Recycling of organic waste
Practical Action
Large-scale, centralised composting has tended to be unsuccessful in developing countries
for a number of technical and organisational reasons. It is not dealt with in this paper.
Medium scale biogas and compost production from market garbage in Colombo, Sri Lanka
A pilot project being implemented by the Colombo Municipal Council uses organic waste from local
city vegetable markets to produce biogas and compost. The digesters were developed by the
National Energy Research and Development Centre and accept dry batches of organic waste. There
are four 20 foot diameter floating dome digesters (see figure 3) each with a capacity of 40 tonnes
dry waste. The residence time for the organic matter is 4 months and thus the four tanks are able to
deal with a total of 480 tonnes of market waste each year.
The waste produces approximately 1 cubic metre of biogas per tonne per day and this translates to
a total of 7500 kilowatt hours of electricity each year. The system also yields 300 tonnes of
saleable fertiliser each year. Before this, all the waste had to be landfilled outside the city.
The digester is made from concrete with a floating fibreglass cover. The gas is piped from the
digester and is used to power a 220 volt, 5 kilowatt converted engine. There is also a baker’s oven
and a catering size gas burner at the site to demonstrate the uses of the gas.
Now we will look at an example of animal rearing using organic food scraps. This is a typical
example of waste being put to good use and benefiting a number of groups.
Pig-feeding in Metro Manila
In the outlying urban areas of Manila, backyard pig- rearing has long been a traditional source of
income. Commercially produced feed for this activity is expensive and pig raisers often turn to
organic scraps to supplement or replace the commercial product. A network of collectors has
developed that collects organic waste from restaurants in the city centre, and then distribute it
amongst the backyard farmers. The farmers can purchase the scrap at about half the price of the
commercial feed. A cost comparison carried out under the WAREN project (cited in a report titled
‘Recycling activities in Metro Manila’) shows that profit is more than doubled by feeding the pigs on
organic scraps, even after all other costs, such as veterinary costs, transport, fuel, etc., are taken
into consideration.
Such ventures are beneficial not only to the pig raisers, but also to the municipality who would
otherwise have to dispose of the waste in a landfill.
Biogas production
Biogas is produced by means of a process known as anaerobic digestion. It is a process whereby
organic matter is broken down by microbiological activity and takes place in the absence of air
(anaerobic means ‘in the absence of air’). It is a phenomenon that occurs naturally at the bottom of
ponds and marshes and gives rise to ‘marsh gas’ or methane, which is a combustible gas. It also
takes place naturally in landfill sites and contributes to harmful greenhouse gases. Biogas can be
produced by digesting human, animal or vegetable waste in specially designed digesters (see Box
2). Animal waste is particularly suitable for biogas production because it is often available is large
quantities and also has a suitable C:N ratio. The scale of simple biogas plants can vary from a small
‘household’ system to large commercial plants of several thousand cubic metres. The process is
sensitive to both temperature and feedstock (the correct C:N ratio is required as with composting)
and both need to be controlled carefully for digestion to take place. Digestion time varies from a
couple of weeks to a couple of months.
The digestion of waste yields several benefits:
the production of methane for use as a fuel.
the waste is reduced to slurry which has a high nutrient content which makes an ideal
fertiliser; in some cases this fertiliser is the main product from the digester and the biogas
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