16-Civ-A3 Elementary Environmental Engineering · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, December 2016 — 98-Civ-A3 Environmental Engineering. Three hours; closed book with one candidate-prepared 8 × 11 double-sided aid sheet; approved Casio or Sharp calculator only. Seven problems are printed, each worth 20 marks, and any five constitute a complete paper (maximum 100 marks). All seven are solved here, because the set is intended as a study resource rather than an exam script. Section marks are shown in brackets at the left margin of each question and are reproduced below.
Reference texts.
Check: The page-1 marking scheme on this paper is not reliable as printed, but the mark figures printed in the left margin of each question page are internally consistent — every problem's sub-part marks sum to exactly 20, and the parts of the scheme that are given agree with them. The margin figures are adopted throughout: Q1 (6, 7, 7); Q2 (9, 6, 5); Q3 (7, 7, 6); Q4 (10, 10); Q5 (10, 10); Q6 (10, 10); Q7 (5, 6, 3, 3, 3).
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 selection principle is that a control device must be matched to the physical state of the pollutant. Particulate matter is a dispersed solid or liquid phase and is removed by a mechanical separation that exploits inertia, electrostatic charge or interception on a surface. Volatile organic compounds are gases in true solution in the carrier air and cannot be filtered out at all; they must be destroyed by oxidation or transferred to another phase by adsorption or absorption. Odorous organics are chemically similar to VOCs but occur at concentrations three to six orders of magnitude lower, where the human nose is more sensitive than most instruments, so the economic answer is usually a low-cost transfer or biological process rather than combustion. Choosing a baghouse for a solvent stream, or a thermal oxidiser for a dilute odour, are the two classic and expensive errors.
| Pollutant type | Selected control device | Principle of operation and typical performance | Example stationary source |
|---|---|---|---|
| Particulate matter (PM10, PM2.5) | Fabric filter (baghouse) | Dust-laden gas passes through woven or felted fabric tubes; a dust cake builds on the fabric and it is that cake, rather than the weave, which does most of the filtering, so efficiency improves as the cake develops. Bags are cleaned periodically by pulse-jet, shaker or reverse air, and the dislodged dust falls to a hopper. Collection exceeds 99.9 % and, critically, holds that efficiency down into the fine PM2.5 range where cyclones and wet scrubbers fall away. Limits are gas temperature (fabric-dependent, typically 130 to 260 °C), moisture and acid dewpoint, which cause blinding, and a pressure drop of 1 to 2 kPa that must be paid for in fan power. An electrostatic precipitator is the common alternative for very large, hot gas volumes, with lower pressure drop but poorer fine-particle capture and a sensitivity to dust resistivity. | Cement kiln or coal-fired boiler; also a foundry, an asphalt plant or a grain-handling facility. |
| Volatile organic compounds (solvent vapours) | Regenerative thermal oxidiser | The contaminated stream is preheated by passing through a hot ceramic bed, oxidised in a combustion chamber at 760 to 870 °C with about one second of residence time, and the hot products are then passed through a second ceramic bed which recovers their heat for the next cycle. Flow is periodically reversed between beds, giving thermal energy recovery above 95 % and destruction and removal efficiency of 95 to 99 %, with the organics converted to carbon dioxide and water. The high heat recovery is what makes the device economic on dilute streams that would otherwise need continuous auxiliary fuel; the constraints are that halogenated solvents produce acid gases requiring a downstream scrubber, and that streams must be kept well below the lower explosive limit. Carbon adsorption with steam regeneration is preferred instead where the solvent is valuable enough to recover. | Automotive or appliance paint-spray booth and bake oven; also a printing works, a coating line or a chemical reactor vent. |
| Odorous organic compounds (reduced sulfur and nitrogen compounds, amines, mercaptans) | Biofilter (packed compost, wood-chip or engineered inert medium) | Humidified foul air is drawn slowly upward through a moist packed bed on which a mixed microbial population is established. Odorants partition from the gas into the water film on the packing and are then metabolised by the biofilm to carbon dioxide, water, sulfate and nitrate, so the contaminant is destroyed rather than merely transferred. Empty-bed residence times of 30 to 60 seconds give odour removal of 90 to 99 % at very low operating cost, since the only significant energy input is the fan. Requirements are careful humidity control — a dried-out bed loses activity quickly and is slow to recover — pH control where hydrogen sulfide loading is high, and periodic media replacement every three to five years. Chemical scrubbing and activated-carbon polishing are the alternatives where the footprint is not available or loadings vary sharply. | Wastewater treatment plant headworks, primary clarifier and biosolids handling building; also a rendering plant, a composting facility or a landfill gas collection system. |
The 3R hierarchy — Reduce, Reuse, Recycle — ranks waste management options in decreasing order of environmental preference, and its logic is that the options are ordered by how much of the material's embodied energy and value each preserves. It is the organising framework of provincial waste diversion policy across Canada and of the federal Canada-wide Action Plan for Extended Producer Responsibility.
Reduce stands at the top because a material that is never produced imposes no extraction, no manufacturing energy, no transport, no collection and no disposal burden. It is source reduction: designing products to use less material, to last longer and to be repairable; eliminating unnecessary packaging; substituting a service for a product; and buying in bulk or in concentrate. Reduction is the only tier that decouples material consumption from economic activity outright, and it is also the only one that costs nothing to operate once designed in. Its difficulty is that it depends on decisions taken at the design and purchasing stage, largely outside the waste manager's control, which is why extended producer responsibility exists — it places the disposal cost on the party who makes the design decision.
Reuse ranks second because the product is used again in its existing form, so the manufacturing energy embodied in it is retained entirely and only cleaning and transport energy are expended. Refillable containers, returnable pallets and totes, repair and refurbishment of equipment, and the resale of durable goods through second-hand markets all fall here. Reuse preserves far more value than recycling does, since recycling destroys the product's form and recovers only the material.
Recycle ranks third because the product is destroyed and reprocessed into new feedstock, which requires collection, sorting, cleaning and remanufacture, all of which consume energy and all of which lose some material at each pass. It is nonetheless far preferable to disposal: recycled aluminium requires about 95 % less energy than primary smelting from bauxite, recycled steel about 60 to 74 % less, and recycled paper about 40 % less. Composting and anaerobic digestion of organics are properly regarded as recycling of the biological nutrient stream, and in most Canadian municipalities the organic fraction is the single largest remaining diversion opportunity. Below the three Rs, and outside the hierarchy proper, sit energy recovery through incineration or landfill-gas capture, and last of all landfill disposal.
A worked example — beverage containers in an office building. Consider a 400-person office that currently sends about 60,000 single-use plastic water bottles to landfill each year. Applying the hierarchy in order rather than jumping to the recycling bin gives the following programme. Reduce: install filtered drinking-water fountains with bottle-filling stations and issue every employee a reusable bottle at induction, then discontinue the purchase of single-use bottled water for meetings and the cafeteria. This alone eliminates the great majority of the stream — perhaps 50,000 bottles a year — because the waste is never created and no collection or processing infrastructure is needed at all. Reuse: for the remaining beverage service, switch to refillable glass or stainless containers and to a returnable-keg or bulk-dispenser system for coffee and juice, so the containers cycle through washing rather than through the waste stream, perhaps a further 6,000 units. Recycle: the residual containers that cannot be avoided or reused — visitor bottles, off-site purchases, the occasional event — are captured in clearly labelled, co-located three-stream stations at every point of disposal and sent to the municipal recycling programme, recovering the PET and aluminium as feedstock. Only what escapes all three tiers, perhaps a few hundred contaminated containers, goes to disposal.
The example makes the hierarchy's economics visible. The reduce step is the largest by far in tonnage avoided and is also the cheapest — the fountains pay back in avoided bottled-water purchasing within a year or two, whereas the recycling step requires bins, floor space, custodial labour and a hauling contract in perpetuity. It also illustrates the most common practical failure in applying the 3Rs, which is treating them as three parallel options rather than as a ranked sequence: an organisation that installs recycling bins and declares the problem solved has adopted the least preferred of the three tiers while leaving the first two, which offer the greater part of the benefit, entirely untouched.
