Method for decreasing N-phosphonomethyliminodiacetic acid waste water and reducing salt content of the waste water

The invention relates to a method for reducing PMIDA waste water and reducing salinity in the waste water. The method comprises the following steps: after the alkaline hydrolysis of iminodiacetonitrile, adding hydrochloric acid or hydrochloric acid gas to generate iminodiacetic acid through acidification, then carrying out dehydration in a reduced pressure distillation mode, and carrying out high-temperature filtration after the dehydration is completed to obtain filter cake and filter liquor; preparing PMIDA through the condensation reaction of the filter liquor. The processing method which is simple and easy can not only reduce the amount of the waste water, but also improve the product yield and reduce the difficulty of waste water treatment, thereby bringing about higher economic and environmental benefits.