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18-Env-A1 Principles of Environmental Engineering · May 2018

Question 2 of 7: Environmental Impact Assessment and Acid Precipitation

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

Notes on this paper

National Exams — May 2018 — 04-Env-A1 / Principles of Environmental Engineering. 3 hours duration; closed book with a candidate-prepared 8.5×11 in double-sided aid sheet; Casio or Sharp approved calculator only. Any five questions constitute a complete paper (first five answers marked); all seven are solved below for completeness. Each question is worth 20 marks.

Reference texts. Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.); MWH’s Water Treatment: Principles and Design (3rd ed.); Sawyer, McCarty & Parkin, Chemistry for Environmental Engineering and Science; Guidelines for Canadian Drinking Water Quality (Health Canada); Canadian Council of Ministers of the Environment (CCME) water-quality and municipal solid-waste guidelines; Canadian Environmental Protection Act, 1999 (CEPA) and Canadian Environmental Assessment Act (CEAA 2012); Bies & Hansen, Engineering Noise Control; Andrews, Canadian Professional Engineering and Geoscience (professional ethics).

Question 2: Environmental Impact Assessment and Acid Precipitation (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.

(i) Applying an Environmental Impact Assessment to a Forested-Area Gold Mine

An environmental impact assessment (EIA) reduces pollution from a gold-mining development by systematically identifying the environmental effects of each major project phase before construction begins, so that mitigation is designed into the process rather than retrofitted after damage occurs. For a gold mine in a heavily forested area, the three key process steps, their key issues, and the mitigating actions are:

EIA process steps, issues and mitigation actions — forested-area gold mine
Process StepKey IssueAction to Address the Issue
Site preparation & forest clearingDeforestation, habitat fragmentation and loss of biodiversity; increased erosion and sediment loading to nearby streams once forest cover is removedMinimize the cleared footprint and stage clearing progressively; establish vegetated buffer zones along all watercourses; install sediment/erosion controls (silt fences, sediment retention ponds) before clearing begins; conduct pre-construction wildlife/habitat surveys and secure a habitat offset or compensation plan
Ore processing (cyanide leaching & tailings management)Cyanide and heavy-metal contamination of surface and groundwater; risk of tailings-storage-facility failure; fugitive dust and process air emissionsUse a lined, engineered tailings storage facility with leak-detection monitoring; treat process water (e.g., cyanide destruction such as the INCO SO2/air process) before any discharge or recycle it in a closed-loop circuit; continuous downstream water-quality monitoring against CCME guidelines; dust suppression and an approved air-emissions permit
Mine closure & reclamationAcid rock drainage (ARD) from newly exposed sulfide-bearing rock and waste rock piles; long-term degradation of water quality after mine life ends; failure to restore forest/land productivityConduct acid-base accounting (net acid-producing potential) on waste rock before disposal and use engineered covers/water covers to limit oxygen and water ingress to sulfide material; progressive (concurrent) reclamation and revegetation with native species rather than waiting until closure; require a closure and financial-assurance (bonding) plan covering long-term water treatment and monitoring

Applied this way, the EIA process converts pollution control from an after-the-fact remediation exercise into a design requirement carried through every phase of the mine’s life — construction, operation and closure — which is precisely what distinguishes impact assessment (predict, then design out) from impact response (react after the fact).

(ii) Principal Causes of Acid Precipitation and Root-Cause Engineering Solutions

Acid precipitation forms when SO2 and NOx emitted to the atmosphere oxidize and combine with water vapour to form sulfuric and nitric acid, which then fall as acidic rain, snow or fog. In a developed country, three principal causes are:

  1. SO2 from coal- and oil-fired electricity generation. Combustion of sulfur-bearing fossil fuels at large central power stations is historically the single largest SO2 source in industrialized countries.
  2. NOx from motor-vehicle traffic and other high-temperature combustion. NOx forms whenever air (N2 + O2) is heated to high temperature in an internal-combustion engine or industrial boiler/furnace, regardless of the fuel’s own sulfur or nitrogen content — a large, diffuse, mobile source base that is harder to control than a handful of point sources.
  3. SO2 from non-ferrous metal smelting. Smelting of sulfide ores (e.g., copper, nickel, zinc) liberates the sulfur bound in the ore as SO2, making metal smelters a major regional point source in mining-intensive developed economies.

Two different types of engineering solutions that act at the root cause — the precursor emission itself, rather than the resulting acidic deposition:

  1. Flue-gas desulfurization (wet limestone scrubbing) at power plants and smelters. A wet scrubber reacts stack SO2 with a limestone/lime slurry to form gypsum, removing the precursor before it ever reaches the atmosphere — addressing cause (1) and (3) directly at the point source.
  2. Selective catalytic reduction (SCR) and combustion/vehicle-emission controls for NOx. SCR converts NOx to N2 and water over a catalyst in stationary boiler/furnace exhaust, while three-way catalytic converters (with progressively tighter vehicle-emission standards) achieve the same for mobile sources — addressing cause (2) at both the stationary and mobile combustion source.

Both solution types share the same engineering logic: intercept and convert the acid-forming gas at (or very near) its point of formation, rather than attempting to neutralize acidity after it has already been deposited across a watershed (e.g., liming lakes), which treats the symptom rather than the cause.