1. Field of Invention
The invention relates to a sewage treatment system and, more particularly to a sewage treatment system characterized by removal of solid components in sewage by high-rate filtration of floating filters media, cleaning of filter adhered with solid components, and biological treatment of sewage after having removed the solid components.
2. Description of the Related Art
Generally, the conventional sewage treatment process, which cleanses the sewage and releases the proceeds into public waters such as river and lake, consists of two parts; The first process of natural sedimentation of solid components in the sewage in the primary sedimentation tank, and the second process of biological treatment by microbes in the activated sludge treatment system. The activated sludge in the sewage which have been treated biologically is sediment in the final sedimentation tank, and the supernatant liquid of the sewage is released and a part of the activated sludge is taken out as surplus sludge, and then the remaining part of the activated sludge is sent to the activated sludge treatment system. After the sewage has been treated to conform to the water quality standards for public waters, the water quality is then improved.
However, recently, as urbanization has advanced and the living standards have risen, the concentration of water contamination, and the increased inflow of sewage, and the various sources of sewage have increased.
In the conventional sewage treatment system comprising the primary sedimentation tank and the activated sludge treatment system, the conventional sewage treatment system cannot meet the treatment needs since the solid component removal capacity by natural sedimentation in the primary sedimentation tank is limited. Therefore, a filtration system using a high-rate filtration process has been developed to remove solid components quickly. According to the filtration system, a layer of floating filter media, which is made of granular filter media of which specific gravity is smaller than the sewage, is provided and the sewage is filtered through the floating filter media layer. When the filtering performance declines by sticking solid components in the sewage to the filter medium, the filter medium are cleaned to reactivate the filtering capability. Therefore, the filtration rate is higher than that of the primary sedimentation tank. For example, patents on the filtration systems are disclosed in Japanese Patent Application Laid-open No. 51-85257, Japanese Patent Application Laid-open No. 60-193508, and Japanese Utility Model Application Laid-open No.2-12405.
The conventional filtration system using a high-rate filtration process has a higher filtration rate than the primary sedimentation tank, therefore, only small area is required for the treatment system. However, the filter medium can be blocked easily and break through pint is shortened, at which the filtrating performance of the filter medium gets lost, is short and, therefore, there is a disadvantage in that the filter medium must be cleaned frequently. Further, there is another disadvantage in that large effluent of filter backwash water is generated. And, there is a further disadvantage in that the insufficient filtration rate cannot be achieved when the flow resistance is strong.
In the sewage treatment system the junction-type sewage treatment area, when extraordinary amount of rain water exceeding the activated sludge treatment capacity due to stormy weather flow into the excess sewage must be released to public waters merely after removing the solid component, therefore, the solid components in the sewage must be removed as much as possible at the time of release. However, in the conventional sewage treatment system the sewage is released into public waters without sufficient solid component removal due to limited treatment capacity for excess inflow at a stretch, and absence of adequate treatment technology for sudden increase of inflow, therefore, there is a problem in that the quality of public waters deteriorates.
Under these circumstances, the following capabilities have been desired to the sewage treatment system:
(1) A sewage treatment system must be developed, wherein efficient filtration of solid components in sewage, efficient cleaning of filter medium for recycling, and the system can become compact. PA1 (2) A sewage treatment system in which high-rate filtration and biological treatment are incorporated, and sewage treatment technology which can meet the amount of the inflow sewage to the sewage treatment system and the condition thereof.