1. Field of the Invention:
The present invention relates to apparatus and a method for processing excess sludge which is necessarily generated in and sent out of various sewage treatment plants as a result of processing of sewage water such as industrial effluent, discharges from homes or the like, using micro-organisms.
2. Background of the Prior Art:
The process of eliminating water polluting materials such as BOD, or COD contained in sewage water, by oxidatively decomposing them with micro-organisms, includes the activated sludge method, trickling filter method and the catalytic oxidation method. It is well known that excess sludge is produced according to the purification process and deposited on, for example, the bottom of a tank of the processing unit. If the excess sludge is left as it deposited, purification performance of the treating equipment is lowered, eventually resulting in incapability of the equipment. Accordingly, it is essential to remove the excess sludge out of the tank and process it.
Hitherto, there have been proposed several ways to process the deposited sludge, e.g., a method in which sludge is dewatered, dried and incinerated; a method in which dewatered sludege is buried under the ground or dumped into the ocean; a method in which dewatered sludge is made into compost to be effectively utilized as fertilizer, or a method in which sludge is quantitatively reduced by digestion with anaerobic bacteria, and others.
In the above known methods, however, there are serious drawbacks in that there is high equipment cost and also a fuel cost in the first method of dewatering, drying and incinerating the sludge. High transportation cost is incurred in the second method of burying the sludge under the ground because a large amount of sludge must be carried to the dumping site, and moreover it is rather difficult to get an extensive dumping site. Likewise, in the third method of dumping the sludge into the ocean, there is a drawback of transportation cost and moreover it raises the problem of ocean pollution. In the fourth method of utilizing the sludge by transforming it into compost, there is a drawback in that a high construction cost of processing facilities is required; and another drawback is that the utility of sludge transformed into the compost is lower than that of chemical fertilizer industrially produced.
On the other hand, in the method of reducing the quantity of sludge, utilizing digestion by anaerobic bacteria, processing equipment of rather simple construction can be used and its operation is also relatively simple, and therefore this method has been traditionally adopted in the typical sludge treatment process in sewage plants. In this method, however, it takes long for the sludge to be treated, e.g., over 6 to 8 months, because it is bacteria that carries out the treatment in the form of micro-biological decomposition. Accordingly, this method also results in a high construction cost for the treating equipment since a large scale processing facility is essential for the purpose of carrying out large-scale treatment. Moreover, during the period of digestion of sludge by the anaerobic bacteria, methane gas is continuously generated, and during the period of acidity reduction, large amount of offensive malodorous gases such as hydroxide, mercaptan, indole, etc. is generated. Accordingly, routine operation control of the processing facility is very important and troublesome. Besides, organic material of high concentration is contained in supernatant liquer left after the sludge treatment, and therefore a further problem exists in that some secondary process has to be applied to this supernatant liqueur.