Question 13 of 15: Gases from Decomposing Manure

Question 13: Gases from Decomposing Manure (5 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.

Anaerobic decomposition of stored manure commonly produces four gases of concern: methane (CH₄) — flammable/explosive and an asphyxiant; hydrogen sulphide (H₂S) — highly toxic even at low concentrations and the most common cause of acute manure-gas fatalities, especially released in a sudden pulse during agitation; ammonia (NH₃) — a respiratory irritant harmful to both workers and livestock at elevated concentrations; and carbon dioxide (CO₂) — an asphyxiant that displaces oxygen in a confined or poorly-ventilated space.

Precautionary measures: (1) run forced ventilation (exhaust fans at full capacity) BEFORE and DURING any manure agitation or pit disturbance, and never agitate manure while animals remain in the barn without full ventilation running, since agitation releases a large pulse of trapped gas very quickly; (2) treat any manure storage/pit entry as a confined-space entry — use a calibrated multi-gas detector to test the atmosphere before entry, follow lockout/permit procedures, and never enter alone (an attendant with rescue means must be present), since a worker who is overcome by H₂S or CO₂ cannot self-rescue and a would-be rescuer entering without protection is the single most common cause of multiple-fatality manure-gas incidents.