16-Civ-A3 Elementary Environmental Engineering · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, May 2016 — 98-Civ-A3 Environmental Engineering. Three hours, closed book with one candidate-prepared double-sided aid sheet and an approved Casio or Sharp calculator. Seven problems of 20 marks each; any five constitute a complete paper and only the first five answered are marked, for a maximum of 100 marks. Section marks appear in brackets in the left margin and are repeated in the Marking Scheme on page 6. All seven problems are solved here, because the set is a study resource rather than an exam script.
Reference texts. Davis & Cornwell, Introduction to Environmental Engineering (McGraw-Hill); Mihelcic & Zimmerman, Environmental Engineering: Fundamentals, Sustainability, Design (Wiley); Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery; Crittenden et al. (MWH), Water Treatment: Principles and Design; Health Canada, Guidelines for Canadian Drinking Water Quality (GCDWQ); CCME, Canadian Environmental Quality Guidelines; the federal Impact Assessment Act and IAAC guidance; Engineers Canada / EGBC Code of Ethics.
Check: assumed data. Two readings are adopted and used consistently throughout. (1) In Problem 1(ii) the decomposition is taken as the stoichiometric reaction 2 N2O5 → 2 N2O4 + O2, the only balanced route from N2O5 to the two named products, and the vessel is closed at fixed volume and temperature so that pressure tracks total moles. (2) In Problem 2(ii) the printed atomic weights (Ca = 40, Mg = 24, Fe = 56, H = 1, C = 12, O = 16) are used exactly as given rather than the textbook values, and the printed line “mg2+ 40 mg/L” is read as Mg2+ = 40 mg/L. Note 1 on page 1 expressly invites the candidate to state such interpretations.
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.
Particles in a raw surface water span roughly seven orders of magnitude in size, from dissolved organic molecules and colloidal clay of a few nanometres, through bacteria and algal cells of one to ten micrometres, up to silt and detritus that settle out unaided. The three processes named in the question are not alternatives; they are a sequence engineered so that each one hands the next a particle population it can actually handle, and the reason the sequence exists at all is that the smallest and most numerous particles — the colloids — will neither settle nor strain out in any economic time or through any economic medium.
Coagulation addresses the reason colloids persist. Clay, silica and natural organic matter carry a net negative surface charge in natural waters, and the resulting electrostatic repulsion between the diffuse double layers keeps particles apart indefinitely; Brownian motion is more than sufficient to hold them in suspension. Coagulation is the chemical destabilisation of that suspension. A hydrolysing metal salt — aluminium sulphate, polyaluminium chloride or ferric chloride — is dosed into a rapid-mix basin with a velocity gradient of several hundred reciprocal seconds and a detention time of well under a minute, so that the coagulant is dispersed before its hydrolysis products have finished forming. Destabilisation occurs by two mechanisms that operate at different doses: at low dose, the positively charged hydrolysis species adsorb on the particles and compress or neutralise the double layer; at higher dose, the metal hydroxide precipitates as a voluminous floc and physically enmeshes the colloids, the mechanism known as sweep coagulation, which is what most Canadian surface-water plants actually run. Coagulant dose and pH are selected together on the jar test, because each coagulant has a pH window in which its hydroxide is least soluble. Coagulation removes essentially nothing by itself — it only makes removal possible.
Flocculation then converts the destabilised primary particles into settleable and filterable aggregates. It is a transport step, not a chemical one: gentle mixing at velocity gradients of the order of 10 to 60 s−1 over 20 to 40 minutes brings particles into contact often enough for the now-unrepelled surfaces to stick. Too little mixing and the collision rate is inadequate; too much and the shear tears the growing floc apart, so basins are commonly tapered, with the highest gradient in the first stage and the gentlest in the last, so that fragile large floc is not broken just before it enters the settling basin. The floc leaving flocculation is large enough for gravity sedimentation to remove the bulk of the solids load, typically taking a raw turbidity of tens of NTU down to a few NTU, and that is the point of the step: sedimentation is far cheaper per kilogram of solids removed than filtration, so the flocculation basin exists largely to protect the filters from a solids load they would otherwise have to carry.
Filtration is the final particle barrier and the one that determines whether the water is safe. Water passes down through a granular bed — conventionally sand, more usually a dual anthracite-over-sand medium so that the coarse layer sits on top and the bed filters in depth rather than clogging at the surface. Removal is by depth filtration, in which particles are transported to the grain surfaces by interception, sedimentation and diffusion and then attach there; straining alone accounts for very little, because the pore openings are far larger than the particles being removed. This is precisely why filtration cannot be run on an uncoagulated water: attachment depends on the same surface destabilisation that coagulation provides, and a filter fed with charged colloids passes them straight through. Filtration is credited with substantial pathogen log-removal, and it is the principal barrier against chlorine-resistant Cryptosporidium oocysts. The integrated train is therefore: coagulation destabilises, flocculation aggregates, sedimentation removes the mass, filtration polishes to the safety target, and disinfection inactivates whatever survives. Removing any one step degrades the next, which is the practical meaning of the multi-barrier principle that underlies Canadian drinking-water policy after Walkerton and North Battleford.
Why turbidity indicates filtration performance. Turbidity is an optical measurement of light scattered at 90 degrees by suspended particles, reported in nephelometric turbidity units. It is a good surrogate for filter performance for four related reasons. It is continuous and instantaneous: an on-line turbidimeter on each filter effluent reports every few seconds, whereas a microbiological assay takes 18 to 24 hours, by which time the water is in the distribution system and possibly in consumers. It is sensitive in the right size range, because light scattering is most efficient for particles of roughly the wavelength of light, one to ten micrometres — which is the size of Giardia cysts, Cryptosporidium oocysts and the floc fragments that carry them. It is diagnostic of failure modes: the characteristic turbidity spike at the start of a filter run after backwash, and the slow rise that signals breakthrough near the end of a run, are both visible in the trace and are managed by filter-to-waste and by terminating the run on turbidity rather than on head loss alone. And it is regulated as a performance standard: the GCDWQ require chemically assisted filtration to produce effluent turbidity of 0.3 NTU or less in at least 95 % of measurements in each month and never above 1.0 NTU, with individual filters monitored continuously. The important qualification is that turbidity is a surrogate, not a pathogen measurement — low turbidity does not prove absence of pathogens, and a filter can pass oocysts at compliant turbidity — which is exactly why it is paired with disinfection CT credit and indicator-organism monitoring rather than relied on alone.
Given. The reported average concentrations of divalent cations in the lake water near the salt mine, with the atomic weights the question supplies.
| Species | Concentration (mg/L) | Atomic weight (given) | Charge | Equivalent weight (mg/meq) |
|---|---|---|---|---|
| Ca2+ | 80 | 40 | 2+ | 20 |
| Mg2+ | 40 | 24 | 2+ | 12 |
| Fe2+ | 20 | 56 | 2+ | 28 |
| CaCO3 (reference) | — | 100 (from Ca 40 + C 12 + 3 × O 16) | 2 | 50 |
Find. Total hardness expressed in mg/L as CaCO3, and the classification of the water as soft or hard.
Approach. Build the CaCO3 reference from the given atomic weights, convert each divalent cation to its CaCO3 equivalent through equivalent weights, and sum.
Check: data plausibility. The concentrations given are not representative of open Lake Ontario water, which runs about 40 mg/L Ca, 8 mg/L Mg and total hardness near 130 mg/L as CaCO3; more pointedly, 20 mg/L of dissolved ferrous iron cannot persist in an oxygenated surface water near pH 8, where Fe(II) oxidises and precipitates as ferric hydroxide within minutes. The values are treated here exactly as given, as a localised plume near the salt mine, and are not adjusted. The classification is unaffected: calcium and magnesium alone put the water well into the hard band.
The engineering consequence is worth one sentence beyond the arithmetic. At 367 mg/L as CaCO3 the water would justify softening, and because magnesium supplies nearly half the hardness the softening would have to be the excess-lime process taken to pH 11 or above to precipitate Mg(OH)2, rather than the simpler single-stage lime treatment that removes carbonate hardness as CaCO3 near pH 10.3. That distinction drives both the chemical demand and the sludge volume, so it is the practical reason the calculation is split by cation rather than reported only as a total.
| Contribution | mg/L as CaCO3 |
|---|---|
| Ca2+ (80 mg/L) | 200.00 |
| Mg2+ (40 mg/L) | 166.67 |
| Fe2+ (20 mg/L) | 35.71 |
| Total hardness, Ca + Mg (conventional) | 366.67 |
| Total hardness including Fe (strict) | 402.38 |
| Classification | Hard (three-band) / very hard (GCDWQ four-band) |
Ammonia — impact. Ammonia is released from the hydrolysis of urea and the anaerobic degradation of proteins in sewage. In the sewer atmosphere and at manhole or wet-well openings it is a severe respiratory and ocular irritant: it is extremely soluble, so it deposits in the upper airway and on the moist surfaces of the eyes, producing burning, lacrimation and, at concentrations of a few hundred parts per million, chemical burns and pulmonary oedema. The occupational exposure limits applied in Canadian jurisdictions are of the order of 25 ppm as an eight-hour time-weighted average and 35 ppm short-term, and its sharp odour gives warning well below those levels — which is the important contrast with hydrogen sulphide below. The environmental impact is separate and arises where ammonia-bearing effluent or sidestream reaches the receiving water: un-ionised ammonia is acutely toxic to fish at well under 1 mg/L, and its nitrification exerts an oxygen demand roughly 4.6 g O2 per gram of nitrogen oxidised, depressing dissolved oxygen downstream, while the nitrogen itself contributes to eutrophication.
Ammonia — engineering strategy. The controlling measure is forced ventilation of the sewer and wet-well headspace, sized to a defined number of air changes per hour and discharged through a treatment unit rather than to the street: a chemical scrubber using dilute sulphuric acid converts ammonia to ammonium sulphate with very high efficiency, and biofilters or biotrickling filters achieve the same result biologically at lower operating cost. The measure is a source-and-path control, and it is paired with confined-space entry procedure — continuous gas monitoring, purge before entry, supplied-air respiratory protection, attendant and retrieval — because ventilation reduces but does not eliminate the hazard. On the receiving-water side, the corresponding engineering answer is biological nitrification, and where nitrogen limits are strict, nitrification–denitrification at the wastewater treatment plant.
Hydrogen sulphide — impact. Hydrogen sulphide is generated by sulphate-reducing bacteria in the anaerobic slime layer of the submerged sewer wall and is released where turbulence strips it from solution, typically at drops, junctions and force-main discharge points. Its health impact is more insidious than ammonia's for a specific physiological reason: at low concentration it has an unmistakable rotten-egg odour, but at roughly 100 ppm and above it rapidly paralyses the olfactory nerve, so the warning property disappears exactly when the hazard becomes serious. It is a chemical asphyxiant that inhibits cytochrome oxidase, and at concentrations above about 700 ppm a single breath can cause immediate collapse and death; it is the leading cause of fatal confined-space incidents in the collection system. Its environmental and asset impact is crown corrosion — hydrogen sulphide released to the sewer headspace is oxidised by Thiobacillus on the moist unsubmerged concrete to sulphuric acid, which destroys the crown of concrete sewers and can consume a pipe wall in a decade.
Hydrogen sulphide — engineering strategy. The preferred control acts at the source, in the liquid phase, before the gas ever forms: dosing the upstream sewer or force main with an oxidant or a precipitant — nitrate salts to give the biofilm an alternative electron acceptor so that sulphate reduction does not occur, or iron salts to precipitate sulphide as insoluble FeS — and reducing the septicity that drives generation by limiting force-main detention time and by adding air or pure oxygen. Where release cannot be prevented, the path and receptor controls are headspace ventilation to a scrubber or biofilter, and, for asset protection, corrosion-resistant materials such as PVC, HDPE or calcareous or epoxy-lined concrete at known release points. As with ammonia, confined-space entry procedure with continuous multi-gas monitoring is the final receptor-level control, and it is mandatory under provincial occupational health and safety regulation rather than discretionary.
