Explain the principles involved in the generation of methane from biomass.

State the role of Rhizobium in the nitrogen cycle.

organic matter/manure/waste/agricultural material/seaweed used;
bacteria in digester transform biomass/raw material;
anaerobic conditions / constant temperature / neutral pH in the digester;
bacteria convert organic material to organic acids/alcohol;
other bacteria convert organic acids/alcohols into acetate;
methanogenic bacteria convert acetate to methane

nitrogen fixation / changes (free) nitrogen to ammonia

Most candidates only scored one or two marks in this question as they only explained that bacteria are used to transform organic matter. No further detail of the process was provided.

(c) (i) and (ii) almost all scored full marks.