Liquid purification or separation – Processes – Treatment by living organism
Patent
1996-06-27
1998-03-03
McCarthy, Neil
Liquid purification or separation
Processes
Treatment by living organism
210609, 210612, 210613, C02F 328
Patent
active
057230480
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method of efficient anaerobic digestion of sewage sludge to utilize the sewage sludge as energy resources.
BACKGROUND ART
Methane fermentation of sewage sludge has been known as an important unit process of sewage treatment since older times. However, the method has disadvantages such as large equipment, low digestion rate and inefficiency, so that the method is likely to be kept at a distance. Recently, with a rise in consciousness of the global environment, people have reconsidered the importance of methane fermentation of the sewage sludge which is regenerable energy resources. Technological innovation based on new conception is required in order to utilize the sewage sludge as energy resources.
As a means to promote efficiency of methane fermentation, there can be given 1) high-temperature fermentation, 2) heat treatment of sludge and 3) increasing concentration of sludge. However, 1) and 2) closely relate to 3) in terms of heat balance, and 3) is necessary in order to carry out 1) and 2). Therefore, 3) increasing the concentration of sludge is the most important matter of all.
In this respect, as shown in FIG. 11, there has been a method of anaerobic digestion of sewage sludge wherein a part of digested sludge is discarded after methane fermentation, while the rest of the digested sludge is circulated, raw sludge is added to the circulating digested sludge and the mixture is subjected to methane fermentation, the method comprising the steps of dewatering the above raw sludge to 10% or higher of concentration of solid matter, subjecting the resulting dewatered sludge to heat treatment at 120.degree. to 180.degree. C., adding the heat-treated sludge to the above circulating digested sludge, mixing them uniformly and subjecting the mixture to methane fermentation (Japanese Laid-open Patent Publication No. 2-31898).
However, in this method, the sludge is subjected to heat treatment after dewatering the sludge, so that the method had the following problem. In the case of sewage sludge containing much surplus sludge, subjecting dewatered sludge whose sludge concentration is 10% or higher to heat treatment, total soluble organonitrogen compounds formed by heat treatment are included in the heat-treated sludge. Since the organonitrogen compounds are converted into concentrated NH.sub.3 by anaerobic digestion, the digestion rate is depressed low by NH.sub.3 concentration obstacle.
DISCLOSURE OF INVENTION
In view of the above point, the object of the present invention is to provide a method of improving gas generation efficiency while decreasing capacity of a digestion vessel in an anaerobic digestion process of sewage sludge.
The method of anaerobic digestion of sewage sludge of the invention is devised in order to achieve the above object. The method comprises the steps of subjecting surplus sludge to preliminary heat treatment at 60.degree. C. or higher, dewatering the surplus sludge to make dewatered sludge of 10 to 25% by weight, mixing the dewatered sludge with digested sludge which is taken out from a methane fermentation vessel of dewatered sludge, and supplying the resulting mixture to the methane fermentation vessel of dewatered sludge while discarding digested sludge whose amount is equal to that of the dewatered sludge supplied.
In the method of anaerobic digestion of sewage sludge of the invention, since the heat treatment has an effect to reduce nitrogenous component on the surplus sludge, the surplus sludge may be subjected to heat treatment singly and the heat-treated surplus sludge may be dewatered singly. A mixture of the surplus sludge with initial settling sludge may also be subjected to heat treatment and dewatered. In addition, the heat-treated surplus sludge and the initial settling sludge may be dewatered separately and they may be mixed each other, that is, the resulting dewatered sludge may be mixed with the dewatered initial settling sludge, to make dewatered sludge of the required concentration.
According to the second embodim
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Akita Minoru
Kobayashi Yoshio
Green Theodore M.
Hitachi Zosen Corporation
McCarthy Neil
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