JPH0417120B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for removing heavy metals from sewage sludge, and is particularly applicable to a wide range of applications from household sewage sludge to factory wastewater and human waste treatment sludge. The present invention relates to a method for removing heavy metals from sewage sludge that can be used to transform sewage sludge into green farmland as part of the ecosystem.

[Prior Art] With the recent dramatic increase in investment in sewerage development, the number of local governments implementing sewerage projects has increased, and as the penetration rate has increased, the amount of treated sewage has also increased. As a result, the amount of sewage sludge generated continues to increase.
This trend is expected to increase further in the future.

On the other hand, due to the lack of available organic matter resources, the amount of organic matter applied to green farmland is gradually decreasing, which is one of the causes of a decline in soil fertility.

Since sewage sludge contains organic matter and is rich in fertilizer components, there is interest in its return use for green farmland from the perspective of resource recycling.

However, when applying sewage sludge to green farmland, it is assumed that the sewage sludge is effective as a fertilizer and/or soil improvement material and that it does not contain harmful components that could lead to soil contamination.

The types, forms, and amounts of heavy metals in sewage sludge are affected by the conditions of inflowing sewage, i.e., the inflow ratio of industrial and domestic wastewater, the size of the sewage treatment plant, etc., but in general, there are many types of heavy metals. It is diverse and has a certain concentration. Figure 2, attached for reference, is a table showing the properties of sewage sludge in the fiscal year, and Figure 3 is a table showing the results of multiple property measurements within the year.

[Problems to be Solved by the Invention] Conventional methods for treating and disposing of sewage sludge have mostly involved incineration, landfilling, etc., since there is almost no equipment to remove heavy metals from sewage sludge.
However, these methods have limitations, and problems have been raised from the aspect of environmental conservation.

Until now, attempts have been made to recycle sewage sludge into green farmland, but it has only been limited to domestic wastewater, and only so-called clean sewage sludge has been targeted. However, in reality, the amount and type of such sewage sludge are small, and many general sewage sludges are thought to contain heavy metals at a certain concentration or higher. Furthermore, even in domestic wastewater, the content of heavy metals seems to be increasing as the properties of organic matter change as people's eating habits have become more Western-style.

From the above viewpoint, when returning sewage sludge to green farmland, it is first necessary to remove heavy metals, which is technically difficult.

Therefore, in order to solve the above-mentioned problems, the present invention matches the properties of various sewage sludges, can remove the heavy metals contained therein with high efficiency, and can miniaturize the device.
The objective is to provide a method for removing heavy metals from sewage sludge that can be returned to green farmland.

[Means for Solving the Problems] The method for removing heavy metals from sewage sludge according to the present invention includes a foam flotation step in which sludge that has been hydrolyzed by adding a strong alkalizing agent is foamed by aeration, and the foam flotation separation step. The method includes an acid treatment step in which acid is added to the precipitated sludge precipitated in the process to make it into a strongly acidic state, and a treated sludge separation step in which the treated sludge precipitated in the acid treatment step is separated.

[Function] In the method for removing heavy metals from sewage sludge configured as described above, sludge that has been hydrolyzed by adding a strong alkalizing agent is foamed by a foam flotation separation step in which the sludge is foamed by aeration. In this way, fats and oils and heavy metals are attached to the foam and the sludge is allowed to settle, whereas oil and fats and heavy metals are floated while attached to the foam and separated from the sludge.

Then, in an acid treatment step in which an acid is added to the precipitated sludge precipitated in the foam flotation separation step to make it into a strongly acidic state, concentrated hydrochloric acid is added to bring the pH to about 1, and the sludge is left to stand for hours. That is, it decomposes with acid the parent organic substance that is bound to heavy metals and the like. That is, the heavy metal-organic bond is dissociated.

In this way, heavy metals in sewage sludge are removed from the sludge through the treated sludge separation step in which the treated sludge precipitated in the acid treatment step is separated.

[Example] FIG. 1 is an overall configuration diagram of a first example of the method for removing heavy metals from sewage sludge of the present invention.

First, sewage discharged from homes, factories, etc. is treated with sewage treatment equipment such as the activated sludge method, and sewage sludge that has been thickened by gravity thickening is further converted into thickened sludge through a physical thickening process. be done. The concentration treatment step is performed to improve treatment efficiency, and may be performed by mechanical concentration using a centrifugal concentrator or the like.
That is, the purpose of the sludge concentration performed here is to significantly reduce its volume, thereby saving energy and space. For example, if sludge with a moisture content of 98% is concentrated to 96%, its volume will be halved.

However, when the process of a sewage treatment apparatus such as an activated sludge method and the treatment process of the present invention are continuous, the water content of the thickened sludge obtained in the thickening process does not necessarily need to be reduced. In particular, when matching with a water treatment facility, the water content may be left to the capacity of the water treatment facility.

Therefore, just to be sure, the apparatus for obtaining thickened sludge includes sludge of such a concentration that the thickened sludge is extracted from a sludge thickening tank using a sludge pump.

The thickened sludge is put into a mixing tank 1, and while stirring, caustic potassium KOH is added thereto to make it strongly alkaline with a pH of about 10 as measured by a PH meter. At this time, as described above, they are mixed in a suitable aqueous solution state. The thickened sludge to which caustic potassium KOH has been added is preferably stirred during this period to make the concentration distribution uniform.

Add caustic potassium KOH to the mixing tank 1 above to make it strongly alkaline with a pH of about 10, and then use the slurry pump P1.
The foam is then sent into the foam flotation separation device 2.

In the foam flotation separator 2, compressed air is discharged into the thickened sludge by a gas diffusion method using a compressed air supply pump P2 to perform foaming and aeration. In this aeration, concentrated sludge hydrolyzed with caustic potassium KOH is foamed by aeration, and the alkali-treated sludge is allowed to settle. On the other hand, fats and oils and heavy metals are attached to the foam, and the foam is floated and separated by being allowed to rise. At this time, the time for foam flotation separation treatment is determined by the amount of sludge.

In addition, as the aeration method, water turbine,
Although there is a mechanical method using impellers or the like, since the purpose of this foam flotation separation process is to perform foam flotation separation, it is preferable to foam by the aeration method. However, this does not negate other methods.

In addition, the oils and heavy metals attached to the foam in this foam flotation separation device 2 are raised, and the foam is collected in the gutter 2a and guided to the defoaming device 3.
There, the foam is extinguished and turned into an aqueous solution, which is then sent to the water treatment process as a recovered liquid.

The sludge precipitated by the foam flotation separation is taken out from the lower part 2b of the main body of the foam flotation separator 2 by the sludge pump P3 via the discharge valve V2, and is sent to the mixing tank 4.

The step of aeration within the foam flotation separation device 2 constitutes a foam flotation separation step.

At this time, the concentrated sludge is put into the foam flotation separator 2, and while stirring, caustic potassium KOH is added to make it strongly alkaline to about PH10. After stirring to make the concentration distribution uniform, aeration is carried out. You may go. Alternatively, caustic potassium KOH may be added to the concentrated sludge while it is being sent into the foam flotation separator 2 to make it strongly alkaline to about pH 10.

In addition, in the foam flotation separation process, since it seems that heavy metals are organically bound to various types of sewage sludge, organic substances are peptized using caustic potassium KOH with a strong alkalinity of about PH10. It also has the effect of breaking down the matrix of heavy metal bonds that seem to have a bonding relationship with organic matter as a matrix, making it easier for heavy metals to separate. That is, by making the sludge alkaline, the humus-like portion is eluted to the solution side. Since heavy metals in sludge are thought to be bound to some extent with humus-like parts, a removal effect can be expected by dissolving them. In addition, the effect of dissociating metal-organic bonds can also be expected.

The precipitated sludge taken out from the lower body part 2b of the foam flotation separator 2 is added with concentrated hydrochloric acid HCl in the mixing tank 4 to make it strongly acidic with a pH of about 1 as measured by a PH meter. That is, the parent organic matter that is bound to heavy metals and the like that could not be separated by the alkali treatment is acid-decomposed. That is, the heavy metal-organic bond is dissociated.

The step of taking out the precipitated sludge from the foam flotation separator 2 and adding concentrated hydrochloric acid HCl in the mixing tank 4 to make it strongly acidic to about PH1 constitutes an acid treatment step.

The acid-treated concentrated sludge that has undergone an acid treatment process in the mixing tank 4 to be made strongly acidic to about pH 1 by adding concentrated hydrochloric acid HCl is sent to the dehydrator 5 by the sludge pump P4.

The dehydrator 5 is a mechanical dehydrator or dewatering equipment such as vacuum filtration, pressure filtration, or centrifugation. The acid-treated concentrated sludge treated by the dehydrator 5 becomes treated sludge and filtrate, and the treated sludge is returned to green farmland.

In this way, the process of separating treated sludge precipitated in the acid treatment process constitutes a treated sludge separation process, and heavy metals in sewage sludge are removed from the sludge by this process of separating treated sludge. I can do it.

As described above, the embodiments of the present invention utilize sewage sludge as organic matter or fertilizer material, and it is extremely desirable from the standpoint of resource utilization and resource saving to be able to use it for return to green farmland. As something that fits the logic of recirculation, its social significance is considered to be extremely large.

4 to 12 show the experimental results of this example.

In the foam flotation process in which the treated sludge is foamed by aeration using the foam flotation device 2 of the above embodiment, results as shown in the table of FIG. 4 showing the removal rate of heavy metals by the foam flotation method were obtained.

The test subject added 1, 5, 10, and 20 [g] of caustic potassium KOH to 500 [g] of sewage sludge, and
[ ] and aerated for 1 hour.
The relationship between the amount of caustic potassium KOH added to sludge and the pH is as follows: when the amount of caustic potassium KOH added is 0 [gKOH], when the amount of caustic potassium KOH added is 0 [gKOH], when the amount is PH5.05 and 1 [gKOH], and when the amount is PH9.60 and 5 [gKOH]. , when PH12.62 was 10 [gKOH], when PH12.90 was 20 [gKOH], PH13.09.

Based on this result, 10% of caustic potassium KOH
It can be seen that when [g] is added, there is a peak in the removal rate of heavy metals, except for iron (Fe). Considering the overall removal rate of heavy metals, when 10 [g] of caustic potassium KOH is added, the test specimen has a pH of 10 or more. In the separation process, it is found that it is desirable to carry out foam flotation separation in a strongly alkaline state with a pH of about 10.

With only acid treatment, the results shown in the tables shown in FIGS. 5 to 7 showing the removal rate of heavy metals by acid treatment were obtained.

The subject added hydrochloric acid HCl to 1000 [g] of sewage sludge to make it PH1, PH2, or PH3, respectively, and
The mixture was left to stir for 1, 3, 6, and 12 hours, centrifuged at 7000 rpm for 10 minutes, and heavy metals were measured by atomic absorption spectrometry after aqua regia decomposition.

Since the standing time required for acid treatment reaches a substantially saturated region in about 6 to 12 hours, if acid treatment is performed for about 6 hours, the removal rate of heavy metals will be high. However, it can be seen that even if the leaving time is 0 hours, that is, about 30 minutes or less at a pH of about 1, the removal rate is around 50%, although the measurement error seems to be large. When designing a system for removing heavy metals from sewage sludge in combination with other treatment processes,
A removal rate of over 50% can be expected even after leaving it for about one hour.

There are several reports regarding metal removal by acid treatment (DJWozniak and JYCHuang, M.
S.Gould etc.). According to it, the removal rate is PH
The removal rate also increases as the pH decreases. However, assuming the return of green farmland, the pH needs to be about 1 or higher so that organic matter is not destroyed.

Therefore, how long to leave the sludge in a strongly acidic state during the acid treatment process depends on the content of heavy metals, the combination with other treatment processes, and the design of the system for removing heavy metals from sewage sludge. It is determined.

In the foam flotation separation process and acid treatment process,
The results shown in the table shown in FIG. 8 showing the removal rate of heavy metals by the continuous treatment of foam flotation and acid treatment were obtained.

The subject was exposed to caustic potassium in 500 [g] of sewage sludge.
The sludge obtained by adding 10 [g] of KOH to make the liquid volume 1 [] and aerating for 1 hour was treated in the same manner as in the acid treatment experiment.

The value obtained at this time is approximately the time left for acid treatment.
This was done for 1 hour at a pH of about 1. Also, since it is not destroyed as an organic substance at this time,
It can be recycled as a completely organic fertilizer.

Note that caustic potassium KOH is used in the foam flotation separation process of the above embodiment of the present invention because it is relatively inexpensive as a strong alkalizing agent to obtain a strongly alkaline state, and when returning to green farmland, This will replenish potassium, a fertilizer ingredient, that is lacking in sewage sludge. However, when carrying out the present invention, the strong alkalizing agent used in the foam flotation separation process is not limited to caustic potassium KOH.

FIG. 9 to FIG. 12 are tables showing the existence forms of heavy metals.

In particular, when the treated sludge of this example is returned to green farmland, it is necessary that heavy metals do not dissolve from the treated sludge in a natural state.

For example, as standard values for substances contained in sewage sludge, the dry matter concentration for Hg is 2.0 (mg/Kg), the dry matter concentration for Cd is 5.0 (mg/Kg), the dry matter concentration for As is 50.0 (mg/Kg), and the standard value for eluted water. As, Concentration of Cd: 0.3 (mg/) Concentration of Pb: 3.0 (mg/) Concentration of Cr: 1.5 (mg/) Concentration of As: 1.5 (mg/) Concentration of Hg: 0.005 (mg/) Concentration of CN: 1.0 (mg/) The concentration of PCB is 0.003 (mg/).

Naturally, values exceeding these standard values do not dissolve out of the treated sludge, and preferably, heavy metals do not dissolve out of the treated sludge in a natural state.

Therefore, we treated it with water, ammonium acetate, hydrochloric acid, and sodium hydroxide, and looked at the presence of heavy metals in order to determine the possibility of this.

From the table showing the existence form of heavy metals in untreated sludge in Figure 9 to the table showing the existence form of heavy metals in treated sludge after foam flotation separation (foam flotation separation only) in Figure 10, As can be seen from the table showing the existence form of heavy metals in treated sludge after acid treatment (acid treatment only), and further from the table showing the existence form of heavy metals in treated sludge after foam flotation separation and acid treatment in Figure 12. It can be seen that those that have undergone the foam flotation separation process and the acid treatment process have the most poorly soluble substances, making it difficult to dissolve from the treated sludge.

[Effects of the Invention] As described above, the method for removing heavy metals from sewage sludge of the present invention includes a foam flotation step in which sludge that has been hydrolyzed by adding a strong alkalizing agent is foamed by aeration, and a foam flotation separation step in which the sludge is foamed by aeration. This process consists of an acid treatment step in which acid is added to the precipitated sludge to make it into a strongly acidic state, and a treated sludge separation step in which the treated sludge precipitated in the acid treatment step is separated.
The heavy metals contained in the farm can be efficiently removed by processing based on the premise of returning green farmland without destroying organic matter.

In addition, when a strong alkalizing agent is added to sludge for hydrolysis, the aeration during the foam flotation separation process forces the bonds to break and the humus-like parts are eluted into the solution, which requires some standing time. Since the processing time is not reduced, the processing time can be shortened, and the entire apparatus can be made smaller.

Since caustic potassium is added and hydrolyzed, it is possible to replenish the potassium fertilizer that sewage sludge lacks in fertilizer components, and is effective in returning sewage sludge to green farmland.

[Brief explanation of drawings]

FIG. 1 is an overall configuration diagram of a method for removing heavy metals from sewage sludge according to an embodiment of the present invention, and FIG. 2 is a table showing the properties of sewage sludge in each year in the method for removing heavy metals from sewage sludge according to an embodiment of the present invention. FIG. 3 is a table showing the results of multiple property measurements within the fiscal year in the method for removing heavy metals from sewage sludge according to the embodiment of the present invention;
FIG. 4 is a table showing the removal rate of heavy metals by foam flotation separation method in the method for removing heavy metals from sewage sludge according to an example of the present invention, and FIGS. 5 to 7 show heavy metals from sewage sludge according to an example of the present invention. A table showing the removal rate of heavy metals by the acid treatment method in the removal method, FIG. 8 shows the removal rate of heavy metals by continuous treatment of the foam flotation separation and acid treatment methods in the method for removing heavy metals from sewage sludge in the example of the present invention. The table and FIGS. 9 to 12 are tables showing the existence forms of heavy metals in the method for removing heavy metals from sewage sludge according to the embodiments of the present invention. In the figure, 1 and 4 are a mixing tank, 2 is a foam flotation separation device, 3 is a defoaming device, and 5 is a dehydrator. In addition, in the figures, the same reference numerals and symbols indicate the same or equivalent parts.

Claims (1)

[Claims] 1. A foam flotation separation step in which sludge that has been hydrolyzed by adding a strong alkalizing agent is foamed by aeration, and an acid is added to the precipitated sludge precipitated in the foam flotation step to make it in a strongly acidic state. A method for removing heavy metals from sewage sludge, comprising: an acid treatment step, and a treated sludge separation step, which separates treated sludge precipitated in the acid treatment step. 2. The method for removing heavy metals from sewage sludge according to claim 1, wherein hydrochloric acid is added to the sludge to make it strongly acidic. 3. The method for removing heavy metals from sewage sludge according to claim 2, wherein the acidity of the sludge is about PH1 or higher. 4. Any one of claims 1 to 3, wherein the sludge is thickened sludge.
Method for removing heavy metals from sewage sludge described in . 5. The method for removing heavy metals from sewage sludge according to any one of claims 1 to 4, wherein the acid treatment step comprises leaving the sludge in a strongly acidic state for a predetermined period of time. 6. Heavy metals in sewage sludge according to claim 5, characterized in that the time for adding acid to the sludge precipitated in the foam flotation separation step and leaving it in a strongly acidic state is within 6 hours. Removal method.