The cactus solution involves a species of Prickly Pear called Opuntia ficus-indica. Although this species is an occasional weed in Australia it is not the species that invaded thousands of square kilometres of farmland in the nineteenth century. That was mostly Opuntia stricta, one of the first plants introduced deliberately into Australia.

(Captain Phillip included this species among a range of ecomonically useful plants brought from Brazil to Australia in 1788, planting it in what is now the Royal Botanic Gardens. The idea was to harvest the opuntia-loving cochineal bug to extract carmine dye for important purposes such as colouring the red soldier's 'red coats'.)

I think many of the opuntia species would work. Audrey Buttice and colleagues from the University of South Florida extracted what they call 'mucilage gum', and I call goo, from freshly cut segments of the Opuntia ficus-indica. Their results are published in the journal Environmental Science and Technology.

The cactus slices were steamed then blended to extract two different kinds of goo, a gelling and a non-gelling kind. Apparently there are up to 55 different sugars in Prickly Pear goo and each component has different quantities and types of these sugars.

The Floridians tested the extract in hard (ion-rich) and soft water dosed with bacteria or sediment. The sediment - kaolin (aluminium silicate) - settled at a rate of up to 13 centimetres per minute, which I gather is good.

More importantly, they found that 96-99% of bacteria were removed by the goo, with gelling and non-gelling extracts pretty similar. Although this is a good result, the authors note that the water would still be considered unsafe to drink.

However the quantities of bacteria added to the water were extremely high, much greater than occur naturally. This was so they could track the removal of the bacteria visually in the lab. The next set of experiments will use real-world levels.

Prickly Pear purification is likely to well suited to cleaning up hard water. As the authors note, it's a common species in places with this kind of water chemistry, it's cheap and it's likely to be accepted culturally.

According to a New Scientist report on this paper, the same species was used to purify water by Mexican communities in the nineteenth century. We just need to make sure it meets our twenty-first century water quality standards.

Image: My son Jerome and friends in Spain last year, tasting Prickly Pear fruits for the first time.