Waste Materials and Their Effective Management
Senior 1 · Chemistry
Waste Materials and Their Effective Management
Lesson 5 of 5
25 min
On a cold morning, steam rises from a well-made compost heap, as if something inside were cooking. In some Rwandan prisons, the toilets are connected to underground tanks, and the gas from them cooks the prisoners' food. Both use the same idea: the biodegradable waste that makes up most of what we throw away (Lesson 2) can be turned into fertiliser and fuel by microorganisms. How?
Definition
Composting: the breakdown of biodegradable waste by microorganisms with air (aerobic decomposition) into a dark, crumbly material, compost, that improves soil. Anaerobic digestion is the breakdown of biodegradable waste by microorganisms without air, producing biogas, a fuel that is mostly methane and carbon dioxide.
Build the heap in layers (Figure 5.1), on bare ground in a shady corner, about 1 m wide. Mix carbon-rich "browns" with nitrogen-rich "greens", roughly two to three parts browns to one part greens by volume, so that the microorganisms have both energy and the nitrogen they need to grow. Keep it moist and turn it every two weeks. After two to three months it becomes dark, crumbly and earthy-smelling, and is ready to dig into the school garden.
Figure 5.1: A compost heap built in layers.
Why the heap gets hot. Bacteria and fungi respire, using oxygen to break down the sugars and other compounds in the waste. Respiration is exothermic, so the heap warms up, often to or more in the middle, hot enough to kill many weed seeds and germs. For glucose:
If the heap is too wet or never turned, air cannot get in, anaerobic bacteria take over, and the heap starts to smell of rotten eggs.
Table 5.1: What to put in a school compost heap
| Put in | Keep out | Why keep it out |
|---|---|---|
| fruit and vegetable peels, food leftovers without meat | meat, fish, bones, oily food | smell, attract rats and dogs |
| dry leaves, grass cuttings, maize stalks, straw | plastic, glass, metal | not biodegradable |
| paper, cardboard, sawdust, wood ash in small amounts | batteries, chemicals, e-waste | hazardous: poison the compost |
| cow and goat dung | dog and cat faeces, human waste | can carry germs of disease |
In a biogas digester (Figure 5.2), dung, food waste or toilet waste is mixed with water and sealed away from air. Anaerobic bacteria break it down in stages, finally producing methane and carbon dioxide:
The gas collects under the dome and is piped to a stove, where the methane burns with a clean blue flame:
The digested slurry pushed out into the outlet tank has lost its smell and most of its germs, and it still contains the nitrogen, phosphorus and potassium from the dung, so it is a good fertiliser. A digester therefore gives three things at once: fuel, fertiliser and cleaner surroundings.
Figure 5.2: A fixed-dome biogas digester in cross-section.
Definition
Biogas: a fuel gas made by the anaerobic digestion of biodegradable waste, containing mainly methane (typically about half to three-quarters) and carbon dioxide, with traces of hydrogen sulfide. Example: gas from a household digester fed with cow dung.
In the early 2000s Rwanda's large prisons each held thousands of people. Their toilet waste was a serious health hazard, and cooking for them burned great amounts of firewood. The Kigali Institute of Science, Technology and Management (KIST) designed and built large biogas plants for the prisons: linked underground digesters, each of 50 or 100 m³, into which the toilet waste flows, so that the sewage is out of sight and smell. The biogas is piped to the prison kitchens and halves the use of firewood, and the digested effluent is used as fertiliser for crops and tree plantations. By 2005 the plants served about 30,000 people and cut firewood use by about 27,000 m³ a year, and KIST won an Ashden Award for the work (Ashden, 2005).
Worked example
Problem. In the school of Lesson 2, the kitchen produces 38.5 kg of food waste each school day. How much waste is kept out of the landfill in a 12-week term of 5-day weeks if all of it is composted?
Solution.
Answer: 2310 kg, about 2.3 tonnes a term, turned into free fertiliser instead of landfill.
Worked example
Problem. A family's digester produces 1.5 m³ of biogas a day, of which 60% is methane. Their stove burns 0.4 m³ of biogas an hour. (a) What volume of methane is produced each day? (b) For how many hours a day can they cook?
Solution.
Answer: 0.9 m³ of methane; enough gas to cook for about three and three-quarter hours a day, without buying charcoal.
Worked example
Problem. A school with 20 cows and a large kitchen must choose between a compost heap and a biogas digester. Compare the two and recommend one.
Solution.
| Compost heap | Biogas digester | |
|---|---|---|
| Process | aerobic, needs air | anaerobic, sealed from air |
| Products | compost | biogas fuel and liquid fertiliser |
| Cost | almost nothing | building costs; needs skilled builders |
| Best inputs | kitchen and garden waste | dung, toilet and food waste, with water |
Answer: with plenty of dung and a kitchen that burns fuel, a digester pays back by replacing firewood or charcoal and gives fertiliser too; a compost heap is still the cheapest way to handle garden waste. Many schools can use both.
Warning
Safety. Biogas is flammable. Check pipes for leaks with soapy water, never with a flame, and close the valve after cooking. Never climb into an empty digester: it can contain no oxygen and poisonous hydrogen sulfide. Wear gloves when handling dung and compost and wash your hands afterwards.
Tip
With air, compost; without air, gas. Aerobic decomposition gives compost and heat; anaerobic digestion gives methane.
Science Trek (Idaho Public Television): Composting, how it works (6 min). Listen for the decomposers that do the work and what they need.
Practice
Further reading
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
