Water and sludge were carried through the upflow pipe to the gas–liquid
separator by the biogas. At the separator, the water-sludge mixture
was separated from the gas and dropped directly into the mixed
zone at the bottom of the reactor, resulting in an internal circulation
flow [11–13]. In the 1st UASB, the waste was recycled from the
top to the bottom by a peristaltic pump, resulting in the sludge
being suspended in the reactor, which was known as the external
circulation system.