16-Civ-A3 Elementary Environmental Engineering · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, December 2014 — 98-Civ-A3 Environmental Engineering. Three hours, closed book with one candidate-prepared double-sided aid sheet. Seven problems, each worth 20 marks; any five constitute a complete paper (maximum 100 marks), and only the first five answers in the work book are marked. All seven problems are solved below, because the set is a study resource rather than an exam attempt.
Reference texts. Davis & Cornwell, Introduction to Environmental Engineering; Mihelcic & Zimmerman, Environmental Engineering: Fundamentals, Sustainability, Design; Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery; Crittenden et al. (MWH), Water Treatment: Principles and Design; CCME Canadian Environmental Quality Guidelines; Impact Assessment Agency of Canada, Impact Assessment Act guidance.
Check: two source inconsistencies are carried through deliberately. (1) Problem 1(i) prints the dipropylene glycol formula as C6H14O2 (118.2 g/mol); the actual compound is C6H14O3 (134.2 g/mol). (2) The same sentence states the dose as “76 kg (1000 mol)”, which implies a molar mass of 76 g/mol and matches neither formula — 1000 mol of the real compound is 134 kg. The mole quantity is the load-bearing datum for a closed-system balance, so 1000 mol is adopted and both molar masses are reported where a mass concentration is asked for. NOTE 1 on page 1 expressly invites this kind of stated assumption.
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.
The two air toxics selected are oxides of nitrogen (NOx), formed thermally in the engine and responsible for ground-level ozone and respiratory irritation, and diesel particulate matter, a IARC Group 1 human carcinogen carrying adsorbed polycyclic aromatic hydrocarbons deep into the lung. The three methods span the full control hierarchy: aftertreatment, in-cylinder modification, and fuel and source substitution.
| Air toxic | Method 1: catalytic aftertreatment | Method 2: in-cylinder and engine modification | Method 3: fuel substitution and source displacement |
|---|---|---|---|
| Air toxic 1 — NOx | How: a three-way catalyst on spark-ignition engines reduces NO over rhodium at stoichiometric air-fuel ratio; on diesels, selective catalytic reduction injects urea to convert NOx to N2 and water over a vanadium or zeolite catalyst. Advantage: very high efficiency, 90 % and above, with no penalty to fuel economy and in the SCR case an improvement. Limitation: requires precise closed-loop control and a light-off temperature, so it is largely ineffective during cold start and at low load, and the catalyst is permanently poisoned by sulphur or lead. Best applied: light-duty vehicle fleets and highway trucks operating at steady load, where exhaust temperature is reliably above light-off. | How: exhaust gas recirculation returns inert exhaust to the intake, lowering peak flame temperature and suppressing the strongly temperature-dependent thermal NOx formation; combined with retarded injection timing, high-pressure common-rail injection and turbocharging with charge-air cooling. Advantage: prevention at source, effective immediately from cold start, and requiring no consumable reagent. Limitation: the classic NOx–particulate trade-off — lowering flame temperature raises soot and generally costs a few per cent in fuel efficiency and engine durability. Best applied: off-road and stationary-duty diesel engines and older fleets where a full aftertreatment retrofit is not economic. | How: substitute compressed natural gas, renewable diesel or electric drive for conventional diesel, and displace the trip itself through transit, rail freight and land-use planning. Advantage: eliminates rather than treats the emission, and removes the co-pollutants and greenhouse gases at the same time; an electric bus emits no NOx at the roadside at all. Limitation: high capital cost and dependence on refuelling or charging infrastructure and on the emissions intensity of the electricity grid. Best applied: centrally fuelled municipal fleets — transit buses, refuse trucks, delivery vans — which return to a single depot nightly and operate in the dense urban areas where exposure is highest. British Columbia's largely hydroelectric grid makes electrification unusually effective there. |
| Air toxic 2 — diesel particulate matter | How: a wall-flow diesel particulate filter of cordierite or silicon carbide traps over 95 % of particles by mass and over 99 % by number, with an upstream oxidation catalyst and periodic active or passive regeneration to burn off the accumulated soot. Advantage: the single most effective particulate control available, and it removes the ultrafine fraction that dominates the health risk. Limitation: requires ultra-low-sulphur fuel, imposes backpressure and a small fuel penalty, and demands regular duty cycles hot enough to regenerate — filters plug on stop-start urban service. Best applied: highway trucks and any retrofit programme where low-sulphur fuel is guaranteed. | How: high-pressure common-rail injection with multiple injection events and optimised spray and swirl improves fuel-air mixing so that less soot forms in the fuel-rich core of the spray. Advantage: reduces the pollutant before it exists, and improves efficiency and noise at the same time. Limitation: bounded by the same NOx trade-off in the opposite direction, and confined to new engine designs since it cannot be retrofitted meaningfully. Best applied: new engine platforms, where it is the enabling technology that makes the aftertreatment package affordable. | How: ultra-low-sulphur diesel, biodiesel blends and natural gas or electric drive, together with idle-reduction measures such as anti-idling bylaws and truck-stop electrification. Advantage: low-sulphur fuel is a prerequisite that simultaneously cuts sulphate particulate directly and unlocks every catalytic control; anti-idling measures cost essentially nothing to implement. Limitation: refinery investment and modest fuel-cost increase; biodiesel can raise NOx slightly and has cold-weather handling limits relevant across most of Canada. Best applied: fleet-wide fuel specification as a regulatory measure, and idle control at schools, hospitals and freight terminals where receptors are close to the source. |
The methods are complementary rather than competing, and modern practice uses all three simultaneously: in-cylinder measures set the engine-out emission, aftertreatment removes what remains, and fuel and source substitution addresses the fraction that neither can reach.
The strategies are presented in the priority order that a 25-year assessment of environmental benefit and cost recovery supports, which follows the waste hierarchy: source reduction first, organics diversion second, materials recovery third.
| Solid waste | Strategy 1 (highest priority): source reduction and pay-as-you-throw | Strategy 2: organics diversion to composting and anaerobic digestion | Strategy 3: materials recovery and extended producer responsibility |
|---|---|---|---|
| Generation rate | Volume-based user fees, a bag or cart limit, mandatory recycling with disposal bans on divertible materials, reuse and repair centres, and packaging reduction agreements. Directly changes household behaviour and typically reduces disposed tonnage by 20 % to 40 % within two years of implementation. | Does not reduce gross generation, but removes the 30 % to 40 % of the residential stream that is food and yard waste from the disposal path through separate collection and processing. | Removes paper, containers, metals and glass — a further 20 % to 30 % — from disposal; extended producer responsibility additionally creates a manufacturer incentive to reduce packaging at design stage, which is the only mechanism in the table that lowers generation upstream of the household. |
| Conserve landfill needs | Every tonne not generated is a tonne of airspace never consumed and requires no processing facility whatsoever — the cheapest airspace a municipality will ever obtain. | Organics are dense, wet and are the source of essentially all landfill methane and leachate strength; removing them conserves airspace, cuts greenhouse gas emissions sharply, and improves the performance of the remaining landfill. | Substantial volume conservation, particularly from bulky low-density materials such as cardboard and film plastics, which consume airspace out of all proportion to their mass. |
| Environmental benefit (25 yr) | Highest. Avoids the full life-cycle burden — extraction, manufacture, transport and disposal — of goods never produced. Nothing downstream can match a tonne that was never made. | High. Methane has roughly 28 times the hundred-year warming potential of carbon dioxide, and diverted organics also return carbon and nutrients to soil, displacing synthetic fertiliser. | Moderate to high. Recycled aluminium requires about 5 % of the energy of primary production and recycled paper roughly 60 %; benefit varies widely by material and is sensitive to transport distance and contamination rate. |
| Cost recovery (25 yr) | Best. Low capital requirement, revenue-positive through user fees, and avoided disposal cost accrues from the first year. Payback is typically under three years. | Good. Moderate capital for a compost pad or digester, offset by tipping fees, compost sales, and in the anaerobic digestion case renewable natural gas or electricity revenue. Payback typically seven to twelve years, and the avoided cost of landfill gas management and post-closure care over 25 years is substantial. | Variable. Materials recovery facility capital is high and commodity revenue is volatile, so the programme is rarely self-financing on commodity sales alone; extended producer responsibility is what makes the economics work, by shifting the net cost to producers. Landfill airspace savings remain the dominant benefit. |
Justification of the ranking. The order follows the waste hierarchy for sound economic reasons, not merely conventional ones. Source reduction has the lowest capital intensity, the shortest payback and the largest avoided life-cycle burden, so it dominates on both axes. Organics diversion ranks second because it addresses the largest single divertible fraction and, uniquely, eliminates the methane and leachate liabilities that drive landfill operating and post-closure cost — a benefit that compounds strongly over a 25-year horizon. Materials recovery ranks third not because it lacks value but because its economics depend on commodity markets the municipality does not control, and its capital intensity is highest.
For a municipality that is genuinely land-constrained the three should be implemented together and in this sequence, with the landfill positioned explicitly as residual disposal only. Sequencing matters practically as well: source reduction and organics diversion should be established before sizing any new recovery facility or landfill cell, because both permanently reduce the tonnage that facility must be built to handle, and a facility sized on pre-diversion tonnage becomes a stranded asset.
