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.
Particulate control devices are selected by particle size distribution, gas temperature and chemistry, dust loading, dust properties such as resistivity and stickiness, and the required outlet concentration. The three methods below span the practical range from coarse pre-cleaning to high-efficiency fine-particle capture, and each is characterised by the capture mechanism that governs its performance.
| Method | Capture mechanism | Advantage | Limitation | Most appropriate use |
|---|---|---|---|---|
| 1. Cyclone (inertial separator) | Gas enters tangentially into a cylindrical-conical body; the vortex imposes a centrifugal acceleration many times gravity, throwing particles to the wall where they slide to the hopper while the cleaned gas reverses and exits through the central vortex finder. Cut size is typically 5–10 µm for a conventional unit, 2–5 µm for a high-efficiency small-diameter design. | Simple, rugged and cheap — no moving parts, no consumables, low capital and maintenance cost, tolerant of high temperature, high dust loading and abrasive or corrosive dust. | Poor efficiency on fine particulate: collection falls off sharply below about 5 µm, so it captures little PM2.5 and cannot meet a modern outlet standard on its own. | As a pre-cleaner ahead of a baghouse or precipitator, to strip the coarse and abrasive fraction and reduce the load on the downstream device — for example on a sawmill, grain-handling, cement-kiln or wood-fired boiler gas stream. |
| 2. Fabric filter (baghouse) | Dust-laden gas passes through woven or felted fabric bags; capture occurs by interception, impaction and diffusion, and once a dust cake forms the cake itself becomes the filtering medium and does most of the work. Bags are cleaned periodically by pulse-jet, reverse air or shaker action. Efficiencies exceed 99.9 % including on submicron particles. | Very high and consistent efficiency across the whole size range, including PM2.5 and submicron particulate, and largely independent of the electrical resistivity of the dust; outlet concentrations of a few milligrams per cubic metre are routine. | Meaningful pressure drop and therefore fan energy; bags have finite life and are a recurring cost; the fabric is vulnerable to high temperature, to moisture and condensation below the acid dew point (blinding), and to sparks or sticky dust. | Where a high-efficiency fine-particle standard must be met on a dry, moderate-temperature stream — municipal waste incinerators, cement plants, metallurgical furnaces and coal-fired boilers, usually with lime and activated-carbon injection upstream so acid gases and mercury are captured in the same cake. |
| 3. Electrostatic precipitator, and its mobile-source analogue the diesel particulate filter | ESP: a high-voltage corona charges the particles, which then migrate to grounded collecting plates under the electric field and are periodically rapped into hoppers; efficiency follows the Deutsch–Anderson relation, $\eta = 1 - e^{-wA/Q}$, where $w$ is drift velocity, $A$ collecting area and $Q$ gas flow. DPF: a wall-flow ceramic monolith on a diesel exhaust traps soot in the porous wall and is regenerated by periodic oxidation. | Very high efficiency at very low pressure drop, so the fan energy penalty is small even on enormous gas volumes; handles high temperature and high flow continuously with low maintenance. The DPF likewise removes over 90 % of diesel soot, including the black-carbon fraction that matters for climate. | High capital cost and a large footprint; performance depends strongly on dust resistivity, so it degrades on very high- or very low-resistivity ash and is sensitive to changes in fuel or process conditions. The DPF requires low-sulphur fuel and a reliable regeneration strategy, and plugs if either fails. | ESP: very large fixed sources with high gas volumes — coal-fired utility boilers, cement kilns, pulp-mill recovery boilers. DPF: the mobile-source case the question asks about — on-road heavy trucks and buses and off-road construction equipment, retrofitted or factory-fitted with ultra-low-sulphur diesel. |
Two selection principles emerge from the matrix and are worth stating explicitly. First, the methods are complementary rather than competing: a real installation commonly places a cyclone ahead of a baghouse or ESP, so that the cheap device removes the mass and the expensive device removes the fine fraction that determines compliance and health impact. Second, the health and climate significance is concentrated in exactly the size range the cyclone cannot reach — PM2.5 penetrates to the alveoli and carries the black carbon that forces climate directly — so a control strategy judged by mass removed can look excellent while achieving very little of what matters. Efficiency should be assessed by size fraction, not by total mass.
The three approaches are ordered below by the waste-management hierarchy, and the prioritisation the question asks for follows the same order on both criteria — environmental benefit and cost recovery — when assessed over the 30-year life cycle of the site. The reason they coincide is that airspace in an engineered landfill is a capital asset with a finite quantity, so every cubic metre not consumed defers the cost of the next cell and ultimately the next site.
| Priority | Approach | What it does | Environmental benefit | Cost recovery over 30 years |
|---|---|---|---|---|
| 1 (highest) | Source reduction and reuse — waste minimisation at the generator | Extended producer responsibility and packaging stewardship, pay-as-you-throw variable-rate collection, reuse and repair networks and building-material salvage, procurement standards, public education. Typically diverts 10–20 % of the stream. | Highest. It is the only approach that avoids the impact entirely rather than managing it: no collection, no processing, no residue, and it also avoids the upstream extraction, manufacturing and transport burdens embodied in the product. Nothing downstream can match a tonne never generated. | Highest, and the least capital-intensive: essentially no infrastructure, funded by policy and program cost, while avoided collection, tipping and airspace costs accrue every year for the full 30 years. Pay-as-you-throw shifts cost to the generator and typically cuts disposal 15–20 % on its own. |
| 2 | Recycling and organics diversion — materials recovery and composting or anaerobic digestion | Curbside source separation with a materials recovery facility for containers, paper, metals and plastics; separate organics collection to windrow or in-vessel composting or to anaerobic digestion producing biogas and digestate. Together these commonly divert 40–60 % of the municipal stream by mass. | High and, for organics, disproportionate: organics are the source of essentially all landfill methane, so diverting them attacks the site's largest climate impact and also its leachate strength, since readily degradable organics drive the organic load. Recycled metals and paper avoid large upstream energy burdens. | Moderate to good. Capital is required for the MRF, the organics facility and the collection fleet, and commodity revenue is volatile, but tipping-fee avoidance, compost and digestate sales, and biogas energy or renewable-fuel credits build a return that is normally realised within the 30-year horizon. Organics diversion also extends site life directly, since organics are a large volume fraction. |
| 3 | Volume reduction and energy recovery at the site — compaction and daily-cover practice, with landfill gas recovery and waste-to-energy for the residual | High-compaction equipment and thin-lift placement to raise in-place density; alternative daily cover to save airspace otherwise consumed by soil; landfill gas collection with flaring or electricity or renewable-natural-gas production; combustion of the residual stream with energy recovery, reducing mass by about 75 % and volume by about 90 %. | Real but lowest of the three, because it manages the waste rather than avoiding it. Gas capture is nevertheless the single highest-value climate measure available at the site, converting methane to CO2 and displacing fossil energy; combustion residues (bottom and fly ash) still require managed disposal and emission control. | Good but capital-intensive, and the payback is long. Compaction improvements pay back quickly in deferred airspace consumption; gas systems earn energy revenue and carbon credits and are in any event required by regulation at larger sites; waste-to-energy has the highest capital cost per tonne of any option and can only be justified at scale and with secure long-term tonnage, which itself conflicts with priority 1. |
Justification of the ranking. Environmental benefit follows the hierarchy directly: avoiding waste eliminates every downstream impact, recovering materials and organics displaces virgin production and removes the methane precursor, and site-based measures only mitigate what remains. Cost recovery ranks the same way over a 30-year horizon, for a reason specific to landfills. Airspace is the site's saleable asset, and it is consumed irreversibly; extending site life by diversion defers the capital cost of the next cell, of eventual siting and construction of a replacement facility — the single largest and most politically difficult expenditure a municipality faces — and of the post-closure care and monitoring period, which in Canadian practice runs 25 years or more after closure and is funded from a reserve accumulated during operation. A tonne diverted in year 5 therefore avoids not only that year's tipping cost but a proportionate share of closure and perpetual-care liability. Set against that, the site-based measures at priority 3 carry the largest capital exposure and the longest payback, and waste-to-energy in particular creates a contractual incentive to keep tonnage high, directly opposing priorities 1 and 2 — which is why it belongs last even though it is technically effective.
An integrated programme runs all three concurrently rather than choosing among them, because the residual stream always exists and must be managed to modern standards: composite liner, leachate collection and treatment, gas capture, progressive closure and long-term monitoring. The prioritisation is about where the incremental dollar goes, and over a 30-year life cycle that dollar buys the most environmental benefit and the most avoided cost at the top of the hierarchy.
