Problem: The anaerobic digester supernate has a rancid-butter odor and the COz has increased to a point where no burnable gas is produced. The sludge feed, sludge withdrawal and VA:Alk ratio are all within normal operating ranges. What could be the cause of the digester upset? O The feed sludge from the gravity thickener was septic • The WAS from the secondary clarifier was septic • Toxic amounts of heavy metals entered the digester in the feed sludge • Too much fats, oil and grease (FOG) were fed into the digester