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.
Issue 1 — vulnerability to contamination and the consequent treatment obligation. A surface water source is open to the atmosphere and directly connected to its whole catchment, so it receives runoff, agricultural drainage, wildlife and livestock faecal loading, and any upstream discharge, essentially without delay. Its quality is therefore variable on the time scale of a single storm: turbidity, colour and pathogen loading can rise by an order of magnitude within hours of a rainfall event, and Giardia and Cryptosporidium must be presumed present. Canadian practice consequently requires a full multi-barrier train — coagulation, flocculation, sedimentation, filtration and disinfection — with continuous turbidity monitoring and defined log-removal credits. Groundwater from a properly constructed well in a confined aquifer is naturally protected: the overlying material provides filtration and long residence time, so it is usually microbiologically secure and of stable quality year-round, and it may require only disinfection. The management issue for the small community is therefore capital and operational: the surface source imposes a plant whose complexity is disproportionate to the population and demands a certified operator on site, which is the classic failure mode for small Canadian systems, whereas the groundwater source can often be run with a simple, robust installation. The qualification is that groundwater under the direct influence of surface water — a shallow or poorly sealed well, or one in fractured rock or karst — must be treated as a surface water; the hydrogeological determination is itself a management obligation, and it is precisely what was missed at Walkerton.
Issue 2 — quantity, reliability and the character of the contaminants that remain. Surface water availability is seasonal and can be severely constrained in late summer or under ice, and abstraction competes directly with instream flow needs, downstream users and, in many parts of Canada, licensed water rights and Indigenous rights and title; drought and low-flow conditions bind exactly when demand peaks. Groundwater offers a large natural storage buffer that smooths seasonal variation, but it is finite in a different sense: sustained abstraction above the recharge rate mines the aquifer, drawing down water levels, dewatering neighbouring domestic wells, inducing land subsidence and, in coastal or interior saline settings, drawing in saltwater or connate brine, all of which are slow to appear and very slow to reverse. The contaminant character also differs: groundwater is more likely to carry dissolved constituents acquired from the geology — hardness, iron and manganese, arsenic, fluoride, radionuclides, hydrogen sulphide — which require different and sometimes expensive processes such as oxidation and filtration, ion exchange, adsorption or membranes, and which do not respond to disinfection at all. So the trade is real rather than one-sided: surface water tends to present a microbiological and turbidity problem that is variable but well understood and treatable by a conventional train, while groundwater tends to present a chemical problem that is stable but may be intractable and, once the aquifer is over-drafted or contaminated, effectively permanent. For the small rural community, the decision turns on which risk the community can actually operate and finance over decades, and the defensible answer usually includes source-water protection planning — wellhead protection areas or watershed control — because prevention is far cheaper than treatment at that scale.
| Contaminant type | Treatment method | How it works and why it suits this contaminant |
|---|---|---|
| Suspended solids | Coagulation, flocculation, sedimentation and granular media filtration | A hydrolysing metal coagulant destabilises the negatively charged colloids, gentle flocculation aggregates them into settleable floc, sedimentation removes the bulk of the mass, and a dual-media anthracite-over-sand filter captures the remainder by depth filtration. This is the only economic route to the colloidal fraction, which will neither settle nor strain out untreated, and performance is verified continuously by filter-effluent turbidity against the GCDWQ limit of 0.3 NTU in 95 % of measurements. |
| Microbial contaminants | Disinfection — ultraviolet irradiation, with a chlorine residual for the distribution system | UV at 254 nm is absorbed by nucleic acids and forms pyrimidine dimers that prevent replication, inactivating bacteria, viruses and — critically — the chlorine-resistant protozoa Giardia and Cryptosporidium, at doses of about 40 mJ/cm2, with no disinfection by-product formation. It requires low turbidity and good UV transmittance, which is why it follows filtration, and it leaves no residual, so a chlorine dose is added afterwards to protect the distribution system. Chlorination alone, delivering the required CT, is the alternative where UV is not practical. |
| Metals | Oxidation followed by precipitation and filtration — with ion exchange, adsorption or membranes for the dissolved metals that route cannot reach | Dissolved iron and manganese are oxidised by aeration, chlorine or permanganate to insoluble Fe(OH)3 and MnO2, then removed as particles by filtration; pH adjustment with lime precipitates many heavy metals as hydroxides, each with its own minimum-solubility pH. Metals that remain dissolved — arsenic, chromium, lead at trace level — are removed instead by adsorption on activated alumina or granular ferric media, by ion exchange, or by reverse osmosis or nanofiltration. The distinguishing point is that metals are dissolved species, so they must first be converted to a particulate or captured on a solid phase; no amount of filtration or disinfection touches them in solution. |
The three methods are deliberately different in kind — a physical-chemical particle-removal train, a physical inactivation process, and a chemical phase-transfer process — which is what the question's phrase “a different treatment method for each type of contaminant” is testing. They are also ordered correctly as a train: solids removal must precede disinfection, because particles shield organisms and absorb UV, and metals oxidation is placed so that the resulting precipitate is captured by the same filter that removes the natural turbidity.
(a) Anoxic versus aerobic treatment (3 marks). Electron acceptor: aerobic treatment uses dissolved molecular oxygen, maintained at roughly 1.5 to 2 mg/L in the mixed liquor; anoxic treatment operates in the absence of dissolved oxygen (below about 0.2 mg/L) but in the presence of combined oxygen, with nitrate or nitrite serving as the terminal electron acceptor. Reactions and products: aerobic conditions oxidise carbonaceous BOD to CO2 and water and, with sufficient sludge age, allow autotrophic nitrification of ammonia to nitrite and nitrate; anoxic conditions drive heterotrophic denitrification, reducing nitrate to nitrogen gas, which leaves the system to atmosphere — so anoxic zones are where nitrogen is actually removed rather than merely converted. Energy and process implications: aeration is the dominant energy consumer in a wastewater plant, often more than half of total site power, whereas an anoxic zone needs only mixing; denitrification also recovers roughly 2.9 g of oxygen equivalent and about half the alkalinity consumed by nitrification per gram of nitrate-nitrogen reduced, which is why pre-anoxic configurations such as the Modified Ludzack–Ettinger process place the anoxic zone first and recycle nitrate to it, using the incoming raw BOD as the carbon source instead of purchased methanol. Anoxic is distinct from anaerobic, in which neither free nor combined oxygen is available and sulphate reduction and methanogenesis occur.
(b) Primary versus secondary disinfection (3 marks). Purpose: primary disinfection achieves the required inactivation of pathogens within the treatment plant, meeting a defined log-reduction target; secondary disinfection maintains a residual throughout the distribution system to suppress regrowth, control biofilm, and provide a sentinel that reveals contamination from a main break, cross-connection or intrusion event. Location and time scale: primary disinfection occurs in a contact chamber over minutes to tens of minutes and is measured as a CT product; secondary disinfection must persist for hours to days of water age, out to the most remote dead end in the system. Agents: primary disinfection may use free chlorine, ozone, chlorine dioxide or UV — ozone and UV are powerful primary disinfectants but leave no residual and therefore cannot serve as secondary; secondary disinfection uses free chlorine or, increasingly, chloramine, which is a weaker disinfectant but far more persistent and forms fewer regulated trihalomethanes, making it the common choice for large or long distribution systems. The practical corollary is that a plant using UV as its primary barrier must still add a chlorine-based residual afterwards.
(c) Primary versus tertiary wastewater treatment (3 marks). Mechanism: primary treatment is purely physical — screening, grit removal and plain gravity sedimentation of settleable solids and floating grease; tertiary treatment is whatever advanced physical, chemical or biological process follows secondary treatment, such as effluent filtration, chemical phosphorus precipitation, biological nutrient removal, membrane filtration, activated carbon, or disinfection. Performance: primary sedimentation removes roughly 50 to 70 % of suspended solids and 25 to 40 % of BOD and essentially no dissolved nutrients; tertiary treatment targets the residual, producing effluent with BOD and suspended solids of a few milligrams per litre and total phosphorus below 1 mg/L, often to 0.1 mg/L, with nitrogen removal to single digits where required. Purpose, cost and position: primary treatment is the first liquid-stream process, cheap, energy-light and universal, and it exists largely to protect and unload the biological stage that follows; tertiary treatment is the last stage, is driven by a specific receiving-water sensitivity or a reuse standard rather than by the plant's own needs, and carries by far the highest incremental capital and operating cost per kilogram of pollutant removed. Between them sits secondary treatment, the biological step — activated sludge, trickling filter or lagoon — that removes the soluble and colloidal organic matter neither of the other two addresses. In the Canadian regulatory frame, the federal Wastewater Systems Effluent Regulations set a national secondary-equivalent baseline, and tertiary requirements come from provincial or site-specific receiving-water objectives.
