18-Env-A4 Water and Wastewater Engineering · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2014 — 04-Env-A4 / Water and Wastewater Engineering. 3 hours duration; closed book with one aid sheet written on both sides; an approved calculator is permitted. Question 1 is compulsory; any three of the remaining four questions constitute a complete paper (only the first four of Questions 2–5 in the work book are marked); all five questions are solved below for completeness. Each question is worth 25 marks.
Reference texts. Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.); MWH's Water Treatment: Principles and Design (3rd ed.); Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); Guidelines for Canadian Drinking Water Quality (Health Canada); Canadian Council of Ministers of the Environment (CCME) water-quality guidelines.
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 ammonia nitrogen (TAN) is the sum of the un-ionized ammonia molecule, $\text{NH}_3(aq)$, and the ionized ammonium ion, $\text{NH}_4^+$, present in a water sample — it is what a standard laboratory ammonia test (ion-selective electrode or colorimetric method) reports directly. Free ammonia is the un-ionized $\text{NH}_3$ fraction alone. The two forms are linked by the pH- and temperature-dependent acid-base equilibrium $\text{NH}_4^+ \rightleftharpoons \text{NH}_3 + \text{H}^+$ (pKa ≈ 9.25 at 25 °C), so the free-ammonia fraction of TAN rises sharply as pH climbs toward and past the pKa, and also increases with temperature. The distinction matters because un-ionized $\text{NH}_3$ is the toxic species to fish and other aquatic life (it crosses gill membranes readily), while $\text{NH}_4^+$ is comparatively benign; a receiving-water or effluent toxicity criterion is therefore always expressed as an un-ionized ammonia limit, calculated from the measured TAN together with the sample's pH and temperature, rather than as a limit on TAN alone.
Biochemical Oxygen Demand (BOD5) is a 5-day, 20 °C bioassay that measures the dissolved oxygen consumed by microorganisms as they biologically oxidize the biodegradable organic matter in a sample. Chemical Oxygen Demand (COD) measures the oxygen equivalent of essentially all oxidizable material — biodegradable and non-biodegradable/refractory alike — using a strong chemical oxidant (dichromate reflux) rather than living organisms, and returns a result in about 2–3 hours instead of 5 days. Because COD oxidizes material that BOD5's bacteria cannot touch (and even readily biodegradable organics rarely complete their oxidation within 5 days), COD is always numerically greater than or equal to BOD5 for the same sample. The ratio COD/BOD5 is used as a rough biodegradability index: domestic sewage typically runs 1.5–2.5 (highly biodegradable, well suited to biological treatment), while industrial wastewaters containing refractory or inhibitory compounds can run 5 or higher, signalling that a purely biological process will leave significant residual COD.
Total alkalinity in natural waters is carried by up to three species depending on pH: hydroxide ($\text{OH}^-$), carbonate ($\text{CO}_3^{2-}$), and bicarbonate ($\text{HCO}_3^-$). Hydroxyl (hydroxide) alkalinity is the portion attributable to free $\text{OH}^-$ and is present only at elevated pH, typically above about 9.5–10 (e.g., after lime addition in softening). Carbonate alkalinity is the portion attributable to $\text{CO}_3^{2-}$ and is significant over roughly pH 8.3–10.5; below pH 8.3 essentially all alkalinity is bicarbonate. The three forms are separated by titrating to two endpoints: the phenolphthalein endpoint (pH 8.3) captures hydroxide plus half of the carbonate, while the total/methyl-orange endpoint (pH 4.5) captures hydroxide, carbonate and bicarbonate together; the standard P/T alkalinity relationship table then partitions the three species from just those two titration readings.
Temporary (carbonate) hardness is the portion of total hardness (dissolved $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$) that is balanced by bicarbonate/carbonate anions; it is called "temporary" because boiling drives off dissolved $\text{CO}_2$ and shifts the carbonate equilibrium, precipitating the calcium and magnesium as $\text{CaCO}_3$/$\text{Mg(OH)}_2$ scale and removing that hardness from solution. Permanent (non-carbonate) hardness is the remaining portion balanced by non-carbonate anions such as sulfate, chloride, or nitrate; because there is no bicarbonate to drive off, boiling cannot precipitate it, and it can only be removed by chemical treatment — lime-soda softening (which supplies the missing carbonate to precipitate it) or cation-exchange softening. Total hardness is simply the sum of the temporary and permanent fractions, and this distinction is why a water utility must know both the total hardness and the alkalinity of a source before selecting a softening process and dosing it correctly.
Mineral acidity is acidity contributed by strong, fully-dissociated mineral acids (e.g., $\text{H}_2\text{SO}_4$, $\text{HCl}$, $\text{HNO}_3$) and is titrated with a base to a low pH endpoint (methyl orange, ≈ pH 3.7–4.5), the point at which the strong-acid protons are neutralized. Carbon dioxide (CO2) acidity is acidity contributed by dissolved $\text{CO}_2$/carbonic acid, a weak acid; it is titrated to the higher phenolphthalein endpoint (pH 8.3), the point at which essentially all dissolved $\text{CO}_2$ has been converted to bicarbonate. Total acidity is the sum of the two, and — mirroring the alkalinity titration — the two endpoints (4.5 and 8.3) separate the strong-acid (mineral) contribution from the weak-acid ($\text{CO}_2$) contribution; a water with high CO2 acidity but negligible mineral acidity is simply CO2-charged (e.g., groundwater or a lime-softened water that has not been fully degasified), not industrially contaminated.
