18-Env-A4 Water and Wastewater Engineering · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams / EGBC — May 2015 — 04-ENV-A4 Water and Wastewater Engineering. Three-hour exam; Question 1 is compulsory (25 marks) and any three of the remaining four questions are required (25 marks each); all five are solved below for completeness. Closed book, one double-sided aid sheet permitted, approved calculator permitted.
Reference texts: Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.) — nitrogen and solids characterization, BOD test theory, nitrification/alkalinity, disinfection chemistry, activated-sludge and sludge-processing design; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — the Streeter–Phelps oxygen sag, pH and coagulation–flocculation chemistry, jar testing; MWH's Water Treatment: Principles and Design (3rd ed.) — granular filtration (headloss, backwash) and chemical phosphorus removal.
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.
Nitrification is the two-step biological oxidation of ammonia to nitrate, $NH_4^++2O_2\rightarrow NO_3^-+2H_2O+2H^+$ (via Nitrosomonas to nitrite, then Nitrobacter/Nitrospira to nitrate), and it is acid-producing: each mg/L of $NH_4^+$-N oxidized destroys about 7.14 mg/L of alkalinity (as $CaCO_3$), from the stoichiometric $2\ \text{mol}\ H^+$ produced per mole N. Nitrifying organisms are also strongly pH-sensitive, with an optimum in the range pH 7.5–8.5 and significant inhibition below about pH 6.5–7.0. If the wastewater's buffering capacity (alkalinity) is insufficient to absorb the acid produced, the reactor pH falls, nitrification slows or stalls, and the process can become unstable (a self-limiting feedback). Design practice is therefore to check the alkalinity mass balance and keep a residual alkalinity of roughly 50–80 mg/L as $CaCO_3$ in the aeration basin/effluent, adding an external alkalinity source (lime, soda ash, caustic) if the raw wastewater's own alkalinity is insufficient to cover the consumption. Temperature governs the specific growth rate of the (slow-growing) nitrifying bacteria through an Arrhenius-type relationship; growth rate roughly halves for every 10 °C drop below the ~20 °C optimum, so the minimum aerobic solids retention time (SRT) needed to sustain a stable nitrifier population must be increased substantially in cold-weather operation — winter design SRT is typically 2–3× the summer value with an added safety factor. Alkalinity is measured by acid–base titration: a standardized strong acid (usually 0.02 N $H_2SO_4$) is added to a known sample volume to two successive endpoints — the phenolphthalein endpoint at pH 8.3 (marking conversion of carbonate to bicarbonate, "phenolphthalein alkalinity") and the methyl-orange/mixed-indicator endpoint at pH 4.5 (marking conversion of bicarbonate to carbonic acid, "total alkalinity"); alkalinity as $CaCO_3$ (mg/L) is then computed from the titrant volume and normality, $\text{Alk}=\dfrac{V_\text{acid}\times N_\text{acid}\times 50{,}000}{V_\text{sample}}$.
Organic compounds in the water consume part of the applied chlorine dose (the "chlorine demand") by oxidation side-reactions, and critically react with free chlorine to form disinfection by-products (DBPs) such as trihalomethanes and haloacetic acids, which are regulated carcinogens/health concerns — so a higher-organics water needs pre-treatment (coagulation, activated carbon) to control DBP formation rather than simply raising the chlorine dose. Ammonia reacts preferentially and rapidly with free chlorine to form chloramines (mono-, di-, trichloramine) before the breakpoint is reached; because combined chlorine is a much weaker, slower disinfectant than free chlorine, ammonia-bearing water requires breakpoint chlorination (dosing well past the point where all ammonia is oxidized to $N_2$/$NCl_3$ and true free residual reappears) to achieve reliable primary disinfection, substantially increasing the required chemical dose and cost.
UV advantages over chlorination: (1) UV produces no halogenated disinfection by-products because no chemical residual is added to the water; (2) UV is effective against chlorine-resistant protozoan pathogens (Cryptosporidium, Giardia oocysts), which survive typical chlorine CT values but are readily inactivated by UV dose. UV disadvantages versus chlorination: (1) UV leaves no disinfectant residual, so it provides no protection against regrowth or recontamination in the downstream distribution/discharge system; (2) UV performance depends strongly on water clarity (UV transmittance) — turbidity and particulates shield organisms from the UV dose and can cause fouling of the lamp sleeves, so UV disinfection normally requires the water to already be well filtered.
