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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

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 The apparatus has a means for obtaining a withdrawn sludge from a part of the sludge and an alkali-treatment tank, wherein an alkali-treatment of the withdrawn sludge is carried out.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.

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What is claimed is: 1. A method for treating a wastewater comprising: (a) treating a wastewater biologically in a biological treatment tank; (b) separating a solid from a liquid in the wastewater treated biologically from step (a), to obtain a treated wastewater and a sludge; (c) withdrawing a port

What is claimed is: 1. A method for treating a wastewater comprising: (a) treating a wastewater biologically in a biological treatment tank; (b) separating a solid from a liquid in the wastewater treated biologically from step (a), to obtain a treated wastewater and a sludge; (c) withdrawing a portion of the sludge from step (b) to obtain a withdrawn sludge; (d) treating the withdrawn sludge from step (c) by an alkali-treatment to reduce the volume of the sludge; (e) biologically solubilizing and decomposing the withdrawn sludge from step (d), under an anaerobic, an anoxic or a microaerophilic condition and in the presence of a sludge decomposing microorganism to produce a solubilized sludge; and (f) returning the solubilized sludge from step (e) to the biological treatment tank of step (a). 2. The method according to claim 1, wherein in step (d), withdrawn sludge from step (c) is treated by an alkali-treatment at a pH value of 7 to less than 9 and for a retention time of 3 hours to 24 hours; and in step (e), the solubilizing comprises solubilizing the withdrawn sludge from step (d), at a temperature of 5° C. to 35° C., at atmospheric pressure and for a retention time of 1 day to 3 days. 3. The method according to claim 1, further comprising the step of thickening the sludge before treating the withdrawn sludge by the alkali-treatment. 4. The method according to claim 1, wherein step (e) is carried out under an anaerobic condition. 5. A method for treating a sludge comprising: (a) biologically treating a wastewater to generate the sludge; and (b) treating the sludge from step (a) by an alkali-treatment, by intermittently adding an alkali to the sludge at an interval of 0.5 hours to 48 hours. 6. The method according to claim 5, further comprising a step of biologically solubilizing a sludge after treating the sludge by the alkali-treatment under an anaerobic, an anoxic or a microaerophilic condition. 7. The method according to claim 6, wherein the alkali-treatment is carried out at a temperature of 5° C. to 35° C. and at atmospheric pressure; and the solubilizing of the sludge is carried out at a temperature of 5° C. to 35° C. and at atmospheric pressure. 8. The method according to claim 6, wherein a retention time of the sludge in the alkali-treatment step (c) is 3 hours to 24 hours and a retention time of the sludge in the biological solubilization step (d) is 1 day to 3 days. 9. The method according to claim 6, wherein step (b) is carried out under a microaerophilic condition. 10. A method for treating a sludge comprising: (a) biologically treating a wastewater to generate the sludge; and (b) treating the sludge from step (a) by an alkali-treatment, by intermittently adding the alkali to the sludge at a determined interval, wherein a retention time of the sludge in the alkali-treatment step (b) is 3 hours to 24 hours and a retention time of the sludge in the biological solubilization step (a) is 1 day to 3 days. 11. A method for treating a sludge comprising: (a) biologically treating a wastewater to generate the sludge; and (b) treating the sludge from step (a) by an alkali-treatment, by intermittently adding the alkali to the sludge at a determined interval, wherein a retention time of the sludge in the alkali-treatment step (b) is 3 hours to 24 hours and a retention time of the sludge in the biological solubilization step (a) is 1 day to 3 days, and wherein the alkali-treatment is carried out at a temperature of 5° C. to 35° C. and at atmospheric pressure; and the solubilizing of the sludge is carried out at a temperature of 5° C. to 35° C. and at atmospheric pressure. 12. A triethod for treating a wastewater comprising: (a) biologically treating a wastewater in a biological treatment system; (b) separating a solid from a liquid after treating the wastewater biologically, to obtain a treated wastewater and a sludge, and to obtain a withdrawn sludge from a part of the sludge; (c) treating the withdrawn sludge from step (b) in an alkali-treatment to obtain an alkali-treated sludge, by intermittently adding an alkali at a determined interval, wherein a retention time after adding the alkali is 3 hours to 24 hours; (d) biologically solubilizing the alkali-treated sludge from step (c) at a temperature of 5° C. to 35° C. and at atmospheric pressure in an anaerobic, an anoxic, or a microaerophilic atmosphere, wherein a retention time after adding the alkali is 1 day to 3 days, to obtain a solubilized sludge; and (e) recycling the solubilized sludge from step (d) to the biological treatment system of step (a). 13. A method for treating a wastewater comprising: (a) biologically treating a wastewater in a biological treatment tank to obtain a biologically treated wastewater; (b) separating a solid from a liquid in the biologically treated wastewater from step (a) to obtain a treated wastewater and a sludge; (c) treating a part of the sludge from step (b) by an alkali-treatment, within a pH range of 9 to 12.5 to reduce the volume of the part of the sludge; (d) biologically solubilizing the sludge from step (c) in an anaerobic, an anoxic or a microaerophilic condition and in the presence of a sludge decomposing microorganism to reduce the volume of the sludge to obtain a solubilized sludge; and (e) returning the solubilized sludge from step (d) to the biological treatment tank of step (a). 14. The method according to claim 13, further comprising a step of thickening the sludge before treating the sludge by the alkali-treatment. 15. The method according to claim 13, wherein a retention time of the sludge in the alkali-treatment step (c) is 3 hours to 24 hours and a retention time of the sludge in the biological solubilization step (d) is 1 day to 3 days.