Outline the role of saprotrophic bacteria in the treatment of sewage.

Explain the formation of methane from biomass.

sewage trickled over bed of rocks with (biofilm of) saprotrophs and oxygen added;

saprotrophic bacteria feed on/break down organic matter (found in sewage);

transforming it into harmless/re-usable products/ \({\text{C}}{{\text{O}}_2}\), \({{\text{H}}_2}{\text{O}}\), ammonia;

bioreactor with anaerobic conditions;

bacteria convert organic matter into organic acids/alcohol/acetate/ \({\text{C}}{{\text{O}}_2}\) and \({{\text{H}}_2}\);

methanogenic bacteria produce methane from breakdown of acetate/ \({\text{C}}{{\text{O}}_2}\) and \({{\text{H}}_2}\);

(Accept correct word or chemical equations)

As was stated in the N12 Examiners’ Report, candidates did not do well in this question on the treatment of sewage. One mark for saprotrophic bacteria feed on/break down organic matter (found in sewage) was common but seldom was a second mark awarded.

This proved to be a discriminating question as 1 mark may have been given but seldom 2 marks and not 3.