Enhanced methane production from algal digestion using free nitrous acid pre-treatment


The methane yield from algae was significantly enhanced by FNA pre-treatment.

FNA pre-treatment enhanced both rapidly and slowly biodegradable fractions.

Algal biomass solubilisation increased up to 5.7 times with FNA pre-treatment.

Higher levels of nitrite present had an inhibitory/toxic effect to anaerobic digestion.


The methane yield from the digestion of algae is typically much lower than the theoretical methane yield, and lower than yields reported for other organic substrates. This study presents a novel free nitrous acid (FNA) pre-treatment technique to improve methane production from algal biomass. The methane production yield through anaerobic digestion was found to be dramatically enhanced by FNA pre-treatment (2.31 mg HNO2–N L−1), with a 51% increase in the methane yield (from 161 to 250 L CH4 per kg VS added). A two substrate model was used to describe the apparent presence of rapid and slowly degradable material. Model-based analysis revealed that with FNA pre-treatment (2.31 mg HNO2–N L−1), the availability of both rapid and slowly biodegradable substrates were increased. Higher levels of nitrite (159 and 1006 mg N L−1) had an inhibitory/toxic effect. For this reason, coupled with the fact that denitrification of nitrite consumes organic substrate, it is concluded that pre-treatment liquor should be removed before digestion.

Graphical abstract


  • Free nitrous acid;
  • Algae;
  • Anaerobic digestion;
  • Biogas;
  • Inhibition

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