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.
When chlorine is added progressively to a water containing ammonia and oxidizable organic matter, the measured chlorine residual does not rise linearly with dose — it traces the characteristic four-zone curve below.
Chlorine demand is the quantity of applied chlorine consumed by reactions with reducing agents in the water — ferrous iron, manganous manganese, sulfide, nitrite, and readily oxidizable organic matter — before any measurable residual appears. This is Zone I of the curve: chlorine dose rises from zero but the residual stays essentially at zero because every increment of applied chlorine is immediately consumed by these instantaneous oxidation reactions; demand is simply the applied dose at which the curve first departs from the zero-residual axis.
Formation of chloramines and organochlorines occurs in Zone II, once the instantaneous reducing-agent demand is satisfied and further chlorine begins reacting with ammonia and organic nitrogen/organic matter present in the water. Chlorine reacts stepwise with ammonia, $NH_3+HOCl\rightarrow NH_2Cl+H_2O$ (monochloramine), and, as the $Cl_2$:$NH_3$-N molar ratio rises, further to dichloramine ($NHCl_2$) and eventually trichloramine ($NCl_3$); chlorine also reacts with natural organic matter to form organochlorines (including regulated disinfection by-products such as trihalomethanes and haloacetic acids). Because chloramines and organochlorines do register on a chlorine residual test, the measured residual rises through Zone II up to a local maximum at point A, the "hump," which occurs at roughly a 5:1 $Cl_2$:$NH_3$-N mass ratio where monochloramine formation is maximal.
Beyond point A (Zone III), additional applied chlorine begins to oxidize the chloramines themselves — through reactions producing nitrogen gas, nitrogen trichloride and other oxidized by-products — so the measured residual actually falls even though dose keeps increasing, reaching a minimum at point B, the breakpoint. At the breakpoint essentially all the ammonia-derived combined residual and the initial demand have been satisfied and destroyed. Breakpoint chlorination is the practice of dosing chlorine deliberately past point B (into Zone IV) so that any further applied chlorine appears as free available chlorine ($HOCl$/$OCl^-$), which rises roughly linearly with dose beyond the breakpoint; operating past the breakpoint is the standard way utilities guarantee a free (not combined) chlorine residual, since free chlorine is a far stronger and faster disinfectant, at the cost of a higher chlorine dose and typically greater organochlorine (DBP) formation than stopping in the combined-residual zone.
Given.
| Parameter | Reported | GCDWQ reference value | Assessment |
|---|---|---|---|
| Sulfate ($SO_4^{2-}$) | 150 mg/L | Aesthetic objective (AO) 500 mg/L | Well below AO — acceptable |
| Nitrate (as $NO_3^-$) | 10 mg/L | Health-based MAC 45 mg/L as $NO_3^-$ (equivalently 10 mg/L as N) | Well below MAC if reported as $NO_3^-$; AT the MAC if the 10 mg/L is actually reported as N — unit must be confirmed |
| Chloride ($Cl^-$) | 100 mg/L | Aesthetic objective (AO) 250 mg/L | Well below AO — acceptable |
Find. Suitability of the sample for its intended (drinking-water) use, with comments and recommendations as the consulting engineer of record.
Approach. Compare each reported concentration to the corresponding GCDWQ aesthetic objective (taste/corrosion/staining concern) or maximum acceptable concentration (health-based limit), and give a combined suitability recommendation with monitoring guidance.
| Parameter | % of GCDWQ reference value | Recommendation |
|---|---|---|
| Sulfate | 30% of AO (500 mg/L) | Suitable, no action |
| Nitrate | 22% of MAC (45 mg/L as $NO_3^-$) | Suitable as reported; confirm reporting basis (as $NO_3^-$ vs. as N) and monitor trend |
| Chloride | 40% of AO (250 mg/L) | Suitable, no action |
As consulting engineer, my comments and recommendations are: (1) as sampled and reported, all three parameters comply with the GCDWQ and the water is suitable for its intended use with respect to these three parameters – no treatment modification is required on their account alone. (2) I would formally confirm with the testing laboratory whether the nitrate result is reported as $NO_3^-$ or as N before closing this file, since the "as N" interpretation would place the result exactly at the health-based MAC rather than comfortably under it — a materially different risk position, especially if the supply serves infants or is used for infant formula preparation. (3) Because nitrate, sulfate and chloride together are classic indicators of agricultural, septic or road-salt influence on a source, I would recommend establishing (or continuing) routine quarterly monitoring of all three, particularly around spring runoff/fertilizer-application season, rather than treating this single sample as a permanent clearance — a rising trend in any of the three, even while still below its respective guideline, is an early warning that source protection or treatment upgrades should be evaluated proactively rather than reactively.
