Question 14 of 20: Factors Affecting LFG Production

Question 14: Factors Affecting LFG Production (4 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.

Four factors, distinct from the three core biological-process variables of Question 13: (1) refuse density/compaction — denser compaction restricts oxygen ingress (favouring the anaerobic conditions gas production needs) but can also restrict moisture distribution; (2) pH within the waste mass — methanogenic bacteria are inhibited outside a roughly neutral pH range, so accumulation of volatile fatty acids early in decomposition (which depresses pH) can suppress methanogenesis until buffering capacity develops; (3) presence of toxic or inhibitory substances (heavy metals, high salt/ammonia concentrations, certain industrial co-disposed wastes) that can poison the methanogenic population; and (4) oxygen availability/depth of cover — methanogenesis is strictly anaerobic, so any oxygen ingress (poor cover, shallow placement) suppresses gas generation locally until anaerobic conditions re-establish.

These four secondary factors explain why real landfill gas-generation curves diverge substantially from the idealized first-order decay model in practice: the pH-buffering delay after fresh waste placement produces the characteristic acid-phase lag before methanogenesis ramps up, and localized zones of poor compaction or shallow cover create pockets of aerobic or transitional conditions that generate little or no methane even while the bulk of the cell is fully anaerobic and productive. A gas-collection system design that ignores this spatial and temporal heterogeneity — assuming uniform production across the whole cell from day one — will systematically mis-predict both the timing and the magnitude of recoverable gas, undermining the economic case for any planned beneficial-use project relying on that gas. The practical implication is that a well field should be staged to match the cell's actual filling sequence rather than installed as a uniform grid at closure, since wells placed over freshly filled areas will draw little useful gas for months while the pH-buffering lag plays out.