Water treatment

In a method of sanitizing and disinfecting swimming pool water by adding an effective sanitizing and disinfecting amount of a sanitizing and disinfecting agent to the water, the improvement which comprises passing the water through or over a bed of said agent consisting of particles of trichloroisocyanuric acid having a mass median particle diameter of from 1 to 20 mm, not more than 10% by weight of which consists of particles of a diameter above the upper limit of the specified mass median particle diameter range, and not more than 10% by weight of which consists of particles of a diameter below the lower limit of the specified mass median particle diamerter range, said bed being contained in a chlorinator unit through which the water is passed, and thereby smoothing out variations in chlorine concentration in said swimming pool water as the particles dissolve and wherein the flow rate of the water is such as to dissolve 5 kg of the trichloroisocyanuric acid over a period of from 30 minutes to 12 hours.