An evaporator having an external vessel and an inner cavity, and a heat exchanging tube assembly, the external vessel comprising an opening closable by a hatch, the heat exchanging tube assembly being insertable into and removable from the inner cavity through the opening. In oil production and in other industrial processes, waste water is often produced which requires treatment. This water may be contaminated with amongst other contaminants Oil & Grease, minerals, Silica, and organic contamination.