Combined treatment method for phosphorus-containing wastewater and heavy metal-containing wastewater

Abstract

The invention relates to the technical field of wastewater treatment, in particular to a combined treatment method for phosphorus-containing wastewater and heavy metal-containing wastewater. According to the combined treatment method for the phosphorus-containing wastewater and the heavy-metal-containing wastewater, the phosphorus-containing wastewater and the heavy-metal-containing wastewater are mixed, an iron element in the mixed wastewater is oxidized through an oxidizing agent, the phosphorus element and the oxidized iron element generate an iron phosphate precipitate, and the phosphorus element and the iron element both leave a reaction system; carrying out precipitation reaction on the ferric phosphate precipitate and a sodium hydroxide solution to generate ferric hydroxide precipitate and a sodium phosphate solution, so as to respectively obtain phosphorus-containing sodium phosphate and iron-containing ferric hydroxide precipitate; the removal effect of the phosphorus element and the iron element is improved, and the phosphorus element and the iron element are recycled while the phosphorus element and the iron element are removed.

Description

Technical Field

[0001] The present application relates to the technical field of wastewater treatment, and in particular to a method for jointly treating phosphorus-containing wastewater and heavy metal-containing wastewater. Background Art

[0002] Chemical polishing waste liquid is a kind of industrial hazardous waste rich in phosphorus. Most of the existing treatment methods for chemical polishing waste liquid are wastewater treatment for the purpose of removing phosphorus, without considering the recovery of phosphorus resources. In terms of phosphorus resource recovery, there are still problems such as low recovery rate or high equipment investment and operating costs, which cannot achieve efficient recovery of phosphorus resources.

[0003] Stainless steel pickling wastewater contains a large amount of H + and Fe 2+ Cr 3+ Heavy metal ions such as chlorinated metal and chlorinated metal have been listed in the national hazardous waste list. The existing technology mainly uses the neutralization precipitation method to treat stainless steel pickling wastewater. This method has the advantages of simple process, small investment, and easy operation, but it will produce a large amount of heavy metal sludge. The disposal cost of this heavy metal sludge is high, and it poses a great threat to the environment. There is currently no reasonable way to utilize it.

[0004] Therefore, how to economically and efficiently realize the recovery and utilization of phosphorus resources in chemical polishing wastewater and the separation and resource utilization of iron and chromium elements in stainless steel pickling wastewater is the key issue in the resource utilization of chemical polishing waste liquid and stainless steel pickling wastewater. Summary of the invention

[0005] Based on this, the present application provides a method for the combined treatment of phosphorus-containing wastewater and heavy metal-containing wastewater to solve the technical problem that the prior art cannot achieve the recovery of phosphorus and iron elements.

[0006] An embodiment of the present application provides a method for jointly treating phosphorus-containing wastewater and heavy metal-containing wastewater, comprising:

[0007] The phosphorus-containing wastewater is mixed with the heavy metal-containing wastewater, wherein the molar ratio of iron to phosphorus in the obtained mixed wastewater is 0.9 to 1.1;

[0008] The mixed wastewater is subjected to a first reaction with an oxidant to oxidize the iron element in the mixed wastewater and generate iron phosphate precipitation;

[0009] performing a second reaction between the ferric phosphate precipitate and the first sodium hydroxide solution to generate an ferric hydroxide precipitate;

[0010] The reaction liquid generated after the second reaction is subjected to solid-liquid separation to obtain the iron hydroxide precipitate and the sodium phosphate solution.

[0011] In some embodiments, the pH value of the first reaction is 1.5-2.5, and the oxidant is hydrogen peroxide.

[0012] In some embodiments, in the first reaction, the pH value of the mixed wastewater is adjusted to 1.5-2.5, and an excess of hydrogen peroxide solution is added to the pH-adjusted mixed wastewater to oxidize the iron element in the mixed wastewater and generate iron phosphate precipitate.

[0013] In some embodiments, the pH value of the mixed wastewater is adjusted to 1.5-2.5 using an alkaline reagent, and the alkaline reagent is a sodium hydroxide solution with a mass percentage of 25%-50%.

[0014] In some embodiments, the temperature of the second reaction is 60-85° C., and the molar ratio of the sodium element in the first sodium hydroxide solution to the iron phosphate precipitate is 3.0-3.5.

[0015] In some embodiments, the method for jointly treating phosphorus-containing wastewater and heavy metal-containing wastewater further comprises:

[0016] The first filtrate is subjected to a third reaction with sodium hydroxide to generate chromium hydroxide precipitate, wherein the first filtrate is obtained by performing solid-liquid separation on the reaction liquid generated after the first reaction;

[0017] The reaction liquid generated after the third reaction is subjected to solid-liquid separation to obtain the chromium hydroxide precipitate and high-salt wastewater.

[0018] In some embodiments, the temperature of the third reaction is 50-85° C., and the pH value of the third reaction is 6.0-7.5.

[0019] In some embodiments, the third reaction comprises:

[0020] adding a second sodium hydroxide solution or solid sodium hydroxide to the first filtrate to adjust the pH value of the first filtrate to 6.0 to 7.5;

[0021] The first filtrate is reacted with sodium hydroxide at 50-85° C. to generate chromium hydroxide precipitate.

[0022] In some embodiments, the method for jointly treating phosphorus-containing wastewater and heavy metal-containing wastewater further comprises:

[0023] Concentrating the sodium phosphate solution by evaporation until the solution density of the sodium phosphate solution is 1.15 to 1.3 g / cm3;

[0024] The evaporated and concentrated sodium phosphate solution is cooled and crystallized to precipitate trisodium phosphate dodecahydrate crystals.

[0025] In some embodiments, the method for jointly treating phosphorus-containing wastewater and heavy metal-containing wastewater further comprises:

[0026] The iron hydroxide precipitate is added into a chemical polishing solution.

[0027] The combined treatment method of phosphorus-containing wastewater and heavy metal-containing wastewater in the embodiment of the present application comprises mixing phosphorus-containing wastewater and heavy metal-containing wastewater, oxidizing the iron element in the mixed wastewater by an oxidant, and the phosphorus element and the oxidized iron element form an iron phosphate precipitate, and both the phosphorus element and the iron element leave the reaction system; then, the iron phosphate precipitate is subjected to a precipitation reaction with a sodium hydroxide solution to generate an iron hydroxide precipitate and a sodium phosphate solution, and sodium phosphate containing phosphorus and iron hydroxide precipitate containing iron are obtained respectively; this is beneficial to improving the removal effect of phosphorus and iron elements, and while removing phosphorus and iron elements, the recovery of phosphorus and iron elements is achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a schematic diagram of a method for jointly treating phosphorus-containing wastewater and heavy metal-containing wastewater according to an embodiment of the present application. DETAILED DESCRIPTION

[0029] In order to facilitate the understanding of the present application, the present application will be described more fully below with reference to the relevant drawings. The preferred embodiments of the present application are given in the drawings. However, the present application can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present application more thorough and comprehensive.

[0030] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0031] In the embodiments of the present application, it should be noted that, in this article, relational terms such as first and second, etc. are merely used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations.

[0032] Moreover, the terms "comprises," "comprising," or any other variation thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements, but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of additional identical elements in the process, method, article, or apparatus that includes the element.

[0033] In the description of the embodiments of the present application, words such as "example" or "for example" are used to indicate examples, illustrations or descriptions. Any embodiment or design described as "example" or "for example" in the embodiments of the present application is not to be interpreted as being more preferred or having more advantages than another embodiment or design. The use of words such as "example" or "for example" is intended to present relative concepts in a clear manner.

[0034] In addition, the "plurality" in the embodiments of the present application refers to two or more than two. In view of this, in the embodiments of the present application, "plurality" can also be understood as "at least two". "At least one" can be understood as one or more, for example, one, two or more. For example, including at least one means including one, two or more, and there is no limit on which ones are included. For example, including at least one of A, B and C, then A, B, C, A and B, A and C, B and C, or A, B and C can be included.

[0035] It should be noted that, in the embodiments of the present application, "and / or" describes the association relationship of the associated objects, indicating that three relationships may exist. For example, A and / or B may represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / ", unless otherwise specified, generally indicates that the associated objects before and after are in an "or" relationship.

[0036] As used herein, the description of “a1-a2” means “greater than or equal to a1” and “less than or equal to a2”, for example, x is 0-1, that is: 0≤x≤1.

[0037] like Figure 1 As shown, it is a method for jointly treating phosphorus-containing wastewater and heavy metal-containing wastewater according to an embodiment of the present application.

[0038] like Figure 1 As shown, the combined treatment method of phosphorus-containing wastewater and heavy metal-containing wastewater includes steps S11 to S14:

[0039] Step S11: mixing the phosphorus-containing wastewater with the heavy metal-containing wastewater, wherein the molar ratio of iron to phosphorus in the obtained mixed wastewater is 0.9 to 1.1.

[0040] Phosphorus-containing wastewater can be chemical polishing waste liquid, which is a kind of industrial hazardous waste rich in phosphorus and has a high acid concentration. Heavy metal-containing wastewater can be stainless steel pickling wastewater, which contains a large amount of H + and Fe 2+ Cr 3+ Heavy metal ions such as chlorinated bicarbonate have higher acid concentrations.

[0041] The mixing ratio of phosphorus-containing wastewater and heavy metal-containing wastewater needs to be configured so that the molar ratio of iron to phosphorus in the resulting mixed wastewater is 0.9 to 1.1. Controlling the phosphorus-iron molar ratio within the above range is beneficial to the sufficient precipitation of phosphate ions in the chemical polishing waste liquid by iron ions, which can fully remove phosphorus and iron elements and improve the recovery rate of phosphorus.

[0042] Step S12: performing a first reaction between the mixed wastewater and an oxidant to oxidize the iron element in the mixed wastewater and generate iron phosphate precipitate.

[0043] The iron element in heavy metal wastewater is ferrous ion Fe 2+ The phosphorus in phosphorus-containing wastewater exists in the form of phosphate PO4 3- In the form of ferrous ions Fe 2+ Oxidized to iron ion Fe 3+ , then, the resulting iron ions Fe 3+ With phosphate PO4 3- A precipitate of iron phosphate is formed.

[0044] The iron phosphate precipitate can be separated from the reaction liquid after the first reaction by solid-liquid separation to obtain the iron phosphate precipitate and the first filtrate respectively. The first filtrate contains most of the chromium element in the heavy metal-containing wastewater (steel pickling wastewater), which has further recovery value and does not belong to waste acid. The chromium element will be recovered in subsequent steps.

[0045] As an embodiment, the oxidant is hydrogen peroxide, which has strong oxidizing properties, and another product generated by the oxidation reaction of hydrogen peroxide is water, and no new impurities are introduced into the reaction system.

[0046] In order to promote the oxidation reaction and the iron phosphate precipitation reaction, the pH value of the first reaction is 1.5-2.5.

[0047] Since the acidity of phosphorus-containing wastewater and heavy metal-containing wastewater is relatively strong, the pH value of the mixed wastewater is adjusted to 1.5-2.5 before adding the oxidant; then, hydrogen peroxide solution (hydrogen peroxide) is added to the mixed wastewater after the pH value is adjusted to oxidize the iron element in the mixed wastewater and generate iron phosphate precipitation. 2+ Fully oxidized to iron ion Fe 3+, the added hydrogen peroxide solution is in excess.

[0048] By controlling the pH value of the oxidation reaction to 1.5-2.5, the reaction is facilitated to proceed fully, so that the phosphate ions in the chemical polishing waste liquid are fully precipitated by the iron ions, thereby improving the recovery rate of the phosphorus element.

[0049] The pH value of the mixed wastewater can be adjusted to 1.5-2.5 by using an alkaline agent, and the alkaline agent is a sodium hydroxide solution with a mass percentage of 25%-50%. For example, the alkaline agent can be a sodium hydroxide solution with a mass percentage of 25% or a sodium hydroxide solution with a mass percentage of 50%.

[0050] Step S13: subjecting the ferric phosphate precipitate to a second reaction with the first sodium hydroxide solution to generate ferric hydroxide precipitate.

[0051] Ferric phosphate reacts chemically with sodium hydroxide to generate ferric hydroxide and sodium phosphate. The iron element is recovered by recovering the ferric hydroxide, and the phosphorus element is recovered by recovering the sodium phosphate.

[0052] As an embodiment, in the second reaction, in order to promote the formation of iron hydroxide precipitation, the temperature of the second reaction is controlled to be 60-85°C.

[0053] In some embodiments, the molar ratio of the sodium element in the first sodium hydroxide solution to the iron phosphate precipitate is 3.0 to 3.5, that is, the molar ratio of the sodium element to the phosphorus element is 3.0 to 3.5, so that the sodium element is completely converted into sodium phosphate, so as to fully recover the phosphorus element. Specifically, by controlling the molar ratio of the sodium element to the phosphorus element to be 3.0 to 3.5, the iron element is fully reacted into iron hydroxide, and the phosphate radical can be fully crystallized to form sodium phosphate solid in the subsequent steps.

[0054] Step S14: performing solid-liquid separation on the reaction liquid generated after the second reaction to obtain the iron hydroxide precipitate and the sodium phosphate solution.

[0055] Among them, the iron hydroxide precipitate can be separated from the reaction liquid after the second reaction by solid-liquid separation to obtain the iron hydroxide precipitate and the second filtrate respectively, and the second filtrate is the sodium phosphate solution.

[0056] The iron element is recovered by recycling ferric hydroxide, and the phosphorus element is recovered by recycling sodium phosphate. The phosphorus element in the chemical polishing waste liquid is converted into sodium phosphate solution, and the phosphorus element is fully recovered. There is no requirement for the phosphorus content in the chemical polishing waste liquid, and the chemical polishing waste liquid with a low phosphorus content has a good phosphorus recovery rate. The iron and chromium elements in the steel pickling wastewater are effectively separated, which solves the problem of resource utilization of steel pickling wastewater. Through further processing and recycling, it has the economic benefit of repeated use and will not cause secondary waste acid pollution.

[0057] In this embodiment, phosphorus-containing wastewater is mixed with heavy metal-containing wastewater, and the iron element in the mixed wastewater is oxidized by an oxidant, and the phosphorus element and the oxidized iron element form iron phosphate precipitate, and both the phosphorus element and the iron element leave the reaction system; then, the iron phosphate precipitate is subjected to a precipitation reaction with a sodium hydroxide solution to generate an iron hydroxide precipitate and a sodium phosphate solution, and sodium phosphate containing phosphorus and iron hydroxide precipitate containing iron are obtained respectively; this is beneficial to improving the removal effect of phosphorus and iron elements, and while removing phosphorus and iron elements, the recovery of phosphorus and iron elements is achieved.

[0058] Through the combined treatment method of the embodiment of the present application, the phosphorus recovery rate of chemical polishing wastewater is greater than or equal to 95%, the recovery rate of iron in stainless steel pickling wastewater is greater than or equal to 95%, and the recovery rate of chromium in stainless steel pickling wastewater is greater than or equal to 70%.

[0059] As an implementation manner, step S12 further includes the following steps:

[0060] Step S151: performing a third reaction on the first filtrate and sodium hydroxide to generate chromium hydroxide precipitate.

[0061] The first filtrate contains most of the chromium in the heavy metal wastewater (steel pickling wastewater), and the chromium is in the form of chromium ions Cr 3+ Chromium ions exist in the form of Cr 3+ OH formed after dissolving with sodium hydroxide - The reaction generates chromium hydroxide precipitate, which separates the chromium element from the reaction system.

[0062] As an embodiment, in the third reaction, in order to promote the formation of chromium hydroxide precipitation, the temperature of the third reaction is controlled to be 50-85°C.

[0063] In some embodiments, the pH value of the third reaction is 6.0-7.5.

[0064] Therefore, in the third reaction, first, the second sodium hydroxide solution or solid sodium hydroxide is added to the first filtrate to adjust the pH value of the first filtrate to 6.0-7.5. At this time, the filtrate after adjusting the pH value contains sodium hydroxide (Na + and OH - ) ; then, at 50-85°C, the chromium ions Cr in the first filtrate 3+ It reacts with sodium hydroxide added thereto to produce chromium hydroxide precipitate.

[0065] Step S152: performing solid-liquid separation on the reaction liquid generated after the third reaction to obtain chromium hydroxide precipitate and high-salt wastewater.

[0066] The chromium hydroxide precipitate can be separated from the reaction liquid after the third reaction by solid-liquid separation to obtain the chromium hydroxide precipitate and the third filtrate respectively. The third filtrate is high-salt wastewater, which has no value for further recovery.

[0067] In this embodiment, sodium hydroxide is used as an alkaline reaction liquid to react with the acidic first filtrate to obtain chromium hydroxide precipitate and high-salt wastewater. The chromium hydroxide precipitate is further removed to effectively utilize the chromium element in the steel pickling wastewater.

[0068] As an implementation manner, the following steps are further included after step S14:

[0069] Step S161: Evaporate and concentrate the sodium phosphate solution until the density of the sodium phosphate solution is 1.15-1.3 g / cm 3 ;

[0070] Step S162: cooling and crystallizing the evaporated and concentrated sodium phosphate solution to precipitate trisodium phosphate dodecahydrate crystals.

[0071] In this embodiment, phosphate ions and sodium ions in the solution are converted into sodium phosphate dodecahydrate crystals through evaporation concentration and cooling crystallization to fully recover phosphorus in the chemical polishing waste liquid, thereby achieving not only phosphorus removal but also phosphorus recovery. The purity of sodium phosphate dodecahydrate is greater than 99%.

[0072] As an implementation manner, the following steps are further included after step S14:

[0073] Step S17: adding the iron hydroxide precipitate into the chemical polishing solution.

[0074] In this embodiment, the chemical polishing liquid may be phosphoric acid, and the iron hydroxide precipitate can be recycled.

[0075] In this embodiment, the phosphorus element in the chemical polishing waste liquid and the iron and chromium elements in the steel pickling waste water are recycled. The recovery rate of the phosphorus element in the chemical polishing waste liquid is greater than or equal to 95%, and the obtained trisodium phosphate dodecahydrate complies with HG / T2517-2009. The recovery rate of the iron element in the steel pickling waste water is greater than or equal to 95%; the recovery rate of the chromium element is greater than or equal to 70%.

[0076] In some embodiments, the solid-liquid separation method can be centrifugation or filtration.

[0077] The method for preparing phosphate products by combining chemical polishing waste liquid with steel pickling wastewater in this embodiment has a simple process, low mud production, no secondary waste acid pollution, high economic value, no special requirements for the phosphorus concentration in chemical polishing phosphorus-containing waste acid, and strong raw material adaptability; at the same time, the iron and chromium elements in steel pickling wastewater can be well separated, solving the problem of resource utilization of steel pickling wastewater. The method has a phosphorus recovery rate of more than 95% in chemical polishing phosphorus-containing waste acid; the recovery rate of iron in steel pickling wastewater is greater than or equal to 95%, and the recovery rate of chromium is greater than or equal to 70%; it can be widely used in the resource utilization and disposal of chemical polishing phosphorus-containing waste acid and stainless steel pickling wastewater.

[0078] Embodiment 1:

[0079] Take 100 mL of chemical polishing waste acid, in which the total phosphorus content is 25 g / L, add 140 mL of steel pickling wastewater, in which the total iron content is 34.4 g / L, control Fe / P=1.05 (molar ratio), slowly add sodium hydroxide solution (50%) to control the solution pH to 2.0, add hydrogen peroxide, react for 60 min, and filter to obtain iron phosphate solid and the first filtrate; mix the iron phosphate solid and NaOH solution (mass percentage 30%) according to the ratio of Na / P=3.1, react at 80°C for 10 min, then filter to obtain sodium phosphate solution, and evaporate and concentrate to control the solution density to 1.2 g / cm 3 , stop concentrating, cool and crystallize to obtain sodium phosphate dodecahydrate crystals with a purity of 99.5%, meeting the standard HG / T2517-2009, and a P recovery rate of 95%. The total iron content in the first filtrate is 1.17 g / L, the iron element separation rate is 97%, the chromium content is 3.5 g / L, and the chromium retention rate is 72%.

[0080] Embodiment 2:

[0081] Take 100 mL of chemical polishing waste acid, in which the total phosphorus content is 70 g / L, add 390 mL of steel pickling wastewater, in which the total iron content is 34.4 g / L, control Fe / P=1.05 (molar ratio), slowly add sodium hydroxide solution (50% by mass) to control the solution pH to 2.0, add hydrogen peroxide, react for 60 minutes, and filter to obtain iron phosphate solid and the first filtrate; mix the iron phosphate solid with NaOH solution (30% by mass) according to the ratio of Na / P=3.1, react at 80°C for 10 minutes, then filter to obtain sodium phosphate solution, and evaporate and concentrate to control the solution density to 1.2 g / cm 3 , stop concentrating, cool and crystallize to obtain sodium phosphate dodecahydrate crystals with a purity of 99.7%, in line with standard HG / T 2517-2009, and a P recovery rate of 97%. The total iron content in the first filtrate was 1.34 g / L, the iron element separation rate was 96.5%, the chromium content was 3.75 g / L, and the chromium retention rate was 75.3%.

[0082] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0083] The above embodiments only express the preferred implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the patent application. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent application shall be subject to the attached claims.

Claims

1. A method for the combined treatment of phosphorus-containing wastewater and heavy metal-containing wastewater, characterized in that: include: The phosphorus-containing wastewater is mixed with the heavy metal-containing wastewater, wherein the molar ratio of iron to phosphorus in the obtained mixed wastewater is 0.9 to 1.1; The mixed wastewater is subjected to a first reaction with an oxidant to oxidize the iron element in the mixed wastewater and generate iron phosphate precipitation; performing a second reaction between the ferric phosphate precipitate and the first sodium hydroxide solution to generate an ferric hydroxide precipitate; The reaction liquid generated after the second reaction is subjected to solid-liquid separation to obtain the iron hydroxide precipitate and the sodium phosphate solution.

2. The combined treatment method for phosphorus-containing wastewater and heavy metal-containing wastewater according to claim 1, characterized in that: The pH value of the first reaction is 1.5-2.5, and the oxidant is hydrogen peroxide.

3. The combined treatment method of phosphorus-containing wastewater and heavy metal-containing wastewater according to claim 2, characterized in that: In the first reaction, the pH value of the mixed wastewater is adjusted to 1.5-2.5, and an excess amount of hydrogen peroxide solution is added to the mixed wastewater after the pH value is adjusted, so as to oxidize the iron element in the mixed wastewater and generate iron phosphate precipitation.

4. The combined treatment method for phosphorus-containing wastewater and heavy metal-containing wastewater according to claim 3, characterized in that: The pH value of the mixed wastewater is adjusted to 1.5-2.5 by using an alkaline reagent, wherein the alkaline reagent is a sodium hydroxide solution with a mass percentage of 25%-50%.

5. The combined treatment method for phosphorus-containing wastewater and heavy metal-containing wastewater according to claim 1, characterized in that: The temperature of the second reaction is 60-85° C., and the molar ratio of the sodium element in the first sodium hydroxide solution to the iron phosphate precipitate is 3.0-3.

5.

6. The combined treatment method of phosphorus-containing wastewater and heavy metal-containing wastewater according to claim 1, characterized in that: The combined treatment method of phosphorus-containing wastewater and heavy metal-containing wastewater also includes: The first filtrate is subjected to a third reaction with sodium hydroxide to generate chromium hydroxide precipitate, wherein the first filtrate is obtained by performing solid-liquid separation on the reaction liquid generated after the first reaction; The reaction liquid generated after the third reaction is subjected to solid-liquid separation to obtain the chromium hydroxide precipitate and high-salt wastewater.

7. The combined treatment method for phosphorus-containing wastewater and heavy metal-containing wastewater according to claim 6, characterized in that: The temperature of the third reaction is 50-85° C., and the pH value of the third reaction is 6.0-7.

5.

8. The combined treatment method for phosphorus-containing wastewater and heavy metal-containing wastewater according to claim 7, characterized in that: The third reaction comprises: adding a second sodium hydroxide solution or solid sodium hydroxide to the first filtrate to adjust the pH value of the first filtrate to 6.0 to 7.5; The first filtrate is reacted with sodium hydroxide at 50-85° C. to generate chromium hydroxide precipitate.

9. The combined treatment method of phosphorus-containing wastewater and heavy metal-containing wastewater according to claim 1, characterized in that: The combined treatment method of phosphorus-containing wastewater and heavy metal-containing wastewater also includes: The sodium phosphate solution is evaporated and concentrated until the solution density of the sodium phosphate solution is 1.15-1.3 g / cm 3 ; The evaporated and concentrated sodium phosphate solution is cooled and crystallized to precipitate trisodium phosphate dodecahydrate crystals.

10. The combined treatment method of phosphorus-containing wastewater and heavy metal-containing wastewater according to claim 1, characterized in that: The combined treatment method of phosphorus-containing wastewater and heavy metal-containing wastewater also includes: The iron hydroxide precipitate is added into a chemical polishing solution.

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