23-Chem-B2 Environmental Engineering · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 16-Chem-B2, Environmental Engineering — May 2019. 3 hours, Closed-Book Exam with a candidate-prepared 8½×11" double-sided aid sheet. Any five (5) of the seven questions constitute a complete paper (100 marks); all seven are solved below for completeness.
Reference texts: Metcalf & Eddy (Tchobanoglous, Burton, Stensel), Wastewater Engineering: Treatment and Reuse, 4th ed.; Davis & Cornwell, Introduction to Environmental Engineering, 5th ed.; Turner, Workbook of Atmospheric Dispersion Estimates, 2nd ed.; Cooper & Alley, Air Pollution Control: A Design Approach, 4th ed.
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.
| Technology | Main principle | Advantages | Limitations | Application |
|---|---|---|---|---|
| (i) Electrostatic precipitator (particulates) | Dust-laden gas passes between charged discharge electrodes and grounded collection plates; particles acquire a negative charge via corona discharge, migrate under the electric field, and are collected on the plates before being periodically rapped off into a hopper. | 1) Very high collection efficiency (often >99%) for PM₂₀ and much finer particulates (PM₁₀, PM₂.₅), unlike purely inertial devices. 2) Low pressure drop (low fan energy cost) compared to a high-efficiency fabric filter for the same gas volume. | 1) Efficiency is sensitive to particle resistivity — very high- or very low-resistivity dust (e.g. after a fuel or process change) can sharply reduce collection. 2) High capital cost and a large footprint, plus efficiency degrades quickly if plate rapping/cleaning is neglected (dust re-entrainment). | Coal-fired boiler or cement-kiln flue gas, where a large, fairly steady particulate load must be removed at high efficiency with low pressure drop. |
| (ii) Air (wet) scrubber (gases) | The gas stream is contacted with a liquid (usually an alkaline scrubbing solution for an acid gas like SO₂) in a packed or spray tower; the pollutant gas is absorbed into the liquid phase and reacts chemically (e.g. SO₂ with lime/limestone slurry to form gypsum), removing it from the gas stream. | 1) High removal efficiency for soluble/reactive acid gases such as SO₂ (often >90–95% with a well-designed wet limestone system). 2) Can simultaneously knock down some particulate matter and cool the gas stream. | 1) Generates a wet waste/byproduct stream (scrubber liquor or gypsum sludge) requiring further handling or disposal, an added carbon/cost burden. 2) High pressure drop and pumping/reagent energy demand relative to a dry control technology. | Coal- or oil-fired utility boiler SO₂ control (wet flue-gas desulfurization), or acid-gas control on a smelter or chemical-process off-gas. |
| (iii) Adsorption (odorous vapours/VOCs) | The vapour stream is passed through a bed of activated carbon (or other high-surface-area sorbent); VOC molecules are physically adsorbed onto the internal pore surface until the bed approaches saturation (breakthrough), at which point it is replaced or thermally regenerated. | 1) Very high removal efficiency for a wide range of non-polar organic vapours, including many odorous compounds not easily biodegraded. 2) Compact, simple, low-energy operation for a dilute, intermittent vapour stream. | 1) Capacity is limited and humidity/competing-compound sensitive; a saturated bed breaks through and stops removing the target compound without warning unless outlet monitoring is in place. 2) Spent carbon is a waste stream requiring regeneration or disposal, and regeneration (steam or thermal) is itself an energy cost. | Solvent-recovery, tank-farm vent, or odour-control polishing stage handling a dilute, intermittent VOC or odorous-vapour stream downstream of a primary control device. |
The three technologies target different phases and particle/molecule size ranges: the ESP is a physical particulate collector whose efficiency depends on electrostatic charging rather than particle inertia (so it remains highly efficient down to sub-micron particulates, unlike a cyclone), the wet scrubber removes gas-phase pollutants by mass transfer into a reactive liquid (so its performance hinges on gas solubility/reactivity, not particle size at all), and carbon adsorption removes vapour-phase organics by physical sorption onto a solid surface, making it well suited to compounds too dilute or non-biodegradable for a scrubber or biofilter. Selecting among them (or combining them in series, e.g. ESP upstream of a wet scrubber) is therefore driven first by contaminant phase and chemistry, and only secondarily by capital/operating cost.
