Water Terms, Treatment, Decay and Demand: 23-CS-3 Sustainability, Engineering and the Environment, December 2014, Q4

Question 4: Water Terms, Treatment, Decay and Demand (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) Definitions (any four)

Turbidity: the cloudiness of water caused by suspended particles (measured by light scattering, in NTU). BOD (biochemical oxygen demand): the dissolved oxygen consumed by micro-organisms decomposing organic matter in water over a set period—a measure of organic pollution strength. Watershed: the area of land that drains to a common outlet (river, lake). Embodied water: the total water used, directly and indirectly, to produce a product (also called virtual water). Water footprint: the total volume of freshwater used to produce the goods and services consumed by a person, business, or nation. Hydrologic cycle: the continuous movement of water through evaporation, condensation, precipitation, infiltration, runoff, and return to the sea.

(b) Surface-Water Treatment Flow Diagram

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

(c) Bacterial Decay Calculation

t = \frac{1}{k}\ln\!\left(\frac{N_0}{N}\right) = \frac{1}{2.3}\ln\!\left(\frac{2\times10^{7}}{10}\right) = \frac{\ln(2\times10^{6})}{2.3} = \frac{14.509}{2.3} \approx \boxed{6.31\ \text{days}}

(d) Future Water-Demand Forecast

P = 5000\,(1.012)^{20} = 5000 \times 1.269 \approx 6{,}347\ \text{people}

q = 350\,(1 + 0.006\times20) = 350 \times 1.12 = 392\ \text{L/person/day}

Q = 6{,}347 \times 392 \approx 2{,}488{,}000\ \text{L/day} \approx \boxed{2.49\ \text{ML/day}}

(e) Greatest Water Use and Reduction Technologies

Worldwide, irrigation (agriculture) is the largest use of water—about 70% of freshwater withdrawals. Two reduction technologies: drip (micro) irrigation, delivering water directly to roots and cutting evaporation and runoff; and water reuse/recycling of treated greywater or effluent for irrigation, alongside efficiency measures such as low-flow fixtures and leak reduction.

Question 4 of 5: Water Terms, Treatment, Decay and Demand