A method for disinfecting microorganisms in metalworking fluids without leaving behind unacceptable amount of chemicals in the treated fluids. The method comprises passing the fluid in a recirculating cooling system through a filter, which contains a demand disinfectant, preferably an iodinated resin. The iodinated resin inhibits the growth of microorganism in the fluid, which is then cycled back into the system. The present invention also relates to a filter and system for disinfecting microorganisms in metalworking fluids.