Rate of decay, compost and biogas (HT)

Higher-tier students need to be able to investigate how decay rate depends on environmental conditions, calculate rates from data, and explain the use of decomposition in composting and biogas generation.

What is rate of decay?

Decomposition is carried out by bacteria and fungi (decomposers). They release enzymes onto dead material and absorb the soluble products (extracellular digestion). The rate of decay can be measured by:

• The mass loss of the dead material over time.
• The volume of CO₂ produced.
• The pH change as acids are released.

A typical exam graph: % mass remaining of a leaf litter sample plotted against time. The gradient gives rate of decay (e.g. % mass loss per day).

Factors affecting rate (the required practical question)

A school-friendly experiment: leave milk samples (or fruit, leaf disks) in different conditions and time how long it takes them to decay (e.g. for the pH to fall by a set amount).

Independent variable: temperature, moisture, oxygen availability or pH. Dependent variable: time taken to spoil OR mass loss OR pH change OR CO₂ production. Controls: keep all other variables constant; same starting mass; same starting pH.

Temperature

• Decay enzymes work faster as temperature increases (until denaturation, ~50 °C+).
• Below freezing, decomposers stop activity (food in freezers keeps for months).

Moisture

• Bacteria/fungi need water for their enzymes to function.
• Dry conditions slow decay (mummies in desert tombs).

Oxygen

• Aerobic decomposers respire faster with O₂.
• Anaerobic decay still occurs but is much slower (and produces methane).

Compost — gardener's friend

Compost is decayed plant material used as a natural fertiliser. To speed up composting:

• Mix plant waste with grass cuttings (high nitrogen).
• Keep moist but not waterlogged.
• Turn regularly to add oxygen (aerobic).
• Cover lightly to retain heat.

Heat builds up in a well-tended heap (50–65 °C) — this is metabolic heat from decomposers' respiration.

Biogas — anaerobic decomposition for fuel

A biogas generator uses anaerobic bacteria to break down organic waste (animal manure, sewage, food waste). The product is biogas, mainly methane (CH₄).

Biogas can be burnt for heating, cooking or generating electricity. Two scales:

• Small-scale (village in India / China) — a brick-lined pit fed with kitchen and animal waste. Provides cooking gas and reduces firewood demand.
• Large-scale industrial — sealed tanks digest sewage or farm waste; biogas captured and burnt.

Why is biogas useful?

• Renewable — uses constantly produced waste.
• Reduces methane release — methane would otherwise leak from rotting waste and contribute to global warming. Capturing and burning it converts CH₄ to CO₂ (less potent).
• Local energy supply in regions without grid electricity.

Anaerobic digestion — conditions

• Temperature: ideally 30 °C (mesophilic) or 50–60 °C (thermophilic).
• pH: near neutral.
• No oxygen.
• Continuous supply of fresh organic matter.

Calculating rate from a graph (worked example)

A leaf-litter sample loses 60 % of its mass in 30 days at 20 °C and 40 % in 30 days at 10 °C.

• Rate at 20 °C = 60 % / 30 d = 2 % per day.
• Rate at 10 °C = 40 % / 30 d = 1.3 % per day.
• The rate is roughly 1.5× faster at the higher temperature.

A 10 °C rise often roughly doubles enzyme-controlled rates (the Q₁₀ rule).

Links

Builds on B7.3 (carbon cycle, decomposition). Connects to B7.6 (greenhouse gases — methane is a major greenhouse gas).