JP7660244B2 - Waste liquid treatment method - Google Patents

Description

The present invention relates to a method for treating waste liquid.

Phosphorus is a type of nutrient, and when the phosphorus concentration in water is high, it causes the mass proliferation of algae, which is a type of phytoplankton. When the algae proliferates in large quantities, the algae consume the oxygen in the water, causing the water to become anaerobic, resulting in a deterioration in water quality. In particular, phosphorus is likely to cause a deterioration in water quality in closed water areas such as lakes and bays.
The Ministry of the Environment has set the wastewater standard for phosphorus content in wastewater at 8 mg/liter per day on average.
As a method for reducing the phosphorus concentration in wastewater, Patent Document 1 describes a method for reducing the fluorine and phosphorus concentrations in wastewater by decomposing a fluorophosphate compound in the wastewater, the method comprising the steps of: adjusting the temperature of the wastewater to 65 to 85° C. while adding sulfuric acid to the wastewater so that the sulfuric acid concentration is 10 to 20% by weight; maintaining the wastewater within the temperature range; and adding a calcium compound to the wastewater.

JP 2010-94573 A

In closed water areas, etc., stricter standards may be set for the phosphorus concentration (mg/liter) in waste liquid (wastewater) in order to prevent deterioration of water quality.
In addition, some wastewater contains sulfate ions in addition to phosphorus (especially phosphate ions). In this case, it is necessary to carry out a treatment to remove phosphorus from the wastewater, assuming that sulfate ions are present.
An object of the present invention is to provide a method for treating waste liquid, which is capable of reducing the amount of phosphate ions contained in waste liquid containing phosphate ions and sulfate ions.

As a result of intensive research into solving the above-mentioned problems, the present inventors have found that the above-mentioned object can be achieved by a method for treating waste liquid containing phosphate ions and sulfate ions, which comprises adding to the waste liquid an amount of a calcium compound such that the molar ratio (Ca/P) of calcium (Ca) to phosphorus (P) in the waste liquid becomes a specific value or more, and adding a pH adjuster to the waste liquid to adjust the pH of the waste liquid to 10.5 or more, and have completed the present invention.
That is, the present invention provides the following [1] to [4].

[1] A method for treating a waste liquid containing phosphate ions and sulfate ions, comprising: a phosphorus-containing solid production step of producing a phosphorus-containing solid in the waste liquid by: (A) a method of carrying out the following step (a) followed by the following step (b); (B) a method of carrying out the following step (b) followed by the following step (a); or (C) a method of carrying out the following steps (a) and (b) simultaneously:
(a) a step of adding a calcium compound to the waste liquid, the step being to adjust the amount of the calcium compound so that the molar ratio (Ca/P) of calcium (Ca) to phosphorus (P) in the waste liquid is 2.1 or more; (b) a pH adjustment step of adding a pH adjuster to the waste liquid to adjust the pH of the waste liquid to 10.5 or more. [2] The method for treating waste liquid described in [1] above, wherein the calcium compound is one or more selected from the group consisting of calcium chloride, calcium sulfate, calcium hydroxide, calcium oxide, and calcium carbonate.
[3] The method for treating a waste liquid according to the above [1] or [2], wherein the pH adjuster is an alkali metal hydroxide, an alkaline earth metal hydroxide, or an inorganic acid.
[4] The method for treating a waste liquid according to any one of [1] to [3] above, further comprising a solid-liquid separation step of subjecting the waste liquid to solid-liquid separation after the phosphorus-containing solid production step to obtain a treated waste liquid which is a liquid fraction having a reduced concentration of phosphate ions.

The wastewater treatment method of the present invention can reduce the amount of phosphorus contained in the wastewater.

The method for treating waste liquid of the present invention is a method for treating waste liquid containing phosphate ions and sulfate ions, and includes a phosphorus-containing solid production step of producing a phosphorus-containing solid in the waste liquid by: (A) a method of carrying out the following step (a) followed by the following step (b); (B) a method of carrying out the following step (a) followed by the following step (b); or (C) a method of carrying out the following steps (a) and (b) simultaneously.
(a) a step of adding a calcium compound to the waste liquid, the amount of the calcium compound being adjusted so that the molar ratio (Ca/P) of calcium (Ca) to phosphorus (P) in the waste liquid is 2.1 or more; (b) a pH adjustment step of adding a pH adjuster to the waste liquid to adjust the pH of the waste liquid to 10.5 or more.

In the present invention, the waste liquid to be treated is not particularly limited as long as it contains phosphate ions and sulfate ions, and examples of the waste liquid (wastewater) include filtrate obtained by dehydrating sewage in a sewage treatment plant (water generated when dehydrating excess sludge in the process of treating sewage in a sewage treatment plant), water discharged from factories such as food manufacturing plants, water discharged from agriculture or the livestock industry, water discharged in daily life (domestic sewage), etc. Furthermore, the target may be a diluted liquid obtained by diluting the above-mentioned waste liquid with water.
Considering the object of the present invention to reduce the amount of phosphorus contained in the waste liquid, the phosphorus concentration in the waste liquid is preferably 10 mg/L or more, more preferably 100 mg/L or more, even more preferably 500 mg/L or more, and particularly preferably 800 mg/L or more.
The upper limit of the phosphorus concentration is not particularly limited, but taking into consideration the phosphorus concentration in the wastewater that is actually to be treated, it is usually 5,000 mg/L, preferably 4,000 mg/L, and more preferably 3,000 mg/L.
Incidentally, phosphorus in the waste liquid is usually present in the form of phosphate ions.

The concentration of sulfate ions in the waste liquid is preferably 30,000 mg/liter or more, more preferably 32,000 mg/liter or more, further preferably 35,000 mg/liter or more, and particularly preferably 40,000 mg/liter or more.
The upper limit of the sulfate ion concentration is not particularly limited, but taking into consideration the concentration of sulfate ions in the wastewater that is actually to be treated, it is preferably 90,000 mg/L, more preferably 80,000 mg/L, and particularly preferably 75,000 mg/L.
When the sulfate ion concentration is within the above-mentioned preferred range, the effect of the present invention can be further enhanced. Specifically, a larger amount of gypsum can be generated in the waste liquid after the addition of the calcium compound, and a larger amount of phosphorus-containing solids can be adsorbed onto the gypsum before grain growth. The gypsum adsorbing the phosphorus-containing solids can be recovered as a solid by solid-liquid separation. As a result, the phosphate ion concentration in the waste liquid (liquid portion after solid-liquid separation) can be reduced more significantly.

In the present invention, for the purpose of adjusting the concentration of sulfate ions in the waste liquid, a sulfate salt such as sodium sulfate may be appropriately added to the waste liquid.
In addition to phosphate ions and sulfate ions, sodium ions may also be present in the waste liquid. In this case, the concentration of sodium ions in the waste liquid is, for example, 200 to 30,000 mg/L.
An example of the waste liquid to be treated in the present invention is a waste liquid containing phosphate ions at a concentration of 50 to 6,000 mg/L, sulfate ions at a concentration of 30,000 to 90,000 mg/L, and sodium ions at a concentration of 200 to 30,000 mg/L. An example of such a waste liquid is a metal phosphate (e.g., a waste liquid generated in the synthesis of aluminum phosphate).

[Phosphorus-containing solid production process]
The phosphorus-containing solid production step is a step of producing a phosphorus-containing solid in the waste liquid by the following method (A), (B) or (C).
(A) A method of carrying out the following step (b) after the following step (a): (B) A method of carrying out the following step (a) after the following step (b): (C) A method of carrying out the following steps (a) and (b) simultaneously: (a) A calcium compound addition step of adding a calcium compound to the waste liquid, in which the amount of the calcium compound is adjusted so that the molar ratio (Ca/P) of calcium (Ca) to phosphorus (P) in the waste liquid is 2.1 or more; (b) A pH adjustment step of adding a pH adjuster to the waste liquid to adjust the pH of the waste liquid to 10.5 or more; Among these, method (A) is preferred from the viewpoint of further reducing the amount of phosphorus contained in the waste liquid (liquid portion) after treatment according to the present invention.

In the phosphorus-containing solid production process, the phosphate ions in the waste liquid react with calcium to produce phosphorus-containing solids such as hydroxyapatite, thereby reducing the amount of phosphorus contained in the waste liquid.
Furthermore, gypsum (CaSO ₄ ) is produced by the reaction of sulfate ions in the wastewater with calcium. The gypsum adsorbs fine granular phosphorus-containing solids (fine granular materials that are not recovered as solids even after solid-liquid separation and remain in the liquid), and therefore the amount of phosphorus in the wastewater (also referred to as the "liquid after treatment" in this specification) after the solid-liquid separation process, which is a subsequent process of the phosphorus-containing solid production process, can be further reduced. Furthermore, the amount of fine granular phosphorus-containing solids floating in the liquid is reduced, and therefore the processing efficiency of the solid-liquid separation can be improved.
Each of steps (a) and (b) will be described in detail below.

[Step (a)]
Step (a) is a step of adding a calcium compound to the waste liquid, and adjusting the amount of the calcium compound so that the molar ratio of calcium (Ca) to phosphorus (P) (Ca/P) in the waste liquid is 2.1 or more.
The calcium compound can be added in the form of a powder, a solution, or a slurry. Among these, from the viewpoints of reactivity and mixability, the solution or slurry form is preferred.
Examples of calcium compounds include calcium chloride, calcium sulfate, calcium hydroxide, calcium oxide, calcium carbonate, etc. These may be used alone or in combination of two or more.
Among these, calcium chloride is preferred because it has excellent solubility in water and can be added to the waste liquid in the form of an aqueous solution.

The molar ratio (Ca/P) is 2.1 or more, preferably 2.2 or more, and more preferably 2.3 or more. If the molar ratio is less than 2.1, the amount of gypsum produced is small, and the effect of reducing the amount of phosphorus in the liquid after treatment is reduced.
The upper limit of the molar ratio (Ca/P) is not particularly limited, but from the viewpoints that an excessive amount of calcium (Ca) causes the effect of the present invention (removal of phosphorus) to plateau, and that the amount of calcium compound used can be reduced to reduce the cost of the treatment method of the present invention, the upper limit is preferably 3.5, more preferably 3.0, and particularly preferably 2.5.

In the present invention, the molar ratio of calcium (Ca) to phosphorus (P) (Ca/P) means the ratio of the molar amount of calcium (Ca) contained in the water-soluble calcium component present as calcium ions in the waste liquid to the molar amount of phosphorus (P) contained in the water-soluble phosphorus component present as phosphate ions in the waste liquid.
Therefore, phosphorus contained in the poorly soluble phosphorus component and calcium contained in the poorly soluble calcium component are not included in the phosphorus and calcium in the above molar ratio.
In the present invention, when calcium ions are present in the waste liquid before the addition of the calcium compound, the calcium (Ca) of the calcium ions is included in the calcium in the above molar ratio (Ca/P). In this case, the amount of the calcium compound is determined taking into account the amount of the calcium ions.

[Step (b)]
Step (b) is a step of adding a pH adjuster to the waste liquid to adjust the pH of the waste liquid to 10.5 or higher.
Examples of the pH adjuster include hydroxides of alkali metals, hydroxides of alkaline earth metals, and inorganic acids.
Examples of alkali metal hydroxides include sodium hydroxide, potassium hydroxide, and lithium hydroxide.
Examples of the hydroxides of alkaline earth metals include calcium hydroxide and magnesium hydroxide.
Examples of the inorganic acid include hydrochloric acid and sulfuric acid.
Among these, sodium hydroxide, sulfuric acid, and hydrochloric acid are preferred from the viewpoint of availability, etc. The pH adjusters may be used alone or in combination of two or more.

The pH of the waste liquid is adjusted to 10.5 or higher, preferably 10.7 or higher, more preferably 10.9 or higher, even more preferably 11.2 to 13.5, even more preferably 11.5 to 13.0, and particularly preferably 11.5 to 12.5. If the pH is less than 10.5, the amount of phosphorus-containing solids produced will be small, and the effect of reducing the amount of phosphorus in the liquid after treatment will be small. If the pH is 13.5 or lower, it is preferable from the viewpoint of reducing the amount of pH adjuster used, etc.

In the case where step (a) (calcium addition) is carried out after step (b) (pH adjustment), if the pH of the waste liquid changes to less than 10.5 (or to a value less than the above-mentioned preferable lower limit value, or exceeding the above-mentioned preferable upper limit value) during the addition of a calcium compound in step (a) after adjusting the pH to 10.5 or more in step (b), it is preferable to add an appropriate pH adjuster in step (a) to adjust the pH of the waste liquid to 10.5 or more (or to a value equal to or greater than the above-mentioned preferable lower limit value, or equal to or less than the above-mentioned preferable upper limit value).
In addition, when the steps (a) and (b) are carried out simultaneously, it is preferable to add a pH adjuster so that the pH of the waste liquid after the addition of the calcium compound is completed is 10.5 or more (or within the above-mentioned preferred numerical range).
The temperature of the waste liquid during the steps (a) and (b) is not particularly limited, but is usually room temperature (liquid temperature without cooling or heating; for example, 10 to 25° C.).

[Solid-liquid separation process]
This step is an optional step that is provided after the phosphorus-containing solid production step (steps (a) and (b)), and is a step in which the waste liquid is subjected to solid-liquid separation to obtain a treated waste liquid (liquid fraction after treatment), which is a liquid fraction with a reduced concentration of phosphate ions.
From the viewpoint of improving the efficiency of solid-liquid separation, a filter aid may be added to the waste liquid in the solid-liquid separation step.
Examples of filter aids include diatomaceous earth, perlite, cellulose, and polymeric flocculants.
Examples of methods for solid-liquid separation include suction filtration, filter press, and centrifugal dehydration.
By subjecting the waste liquid to solid-liquid separation, the treated waste liquid (liquid fraction after treatment), which is a liquid fraction with a reduced concentration of phosphate ions, can be separated from phosphorus-containing solid matter (solid fraction).

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.
[Materials used]
(1) Waste liquid Waste liquid prepared by adding phosphoric acid, sulfuric acid, and sodium hydroxide to wastewater from an aluminum phosphate manufacturing plant (phosphoric acid phosphorus concentration: 1,000 mg/L, sulfate ion concentration: 45,000 mg/L, sodium ion concentration: 25,000 mg/L).
(2) Calcium chloride aqueous solution: Contains calcium chloride at a concentration of 35% by mass. (3) Sodium hydroxide solution: Contains sodium hydroxide at a concentration of 20% by mass.

[Example 1]
To 1 liter of waste liquid (liquid temperature: about 20° C.), an aqueous calcium chloride solution was added while stirring the waste liquid so that the molar ratio (Ca/P) of calcium (Ca) to phosphorus (P) in the waste liquid became 2.1, thereby obtaining a waste liquid with an adjusted calcium content.
An aqueous sodium hydroxide solution was added to the resulting waste liquid and stirred to obtain a pH-adjusted waste liquid having a pH of 13.0.
Once the pH was adjusted to 13.0, the pH-adjusted waste liquid was stirred for 30 minutes.
Next, the resulting waste liquid was filtered to obtain a filtrate (treated liquid).
The phosphorus concentration in the filtrate (shown as "phosphorus concentration after treatment" in Table 1) was measured using an ICP emission spectrometer.

[Example 2, Comparative Examples 1 and 2]
A filtrate was obtained in the same manner as in Example 1, except that an aqueous calcium chloride solution and an aqueous sodium hydroxide solution were added so that the molar ratio (Ca/P) and pH became the values shown in Table 1.
The phosphorus concentration of the obtained filtrate was measured in the same manner as in Example 1.

[Example 3]
An aqueous sodium hydroxide solution was added to 1 liter of the waste liquid (liquid temperature: about 20° C.) and stirred to obtain a pH-adjusted waste liquid having a pH of 11.0.
Next, an aqueous calcium chloride solution was added to the pH-adjusted waste liquid while stirring the waste liquid so that the molar ratio (Ca/P) of calcium (Ca) to phosphorus (P) in the waste liquid became 2.3, thereby obtaining a waste liquid with an adjusted calcium content.
After the molar ratio (Ca/P) was adjusted to 2.3, the waste liquid was stirred for 30 minutes. The pH of the waste liquid after stirring was 11.0.
Next, the resulting waste liquid was filtered to obtain a filtrate (treated liquid).
The phosphorus concentration in the filtrate was measured in the same manner as in Example 1.

The results are shown in Table 1.
In Table 1, "after Ca addition" means that after the calcium chloride aqueous solution (Ca-containing aqueous solution) was added, a sodium hydroxide aqueous solution was added to adjust the pH. "before Ca addition" means that before the calcium chloride aqueous solution (Ca-containing aqueous solution) was added, a sodium hydroxide aqueous solution was added to adjust the pH. "Phosphorus concentration after treatment" means the concentration of phosphorus contained in the filtrate (liquid after treatment).

From Table 1, it can be seen that the phosphorus concentration (0.2-0.5 mg/liter) of the liquid after treatment in Examples 1-3 (Ca/P: 2.1-2.3, pH: 11.0-13.0) is lower than the phosphorus concentration (3.1-7.5 mg/liter) of the liquid after treatment in Comparative Example 1 (Ca/P: 1.8, pH: 12.0) and Comparative Example 2 (Ca/P: 2.5, pH: 10.0).

Claims (2)

A method for treating wastewater containing phosphate ions and sulfate ions, comprising the steps of:
(A) a method comprising carrying out the following step (a) and then the following step (b):
(B) A method of carrying out the following step (a) after the following step (b), or
(C) A method of simultaneously carrying out the following step (a) and the following step (b):
and generating a phosphorus-containing solid in the waste liquid by the above-mentioned process.
(a) a step of adding calcium chloride, which is a calcium compound, to the waste liquid, and adjusting the amount of the calcium compound so that the molar ratio (Ca/P) of calcium (Ca) to phosphorus (P) in the waste liquid is 2.1 or more; (b) a pH adjustment step of adding a hydroxide of an alkali metal, which is a pH adjuster, to the waste liquid to adjust the pH of the waste liquid to exceed 12.5. The method for treating wastewater according to claim 1 further comprises a solid-liquid separation step of separating the wastewater into solid and liquid after the phosphorus-containing solid production step to obtain treated wastewater, which is a liquid fraction having a reduced concentration of phosphate ions.