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Waste Not, Want Not Culhane used some of the sustainable ideas he learned from other cultures to invent a new device. It provides energy in a diff erent way. Culhane began with his belief that there is


no such thing as “waste” or “garbage.” He says that most of what we call “kitchen waste,” “toilet waste,” “agricultural waste,” and “garden and yard waste” have value. T ey can be reused. T ey can be a source of clean fuel and fertilizer. He calls his invention a biodigester. T e


bio stands for “living” and digester says what it does—digests. T e device uses the same microbes that people and animals have in their guts to break materials down. So, you can put food scraps, used cooking


oil, grass clippings, even what you clean out of the cat’s litter box, into the biodigester. Your bathroom waste can go into it. People in rural areas can add in manure from cattle and sheep.


How it Works Once organic materials are inside the biodigester, the microbes get to work. T ey break down what you put in it in the same way your stomach breaks down food. Over time, the device fi lls up with biogas. Biogas is a combination of methane, carbon dioxide, and sulfur gas. When biogas reacts with oxygen, it releases


energy when burned. T is energy release allows biogas to be used as a fuel. It can be used for heating purposes, such as cooking. It can also be used in a natural gas engine to convert the energy into electricity and heat. Like all systems, the amount of energy put


in relates to how much energy comes out. For a biodigester, the amount of energy released depends on how much organic material is put into it. Culhane uses a plastic bucket to measure how


much energy goes into a biodigester. One full bucket of food scraps a day releases enough gas to light two small rooms for an evening or cook two meals. T rough trial and error, Culhane has learned that the breakdown of some materials releases more gas than others.


20 NATIONAL GEOGRAPHIC EXPLORER


 Microbes break down the contents of the biodigester. Biogas moves out


when other materials are put in.


Culhane’s Biodigester


T.H. Culhane invented a device he calls a biodigester to create energy from waste. Here’s how it works:


 Food scraps, manure, and organic materials are


dumped into the biodigester.


No More Garbage Cans For example, freshly cut grass clippings produce four times more biogas than the same amount of dried cow manure. And, of course, bigger biodigesters use more organic material and supply more gas. Culhane has designed biodigesters in


diff erent shapes and sizes. Some are just a large hole in the ground lined with concrete. Others are big plastic containers. Despite the diff erences in size and design,


all of the biodigesters work in the same way. One pipe is used for putting the organic material into the device. A second pipe carries the biogas to be stored. A third pipe is used to drain off the liquid fertilizer that forms in the process. T is fertilizer can be used in gardens.


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