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.
| Component | Moisture content (percent) |
| Food waste | 46 |
| Paper | 34 |
| Plastic | 12 |
| Textile | 9 |
| Rubber | <1 |
| Leather | 3 |
| Garden waste | 30 |
| Wood | 13 |
| Glass | <1 |
| Metals | <1 |
| Ash and soil | 16 |
| 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
In percentage terms, this is
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