JP2001205298A - Method for treating sewage sludge - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for treating sewage sludge which solved the problem that when nitrate nitrogen and dissolved oxygen in the sludge are eliminated in the process of concentration and dewatering, phosphorus which was accumulated and fixed as polyphosphoric acid in the cells of microorganisms is released into water as orthophosphoric acid to increase the phosphorus concentration of wastewater. SOLUTION: In the method for treating sewage sludge generated in sewage works, in which phosphorus is removed biologically, comprising a concentration treatment process, a dewatering treatment process, and a drying or incineration treatment process, the sludge is concentrated and, without being allowed to stand at ordinary temperature for a long time, refrigerated while nitrate nitrogen is left in the sludge. In this way, the cell membranes of microorganisms in the sludge are broken so that phosphorus contained in the microorganisms is fixed as polyphosphoric acid, and the refrigerated sludge is melted, dewatered, and dried or incinerated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for treating sewage sludge containing a high concentration of phosphorus generated in an activated sludge treatment process for biologically removing phosphorus such as sewage and industrial wastewater treatment.

[0002]

2. Description of the Related Art Generally, sludge discharged from a final sedimentation basin of a sewage treatment plant is first concentrated by gravity concentration, centrifugal concentration, flotation concentration, etc., and then by belt press dehydration, centrifugal dehydration, pressure dehydration, etc. It is dehydrated and then treated such as compost, sludge drying and sludge incineration. In particular, in advanced sewage treatment plants that biologically remove phosphorus, the sludge containing high concentration of phosphorus is concentrated and dewatered. Is a problem.

[0003] In the conventional treatment of sewage sludge, the main purpose is to control the water content in such a manner that the sludge can be easily used for composting, landfilling and incineration, etc. Was returned to the inflow end of the sewage treatment plant and treated as inflow water to the sewage treatment plant. When advanced sewage treatment has begun to spread and sewage sludge contains a high concentration of phosphorus, the wastewater generated by the concentration and dehydration treatments will add a new phosphorus load to the sewage treatment plant, and ensure stable operation for phosphorus removal. Hindering.

[0004]

The sludge discharged from the final sedimentation basin is discharged several times a day, accumulated in a certain amount for a certain period, concentrated by gravity concentration, centrifugal concentration, flotation concentration, etc., and dewatered by belt press. The dehydration treatment is performed by centrifugal dehydration, pressure dehydration and the like. During the period from this concentration process to the dehydration process, the microorganisms in the sludge are active, and when nitrate nitrogen and dissolved oxygen are no longer present in the sludge, the microorganisms are accumulated and immobilized in the microbial cells as polyphosphoric acid. The resulting phosphorus is released into the water as orthophosphoric acid, and there is a problem that high concentrations of phosphorus as orthophosphoric acid are contained in wastewater generated by the concentration treatment and the dehydration treatment.

[0005] The present invention provides a method for treating sewage sludge which suppresses release of polyphosphoric acid in the sludge as orthophosphoric acid into water in the process of discharging sewage sludge from a final sedimentation tank and performing concentration treatment and dehydration treatment. Is to provide.

[0006]

The object of the present invention is to concentrate, dehydrate, dry or incinerate sewage sludge generated from a sewage treatment plant that biologically removes phosphorus. In the method of treating sewage sludge, after concentrating the sewage sludge, do not leave it at room temperature for a long time, and keep the temperature within the temperature range where microorganisms stop, or freeze, while nitrate nitrogen remains in the sludge. Achieved by doing. Here, the temperature at which the microorganism stops its activity is
Generally higher than the temperature at which sewage freezes.

By maintaining the temperature range in which the activity of microorganisms is stopped, the release of polyphosphoric acid in sludge as orthophosphoric acid into water can be suppressed, and the sewage is frozen to increase the size of ice crystals. By growing
The microbial cell membrane in the sewage sludge is destroyed, and the phosphorus contained in the microorganism can be immobilized as polyphosphoric acid. Thereafter, when frozen, the sludge is liquefied through a thawing treatment, and this problem can be solved by a method of treating sewage sludge that is dehydrated, dried or incinerated.

If the concentration of the discharged sludge is high, the concentration process is omitted and the process is directly carried out in a process of maintaining the temperature range in which the microorganism stops its activity, or in a process of freezing and thawing. It is solved by the processing method. . Phosphorus contained in activated sludge is stored in microorganisms in the form of polyphosphoric acid by dephosphorylated bacteria, and dephosphorylated bacteria are in an anaerobic state (molecular oxygen, that is, nitrite nitrogen and nitrate nitrogen, and dissolved oxygen). It is known that polyphosphoric acid is released into water as orthophosphoric acid in the absence of water.

The present inventors have studied the wastewater obtained by dewatering sludge after freezing and thawing and sludge left at room temperature by paying attention to the phosphorus concentration, and have found the following points. First, when the sludge is left at room temperature, the phosphorus-removing bacteria in the sludge are active, the nitrate nitrogen in the sludge decreases with the standing time and becomes anaerobic, and the polyphosphoric acid in the phosphorus-removing bacteria becomes Is released into the water as orthophosphoric acid.

On the other hand, when freezing is performed in the presence of nitrate nitrogen in sludge, phosphorus is present as polyphosphoric acid in the cells of dephosphorylated bacteria due to the presence of nitrate nitrogen. By performing appropriate temperature control during the freezing process, the sewage sludge turns into ice crystals from water molecules, and then the sludge itself freezes. It is self-evident that water molecules increase in volume in the solid state compared to the liquid state, but at this time, the cells of the microorganisms are compressed and destroyed due to the increase in the volume of ice, and the microorganisms die. Was found. When the melting treatment was performed in this state, it was confirmed that phosphorus was present as polyphosphoric acid in the dead microbial cells even when left at room temperature, and was not detected as orthophosphoric acid in water.

In addition, even if a part of the dephosphorylated bacteria in the sludge grows and resumes the activity in the freeze-thaw treatment, phosphorus is not released into the water as orthorin while nitrate nitrogen remains. Therefore, the phosphorus concentration in the dewatered wastewater can be reduced by performing the dehydration treatment during this time. From this examination result, it is possible to solve the problem that phosphorus is contained in a high concentration by inserting a freeze-thaw process between the sewage sludge concentration process and the dehydration process.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described with reference to three embodiments. FIG. 1 shows an apparatus configuration of the first embodiment.
In this figure, 1 is a sludge concentrator, 2 is a freeze-thaw device, 3 is a dehydrator, 4 is a thermometer, and 5 is a freezing temperature controller. Raw sludge discharged from the sewage treatment plant is treated by the sludge concentrator 1 and the concentrated sludge is put into the freeze-thaw device 2.
The concentration of orthophosphoric acid in the sludge solution at this time was 1.2 mg /
L. During the freezing process in the freeze-thaw device 2, the temperature of the sludge is measured by the thermometer 4, and the temperature of the freeze-thaw device 2 is adjusted by the freezing temperature control device 5 to a temperature of about 0 ° C. to −5 ° C. at which ice crystals grow large. Was controlled within the range. After confirming freezing, the concentration of orthophosphoric acid in the dewatered wastewater obtained by melting and dewatering the sludge was 2.2 mg / L.

The concentration of orthophosphoric acid in the dewatered wastewater obtained by subjecting the same sludge to dewatering by the conventional method without performing freeze-thaw treatment is 3
It was 0.2 mg / L. Thus, according to the present invention,
This means that the concentration of orthophosphoric acid in the dewatered wastewater can be reduced to about one tenth in comparison with the conventional method. Next, FIG. 2 shows an apparatus configuration of a second embodiment of the present invention. In this embodiment, since the concentration of orthophosphoric acid in raw sludge is high, the sludge concentration apparatus 1 is omitted from the first embodiment.

Raw sludge discharged from the sewage treatment plant is directly put into the freeze-thaw apparatus 2. At this time, the concentration of orthophosphoric acid in the sludge solution was 1.4 mg / L. In the freezing process in the freeze-thaw device 2, the temperature of the sludge is measured from the thermometer 4, and the temperature of the freeze-thaw device 2 is adjusted by the freezing temperature control device 5 to a temperature of about 0 ° C. to −5 ° C. at which ice crystals grow large. Controlled by range. After confirming freezing, the concentration of orthophosphoric acid in the dewatered wastewater obtained by melting and dewatering the sludge was 1.7 mg / L.

The orthophosphoric acid concentration of the dewatered wastewater obtained by directly dewatering the same sludge by the conventional method without performing freeze-thaw treatment is 3
It was 3.6 mg / L. Thus, according to the present invention,
This means that the concentration of orthophosphoric acid in the dewatered wastewater can be reduced. FIG. 3 shows a third embodiment of the present invention.
This will be described below. In this embodiment, the apparatus is configured by adding a nitrate nitrogen meter 6 to the sludge concentration apparatus 1 in the apparatus configuration of the first embodiment.

Raw sludge discharged from the sewage treatment plant is treated by the sludge concentrator 1 and the concentrated sludge is put into the freeze-thaw apparatus 2. The concentration of orthophosphoric acid in the sludge solution at this time was 1.5 m
g / L, and the value of the nitrate nitrogen analyzer 6 is 7.3 mg / L.
L. During the freezing process in the freeze-thaw device 2, the temperature of the sludge is measured from the thermometer 4, and the temperature of the freeze-thaw device 2 is adjusted by the freezing temperature control device 5 to a temperature of about 0 ° C. to −5 ° C. at which ice crystals grow large. Was controlled within the range. After confirming the freezing, the concentration of orthophosphoric acid in the dewatered wastewater obtained by melting and dewatering the sludge was 2.4 mg / L.

The same sludge has a nitrate-nitrogen meter 6 value of 1.4 m.
At the time when the concentration became g / L, a freeze-thaw treatment was performed, and the concentration of orthophosphoric acid in the dewatered dewatered wastewater was 28.2 mg / L. Therefore, by measuring the nitrate nitrogen concentration of the concentrated sludge and controlling the interval of freeze-thaw treatment of the concentrated sludge based on the measured value, the phosphorus concentration of the dewatered wastewater could be reduced.

[0018]

According to the present invention, as described above, the conventional process includes a process of maintaining the temperature range in which the microorganism stops its activity, or a process of freeze-thawing.
A method for treating sewage sludge having a low phosphorus concentration in wastewater discharged in the dehydration process can be provided. In fact, according to the present invention, polyphosphoric acid in sludge can be immobilized on dehydrated sludge,
Sludge that can be easily used for composting sludge can be provided.

Furthermore, the water content of the dewatered sludge can be made lower than in the conventional method. It also has an effect of preventing bad smell and sludge solidification generated in the sludge treatment process.

[Brief description of the drawings]

FIG. 1 is an apparatus configuration diagram illustrating a first embodiment of the present invention.

FIG. 2 is an apparatus configuration diagram illustrating a second embodiment of the present invention.

FIG. 3 is an apparatus configuration diagram illustrating a third embodiment of the present invention.

[Explanation of symbols]

 1: Sludge concentrator 2: Freezing and thawing device 3: Dehydrating device 4: Thermometer 5: Freezing temperature controller 6: Nitrate nitrogen meter

Claims (4)

[Claims] 1. A method for treating sewage sludge generated from a sewage treatment plant that biologically removes phosphorus, wherein at least a treatment step for maintaining a temperature range in which microorganisms stop their activity, a dehydration treatment step, a drying or A method for treating sewage sludge, comprising an incineration process. 2. A freezing process according to claim 1, wherein the temperature range in which the microorganism stops its activity in the process of maintaining the temperature range in which the microorganism stops its activity is the temperature range in which the sewage freezes. A method for treating sewage sludge, which is characterized in that: 3. The method according to claim 1, further comprising a concentrated sludge treatment step preceding the treatment step of keeping the temperature range in which the microorganism stops its activity or the freezing treatment step. Sewage sludge treatment method. 4. The method according to claim 3, wherein the nitrate nitrogen in the apparatus for performing the concentrated sludge treatment step is measured, and the activity of the microorganisms is increased while the nitrate nitrogen in the concentrated sludge remains. A method for treating sewage sludge, comprising performing a process of maintaining a temperature range in which the process is stopped, or performing the freezing process.