18-Env-A6 Solid Waste Engineering and Management · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
18-Env-A6, Solid Waste Engineering and Management — National Exam, May 2019. 3 hours, closed book (one double-sided aid sheet permitted). The paper's own notes state that Question 1 is compulsory and any three of the remaining four questions complete the paper; all five questions are answered in full below.
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) Phase 3 (acid phase) — biological activity and leachate composition. A landfill's biodegradation proceeds through five recognized phases (initial aerobic adjustment, transition, acid formation, methane fermentation, and maturation); Phase 3, the acid phase, follows shortly after the initially trapped oxygen is consumed and strictly anaerobic conditions become established. Biological activity in this phase is dominated by facultative and obligate anaerobic bacteria performing hydrolysis and fermentation: complex organic polymers (cellulose, proteins, lipids) are hydrolyzed into soluble sugars, amino acids and fatty acids, which fermentative (acidogenic) bacteria then convert into short-chain volatile fatty acids (acetic, propionic, butyric acid), alcohols, CO2 and H2; the strict methanogenic (methane-forming) archaea that will dominate the next phase are still only sparsely established, because they are highly sensitive to the low pH this phase itself produces and are slow-growing relative to the acid-formers. Leachate composition during this phase is characterized by a sharp drop in pH (often to 5–6 or lower, from the accumulating volatile fatty acids), a correspondingly high biochemical oxygen demand (BOD) and chemical oxygen demand (COD) with a high BOD:COD ratio (indicating the readily biodegradable acid intermediates dominate), elevated concentrations of volatile fatty acids, and increased solubility/mobility of heavy metals and inorganic species (the low pH mobilizes metals that are far less soluble at neutral pH). This is typically the most aggressive, highest-strength leachate the landfill will produce, and is a key design driver for the leachate collection and treatment system's peak capacity.
(b) Trench Method vs. Area Method. Both are standard construction sequencing methods for placing refuse in an engineered landfill cell; they differ chiefly in whether the waste is placed below or built up above the original ground surface, and in where the daily cover soil is sourced from.
| Aspect | Trench Method | Area Method |
|---|---|---|
| Refuse placement | Excavated below original grade, into an engineered trench | Placed directly on the prepared (lined) original ground surface, built up in lifts |
| Cover soil source | Native soil excavated from the trench itself | Imported/hauled from an off-site borrow source |
| Site suitability | Requires deep, easily excavatable soil and a water table well below the trench floor | Suited to sites with a high water table, shallow bedrock, or soil unsuitable for clean excavation |
| Liner installation | Lines the excavated trench walls and floor | Lines the prepared original ground surface before the first lift is placed |
| Typical application | Flat sites with deep, well-drained, easily excavated soils | Low-lying land needing above-grade fill, or reclaimed quarries/pits above the water table |
In the trench method, a trench is excavated ahead of the working face, lined along its walls and floor, and refuse is placed and compacted in lifts within it; the excavated soil becomes the daily cover for that same trench, an economical arrangement wherever geotechnical conditions permit deep excavation without encountering groundwater. In the area method, no excavation into native grade occurs — often because a shallow water table or exposed bedrock rules it out — the liner is placed on the prepared original surface, and refuse is built up in successive lifts to the design height, progressively raising the land surface; because no soil is generated on site, all daily/final cover must be hauled in from an off-site borrow source, which is the method's principal added operating cost. The choice between the two is therefore driven by geotechnical and hydrogeological site conditions rather than preference, and a single large facility often uses both methods across different phases of its life as those underlying conditions vary across the site.
