16-Civ-A3 Elementary Environmental Engineering · May 2014

Question 5 of 7: Air Pollution Control and Solid Waste Management

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

Paper format. National Exams 98-Civ-A3 Environmental Engineering, May 2014 — 3 hours, closed book with one candidate-prepared double-sided aid sheet, approved Casio or Sharp calculator only. Seven questions are offered; any five constitute a complete paper (20 marks each, 100 marks maximum), and only the first five answers in the work book are marked. All seven are solved here, because the set is intended as a study resource rather than an examination script. Section marks are shown in brackets at the left margin of each part, and the marking scheme on page 6 confirms the split.

Reference texts. Davis & Cornwell, Introduction to Environmental Engineering (5th ed.); Mihelcic & Zimmerman, Environmental Engineering: Fundamentals, Sustainability, Design (3rd ed.); Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.); Crittenden et al., MWH’s Water Treatment: Principles and Design (3rd ed.). Canadian regulatory frame: the federal Impact Assessment Act (2019) and the Impact Assessment Agency of Canada, the Canadian Environmental Protection Act (CEPA 1999), CCME Canadian Environmental Quality Guidelines, and Health Canada’s Guidelines for Canadian Drinking Water Quality (GCDWQ).

Check: Henry’s law constant units in Question 1(i). The paper writes the constant as “0.30 (mol/atm)”, which is dimensionally incomplete — a Henry’s constant in the concentration/pressure form must carry a volume in the denominator. It is taken here as 0.30 mol/(L·atm), i.e. the aqueous-concentration form $C_{aq}=K_H\,p$. That reading is confirmed by the published value for ethyl acetate, $H \approx 1.3\times10^{-3}\ \text{atm}\cdot\text{m}^3/\text{mol}$, whose reciprocal is $\approx 0.77\ \text{mol}/(\text{L}\cdot\text{atm})$ — the same order of magnitude. Per NOTE 1 on page 1, this assumption is stated with the answer.

Question 5: Air Pollution Control and Solid Waste Management (20 marks)

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.

Part (i) — Three VOC control methods (10 marks)

VOC control methods for industrial fixed sources
Method	Principle	Advantage	Limitation	Most appropriate application
1. Thermal / regenerative thermal oxidation (RTO)	Destroys VOCs by oxidation to $\text{CO}_2$ and water at 760–870 $^\circ\text{C}$ with 0.5–1 s residence; ceramic beds recover 95 % of the heat.	Very high and reliable destruction efficiency (98–99+ %) across essentially any mixture of organic compounds, including odorous and toxic species.	High capital cost and supplemental fuel demand at low VOC loading; halogenated VOCs generate $\text{HCl}$ and dioxin risk, requiring downstream acid-gas scrubbing; NO_x is created by the burner.	High-volume, continuous, moderately concentrated streams — a paint or coil-coating line, a printing press with solvent-laden air, a chemical reactor vent where destruction rather than recovery is the objective.
2. Adsorption on granular activated carbon	VOC molecules partition onto the internal surface of activated carbon (typically 800–1500 m²/g) by physical adsorption; the bed is regenerated with steam or hot nitrogen and the desorbed solvent condensed and reused.	Permits recovery and reuse of valuable solvent rather than destruction, so the control can pay for itself; effective at very low concentrations where oxidation would need continuous fuel.	Humidity above roughly 50 % competes for adsorption sites; high-boiling or polymerising compounds foul the carbon irreversibly; exothermic adsorption of ketones creates a bed-fire risk; spent carbon is itself a waste requiring management, and breakthrough must be monitored.	Dilute streams of a single recoverable solvent — degreasing with a chlorinated solvent, a dry-cleaning operation, a soil-vapour-extraction off-gas, or a pharmaceutical vent where the solvent has real value.
3. Biofiltration	The contaminated air passes slowly through a moist packed bed of compost, peat, bark or engineered media on which a biofilm of heterotrophic bacteria metabolises the VOCs to $\text{CO}_2$, water and biomass at ambient temperature.	Very low operating cost and energy demand, no combustion by-products or NO_x, and outstanding performance on odorous reduced-sulphur and nitrogen compounds — the lowest-carbon option available.	Large footprint and long empty-bed residence time; slow to recover from shock loads, dry-out, or shutdowns because the biomass must stay alive; poor on chlorinated and other recalcitrant compounds; media must be replaced every few years, and pH must be controlled where acids are produced.	High-volume, low-concentration, readily biodegradable and odorous streams — wastewater treatment plant headworks and biosolids handling, composting facilities, rendering plants, and food processing.

The selection among these three is governed chiefly by the concentration–flowrate position of the stream and by whether the organic has recovery value. Oxidation suits high concentration where the VOC itself provides the fuel; adsorption suits low concentration where a valuable single solvent can be recovered; biofiltration suits high flow at very low concentration where the compounds are biodegradable and the driver is odour. Two further options deserve mention: condensation, by refrigeration or cryogenic cooling, is economic only for high-concentration streams of high-boiling compounds, and absorption in a packed scrubber suits water-soluble VOCs but transfers the problem to a liquid stream that must then be treated. In all cases the first question a Canadian practitioner should ask is whether the emission can be avoided altogether — through low-VOC or waterborne coating reformulation, closed transfer systems, floating-roof tanks and leak detection and repair — because pollution prevention avoids the capital, the energy and the residual waste that every control device creates.

Topic. Volatile organic compound control from fixed industrial sources.

Key relations. Destruction and removal efficiency $DRE = (C_{in}-C_{out})/C_{in}$; the three T’s of oxidation — temperature, turbulence and time; Freundlich and Langmuir adsorption isotherms and the mass-transfer-zone breakthrough model; empty-bed residence time $EBRT = V_{bed}/Q$ and elimination capacity for biofilters; VOCs as ozone precursors under CEPA 1999 Schedule 1.

Why this works. The three methods represent the three physically distinct things one can do with a pollutant — destroy it thermally, concentrate and recover it, or destroy it biologically — and each is economic in a different region of the concentration–flowrate plane. Recognising that a control device does not make a pollutant disappear but converts it into another form (combustion products, spent carbon, biomass and acidic drainage) is what makes the selection an engineering judgement rather than a catalogue choice.

Common pitfall. Recommending thermal oxidation for a very dilute, very high-flow stream, where the auxiliary fuel needed to heat enormous volumes of air containing almost no fuel value makes both cost and net greenhouse-gas emissions absurd — the classic case where biofiltration or carbon adsorption is correct. The mirror-image error is proposing carbon adsorption for a hot, humid, high-concentration stream that would saturate the bed within hours.

Source. Davis & Cornwell, Introduction to Environmental Engineering, 5th ed., Ch. 9 (Air Pollution Control); Mihelcic & Zimmerman, 3rd ed., Ch. 12; Environment and Climate Change Canada VOC control guidance under CEPA 1999.

Part (ii) — Three municipal solid waste strategies, prioritised (10 marks)

3 × 2 matrix — strategy ranked by environmental benefit and by cost recovery over 25 years
Strategy	Environmental benefit (25-year horizon)	Cost recovery (25-year horizon)
1. Source reduction and reuse — extended producer responsibility, packaging reduction, pay-as-you-throw volume-based user fees, repair and reuse networks, procurement standards, bans on single-use items.	Rank 1 (highest). Avoids the entire upstream life cycle — extraction, processing, manufacture and transport — not merely the disposal step, so the avoided energy and greenhouse-gas burden per tonne is several times that of recycling the same material. It generates no residual, needs no facility, and is the only strategy that reduces waste in absolute terms rather than redirecting it.	Rank 1 (highest). Capital cost is essentially nil; the programme is administrative and regulatory. Under extended producer responsibility the cost shifts to producers, and pay-as-you-throw generates revenue while cutting collection tonnage. Avoided landfill capital — on the order of hundreds of dollars per tonne of airspace consumed — accrues from year one and compounds over 25 years as landfill life is extended.
2. Recycling and organics diversion — source-separated blue-box recyclables plus a green-bin organics programme feeding composting or anaerobic digestion.	Rank 2. Substantial: it displaces virgin material production (aluminium recycling saves roughly 95 % of the primary smelting energy) and, critically, removes putrescible organics from landfill, which is where landfill methane originates — methane having a global warming potential about 28 times that of $\text{CO}_2$ over a century. Anaerobic digestion recovers that methane as usable biogas instead. The limitation is that collection, processing and residual disposal all carry their own footprint.	Rank 2. Moderate. Capital for a materials recovery facility and organics plant is significant, and commodity revenue is genuinely volatile — the 2018 restriction of Chinese import markets stranded many programmes. Over 25 years, however, avoided tipping fees, digestate and compost sales, biogas or renewable-natural-gas revenue, and EPR stewardship payments typically bring net cost close to or below that of disposal, and the position improves as carbon pricing rises.
3. Residual management — energy recovery and engineered landfill — waste-to-energy combustion or landfill with a composite liner, leachate collection and treatment, and landfill-gas capture with electricity or RNG production.	Rank 3 (lowest). Necessary but least beneficial: the material and its embodied energy are lost, combustion produces $\text{CO}_2$ and requires air pollution control for dioxins, acid gases and metals, and even the best-engineered landfill captures only 60–90 % of its gas, leaves a leachate liability, and imposes post-closure care for at least 25–30 years. It is the residual step in the waste hierarchy for good reason.	Rank 3 (lowest). The highest capital and the longest liability. A waste-to-energy plant is a major capital project with high operating cost, and a modern landfill carries liner, leachate treatment and post-closure financial assurance obligations that extend decades past the last tonne received. Electricity and RNG revenue offset only part of this. It is also the strategy most exposed to future carbon pricing and to the siting and community opposition that make replacement capacity so costly.

Prioritisation and its justification. The ranking is identical on both criteria — source reduction first, diversion second, residual management third — which is precisely the classical waste hierarchy (reduce, reuse, recycle, recover, dispose). This alignment is not accidental. Each step up the hierarchy avoids not only the disposal cost but the entire upstream burden of producing the material in the first place, so environmental benefit and avoided cost move together. The convergence is stronger over the 25-year horizon the question specifies than over a short one, because the largest financial benefit of the upper tiers — deferring or avoiding the construction of new landfill or thermal capacity — is realised only in the long term, and because the carbon price applied to landfill methane and combustion emissions is expected to rise over that period.

Two qualifications matter in practice. First, the strategies are complementary, not alternatives: no municipality can eliminate residual waste, so an engineered disposal facility is required regardless of how successful the upper tiers are, and the goal is to minimise the tonnage reaching it and extend its life. Second, the 25-year framing rewards an approach that many councils resist, because source reduction and diversion require sustained programme spending and behavioural change with benefits that accrue slowly, whereas a landfill delivers visible capacity immediately. Presenting the analysis on a full life-cycle-cost basis, including post-closure care and avoided future capital, is what makes the case defensible — and it is the analysis a professional engineer is expected to bring to the council table.

Topic. Integrated municipal solid waste management and the waste hierarchy.

Key relations. The waste hierarchy (reduce > reuse > recycle > recover > dispose); diversion rate $=$ (diverted tonnage)/(total generated); life-cycle assessment and life-cycle costing including post-closure care; landfill gas generation modelled by first-order decay $Q = kL_0M e^{-kt}$; net present value over the 25-year planning horizon.

Why this works. The hierarchy is not an arbitrary preference list — it follows from the observation that disposal cost is only the last and smallest part of a material’s environmental burden. Preventing a tonne of waste avoids the extraction, refining, manufacturing and transport energy embodied in it, which typically exceeds the disposal impact several times over. Because those upstream burdens also carry real costs somewhere in the economy, the environmental ranking and the long-horizon economic ranking coincide. Evaluating over 25 years rather than one budget cycle is what allows deferred landfill capital and post-closure liability to appear in the comparison at all.

Common pitfall. Listing the three strategies without ranking them, when the question explicitly asks for prioritisation on two stated criteria — or ranking waste-to-energy first because it “recovers energy”, which mistakes recovering a fraction of a material’s embodied energy for having never spent it.

Source. Davis & Cornwell, Introduction to Environmental Engineering, 5th ed., Ch. 10 (Solid Waste Management); Mihelcic & Zimmerman, 3rd ed., Ch. 13; CCME Canada-wide Action Plan for Extended Producer Responsibility.

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