JPS6231632B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for removing phosphates present in tap water, sewage, human waste water, industrial water, factory wastewater, boiler water, and all other liquids. Generally, in the various liquids mentioned above that are discharged into natural water systems, inorganic phosphates such as orthophosphates, various condensed phosphates, and organic phosphates exist in various states. The presence of phosphates is a factor that induces the occurrence of "blue water" and "red tide" in closed or stagnant waters such as lakes, inland seas, and inner bays. Furthermore, when water is used for various purposes, living organisms may be present in equipment and piping. Chemical slime is generated and chemical scale is formed, which is a major cause of accidents. Therefore, various phosphorus removal methods are being considered in order to remove the phosphates present in these liquids, and the representative ones include biological treatment, ion exchange resin method, and chemical treatment. Among these methods, the chemical coagulation-sedimentation method is regarded as a somewhat completed treatment technology, and treatment equipment on an actual treatment scale is already in operation in many places. The removal of phosphates by this chemical coagulation-precipitation method is a method in which phosphates present in the liquid are removed as insoluble phosphates by adding a specific flocculant. Usually, slaked lime [Ca(OH) ₂ ], aluminum sulfate [Al ₂ (SO ₄ ) ₃ ], ferric chloride [FeCl ₃ ], etc. are used. However, the biggest drawback of chemical coagulation and precipitation is that it generally requires a large amount of flocculant, which is expensive, regardless of the type of flocculant used. A large amount of sludge is generated in proportion to the large amount of chemical injection, and the settling and thickening properties of this sludge are extremely poor (with the exception of lime sludge). Furthermore, this sludge has poor dewatering properties (lime sludge is an exception). However, because it requires a large sludge treatment facility and processing costs, it has many practical problems despite being a current technology. In addition, a cylindrical or cone-shaped dephosphorization tower is filled with phosphate minerals containing calcium phosphate having a certain particle size, and the pH of the liquid to be treated is adjusted to a range of 6 to 11. By adding a calcium agent such as calcium chloride according to the concentration of soluble phosphates contained in the mineral and bringing it into contact with the phosphate mineral at a constant flow rate, calcium is added to the surface of the filled phosphate mineral. A method of removing soluble phosphates by crystallizing and fixing hydroxyapatite crystals has also been proposed. in this case,
Typical chemical reactions on the surface of phosphate minerals are as follows. 5Ca ²⁺ +OH ^- +3PO ₄ ^3- = Ca ₅ (OH) (PO ₄ ) ₃ ...(1) If this dephosphorization method is applied, the separation and dehydration of phosphate minerals to which calcium hydroxyapatite is fixed will be possible. It is extremely easy to use, and compared to so-called coagulated sludge made using conventional chemical flocculation and sedimentation methods, it not only does not require preconceived sludge treatment facilities such as thickeners, dewatering machines, and drying equipment, but it is also useful as a resource. This is an excellent dephosphorization technology that can recover phosphorus. However, in this new catalytic dephosphorization method,
In order to fix the soluble phosphates contained in the liquid as calcium hydroxyapatite [Ca ₅ (OH) (PO ₄ ) ₃ ] crystals, it is necessary to react with calcium ions under appropriate pH conditions. If there are not enough calcium ions in the liquid, calcium chloride [CaCl ₂ ], slaked lime [Ca(OH) ₂ ], etc. are added as calcium agents, but if the liquid to be treated contains carbonic substances, Calcium carbonate is produced according to the reaction equation shown below. Ca ²⁺ + HCO ₃ ^- + OH ^- = CaCO ₃ + H ₂ O...(2) This calcium carbonate is produced only if the pH of the liquid is within the pH range suitable for the production of calcium hydroxyapatite or if it has been adjusted. , apatite is not produced because the production rate is higher than that of calcium carbonate, but it is difficult to completely control the pH in actual processing equipment, and when carbonic substances are concentrated, the moment a calcium agent is added It is inevitable that the reaction of formula (2) above proceeds to some extent. Although the amount of calcium carbonate crystals produced under such conditions is extremely small, over a long period of time, the calcium carbonate crystals gradually grow fixedly on the surface of the phosphate mineral. As a result, the surface activity of the phosphate mineral deteriorates, the dephosphorization function decreases, and the dephosphorization effect can no longer be expected to be as high as it was initially. In this way, the catalytic dephosphorization method has a limited applicability to liquids that contain a large amount of carbonic acid components and also soluble phosphates.
If carbonic substances are removed by some method, the scope of application of the catalytic dephosphorization method will be expanded. Conventional methods for removing carbonate substances from liquids include biological methods (biological nitrification method or biological nitrification denitrification method, which is applied only when the liquid contains nitrogen components). There are chemical and physicochemical methods, and biological methods are capable of effective decarboxylation, but require large treatment facilities and are not always easy to manage. On the other hand, chemical or physicochemical methods require agents to remove carbonated substances, and some methods generate large amounts of sludge, but carbonation can be easily achieved using simple treatment facilities and simple methods. Because it can remove substances, it is commonly used as a water treatment technology. The biggest drawback of biological methods is that they cannot be applied unless the wastewater to be treated contains nitrogen compounds (preferably NH ₄ -N), whereas chemical or physicochemical methods This method can be applied to any kind of wastewater, and has the advantage of ensuring a reliable carbonate removal effect. The main chemical or physicochemical method is a stripping method under liquid acidity.
In this method, acid is added to the liquid to be treated, and the pH of the liquid is adjusted to 3 depending on the type and concentration of carbonic substances to be removed.
-6, and then introduces the acidified liquid into a stripping device to diffuse carbonic substances into the atmosphere. It is an object of the present invention to provide a method for efficiently and stably removing phosphates from a liquid containing phosphates and carbonic substances over a long period of time by making full use of the above-mentioned advantages. . Another object of the present invention is to eliminate various drawbacks of conventional methods, to economically obtain treated water of extremely high quality with extremely simple operations, and to significantly reduce treatment equipment and treatment costs. The goal is to find an effective way to achieve this goal. That is, in the present invention, after adding a required amount of mineral acid to the liquid to be treated and bringing it into contact with air to remove carbonated substances, an alkaline agent is added to maintain the liquid to be neutral or weakly basic while removing calcium ions. In a method for removing soluble phosphates in a liquid to be treated by contacting it with a phosphate mineral containing calcium phosphate in the presence of mineral acids, the amount of the mineral acid added is determined based on the M-alkalinity of the liquid to be treated. This is a method for treating a phosphorus compound-containing waste liquid, characterized in that the amount is set to an amount that is equal to or higher than the chemical equivalent and can deliver hydrogen ions at a ratio of 4 gram ions to 1 gram phosphate ions in the liquid to be treated. One embodiment of the present invention will be described with reference to FIG. 1. For example, secondary treated sewage water (PO ₄ ^3-20mg /
, M-alkalinity 50mg/) as the target raw water,
After coarse solids and other impurities are removed through appropriate pretreatment, the activated sludge method is applied. The treated water (secondary treated water) by this activated sludge method is passed from the secondary treated water inflow pipe 1 to the decarbonator 2.
to be introduced. This decarboxylation device 2 is based on the acid stripping method, that is, it is a device that adds mineral acid to the liquid to be treated to lower the pH of the liquid, and then brings it into contact with air to expel carbonic substances in the liquid into the gas phase. ,
Either a bubble column in which air is blown into the acidified liquid or a packed column in which the liquid is spread over a packed material and brought into contact with air may be used, or both may be used in combination. FIG. 1 shows a bubble column type decarboxylation device.
The decarboxylation device 2 includes an air diffuser 6 that disperses air supplied from an air compressor 5 into water, and a pH
An indicating controller 3 is provided, and a mineral acid supply device 4 linked thereto is also attached. The mineral acid supplied in this decarboxylation process not only lowers the pH to a value that allows effective decarboxylation, but also has enough mineral acid to sufficiently dissolve the slaked lime added in the next process as calcium ions in the liquid. amount needs to be added. Furthermore, the effect is the same whether the mineral acid is sulfuric acid or hydrochloric acid. The secondary treated water that has been decarboxylated in the decarboxylation device 2 is led to the lime mixing PH adjustment tank 7, where the PH is adjusted to the optimum PH conditions for the catalytic dephosphorization reaction by adding slaked lime.
It is adjusted to 6.0 or more, and is led to the catalytic dephosphorization tank 12 via the next rapid sand filter tank 11. The lime mixing PH adjustment tank 7 is equipped with a stirrer 10, a PH indicator controller 8, and a slaked lime supply device 9 linked thereto.The rapid sand filter tank 11 is equipped with filter sand (silica sand, anthracite, etc.) with a constant particle size site, etc.), and the SS contained in the secondary treatment water and the SS entrained when adding slaked lime are removed by filtration, preventing clogging of the catalytic dephosphorization tank 12, and further, This prevents the dephosphorization filter media from becoming contaminated and deteriorating. The filtered secondary treated water is led to the catalytic dephosphorization tank 12, but in this catalytic dephosphorization step, a packed tower filled with phosphate minerals containing calcium phosphate having a certain particle size is used as the dephosphorization tower. Various phosphates present in the liquid are removed by passing a liquid to be treated containing phosphates through this packed column at a constant flow rate. In this case, the pH of the liquid to be treated is adjusted to the optimum pH range of 6.0 or higher for the chemical reaction of phosphorus removal, and an appropriate amount of calcium agent is added in accordance with the phosphate concentration in the liquid to be treated, so that the calcium agent is It is effective to flow into the packed column in an ionic state. Also, depending on the type of liquid to be treated,
Metal hydroxides or insoluble carbonates contained in the liquid may precipitate on the surface of the phosphate mineral in the packed tower and reduce its surface activity. Adjust the pH of the liquid to 6.0~
It is best to adjust within the range of 11.0, and more preferably
A range of 8.8 to 9.2 is good. In addition, 13 in FIG. 1 is a treated water outflow pipe. Phosphate ions and calcium ions, or slaked lime and mineral acids cause the following reaction depending on the pH of the reaction solution. 5Ca ²⁺ +OH ^- +3PO ₄ ^3- = Ca ₅ (OH) (PO ₄ ) ₃
…(1) (previously) 3Ca ²⁺ +2PO ₄ ^3- =Ca ₃ (PO ₄ ) ₂ ………(3) Ca ²⁺ +H ⁺ +PO ₄ ^3- =CaHPO ₄ ………(4) Ca (OH) ₂ +2H ⁺ =Ca ²⁺ +2H ₂ O ………(5) In other words, in the neutral to alkaline region, (1), (3)
In the weakly acidic region, the reactions of equations (4) and (5) occur, respectively. Among these, the reaction of formula (1) has the lowest solubility of the reaction product and therefore the lowest concentration of residual phosphorus in the liquid. Therefore, the PH conditions (pH approximately 9) is preferable. By the way, from equation (1), in order for phosphate ion (PO ₄ ^3- ) to become apatite [Ca ₅ (OH) (PO ₄ ) ₃ ], the reaction coefficient and the chemical formula weight of phosphate ion and calcium ion are required. From this, it can be seen that about 7 times as much calcium ion as phosphate ion is required in terms of weight ratio. On the other hand, the following reaction formula can be derived from the above formulas (1) and (5). 5Ca(OH) ₂ +10H ⁺ +3PO ₄ ^3- +OH ^- = Ca ₅ (OH) (PO ₄ ) ₃ +10H ₂ O …………(6) From formula (6), when apatite is produced, it is supplied from mineral acid. The hydrogen ions that
It can be seen that 10/3 times the amount is required, but as a result of the inventors' investigation and consideration of various wastewaters, the amount of hydrogen ions must be at least 4 times the amount of phosphate ions in order to effectively remove phosphorus. This knowledge led to the completion of the present invention. In other words, the amount of mineral acid that can deliver at least 4 grams of hydrogen ions per 1 gram of phosphate ions in the liquid to be treated is calculated, and the pH of the liquid to be treated is calculated for each amount of added mineral acid determined in advance. If mineral acid is added using a mineral acid supply device that is linked to a PH indicator set to a certain value, extremely efficient and stable decarboxylation can be performed. In this case, it goes without saying that the PH setting value that indicates the amount of mineral acid added must be 5 or less, which is the PH value when adding an amount of mineral acid equivalent to M-alkalinity. If the purpose is only to decarboxylate, the pH should be approximately 5 so that all the carbonic substances in the water are in the form of free carbonic acid, but after decarboxylation, pH adjustment and calcium ion supply are performed. Therefore, PH must be greater than 5. In the embodiment, the phosphate ion concentration of the raw water is 20 mg/, the M-alkalinity is 50 mg/,
The amount of sulfuric acid needed to be added to adjust the pH to 5 is approximately 50 mg/ml, based on the PH titration curve shown in Figure 2. After this, for example, when the pH is adjusted to 9.0 with slaked lime, the amount of calcium ions added will be about 10mg/.
In this case, the weight ratio of calcium ions and phosphate ions is 0.5, and as mentioned above, this weight ratio is about 7.
Considering that it is necessary that the amount of calcium ions added is clearly insufficient. In conventional methods, neutral water-soluble calcium salts are generally used to supply this insufficient amount of calcium ions, but this method reduces the types of chemicals added, simplifies operations, and facilitates pH adjustment. For this purpose, it is effective to use slaked lime as a source of calcium ions and to also use this as the alkali agent. In this way, in the present invention, the amount of hydrogen ions added to the mineral acid in the decarboxylation step is set to be at least four times the amount of phosphate ions, and as a result, slaked lime is added in the next neutralization step. The required amount of calcium ions can be sufficiently supplied only by adjusting the pH using . In this case, if you try to add calcium ions only by adding slaked lime, the pH of the solution will rise too much, sometimes exceeding the optimum pH range. Adding mineral acid during the decarboxylation process serves the purpose of suppressing the increase in pH after adding slaked lime and securing calcium ions. In the present invention, the phosphate minerals for the catalytic dephosphorization process may be foreign products such as those produced in Jordan, Florida, or Mexico, or domestic products such as those produced in Kurokura, Kanagawa Prefecture, or Ashio Copper Mine, Tochigi Prefecture. Can be applied. Further, in the catalytic dephosphorization step, a part of the dephosphorized water can be returned to the decarboxylation step or the catalytic dephosphorization step for treatment. As described above, the present invention efficiently and economically removes carbonic substances contained in a liquid by a chemical or physicochemical method, and also stably and efficiently removes carbonic substances from a liquid for a long period of time. It is possible to remove phosphates from the liquid, and when it comes into contact with phosphate minerals to remove phosphates from the liquid, it is possible to maintain an appropriate pH level.
The conventional chemical coagulation-sedimentation method is achieved by maintaining the same conditions, preventing direct contact between the calcium agent and concentrated soluble phosphates, and allowing the calcium agent to flow into the catalytic dephosphorization device in its ionic state. It eliminates the drawbacks of the treatment method, efficiently removes phosphates from the liquid, is easy to operate, enables stable treatment, is suitable for large-scale treatment, requires less land and equipment installation space, and requires less processing equipment and treatment. It is possible to accurately measure cost savings. Next, an embodiment of the present invention will be described. The sewage from which coarse solids have been separated is treated using the conventional activated sludge method, and an appropriate amount of hydrochloric acid is added to this secondary treated water to lower the pH of the liquid to 1.5 or less, and then aeration is performed to remove the carbon dioxide in the liquid. The substance was released into the atmosphere in the form of free carbonate. Through this treatment (decarboxylation), carbonic substances contained in the secondary treated water were removed to a range of 30 mg/ to 10 mg/(as CaCO ₃ ) in one example of the experiment. This was then adjusted to pH 8.5 to 8.8 with slaked lime and placed in a filter tank filled with silica sand with an effective diameter of 0.6mm and a uniformity factor of 1.4.
After removing the SS content in the liquid by passing water at LV5 m/h, the phosphate rock was crushed and sieved and placed in a catalytic dephosphorization reaction tank filled with particles with a particle size of 0.42 to 0.54 mm to a thickness of 2 m.
Water flowed at LV5m/h. This experiment was continued for about 3 months, in order to check the deterioration of the activity of phosphate rock when water was passed for a long period of time.
Precise analysis was conducted on the final treated water one week and three months after the start of operation. The results are shown in Table 1, and according to the present invention, no reduction in the dephosphorization effect was observed either one week or three months after the start of water flow. On the other hand, as a comparative example, activated sludge treated water (secondary treated water) of the sewage was aerated to a pH of 4.5 to 5 during decarboxylation and continuously passed to a catalytic dephosphorization reaction tank under the same water flow conditions for three months. The results of watering are shown in Table 2. Although the dephosphorization effect was remarkable at the beginning of watering, the dephosphorization effect deteriorated significantly after 3 months.

【table】

【table】 [Brief explanation of the drawing]

FIG. 1 is a system explanatory diagram showing one embodiment of the present invention, and FIG. 2 is a PH titration curve when PH is adjusted by adding sulfuric acid to raw water. 1...Secondary treated water inflow pipe, 2...Decarboxylation device, 3...
PH indicating controller, 4... Mineral acid supply device, 5... Air compressor, 6... Aeration device, 7... Lime mixing PH adjusting tank, 8... PH indicating controller, 9... Slaked lime supply device, 1
0... Stirrer, 11... Rapid sand filter tank, 12... Catalytic dephosphorization tank, 13... Treated water outflow pipe.

Claims (1)

[Claims] 1. After adding the required amount of mineral acid to the liquid to be treated and bringing it into contact with air to remove carbonic substances, an alkaline agent is added to maintain neutrality or weak basicity while calcium is removed. In a method for removing soluble phosphates in a liquid to be treated by contacting it with a phosphate mineral containing calcium phosphate in the presence of ions, the amount of the mineral acid added is adjusted to the M-alkalinity of the liquid to be treated. A method for treating a phosphorus compound-containing waste liquid, characterized in that the amount is set at a chemical equivalent or higher and at a ratio of 4 gram ions to 1 gram ion of phosphate ions in the liquid to be treated. 2 The addition of the mineral acid is performed by a mineral acid supply device that is linked to a PH indicator controller that is set to a PH value of the liquid to be treated after addition of the mineral acid corresponding to the amount of mineral acid added. The method described in Scope 1.