14/02/2013 12:35 GMT | Updated 16/04/2013 06:12 BST

Slaughterhouse Waste - It All Has to Be Dealt With: A Case for the Vegan Option Continued

Thanks to the horse meat scandal tons of processed meat products are being removed from supermarket cabinets. The waste is huge.

But the meat industry usually strives to avoid waste. Any part of an animal that can possibly be used for human consumption is made fit to eat, right down to the stripping of bones with high pressure hoses (though since the BSE epidemic meat is only recovered from pigs this way and not from beef, sheep or goats). The resulting sludge is formed into a pink paste. Known as mechanically recovered meat (or MRM) it is recognisable in the cheapest pies but also (and more cleverly disguised) in reformed meat.

But what of the inedible parts? Parts like bones, bladders, diaphragms, lungs, sphincter muscles, fat and gristle. Or external parts: heads, feet, hooves, skin, hair or udders? Or the liquids - the blood and urine? And solids - the faeces and stomach contents? None of this is of any use to the human food industry. Yet it accounts for about 49% of a cattle carcass; 44% of a pig; 37% of a chicken; and 57% of most fish species.

As much of this waste that can possibly be recycled is. And the industry that handles it is vast.

Metal screens trap solids like skin trimmings, hair, bones and hooves. Grease traps capture fat. Outside slaughterhouses enormous containers hold all these leftovers until tankers come to take them away. In most countries this type of waste has to be heat-treated and sanitised to make sure it poses no danger to human or animal health. Only then can it be disposed of in municipal sewers, in incineration plants or, if possible, made into 'value-added' products, including feed for animals.

Fat is made into tallow and used as a lubricant in steel rolling. In the cosmetic industry tallow is a key ingredient in lipstick and eye make-up. Blood and bones are rendered into blood and bone meal for fertiliser. Feathers are rendered into 'feather meal' to add to farm animal feed.

For pet food rendered waste might include beaks, feathers and hair. But whatever the contents they will, like food processed for human consumption, have gone through a thorough cooking process designed to eliminate bacteria and moulds and ensure a hygienic end product, appealingly presented.

And then there is blood (after stunning, animals have their throats slit so they bleed to death). The quantity is vast. Usually it is stored in huge vats until tankers come to collect it - normally daily but at least once a week because of the 'offensive odour' that develops. Removed by 'milk round' tankers it is taken either to blood processing facilities or to treatment plants. Treatment ensures there is no threat to human or animal health when it is disposed of in sewers, in landfill or spread over the land. But as much as possible is made into by-products like blood meal for animal feed. Waste blood is also used in pet food, or, for human consumption, in mechanically recovered meat. It is also used to make blood sausage (black pudding).

Hair and wool are rendered into fertiliser (which the EU permits in organic farming). The leather industry processes cattle hides and pig skin. Gelatin (made by boiling skin, tendons, ligaments and bones with water) is used in the food industry - in jellies, sweets and chewing gum for example - and also in cosmetics, like shampoo and face masks. Keratin from horns, hooves, feathers and hair are also used in hair-care products as well as body washes, lotions and make up.

Hatchery waste - the discarded male chicks from the egg industry - is made into fertilizer and farm animal feed, including feed for chickens. Hatchery waste is also fed to animals in zoos and wildlife parks. And some is sent to landfill.

Trimmings from the fish processing sector are rendered into fish meal. Used in feed for farm animals including farmed sea fish and prawns. It is useful in all animal feed because it aids digestion and makes unnatural ingredients taste more palatable. Being high in protein animals grow more quickly on less feed.

All over the world 60 billion farmed animals are slaughtered every year. When that number doubles - as it is predicted to do by 2050 - how will the industry cope with the waste? And what new uses might it be put to?