JPH0671296A - Solid-liquid separation method for digested sludge - Google Patents

Abstract

PURPOSE:To sufficiently and rapidly perform solid-liquid separation of digested sludge without requiring much power consumption and complicated equipment by lowering the pH of digested sludge to discharge carbon dioxide and floating and separating the digested sludge by the gas to subject it to solid-liquid separation. CONSTITUTION:Alkali is added to an anaerobic digestion tank 1 from an alkali storage tank 2 to rapidly increase the total concentration of carbonate ions, bicarbonate ions and dissolved carbon dioxide dissolved in digested sludge. The digested sludge leaving the anaerobic digestion tank 1 is introduced into a mixing tank 3 and a pH lowering agent is added from a pH lowering agent storage tank 4 to turn the dissolved carbonate ions, bicarbonate ions, dissolved carbon dioxide, etc., into gaseous CO2 and the mixture of gas and digested sludge is thrown into a solid-liquid separation tank 5. Then, the generated carbon dioxide is stuck to solid components in the digested sludge to float them. As a result, the digested sludge is subjected to solid-liquid separation into concentrated digested sludge of two or more times concentration and a clean digested desorbed liquid.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid-liquid separation method for digested sludge.

[0002]

2. Description of the Related Art Conventionally, a gravity concentration method has been mainly used as a solid-liquid separation method for digested sludge. However, since the gravity concentration method cannot sufficiently perform solid-liquid separation, it has the following problems. The concentration of concentrated digested sludge was low, and the ability of dehydration and drying processes in the next process was reduced. Since solid substances and the like are mixed in the digested and desorbed liquid, the load on the water treatment system increases when it is returned as return water to the water treatment system. In the anaerobic digestion process, ortholine phosphorus was eluted, and the concentration of ortholine phosphorus in the digestive desorbate was high. Further, the gravity concentration method has a problem that the size of the solid-liquid separation tank has to be as large as that of the anaerobic digestion tank because the solid-liquid separation speed is slow.

On the other hand, as a solid-liquid separation method that solves the drawbacks of the gravity concentration method, a method called pressure floating concentration method is known. This method is mainly used in the sludge concentrating process.When pressure is released in the pressure vessel to dissolve gas such as air and then the pressure is released, the gas becomes bubbles and adheres to the sludge, causing the sludge to float. This is a solid-liquid separation method that utilizes the phenomenon of causing. However, this method requires a lot of electric power to apply a high pressure to the sludge and has a drawback that the equipment is complicated.

[0004]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems of the prior art and is capable of solid-liquid separation of digested sludge at a sufficiently high speed without requiring a lot of electric power and complicated equipment. It has been completed to provide a solid-liquid separation method for digested sludge that can be obtained.

[0005]

Means for Solving the Problems The present invention, which has been made to solve the above-mentioned problems, is directed to anaerobic digestion of organic sludge in a sealed anaerobic digestion tank under alkaline conditions having a pH of 7.3 or higher. By increasing the total amount of carbonate ions, bicarbonate ions, and dissolved carbon dioxide concentration in the obtained digested sludge, the carbon dioxide gas is released by lowering the pH of the digested sludge, and this gas is used. The method is characterized in that the digested sludge is floated and separated to perform solid-liquid separation into concentrated digested sludge and digested desorbed liquid. In addition, a preferable result can be obtained by adding a coagulant to the concentrated digested sludge obtained by this method and performing a dehydration treatment.

[0006]

The present invention will be described in more detail below with reference to the illustrated embodiments. In FIG. 1, 1 is a closed anaerobic digestion tank, and organic sludge as it is or concentrated to a concentration of about 2 to 4% is put into the anaerobic digestion tank 1 and digested anaerobically. And becomes digested sludge. At this time, carbon dioxide and methane gas are mainly generated in the anaerobic digestion tank 1.

Alkali is added to the anaerobic digestion tank 1 from the alkali storage tank 2, and the inside of the anaerobic digestion tank 1 is maintained under alkaline conditions with a pH of 7.3 or higher. The addition of alkali may be performed on the organic sludge added to the anaerobic digestion tank 1. Figure 2
Is a graph showing the relationship between the pH in the anaerobic digestion tank 1 and the ratio of carbon dioxide and methane gas in the generated gas. When the pH is increased, a digested gas with a high methane gas ratio is obtained. On the contrary, the ratio of carbon dioxide is lower than that of the normal digestion where the pH is around 7, and it becomes about 15% at pH 8. From this, the partial pressure of carbon dioxide in the anaerobic digester 1 is about 0.15 atm.

The carbon dioxide generated in the anaerobic digester 1 dissolves into the digested sludge in the anaerobic digester 1 according to Henry's law. Henry's constant varies with temperature, so
The dissolved carbon dioxide concentration [CO ₂ ] is controlled by the partial pressure of carbon dioxide in the atmosphere in the anaerobic digester 1 and the temperature in the anaerobic digester 1. Further, the dissolved carbon dioxide ^{_{CO 2 + H 2 O ⇔H +}} + HCO 3 - ⇔2H + + CO 3 dissociates ² form, an alkaline side is [HCO ₃ ^-] and [CO ₃ ^-] is increased equilibrium However, when the pH drops, carbonate ions and bicarbonate ions return to carbon dioxide. Therefore, the concentrations of carbonate ions, bicarbonate ions, and dissolved carbon dioxide in the digested sludge are governed by the partial pressure of carbon dioxide in the anaerobic digestion tank 1, the temperature in the anaerobic digestion tank 1, and the pH. Figure 3 shows how the total concentration of carbonate ions, bicarbonate ions, and dissolved carbon dioxide in clear water changes with pH when the partial pressure is 0.15 atm. As you can see from this Figure 3,
On the alkaline side where the pH is 7.3 or higher, the total concentration of carbonate ions, bicarbonate ions and dissolved carbon dioxide dissolved in the digested sludge rapidly increases.

Next, as shown in FIG. 1, the digested sludge discharged from the anaerobic digestion tank 1 is introduced into a mixing tank 3 and a pH lowering agent is added from a pH lowering agent storage tank 4. As a result, carbonate ions, bicarbonate ions, dissolved carbon dioxide, etc. dissolved in the digested sludge become carbon dioxide gas, and the mixed liquid of carbon dioxide gas and digested sludge is put into the solid-liquid separation tank 5. The amount of carbon dioxide gas generated is theoretically in the range of pH 8 to 7.
It becomes 7333 mg / L.

In the solid-liquid separation tank 5, the generated carbon dioxide gas adheres to the solid components in the digested sludge to reduce the apparent specific gravity and float the solid components. If the mixing in the mixing tank 3 is complete, the flotation is completed in about 30 to 60 minutes, and the digested sludge having a concentration twice or more and the clean digested desorbed liquid are separated.
A coagulant is added to the concentrated digested sludge from the coagulant storage tank 6 and dehydrated by the dehydrator 7, and the digested desorbed liquid is sent to the water treatment system.

FIG. 4 shows the concentration ratio and the concentrated digested sludge depending on the gas-solid ratio in the solid-liquid separation tank 5 when the floating time is set to 30 minutes.
It is a graph showing how the SS recovery rate changes. As can be seen from this figure, it is necessary to set the gas-solid ratio to 0.1g-CO ₂ / g-ss in order to increase the concentration ratio to 2 or more. When carbon dioxide gas is generated, the gas-solid ratio is 0.36 g-CO ₂ / g-ss, which means that a high concentration factor can be obtained.

Ferric chloride is used as the above-mentioned pH lowering agent.
By using (hereinafter abbreviated as salt iron) or PAC, the concentration of ortholine phosphorus in the digestive desorbed liquid can be reduced as shown in FIG. As shown in Fig. 5, theoretically Fe / P
If salt iron is added so that the O ₄ -P ratio becomes 1, it should be possible to remove 100% of orthophosphorus phosphorus, but in reality it is necessary to add an excessive amount. For example, as shown in FIG. 5, when the salt iron addition rate is 1.62 [Fe / PO ₄ -P], the PO ₄ -P removal rate is about 64%.

The concentrated digested sludge obtained as described above is superior to the filter press dehydrator and the belt press dehydrator when it is added in a smaller amount than the commonly used coagulant. Shows dehydration. The results are shown in Tables 1 and 2. The concentrated digested sludge of the present invention can greatly improve the dewatering ability in both the filter press dewatering machine and the belt press dewatering machine, reduce the water content of the cake, and improve the releasability from the filter cloth.

[0014]

[Table 1]

[0015]

[Table 2]

[0016]

As described above, according to the method for solid-liquid separation of digested sludge of the present invention, organic sludge is anaerobically digested in an anaerobic digester under alkaline conditions of pH 7.3 or higher. After that, the digested sludge is floated and separated by using the carbon dioxide gas released by lowering the pH, so unlike the conventional pressure flotation method, it requires a lot of power and complicated equipment. Therefore, the digested sludge can be solid-liquid separated at a sufficiently high speed. According to the invention,
The capacity in the dehydration process can be improved, and the concentration of orthophosphorus phosphorus in the digested and desorbed liquid returned to the water treatment system can be reduced by using a coagulant such as iron salt or PAC as a pH lowering agent. Can be made.

[Brief description of drawings]

FIG. 1 is a flow sheet showing the steps of the present invention.

FIG. 2 is a graph showing the relationship between pH in the anaerobic digester and the ratio of carbon dioxide and methane gas in the generated gas.

[Fig. 3] Fresh water when the carbon dioxide partial pressure is 0.15 atm
3 is a graph showing the relationship between pH and the total concentration of carbonate ions, bicarbonate ions, and dissolved carbon dioxide.

FIG. 4 is a graph showing a relationship between a gas-solid ratio, a concentration ratio, and a concentrated digested sludge SS recovery rate.

FIG. 5 is a graph showing the relationship between the amount of iron salt, which is a pH-lowering agent, and the removal rate of ortholine phosphorus in the digestive desorption solution.

[Explanation of symbols]

 1 Anaerobic digestion tank 2 Alkali storage tank 3 Mixing tank 4 pH lowering agent storage tank 5 Solid-liquid separation tank 6 Coagulant storage tank 7 Dehydrator

Claims (2)

[Claims] 1. A digestion sludge obtained by subjecting an organic sludge to an anaerobic digestion treatment in an enclosed anaerobic digestion tank under alkaline conditions having a pH of 7.3 or more, carbonate ions, bicarbonate ions, and dissolved dioxide. After increasing the total carbon concentration, the pH of the digested sludge is lowered to release carbon dioxide gas, and the digested sludge is floated and separated by using this gas to solidify the digested sludge and the digested desorbed liquid. A solid-liquid separation method for digested sludge, characterized by liquid separation. 2. A method for solid-liquid separation of digested sludge, which comprises adding a flocculant to the concentrated digested sludge obtained by the method of claim 1 and performing dehydration treatment.