Which of the following could cause foaming in an anaerobic digester?

Foaming in an anaerobic digester can primarily be attributed to organic overload. When the organic material fed into the digester exceeds the system’s processing capacity, it can lead to the rapid growth of microorganisms. These microorganisms produce gas at an accelerated rate, which can trap liquid in foam, leading to the formation of scum on the surface. This foaming can interfere with the digester's efficiency and stability, as it can impede the mixing of the substrate and hinder the digestion process.

While other factors, such as excessive nutrient input or high water flow rates, can also impact digester performance, they are less directly associated with the intense foaming seen with organic overload. Heating the digester above normal temperatures could affect microbial activity but is not a common cause of foaming per se. Understanding the dynamics around organic loading is crucial for maintaining optimal operation conditions in anaerobic digestion.