Question 2 of 5: Composting Process, Odour Control and Compost Quality

Question 2: Composting Process, Odour Control and Compost 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) Waste streams required for composting. A well-functioning compost mix needs a balance of carbon-rich and nitrogen-rich feedstocks plus adequate structure and moisture: yard waste/green waste (leaves, grass clippings, brush) supplies both a carbon source and, once shredded, bulking structure for air passages; food waste (residential or institutional/commercial organics) is nitrogen-rich and highly biodegradable but wet and low in structure on its own; compostable paper/cardboard (soiled paper unsuitable for recycling) is a high-carbon, dry bulking agent that helps balance the C:N ratio and moisture of wet food waste; biosolids (dewatered wastewater treatment sludge), where accepted, supply nitrogen and moisture but require careful blending and pathogen control; and a bulking agent (wood chips, shredded pallets) is often added specifically to maintain porosity and free air space in an otherwise dense, wet mix. Successful composting depends on blending these streams to hit target ranges for C:N ratio (~25–30:1), moisture (~50–60%) and free air space, rather than composting any single stream alone.

(b) Key causes of odour in a composting facility. Odour is overwhelmingly a symptom of anaerobic (oxygen-starved) conditions developing somewhere in the pile, which produces malodorous reduced compounds instead of the largely odourless CO₂ and water vapour of healthy aerobic composting. The specific causes are: insufficient aeration/turning, allowing oxygen demand in the pile interior to outstrip the oxygen supplied by passive diffusion or mechanical aeration, so anaerobic pockets form and generate hydrogen sulphide (H₂S, "rotten egg"), organic sulphides/mercaptans, and volatile fatty acids; excess moisture, which fills the pore space that would otherwise carry air, has the same effect as under-aeration and is a very common root cause when feedstock (especially food waste or biosolids) is too wet or the pile is over-compacted; excess nitrogen relative to carbon (low C:N ratio), which drives excess ammonia (NH₃) release, a distinct pungent odour from the sulphur-compound family; feedstock storage/receiving, since raw food waste or biosolids left too long before incorporation into an active pile begin anaerobic decomposition on their own; and curing-pile odour from material that was never adequately aerated during the active thermophilic phase and is only now releasing the accumulated anaerobic byproducts. Facility design and operating practice (adequate turning frequency, moisture control, proper C:N blending, and biofilter/enclosed-hall treatment of process air at larger facilities) directly target each of these causes.

(c) Key parameters defining final compost product quality. Regulatory and end-use compost standards (e.g. under CCME guidance and provincial organic-matter regulations) evaluate finished compost against several parameter groups: stability/maturity — typically measured by respiration rate (oxygen uptake or CO₂ evolution) or a self-heating test, confirming the readily biodegradable fraction has been consumed so the product will not continue to decompose (and rob nitrogen or generate phytotoxins) once applied to soil; pathogen reduction — confirmation the thermophilic phase (temperature/time exposure, e.g. 55 °C for 3 consecutive days with turning per common standards) achieved adequate pathogen kill; nutrient content (total N, P, K and organic matter/carbon content), which determines the product's fertilizer value; particle size/physical contaminants — percent passing a specified screen size, and the concentration of visible physical contaminants (glass, plastic film, metal fragments) that a poorly source-separated feedstock can carry through; trace metal/contaminant concentrations (e.g. Cd, Pb, Hg, Cu, Zn) against regulatory maximum limits, particularly important where biosolids are an input; pH and soluble salt content (electrical conductivity), both of which affect plant growth if the finished compost is applied at high rates; and moisture content of the finished product, which affects handling, storage and shelf life. Together these parameters confirm the product is stable, safe (pathogen- and contaminant-free) and agronomically useful, the three properties a compost quality standard is designed to guarantee.