JPH0117753B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for removing phosphoric acid from water containing phosphoric acid.

Phosphoric acid in water is said to be a eutrophicating component along with ammonia, and is considered to be the source of eutrophication in rivers, lakes, etc., and an effective method for its removal is desired.
Advanced treatment technologies for sewage and wastewater are also aimed at removing phosphoric acid.

In recent years, there has been a growing demand for the development of advanced wastewater treatment technology, and its purpose is firstly to prevent environmental pollution, secondly to save valuable resources and water resources, and to reuse reclaimed water. However, the primary problem when reusing wastewater or discharging it outside the system is the phosphorus content in the wastewater. If phosphorus in wastewater is directly discharged into lakes, rivers, etc., algae will proliferate, making the lake water, rivers, etc. anoxic, leading to the death of useful organisms.
It is often used as a lake or river of death. for example,
Examples of this include Lake Kasumigaura and Lake Biwa in Japan.
Countermeasures are being considered from various angles. Furthermore, when river water contaminated with phosphorus components is used for agricultural purposes, the crops only grow taller, but do not bear fruit, and in many cases, yields are reduced.

Conventionally, in order to remove phosphorus from wastewater, metal ions such as iron and aluminum were added to the wastewater.
One example is the method of removing phosphorus, which often involves adding multiple metal salts to the wastewater.
In many cases, it coagulates and precipitates at approximately 100 to 300 mg/metal salt. In addition, recently, human waste has been treated without dilution, and if dephosphorization is not performed in biological treatment, ₇₀₀ to 800
mg/mg of phosphoric acid compound, so if you try to treat it by adding iron salts as mentioned above, it will be more than 1,000
It is necessary to add about ppm. Moreover, the resulting precipitate is extremely bulky and difficult to concentrate, making solid-liquid separation difficult. Moreover, if you try to dephosphorize by adding these metal salts, the PH will drop extremely due to the by-produced acid, so in order to neutralize it, an alkaline agent is added from outside the system to neutralize the PH. You need to adjust it to suit your sexuality. In other words, the current situation is that extremely uneconomical processing is being carried out. Another method is to add slaked lime, quicklime, dolomite, etc. to precipitate and separate phosphorus into a compound with calcium, such as calcium apatite, and then blow carbon dioxide gas to precipitate excess lime as carbonate to lower the pH. However, when removing dilute phosphoric acid as precipitate, a large excess of lime is required, resulting in a pH of around 11.5. Therefore, an acid agent is required for neutralization, which is extremely uneconomical.
The present invention addresses the drawbacks of both of these conventional methods, namely, the reduction in pH due to the addition of a considerable amount of metal salts in the dephosphorization method by adding metal salts, and the reduction in PH due to the addition of a considerable amount of metal salts in the dephosphorization method by adding lime and dolomite. After repeated research to eliminate the increase in PH caused by the addition of lime and dolomite, we found that by using these two dephosphorizing agents in any ratio, it was possible to eliminate the conventional drawbacks, and also to solve the conventional problems. It was found that the problems of precipitation concentration and dehydration could be significantly improved. In other words, phosphoric acid compounds dissolved in water exist in the form of phosphates other than phosphoric acid, and when removing phosphoric acid and phosphates from water, at least one of iron salts and aluminum salts, calcium and magnesium oxides, and water are used to remove phosphoric acid and phosphates. It is characterized by adding at least one of an oxide and a carbonate at the same time or separately.

Estimating the in-system reactions in the present invention, it appears that the following reactions proceed to effect dephosphorization. That is, 2H ₃ PO ₄ +2Fecl ₃ →2FePO ₄ ↓+6Hcl 6Hcl+3ca(OH) ₂ →3CaCl ₂ +6H ₂ O 2Na ₃ PO ₄ +3CaCl ₂ →Ca ₃ (PO ₄ ) ₂ ↓+6Nacl Therefore, the total reaction 2H ₃ PO ₄ +2FeCl ₃ +2Na ₃ PO ₄ +3Ca(OH) ₂ → 2FePO ₄ +Ca ₃ (PO ₄ ) ₂ +6NaCl+6H ₂ O Of course, depending on the pH, phosphoric acid may also exist as an acidic phosphate ion in water, but basically In general, it is thought that the reactions listed here will proceed.

In other words, in the conventional method, dephosphorization was performed using the reaction or the reaction shown below, that is, 2H ₃ PO ₄ +3Ca(OH) ₂ →Ca ₃ (PO ₄ ) ₂ ↓+6H ₂ O. This by-product causes a significant drop in pH, and in order to prevent this drop, caustic soda and soda carbonate have been added. In addition, in the reaction, the pH increases due to the need to add excessive Ca(OH) ₂ , and in order to neutralize it,
They were forced to add acid agents. In the present invention, the disadvantages of the conventional methods are solved at once by utilizing the reaction of , , which is the total reaction, and oxides of calcium and magnesium such as slaked lime are used to remove the acid by-produced by the reaction of . This is a reaction that uses at least one of hydroxides and carbonates, and further uses metal salts as by-products to remove phosphorus bonded to phosphates. In this way, in the present invention, dephosphorization can proceed in the vicinity of neutrality without the presence of free acid or excess alkali. Moreover, the produced precipitate has advantages such as easy separation, good dehydration properties, and extremely easy handling.

The metal salts used in the present invention are limited to iron salts and aluminum salts. Other substances cannot be neutralized by calcium or magnesium oxides, hydroxides, or carbonates added at the same time or separately.

There is no particular restriction on the order in which these two are added, and they may be added at the same time or at different times. The reaction time depends on the concentration of phosphate ions, but at least 5 minutes is required.

Next, examples of the present invention will be described.

750 mg of ferric chloride was added to the biologically treated organic wastewater containing 760 mg of phosphate ion, and after stirring for 2 minutes, 510 mg of slaked lime was added, and after further stirring for 5 minutes, it was subjected to sedimentation and separation. Liquid separation was performed. The separated water was collected, filtered, and the concentration of dissolved phosphate ions was measured.
It was 5 mg/PO ₄ and pH 6.0. As a control example, the same level of dephosphorization was carried out by adding 1400 mg/ferric chloride, but the phosphate ions in the filtered water of the separated water were 4 to 5 mg/, as _PO4 .
The PH was 2.5 to 2.7, and it became necessary to add 1010 mg of caustic soda to adjust the PH of the treated water to 6.8 to 7.0.

Although a permeable membrane device was used as a means for solid-liquid separation, similar results were obtained. As the permeation membrane, an ultrafiltration membrane with a molecular weight cutoff of 6000 was used. The reaction time after addition of the dephosphorizing agent was the same as in the previous example.

In this way, the present invention can correct the drawbacks of using metal salts such as iron salts alone or calcium compounds alone, and can be used for the purpose of pH adjustment without necessarily requiring injection of chemicals. Even if it does, it can be done in an extremely small amount compared to the conventional method, which is economically advantageous.

Claims (1)

[Claims] 1. At least one of iron salts and aluminum salts that produce an insoluble phosphoric acid compound in a phosphoric acid-containing liquid;
Calcium, magnesium oxides, hydroxides,
A method for removing phosphoric acid dissolved in water, which comprises adding at least one type of carbonate at the same time or separately, and dephosphorizing the water at a state near neutrality.