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.