PURPOSE: To efficiently form the calcium salt of alginic acid and to form secure and large flocculated flocs by adding a soln. of sodium alginate to waste water, then adding the calcium salt which is not alkaline and has high solubility thereto.

CONSTITUTION: The sodium alginate is added to the waste water contg. the suspended solid (SS) to crosslink and bond the SS. A large amt. of the calcium salt such as CaSO4.2H2O, Ca(H2PO4)2.H2O or Ca(CH3COO)2.H2O which has large solubility and is not strongly alkaline is thereafter added thereto. The reaction for forming calcium salt of the alginic acid is thereby extremely effectively executed. The securely flocculated flocs are thereafter stably formed and deposited from the inside of the weakly alkaline waste water simply by adding a small amt. of slaked lime thereto.