State one example of a bacterium that forms aggregates.

Outline the process of nitrogen fixation by a named free-living bacterium.

The image shows part of a sewage treatment plant.

Outline the role of bacteria in trickling filter bed treatment of sewage.

Pseudomonas aeruginosa / Vibrio fischeri

Accept other correct answers.

a. (atmospheric) nitrogen is converted to ammonia;

b. by Azotobacter;

Do not accept Rhizobium.

a. (saprotrophic) bacteria/biofilm fix on the surface of the rocks/material in the trickling filter;

b. bacteria decompose the sewage/organic matter as it runs over the filter bed;

c. bacteria break down organic matter aerobically;

d. the rocks increase the surface area for the decomposition of organic matter;

e. filter bed can treat high amounts of sewage quickly;

Many mentioned Vibrio fischeri or Pseudomonas aeruginosa as an example of bacteria forming aggregates, but some gave far too vague categories, such as methanogenic bacteria.

Most could state halophiles and Azotobacter, although there was some confusion with Rhizobium.

Many knew only that bacteria fix on the rocks, but did not give enough detail for more, showing poor understanding of the treatment of sewage.