They are far from appetising, and out-of-date meat and dairy products may even induce the gagging reflex, but the food trimmings, leftovers and beyond-best-befores that go into your garbage are still high in energy. Even though they might be in the process of breaking down, your mouldy oranges, crusts of bread and rancid walnuts are still chock full of calorie-giving molecules such as sugars, starches, fats, proteins and organic acids. In the context of global warming we all want to be carbon neutral (or even good little carbon sinks), so the responsible thing to do is to capture and re-use all that trapped energy.
In the old days when we all lived on farms and when even urban dwellers had pigs and chickens to deal with slops, very little food waste was sent to landfill. At the very least, back in our grandparents’ time potato skins, gristle and the like was put on the compost heap to eventually be recycled back into the soil. At present we are throwing out worrying amounts of food, to the extent that even swivel-eyed Brexit politicians and the Pope are expressing concern. Most of the food waste we dump goes to landfill, meaning that the carbon that cost so much in terms of energy, water and other resources to incorporate into our munchables returns slowly to the atmosphere with nobody benefiting except the dumpsite decomposers (beetles, worms, fungi and bacteria) whose job it is to gobble up macromolecules and spit out water, carbon dioxide and a bit of methane and hydrogen sulphide.
What can be done to recapture the energy we are losing through discarding so much food? One excellent option is to convert it to biogas. And guess who are the stars in the story of the conversion of old soggy spinach and crusty peanut butter to methane? You guessed it, our old friends, bacteria. And not just one bacterium or one group of bacteria, but a whole bunch of them working in co-operative layers (called “trophic” layers). Here’s a helicopter view of the process, which takes place in a huge tank known as an anaerobic digester. Your green waste management company collects you and thousands of others’ food waste and transports it to its site where after some pre-processing it is packed into the temperature-controlled digester. The bugs get to work on it immediately. The first group of bugs, hydrolysers, chop up the big molecules in the waste (e.g. starches) into smaller ones (e.g. sugars). These simpler molecules then become the foodstuff for acidogenic bacteria, who produce acidy compounds like those found in pickles. A third group of bugs work on the acids to form primarily acetic acid (found in vinegar), carbon dioxide and hydrogen. Finally, methanogenic bacteria mop up the waste products of the other three groups and produce — you guessed it — methane. This gas is our biofuel and can be burned to produce heat or drive turbines that generate electricity. It’s not just waste disposal companies who are getting into anaerobic digestion: many factories now have their own on-site anaerobic digestors to convert their own waste into usable energy.
You may be wondering why “anaerobic” digestion? Anaerobic means “in the absence of air. More correctly, we are talking about the absence of oxygen. Aerobic digestion carried out by aerobic bacteria is basically like slow-burning your waste: you oxidise (burn) your macromolecules and generate heat, carbon dioxide and water, but no usable high-energy compounds such as methane are left behind by the process.
Finally, a special form of recycling food waste, is the conversion of cooking oil to biodiesel. Companies such as Olleco collect used cooking oil from chippers, Chineses and caffs and, using a chemical process (no bacteria involved, unfortunately), turn what would be a potent building block for sewer fatbergs into fuel.
Waste not want not!