The graph shows the development of biogas production and substrate utilization at Svensk Biogas (Sweden) from 1997 to 2009. 
Key:
- Biogas production Types of substrates: Manure Slaughterhouse waste Other Residue from ethanol production Food industry [Source: L Vallin, (2012), Svensk Biogas AB]
Biogas production in a fermenter requires a substrate. State another requirement for this process.
lack of oxygen/anoxic/anaerobic conditions / acidic pH / warm temperature / methanogens / acidogenic bacteria ✔
Suggest reasons based on the data in the graph for increases in biogas production at Svensk Biogas.
a. increased variety of substrates used ✔ b. change in the proportion of substrates used OR from 1997 to 2004 increase in slaughterhouse waste ✔ c. less reliance on manure/increase use from food industry ✔ d. waste from food industry results in higher biogas yield ✔
Outline the principles of fermentation by continuous culture.
a. microbial population can be maintained in a state of exponential growth for a long time OR concentration of microorganisms in fermenter stable ✔ b. «balanced growth is» maintained by keeping nutrients/medium/pH/ temperature/ oxygen level constant ✔ c. nutrients are added AND products removed «at steady rate» ✔ d. probes used to monitor conditions within fermenters ✔ e. open fermentation/fermenter ✔
