The present invention relates to the anaerobic digestion treatment of biochemical wastes. More particularly, the invention relates to a possess for removing BOD (biological oxygen demand) and nitrogen and phosphorus from waste water discharged from the process of the anaerobic digestion treatment of a biochemical waste.
The anaerobic digestion treatment of biochemical wastes have heretofore been adopted for treating wastes having a high BOD value, for example, excessive activated sludges, excretions and waste waters from the alcohol distillation process. The anaerobic digestion treatment has recently attracted attention in the art because of various advantages. For example, since the digestion gas generated as a by-product can be used as the energy source of the treatment apparatus, the treatment cost is low, and the digestion sludge discharged as a by-product can be used as a high quality organic fertilizer.
One example of such anaerobic digestion treatment is disclosed in the specification of U.S. Patent Application Ser. No. 685,901 filed on May 12, 1976 now U.S. Pat. No. 4,067,801, and this treatment process comprises the following two main steps; the primary treatment step of subjecting the starting waste water to pulverization, heating and acid addition to form an organic slurry, and the secondary treatment step of subjecting the organic slurry to anaerobic fermentation in the presence of bacteria to decompose it to digested sludge and methane gas. In general, since BOD is left at a concentration of several hundred ppm in the anaerobically treated water (the effluent separated from the digested sludge by solid-liquid separation), the treated water is diluted and subjected to the aerobic digestion treatment to remove BOD therefrom and then, the treated water is discharged into sewerage or the like. This discharged water still contains 500 to 800 ppm of nitrogen and about 10 ppm of phosphorus. Accordingly, the anaerobic digestion treatment involves a problem of occurrence of secondary pollution by promotion of eutrophication of rivers, seas and the like by nitrogen and phosphorus contained in discharged water. As means for removal of nitrogen and phosphorus, there are known physical, chemical and biological processes, but there is not known a process for removing nitrogen and phosphorus from waste waters from the anaerobic digestion treatment process.