Process for recovering water nutrients from sludge hydrothermal carbonization

Abstract

The application discloses a sludge hydrothermal carbonization process water nutrient salt recovery method, and belongs to the technical field of sludge treatment. The method comprises the following steps: adding calcium salt and magnesium salt into process water and heating; synchronously adding alkali liquor, adding ferrous salt and iron salt; heat preservation stirring reaction to produce granules containing nitrogen and phosphorus; magnetic separation of the granules containing nitrogen and phosphorus, solid-liquid separation to recover magnetic nutrient salt and drying for standby; further treatment of the separated liquid and recycling. By adding alkali liquor into process water to adjust the Ph value, adding calcium salt, magnesium salt, ferrous salt and iron salt, and heating stirring reaction, granules containing nitrogen and phosphorus are formed; magnetic separation is used to quickly recover magnetic nutrient salt granules; the liquid is returned to a sewage treatment system after treatment, and the magnetic granules can be used as slow-release fertilizer for soil improvement and as magnetic material containing nutrients for preparing sewage treatment microbial carrier material. The application has the advantages of simple process, good effect, recovery of nutrients in process water and resource recycling.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sludge treatment, and particularly relates to a method for recovering water nutrient salts in a sludge hydrothermal carbonization process. BACKGROUND

[0002] Sludge is a byproduct of sewage treatment, and the composition of sludge is very complex, containing a large amount of microorganisms, organic matter, and rich nutrients such as nitrogen, phosphorus, and potassium. At the same time, sludge has the characteristics of high water content, easy rotting, and foul odor. The sludge of some sewage treatment plants also contains excessive heavy metals, pathogenic microorganisms, and non-degradable toxic and carcinogenic substances. If the sludge is randomly piled up without treatment, it will cause pollution to groundwater after being eroded and leaked by rainwater, directly endangering human health.

[0003] At present, the commonly used methods for sludge treatment in China include aerobic composting, anaerobic digestion, drying incineration, and drying carbonization technology. Sludge anaerobic digestion is only suitable for large-scale sewage plants with high organic matter content in the original sewage, but it occupies a large area and still needs further dewatering treatment after sludge anaerobic digestion. The sludge cake has poor drainage, so this process is not recommended. Sludge composting also occupies a large area, has a difficult odor problem, and the heavy metals are unstable. The product has a large problem of outlet. The investment and operation cost of sludge pyrolysis carbonization is high, and the calorific value of biochar is low, which affects the end disposal destination. The energy consumption of sludge drying-incineration is high, and the investment and operation cost is the highest among various technologies. The flue gas treatment of fly ash has strict requirements, the process is complex, the management is difficult, the approval procedures are complicated, and the environmental impact assessment requirements are high. Therefore, the above technologies are not suitable for the current situation of municipal sludge treatment and disposal.

[0004] Sludge hydrothermal carbonization technology is one of the research and application hotspots of sludge treatment and disposal in recent years. It is a new technology for efficient recycling of resources and energy in sludge, which meets the double carbon target. Hydrothermal carbonization (HTC) technology is based on the high-pressure chemical theory proposed by Friedrich Bergius, the German chemist who won the Nobel Prize in Chemistry in 1931. This method simulates the process of the formation of coal, oil, and natural gas in nature, and reproduces the reaction process that takes millions of years in nature within a few hours under appropriate temperature, pressure, and pH conditions. In the process of sludge hydrothermal carbonization, catalytic cracking of sludge occurs. Under the conditions of lack of oxygen and appropriate catalysis, the molecular structure of biomass in sludge is broken, and carbonization occurs. Finally, through mechanical dewatering, it is converted into biochar with a water content of 30%, with a reduction of more than 70%, which can be directly used as a biomass solid fuel.

[0005] However, a certain amount of process water is generated in the sludge hydrothermal carbonization process, and the process water is rich in nutrients such as nitrogen and phosphorus, and direct discharge will pollute the environment. The traditional water treatment process is to remove pollutants from the process water, which is complex, has poor treatment effect, is high in cost, and cannot realize resource utilization, wasting valuable material resources. SUMMARY

[0006] The purpose of the present application is to provide a sludge hydrothermal carbonization process water nutrient salt recovery method, aiming to solve the technical problems of the prior art that the hydrothermal carbonization process water treatment process is complex, the treatment effect is poor, the cost is high, and resource utilization cannot be realized.

[0007] To solve the above technical problems, the technical solution adopted by the present application is:

[0008] A sludge hydrothermal carbonization process water nutrient salt recovery method, comprising the following steps:

[0009] Step 1: Add calcium salt and magnesium salt to the sludge hydrothermal carbonization process water, stir and heat;

[0010] Step 2: Simultaneously add alkali, ferrous salt and ferric salt to the sludge hydrothermal carbonization process water;

[0011] Step 3: After adding, keep stirring and reaction to produce granular material containing nitrogen and phosphorus;

[0012] Step 4: Magnetically separate the granular material containing nitrogen and phosphorus, recover the magnetic nutrient salt and separate the solid and liquid, dry the obtained magnetic nutrient salt granular material for standby; the separated liquid is further treated and reused;

[0013] The sludge hydrothermal carbonization process water refers to the filtrate obtained by filtering the pre-dehydrated sludge after hydrothermal carbonization reaction at 160-220 DEG C under high temperature and high pressure for 2-6 h, and the indexes of the sludge hydrothermal carbonization process water are as follows: pH 1-7, COD 10000-100000, ammonia nitrogen 200-1000, total nitrogen 400-2000, and phosphate 200-1000.

[0014] Preferably, the calcium salt in step one is one or a mixture of two or more of calcium nitrate, calcium chloride, calcium sulfate and calcium carbonate, the magnesium salt is one or a mixture of two or more of magnesium nitrate, magnesium chloride, magnesium sulfate and magnesium carbonate, the total amount of calcium ions and magnesium ions added is 1000-5000 ppm, and the mass ratio of calcium salt to magnesium salt is 1:99-50:50.

[0015] Preferably, the reaction temperature of steps one to four is 40-100 DEG C, and the reaction time of step four is 1-5 h.

[0016] Preferably, the base solution in step two refers to one or more than two mixture of alkali metal hydroxide and alkali earth metal hydroxide, and the pH is adjusted to 8-12.

[0017] Preferably, the ferrous salt in step two refers to one or two mixture of ferrous sulfate and ferrous chloride, and the ferric salt refers to one or two mixture of ferric sulfate and ferric chloride, and the total amount of the ferric salt and the ferrous salt is 10000-100000 ppm, and the mass ratio of the ferric salt and the ferrous salt is 1:1-2:1.

[0018] Preferably, after the recovery of the nutrient salt, the COD reduction rate of the sludge hydrothermal carbonization process water is 10-20%, the ammonia nitrogen reduction rate is 80-100%, and the phosphate reduction rate is 95-100%.

[0019] Preferably, the magnetic nutrient salt obtained in step four is used for making slow-release fertilizer, or as a magnetic material with nutrient substances for preparing sewage treatment microbial carrier material.

[0020] Preferably, the liquid separated in step four is treated and reused to the sewage treatment system as a denitrification carbon source.

[0021] The beneficial effects produced by the above technical solution are that, compared with the prior art, by adding calcium salt and magnesium salt, the calcium salt reacts with phosphate in the process water to form calcium phosphate precipitate, and the magnesium salt reacts with ammonium and phosphate to form struvite; the base solution is added dropwise to adjust the Ph value, and then the ferrous salt and the ferric salt are added, and the material is fully reacted under insulation and stirring to form granular material containing nitrogen and phosphorus; the magnetic nutrient salt granular material is quickly recovered by magnetic separation; the remaining liquid is further treated and returned to the sewage treatment system; the magnetic granular material can be used as slow-release fertilizer for soil improvement, and can also be used as a magnetic material with nutrient substances for preparing sewage treatment microbial carrier material. The process for recovering nutrient salt is simple and effective, which can pretreat the sludge hydrothermal carbonization process water, reduce nitrogen and phosphorus pollutants, facilitate subsequent treatment, recover nitrogen and phosphorus nutrients in the process water, and realize resource recycling. BRIEF DESCRIPTION OF DRAWINGS

[0022] The application will be further described in detail below with reference to the drawings and specific embodiments.

[0023] Figure 1 is a process schematic diagram of a sludge hydrothermal carbonization process water nutrient salt recovery method provided by the application;

[0024] In the figure: 00-reaction container, 1-Ph meter; 2-stirrer; 3-thermometer; 4-heater; 5-magnetic nutrient salt granular material. DETAILED DESCRIPTION

[0025] With reference to the accompanying drawings, the technical solutions in the embodiments of the present application will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all the other embodiments obtained by a person of ordinary skill in the art without creative effort belong to the protection scope of the present application.

[0026] The present application provides a sludge hydrothermal carbonization process water nutrient salt recovery method, and the specific recovery process is shown in Figure 1 , which comprises the following steps:

[0027] First step: calcium salt and magnesium salt are added into the sludge hydrothermal carbonization process water in the reaction container 00, and stirring and heating are performed. After the addition, the stirrer 2 and the heater 4 are started, and sufficient stirring, dissolution and heating are performed. The calcium salt reacts with the phosphate in the process water to generate calcium phosphate precipitate, and the magnesium salt reacts with the ammonium and phosphate to generate struvite.

[0028] The calcium salt refers to one or more than two mixtures of calcium nitrate, calcium chloride, calcium sulfate and calcium carbonate, the magnesium salt refers to one or more than two mixtures of magnesium nitrate, magnesium chloride, magnesium sulfate and magnesium carbonate, the total addition amount of calcium ions and magnesium ions is 1000-5000 ppm, and the mass ratio of the calcium salt to the magnesium salt is 1:99-50:50.

[0029] Second step: simultaneously, alkali, ferrous salt and iron salt are added dropwise into the sludge hydrothermal carbonization process water.

[0030] The alkali refers to one or more than two mixtures of alkali metal hydroxide and alkaline earth metal hydroxide, the pH is adjusted to 8-12 to provide alkaline reaction adjustment. The ferrous salt refers to one or two mixtures of ferrous sulfate and ferrous chloride, and the iron salt refers to one or two mixtures of ferric sulfate and ferric chloride. The total addition amount of the iron salt and the ferrous salt is 10000-100000 ppm, and the mass ratio of the iron salt to the ferrous salt is 1:1-2:1.

[0031] Third step: after the addition, the stirring reaction is performed under insulation to produce the nitrogen and phosphorus containing granular material, which contains calcium phosphate, ammonium magnesium phosphate, ferric oxide composite and other substances adsorbed therein.

[0032] Fourth step: magnetic separation of the nitrogen and phosphorus containing granular material: the nitrogen and phosphorus containing granular material is adsorbed and recovered by using a permanent magnet or an electromagnet, and the magnetic nutrient salt is obtained by solid-liquid separation. The obtained magnetic nutrient salt granular material 5 is dried for standby use. The separated liquid is further treated and reused.

[0033] The recovered magnetic nutrient salt can be used to make slow-release fertilizer, or as a magnetic material with nutrients to prepare microbial carrier materials for sewage treatment. The separated liquid is treated and reused as a denitrification carbon source in the sewage treatment system.

[0034] The sludge hydrothermal carbonization process water refers to the filtrate obtained by filtering the pre-dewatered sludge after hydrothermal carbonization reaction at a high temperature of 160-220°C and a high pressure for 2-6h. The indexes of the sludge hydrothermal carbonization process water are as follows: pH 1-7, COD 10000-100000, ammonia nitrogen 200-1000, total nitrogen 400-2000, and phosphate 200-1000. The reaction temperature in each of steps one to four is 40-100°C, and the reaction time in step four is 1-5h.

[0035] After the reaction for recovering the nutrient salt, the COD reduction rate of the sludge hydrothermal carbonization process water is 10-20%, the ammonia nitrogen reduction rate is 80-100%, and the phosphate reduction rate is 95-100%.

[0036] The following are several specific examples, and the raw materials are as follows:

[0037] Sludge: dewatered sludge from a sewage treatment plant in Chuzhou, with a moisture content of 80%, and sludge dry basis organic matter content of 52% magnesium chloride, ferrous chloride, ferric chloride, and sodium hydroxide, all of which are products produced by the National Medicine Group.

[0038] Water: tap water

[0039] Sludge hydrothermal carbonization: 200g of dewatered sludge from a sewage treatment plant in Chuzhou was selected, and the moisture content was tested to be 80%. The sludge was added to a hydrothermal carbonization reactor, and the reactor was heated and maintained at 200°C for 4h. After natural cooling to room temperature, the sludge was filtered, and the filter cake was sludge biochar. The filtrate was sludge hydrothermal carbonization process water. The quality of the process water was tested, and the results are shown in Table 1.

[0040] Example 1

[0041] First step: 100ml of the above sludge hydrothermal carbonization process water was added with calcium chloride solution and magnesium chloride solution under stirring, with the addition amount of calcium ions being 500ppm and the addition amount of magnesium ions being 1000ppm.

[0042] Second step: heating to 60°C under stirring;

[0043] Third step: simultaneously adding ferric chloride and ferrous chloride solutions, and adding sodium hydroxide solution to maintain the pH of the system at 10. The addition amounts of the ferric salt and the ferrous salt were 20000ppm and 10000ppm, respectively.

[0044] Fourth step: after the addition was completed, the system was maintained at 60°C for 1h under stirring.

[0045] Fifth step: cooling to room temperature, magnetic separation of black precipitate, liquid test water quality parameters, results are shown in Table 1

[0046] Observation of the magnetic strength of solid particles, results are shown in Table 2.

[0047] Example 2

[0048] Step one magnesium ion addition amount is 5000ppm, other methods and parameters are the same as example 1;

[0049] Magnetic separation of black precipitate, liquid test water quality parameters, results are shown in Table 1,

[0050] Observation of the magnetic strength of solid particles, results are shown in Table 2.

[0051] Example 3

[0052] Step two temperature is 100℃, other methods and parameters are the same as example 1;

[0053] Magnetic separation of black precipitate, liquid test water quality parameters, results are shown in Table 1,

[0054] Observation of the magnetic strength of solid particles, results are shown in Table 2.

[0055] Example 4

[0056] Step three iron salt and ferrous salt addition amount is 50000ppm and 20000 respectively, other methods and parameters are the same as example 1;

[0057] Magnetic separation of black precipitate, liquid test water quality parameters, results are shown in Table 1,

[0058] Observation of the magnetic strength of solid particles, results are shown in Table 2.

[0059] Example 5

[0060] Step four reaction time is 3h, other methods and parameters are the same as example 1;

[0061] Magnetic separation of black precipitate, liquid test water quality parameters, results are shown in Table 1,

[0062] Observation of the magnetic strength of solid particles, results are shown in Table 2.

[0063] Comparative example 1 (no magnesium salt)

[0064] First step: take 100ml of the above sludge water heat carbonization process, heated to 60℃ under stirring;

[0065] Second step: synchronous drop of ferric chloride and ferrous chloride solution, and drop of sodium hydroxide solution to maintain the system pH10, iron salt and ferrous salt addition amount is 20000ppm and 10000 respectively;

[0066] Third step: After adding, keep the temperature at 60°C and stir for 1h;

[0067] Fourth step: Cool to room temperature, magnetically separate the black precipitate, and test the water quality parameters of the liquid. The results are shown in Table 1

[0068] Comparative Example 2 (room temperature reaction)

[0069] First step: Take 100ml of the sludge hydrothermal carbonization process water described above, and add magnesium chloride solution under stirring. The amount of magnesium ions added is 1000ppm;

[0070] Second step: Simultaneously add ferric chloride and ferrous chloride solutions under stirring at room temperature 25°C, and add sodium hydroxide solution to maintain the pH of the system at 10. The amounts of ferric salt and ferrous salt added are 20000ppm and 10000 respectively;

[0071] Third step: After adding, keep the temperature at 25°C and stir for 1h;

[0072] Fourth step: Cool to room temperature, magnetically separate the black precipitate, and test the water quality parameters of the liquid. The results are shown in Table 1

[0073] Observe the magnetic strength of the solid particles, and the results are shown in Table 2.

[0074] Comparative Example 3 (without adjusting pH)

[0075] First step: Take 100ml of the sludge hydrothermal carbonization process water described above, and add magnesium chloride solution under stirring. The amount of magnesium ions added is 1000ppm;

[0076] Second step: Heat to 60°C under stirring;

[0077] Third step: Simultaneously add ferric chloride and ferrous chloride solutions. The amounts of ferric salt and ferrous salt added are 20000ppm and 10000 respectively;

[0078] Fourth step: After adding, keep the temperature at 60°C and stir for 1h;

[0079] Fifth step: Cool to room temperature, magnetically separate the black precipitate, and test the water quality parameters of the liquid. The results are shown in Table 1

[0080] Observe the magnetic strength of the solid particles, and the results are shown in Table 2.

[0081] Comparative Example 4 (ferric salt and ferrous salt ratio is not suitable)

[0082] First step: Take 100ml of the sludge hydrothermal carbonization process water described above, and add magnesium chloride solution under stirring. The amount of magnesium ions added is 1000ppm;

[0083] Second step: Heat to 60°C under stirring;

[0084] Third step: synchronously drop iron chloride and ferrous chloride solution, and drop sodium hydroxide solution to maintain system pH 10, and the adding amount of iron salt and ferrous salt is 10000ppm and 20000 respectively;

[0085] Fourth step: after adding, keep 60℃ and stir for 1h;

[0086] Fifth step: cool to room temperature, magnetically separate black precipitate, and test water quality parameters of liquid, and the results are shown in Table 1

[0087] Observe the magnetic strength of solid particles, and the results are shown in Table 2.

[0088] Table 1

[0089] Indicators [NH3-N (mg / L)] TP (mg / L) COD Cr (mg / L) TN (mg / L) pH Process water 800 400 50000 1000 3.5 Example 1 50 0 46000 210 10 Example 2 20 0 45000 160 10 Example 3 30 0 48000 160 10 Example 4 40 0 47000 180 10 Example 5 40 0 47000 180 10 Comparative Example 1 650 80 46000 820 10 Comparative Example 2 660 100 45000 850 10 Comparative Example 3 750 220 44000 930 3.5 Comparative Example 4 80 20 48000 250 10

[0090] Table 2

[0091] Number Magnetic Example 1 ◎ Example 2 ◎ Example 3 ◎ Example 4 ◎ Example 5 ◎ Comparative Example 1 ◎ Comparative Example 2 × Comparative Example 3 × Comparative Example 4 ○

[0092] As shown in Table 1, the sludge water heat carbonization process water is rich in nutrients such as nitrogen and phosphorus, and after the treatment of the present application examples 1-5, the COD in the water is reduced to within 10%, the ammonia nitrogen is reduced by more than 90%, and the phosphate is reduced to 0, showing excellent nutrient recovery efficiency, and the generated granular precipitate has strong magnetism and is easy to be magnetically separated, which provides better performance for subsequent nutrition. Compared with comparative examples 1-4 outside the present application, the nutrient recovery efficiency is obviously lower than that of the present application, and the granular precipitate has weak or no magnetism.

[0093] In summary, the beneficial effects of the present application are:

[0094] 1. The present application can realize the purpose of recycling and removing nutrients such as ammonia nitrogen and phosphate in sludge water heat carbonization process water;

[0095] 2. The present application forms magnetic particles rich in nutrients, which can be used as slow-release fertilizer and as magnetic material with nutrients for preparing sewage treatment microbial carrier material;

[0096] 3. The present application has simple process, good effect, low cost, can pretreat sludge water heat carbonization process water, reduce pollutants such as nitrogen and phosphorus, facilitate subsequent treatment, recycle nutrients such as nitrogen and phosphorus in process water, realize resource recycling, and meet the goal of environmental protection and emission reduction.

[0097] Many specific details are set forth in the above description in order to provide a thorough understanding of the application. However, the application can be practiced according to other embodiments that depart from these specific details without departing from the intended scope of the application. Accordingly, the scope of the application is not intended to be limited by the specific details presented above.

Claims

1. A method for recovering water nutrients from a sludge hydrothermal carbonization process, characterized in that, It comprises the following steps: Step 1: calcium salt and magnesium salt are added to the sludge hydrothermal carbonization process water, stirred and heated, calcium salt reacts with phosphate in the process water to form calcium phosphate precipitate, and magnesium salt reacts with ammonium and phosphate to form struvite; Step 2: alkali, ferrous salt and ferric salt are added to the sludge hydrothermal carbonization process water simultaneously; Step 3: after the addition is completed, the reaction is carried out under stirring and insulation, and nitrogen and phosphorus-containing particles are produced, which contain calcium phosphate, ammonium magnesium phosphate, ferric oxide composite and other substances adsorbed therein; Step 4: the nitrogen and phosphorus-containing particles are separated by magnetic separation, the magnetic nutrient salt is recovered and solid-liquid separation is carried out, the obtained magnetic nutrient salt particles are dried for standby use, and the separated liquid is treated and reused; The sludge hydrothermal carbonization process water refers to the filtrate obtained by filtering the pre-dehydrated sludge after hydrothermal carbonization reaction at 160-220℃ under high temperature and high pressure for 2-6h, and the indexes of the sludge hydrothermal carbonization process water are as follows: pH 1-7, COD 10000-100000mg / L, ammonia nitrogen 200-1000mg / L, total nitrogen 400-2000mg / L, and phosphate 200-1000mg / L.

2. The method of claim 1, wherein the method is characterized by: The calcium salt in step 1 refers to one or more than two mixtures of calcium nitrate, calcium chloride, calcium sulfate and calcium carbonate, the magnesium salt refers to one or more than two mixtures of magnesium nitrate, magnesium chloride, magnesium sulfate and magnesium carbonate, the total addition amount of calcium ions and magnesium ions is 1000-5000ppm, and the mass ratio of calcium salt to magnesium salt is 1:99-50:

50.

3. The method of claim 1, wherein the method further comprises: The reaction temperature in steps 1-4 is 40-100℃, and the reaction time in step 4 is 1-5h.

4. The method of claim 1, wherein the method further comprises: The alkali in step 2 refers to one or more than two mixtures of alkali metal hydroxide and alkaline earth metal hydroxide, and the pH is adjusted to 8-12.

5. The method of claim 1, wherein the method further comprises: The ferrous salt in step 2 refers to one or more than two mixtures of ferrous sulfate and ferrous chloride, and the ferric salt refers to one or more than two mixtures of ferric sulfate and ferric chloride, the total addition amount of ferric salt and ferrous salt is 10000-100000ppm, and the mass ratio of ferric salt to ferrous salt is 1:1-2:

1.

6. The method of claim 1, wherein the method further comprises: After the recovery of the nutrient salt, the COD reduction rate of the sludge hydrothermal carbonization process water is 10-20%, the ammonia nitrogen reduction rate is 80-100%, and the phosphate reduction rate is 95-100%.

7. The method of claim 1, wherein the method further comprises: The magnetic nutrient salt obtained in step 4 is used for preparing slow-release fertilizer, or as a magnetic material containing nutrient substances for preparing sewage treatment microbial carrier material.

8. The method of claim 1, wherein the method further comprises: The separated liquid in step 4 is treated and reused in the sewage treatment system as a denitrification carbon source.

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