This unit includes:

• at least one receptacle formed to extend directly from the bottom of a filtration compartment, delimiting a peripheral space in which a pipeline is formed.
• Two spacer webs link the receptacle to the interior of the compartment over the entire height of the said receptacle, thus delimiting the pipeline.
• The bottom has an opening for communication with the pipeline and an
• upper end of the pipeline fits onto a connector to a suction intake of a pump.