16-Civ-A3 Elementary Environmental Engineering · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, December 2018 — 16-Civ-A3 Elementary Environmental Engineering. Three hours; closed book with one candidate-prepared 8½ × 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 from the final-page Marking Scheme.
Reference texts.
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.
(a) Settling chamber (particulates). A gravity settling chamber is an enlarged duct that slows the gas stream so that coarse particles have residence time to fall out under gravity, since terminal settling velocity (Stokes’ law) rises with the square of particle diameter. It is simple and robust but only effective for large particles (> ~50 µm), so it serves as a pre-cleaner ahead of a high-efficiency device (cyclone, ESP or baghouse). (b) Packed tower (SO2). A packed-bed absorber (wet scrubber) passes the flue gas up through packing while an alkaline scrubbing liquor (lime/limestone slurry or caustic) flows down; the large gas–liquid interfacial area drives mass transfer of SO2 into the liquid where it reacts (SO2 + alkali → sulfite/sulfate), removing it from the gas. Countercurrent flow maximises the driving-force gradient. (c) Combustion control of NOx. Thermal NOx forms from N2 and O2 at high flame temperature, so combustion-side controls lower peak temperature and excess oxygen: staged/low-NOx burners, flue-gas recirculation and lean/reduced-temperature combustion cut formation. Post-combustion, selective catalytic (SCR) or non-catalytic (SNCR) reduction injects ammonia/urea to reduce NOx to N2. The principle is prevention (control the flame) before treatment.
(a) Source reduction and separation. Technical: design for less packaging, product stewardship and at-source separation (recyclables, organics, residual) via multi-stream collection. Non-technical: extended-producer-responsibility regulation, pay-as-you-throw pricing, deposit-return, and public education to change behaviour — sitting at the top of the waste hierarchy (reduce, reuse, recycle). (b) Transportation routes and transfer stations. Efficient collection-route optimisation (GIS routing) reduces haul distance, fuel and emissions; where disposal or recycling sites are distant, a transfer station consolidates collection-vehicle loads into larger long-haul trailers, cutting the number of trips, cost and road impact. Non-technical drivers include regional cost-sharing agreements and siting acceptance. (c) Landfill volume over its life. Estimate the volume from a mass balance: annual waste mass = population × per-capita generation rate, adjusted for the diversion (recycling/composting) fraction; divide by the compacted in-place density to get annual airspace, add daily/intermediate cover (typically ~10–20% extra), and multiply by the design life to obtain total required airspace — Vtotal = Σ [ (P × g × (1−diversion)) / ρcompacted × (1+cover) ] over the design years, allowing for population growth. This sizes the cell and cap volumes.
