18-Env-A4 Water and Wastewater Engineering · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2013 — 04-Env-A4 / Water and Wastewater Engineering. 3 hours duration; closed book with one double-sided aid sheet; approved 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.) — hardness, alkalinity, chlorination chemistry; MWH’s Water Treatment: Principles and Design (3rd ed.) — rapid 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.
BOD₅ (5-day biochemical oxygen demand) is the mass of dissolved oxygen consumed by microorganisms oxidizing biodegradable organic matter in a sample incubated at 20°C for exactly 5 days — a standardized, reproducible point on the oxygen-demand curve, not the full demand. BODₖ (ultimate BOD, also written BODL) is the total oxygen that would be consumed if the carbonaceous reaction were allowed to go to completion (theoretically infinite time). The two are related through first-order BOD kinetics, $\text{BOD}_t = \text{BOD}_u\left(1-e^{-kt}\right)$, where $k$ is the deoxygenation rate constant (typically 0.1–0.3 d-1 base-e for domestic sewage); BOD₅ is therefore always smaller than BODₖ, and the ratio BOD₅/BODₖ depends entirely on $k$ (about 0.68 at $k=0.23$ d-1). BOD₅ is the regulatory/design number because a 5-day test is practical; BODₖ is the more fundamental quantity used to characterize a waste's true oxygen-demanding strength and to project long-term receiving-water impact.
pH is the negative base-10 logarithm of the hydrogen-ion activity, $\text{pH}=-\log_{10}[\text{H}^+]$ — an intensity factor describing how acidic or basic the water is at the instant it is measured (a single number, 0–14, temperature-dependent). Alkalinity is a capacity factor: the water's ability to neutralize (buffer) added acid without a large pH swing, expressed as mg/L as CaCO₃ and contributed mainly by bicarbonate, carbonate and hydroxide species ($\text{Alkalinity}=[\text{HCO}_3^-]+2[\text{CO}_3^{2-}]+[\text{OH}^-]-[\text{H}^+]$, all in equivalents). A water can be high-alkalinity and still read a modest, near-neutral pH, because alkalinity measures the reservoir of buffering species present, not the free hydrogen-ion concentration at that moment; conversely two waters with identical pH can have very different alkalinities and therefore very different resistance to pH shock from an acidic discharge or from coagulant dosing.
TKN (Total Kjeldahl Nitrogen) is the sum of organic nitrogen (protein-, urea- and amino-acid-bound N) plus free ammonia nitrogen, measured together by the Kjeldahl digestion procedure (it does NOT include nitrite or nitrate — the oxidized forms). Ammonia Nitrogen ($\text{NH}_3\text{-N}/\text{NH}_4^+\text{-N}$) is only the reduced, inorganic fraction — already-mineralized nitrogen present as free ammonia or the ammonium ion, whose split depends on pH and temperature via the $\text{NH}_3+\text{H}_2\text{O}\rightleftharpoons\text{NH}_4^++\text{OH}^-$ equilibrium. So $\text{TKN} = \text{Organic-N} + \text{Ammonia-N}$: ammonia nitrogen is a subset of TKN, and the difference (Organic-N $=$ TKN $-$ Ammonia-N) indicates how much nitrogen is still bound in undegraded organic solids versus already mineralized and available for nitrification.
TSS is the mass of solid material retained on a standard glass-fibre filter after drying a known sample volume at 103–105°C — everything not dissolved, both organic and inorganic (grit, silt, biological floc, organic debris). VSS is the fraction of that TSS residue that combusts (ignites off) when the dried filter is subsequently fired at 550‡°C in a muffle furnace — approximately the organic portion of the suspended solids, since volatilization at that temperature destroys organic matter but leaves inorganic ash (fixed suspended solids, FSS) behind: $\text{TSS} = \text{VSS} + \text{FSS}$. VSS is the more useful process-control number in biological treatment because it approximates the concentration of active/degradable biomass and organic solids (e.g. in MLVSS monitoring), whereas raw TSS also carries inert grit and mineral solids that add mass without biological significance.
Free (available) residual chlorine is chlorine remaining in water as hypochlorous acid ($\text{HOCl}$) and hypochlorite ion ($\text{OCl}^-$) — the strong, fast-acting disinfecting species that has not reacted with ammonia or organic nitrogen. Combined residual chlorine is chlorine tied up as chloramines ($\text{NH}_2\text{Cl}$, $\text{NHCl}_2$, $\text{NCl}_3$), formed when free chlorine reacts with ammonia present in the water; combined chlorine is a much weaker, slower-acting (but more persistent/less taste-and-odour-forming) disinfectant than free chlorine, typically 25–100× less effective per unit concentration against the same organisms at the same contact time. Total residual chlorine is the sum of the two ($\text{Total} = \text{Free} + \text{Combined}$); distinguishing them matters because regulatory CT (concentration × time) disinfection credit is normally based on the FREE residual, not total.
