TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Process: In (M1), the organic waste-liquid is a liquid containing starch and nitrogen contents. The liquid after performing electrolytic-oxidation is returned to methane-fermentation process. The liquid after performing coagulation and sedimentation is returned to methane fermentation process. The stock solution is pretreated using ozone, heat or hydrogen-peroxide solution before subjecting to methane fermentation processing. The organic waste-liquid is a waste-liquid of sewage-sludge. The stock solution is concentrated before subjecting to methane fermentation and the supernatant obtained after concentration process is subjected to electrolytic-oxidation process with a digestive liquor after methane fermentation. The electric power used for electrolytic-oxidation process is obtained from electric power generated by the solar cell and from the methane gas obtained at the methane fermentation process.Preferred Plant: In (I), the liquid after electrolytic-oxidation process and coagulation-sedimentation process is returned to methane-fermentation tank. (I) further comprises a pre-processing tank which pretreats the stock solution to the opening side of the methane fermentation tank using ozone, heat or hydrogen-peroxide solution. (I) further comprises a thickener tank for concentrating the stock solution before introducing into the methane fermentation tank. The supernatant liquid obtained from the concentration process is introduced into electrolytic-oxidation tank with a digested liquor. After performing methane fermentation in the electrolytic-oxidation tank. EXAMPLE - A stock solution containing organic waste liquid with starch and phosphorus was introduced into a methane-fermentation tank and the produced methane gas was collected. The solution was transferred to electrolytic-oxidation tank for decomposing organic substances containing carbon and nitrogen. The electric power used in the electrolytic-oxidation process was obtained from power generated from methane gas. Coagulation and sedimentation processing was performed efficiently to remove phosphorus in a non-alkaline condition and thus a significant amount of compost was obtained from the sludge.