Question 3 of 5: Composting Process, Odour and Product Quality

Question 3: Composting Process, Odour and Product Quality (25 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.

(a) Steps in composting. (1) Receiving and preprocessing — incoming feedstock is inspected and sorted to remove contaminants (film plastic, glass, metal) and mechanically size-reduced to increase surface area for microbial attack. (2) Mixing and amendment — feedstock is blended, usually with a bulking agent (wood chips, yard trimmings), to bring the mix to a target C:N ratio of roughly 25–30:1 and a moisture content of 50–60%, while improving porosity for aeration. (3) Active (high-rate) composting — the mix undergoes rapid aerobic decomposition through a mesophilic stage followed by a thermophilic stage (40–65°C, where most decomposition and pathogen destruction occurs), typically over 3–8 weeks with regular turning or forced aeration. (4) Curing/maturation — the partially stabilized material is held at near-ambient temperature for several weeks to months so remaining degradable material stabilizes and phytotoxic compounds break down. (5) Screening and refining — the cured compost is screened to remove oversize material and residual contaminants. (6) Storage and marketing/distribution — the finished product is stockpiled and distributed to its end market.

(b) Key causes of odour in a composting facility. The dominant cause is the formation of anaerobic microsites within the pile: whenever aeration (turning frequency, or air-supply rate in an ASP) is insufficient relative to the oxygen demand of the decomposing material, localized oxygen-depleted pockets generate malodorous anaerobic by-products — hydrogen sulphide, organic acids (butyric, valeric) and amines — the classic "sour" composting odour. Excess moisture compounds this directly, since water filling the pile's pore space physically excludes the oxygen that turning or forced air is trying to supply. A second cause is feedstock imbalance: an overload of high-nitrogen, high-moisture material (food waste, manure) relative to carbon/bulking material overwhelms the pile's aerobic capacity and again drives it anaerobic. Poor pile management — infrequent turning, inadequate initial mixing leaving un-blended pockets of raw feedstock, or an oversized pile whose core air cannot reach — produces the same effect mechanically. Odour can also originate outside the active pile, from putrescible feedstock sitting in the receiving area before processing, or from ponded leachate/runoff at the site.

(c) Key parameters defining final compost product quality. Maturity/stability (assessed via respiration rate, a finished C:N ratio of roughly 15–20:1, or a self-heating test); moisture content (target ≈40–50% for stable handling and storage); particle size and uniformity after screening; pH (near neutral, roughly 6–8, for direct plant-growth use); nutrient content (total N–P–K and organic matter percentage, determining its value as a soil amendment); physical and chemical contaminant levels (residual glass/plastic fragments, heavy metals, and pathogen indicators such as fecal coliform held below the regulatory threshold, confirming the thermophilic stage achieved adequate pathogen kill); absence of objectionable odour (a well-cured product should smell earthy, not sour or ammoniacal); and a germination/phytotoxicity bioassay confirming no residual phytotoxic compounds remain.