Which of the following organisms produce biogas from sludge in the digester tank of a wastewater treatment plant? 

Anaerobic bacteria

The correct answer is option 1. Anaerobic bacteria.

Sludge, as we all know, is the term for the solid portion of sewage. The digester tank is a sizable, enclosed tank where the sludge from the first sedimentation tank's bottom is transferred. Numerous varieties of "anaerobic bacteria" in the digester tank break down the organic material in the sludge to create biogas, also known as sewage gas.