JPS632236B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating wastewater containing lower phosphate ions.

Generally, phosphorus compounds in wastewater are regulated because if they are released as they are, they will degrade the environment due to problems such as eutrophication of plants. Therefore, many proposals have been made to remove phosphorus from wastewater, and a typical method is to remove phosphate ions from wastewater by using a calcium agent to remove insoluble calcium phosphate, such as hydroxyapatite. There is a method of generating it and then separating and removing it.

However, the concentrations and types of phosphorus compounds contained in factory wastewater vary widely, and the above method cannot often be applied uniformly.

In the conventional method of removing phosphorus from these wastewaters as hydroxyapatite using calcium, orthophosphorus compounds and condensed phosphorus compounds can be removed relatively well, but lower phosphorus compounds are hardly removed.

Therefore, the present inventors conducted intensive research in order to dephosphorize wastewater that also contains lower phosphorus compounds, and found that they were converted to orthophosphate ions using an oxidizing agent under specific pH conditions, and were used as calcium agents, etc. The present invention was completed based on the finding that all phosphorus compounds in wastewater can be substantially removed by applying a poorly soluble phosphate forming agent.

That is, the gist of the present invention is to add an oxidizing agent to wastewater containing lower phosphate ions, oxidize the lower phosphate ions to orthophosphate ions under conditions of pH 3 or less, and then add a sparingly soluble phosphate forming agent. This is a method for treating wastewater containing lower phosphate ions, which is characterized by separating and removing the precipitate produced.

In the present invention, any wastewater to be treated is applicable as long as it contains lower phosphate ions, regardless of its concentration, even if it contains ordinary orthophosphate ions or polyphosphate ions. There is no problem and it may contain Cl ^- or F ^- .

Here, the lower phosphate ion refers to a reducing phosphorus compound having a lower valence than a pentavalent phosphate ion such as orthophosphate ion, and includes, for example, phosphite ion, hypophosphite ion, Examples include lower phosphoric acids such as pyrophosphite ion and hypophosphate ion, and sodium salts, potassium salts, ammonium salts, and magnesium salts of these acids.

An oxidizing agent is applied to the phosphorus-containing wastewater to oxidize it to pentavalent orthophosphate ions.

Examples of oxidizing agents include chlorine gas, chlorine water,
Chlorine-based oxidizing agents such as sodium hypochlorite and calcium hypochlorite (salt powder) are suitable and typical, but other oxidizing agents include hydrogen peroxide, calcium peroxide, and persulfates. or ozone. .

In this case, the oxidation conditions vary depending on the nature and condition of the wastewater or the type of oxidizing agent, but the liquid nature of the wastewater must be at least PH3 or lower.

The reason is that lower phosphate ions are not effectively oxidized when the pH exceeds 3.

Further, the oxidizing agent needs to be added in an amount greater than the stoichiometric amount required to oxidize lower phosphate ions.

For example, the oxidation rate when treating lower phosphate ions in wastewater using chlorine gas as an oxidizing agent is
As shown in the figure.

As shown in Figure 1, increasing the amount of chlorine speeds up the oxidation reaction, but also increases the amount of chlorine that evaporates out of the system.
To prevent this, chlorine can be used efficiently by connecting two or more chlorine oxidation tanks. Figure 2 shows the oxidation rate of lower phosphate ions in each oxidation tank when three chlorine oxidation tanks each containing 500 ml of waste water were connected at a chlorine injection rate of 40 ml/min.

The time required for the oxidation rate to reach 98% or more is 1 hour for the first unit, 2 hours for the second unit, and 3 hours and 30 minutes for the third unit.

At this time, the color tone of the wastewater in each oxidation tank is yellow, and the dissolved chlorine is 1800mg/, 2500mg/, and 2300mg/, respectively.
In order to completely oxidize lower phosphate ions, more than 1000 mg of dissolved chlorine is required. However, there is virtually no need to increase the oxidation rate to 8% or higher, and dissolved chlorine in the wastewater after phosphorus removal is 200 mg/day.
It is enough.

Since the color of the wastewater at this time is pale yellow, the end point of oxidation can be easily determined.

The above example is an example of the oxidation of lower phosphate ions using chlorine as an oxidizing agent, but if another oxidizing agent is used, the oxidation may be performed in accordance with the actual condition of the wastewater to be treated.

Then, after oxidizing the lower phosphate ions, the orthophosphate ions in the wastewater are treated with a salt-forming agent to form poorly soluble phosphates, as is commonly done. Add. As the poorly soluble phosphate forming agent, calcium agents such as slaked lime, quicklime, calcium chloride, and gypsum are most preferred, but other agents such as magnesium salts, aluminum or iron salts may also be used. When a calcium agent is used, it is preferable to form a precipitate as apatite, as shown in the following formula, and to separate and remove this precipitate.

5Ca ²⁺ +OH ^- +3PO ₄ ^3- →Ca ₅ (OH) (PO ₄ ) ₃ In this case, excess chlorine in the wastewater is simultaneously removed as calcium hypochlorite, and even if fluorine ions are included. , fluoroapatite, and therefore halogens in wastewater can be removed simultaneously without any problems by the method of the present invention.

Note that the above reaction is preferably carried out on the alkaline side,
It goes without saying that the acidic wastewater after oxidation must be neutralized with an alkali to adjust the pH when forming the precipitate, since it is carried out at a pH of 8 or above.

In this way, substantially all dissolved phosphorus compounds in the wastewater are converted into precipitates and can be removed by performing the desired separation operation.

In this case, in the above precipitate formation, if necessary, an inorganic flocculant such as aluminum salt or iron salt, an adsorbent such as activated carbon, or an organic polymer flocculant such as polyacrylamide may be used to make the separation operation more efficient. can be done.

In addition, if the treated wastewater is neutralized by appropriately adjusting the pH, it becomes harmless wastewater as it is, and can be recycled as water if necessary.

Thus, according to the present invention, lower phosphate ions, which could not be removed by conventional methods, as well as orthophosphate ions and polyphosphate ions, as well as all phosphorus compounds can be effectively removed at once.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

Example 1 PH1.89, orthophosphate ion as P190
mg/, lower phosphate ion as P 2530mg/
, other phosphorus compounds as P 190mg/,
Chlorine was blown into 500 ml of wastewater containing 2910 mg of total phosphorus while stirring until it turned pale yellow. Slaked lime was added and aged for about 30 minutes until the pH reached 12. The amount of slaked lime added at this time was 10.4 g, and dissolved chlorine was 160 mg. Phosphorus removal rate 99.2 with total phosphorus 23mg/after filtration
%, and dissolved chlorine was 0 mg/.

Example 2 PH1.43, orthophosphate ion as P170
mg/, lower phosphate ion as P 4020mg/
, other phosphorus compounds as P 150mg/,
Chlorine was injected into 500ml of wastewater containing 4340mg of total phosphorus at room temperature while stirring, and the point at which the color of the wastewater became pale yellow was considered the end of chlorine infusion.

Slaked lime was added until the pH reached 12, and the mixture was aged for about 30 minutes.

The amount of slaked lime added at this time was 15.3g, dissolved chlorine 210
mg/, total phosphorus after filtration is 12 mg/, phosphorus removal rate
99.7%, dissolved chlorine 6 mg/.

Example 3 PH1.52, orthophosphate ion as P210
mg/, lower phosphate ion as P 2610mg/
, other phosphorus compounds as P 180mg/,
Chlorine was injected into 500 ml of wastewater containing 3000 mg of total phosphorus with stirring, and the point at which the color of the waste water turned yellow was considered the end of chlorine infusion.

Slaked lime was added until the pH reached 12, and the mixture was aged for about 30 minutes. The amount of slaked lime added at this time was 10.7g, dissolved chlorine.
It was 1680mg/.

Total phosphorus after filtration is 1.6mg/, phosphorus removal rate 99.9
% dissolved chlorine was 610 mg/.

Example 4 PH1.85, orthophosphate ion as P130
mg/, lower phosphate ion as P 1820mg/
, other phosphorus compounds as P 160mg/,
Adding 9.9 g of highly bleached powder and 5.3 g of hydrochloric acid to wastewater 1 containing 2110 mg of total phosphorus with stirring to adjust the pH to 2.5. After stirring for about 2 hours, adding 7.5 g of slaked lime to the pH.
It became 11.9. It was aged for about 1 hour and filtered.

The total phosphorus after filtration was 28 mg/, and the phosphorus removal rate was 98.7%.

Example 5 PH2.01, orthophosphate ion as P180
mg/, lower phosphate ion as P 1620mg/
, 8.7 g of highly bleached powder and 5.3 g of hydrochloric acid were added to wastewater 1 containing 15 mg of other phosphorus compounds as P and 1720 mg of total phosphorus with stirring to bring the pH to 2.0.
After stirring for an hour, 10.5g of slaked lime was added, resulting in a pH of 12.3. It was aged for about 1 hour and filtered.

The total phosphorus after filtration was 2 mg/, and the phosphorus removal rate was 99.9%.

Example 6 PH1.45, orthophosphate ion as P230
mg/, lower phosphate ion as P 4150mg/
, other phosphorus compounds as P 420mg/,
When approximately 114 g of a 10% sodium hypochlorite solution is added to wastewater 1 containing 4800 mg of total phosphorus while stirring, the PH
It became 2.1.

After stirring for about 4 hours, 34.4g of slaked lime was added and the pH was 12.4.
It became. It was aged for about 1 hour and filtered.

The total phosphorus after filtration was 54 mg/, but after correcting the increase in liquid volume due to the addition of the sodium hypochlorite solution, the total phosphorus was 60 mg/, giving a phosphorus removal rate of 98.8%.

Comparative example PH1.90, 65mg/orthophosphate ion as P
About 3.7 g of highly bleached powder and 0.5 g of hydrochloric acid were added while stirring to wastewater 1 containing 645 mg of lower phosphate ions as P and 710 mg of total phosphorus to adjust the pH to 7.2. After stirring for about 2 hours, 0.6 g of slaked lime was added. When added, PH
It became 12.2. It was aged for about 1 hour and filtered. The total phosphorus after filtration was 366 mg/, and the phosphorus removal rate was 48.5%.

[Brief explanation of the drawing]

Figures 1 and 2 show the amount of chlorine injected and the oxidation rate of lower phosphorus in wastewater containing lower phosphoric acid, respectively.
It is a graph showing the relationship between the oxidation rate of lower phosphorus in each tank when three chlorine oxidation tanks are connected.

Claims (1)

[Claims] 1. Adding an oxidizing agent to wastewater containing lower phosphate ions to oxidize the lower phosphate ions to orthophosphate ions under conditions of pH 3 or less, and then adding a sparingly soluble phosphate forming agent. A method for treating wastewater containing lower phosphate ions, the method comprising separating and removing a precipitate generated by the process. 2. The method for treating lower phosphoric acid-containing wastewater according to claim 1, wherein the oxidizing agent is a chlorine-based oxidizing agent.