7.3.1 Chemical composition of solid wastes

Knowing the chemical composition of solid waste is very important when planning for different waste management options. Key aspects of chemical composition are:

  • moisture content – the percentage of water in the waste
  • ash content – the amount of material remaining after burning the waste
  • heat content – also known as calorific value, which is the amount of heat energy produced when the waste is burned.

Full analysis could also identify the different chemical elements present in the waste.

In practice, the composition of a mixture of wastes is found by separating the components and testing each different material to find out its moisture, ash content and chemical composition, and combining the values to get the overall composition of the waste.

The moisture content of municipal solid wastes varies depending on the season of the year, humidity and weather conditions, as well as on the composition of the waste. It is calculated by weighing an amount of waste, drying it in an oven which causes the moisture to evaporate, then weighing it again. The mass lost in the drying process is equivalent to the moisture content and is calculated as a percentage of the initial mass. Table 7.3 shows typical moisture content of the major solid waste components generated in Bahir Dar during the 2010 study. As you can see from the data, organic wastes such as food, paper and garden wastes have high moisture contents.

Table 7.3  Typical data for moisture content of residential solid waste in Bahir Dar, 2010. (Forum for Environment/UNEP, 2010)

ComponentMoisture content (percent)
Food waste46
Paper34
Plastic12
Textile9
Rubber<1
Leather3
Garden waste30
Wood13
Glass<1
Metals<1
Ash and soil16
Stone<1
Other<1
  • A sample of food waste weighs 460g. After drying it weighs 275g. What is the moisture content of the sample?

  • The moisture content is equation left hand side 460 times g minus 275 times g equals right hand side 185 times g full stop

    In percentage terms, this is 185 divided by 460 multiplication 100 equals 40

Analysing waste to determine its chemical composition and calorific value is highly specialised work that needs laboratory equipment to prepare the samples and then carry out the analyses. As an urban WASH worker you are unlikely do this type of work, but you should be aware that the results can be used to assess the suitability of a waste for composting, anaerobic digestion, recycling and energy recovery.

  • Thinking back to Study Session 1, what do you understand by the term ‘recycling’?

  • Recycling means taking materials from waste and transporting them to a factory where they are processed to make new raw materials that can be then be used.

7.3  Composition of residential solid waste

7.4  Quantities of residential waste