What is aquamation? The process behind Desmond Tutu?s ?green cremation?

The body of Archbishop Desmond Tutu will undergo aquamation, an increasingly popular and environmentally friendly alternative to traditional cremation methods, using water instead of fire.

With aquamation, or "alkaline hydrolysis", the body of the deceased is immersed for three to four hours in a mixture of water and a strong alkali, such as potassium hydroxide, in a pressurised metal cylinder and heated to around 150C.

The process liquifies everything except for the bones, which are then dried in an oven and reduced to white dust, placed in an urn and handed to relatives.

Like human composting, a technique of composting bodies with layers of organic material like leaves or wood chips, aquamation is still authorised only in certain countries. In South Africa, where Tutu died last Sunday, no legislation at all governs the practice.

First developed in the early 1990s as a way to discard the bodies of animals used in experiments, the method was then used to dispose of cattle during the mad cow disease epidemic, said US-based researcher Philip R Olson.

In the 2000s, US medical schools used aquamation to dispose of donated human cadavers, before the practice made its way into the funeral industry, Olson wrote in a 2014 paper.

Tutu, who died on Boxing Day aged 90, was known for his modest lifestyle. He left instructions that his funeral ceremony should be simple and without frills.

The anti-apartheid hero, whose funeral was held on Saturday, specifically asked for a cheap coffin and an eco-friendly cremation.

With burial space in urban areas worldwide becoming increasingly scarce and expensive, aquamation has obvious attractions. Its advocates say water is a gentler way to go than flames.

They also claim a liquid cremation consumes less energy than a conventional one, and emits less greenhouse gases.

According to UK-based firm Resomation, aquamation uses five times less energy than fire, and reduces a funeral's emissions of greenhouse gases by about 35%.

Aquamation is also used to dispose of animal carcasses in slaughterhouses, where it is considered to be more efficient and hygienic.