JPH10156391A - Treatment of phosphorus recovered from treated water of sewerage - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively utilize a phosphorus and heavy metallic compd. incorporated in a treated water and simultaneously to detoxicate at a sewerage disposal plant. SOLUTION: Metallic salts are added to the treated water of the sewerage to fix a water soluble phosphorus incorporated in the treated water and to recover together with a precipitated sludge at the final settling tank, then ferric chloride and sodium hypochlorite are added to the sludge to elute the heavy metal compd. in the sludge, then hydrogen sulfide is allowed to react with the sludge to convert the heavy metals to sulfide, and also to convert the phosphorus compd. previously converted into insoluble to a soluble phosphoric acid. The phosphorus is incorporated as the soluble phosphoric acid and the heavy metals are incorporated as the insoluble compd. in a cake obtained by the final solid-liq. separation, and the cake is used as a fertilizer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for recovering soluble phosphorus contained in treated water from a sewage treatment plant or the like and being discharged as it is together with sludge as an insoluble phosphorus compound, and then transferring the sludge to green agricultural land or the like. The present invention relates to a method for converting an insoluble phosphorus compound into soluble phosphorus again while simultaneously converting heavy metals in sludge into insoluble sulfide.

[0002]

2. Description of the Related Art In a sewage treatment plant, after a suspended sediment of inflowing sewage is settled and separated, activated sludge is used to purify soluble substances in the sewage to a standard value or less and discharge the same to public waters. Has become. However, in the activated sludge method,
It is difficult to remove the total amount of inflowing phosphorus, and about 45 to 50% of the amount of inflowing phosphorus is usually contained in treated water and discharged to public waters. In addition, sewage such as agricultural wastewater and domestic wastewater flows directly into public waters, and water pollution and deterioration of water quality are becoming a problem. It is desirable that the amount of nutrients such as nitrogen be small. On the other hand, iron salts such as iron sulfate and iron chloride, or aluminum salts such as polyaluminum chloride and aluminum sulfate are added to the influent wastewater to reduce the amount of phosphorus contained in the wastewater. A method has also been practiced in which after an insoluble phosphoric acid compound is formed, an activated sludge treatment is performed, and the insolubilized phosphorus is mixed into the sludge and recovered. Since the phosphorus recovered by this method is insoluble, when composting sludge and applying it to green farmland or the like, the effectiveness of phosphorus is reduced.

[0003]

In recent years, a large amount of sewage containing nutrients such as nitrogen and phosphorus flows into public waters and closed waters, and the deterioration of water quality caused by abnormal growth of aquatic plants and algal microorganisms has become a problem. ing. In order to prevent the deterioration of water quality in these water areas, it is important to reduce inflowing sewage and to reduce nitrogen and phosphorus contained in effluent discharged from sewage treatment plants where intensive treatment is performed as much as possible. However, in the sewage treatment method using activated sludge currently being carried out in a sewage treatment plant, it is difficult to remove the entire amount of inflowing phosphorus, and about 45 to 50% of the amount of inflowing phosphorus is discharged as treated water. Is usually there.

On the other hand, when the treated water discharge water area is a closed water area, or when used together with a soil water intake water area,
In some sewage treatment processes, iron salts or aluminum salts are added to remove soluble phosphorus in the effluent as an insoluble phosphorus compound. When sludge is reused as an organic fertilizer, the phosphorus in the sludge is insoluble, so that the phosphorus fertilizing effect is reduced.

The present inventors have paid attention to these problems,
Significantly reduce the phosphorus contained in the effluent from the sewage treatment plant, and improve the fertilizing effect of phosphorus when the recovered phosphorus is changed to soluble and the sludge is reused as organic fertilizer for green farmland etc. Another object is to provide an economical method for converting heavy metals contained in sludge into insoluble sulfide.

[0006]

Means for Solving the Problems The treated water obtained by purifying sewage using activated sludge at a sewage treatment plant is added to the supernatant water obtained by settling and separating a suspended substance in a final sedimentation basin. 50
% Is released as it is. Approximately 60 to 70% of the phosphorus contained in the effluent is soluble phosphate ions (PO4-3). It is easily predicted that if this soluble phosphorus ion is coagulated and precipitated as an insoluble compound and separated, the phosphorus in the effluent can be significantly reduced. As a method for converting soluble phosphorus ions into an insoluble compound, there are a method of adding calcium salts or magnesium salts, and a method of adding iron salts and aluminum salts. Considering the processing method described later, ferrous chloride (Fecl2) and ferric chloride (Fec2)
The method using l3) is the most suitable.

[0007] When ferrous chloride or ferric chloride is added to the treated water in a process before the sewage treated water enters the final sedimentation basin, the soluble phosphate ion becomes ferrous phosphate (Fe3 (P
o4) 2) or ferric phosphate (FePo4) is generated and sedimented in the final sedimentation tank together with the sludge contained in the treatment liquid. Usually, the final sedimentation tank sludge is first mixed with the sedimentation tank sludge, subjected to sludge conditioning through a concentration step, and then subjected to solid-liquid separation using a dehydrator. When the dewatered sludge separated into solid and liquid is reused as an organic fertilizer for green farmland and the like, it is important that the sludge contains a small amount of heavy metals, particularly zinc, which is most frequently contained.

The present inventor has proposed that heavy metals contained in sludge,
In particular, as a method for removing zinc, a method has been developed to add ferric chloride and sodium hypochlorite to concentrated sludge and elute it to separate and remove it.By treating with this method, zinc in sludge is removed. Is largely removed. However, heavy metals and zinc also remain in the form of chlorides in small amounts in the dewatered sludge treated by this method, and it is important to convert the remaining metal chlorides into insoluble sulfides.
As described above, dehydrated sludge obtained by recovering soluble phosphorus from effluent discharged from a sewage treatment plant and removing heavy metals, particularly zinc, is insoluble iron phosphate compound and a small amount of residual heavy metal. It has a structure containing chloride and zinc chloride.

When the dehydrated sludge is separately reacted with hydrogen sulfide generated by iron sulfide and hydrochloric acid, the insoluble iron phosphate compound generates iron sulfide and soluble phosphoric acid. On the other hand, heavy metal chlorides and zinc chlorides become insoluble sulfides. Since the residue from the production of hydrogen sulfide from iron sulfide and hydrochloric acid is ferrous chloride, this ferrous chloride can be recycled to recover soluble phosphate ions from the effluent. it can.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Conventionally, it is said that when soluble phosphorus is subjected to coagulation precipitation and separation as an insoluble salt, those having a low soluble phosphorus concentration are difficult to recover. And the concentration of phosphorus in the effluent discharged from the sewage treatment plant is also several ppm.
Since it is diluted at a concentration of about several tens of ppm, the method of adding an iron salt to the effluent for coagulation, sedimentation and separation has a very low recovery rate. However, in the sewage treatment plant process, the treated water before entering the final sedimentation basin contains a large amount of sludge particles from the aeration tank, so by adding an iron salt immediately before entering the final sedimentation basin, The generated iron phosphate adheres to sludge particles or generates flocculated floc, coprecipitates, and can be efficiently settled and separated.

Furthermore, sludge fine particles which have not settled conventionally can also form flocculated flocks of iron chloride and can be settled and separated, thereby improving the quality of the discharged water.
In this way, the iron phosphate added and recovered is insoluble, so when dewatered sludge is used as an organic fertilizer, the fertilizer effect of phosphorus is low. I need to change. In addition, dehydrated sludge obtained by treating heavy metals and zinc in sludge with ferric chloride and sodium hypochlorite is a soluble metal chloride, so the removal effect is reduced unless it is made an insoluble compound. It will be.

In order to achieve the above object, if dehydrated sludge is reacted with hydrogen sulfide, iron phosphate becomes soluble phosphoric acid, and metal chloride becomes insoluble sulfide. Then, since hydrogen sulfide and ferrous chloride are generated by the reaction between iron sulfide and hydrochloric acid, ferrous chloride is circulated and used as a collecting agent for phosphoric acid. The above action is described using a chemical formula as follows.

[0013]

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[0014]

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[0015]

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[0016]

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[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a flow for carrying out the present invention.
It is as follows. In Fig. 1, the initial settling, aeration tank, and final settling are
This shows a sewage treatment process in a sewage treatment plant, and inflow sewage A is treated and discharged B through this process using activated sludge. During this treatment step, initial sludge is generated from the initial settling, and excess sludge is generated from the final settling. Both of them are concentrated by gravity to form mixed raw sludge.

Ferric chloride and sodium hypochlorite are added to the mixed raw sludge to elute heavy metals, particularly zinc compounds, in the sludge. The pH of the sludge that has been subjected to the elution treatment is adjusted using an alkali agent, and a dehydration operation is performed using a dehydrator. Heavy metals, particularly a dehydrated cake having a reduced zinc content, and heavy metals and zinc that have been eluted and separated are separated. A dehydrated filtrate containing a large amount is obtained.

In the dewatered cake, heavy metals and zinc contained in the dewatered cake are converted into insoluble sulfides and iron phosphate is converted into soluble phosphoric acid through hydrogen sulfide separately generated by iron sulfide and hydrochloric acid. The dewatered cake subjected to the sulfidation operation is neutralized with an alkali agent to facilitate handling of fertilizer and composting.

On the other hand, the separated dehydrated filtrate containing a large amount of heavy metals and zinc is neutralized with an alkali agent to precipitate and precipitate dissolved heavy metals and zinc as hydroxides.
After dehydration, a dehydrated cake containing a large amount of heavy metals and zinc is separated and collected. Since the separated water generated by this operation becomes clear, the separated water is returned to the concentration tank and treated together with the concentrated separated water. Further, since the residue from the generation of hydrogen sulfide using iron sulfide and hydrochloric acid is ferrous chloride, this ferrous chloride is reused for the recovery of phosphoric acid in the final precipitation.

[0021]

According to the present invention, phosphorus discharged from a sewage treatment plant can be significantly reduced,
Moreover, it is possible to reduce heavy metals in the sludge, in particular to remove most of the zinc, to make the remaining heavy metals insoluble, and to reuse the dehydrated cake as an organic fertilizer. Can be improved.

[Brief description of the drawings]

FIG. 1 is a flow chart of a method according to the present invention.

Claims (2)

[Claims] 1. A metal salt is added to treated water of a sewage treatment plant or the like, and soluble phosphorus contained therein is recovered as an insoluble metal compound together with the sludge of the final sedimentation basin. Sodium chlorite is added to elute heavy metal compounds in the sludge, followed by solid-liquid separation, and hydrogen sulfide is reacted with the separated sludge containing the insoluble iron phosphate compound, heavy metal chloride and zinc chloride. A method for treating phosphorus recovered from sewage water in which an insoluble phosphate compound is soluble phosphoric acid and heavy metals chloride and zinc chloride are insoluble sulfides. 2. The hydrogen sulfide to be reacted with the separated sludge containing the insoluble iron phosphate compound, heavy metal chloride and zinc chloride is hydrogen sulfide produced by reacting iron sulfide and hydrochloric acid. The method for treating phosphorus recovered from sewage treatment water according to claim 1, wherein ferrous iron is used as a metal salt for converting soluble phosphorus into an insoluble metal compound.