18-Env-A6 Solid Waste Engineering and Management · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Reference texts: Tchobanoglous, Theisen & Vigil, Integrated Solid Waste Management: Engineering Principles and Management Issues; Vesilind, Worrell & Reinhart, Solid Waste Engineering; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); CCME, Guidance Document on Landfill Gas Management; Canadian Environmental Protection Act, 1999.
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) Biological activity and leachate composition of Phase III (acid formation). Phase III begins once free oxygen and nitrate/sulphate electron acceptors trapped or entrained in the fresh refuse are exhausted (the end of Phase II, transition), and the community shifts to obligate and facultative anaerobic bacteria. These organisms carry out acidogenesis and acetogenesis: the soluble sugars, amino acids and fatty acids produced by earlier hydrolysis of the biodegradable fraction are fermented into short-chain volatile organic (fatty) acids — predominantly acetic acid, with smaller amounts of propionic and butyric acid — along with carbon dioxide, hydrogen and ammonia. This is the phase of most intense biological activity and the highest rate of organic-matter conversion in the whole landfill life cycle.
The resulting leachate is markedly acidic (pH commonly 5–6, sometimes lower) and carries the highest organic strength of any phase — BOD₅ and COD both peak, often reaching several tens of thousands of mg/L, with a high BOD₅/COD ratio (roughly 0.5–0.7) reflecting the readily biodegradable acid content. The low pH also increases the solubility of inorganic constituents in the refuse: heavy metals and nutrients (ammonia-N, phosphorus) leach into solution at their highest concentrations of the whole landfill life, making Phase III leachate the most difficult to treat and the greatest environmental risk if the collection system is not fully functional. Methane production is not yet significant — the accumulating hydrogen and acetic acid are, instead, the substrate that the slower-growing methanogenic population (dominant in Phase IV, Question 5) needs time to establish itself on.
(b) Sanitary landfill cross section. A modern engineered cell is built up from a base liner system through successive lifts of compacted refuse to a final cover, with dedicated ancillary systems for leachate, gas and groundwater monitoring layered throughout, as sketched below.
From base to surface, the essential components are: the native subgrade, prepared and graded to receive the liner; the composite liner (compacted clay plus a synthetic geomembrane), the primary barrier preventing leachate migration into groundwater; the leachate collection and drainage layer immediately above the liner, sloped to convey leachate by gravity to a collection sump for removal and treatment; successive compacted refuse lifts, each capped with a thin layer of daily/intermediate soil cover to control odour, vectors, litter and fire risk between working periods; a gas vent/extraction riser penetrating the refuse mass to safely collect and route the landfill gas described in Question 4(a); a final cover cap (a low-permeability barrier and drainage layer beneath vegetated topsoil) minimizing long-term infiltration once the cell is closed; a groundwater monitoring well down-gradient of the cell confirming the liner is performing as designed; and a perimeter berm defining the cell boundary and containing the active working face.
