JPH0227002B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD The present invention relates to a method for solid-liquid separation of a suspension, which comprises adding a polymer flocculant to the suspension and concentrating the suspension. Conventional technology and its problems Conventionally, surplus sludge discharged from the activated sludge treatment process, digested sludge discharged from an anaerobic digestion tank, or suspensions such as mine wastewater are reduced in volume by sedimentation concentration or mechanical dewatering. solid content is concentrated. In this type of method, a polymer flocculant is used in combination to improve the concentration rate or to improve the transient nature of the suspension. In conventional methods, the optimum amount of polymer flocculant is added in order to achieve stable concentration by taking into account various conditions such as the properties of the suspension, fluctuations in water temperature, and fluctuations in the solid content concentration in the suspension. It is set higher than the amount. Furthermore, even if this concentration is performed using the optimum amount of flocculant added in the conventional method, a large amount of unreacted polymer flocculant remains in the separated water. As described above, in the conventional method, the added flocculant is not effectively utilized. OBJECTS OF THE INVENTION An object of the present invention is to provide a method for concentrating a suspension that stably provides a high concentration rate with a much smaller amount of polymer flocculant than conventional methods. Summary of the Invention That is, the present invention involves: separating a portion of the suspension;
This fractionated suspension is supplied to a flocculant addition zone,
Here, a polymer flocculant is added and mixed; the effluent is supplied from the flocculant addition zone to the mixing zone, where it is mixed with the fractionated residual suspension; then, it is mixed in the solid-liquid separation zone. The effluent from the zone is separated into a separated liquid and a concentrate; this is a solid-liquid separation method for a suspension consisting of each step. Furthermore, the present invention provides a method characterized in that a polymer flocculant is added to the suspension separated in the above method and mixed at an elevated temperature, or the polymer flocculant is added and mixed, and then the temperature is raised. It is. Embodiments of the Invention The present invention will be described in detail below with reference to the accompanying drawings. 1st
The figure is a process diagram showing an example of the method of the present invention. The suspension 1 treated according to the present invention includes surplus sludge, digested sludge, or thick vertical sludge thereof, human waste, sewage,
The wastewater may be any wastewater such as mine wastewater or tunnel construction water change wastewater. When mechanical dewatering is used as a concentration means, surplus sludge or digested sludge is a suitable suspension. The solid content concentration in the suspension can be applied over a wide range from several hundred ppm to tens of thousands of ppm. If the suspension is excess sludge or digested sludge, the solids concentration is
5000 to 15000ppm, preferably 6000 to 15000ppm
It is 10000ppm. Suspension 1 is divided and the divided suspension is transferred to line 2.
and send the remaining suspension to line 3. The proportion of the flow rate to be diverted varies depending on the type of suspension, its solid content concentration, etc., but it is between 20% and 80% of the total flow rate of the suspension.
is preferably diverted to line 2. When the suspension is excess sludge, a diversion ratio of 40 to 60% is suitable. The divided suspension is sent to a flocculant addition zone, where a polymer flocculant is added through line 4. The polymer flocculant used in the present invention is not particularly limited, and cationic, anionic, and nonionic polymer flocculants can be used. A polymer flocculant having a charge opposite to that of the solid content in the suspension is preferred, and for example, in the treatment of excess sludge, it is preferred to use an acrylamide-based cationic polymer flocculant. In addition, polyvinylpyridine hydrochloride, sodium polyacrylate, etc. can also be applied. The flocculant addition zone 5 may be a tank if equipped with a stirrer, and the separated suspension reacts with the polymeric flocculant added in this mixing tank to form flocs. The amount of polymeric flocculant added may be about 0.1 to 1.0% of the suspension solids weight. The residence time of the suspension in the mixing tank can be set to any desired value, but the desired effect can be achieved in about a few minutes to 30 minutes, preferably 5 to 15 minutes. The stirring speed of the stirrer should be suitable to achieve sufficiently uniform contact between the suspension and the polymeric flocculant, and should be between 10 and 10 mm.
This can be done at a rotation speed of 100 ppm. It is also possible to use two mixing tanks, with the first tank being a rapid stirring tank and the second tank being a slow stirring tank. Alternatively, the flocculant addition zone 5 may be a combination of a concentrator followed by a mixing tank, into which the polymeric flocculant is added. In this case, the divided suspension is concentrated to a predetermined solid content concentration in a concentrator, and then mixed with a polymer flocculant in a mixing tank. A preferred concentrator is a flotation concentrator, and most preferably an atmospheric flotation concentrator. The atmospheric flotation concentration equipment includes: a foaming section that blows air under normal pressure into a liquid phase containing a flocculant and a foaming agent, a mixing section that mixes the air bubbles generated in this foaming section and the suspension, and a concentration section. It mainly consists of a flotation zone that separates into a liquid and a separated liquid. A polymer flocculant is added to this concentrated solution in a mixing tank. If the suspension is excess sludge or digested sludge, the solid content concentration is 4 to 8% using this normal pressure flotation thickener.
Concentrate up to Generally, when dewatering sludge generated in sewage treatment plants, etc., the solid content concentration of the sludge supplied to the dehydrator is 2% to 3%, so most dehydrators use sludge with a low concentration (2 to 3%). is designed to handle. If a normal pressure flotation concentrator capable of concentrating to 4% or more is installed in such a treatment plant, the solid content concentration supplied to the dehydrator may be too high, making it impossible to effectively utilize the dehydrator's capacity. When a normal pressure flotation concentration device is used in the present invention,
The supplied sludge concentration can be arbitrarily set to a concentration that maximizes the performance of the dehydrator, and stable dewatering can be performed with the minimum amount of flocculant added. The suspension leaving flocculant addition zone 5 enters mixing zone 7 via line 6 where it is mixed with the remaining suspension from line 3. This mixing zone 7 is not particularly limited, and may be any means that can sufficiently uniformly mix the suspensions from the line and the line 6. As an example, a mixing tank similar to the above-mentioned mixing tank can be used. The mixed suspension from the mixing zone 7 is sent to the solid-liquid separation zone 9 via line 8, the separated liquid is discharged from the system via line 11, and the concentrate via line 10. The solid-liquid separation zone 9 can employ conventionally known solid-liquid separation means such as centrifugation, sedimentation concentration, flotation concentration, filtration, and belt press type dehydrators. When the concentration of the suspension to be treated is low, sedimentation concentration is preferred, and when the concentration is high, it is preferred to use a belt press type dehydrator or a centrifugal separator. Another aspect of the invention is that the flocculant addition zone 5 includes heating means. The suspension from line 2 may be heated and then reacted with the polymer flocculant in a mixing tank, or the mixing tank may be heated or the suspension from the mixing tank to line 6 may be heated. . In either method, it is important to react the suspension and the polymer flocculant at a predetermined elevated temperature. It is preferred that the temperature of the mixing tank be at least 40°C or higher, preferably 60°C or higher. As a heating means, an electric heater, high temperature gas or steam,
Any means can be applied. When the present invention is implemented in a sewage treatment plant, methane combustion exhaust gas from an anaerobic digestion tank can be blown into a mixing tank to serve as a heating means. By increasing the temperature, the amount of polymer flocculant used can be further reduced. It is also possible to downsize the mixing tank capacity. It is not fully understood what kind of agglomeration mechanism this effect of temperature elevation is based on, but it may be that it promotes an increase in the adsorption rate of the flocculant to the solid content, promotes the crosslinking reaction, or that the flocculant It is thought that the utilization rate of the flocculant improved due to a decrease in the viscosity of the coagulant. In the conventional method, a polymer flocculant is added to the entire suspension without dividing the suspension. In this method, a portion of the flocculant is encapsulated between the flocs. If the stirring force is increased or the stirring time is increased in order to expel the encapsulated flocculant, the flocs will be fragmented and this will interfere with the subsequent solid-liquid separation means.
In the method of the present invention, a floc consisting of solids covered with the flocculant is produced by contacting the fractionated suspension with a polymer flocculant, and then this floc is brought into contact with the remaining suspension in a mixing zone. The flocs take up the solids in the remaining suspension and coagulate. Therefore, the flocculant that has already been adsorbed to the solids is effectively utilized, and the remaining solids in the suspension are adsorbed around the flocs covered with the flocculant. The operation of mixing the suspension separated in the flocculant addition zone with the flocculant is for the purpose of covering the surface of the flocs with the flocculant, so there is no problem even if the stirring force is large and the stirring time is long. do not have. Therefore, the adsorption amount of the flocculant to the solid content can be increased, and the concentration of the flocculant in free water can be reduced. When the operation of mixing the fractionated suspension and the flocculant is performed at an elevated temperature, the adsorption rate of the flocculant to the solid content increases, and the concentration of the flocculant in free water can be further reduced. As another embodiment of the present invention, a suspended matter concentration meter may be installed in the line 1, and the fractionation ratio or the amount of polymer flocculant added may be controlled in accordance with changes in the concentration. In this method, the amount of flocculant used can be further reduced. Example 1 Excess sludge produced by the activated sludge method was concentrated by the method shown in FIG. The solid content concentration of the surplus sludge used was 6000 ppm, and a cationic polymer flocculant of acrylamide copolymer salt was used as the flocculant, and the amount of polymer flocculant was 1.0% based on the solid weight of the surplus sludge to be treated. was added to the collected excess sludge. In this example, the test was carried out at room temperature. Figure 2 shows the capillary suction time (CST) of excess sludge flowing out of the mixing zone. From this figure, it can be seen that the dewaterability of sludge is the best when 40 to 60% of the total amount of excess sludge to be treated is separated and a polymer flocculant is added to it. Example 2 The same procedure as Example 1 was carried out, but the amount of flocculant added was changed.
It was varied from 0.7% to 1.0% (based on the solid weight of excess sludge), and the temperature of the mixing tank in the flocculant addition zone was raised to 40°C and 60°C using an electric heater. A gravity sedimentation device was used in the solid-liquid separation zone. The table below shows the CST of excess sludge flowing out of the mixing zone and the concentration of polymer flocculant in the separated liquid discharged from the gravity settling device.

[Table] This table shows that according to the present invention, the amount of flocculant used can be reduced by 20 to 30% compared to the conventional method, and the concentration of flocculant in the separated liquid is also reduced by 30 to 90%. Example 3 Excess sludge from a community plant (solid content concentration 8000 ppm) 2.36 to 2.93 m ³ /h
m ³ /h is fed to a normal pressure flotation thickener and concentrated, a polymer flocculant is added to the thickened sludge in a mixing tank, and the remaining surplus sludge is separated into this thickened sludge, 0.36 to 0.93 m ³ /h. were added, mixed, and dehydrated using a belt press type dehydrator. A polyacrylamide-based strong cationic polymer flocculant was used as the flocculant. A heater was placed in the mixing tank and the temperature was raised to 60°C. Table 2 shows the results when the temperature was not raised (20°C), and Table 3 shows the results when the temperature was raised (60°C).

【table】

[Table] According to the present invention, the concentration of thickened sludge is stable, and the solid content of sludge supplied to the dehydrator can be reduced by adjusting the amount of sludge sent directly to the mixing device without being supplied to the flotation thickener. The concentration can be arbitrarily set to 2% or 3%. Furthermore, by raising the temperature of the sludge in the step of adding the polymer flocculant to the thickened sludge, the amount of polymer flocculant added can be reduced by about 20% compared to the case where the temperature is not raised.

[Brief explanation of drawings]

FIG. 1 is a process diagram showing the method of the present invention, and FIG. 2 is a diagram showing the relationship between fractionated fraction and CST. 5...Flocculant addition zone, 7...Mixing zone, 9...
...Solid-liquid separation zone.

Claims (1)

[Claims] 1. Part of the suspension is separated; the separated suspension is supplied to a flocculant addition zone, where a polymer flocculant is added and mixed; flocculant addition The effluent from the mixing zone is fed to a mixing zone where it is mixed with the fractionated residual suspension; the effluent from the mixing zone is then separated into a separated liquid and a concentrate in a solid-liquid separation zone. ;
A solid-liquid separation method for a suspension consisting of each step. 2. The method according to claim 1, wherein the suspension is excess sludge. 3. Claim 1, in which 40 to 60% of the suspension to be treated is separated and fed to the flocculant addition zone.
The method described in section. 4. The method according to claim 1, wherein the flocculant addition zone comprises a flotation concentration device and a mixing tank. 5. The method according to claim 1, wherein the solid-liquid separation zone comprises a belt press type dehydrator. 6. Separate a portion of the suspension; feed this separated suspension to a flocculant addition zone, where a polymer flocculant is added and mixed at elevated temperature, or after mixing, Warm; supply the effluent from the flocculant addition zone to the mixing zone, where it is mixed with the fractionated residual suspension; then, in the solid-liquid separation zone, the effluent from the mixing zone is concentrated with the separated liquid. A solid-liquid separation method for a suspension consisting of each step. 7. The method according to claim 6, wherein the suspension is excess sludge. 8 Claim No. 6 in which the temperature rise is 40°C or more
The method described in section. 9. The method of claim 6, wherein 40% to 60% of the suspension to be treated is separated and fed to the flocculant addition zone. 10. The method according to claim 6, wherein the flocculant addition zone comprises a flotation concentration device and a mixing tank. 11. The method according to claim 6, wherein the solid-liquid separation zone comprises a belt press type dehydrator.