18-Env-A6 Solid Waste Engineering and Management · December 2016
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.); Freeze & Cherry, Groundwater; 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.
Three governing variables: (1) waste composition — the fraction of readily biodegradable organic material (food waste, yard waste, paper) present, since only the biodegradable organic fraction generates methane and carbon dioxide through anaerobic decomposition; (2) moisture content — anaerobic microbial activity requires water, so moisture content (and its replenishment via leachate recirculation or infiltration) directly controls the rate of decomposition and gas generation; and (3) temperature within the waste mass — mesophilic and thermophilic anaerobic bacteria have optimum temperature ranges, and gas generation rate rises with temperature up to those optima before falling off.
These three variables are also the levers a landfill operator can actively manage to accelerate or decelerate gas production for engineering purposes: leachate recirculation (deliberately re-wetting the waste mass) is used at "bioreactor" landfills specifically to raise moisture content above the passive-fill baseline, driving faster decomposition and a higher, more front-loaded gas-generation curve that can be captured and beneficially used sooner, rather than trickling out over many decades. Conversely, a dry-tomb design (minimizing moisture ingress through an effective final cover) deliberately suppresses decomposition and stretches gas generation out over a much longer period at lower peak flow — the choice between the two design philosophies is a direct application of understanding which variable is the controlling one. Larkspur Bay's own landfill design team would weigh this tradeoff against the intended end use of the captured gas: a bioreactor approach suits a facility with an immediate beneficial-use market for the concentrated early gas, while a dry-tomb approach may better suit a smaller site with no nearby gas-to-energy customer.
