JPH0126756B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to the treatment of phosphate-containing wastewater. The coagulation-sedimentation method has traditionally been used to remove phosphates from wastewater. This is a method in which insoluble calcium phosphate is produced by reacting phosphates in wastewater with calcium ions at a pH of 10 to 12, and the resulting sediment is separated. This method has the disadvantage that not only calcium phosphate but also metal hydroxides are generated at the same time, resulting in a large amount of sludge and that dewatering is extremely difficult. To overcome this problem, a method has been proposed in which phosphate rock, which mainly consists of calcium phosphate, is brought into contact with the liquid to be treated in the presence of Ca ions to remove phosphates from wastewater (Dissertation Abstracts International Vol. 30).
No. 12, Part I). This method has the same level of phosphorus removal performance as the coagulation-sedimentation method described above, and has the advantage of generating extremely little sludge. However, not only is phosphate rock relatively expensive, but also there is a problem that valuable mineral resources must be used to remove phosphorus from wastewater, as there are concerns about the depletion of phosphorus resources. On the other hand, in Japan, which has been aiming to diversify its energy sources since the advent of oil shocks, the construction of coal-fired power plants is progressing. Along with this, an exhaust gas desulfurization device is also installed to prevent pollution. It has been thought that the sludge generated from the wastewater treatment equipment of flue gas desulfurization equipment has no choice but to be disposed of as waste, but as a result of considering the reuse of this sludge, the inventors found that one of the sources of pollution, wastewater We confirmed that this sludge is extremely excellent as a dephosphorizing material for removing phosphates from wastewater, and obtained a method for removing phosphates from wastewater using this sludge and a dephosphorizing material that is convenient and inexpensive to handle. proposed a method (patent application 1982-13764, patent application 1982-13764,
021998). The present invention has discovered that not only the sludge generated from the wastewater treatment equipment described above, but also the filter material, which is a scale-coated solid material obtained from the clarification tower, which is a component of the wastewater treatment process, has dephosphorizing performance. , which is used to treat phosphate-containing wastewater. That is, the present invention is characterized by a method for treating wastewater, in which insoluble solids are brought into contact with flue gas desulfurization wastewater rich in calcium and fluorine compounds, and then the insoluble solids are brought into contact with phosphate-containing wastewater. The object of the present invention is to provide a method for treating phosphate-containing wastewater using a filter material that is an insoluble solid that is inexpensive and easy to handle. The wastewater from coal-fired power plant flue gas desulfurization equipment contains large amounts of calcium and fluorine compounds.
The process of detoxifying wastewater is shown in Figure 1. In the figure, 1 is flue gas desulfurization wastewater, 2 is a coagulation-sedimentation tank, 3 is coagulation-sedimentation treated water, 4 is a clarification filter tower, and 5 is clarified filtration-treated water. The waste water 1 discharged from the flue gas desulfurization equipment is subjected to coagulation and sedimentation treatment in a coagulation and sedimentation tank 2, and then introduced into a clarification filter tower 4 for treatment and discharged as clarified and filtration treated water 5. In this process, when the filter material, which is an insoluble solid used in the clarification filter tower 4, is passed through the waste water for a long period of time, it becomes covered with scale and forms a granular mud mass as shown in FIG. 2. 6 indicates overwood, and 7 indicates scale. The filling material to be filled may be sand, anthracite, activated carbon, plastic balls, Raschig rings, etc., as long as it is insoluble in waste water, has strength as a filler, and can trap suspended matter. An example of the composition of scale in the granular mud is shown in Table 1.

[Table] The above granular mud lumps at the beginning of generation gradually increase the pressure loss in the clarification tower and increase the frequency of washing inside the tower, but eventually the particle size increases further and it becomes impossible to pass wastewater. Therefore, the granular mud in the tower must be removed periodically. Once this granular mud is generated, it is difficult to strip off only the scale and reuse it as lumber, so the only option is to dispose of it. The scale thickness of the granular mud mass produced in this way reaches approximately 0.2 to 2.0 mm in the case of sand with a grain size of 0.6 mm.
In the present invention, this insoluble solid substance, granular mud, is utilized. Although the dephosphorization reaction mechanism is not clear, it is presumed that the following reactions proceed in parallel. (1) Hydroxyapatite (hereinafter abbreviated as HAP)
Generation of 5Ca ²⁺ +7OH ^- +3H ₂ PO ₄ ^- → Ca ₅ (PO ₄ ) ₃ (OH) +6H ₂ O HAP (2) Fluoroapatite (hereinafter abbreviated as FAP)
Generation of 5CaF ₂ +6OH ^- +3H ₂ PO ₄ → Ca ₅ (PO ₄ ) ₃ F+6H ₂ O+9F ^- FAP Ca ₅ (PO ₄ ) ₃ (OH)+F ^- HAP → Ca ₅ (PO ₄ ) ₃ F+OH ^- FAP Above HAP and FAP is a trace amount of dissolved Ca ²⁺ ions and added Ca ²⁺ on the surface of the dephosphorizing material.
It is thought that ions and phosphate ions (H ₂ PO ₄ ^- ) react and crystallize on the particle surface, which is different from a normal coagulation-precipitation reaction. It is not clear what effect Al ₂ O ₃ in the sludge has in the reaction mechanism. The present invention provides the following effects. (1) Because it uses waste, it is a cheaper dephosphorizing material compared to expensive dephosphorizing materials such as phosphate rock. (2) Stable treated water quality with extremely low phosphorus concentration is obtained, and the pH is slightly alkaline, satisfying discharge regulation values. (3) Since it is a granular dephosphorizing material, it can be packed into a column, making it extremely easy to handle, and it also generates almost no sludge, eliminating the need for sludge treatment. (4) At present, when there is concern about the depletion of phosphorus resources, this not only eliminates the need to use valuable mineral resources for wastewater treatment, but also allows the recovered phosphorus to be reused. (5) It is now possible to reuse waste materials that were originally thought to have no choice but to be disposed of as useful materials, which has extremely high economic effects. Examples will be described below. Example 1 The granular mud lumps produced by passing about 2000 times the amount of coagulation and sedimentation treated water from the flue gas desulfurization equipment through a clarification tower filled with sand with a particle size of 0.6 mm are dried and crushed to reduce the particle size to about 0.8 mm. Add 10g/l of PO ₄ -P to a phosphate solution of 5mg/l.
While mixing and stirring, CaCl ₂ was adjusted to 80 mg/l (as Ca) and pH was adjusted to 8.7, and the change in P concentration in the solution over time was measured. The results are shown in Table 2. When granular mud blocks produced in the same manner as above using anthracite and activated carbon with the same particle diameters were tested under the same conditions, almost the same results were obtained.

[Table] Example 2 For comparison with Example 1, unused sand with a particle size of 0.6 mm was mixed and stirred into the phosphate solution under the same conditions as Example 1, and the change in P concentration in the solution over time was measured. .
The results are shown in Table 3. When a similar test was conducted using anthracite and activated carbon of the same particle size, phosphorus was not removed.

[Table] Example 3 The granular mud with a particle size of 0.8 mm used in Example 1 was
80 mg/l (as Ca) of CaCl ₂ was added to small-scale secondary sewage treatment water to adjust the pH to 8.5 while water was passed through the column at a flow rate of 21/h. The results are shown in Table 4.

[Table] As explained above, the granular mud has an excellent dephosphorizing effect.

[Brief explanation of drawings]

FIG. 1 is a flowchart showing the process of detoxifying flue gas desulfurization wastewater, and FIG. 2 is an explanatory diagram showing the state in which scale is attached to insoluble solids.

Claims (1)

[Claims] 1. A method for treating wastewater, which comprises bringing insoluble solids into contact with flue gas desulfurization wastewater rich in calcium and fluorine compounds, and then bringing the insoluble solids into contact with phosphate-containing wastewater.