16-Civ-A3 Elementary Environmental Engineering · December 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, December 2015 — 98-Civ-A3 Environmental Engineering. Three hours; closed book with one candidate-prepared 8½ × 11 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 appearing in the work book are marked, for a maximum of 100 marks. The complete Marking Scheme is printed on page 8. All seven problems are solved here, because this set is a study resource rather than an examination script.
Reference texts.
Check: the mark split for Problem 1 is printed two different ways. The margin figures on page 2 read (7) for part (i), (7) for part (ii) and (6) for part (iii), while the Marking Scheme on page 8 reads “1. (i) 7, (ii) 6, (iii) 7”. Both add to 20, and the discrepancy is confined to parts (ii) and (iii). The margin figures on the question page are used below, since that is what a candidate sees while allocating time. Nothing in the technical content depends on the choice.
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.
Two air toxics are selected that behave quite differently and therefore exercise the three methods properly. Air Toxic 1 is mercury, emitted as elemental and oxidised vapour with a particle-bound fraction from coal combustion, cement kilns, base-metal smelters and waste incineration — a CEPA Schedule 1 toxic substance and the subject of CCME Canada-wide Standards and the Minamata Convention, notable because it is persistent, bioaccumulative, and transported globally. Air Toxic 2 is benzene, a volatile organic compound and Group 1 human carcinogen released from petroleum refining, coking, chemical manufacture and storage-tank breathing losses. Mercury is a trace metal that cannot be destroyed, only captured and sequestered; benzene is an organic molecule that can be destroyed outright. That single difference drives most of what follows.
| Air toxic | Method 1 — Adsorption (activated carbon) | Method 2 — Thermal / catalytic oxidation | Method 3 — Absorption (wet scrubbing) |
|---|---|---|---|
| Air Toxic 1: Mercury (Hg) | How: powdered activated carbon, usually sulphur- or halogen-impregnated, injected into the duct and collected downstream on a fabric filter, or a fixed carbon bed. Chemisorption binds Hg as HgS or HgCl2. Advantage: very high removal — 90 % or more for both elemental and oxidised mercury — at extremely low inlet concentrations where no other method works. Limitation: transfers the mercury to a spent-carbon residue that is itself hazardous waste requiring stabilisation and secure disposal; sorbent cost is significant and performance falls off above about 150 °C. Best used: coal-fired boiler and municipal waste incinerator flue gas, downstream of the acid-gas scrubber and ahead of the baghouse. |
How: not applicable as a control — mercury is an element and oxidation cannot destroy it. Oxidation is used only as a conditioning step: catalytic or halogen-assisted oxidation of Hg0 to Hg2+, which is water-soluble and therefore capturable downstream. Advantage: converts the hard-to-capture elemental fraction into a form that an existing wet scrubber already removes, so it exploits installed equipment. Limitation: achieves no removal by itself and requires a downstream capture device; re-emission of Hg0 from the scrubbing liquor must be suppressed with an additive. Best used: a coal plant already fitted with selective catalytic reduction and wet flue-gas desulphurisation, where the SCR catalyst delivers the oxidation as a co-benefit. |
How: wet scrubbing with an alkaline or chelating liquor, which absorbs oxidised mercury; often integrated with the acid-gas scrubber and paired with a wet electrostatic precipitator for the particle-bound fraction. Advantage: simultaneous control of several pollutants — SO2, HCl, particulate and oxidised mercury — in one vessel, with no sorbent consumable. Limitation: essentially no capture of elemental mercury, which is sparingly soluble; generates a contaminated liquid effluent needing its own treatment and creates a visible plume. Best used: smelter and incinerator gas already requiring acid-gas control, where oxidised mercury dominates the speciation. |
| Air Toxic 2: Benzene (VOC) | How: fixed or rotary-concentrator granular activated carbon beds; the benzene is adsorbed and the bed is regenerated with steam or hot nitrogen, condensing a recoverable solvent stream. Advantage: permits recovery and reuse of the product rather than destruction, which can offset operating cost; excellent at the low concentrations where oxidation is uneconomic. Limitation: humidity competes for adsorption sites; high inlet concentrations cause a dangerous exotherm and bed fires; spent carbon and the regeneration condensate need management. Best used: tank-farm vapour recovery, loading racks, and printing or coating operations with dilute intermittent VOC streams. |
How: regenerative thermal oxidation at 760–870 °C with ceramic heat-recovery beds, or regenerative catalytic oxidation at 320–430 °C over a precious-metal catalyst. Benzene is destroyed to CO2 and water. Advantage: genuine destruction rather than phase transfer — 99 % or better destruction and removal efficiency — leaving no secondary hazardous residue to manage. Limitation: high energy demand and NOx formation at thermal temperatures; catalysts are poisoned by halogens, sulphur and silicones; uneconomic on very dilute streams unless a concentrator is fitted first. Best used: refinery and chemical-plant process vents with continuous, moderately concentrated VOC loading — the reference technology for benzene. |
How: absorption into a non-volatile oil or a surfactant liquor in a packed tower; for water-soluble organics, water with a chemical reagent. Benzene's low water solubility makes oil the practical solvent. Advantage: handles hot, particulate-laden and corrosive gas that would foul a carbon bed or poison a catalyst, and can be combined with acid-gas control. Limitation: poor efficiency for sparingly soluble benzene unless an oil scrubbing medium is used, and the loaded solvent must then be stripped and regenerated — a second unit operation and a liquid waste stream. Best used: coke-oven gas benzol recovery, where wash-oil scrubbing is the long-established industry practice and the recovered benzene has product value. |
The selection logic behind the matrix. Three questions decide the method in nearly every case. First, can the pollutant be destroyed? Organic toxics can be, and oxidation is then preferred because it ends the problem rather than moving it; metals cannot be, so every mercury option is ultimately a capture-and-sequester option and must be judged on where the mercury finally goes. Second, what is the concentration and the flow? Oxidation is thermodynamically sensible when the stream carries enough heating value to sustain itself, while adsorption excels precisely where concentrations are too dilute for that, which is why a rotary concentrator ahead of an oxidiser is such a common pairing. Third, what else is in the gas? Particulate loading fouls carbon beds, halogens and sulphur poison oxidation catalysts, and moisture competes for adsorption sites — so the control device is selected against the whole gas composition, not against the target pollutant alone. In Canadian practice these choices are made within CEPA 1999 for listed toxics, provincial permitting under statutes such as the BC Environmental Management Act, and mandatory National Pollutant Release Inventory reporting, all of which reward the option that reduces the total cross-media release rather than the one that merely moves it from stack to landfill.
| Objective | Strategy 1 — Source-separated organics with regional anaerobic digestion | Strategy 2 — Enhanced recycling: MRF capacity with extended producer responsibility | Strategy 3 — Transfer stations with compaction and baling, plus RDF or energy recovery |
|---|---|---|---|
| Resource recovery | Diverts the 30–40 % organic fraction that drives both landfill methane and leachate strength. Recovers biogas upgraded to renewable natural gas or used in combined heat and power, plus digestate as a soil amendment — recovering carbon, nutrients and energy from one stream. | Recovers paper, metals, glass and rigid plastics as secondary raw materials. The energy saving is largest for metals: recycled aluminium needs roughly 5 % of the energy of primary smelting, and steel about 25 %. EPR shifts the cost to producers and closes the loop through design-for-recyclability. | Recovers energy from the residual fraction after diversion — roughly 500–600 kWh per tonne of MSW in a modern facility — and metals from the bottom ash. It recovers no materials in the upstream sense; its principal contribution is to the transportation objective. |
| Reduced transportation need | Strong. Organics are the wettest and heaviest fraction, so removing them cuts the mass hauled to a remote landfill sharply; regional digesters sited near the generation centre replace a long-haul trip with a short one, and RNG injected into the gas grid moves the recovered energy with no vehicle movement at all. | Moderate and requires care. Diversion cuts residual haul, but adds a separate collection fleet unless co-collection or alternating-week schedules are engineered. Baling recovered material at the MRF raises payload density and cuts outbound trips; poorly designed dual-stream collection can increase total vehicle-kilometres. | Strongest and most immediate. Collection vehicles achieve about 0.5 t/m3 compaction, whereas a transfer station bales or compacts into transfer trailers at 0.8–1.0 t/m3 carrying 3–4 times the payload, typically cutting long-haul vehicle trips by 70 % or more with a corresponding fall in fuel, emissions and road wear. |
Priority over a 25-year horizon. The ranking below weighs environmental benefit and cost recovery together over the full period, which is the horizon at which capital-intensive facilities amortise and at which avoided landfill airspace acquires its real value.
The three are complements, not alternatives, and the ordering is a sequencing recommendation for capital deployment rather than a claim that any one should be omitted. All three sit below source reduction and reuse in the waste hierarchy, which remain the highest-value interventions and require almost no capital — a point worth stating explicitly, because the question asks for strategies to reclaim waste and it is easy to lose sight of the fact that the waste not generated needs neither reclamation nor transport.
