Water Treatment, Demand and Use: 23-CS-3 Sustainability, Engineering and the Environment, May 2014, Q4

Question 4: Water Treatment, Demand and Use (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

Turbidity measures the cloudiness of water caused by suspended particles (how strongly the particles scatter light, in NTU). It relates to microbial quality because particles both harbour and transport micro-organisms and, critically, shield micro-organisms from disinfection—so turbid water disinfects poorly. Low turbidity is required both as an indicator of particle/pathogen removal and to allow effective disinfection.

(b) Surface-Water Treatment Flow Diagram

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

Each stage targets a finer class of contaminant, from gross debris through colloidal turbidity and cysts to bacteria and viruses, with a residual disinfectant protecting the distribution system.

(c) Bacterial Decay Calculation

t = \frac{1}{k}\ln\!\left(\frac{N_0}{N}\right) = \frac{1}{2.4}\ln\!\left(\frac{10^{7}}{10^{2}}\right) = \frac{\ln(10^{5})}{2.4} = \frac{11.513}{2.4} \approx \boxed{4.80\ \text{days}}

About 4.8 days are needed for the bacteria to fall from 10⁷ to 100 cell/mL.

(d) Future Water-Demand Forecast

P = 6000\,(1.015)^{20} = 6000 \times 1.347 \approx 8{,}081\ \text{people}

q = 380\,(1 + 0.005\times20) = 380 \times 1.10 = 418\ \text{L/person/day}

Q = P \times q = 8{,}081 \times 418 \approx 3{,}378{,}000\ \text{L/day} \approx \boxed{3.38\ \text{ML/day}}

The town should plan for about 3.4 ML/day at the end of the design period.

(e) Greatest Water Use and Reduction Technologies

Worldwide, irrigation (agriculture) is by far the greatest use of water, accounting for roughly 70% of global freshwater withdrawals—far more than industrial or domestic (drinking) use. Two technologies to reduce water use: drip (micro) irrigation, which delivers water directly to plant roots and cuts evaporation and runoff losses compared with flood or spray irrigation; and water reuse/recycling—treating and reusing greywater or municipal effluent for irrigation or industrial cooling—together with efficiency measures such as low-flow fixtures and leak reduction in distribution systems.

Question 4 of 5: Water Treatment, Demand and Use