18-Env-A4 Water and Wastewater Engineering · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2017 — 04-Env-A4 / Water and Wastewater Engineering. 3 hours duration; closed book with one double-sided aid sheet; approved calculator permitted. The paper instructs that Question 1 is compulsory and any three of the remaining four questions are required; all five are solved below for completeness.
Reference texts. Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.) — activated-sludge kinetics, solids/hydraulic retention time, nitrogen and phosphorus forms; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — discrete settling theory, indicator organisms, coagulation chemistry; MWH’s Water Treatment: Principles and Design (3rd ed.) — process selection, softening, rapid and slow sand filtration; Guidelines for Canadian Drinking Water Quality (Health Canada).
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.
Total Kjeldahl Nitrogen (TKN) is the sum of organic nitrogen (protein, urea, amino acids) and ammonia nitrogen in a sample, measured by acid digestion that converts organic-bound nitrogen to ammonium sulfate followed by distillation/titration; it does not include the oxidized forms nitrite or nitrate, so total nitrogen $=\text{TKN}+\text{NO}_2^-\text{-N}+\text{NO}_3^-\text{-N}$. Total Ammonia Nitrogen (TAN) is the sub-fraction of TKN present as ammonia species, $\text{TAN}=[\text{NH}_4^+\text{-N}]+[\text{NH}_3\text{-N}]$, split between the ionized ammonium ion and the un-ionized (free) gas dissolved in solution. Free ammonia is specifically the un-ionized $\text{NH}_3$ share of TAN, set by the pH- and temperature-dependent equilibrium $\text{NH}_4^++\text{OH}^-\rightleftharpoons\text{NH}_3+\text{H}_2\text{O}$ (pK$_a\approx9.25$ at 25°C, so free ammonia rises sharply as pH climbs above ∼8–9). The distinction matters because un-ionized $\text{NH}_3$, not $\text{NH}_4^+$, is the acutely toxic species to fish and aquatic life, so receiving-water ammonia criteria are commonly written in terms of un-ionized (free) ammonia even though plant monitoring reports TAN.
Chemical Oxygen Demand (COD) is the oxygen equivalent of essentially all organic matter that a strong chemical oxidant (potassium dichromate under acid reflux) can oxidize, biodegradable and non-biodegradable alike, obtained in a few hours. Five-day Biochemical Oxygen Demand (BOD₅) is the oxygen actually consumed by a live microbial seed as it biologically degrades the readily biodegradable fraction over a standardized 5-day, 20°C incubation, and therefore always understates the true ultimate oxygen demand a waste will eventually exert. Because COD captures both biodegradable and refractory material chemically while BOD₅ captures only what organisms metabolize in five days, $\text{COD}\ge\text{BOD}_u\ge\text{BOD}_5$; the COD/BOD₅ ratio is used as a quick biodegradability screen — low (∼1.5–2.5) for typical domestic sewage, higher where refractory industrial organics dominate.
Orthophosphates ($\text{PO}_4^{3-}$, $\text{HPO}_4^{2-}$, $\text{H}_2\text{PO}_4^-$, pH-dependent) are the simple, fully dissolved, directly bioavailable form measured by the standard colorimetric molybdate-blue test and taken up immediately by algae and bacteria. Polyphosphates are condensed chains or rings of phosphate units, mainly from synthetic detergents and some industrial sources, that must first hydrolyze back to orthophosphate (slowly in the environment, or via treatment) before they are bioavailable. Organic phosphates are phosphorus incorporated into organic molecules within living or decaying biomass, released as orthophosphate only after microbial decomposition. Total phosphorus is the sum of all three; only the ortho fraction drives immediate eutrophication risk in a receiving water, while poly- and organic-P represent a delayed-release load.
Both streams are drawn from the same physical source — the settled biomass collecting in the underflow of the secondary clarifier. Return activated sludge (RAS) is the portion pumped back to the head of the aeration tank to sustain the mixed-liquor suspended solids (MLSS) inventory the biological process needs to operate at its design food-to-microorganism ratio. Waste activated sludge (WAS) is the remaining portion deliberately removed from the system rather than returned, and is the operator's primary lever for setting the solids retention time (SRT): wasting more sludge shortens SRT (younger, faster-growing biomass), wasting less lengthens it. The RAS/WAS split is decided daily to hold SRT (and hence effluent quality and sludge production) at its target value.
Hydraulic retention time is the average time a parcel of liquid spends in a reactor, $\text{HRT}=V/Q$, governed purely by tank volume and flow. Solids retention time (mean cell residence time) is the average time the microbial solids themselves spend in the system, $\text{SRT}=(V\,X)/(Q_w X_u+Q_e X_e)$ — the mass of biomass in the reactor divided by the rate at which it leaves (mainly through wasting). In a conventional activated-sludge plant with clarifier recycle, RAS returns solids to the aeration tank while the bulk of the liquid passes straight through, so SRT is decoupled from and normally several times longer than HRT; this decoupling is precisely what lets the plant retain a slow-growing population (e.g. nitrifiers) at a liquid flow-through time far too short for those organisms to persist on their own.
