1.4.1 Biodegradability of various feedstock

Video: https://www.youtube.com/watch?v=aEXLaGMI9iw&t=9s

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.

Organic WasteTS (%)VS (% of TS)Source
Faeces14 – 3780 – 92(C. Rose, 2015)
Excreta undigested (incl. pour-flush water) *1 – 348 – 80UPM
Faecal sludge delivered to the treatment site *1 – 357 – 73UPM
Faecal sludge from the septic tank *0.3 – 1.350 – 70UPM
Organic household waste (wet season) **12 – 6461 – 95UPM
Organic household waste (dry season) **37 – 8545 – 89UPM
* Refugee Camp Cox’s Bazar, Bangladesh ** Bhola Municipality, Bangladesh
TS & VS Concentrations of Organic Waste

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.

Further Reading:

  • Results of faecal sludge analyses in Kampala, Uganda - Pictures, characteristics, and qualitative observations for 76 samples. (Link)
  • The Characterization of Faeces and Urine: A Review of the Literature to Inform Advanced Treatment Technology. (Link)
  • Database for the physicochemical composition of (treated) lignocellulosic biomass, micro- and macroalgae, various feedstocks for biogas production and biochar. (Link)
  • Sampling for Faecal Sludge and Other Liquid Wastes in Emergency Settings p. 40 - 41 (Link)
  • Methods for Faecal Sludge Analysis  p. 257 – 268 (Link)