PURPOSE: To efficiently treat waste water contg. not only ammonia and components increasing COD but also suspended solids at high concn. by combining liq. phase oxidation in the absence of a catalyst with liq. phase oxidation in the presence of a catalyst and biological treatment.

CONSTITUTION: Waste water contg. ammonia, components increasing COD and suspended solids is successively subjected to liq. phase oxidation with oxygen-contg. gas in the absence of a catalyst and liq. phase oxidation with oxygen-contg. gas in the presence of a catalyst obtd. by supporting at least one kind of component selected among Fe, Co, Mn, Ni, Pd, Pt, Cu, Au, W and water insoluble or slightly soluble compds. of the metals on a granular carrier. The treated water is subjected to anaerobic digestion by anaerobic methane fermentation and/or aerobic treatment by an activated sludge process. The resulting excess sludge is returned to the first liq. phase oxidation stage.