Step 1: Understanding the Concept:
Biogas production is a biological process (anaerobic digestion) that relies on the activity of microorganisms, primarily bacteria. Like most biological processes, the metabolic rate of these bacteria is highly sensitive to temperature. The question asks for the temperature below which the production rate drops sharply.
Step 2: Detailed Explanation:
1. The bacteria responsible for biogas production can be broadly classified based on their optimal temperature ranges:
- Psychrophilic: Low temperatures (< 20° C).
- Mesophilic: Moderate temperatures (20° C to 45° C).
- Thermophilic: High temperatures (45° C to 60° C).
2. Most common, simple biogas digesters (like those used in rural settings) operate in the mesophilic range.
3. Within the mesophilic range, the optimal temperature for biogas production is generally considered to be around 35° C to 40° C. At this temperature, the bacterial activity is at its peak, and gas production is maximized.
4. As the temperature drops, the metabolic activity of the mesophilic bacteria slows down, and consequently, the rate of biogas production decreases.
5. This decrease is gradual at first, but below a certain point, the activity is significantly inhibited. While production occurs even at lower temperatures, there is a sharp decline in the rate.
6. Below 20° C, the activity of the mesophilic bacteria becomes very low, and gas production falls off steeply. Below about 10-15° C, the production may virtually cease for practical purposes in a simple digester.
7. Therefore, 20° C represents a critical threshold below which performance degrades rapidly.
Step 3: Final Answer:
Biogas production, especially in mesophilic digesters, falls very steeply when the temperature drops below 20° C. Therefore, option (D) is the correct answer.