A process for treating organic wastewater which comprises treating organic wastewater
with ozone and successively with alkali and introducing said alkali treated organic
wastewater into an anaerobic digestion tank for anaerobic digestion. Instead of
the ozone and successive alkali treatments, ozone treatment in the presence of
hydrogen peroxide or ozone treatment under UV radiation is applicable. Prior to
introduction into the anaerobic digestion tank, the treated wastewater may be separated
into solids and phosphorus is preferably recovered from the solution. Solubilization
of solids in the organic wastewater and transformation of organic substances into
methane are greatly enhanced and sludge to be disposed is greatly reduced. Moreover,
phosphorus is efficiently eluted out and recovered from solids in the organic wastewater.
