If the manometer indicates the gas pressure in a digester is below normal, what could be a probable cause?

When interpreting a lower-than-normal gas pressure reading in a digester, one plausible explanation is the addition of too much lime. The purpose of lime addition is to control pH, but excessive lime can severely alter the chemical environment within the digester. This alteration can inhibit the activity of the microorganisms responsible for digestion, leading to a reduced rate of biogas production.

In anaerobic digesters, the microorganisms break down organic matter to produce biogas. If their activity is impaired, the production of gas diminishes, resulting in lower pressure readings on the manometer. This change in gas production can be directly associated with the impact of over-application of lime, which can raise the pH to levels that are unfavorable for microbial activity.

The other options provide different scenarios that typically would not lead to lower gas pressure readings in the same direct manner. For instance, increased microorganism activity usually corresponds with greater gas production, and too little sludge would likely result in inadequate feedstock for the digestion process but might not directly correlate to a drop in pressure as an immediate effect. High volatile solids concentration generally indicates there is more organic matter available for digestion, which is likely to enhance gas production rather than suppress it. Therefore, understanding the relationship between lime addition and