| Date | May 2017 | Marks available | 1 | Reference code | 17M.3.SL.TZ1.8 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | State | Question number | 8 | Adapted from | N/A |
Question
The graph shows the development of biogas production and substrate utilization at Svensk Biogas (Sweden) from 1997 to 2009.
Biogas production in a fermenter requires a substrate. State another requirement for this process.
Suggest reasons based on the data in the graph for increases in biogas production at Svensk Biogas.
Outline the principles of fermentation by continuous culture.
Markscheme
lack of oxygen/anoxic/anaerobic conditions / acidic pH / warm temperature / methanogens / acidogenic bacteria
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
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
