16-Civ-A3 Elementary Environmental Engineering · December 2017
Question 3 of 7: Particle Characteristics, Solution Chemistry and Emissions
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
Paper format. National Exams, December 2017 — 16-Civ-A3 Elementary Environmental Engineering. Three hours; closed book with one candidate-prepared 8½ × 11 double-sided aid sheet; approved Casio or Sharp calculator only. Seven problems are printed, each worth 20 marks, and any five constitute a complete paper (maximum 100 marks). All seven are solved here, because the set is intended as a study resource rather than an exam script. Section marks are shown in brackets at the left margin of each question and are reproduced from the final-page Marking Scheme.
Reference texts.
Davis, M.L. & Cornwell, D.A., Introduction to Environmental Engineering, 5th ed., McGraw-Hill — material and energy balances, reactor kinetics, hardness, disinfection, water and wastewater unit processes.
Masters, G.M. & Ela, W.P., Introduction to Environmental Engineering and Science, 3rd ed., Pearson — mass/energy balances on power plants, air-emission estimation, growth–pollution linkages.
Fogler, H.S., Elements of Chemical Reaction Engineering, 5th ed., Prentice Hall — CSTR design and gas-phase stoichiometry with change in total moles.
Canadian context. Answers use the Canadian regulatory frame: the Guidelines for Canadian Drinking Water Quality (GCDWQ) and Canadian Environmental Quality Guidelines (CCME), provincial water and wastewater regulations, the federal Impact Assessment Act / BC Environmental Assessment Act, and the Engineers Canada / EGBC code of ethics whose canons appear in Problem 2(iii).
Question 3: Particle Characteristics, Solution Chemistry and Emissions (20 marks)
Part (i) — Combined primary and secondary clarification (8 marks)
Clarification (sedimentation) removes particles by gravity, but a single stage cannot remove the full range of solids in wastewater, so a conventional plant uses two clarifiers in series with a biological step between them. Primary clarification follows screening and grit removal and targets the readily settleable solids — discrete organic and inorganic particles dense enough to settle in an hour or two of quiescent detention. A primary clarifier typically removes 50–70% of suspended solids and 25–40% of BOD, lightening the load on the biology and collecting raw primary sludge from the floor.
The particles that do not settle — fine colloidal and dissolved organic matter — are too small for gravity alone. The biological (secondary) process, most commonly activated sludge, converts this dissolved and colloidal material into microbial floc: bacteria metabolise the soluble BOD and agglomerate into larger, settleable biological particles. The job of the secondary clarifier is then to separate that floc from the treated water by sedimentation, producing a clear effluent and a concentrated sludge, part of which is returned to the aeration tank (return activated sludge) to maintain the microbial population and the rest wasted. The two clarifiers thus divide the work: primary removes what is already settleable, the biology renders settleable what was not, and secondary clarification removes that. Together they achieve the roughly 85–95% suspended-solids and BOD removal that a single stage cannot.
Simplified schematic of a conventional wastewater train combining primary and secondary (biological) clarification. Primary sludge and waste/return activated sludge are drawn from the two clarifiers.
Find. The total hardness expressed as mg/L CaCO3, and the hardness classification.
Approach. Hardness is the sum of the multivalent (chiefly divalent) metal cations expressed on a common CaCO3 basis. Convert each ion through its equivalent weight relative to that of CaCO3 (equivalent weight 50), i.e. $\text{as CaCO}_3 = C\times\dfrac{50}{\text{EW}_{\text{ion}}}$ with $\text{EW}=\text{atomic weight}/2$.
Equivalent weights. All three cations are divalent, so $\text{EW}_{\text{Ca}}=40/2=20$, $\text{EW}_{\text{Mg}}=24/2=12$, $\text{EW}_{\text{Cu}}=64/2=32$ (g/eq).
Convert calcium.$50\times\dfrac{50}{20}=125\ \text{mg/L as CaCO}_3.$
Convert magnesium.$60\times\dfrac{50}{12}=250\ \text{mg/L as CaCO}_3.$ Note that magnesium dominates despite the modest mg/L, because its equivalent weight is the smallest — each mg of Mg carries about 1.7× the charge of a mg of Ca.
Total (conventional) hardness. Hardness is defined by Ca2+ and Mg2+:
$$\boxed{\text{TH}=125+250=375\ \text{mg/L as CaCO}_3.}$$
The copper term. As a strict divalent metal cation, Cu2+ converts to $30\times\dfrac{50}{32}=46.9\ \text{mg/L as CaCO}_3$, which would raise the all-cation total to about 422 mg/L. Conventionally, however, copper is reported and regulated as a contaminant, not as hardness.
Check: The 30 mg/L of dissolved copper is itself the more serious finding: it is far above the CCME/GCDWQ copper objective (of order 1–2 mg/L) and is toxic to aquatic life. It is quoted here as a hardness-equivalent for completeness, but the water's hardness classification is based on Ca + Mg = 375 mg/L, and the copper should be flagged as a treatment target in its own right (consistent with a "near a gold/copper mine" source).
Results — Problem 3(ii)
Ion
Concentration
As CaCO3
Ca2+
50 mg/L
125 mg/L
Mg2+
60 mg/L
250 mg/L
Total hardness (Ca + Mg)
375 mg/L — very hard
Cu2+ (contaminant, not hardness)
30 mg/L
46.9 mg/L
Classification. On the common three-band scale (soft < 75, moderately hard 75–150, hard > 150 mg/L as CaCO3) the water at 375 mg/L is firmly hard; on the four-band GCDWQ scale (soft < 60, moderately hard 60–120, hard 120–180, very hard > 180) it is very hard.
Part (iii) — Effluent disinfection with ozone: precautions (5 marks)
Selecting ozone (O3): ozone is a powerful oxidant generated on site from air or oxygen by corona discharge and dosed into the final effluent in a contact chamber. It inactivates bacteria, viruses and the more resistant protozoan cysts quickly and, unlike chlorine, leaves no persistent chemical residual and forms no chlorinated organic by-products, so it is attractive where the receiving water is sensitive. Two important precautions in terms of environmental (and worker) impacts are:
Control ozone off-gas. Ozone is toxic and a respiratory hazard; un-reacted ozone leaving the contact chamber must be captured and destroyed (thermal or catalytic off-gas destructor) before venting, to protect operators and avoid a ground-level oxidant release.
Manage oxidation by-products and residual toxicity. Ozone can form bromate from bromide-bearing waters and other oxidation by-products; the process should be monitored and, where bromide is present, dose and contact controlled so that bromate stays within guideline limits. (Had chlorine been chosen, the parallel precautions would be to dechlorinate the effluent — residual chlorine is acutely toxic to fish — and to limit chlorinated disinfection by-products such as trihalomethanes.)
