Question 4 of 5: Drinking-Water Treatment and Waterborne Disease

Question 4: Drinking-Water Treatment and Waterborne Disease (25 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Turbidity and Microbial Water Quality

Turbidity is the cloudiness of water caused by suspended particles. It is related to microbial quality in two ways. First, it is an indicator: the particles that cause turbidity can harbour and transport micro-organisms, so high turbidity often accompanies higher microbial contamination. Second, and more importantly for treatment, turbidity interferes with disinfection: suspended particles physically shield bacteria, viruses, and protozoan cysts from chlorine or UV, so disinfection is far less effective in turbid water. Low turbidity is therefore required both as a surrogate measure of particle (and pathogen) removal and to ensure that disinfection can actually reach and inactivate the micro-organisms—which is why turbidity is continuously monitored to control the treatment process.

(b) Spread of Waterborne Disease

Waterborne diseases spread by the fecal–oral route. Where sanitation is inadequate, human sewage containing pathogens (bacteria such as Vibrio cholerae and Salmonella typhi, viruses, and protozoa such as Giardia and Cryptosporidium) contaminates the water sources—rivers, wells, and lakes—that the community also uses for drinking. People who then drink or cook with the untreated, contaminated water ingest the pathogens and fall ill; their waste, if again untreated, re-contaminates the water, creating a self-sustaining cycle that can produce rapid, widespread outbreaks such as cholera and dysentery. Breaking this cycle requires both treating drinking water (to remove/inactivate pathogens) and safely collecting and treating sewage (to keep pathogens out of the source water).

(c) Exponential Decay Calculation

Bacterial die-off follows first-order (exponential) decay:

with $N_0 = 10^5$ cell/mL, $N = 10^2$ cell/mL, and $k = 2.5\ \text{day}^{-1}$:

It would take about 2.8 days for the viable bacteria concentration to fall from 10⁵ to 10² cell/mL.

(d) Surface-Water Treatment Flow Diagram

 Raw surface water
      │
      ▼
[1 Intake & Screening] ── removes large debris, leaves, fish, trash
      │
      ▼
[2 Coagulation]  ── add alum/coagulant; neutralizes charge on colloids
      │
      ▼
[3 Flocculation] ── gentle mixing; colloids/turbidity aggregate into floc
      │
      ▼
[4 Sedimentation]── floc & suspended solids settle out (turbidity, some pathogens)
      │
      ▼
[5 Filtration]   ── sand/multimedia; removes fine particles, remaining turbidity,
      │              cysts (Giardia, Cryptosporidium)
      ▼
[6 Disinfection] ── chlorine / UV / ozone; inactivates bacteria & viruses
      │              (+ pH adjustment, fluoridation as required)
      ▼
[7 Clear well / Storage & Distribution] ── chlorine residual maintained
      │
      ▼
   To consumers

Each stage targets specific contaminants: screening removes gross debris; coagulation–flocculation–sedimentation together remove colloidal particles and turbidity (and the pathogens attached to them); filtration polishes out fine particles and disinfection-resistant protozoan cysts; and disinfection inactivates the remaining bacteria and viruses, with a residual disinfectant carried into distribution to guard against re-contamination.