1.4.1 Biodegradability of various feedstock

The feedstock consists of both water and solid parts. The solid part is called Total Solids (TS) and can be divided into organic and inorganic fractions. Only the organic fraction, also known as Volatile Solids (VS), contributes to biogas production. VS is used to assess the suitability of organic waste for anaerobic digestion as it impacts the biogas production potential.

TS is expressed as a percentage (%) or in mg/L or g/L to indicate the dry matter content in a feedstock.

VS is expressed as a percentage of TS (% of TS) or in mg/L or g/L to indicate the organic matter content in a feedstock.

The table below shows the total solids (TS) and volatile solids (VS) concentrations for different types of organic waste. It is evident from the table that the concentration can vary significantly between sources but also within the sources.

Substrates that contain VS less than 60 % of TS are generally not considered high value for anaerobic digestion. However, this rationale does not consider the aspect of the climate emergency.

Using both faeces and undigested excreta as a substrate for biogas production is possible. These materials could have a VS > 60% of TS, and even if biowaste has higher biogas yields, human excreta is still a viable option.

When the retention time in the onsite containment is brief, faecal sludge (FS) can be used as a feedstock for biogas recovery at the Faecal Sludge Treatment Plant (FSTP). This was observed at an FSTP in Cox’s Bazar, where the FS delivered contained a significant amount of organic material (VS).

Faecal sludge (septage) from septic tanks is not ideal for biogas production as the organic matter is digested, and the methane has already been released into the environment. Therefore, upgrading a septic tank to capture biogas could be a viable option from a climate perspective. In 2007, the Intergovernmental Panel on Climate Change (IPCC) estimated a CH4 emission factor of 25.5 g-CH4/cap/day for non-gastight/non-biogas-capturing septic tank systems.

Please note that the characteristics of faeces, excreta, faecal sludge, and organic waste can vary significantly between countries, regions, and locations. Therefore, assessing the characteristics before designing a project is essential, especially if designing larger (semi-) centralised systems.

For further information, please click on the Materials tab at the top of the page.