Composting: Process and Substrates

Composting

• Composting is a thermophilic, aerobic decomposition process using solid substrates like municipal waste, sewage sludge, or raw sewage.
• Microorganisms break down these materials over weeks to create stable organic matter (compost).
• Compost is a dark brown, granular, humus-like end product.
• Composting converts unstable, offensive material into a stable product, effectively disposing of domestic refuse and recycling solid waste.
• It differs from anaerobic digestion of sewage sludge, as it's an aerobic process.

Substrates for Composting

• Substrates range from municipal refuse to sewage sludge, paper, garbage, agricultural waste (leaves), and human faeces.
• Stable compost requires a suitable carbon:nitrogen (C:N) ratio of roughly 50.
• Rubbish and faecal material have a high C:N ratio and need mixing with other materials to lower the ratio (e.g., garbage, sewage sludge, cow manure, or fish farm/cannery waste).

Composting Process

• The composting process has five major stages: salvage, grinding/homogenization, degradation, drying/curing, and finishing.

• Salvage: Removal of non-biodegradable materials (glass, metal, plastic, etc.) from the refuse.

• Grinding/homogenization: Substrates are ground to a fine, granular material to allow for proper aeration, mixing, and microbial invasion. Wet pulping can also be used to produce a slurry. This slurry needs drying to a suitable solid content.

• Biological degradation:

  • Large heaps/windrows can be created, or mechanical composting drums can be utilized, to compost materials aerating and mixing the materials.
  • Bulking agents (wood chips or barks) can be added to promote aeration.
  • The process takes about 21 days in simple composting.

• Curing and drying: Critical stability stage to ensure nitrogen isn't lost to degradation. This takes about two weeks in windrows or less time in mechanical systems.

• Finishing: Depending on the use of the compost (for sale or disposal), further treatment, such as drying or pelletizing may be necessary to meet quality standards. Mechanical processes may take up to three weeks for curing.

• Moisture content of 40-50% is ideal for composting. Lower moisture content can hinder microbial action.

• The C:N ratio should be appropriately balanced.

Microbiology of Composting

• Different microorganisms (mesophilic and thermophilic) participate in the decomposition process.
• Mesophiles: decompose materials at ambient temperatures.
• Thermophiles: take over at higher temperatures (60°C), accelerating the decomposition and killing pathogenic organisms and ungerminated weed seeds.
• High temperatures are crucial for effective decomposition. The high temperature process also significantly helps reduce any harmful microorganisms.

Problems and Remedies in Composting

• Composting may encounter challenges concerning contamination from non-biodegradable materials, market issues, and the presence of larvae.
• Specific problems such as the presence of uncompostable materials, the unavailability of a market for the compost, and the presence of night soil, need to be addressed.
• A suitable moisture level needs to be met to allow effective microbial action for the most efficient decomposition process.
• Problems with odors and lack of suitable space may occur depending on the environment.