| Date | May 2015 | Marks available | 6 | Reference code | 15M.3.HL.TZ2.9 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 2 |
| Command term | Describe | Question number | 9 | Adapted from | N/A |
Question
Describe the consequences of releasing raw sewage into rivers.
Markscheme
a. pathogens/toxins may be released/contaminate drinking water;
b. (saprotrophic) bacteria live off sewage;
c. decrease dissolved oxygen/DO / increase the oxygen demand/BOD;
d. animals/other (aerobic) organisms may die;
e. decomposition causes increase in ammonia/nitrates/phosphates/CO2;
f. high levels of nitrate/phosphate can stimulate algal growth/blooms / eutrophication;
g. block light for other algae/plants below;
h. which die and decompose, releasing more nutrients;
i. promote more algal growth;
Examiners report
Most candidates provided good, but often only partial answers about the release of raw sewage into rivers; many omitted to mention the saprotrophic and/or pathogenic nature of bacteria present in sewage and to make references to specific substances such as phosphates, nitrates and ammonia.
