Water Terms, BOD, Treatment and Demand: 23-CS-3 Sustainability, Engineering and the Environment, May 2017, Q4

Question 4: Water Terms, BOD, Treatment 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 three)

Turbidity: cloudiness of water from suspended particles (light scattering, NTU). Embodied (virtual) water: total water used to produce a product. Hydrologic cycle: the continuous movement of water via evaporation, condensation, precipitation, infiltration, and runoff. Water table: the top of the saturated zone. Aquitard: a low-permeability layer restricting groundwater flow. Vadose zone: the unsaturated zone above the water table.

(b) Biochemical Oxygen Demand

BOD means biochemical oxygen demand: the amount of dissolved oxygen consumed by micro-organisms as they biologically decompose the organic matter in water over a specified time. It measures the biodegradable organic material in the wastewater—the "strength" of the organic pollution. Its different forms include the standard five-day BOD (BOD₅) measured at 20 °C, the ultimate BOD (total oxygen demand to decompose all the organic matter), and the split between carbonaceous BOD (CBOD, from oxidation of carbon compounds) and nitrogenous BOD (NBOD, from oxidation of ammonia/nitrogen). BOD is reduced chiefly by secondary (biological) treatment—the activated-sludge process or trickling filters, in which micro-organisms consume the organic matter—preceded by primary sedimentation (which removes settleable organics) and followed by clarification; this is exactly the stage that removes most of the oxygen-demanding load before discharge.

(c) 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

(d) Decay Constant

k = \frac{1}{t}\ln\!\left(\frac{N_0}{N}\right) = \frac{\ln(4{,}000)}{3} = \frac{8.294}{3} \approx \boxed{2.76\ \text{day}^{-1}}

Here "reduced by a factor of 4,000" means N₀/N = 4,000, and the constant is the natural-log (base-e) rate. This corresponds to a half-life of ln 2 / 2.76 ≈ 0.25 day (about 6 hours). If the rate were instead written in base 10 (N = N₀·10^−kt), the same data would give k = log₁₀(4,000)/3 = 1.20 day⁻¹, which is smaller by the factor ln 10 = 2.303. State which base is used.

(e) Current and Future Water Demand

Current: $Q_0 = 8500 \times 350 = 2{,}975{,}000$ L/day ≈ 2.98 ML/day. Future (25 yr): $P = 8500\,(1.004)^{25} = 8500\times1.105 \approx 9{,}392$; $q = 350(1+0.001\times25) = 350\times1.025 = 358.75$ L/person/day:

Q_{25} = 9{,}392 \times 358.75 \approx 3{,}369{,}000\ \text{L/day} \approx \boxed{3.37\ \text{ML/day}}

Demand grows by about 13% over the 25-year design period. Population growth (×1.105) contributes most of this, and the per-capita increase (×1.025) adds the rest.

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