Abstract: The invention provides a primary treatment unit and system for the collection of methane-containing biogas from sewage. The primary treatment unit is configured to separate sewage settling and sludge digestion into separate regions or chamber of the tank. The settling region is adapted to receive the sewage and output liquid effluent and the digestion region is adapted to receive solid components of the sewage from the settling region and output methane-containing biogas substantially generated in the digestion. The primary treatment unit is designed such that the methane-containing biogas is outputted without coming into substantial contact with sewage scum. The system further comprises a biogas collection system configured to receive the methane-containing biogas from the digestion region of the primary treatment unit.