From the primary clarifier, the effluent is piped to a mixing tank where it is combined with the stream of effluent coming from the bleaching process and which is contaminated with different acids. The mixed stream is sent to a cooling tower, as a specific temperature is necessary for the UNOX reactor to properly fulfil the next operation, which is the further removal and digestion of fibrous substances from the effluent. Digestion occurs by introducing a variety of micro-organisms into the effluent. It has a roof made of prefabricated hollow core covers, on top of which is poured a 3-in. The roof is necessary because oxygen is introduced under pressure into the effluent to speed the digestion process and to maintain the living micro-organisms that are used in the process.
Agitators hanging from the roof mix the effluent.
The reactor's infrastructure is composed of two rows of chambers, each consisting of four compartments. The size of the compartments permits a precise flow and a proper mixing velocity in the treatment of the effluent which, at full mill production stream, takes six hours to pass through the UNOX reactor. The mixture then goes to one of the secondary clarifiers, a structure similar to the primary clarifier, where the micro-organisms are separated from the effluent stream. During that process, approximately 50 percent of the micro-organisms are sent back to the UNOX reactor. The rest is sent to the sludge thickener where the moisture is removed, the separated liquids sent back to the treatment system and the sludge is sent to the boiler heaters where it is burned.