A fluid treatment apparatus is described for the treatment of a fluid substance having multiple component substances to control levels of one or more particular component substances. The apparatus has a reactor chamber; a fluid inlet adapted to provide fluid communication from an external supply of a fluid substance to be treated to said reactor chamber whereby said fluid substance may pass into and through said reactor chamber; a fluid outlet adapted to provide a fluid communication from said reactor chamber whereby said fluid substance may pass from said reactor chamber; at least one first electromagnetic radiation (EMR) waveguide, having at least one first waveguide input port operably coupled within said reactor chamber and adapted to couple electromagnetic radiation of a predetermined first wavelength to a fluid substance passing through said reactor chamber. A method for the treatment of a fluid substance is also described.