WATER TREATMENT METHOD

PROBLEM TO BE SOLVED: To provide a technique effective for obtaining stable phosphorus removal performance when performing phosphorus removal treatment of water to be treated containing phosphorus.SOLUTION: A water treatment method is disclosed which is applied for a carrier fluidizing biological filtration tank 130 that performs phosphorus removal treatment of water to be treated containing phosphorus in a water treatment apparatus 100. The water treatment method includes: a first step of supplying prescribed metal ions of the amount to be added to phosphorus in the water to be treated; and a second step of supplying at least one of calcium ions and magnesium ions to the water to be treated in such a manner that the total concentration of calcium ions and magnesium ions contained in the water to be treated exceeds the reference total concentration which is set based on the amount of the metal ions supplied in the first step and the prescribed phosphorus removal reference value.