Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
1999-11-10
2002-01-15
Barry, Chester T. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S714000
Reexamination Certificate
active
06338799
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods for recovering phosphate from sludge when sewage is treated using activated sludge at sewage treatment plants, in which, by treating waste sludge or mixed sludge of waste sludge and sludge from primary settling basins in an anaerobic condition, phosphate contained in the waste sludge are released into solution, and phosphate in the solution are recovered. The invention also relates to systems for recovering phosphate from sludge.
2. Description of the Related Art
In order to control water pollution in rivers, lakes, and enclosed coastal sea areas, sewage treatment plants for treating industrial and municipal sewage have been built in many places.
FIG. 9
shows an example of a system for treating sewage and sludge conventionally adopted in such sewage treatment plants.
With respect to sewage received at a plant, first, sand, coarse refuse, etc., are removed at a sand separation basin.
Next, suspended solids are settled in a primary settling basin, and supernatant is obtained. The supernatant is treated in a biological reaction tank in an aerobic condition, using activated sludge. Organic substances in the supernatant are primarily removed thereby.
Next, in a secondary settling basin, mixed liquor suspended solids are separated into activated sludge and treated water. The activated sludge is fed to a subsequent stage, and the separated treated water is discharged from the plant as effluent.
The separated activated sludge is separated into return sludge and waste sludge. The return sludge is returned to the biological reaction tank and is reused as activated sludge. The waste sludge is treated in a sludge treatment system.
The waste sludge generated in the water treatment system and sludge from the primary settling basin (raw sludge), which is settled in the primary settling basin, are thickened individually or as a mixture of both in the sludge treatment system, and separation between thick sludge and liquid separated by thickening is achieved. The liquid separated by thickening is returned to the water treatment system. The separated thick sludge is subjected to dewatering treatment and is separated into liquid separated by dewatering and dewatered sludge. The liquid separated by dewatering is returned to the water treatment system as sidestreams. The separated dewatered sludge is disposed of in landfills, by incineration, or other appropriate methods.
However, in the conventional treatment method described above, the following problems may occur. That is, the treated water contains substances, such as phosphorus and nitrogen, which are responsible for eutrophication. Therefore, the treated water is a contributing factor to red tide and the like in lakes, enclosed coastal sea areas, etc.
As a countermeasure against the above, in the water treatment system described above, an anaerobic tank may be provided before the aerobic biological reaction tank. Thereby, activated sludge accumulates phosphate (orthophosphate ions) as polyphosphate, and the phosphorus concentration in the effluent is reduced.
After the activated sludge which has accumulated polyphosphoric acid is withdrawn as waste sludge into the sludge treatment system, phosphate may be released from the waste sludge depending on a thickening method, resulting in increases in the phosphate concentration in the liquid separated by thickening and liquid separated by dewatering.
The liquid separated by thickening and liquid separated by dewatering having increased phosphate concentrations are returned to the water treatment system as sidestreams. As a result, effects of reducing the phosphorus concentration in the effluent are reduced.
An anaerobic digestion tank may be provided subsequently to a thickener. In such a case, when digested sludge which has been digested at the anaerobic digestion tank is transported to a dewatering apparatus, reactions take place among phosphorus, magnesium ion, ammonia, etc., released into the solution by anaerobic digestion, and magnesium ammonium phosphate crystals, i.e., struvite are precipitated in pipes and pumps. As the precipitation progresses, accidents such as clogging of the sludge transport facility may occur.
In order to overcome the problems described above, currently, coagulating agents, such as lime, aluminum sulfate, ferric sulfate, and polyaluminum chloride, are used in sludge treatment processes. When coagulating agents are used, the amount of sludge is increased due to the added coagulant. Consequently, final disposal expenses for sludge associated with sewage treatment are increased. Since products formed by reactions between the coagulant and phosphate are chemically stable, they are not reusable for fertilizers. The products are contained in the dewatered sludge at the end. Therefore, when dewatered sludge is used as a raw material for cement, which has been receiving attention as a final disposal technique of sewage sludge, the cement is degraded by the phosphate. As a result, it is difficult to adopt such a final disposal technique.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method and a system for efficiently recovering phosphate from mixed sludge.
It is another object of the present invention to provide a method and a system for recovering phosphate in a form which can be effectively used as a fertilizer.
It is another object of the present invention to provide a method and a system for recovering phosphate, in which the highly efficient recovery of phosphate is simplified.
It is another object of the present invention to provide a method and a system for recovering phosphate, in which clogging of sludge transport pipes is avoided.
It is another object of the present invention to provide a method and a system for recovering phosphate, in which phosphate are recovered from sludge with high efficiency so as to prevent treated water from affecting the environment.
According to a first aspect of the present invention, a method for recovering phosphate from sludge includes the processes of treating sludge discharged from a water treatment system at a sewage treatment plant in an anaerobic condition to release phosphate in the sludge into solution, and recovering phosphate in the solution using a seed crystal material.
The sludge includes, for example, waste sludge generated in the water treatment system, and mixed sludge of waste sludge and raw sludge.
The treatment in an anaerobic condition is performed, for example, by injecting sludge into an anaerobic tank and retaining it in the anaerobic tank for a predetermined period of time. In the treatment in an anaerobic condition, phosphate (polyphosphate) in waste sludge (solids) are subjected to hydrolysis and are released as orthophosphate into sludge solution.
Phosphate released into the solution are recovered, using a seed crystal material, for example, as hydroxyapatite crystals on the surface thereof.
As the seed crystal material, for example, a substantially spherical material containing a calcium silicate hydrate as a major constituent is used. The calcium silicate hydrate may be at least one selected from the group consisting of tobermorite, xonotlite, hillebrandite, and wollastonite. In addition to the above, a seed crystal material disclosed in Japanese Unexamined Patent Publication No. 9-308877 may be used. In this material, a calcium silicate hydrate is attached to the surface of a honeycomb core. As a seed crystal material, autoclaved lightweight concrete ALC, may also be used. In the production of ALC, a calcic material and a silicic material are used as major materials, and after water and a foaming agent are added thereto, autoclave curing is performed.
The treatment in an anaerobic condition and the recovery of phosphate may be performed separately or in the same tank.
According to a second aspect of the present invention, a method for recovering phosphate from sludge according to the first aspect of the invention further includes the process of digesting the sludge treated in an
Fukushima Yuichi
Kawabata Kouichi
Matsumoto Tadashi
Moriyama Katsumi
Barry Chester T.
Mitsubishi Materials Corporation
LandOfFree
Method for recovering phosphate from sludge and system therefor does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for recovering phosphate from sludge and system therefor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for recovering phosphate from sludge and system therefor will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2875035