An apparatus is operated in a method that comprises the steps of providing a first flotation tank (5) with a first riser pipe (11) erected in a first flotation chamber, providing a continuous flow (E) of effluent (2) to the first flotation tank (5) via the first inlet (10), providing a first mixture (Ml) of a first liquid medium (LM1) with microbubbles (15) of a first flotation vehicle (FV1) in form of a first inert, anoxic or low- oxygen gas and pumping the first mixture (Ml) to the first flotation tank (5), and allowing the microbubbles to attach to organics inside the first riser pipe to form aggregates that can float and settle at the top zone of the first flotation tank and can be further digested in the digester of the biogas plant. A further processing and refining of a lower phase can take place in a second flotation tank in a similar manner. Process gas is the gas already present in the digester and dispersion water for microbubbles is water re-circulated from a second lower phase formed in the secon
