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23-Chem-B2 Environmental Engineering · December 2018

Question 4 of 7: Water Contaminant Characterization, BOD and Aeration Design

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

National Exam 16-Chem-B2, Environmental Engineering — December 2018. 3 hours, Closed-Book Exam with a candidate-prepared 8½×11" double-sided aid sheet. Any five (5) of the seven questions constitute a complete paper (100 marks); all seven are solved below for completeness.

Reference texts: Metcalf & Eddy (Tchobanoglous, Burton, Stensel), Wastewater Engineering: Treatment and Reuse, 4th ed.; Davis & Cornwell, Introduction to Environmental Engineering, 5th ed.; Turner, Workbook of Atmospheric Dispersion Estimates, 2nd ed.; Cooper & Alley, Air Pollution Control: A Design Approach, 4th ed.

Problem 4: Water Contaminant Characterization, BOD and Aeration Design (20 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.

(i) Characterizing turbidity, dissolved oxygen and nutrients

CharacteristicWhat it representsMeasurement / indirect quantification
(a) TurbidityLight-scattering caused by suspended and colloidal particles (clays, silts, organic floc, microorganisms) — a surrogate for particulate content, not a direct contaminant mass measure. Nephelometric turbidimeter measures 90° scattered-light intensity from a calibrated light source, reported in NTU (nephelometric turbidity units); calibrated against formazin polymer standards of known NTU.
(b) Dissolved oxygenConcentration of molecular O₂ dissolved in the water, the key indicator of a water body's or treatment unit's capacity to support aerobic biological activity. Membrane (Clark-cell) electrochemical DO probe or optical (luminescent-quenching) DO sensor, reported in mg/L; calibrated against water-saturated air (100% saturation) at the measured barometric pressure and temperature, or against a zero-oxygen sodium-sulfite solution.
(c) NutrientsNitrogen (ammonia, nitrate, organic-N) and phosphorus (ortho-phosphate, total P) species that drive eutrophication and are the target of BNR processes. Colorimetric/spectrophotometric methods (e.g. ascorbic-acid method for phosphate, Nessler or salicylate method for ammonia) or ion chromatography for anions (nitrate/nitrite); each calibrated against a multi-point standard curve of known-concentration reagent solutions.

(ii) BOD₅ test

Given.

QuantityValue
Sample volume (secondary effluent), Vs200 mL
Dilution water added100 mL
Total mixture volume, Vt300 mL
Initial DO, DOi6.0 mg/L
DO after 5 days, DO₅1.0 mg/L
DO after 20 days (stabilized), DO₂₀0.03 mg/L

Find. (a) 5-day cBOD (cBOD₅) of the secondary effluent, mg/L. (b) Ultimate cBOD (L₀), mg/L.

Approach. Because nitrification is inhibited, all oxygen consumed is carbonaceous demand; the dilution factor P (volumetric fraction of undiluted sample in the BOD bottle) converts the observed DO-depletion in the diluted mixture back to the demand of the undiluted effluent, and the 20-day "stabilized" reading already represents the fully exerted (ultimate) carbonaceous demand, so no rate-constant extrapolation is needed.

  1. Compute the dilution factor P.
    $$ P = \frac{V_s}{V_t} = \frac{200}{300} = 0.6667 $$
  2. 5-day cBOD from the standard dilution-test formula.
    $$ \text{cBOD}_5 = \frac{DO_i - DO_5}{P} = \frac{6.0 - 1.0}{0.6667} = \boxed{7.50\ \text{mg/L}} $$
  3. Ultimate cBOD from the stabilized 20-day reading.
    $$ L_0 = \frac{DO_i - DO_{20}}{P} = \frac{6.0 - 0.03}{0.6667} = \boxed{8.96\ \text{mg/L}} $$
QuantityValue
Dilution factor, P0.6667
5-day cBOD, cBOD₅7.50 mg/L
Ultimate cBOD, L₀8.96 mg/L

The ratio cBOD₅/L₀ ≈ 0.837 implies a first-order rate constant k ≈ 0.36 d⁻¹ (from L₀(1−e−5k)=cBOD₅) — on the fast side of the typical municipal 0.1–0.3 d⁻¹ range, consistent with a well-acclimated secondary effluent whose remaining organics oxidize quickly; this is reported as informational context only and does not change either boxed result.

(iii) Aeration system design principles for activated sludge

  1. Oxygen-transfer-rate sizing with field correction factors. The aerator (diffused or mechanical) is sized on the actual oxygen requirement (AOR) computed from the field α (transfer-efficiency ratio of wastewater to clean water) and β (saturation-concentration ratio) factors applied to the standard oxygen transfer rate (SOTR), because fouled diffuser surfaces and dissolved solids/surfactants in real wastewater transfer oxygen far less efficiently than in clean-water shop tests.
  2. Mixing energy to keep solids in suspension. Independent of the oxygen-transfer duty, the aeration system must impart enough velocity gradient/turbulence (typically expressed as a minimum air flow per unit basin floor area or per unit volume for diffused systems) to keep the MLSS fully suspended and prevent solids settling and septic zones on the tank floor.
  3. Diffuser placement and fine-bubble efficiency. Fine-bubble diffusers (higher standard aeration efficiency, kg O₂/kWh, than coarse-bubble) are laid out in a grid/spiral-roll pattern matched to basin geometry and tapered along the basin length (higher density near the inlet, where oxygen demand from fresh substrate is highest) so oxygen supply tracks the actual oxygen-uptake-rate profile rather than over-aerating the effluent end and under-aerating the inlet end.