In waste water treatment apparatus and method, a mineral mixing tank receives biologically treated water, sludge which is generated by biological treatment, and mineral sludge which contains calcium and so on from a settling tank. A mineral pump returns the sludge and the treated water from the mineral mixing tank to a raw water tank. An air-lift pump circulates treatment water between a reaeration tank having a semi-anaerobic section and a denitrification tank. In circulation of the treatment water between the reaeration tank and the denitrification tank, the semi-anaerobic section alleviates the change of environment for microorganisms and thereby realizes the environment easy to propagate for the microorganisms. The air-lift pump allows the agitation with low energy consumption even when the microorganisms are cultured up to a high concentration thereof.
