At temperatures less than ____________, the rate of volatile solids reduction drops so low that aerobic digestion processes are rendered ineffective.

The correct answer is based on the fact that at temperatures below 10°C (50°F), the biological processes involved in aerobic digestion significantly slow down, leading to a drastic reduction in the efficiency of volatile solids reduction. Aerobic digestion relies on the activity of microorganisms that thrive at higher temperatures. As the temperature decreases, the metabolic activity of these microorganisms also drops, which ultimately affects their ability to break down organic matter.

The threshold of 10°C is particularly critical because below this point, not only is the activity of the microbes compromised, but the overall digestion process becomes inefficient. This is why maintaining optimal temperature conditions is essential in wastewater treatment processes to ensure that biological treatment methods, like aerobic digestion, function effectively.

Higher temperatures, such as those indicated in the other options (20°C, 30°C, and 40°C), support enhanced microbial activity and thus effective solid reduction. Therefore, those temperatures do not hinder the aerobic digestion processes in the same way that temperatures lower than 10°C do.