Western Treatment Plant Explorer : Teacher Resources : Resource 21
Resource 21
Methane digester ¹
A methane digester is a device that 'digests' or decomposes manure into simple organic and biogas products. In the classroom, teachers use digesters to simulate gas generation from landfill sites.
A laboratory methane digester.
To construct the methane digester you will need:
- 2 small aquaria and an aquarium heater
- a thermometer (range 0-100°C)
- a 2-litre glass juice bottle
- a rubber stopper with a hole and top diameter of 37 mm
- glass tubing, 150 mm, diameter 7 mm
- 600 mm of 10 mm rubber tubing
- a beehive shelf
- matches
- compost or soil
- 500 mL measuring cylinder or glass soft drink bottle
- 2 test tubes with stoppers
- test tube tongs
- 250 g of plant and/or animal waste
- 2 retort stands with clamps
- an extension ring for retort stand
- lime water
- safety glasses
- a waterproof felt pen or crayon.
Here are some suggestions for making a successful methane digester model.
- Most plant material and animal manures will produce methane - the best materials are pig and fowl manure.
- The ratio of carbon to nitrogen should not exceed 30:1. Most animal manures are suitable. The ratio of liquid to solid should be about 10:1. Remember that most organic material is 50 per cent water.
- A suitable temperature is 30-37°C. Do not operate the digester at or above 40°C or allow the temperature to fluctuate more than a few degrees, or the bacteria will die and methane generation will cease.
- Insulate the aquarium by standing it in a box lined with shredded newspaper. A sheet of plastic air-cushioned packing material (such as Bubble Wrap) placed across the aquarium will insulate the water and reduce water loss.
Safety precautions
When making a methane digester, it is important to remember that a mixture of methane and air is potentially explosive. When conducting this experiment in the science laboratory, ensure that the following procedures are adhered to.
- The room should be well-ventilated and there must be no air leaks in the digester.
- Do not allow naked flames near the digester when it is operating or when the measuring cylinder is being refilled with water.
- Any testing of gases in a test tube should take place well away from the digester.
- Wear safety glasses.
- Conduct the test in a dark place, as methane burns with a pale blue flame which is hard to see in daylight.
- Ensure that the aquarium heater is connected to a safety switch (residual current device).
- Make sure that the power to the aquarium heater is turned off and the plug removed from the power point (a) when replacing lost water in the heated aquarium, and (b) before placing your hands in the water of the heated aquarium.
¹ Based on an activity from the Gould League.