18-Env-A4 Water and Wastewater Engineering · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2019 — 18-Env-A4 / Water and Wastewater Engineering. 3 hours duration; closed book with one double-sided aid sheet; approved Casio/Sharp calculator permitted. Question 1 is compulsory; the paper instructs candidates to attempt any three of the remaining four (100 marks total); all five are solved below for completeness.
Reference texts. Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.) — BOD kinetics, activated-sludge/clarifier design, anaerobic digestion; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — alkalinity chemistry, disinfection, coagulation/flocculation; MWH’s Water Treatment: Principles and Design (3rd ed.) — ozonation, turbidity; Standard Methods for the Examination of Water and Wastewater — alkalinity titration (2320B).
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.
Chlorination is the addition of chlorine (as gas $Cl_2$, sodium hypochlorite, or calcium hypochlorite) to wastewater effluent to inactivate pathogenic micro-organisms before discharge, via hypochlorous acid/hypochlorite ion oxidizing microbial cell components; it is dosed to satisfy the chlorine demand of the water plus a target residual held for a specified contact time (CT) to achieve the required log-inactivation. Dechlorination is the deliberate downstream neutralization of that residual chlorine (typically with sulfur dioxide, sodium bisulfite, or sodium thiosulfate) before the effluent is discharged to a receiving water, because residual chlorine and its by-products (chloramines) are acutely toxic to fish and other aquatic life at concentrations far below what is needed for disinfection. The two are therefore complementary, sequential steps in the same disinfection train: chlorination achieves pathogen kill, dechlorination removes the toxic residual that achieving that kill leaves behind, and a permit will typically specify both a minimum disinfection CT and a maximum total residual chlorine at the point of discharge.
A facultative lagoon is a large, shallow (1.2–2.5 m), unmixed earthen pond relying entirely on natural processes: an aerobic surface layer (oxygen supplied by wind-driven surface reaeration and algal photosynthesis during daylight) overlies an anaerobic bottom layer where settled solids decompose anaerobically, with a facultative zone in between; long detention times (20–180 days) allow BOD removal through this combined aerobic/anaerobic biology with essentially no mechanical energy input. An aerated lagoon is a similar earthen basin but with mechanical (surface or diffused) aeration supplying oxygen directly, allowing a much higher organic loading and shorter detention time (typically 3–10 days) because the process is no longer limited by natural reaeration and photosynthesis; aerated lagoons behave more like a suspended-growth aerobic process (without sludge recycle) and produce a more consistent effluent, at the cost of a continuous power input that a facultative lagoon does not need. Facultative lagoons are favoured where land is cheap and power/O&M budgets are minimal (small, remote communities); aerated lagoons are chosen where land is more constrained or a more reliable, higher-rate treatment is needed.
COD (chemical oxygen demand) is the mass of oxygen equivalent to the oxidizable material in a sample as measured by strong chemical oxidation (typically dichromate reflux, Standard Methods 5220), which oxidizes essentially all organic matter — both biodegradable and non-biodegradable/refractory — plus many inorganic reducing agents, in a matter of hours. BOD5 is the oxygen consumed by living micro-organisms over 5 days at 20 °C, and therefore measures only the biodegradable fraction, and only the portion of it that the seed culture can metabolize within that 5-day window. Because COD captures more of the total oxidizable material and does not depend on slow, variable biological activity, COD is always numerically greater than or equal to BOD5 for the same sample, COD results are available same-day (versus 5 days for BOD5), and the $COD/BOD_5$ ratio is itself a useful indicator of a waste's biodegradability — a low ratio (near 1–2) indicates a highly biodegradable waste, while a high ratio (industrial effluents with refractory organics) signals a waste poorly suited to conventional biological treatment.
Coagulation is the rapid-mix chemical step that destabilizes colloidal particles: a coagulant (alum, ferric chloride, or a cationic polymer) is dosed and flash-mixed at high intensity (velocity gradient $G\approx700$–$1{,}000\ \text{s}^{-1}$, contact time on the order of seconds) so it hydrolyzes and neutralizes the negative surface charge that normally keeps fine clay/organic colloids in stable suspension (charge neutralization/double-layer compression, plus for hydrolyzing metal salts, enmeshment in a growing metal-hydroxide precipitate, "sweep floc"). Flocculation is the subsequent slow-mix step: gentle, prolonged stirring (much lower $G\approx20$–$70\ \text{s}^{-1}$, 15–45 minutes contact time) that brings the now-destabilized micro-particles into gentle contact so they aggregate into larger, settleable floc without the shear that would tear a forming floc apart. The two must be sequential and distinct because they need opposite mixing regimes — coagulation needs violent, near-instantaneous dispersion of the chemical dose throughout the water, while flocculation needs the opposite (slow, patient particle-particle contact) — and combining or reversing them yields either poorly-dispersed coagulant (patchy destabilization) or floc that is built and then immediately shattered.
