A plant for treatment of a waste water stream comprising fat includes pre-treatment apparatus for removal of liquid oil and gross contaminants, a flow-through assembly for electrocoagulation treatment of a liquid, a floc-separation apparatus arranged for removal of a flocculated fat-containing layer from the aqueous dispersion to provide clarified aqueous solution for disposal and a controller arranged to control voltage and/or current across the electrodes and the flow of the waste water stream through the components of the plant. Also disclosed are methods for using the plant to provide efficient separation with minimised power consumption as flow and contaminant level vary.