The objective of the Anaerobic Digester, "PDAN", is to study and understand the stages of the anaerobic digestion, as well as the anaerobic digestion process itself. It is supplied with two packed anaerobic digesters. In this way, the user can work either in only one stage or in two stages, separating the different phases of the digestion process (the processes of hydrolysis, acidogenesis and acetogenesis would take place in the first digester, and the methanogenesis in the second digester).

Both digesters have a heating water circuit with valves to regulate the appropriate temperature in each part of the process and the operation with different ranges depending on the microorganisms used. Thus, it can operate at the psychrophilic range (room temperature), mesophilic range (temperatures around 35 °C) or thermophilic range (temperatures around 55 °C). The heating system of the digesters consists in making hot water from a thermostatic bath flow through the jacket of the reactor.

The unit has two peristaltic pumps to impel the supply to be introduced in the digesters. When working with a two-stage anaerobic digestion, one of the pumps carries the product from one of the digesters to the other, passing through a buffer tank, which collects the excess of flow from the first reactor. Two flowmeters measure the working flows.

Two volumetric tanks are also included for the storage and volume measurement of the generated biogas. The generated biogas flows through a pipe from the upper side of the digesters to these tanks, where the biogas volume is measured by means of a water displacement. Such tanks have two parts: the upper side is where the generated biogas is collected and the second part, smaller than the first one and located below it, is used to collect the displaced water.