18-Env-A4 Water and Wastewater Engineering · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2018 — 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 questions — all five are solved below for completeness.
Reference texts. Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.) — trickling filters, activated-sludge SRT/yield design, nitrogen speciation; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — discrete particle settling theory, water-quality parameters; MWH’s Water Treatment: Principles and Design (3rd ed.) — coagulation-flocculation, adsorption, chlorine chemistry, water treatment plant process design; Guidelines for Canadian Drinking Water Quality (Health Canada/GCDWQ) — sulfate, nitrate and chloride aesthetic/health-based limits.
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.
Free available chlorine is the chlorine residual present as hypochlorous acid ($HOCl$) and hypochlorite ion ($OCl^-$) — the species formed directly by chlorine hydrolysis that have not reacted with ammonia or organic nitrogen in the water. It is the fast-acting, strongly disinfecting fraction of the residual. Combined available chlorine is the residual tied up as chloramines (monochloramine $NH_2Cl$, dichloramine $NHCl_2$, and trichloramine/nitrogen trichloride $NCl_3$), formed when free chlorine reacts with ammonia present in the water. Combined chlorine is a much weaker and slower-acting disinfectant than free chlorine (roughly 1/25 to 1/100 the biocidal strength per unit residual) but persists longer in the distribution system and produces fewer taste/odour complaints and fewer regulated trihalomethane by-products than free chlorine reacting with natural organic matter. Total chlorine residual is the sum of the free and combined fractions, and the two are distinguished analytically by the DPD method (free chlorine reacts with the DPD indicator instantly; combined chlorine requires the addition of iodide as a catalyst and a short reaction time).
Coagulation-flocculation is a two-step chemical/physical destabilization process for removing colloidal and suspended matter. Coagulation is the rapid-mix addition of a hydrolyzing metal salt (alum, ferric chloride) that neutralizes the negative surface charge of colloids (via charge neutralization and/or enmeshment in a metal-hydroxide sweep floc), eliminating the electrostatic repulsion that otherwise keeps particles apart. Flocculation is the subsequent slow, gentle mixing stage that promotes particle–particle collisions (perikinetic and orthokinetic transport) so the destabilized colloids aggregate into larger, settleable flocs. Adsorption is an entirely different, mass-transfer mechanism: dissolved or colloidal contaminants (natural organic matter, taste-and-odour compounds such as geosmin and 2-MIB, some trace organics/metals) migrate to and bind at the surface of a porous solid adsorbent (activated carbon), driven by van der Waals forces or surface chemical bonding rather than charge neutralization. Coagulation-flocculation targets particulate/colloidal turbidity and requires a subsequent solid–liquid separation step (sedimentation, filtration); adsorption targets soluble contaminants and requires no destabilization chemistry, only sufficient adsorbent surface area and contact time.
Total Kjeldahl Nitrogen (TKN) is the sum of organic nitrogen (protein, urea, amino acids) and ammonia nitrogen, measured by acid digestion, distillation and titration — it captures every reduced-nitrogen form but not nitrate/nitrite. Total ammonia nitrogen (TAN) is the sum of the two ammonia species present in solution: the un-ionized, dissolved-gas form ($NH_3$) and the ionized ammonium ion ($NH_4^+$), i.e. $TAN = NH_3 + NH_4^+$; it is a sub-component of TKN. Free ammonia refers specifically to the un-ionized $NH_3$ fraction of TAN — the toxic form to aquatic life and the form of regulatory concern in receiving-water discharge limits. The split between $NH_3$ and $NH_4^+$ within TAN is governed by the ammonia dissociation equilibrium, $NH_4^+ \rightleftharpoons NH_3 + H^+$ ($pK_a\approx9.25$ at $25\,{}^{\circ}\text{C}$), so the free-ammonia fraction rises sharply with increasing pH and, to a lesser extent, temperature — the same TAN concentration can be well within a toxicity limit at pH 7 and out of compliance at pH 8.5.
Standard $BOD_5$ measures the total oxygen consumed by a sample over 5 days at $20\,{}^{\circ}\text{C}$, and if nitrifying bacteria ($Nitrosomonas$, $Nitrobacter$) are present or seeded into the sample, that oxygen demand includes both carbonaceous (organic-carbon) oxidation and nitrogenous oxidation (ammonia oxidized to nitrite then nitrate). $cBOD_5$ is the same 5-day test run with a nitrification inhibitor added (commonly TCMP, 2-chloro-6-(trichloromethyl)pyridine) to suppress the nitrifying organisms, isolating just the carbonaceous oxygen demand. $cBOD_5$ is the more common regulatory/design parameter for effluent permitting because ammonia oxidation is usually regulated and tracked separately (as a total ammonia nitrogen limit), and including it inside an uninhibited $BOD_5$ result double-counts the same nitrogen load under two different permit parameters while also making organic-loading trends harder to read, since nitrification onset (and its oxygen demand) is erratic and temperature/age-dependent in an uninhibited bottle test.
Orthophosphates ($PO_4^{3-}$, $HPO_4^{2-}$, $H_2PO_4^-$, depending on pH) are the simplest inorganic phosphate species and the only form directly usable by algae and aquatic plants without further transformation — they are what is measured directly by the colorimetric (molybdenum blue) method and are the immediate eutrophication-driving fraction. Polyphosphates are condensed inorganic phosphates (chains or rings of phosphate units linked by $P$–$O$–$P$ bonds), used industrially as detergent builders, corrosion inhibitors and water-treatment scale inhibitors; they are not directly bioavailable and must first hydrolyze back to orthophosphate (a slow abiotic or biologically mediated reaction) before algae can use them. Organic phosphates are phosphorus atoms bound within organic molecules of biological origin (cell tissue, detergent residues, decaying plant/animal matter, biosolids); like polyphosphates, they must be biologically or chemically mineralized to orthophosphate before becoming bioavailable. The three fractions sum to total phosphorus, and the distinction matters for eutrophication management because a water body can carry a high total-P load that is mostly non-bioavailable (polyphosphate/organic) in the short term, with the bioavailable orthophosphate fraction the better near-term predictor of algal response.
