A method and an apparatus for treating organic wastewater and sludge which
remarkably reduce the generated amount of sludge at a much lower running
cost, and which reduce the size and the capacity of the solubilization
tank. The method and apparatus for treating sludge employs a biological
treatment system, wherein a biological treatment of wastewater is carried
out, and a solid-liquid separation unit for separating a solid from a
liquid in the wastewater after the biological treatment, to obtain a
treated wastewater and a sludge. The apparatus has a means for obtaining
a withdrawn sludge from a part of the sludge and an alkali-treatment
tank, wherein an alkali-treatment of the withdrawn sludge is carried out.
The apparatus also has a biological solubilization tank which solubilizes
the sludge after the alkali-treatment under an anaerobic, anoxic or
microaerophilic condition, and a means for recycling the solubilized
sludge to the biological treatment system.