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Solar CITIES open-source design for a remarkable system that turns all food waste into natural gas and liquid fertilizer. According to the tutorial, you’ll never have to buy fertilizer again. The gas is piped to a stove for cooking.

The system is a form of biomimicry, essentially recreating an animal’s digestive system. The “stomach” is a 1,000 liter plastic international bulk container (IBC), with three pipes inserted into the top—one for feeding, one for gas outlet and one for liquid effluence.

Every time you add material into the “plug-flow digestor design,” which optimizes hydraulic retention time (the amount of time things are digested), an equivalent amount of fertilizer will come out.

The nutrient-rich “bioslurry” is ideal for hydroponics and can be diluted with graywater at 10:1. Sludge is minimal; the tank can go years without having to be pumped out.

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The gas can be stored in PVC balloons or bladders, which are then piped to a stove.

All food waste can be fed to the system, including meat and grease. Just grind or mash with water, even dirty or soapy. You can install the digestor under a sink dedicated to grinding food scraps.

In hot climates, paint the digestor black to keep the biogas system warm. This will also prevent algae from growing, which would produce oxygen and kill anaerobic organisms doing the digesting.

Insulate the system in a cold climate with Styrofoam and stretch wrap, or, even better, foam spray. You can put it in a greenhouse and use PEX heating coils connected to a source of hot water, like a solar heater.