Method for treating organic waste water by methane fermentation.

Abstract: Proliferation activity of methane bacteria in organic waste water can be improved by maintaining the ratio of the amounts of nickel, iron and cobalt contained in the waste water to the biological oxygen demand (BOD) thereof to not lower than certain values, and various organic waste waters, such as waste liquor formed during the production of beer and the like, can be treated in a high BOD decrease of not lower than 90% while maintaining a high BOD loading rate of not lower than 10 kg/m³·day.

Description: This invention relates to a method for treating organic waste water by the methane fermentation, which method is adapted for treating organic waste water, such as waste liquor formed in the production step of beer or the like, through a biological treatment by the use of mesophilic or thermophilic methane bacteria. As the above described treating method of organic waste water, there are known an aerobic treatment by the use of active sludge and an anaerobic treatment by the use of methane bacteria. Recently, there has been noticed a treating method, wherein an organic substance is decomposed into methane gas and carbon dioxide with the use of mesophilic or thermophilic methane bacteria by means of an anaerobic fixed bed. However, in the case where an alcohol waste liquor, such as waste liquor formed in the production step of beer, is treated by the methane fermentation, when the biological oxygen demand (BOD) loading rate is as low as about 1-3 kg/m³. day, the treatment can be effected without trouble, but when the BOD loading rate is higher than about 3 kg/m³.day, the BOD decrease is gradually reduced, and a satisfactorily high treating effect can not be obtained. Japanese Patent Laid-open Application No. 64-47,498 discloses a method for improving the efficiency of methane fermentation by adding a mineral and a nutrient, such as amino acid or the like, to alcohol waste liquor. However, according to a test of this method by the inventors in practical operation, this method is still unsatisfactory in the BOD loading rate and BOD decrease. According to the investigation by the inventors, in the treating apparatus using an anaerobic fixed bed of this kind, its treating ability is highly influenced by the value of the ratio of the apparent surface area of the carrier, which carries a microorganism thereon, to the effective volume of the anaerobic treating apparatus. That is, in the anaerobic treatment, the BOD contained in waste water is decomposed into organic acid (through acid fermentation) by the action of acid fermentation bacteria contained in the liquid phase, and the resulting organic acid is decomposed into methane gas by the action of the methane bacteria fixed to the surface of a carrier, and therefore it is important that the methane fermentation in the second half stage proceeds in a rate higher than the rate of the acid fermentation in the first half stage in order to proceed that the anaerobic treatment proceeds smoothly. For this purpose, it is necessary that the value of the ratio of the surface area of the carrier, which is the location of residence of methane bacteria, to the volume of the liquid phase, which is the residence of acid fermentation bacteria, is not lower than a certain value, and it has been necessary that the value of the ratio of (surface area of carrier)/(effective volume of liquid) is at least 20 m²/m³ as disclosed in Japanese Patent Laid-open Application No. 63-248,497. Speece et. al. in Water Research, 17 (1983), No. 6 pages 677-683 describe a research programme investigating the effect of nickel concentration on stimulation of anaerobic digestion in a standardized acetate-enriched methanogenic culture. It is mentioned that iron and cobalt are also stimulatory. There is no mention of BOD. One of the objects of the present invention is to solve the above described problems in the conventional methods and to provide a method for treating organic waste waters by methane fermentation, which can treat organic waste liquors, such as waste liquor formed in the production step of beer and the like, at a high BOD decrease, e.g. of not lower than 90%, while maintaining a high BOD loading rate, e.g. of not lower than 10 kg/m³.day. The present invention can provide a method for treating organic waste waters by methane fermentation, which may attain the above described object while using an apparatus having such a value of the ratio of (surface area of carrier)/(effective volume of liquid) that is considerably lower than the value of the ratio in the conventional method. The inventors have made various investigations in order to solve the above described problems, and found out that the reason why the increasing of the BOD loading rate results in the lowering of the BOD decrease lies in the lowering of the proliferation speed of methane bacteria due to the shortage of nutrient, and that the use of the three elements nickel, iron and cobalt in amounts rather larger than the amounts, which have hitherto been considered to be necessary in the conventional method, is necessary for improving the proliferation activity of methane bacteria. The inventors have further ascertained that, when the proliferation activity of methane bacteria, which has been fixed to carrier, is improved by the addition of these elements to a waste water to be treated, the value of the ratio of (surface area of carrier)/(effective volume of liquid) can be decreased to a value lower than the value which has hitherto been considered to be necessary in the conventional method. The present invention has been accomplished based on the above described knowledge. The present invention is set out in claim 1. The method of the present invention is carried out by means of an anaerobic treating apparatus containing a carrier packed therein, and having a value of the ratio of the apparent surface area of the carrier to the effective volume of the apparatus (the ratio of (surface area of carrier)/(effective volume of liquid)) of at least 10 m²/m³. The term "apparent surface area of a carrier" herein used means the surface area of a carrier, which is calculated from its geometrical shape, and does not include the inner surface area of the carrier material itself, which is formed by the fine small holes, cracks and the like extending into the interior of the carrier material. The term "effective volume of liquid" herein used means the volume obtained by subtracting the volume occupied by the carrier from the volume of the inner space of the apparatus, which receives the waste water therein together with the carrier, that is, means the amount of waste water to be practically treated. As used herein, "BOD decrease" is a value calculated by the following equation: BOD decrease % = BOD in untreated waste water - BOD in treated waste water BOD in untreated waste water x 100 For a better understanding of the invention, reference is made to the accompanying drawings, in which: Fig. 1 is a diagrammatical view, partly in section, for explaining the treating method of the present invention; Fig. 2 is a graph illustrating the relation between the value of the ratio of Ni/BOD and the BOD loading rate; Fig. 3 is a graph illustrating the relation between the value of the ratio of Fe/BOD and the BOD loading rate; Fig. 4 is a graph illustrating the relation between the value of the ratio of Co/BOD and the BOD loading rate; Fig. 5 is a graph illustrating the relation between the BOD loading rate and the BOD decrease; and Fig. 6 is a graph illustrating the relation between the value of the ratio of (surface area of carrier)/(effective volume of liquid) and the maximum BOD loading rate. The present invention will be explained in more detail referring to the drawings. Fig. 1 illustrates a system to be used for carrying out a methane fermentation treatment of a waste liquor formed in the production step of beer. The numeral 1 represents an anaerobic treating apparatus; the numeral 2 represents a porous ceramics carrier containing a thermophilic methane bacteria deposited thereon; the numeral 3 represents a storage tank for organic waste water; the numeral 4 represents a gas holder for receiving methane gas generated by the methane fermentation; the numeral 5 represents a tank for receiving a treated organic waste water; the numeral 6 represents a storage tank for alkali to be used for maintaining the PH of the organic waste water in the anaerobic treating apparatus 1 to a given PH; the numeral 7 represents a storage tank for nutrient source; the reference PH represents a PH meter; the reference T represents a thermometer; and the reference P represents a pump. The organic waste waters to be treated in the apparatus of the present invention include waste liquors formed in the production step of beer, such as waste liquor from a sludge trough, waste liquor from malt dewatering process, waste beer, waste juice and their mixtures, and the like, and further include whey waste water from soybean process, waste water formed during the distillation in the whisky production, and the like. It has been found from the analysis of the components of these waste waters that nickel, iron and cobalt are not at all contained in these waste waters, and even in the case where these elements are contained in the waste waters, the amount is very small. Therefore, in the present invention, in addition to a commonly known nutrient source supplied from the storage tank 7 for nutrient source, nickel, iron and cobalt are occasionally added to a waste water to be treated. These elements are added to a waste water such that the waste water, after the addition, contains Ni, Fe and Co in amounts, based on the BOD of the waste water, that the value of the ratio of Ni/BOD is at least 25×10⁻⁶, the value of the ratio of Fe/BOD is at least 60×10⁻⁶ and the value of the ratio of Co/BOD is at least 1.25×10⁻⁶. Further, it is preferable to adjust the concentrations of nitrogen and phosphorus in the waste water to be that the value of the ratio of N/BOD is at least 6×10⁻³ and the value of the ratio of P/BOD is at least 1.5×10⁻³. The reason why the amounts of nickel, iron and cobalt contained in a waste water are limited to the above described amounts will be explained hereinafter referring to the graphs of Figs. 2-6, which are compilation of the data with respect to the treatments of waste liquors, which have been formed in the production step of beer, by a thermophilic methane bacteria. As illustrated in Fig. 2, in the region wherein the value of Ni/BOD is not lower than 25×10⁻⁶, the BOD loading rate exceeds 10 kgm³·day, but in the region wherein the value of Ni/BOD is lower than 25×10⁻⁶, the BOD loading rate lowers, and in the conventional method, wherein Ni is not at all added to the waste liquor, the BOD loading rate is only about 3 kgm³·day. Fig. 3 illustrates that, in the region wherein the value of Fe/BOD is not lower than 60×10⁻⁶, the BOD loading rate exceeds 10 kgm³.day, but in the region wherein the value of Fe/BOD is lower than 60×10⁻⁶, the BOD loading rate lowers as well, and in the conventional method, the BOD loading rate is only about 3 kgm³·day. Further, as illustrated in Fig. 4, when the value of Co/BOD is not lower than 1.25×10⁻⁶, the BOD loading rate excees 10 kgm³.day. It has been ascertained that, when the above described conditions are satisfied, a high BOD decrease of more than 90% can be attained while maintaining a high BOD loading rate of not lower than 10 kgm³.day. Further, it has been able to be ascertained that, although these data are ones with respect to thermophilic methane bacteria, data with respect to mesophilic methane bacteria have the same tendency as the data in the thermophilic methane bacteria. The reason why the addition of such relatively large amounts of nickel, iron and cobalt to a waste water to be treated acts effectively is probably due to the contribution of these elements to the formation of coenzyme during the methane fermentation. In the above described embodiments, the value of the ratio of the apparent surface area of the porous ceramics carrier 2 packed in an anaerobic treating apparatus 1 to the effective volume of the anaerobic treating apparatus 1 (the ratio of (surface area of carrier)/(effective volume of liquid)) was set to 70 m²/m³. The inventors further measured the maximum BOD loading rate which was able to be treated by the method of the present invention, by changing variously the value of the ratio of (surface area of carrier)/(effective volume of liquid). This measurement was carried out under a constant condition that the value of Ni/BOD was 50×10⁻⁶, the value of Fe/BOD was 120×10⁻⁶ and the value of Co/BOD was 2.5×10⁻⁶. For comparison, a conventional waste water not added with nickel, iron and cobalt, and having a value of Ni/BOD of 10×10⁻⁶, that of Fe/BOD of 40×10⁻⁶ and that of Co/BOD of 1.0×10⁻⁴ was used, and the maximum BOD loading rate, which was able to be treated with the waste water, was measured in the same manner as described above by changing variously the value of the ratio of (surface area of carrier)/(effective value of liquid). The obtained results are illustrated in Fig. 6. It can be seen from Fig. 6 that, in the conventional method, wherein the values of Ni/BOD, Fe/BOD and Co/NOB are outside the range defined in the present invention, the maximum BOD loading rate is lower than 10 kgm³·day unless the value of the ratio of (surface area of carrier)/(effective volume of liquid) is at least 20 m²/m³. On the contrary, in the method of the present invention, even when the value of the ratio of (surface area of carrier)/(effective volume of liquid) is 10 m²/m³, a high maximum BOD loading rate can be maintained. However, even in the method of the present invention, when the value of the ratio of (surface area of carrier)/(effective volume of liquid) is lower than 10m²/m³, the maximum BOD loading rate is lower than 10 kgm³·day. As described above, when the values of the ratios of Ni/BOD, Fe/BOD and Co/BOD are within the range defined in the present invention, the methane fermentation ability of methane bacteria fixed to the carrier surface is strengthened, and methane fermentation can be satisfactorily carried out even in the case where the amount of methane bacteria is decreased, and hence even when the value of the ratio of (surface area of carrier)/(effective volume of liquid) is lowered to 10 m²/m³, the methane fermentation in the second half stage is not adversely influenced by the acid fermentation in the first half stage, and a high BOD loading rate can be maintained. As clearly understood from the above described explanation, according to the present invention, the ratio of the amounts of nickel, iron and cobalt, which are nutrient elements necessary for the proliferation and the like of methane bacteria, to the BOD are maintained to given values, whereby the proliferation activity of methane bacteria is increased, and various organic waste liquors, such as waste liquor formed in the production step of beer and the like, can be treated in a high BOD decrease of not lower than 90% while maintaining a high BOD loading rate of not lower than 10 kgm³·day. Moreover, according to the apparatus of the present invention, the methane fermentation ability of methane bacteria fixed to the carrier surface has been strengthened, and hence the value of the ratio of (surface area of carrier)/(effective volume of liquid), which has been necessary to be not lower than 20 m²/m³ in the conventional method, can be lowered to about 10 m²/m³, and moreover various organic waste liquors can be treated in a high BOD decrease while maintaining a high BOD loading rate. As described above, the treating method and apparatus of organic waste water by the methane fermentation according to the present invention are free from the problems in the conventional method, and the present invention has a very high contribution to the development of industry.

Claims: A method for treating an organic waste water by a methane fermentation with the use of an anaerobic fixed bed, comprising effecting the treatment in the presence of nickel, iron and cobalt, which are nutrient elements necessary for the proliferation of methane bacteria, wherein the amounts of Ni, Fe and Co are simultaneously maintained relative to the biological oxygen demand (BOD) of the waste water such that the value of the ratio Ni/BOD is at least 25x10⁻⁶, the value of the ratio Fe/BOD is at least 60x10⁻⁶ and the value of the ratio Co/BOD is at least 1.25x10⁻⁶. A method according to claim 1, performed in an anaerobic treating apparatus containing a carrier packed therein, said apparatus having a value of the ratio of the apparent surface area of the carrier to the effective volume of the apparatus (the ratio of (surface area of carrier)/(effective volume of liquid)) of at least 10m²/m³. A method according to claim 1 or claim 2, wherein said organic waste water treated has a BOD load of at least 10 kg/m³-day. A method according to any one of claims 1 to 3 wherein said organic waste water is treated at a BOD removal ratio of at least 90%.

Inventors: KAWASE MITSUO, MURATA NAOKI, NOMURA TADASHI, KAWASE, MITSUO, MURATA, NAOKI, NOMURA, TADASHI
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1990
Main classification: C02F3
IPC codes: C02F3, C02F11