Method of treating waste water

Claim 1

A waste water-treating method, which comprises the steps of: adjusting the pH value of an aqueous solution containing a nitrous acid radical within a range of 2 to 5; intermittently adding to a quantity of said thus adjusted aqueous solution hydrogen peroxide in an amount far smaller than that which is sufficient but not excessive for complete oxidation of the nitrous acid radical contained in said aqueous solution into a nitric acid radical; measuring, by means of an oxidation-reduction potentiometer, the oxidation-reduction potential of the aqueous solution thus treated with hydrogen peroxide when said potential is in a stable state after each charge of hydrogen peroxide; amplifying the measured potential by an amplifier; when a measurement timer issues a measurement-commencing instruction after addition of the respective amounts of hydrogen peroxide, converting the measured amplified voltage into a digital signal by an analog-digital converter; causing the digital data to be stored in an up-down counter; computing in said up-down counter a variation between the digitally expressed oxidation-reduction potentials measured at the respective adjacent points of time; comparing by means of a comparator a variation between the oxidation-reduction potentials measured at the respective adjacent points of time which indicates a decrease in said potential with a predetermined amount preset in a digital presetter; stopping the addition of hydrogen peroxide when a variation between the oxidation-reduction potentials measured at the respective adjacent points of time is larger than said predetermined amount, thereby indicating a decrease in said oxidation-reduction potential; and throwing away the aqueous solution in the neutralized form.