| Date | May 2012 | Marks available | 2 | Reference code | 12M.3.SL.TZ1.16 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | State | Question number | 16 | Adapted from | N/A |
Question
The sludge produced in sewage treatment plants contains pathogenic microorganisms. In a study, sludge was heated to 80°C in order to kill the pathogens and the effectiveness of this treatment was compared using viruses (coliphages) and bacteria (Salmonella choleraesuis) which were added as indicators. The level of activity of either of these two indicators shows whether pathogenic microorganisms may have survived in the sewage sludge.
The resistance of the indicators to heat treatment was studied and their level of activity is shown in the following graph.
Compare the effect of the 80°C heat treatment on coliphages and S. choleraesuis.
Discuss whether the heat treatment should be continued beyond 60 minutes if this technique were to be used in sewage treatment plants.
In many areas, sewage is discharged directly into the environment. State two potential environmental consequences of releasing sewage into rivers.
Markscheme
number of both types (bacteria and viruses) is reduced;
the reduction of bacteria was greater than for viruses;
in the first 10 minutes reduction of bacteria is large whereas reduction of viruses is gradual;
after 30 minutes less than 0.0001 % of bacteria remain while about 10 % of viruses remain; Accept any other reasonable comparison.
not necessary for bacteria as nearly all have been killed;
necessary for coliphages as they take longer to denature/destroy/are still present;
depends whether pathogens in sewage are heat tolerant;
depends on the cost of the treatment;
depends whether presence of a few microbes in treated sewage is harmful;
anaerobic conditions / increases BOD (biological oxygen demand) / eutrophication;
low dissolved oxygen may kill (aerobic) organisms;
pathogens/cause health hazards (bathing or drinking water);
algal blooms;
diversity falls/favours organisms able to survive low oxygen levels;
Examiners report
Those students who answered this mainly gained only one mark for saying that both bacteria and viruses were reduced in number.
This proved to be very difficult for most, with those who attempted it only gaining one mark by mentioning possible cost factors.
Most candidates could refer to algal blooms and eutrophication.
