16-Civ-A3 Elementary Environmental Engineering · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, May 2015 — 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 answers in the work book are marked, for a maximum of 100 marks. The complete Marking Scheme is printed on page 8 and is reproduced against each question below. All seven problems are solved here, because this set is a study resource rather than an examination script.
Reference texts.
Check: compound naming in Problem 1(i). The question names the spilled liquid “dipropylene glycol” but gives its formula as C3H8O2 and its quantity as 38 kg (500 mol). C3H8O2 has a molar mass of 76.09 g/mol, and 38 000 g / 500 mol = 76.0 g/mol — so the formula, the mass and the mole count agree exactly with each other. It is the name that is wrong: C3H8O2 is propylene glycol (dipropylene glycol is C6H14O3, 134.2 g/mol). The solution therefore uses the self-consistent set (500 mol, 76.09 g/mol) and notes the naming slip, as NOTE 1 on page 1 invites. Nothing in the answer depends on the name.
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.
| Method | Air toxic targeted & principle | Advantage | Limitation | Most appropriate use |
|---|---|---|---|---|
| 1. Fabric filter (baghouse) with activated-carbon injection | Mercury and other trace metals, plus fine particulate. Powdered activated carbon is injected into the flue gas upstream of the filter; gaseous elemental and oxidised mercury adsorbs onto the carbon, and the resulting particle is captured on the filter cake along with metal-bearing fly ash. Filtration is by the dust cake itself, not the fabric, so collection efficiency rises through the cycle. | Very high and near-constant particulate collection efficiency — better than 99.9 % including the submicron fraction that electrostatic precipitators collect poorly — and efficiency is largely independent of the dust's electrical resistivity. | Sensitive to temperature and moisture: the gas must stay above its acid dew point to avoid blinding the bags, and above about 250 °C conventional fabrics fail. Bags are a consumable with a two-to-five-year life, pressure drop and fan energy are significant, and the collected carbon becomes a mercury-bearing solid waste needing disposal. | Coal-fired boilers, municipal waste incinerators, cement kilns and metallurgical furnaces — any dry, particulate-laden stream where a metal air toxic must be captured at high efficiency. |
| 2. Wet scrubber / flue-gas desulphurisation (absorption) | Acid gases — sulphur dioxide, hydrogen chloride, hydrogen fluoride. The gas is contacted with an alkaline liquid, typically a limestone or lime slurry in a spray tower, and the soluble acid gas transfers into the liquid and is neutralised, precipitating as calcium sulphite and then, with forced oxidation, as gypsum. | Simultaneously removes acid gases and residual particulate, handles high inlet concentrations and hot, moist gas streams, and produces a saleable gypsum by-product that can go to wallboard manufacture. | Generates a large liquid waste and sludge stream, so the air problem is partly converted into a water and solid-waste problem; consumes considerable water and reagent; produces a visible saturated plume that usually needs reheat; and creates corrosion and scaling problems demanding costly alloys or linings. | Coal-fired power stations, municipal and hazardous-waste incinerators, sulphide-ore smelters, and pulp-mill recovery boilers — large sources with high acid-gas loading. |
| 3. Regenerative thermal oxidation (thermal destruction) | Volatile organic compounds and organic air toxics — benzene, toluene, formaldehyde, chlorinated solvents. The stream is heated to 800–1000 °C with sufficient residence time and turbulence to oxidise the organics to carbon dioxide and water; ceramic heat-recovery beds alternate direction to recover 95 % or more of the heat. A catalytic variant runs at 300–500 °C over a precious-metal catalyst. | Destruction efficiency above 99 % across essentially any organic species and any mixture, and unlike adsorption or absorption it destroys the contaminant rather than transferring it to another medium, leaving no secondary waste to manage. | High capital cost and, for dilute streams, high auxiliary fuel consumption and associated CO2 emissions; halogenated organics produce HCl and risk dioxin formation on cooling, requiring a downstream scrubber and rapid quench; the catalytic variant is poisoned by sulphur, silicone and heavy metals. | Chemical plants, printing, coating and painting lines, pharmaceutical manufacturing, soil-vapour extraction off-gas — moderate-to-high VOC concentrations in a continuous stream. |
The selection logic behind this table is worth making explicit. Control device choice follows from the phase and physical chemistry of the target contaminant, not from the industry: particulate-bound and condensable toxics call for a collection device, soluble gases for absorption, and combustible organics for destruction. Three further criteria then discriminate among candidates — the required efficiency set by the emission limit, the gas stream's temperature, moisture and flow, and the disposition of the residual. That last criterion is the one candidates most often neglect: a scrubber and a baghouse both create a solid or liquid residual, so the cross-media transfer must be part of the assessment rather than an afterthought, and in Canada it will be an explicit condition of the provincial air permit and of the CCME Canada-Wide Standards for the substance concerned.
The land constraint is decisive: with no room for new landfill, every tonne diverted is worth far more than its avoided tipping fee, because it defers or eliminates the enormous cost of long-haul export or of siting new capacity. The strategies are therefore prioritized by the waste hierarchy, which also happens to be the order of both environmental benefit and 20-year cost recovery.
| Strategy 1 — Source reduction and reuse (highest priority) | Strategy 2 — Organics diversion by anaerobic digestion / composting | Strategy 3 — Materials recovery and energy-from-waste for the residual (lowest priority) | |
|---|---|---|---|
| Production rate | Attacks generation directly and is the only strategy that does. Instruments: extended producer responsibility and product stewardship shifting packaging cost to producers; pay-as-you-throw variable-rate bag or cart charges, which typically cut set-out 15 to 30 %; packaging and single-use-item bans; reuse infrastructure — repair cafés, deconstruction and building-materials exchange, refill systems, textile and furniture reuse networks; and construction-and-demolition waste reduction requirements in the municipal by-law. | Does not reduce generation but removes 30 to 40 % of the residential stream from disposal, which for a land-constrained municipality is functionally equivalent. Instruments: mandatory source-separated organics collection, a centralised anaerobic digestion facility producing biogas and digestate, backyard and community composting, and a disposal ban on organics enforced at the transfer station. | Does not reduce generation; it manages what the first two strategies leave. Instruments: a single-stream or dual-stream materials recovery facility with optical sorting, deposit-return for beverage containers, and a modern energy-from-waste plant with full air-pollution control for the residual fraction, with bottom-ash metals recovery and aggregate reuse. |
| Recycle and reuse potential | Highest quality outcome, because reuse retains the full embodied energy and function of the product rather than reprocessing it into a lower-value material. Also raises the quality of what is recycled downstream by removing problem items from the stream. | Turns a disposal liability into two products: renewable natural gas or electricity from the biogas, and a compost or digestate soil amendment that returns carbon and nutrients to land. Landfill methane, the sector's dominant greenhouse-gas source, is avoided entirely. | Recovers paper, cardboard, metals, glass and rigid plastics at scale, and recovers energy plus ferrous and non-ferrous metals from the residual. Provides the necessary end point for material that genuinely cannot be reduced, reused or composted. |
| Environmental benefit | Highest. Avoids upstream extraction, manufacturing and transport emissions as well as downstream disposal — the largest life-cycle saving per tonne of any option, and the only one that avoids the impact rather than mitigating it. | High. Eliminates the leachate strength and methane generation that organics cause in landfill, displaces fossil natural gas, and builds soil carbon. Organics are the single most damaging fraction to landfill. | Moderate. Real savings in avoided virgin-material production, but the recovery process itself consumes energy, and energy-from-waste, while far better than landfilling, still emits CO2 and requires stringent control of dioxins, mercury and acid gases. |
| 20-year cost recovery | Best. Very low capital, mostly programme, by-law and communication cost. Avoided collection, haulage and disposal cost accrues from year one and compounds as tipping fees escalate; deferring a new landfill or transfer station is worth tens of millions. Payback is typically under 3 years. | Good. Moderate capital for the digestion facility and the collection fleet, offset by biogas or renewable-natural-gas revenue, digestate sales, avoided tipping fees and, in several provinces, carbon credits. Typical payback 7 to 12 years, comfortably inside the 20-year horizon. | Weakest. Highest capital and operating cost, and revenue is exposed to volatile commodity markets — the collapse of recycled-plastic prices after China's National Sword policy in 2018 turned many MRF contracts negative. Energy-from-waste requires a 20-to-30-year put-or-pay tonnage commitment that can perversely lock the municipality out of further diversion. |
The prioritization is the answer, and it rests on a single argument: environmental benefit and cost recovery rank in the same order because both are governed by how far upstream the intervention sits. A tonne never generated carries no collection cost, no processing cost, no disposal cost and no upstream manufacturing impact, whereas a tonne recovered at a materials recovery facility has already incurred all of the upstream burden and must now be reprocessed. For a municipality with no land, the sequencing is also practically important: Strategy 1 and Strategy 2 together determine how much residual remains, and therefore how large and how expensive the Strategy 3 facility must be. Sizing an energy-from-waste plant before the diversion programmes are mature guarantees over-capacity and a contractual disincentive to divert further. The recommended programme is to implement Strategies 1 and 2 first, measure the residual for two to three years, and only then procure Strategy 3 capacity to fit what is actually left — a sequence consistent with the Canadian zero-waste framework and with provincial waste-diversion legislation.
