Municipal sludge ecological treatment method

Abstract

The invention belongs to the technical field of sludge treatment, and particularly relates to a municipal sludge ecological treatment method which aims at solving the core problems that municipal sludge is complex in component and large in treatment difficulty, and through the technologies of composite adsorption, microwave catalytic oxidation, steam stripping-oxidation coupling nitrogen removal, acid phosphorus recovery and classification-mineral reinforced heavy metal solidification and the like, the municipal sludge is efficiently treated. Deep purification, nitrogen and phosphorus resource recovery and safe recycling of the sludge are realized systematically and efficiently at low cost; the bottleneck that deep removal of pollutants, resource recovery and safe utilization of products are difficult to solve at the same time in the traditional technology is broken through, and a new environment-friendly and resource-circulating sludge treatment and disposal path is provided.

Description

Technical Field

[0001] The invention belongs to the technical field of sludge treatment and relates to an ecological treatment method for municipal sludge. Background Art

[0002] Municipal sludge has a complex composition, with high water content (usually >80%), high concentration of heavy metal ions (such as Cd 2+ , Pb 2+ 、Hg 2+ etc.), organic pollutants (COD), high nitrogen and phosphorus nutrients and pathogens. The existing mainstream treatment technology has the following defects:

[0003] 1) Low dehydration efficiency and residual pollutants. Traditional mechanical dehydration (such as centrifugation and belt filter presses) struggles to reduce the moisture content below 60%, and the dehydration process fails to remove dissolved heavy metals, organic pollutants, and nitrogen and phosphorus. Subsequent treatment is burdensome, and landfilling can easily lead to leachate pollution. Incineration consumes a lot of energy and easily produces pollutants such as dioxins.

[0004] 2) Incomplete COD removal and secondary pollution. Anaerobic digestion can recover biogas, but the cycle is long and the COD removal rate is limited. Aerobic composting requires a large amount of auxiliary materials, is prone to odor, and has no effect on solidifying heavy metals. Incineration, while thorough, consumes a lot of energy and has complex tail gas treatment (dioxins, heavy metal fly ash). Existing oxidation technologies often require strong acidic conditions, produce large amounts of iron sludge, have high reagent costs, and have unstable oxidation efficiency for complex organic matter.

[0005] 3) Difficulty in removing ammonia nitrogen: ammonia nitrogen (NH4 + / NH3) concentration is high, and traditional biological denitrification (nitrification and denitrification) is inefficient and subject to many inhibitory factors in sludge treatment; chemical precipitation (such as the struvite method) mainly targets dissolved ammonia nitrogen and has limited effect on solid organic nitrogen and complexed ammonium; the stripping method has high energy consumption, is prone to scaling, and the tail gas ammonia requires additional treatment, which can easily cause air pollution.

[0006] 4) The difficulty of phosphorus recovery conflicts with heavy metal immobilization. Phosphorus is a precious resource, and traditional chemical methods for heavy metal immobilization / stabilization (such as cement immobilization and lime stabilization) also immobilize phosphorus, making it difficult to recycle. Proprietary phosphorus recovery technologies (such as struvite recovery) often ignore phosphorus in the sludge solid phase and fail to address heavy metal contamination, resulting in a trade-off between phosphorus recovery and heavy metal immobilization.

[0007] 5) Heavy metal stabilization is not durable and is an obstacle to resource utilization. Simple chemical stabilization (such as adding lime to form hydroxide precipitation) is easily affected by changes in environmental pH and re-dissolved. Although sulfide precipitation is stable, its efficiency for certain heavy metals is limited. There is a lack of methods that can effectively fix multiple heavy metals at the same time and ensure long-term stability.

[0008] 6) Existing sludge treatment technologies have long processing chains and low levels of resource utilization, often requiring multiple independent units to be connected in series. The process is complex and the cost is high, and the final product often cannot be safely recycled, resulting in resource waste and environmental risks.

[0009] Therefore, providing a municipal sludge ecological treatment method that can simultaneously remove multiple pollutants and is easy to operate has become an urgent problem to be solved. Summary of the Invention

[0010] In order to solve the above problems, the present invention provides a method for ecological treatment of municipal sludge, which specifically comprises the following steps:

[0011] Step 1: Initial removal of pollutants

[0012] Filter municipal sludge with a moisture content of ≥80% to remove insoluble impurities, dehydrate the sludge to a moisture content of 50-60% through a screw extruder, and add a composite adsorbent. Stir and adsorb at 40-50% humidity, 55-65°C, and 120-150rpm for 2-2.5 hours, then filter to remove the composite adsorbent.

[0013] Preferably, the mass ratio of the dewatered sludge to the composite adsorbent is 100:(1-2).

[0014] Preferably, the composite adsorbent is activated carbon, diatomaceous earth, SiO2, Al2O3 and magnetic Fe3O4 particles, with a mass ratio of 10:5:4:2:3.

[0015] During this process, the composite adsorbent will initially adsorb pollutants such as free heavy metals, ammonia nitrogen, phosphorus and COD in the sludge.

[0016] Step 2: Oxidation of COD

[0017] The sludge treated in step 1 was mixed with calcium peroxide in a mass ratio of 100:(3-5), and subjected to microwave treatment at 2400-2500 MHz and 80-95° C. for 15-20 minutes.

[0018] During this process, calcium peroxide is hydrolyzed step by step in water to produce hydrogen peroxide. Under certain conditions, hydrogen peroxide will decompose to produce hydroxyl radicals, removing organic matter such as oxidized COD. The specific principle is as follows:

[0019] (1) Calcium peroxide is hydrolyzed step by step in water: CaO2+2H2O=Ca(OH)2+H2O2.

[0020] (2) Hydrogen peroxide decomposes to produce hydroxyl radicals: H2O2→·OH+H2O.

[0021] Process (2) homolysis requires overcoming the OO bond energy. Conventional heating is inefficient. Microwaves cause molecules to oscillate at high frequencies, instantly increasing the thermal motion of the molecules and reducing the activation energy of H2O2 decomposition. Microwaves polarize H2O2 molecules into H + 、-OOH - Dipole weakens the OO bond. The vibration frequency of the OO bond matches the microwave frequency, causing molecular resonance and direct cleavage to produce active OH. In addition, Ca 2+ Can form short-lived [Ca(OH)] + Complex, prolongs the life of free radicals and improves oxidation efficiency.

[0022] Step 3: Ammonia nitrogen removal

[0023] The sludge treated in step 2 is mixed with the hypochlorite solution and sent to a stripping tower, where 90-120°C steam is introduced with a gas-liquid volume ratio of 10:1. After the steam is condensed, it is adsorbed in an adsorption tower and then discharged.

[0024] Preferably, the mass ratio of the sludge to the hypochlorite solution is (10-20):1.

[0025] Preferably, the hypochlorite solution is a NaClO solution with a mass fraction of 10-12% or a Ca(ClO)2 solution with a mass fraction of 15-20%.

[0026] During this process, ClO - On the one hand, the COD that was not removed in step 2 is further oxidized, and on the other hand, the ammonia nitrogen is oxidized into N2 and discharged with the steam. The steam will also further carry out the excess ammonia nitrogen in the sludge. After condensation and adsorption of ammonia-containing steam, the ammonia nitrogen content will be significantly reduced and can be discharged directly.

[0027] Step 4: Phosphorus recovery

[0028] Filter the sludge treated in step 3, remove the filtrate, add MgCl2·H2O and CaHPO4 to the sludge in sequence, stir at 200-300 rpm for 30-40 minutes, and then filter to remove insoluble impurities.

[0029] Preferably, the mass ratio of the sludge, MgCl2·H2O and CaHPO4 is 100:0.5:1.

[0030] Step 3 only removes free ammonia nitrogen (NH4 + and NH3), but has limited effect on the removal of organic nitrogen (such as protein, amino acids, etc.) and complexed ammonium in the sludge solid phase. In step 4, the hydrolysis of CaHPO4 provides an acidic environment, which promotes the release of NH3 from organic nitrogen and complexed ammonium. 4+ ,HPO4 - , Ca 2+ Mg2+ Will release NH 4+ Struvite is produced, thereby recovering phosphorus.

[0031] Step 5: Heavy metal solidification

[0032] The sludge treated in step 4 is mixed with the mineral material, and an alkaline solution is added to adjust the pH to 10-11. Then, Na2HPO4 solution and NaHCO3 powder are added in sequence, and the mixture is stirred at 200-300 rpm for 30-40 minutes. The mixture is filtered, and the filtrate and the mineral material are removed. An organic acid solution is added to the filtered sludge to adjust the pH to 4.5-6.0, and then Na2S solution is added. The mixture is stirred at 200-300 rpm for 20-30 minutes. The mixture is filtered, the filtrate is removed, and the sludge is retained.

[0033] Preferably, the mass ratio of the sludge, mineral material, Na2HPO4 solution, NaHCO3 and Na2S solution is 100:(8-10):(3-4):(2-3):(0.5-1).

[0034] Preferably, the mineral material includes one or more of zeolite, vermiculite, sepiolite, attapulgite and bentonite.

[0035] Preferably, the alkaline solution is a NaOH solution with a mass fraction of 10-15% or a KOH solution with a mass fraction of 10-15%.

[0036] Preferably, the organic acid solution is one of a 10-12% by mass citric acid solution, a 15-20% by volume acetic acid solution, a 12-15% by mass lactic acid solution, a 10-12% by mass tartaric acid solution and a 10-15% by mass malic acid solution.

[0037] Preferably, the mass fraction of the Na2HPO4 solution is 10-15%, and the mass fraction of the Na2S solution is 5-10%.

[0038] During this process, the mineral material will absorb some of the free heavy metal ions in the solution (such as Hg 2+ 、Ni 2+ 、Cd 2+ and Pb 2+ etc.), PO4 in alkaline environment 3- (Including PO4 that was not removed in step 4 3- ) and Cd 2+ and Pb 2+ Forming insoluble phosphates (such as Pb3(PO4)2), reducing bioavailability, CO3 2- Will and Zn 2+ 、Cu 2+etc. to form basic carbonate precipitation, S 2- and As 3+ 、Hg 2+ Produces extremely stable sulfides.

[0039] Step 6: Product application

[0040] After the treatment in step five, the sludge is dried at 30-40° C. to a moisture content of ≤10%, and then crushed to particles with a size of ≤5 mm, which can be reused as a plant cultivation medium or a raw material for brick making.

[0041] The present invention has the following advantages:

[0042] (1) Efficient dehydration and reduction of pollutant sources. The present invention dehydrates sludge to a moisture content of 50-60%, significantly reducing the subsequent processing volume and energy consumption. The composite adsorbent has the characteristics of large specific surface area, multi-level pores, and multiple active sites. It can simultaneously and efficiently adsorb free heavy metal ions, ammonia nitrogen and organic pollutants, achieving preliminary removal of pollutants and laying the foundation for subsequent deep treatment.

[0043] (2) The present invention uses calcium peroxide and the synergistic effect of microwaves and temperature to deeply oxidize COD. The microwave frequency matches the vibration frequency of the OO bond in H2O2, triggering molecular resonance, which greatly reduces the activation energy of hydrogen peroxide decomposition to generate strong oxidizing hydroxyl radicals. The oxidation efficiency far exceeds that of conventional heating. The Ca generated by the reaction 2+ Can react with hydroxyl radicals to form short-lived [Ca(OH)] + The complex prolongs the life of free radicals, improves oxidation efficiency, has relatively low energy consumption, and has no secondary pollution, avoiding the strong acidic conditions and iron mud problems of the existing technology.

[0044] (3) Stripping-chemical oxidation coupled deep denitrification. The present invention innovatively combines hypochlorite oxidation with steam stripping. ClO- oxidizes residual COD and dissolved ammonia nitrogen into harmless substances such as N2. High-temperature steam significantly increases the volatility of ammonia nitrogen, efficiently blowing it out from the sludge solid phase. The ammonia-containing vapor is discharged after condensation and adsorption tower treatment to avoid air pollution. This process can efficiently remove dissolved ammonia nitrogen.

[0045] (4) Acid-driven struvite crystallization to recover phosphorus is carried out after ammonia nitrogen removal. The acidic environment is cleverly provided by the hydrolysis of CaHPO4. The acidic conditions promote the dissolution of insoluble phosphates in the sludge solid phase and promote the release of NH4 from residual organic nitrogen and complexed ammonium. + and with Mg 2+ PO4 3- This process not only efficiently recovers phosphorus resources from the sludge, but also effectively removes organic nitrogen and complexed ammonium from the solid phase, achieving more thorough denitrification and avoiding the limited effect of existing stripping methods on solid-phase nitrogen.

[0046] (5) Gradual stabilization of heavy metal ions: In the present invention, mineral materials are added to initially adsorb heavy metal ions under alkaline conditions of pH 10-11, PO4 3- (from Na2HPO4 and residual phosphorus) and Cd 2+ , Pb 2+ etc. to form insoluble phosphates, CO3 2- With Zn 2+ 、Cu 2+ The mineral material provides a huge adsorption surface and enhances the stability of the precipitation. Then adjust the pH to weak acidity. When the acidity is weak, S 2- With As 3+ 、Hg 2+ It forms extremely stable sulfides with extremely low solubility (such as As2S3 (orpiment) and HgS (cinnabar)).

[0047] This process is a graded treatment of alkali first and then acid, with heavy metals that are easy to form hydroxides / carbonates / phosphates being treated first under alkaline conditions, and then the most difficult to treat As being stabilized under weak acid conditions. 3+ 、Hg 2+ The results show that the bioavailability and leaching toxicity of heavy metals are significantly reduced, which avoids the problem that existing stabilization methods are difficult to effectively fix multiple heavy metal ions and have insufficient long-term stability.

[0048] (6) The present invention integrates dehydration, oxidation, denitrification, phosphorus recovery, and heavy metal stabilization. The final product can be used as a plant cultivation substrate or brick-making raw material after simple drying and pulverization, truly achieving the ecological cycle goal of turning waste into treasure and treating and utilizing it. This solves the problem that existing technologies are limited to reduction or preliminary stabilization, making it difficult to produce high-value-added, safe resource products. DETAILED DESCRIPTION

[0049] The following is a clear and complete description of the technical solutions in the embodiments of the invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0050] Example 1

[0051] Step 1: Initial removal of pollutants

[0052] Municipal sludge with a moisture content of 80% or higher was filtered to remove insoluble impurities. The sludge was then dehydrated to a moisture content of 55% using a screw extruder and mixed with a composite adsorbent at a mass ratio of 50:1. The adsorption was allowed to proceed for two hours at a humidity of 45% and a temperature of 60°C, with stirring at 120 rpm. The composite adsorbent was then removed by filtration. The composite adsorbent consisted of activated carbon, diatomaceous earth, SiO2, Al2O3, and magnetic Fe3O4 particles in a mass ratio of 10:5:4:2:3.

[0053] Step 2: Oxidation of COD

[0054] The sludge treated in step 1 was mixed with calcium peroxide at a mass ratio of 25:1 and subjected to microwave treatment at 2450 MHz and 90 °C for 20 min.

[0055] Step 3: Ammonia nitrogen removal

[0056] The sludge treated in step 2 and 18% Ca(ClO)2 solution are mixed in a mass ratio of 15:1 and sent to a stripping tower. 105°C steam is introduced with a gas-liquid volume ratio of 10:1. After the steam is condensed, it is adsorbed in an adsorption tower and then discharged.

[0057] Step 4: Phosphorus recovery

[0058] Filter the sludge treated in step 3, discard the filtrate, and sequentially add MgCl2·H2O and CaHPO4 to the sludge. Stir at 250 rpm for 35 minutes, then filter to remove insoluble impurities. The mass ratio of sludge, MgCl2·H2O, and CaHPO4 is 100:0.5:1.

[0059] Step 5: Heavy metal solidification

[0060] The sludge treated in step 4 is mixed with the mineral material, and a 12% mass fraction NaOH solution is added to adjust the pH to 10-11. Then, a 12% mass fraction Na2HPO4 solution and NaHCO3 powder are added in sequence, and the mixture is stirred at 250 rpm for 35 minutes. The mixture is filtered, and the filtrate and the mineral material are removed. A 12% mass fraction citric acid solution is added to the filtered sludge to adjust the pH to 4.5-6.0. Then, an 8% mass fraction Na2S solution is added, and the mixture is stirred at 250 rpm for 30 minutes. The mixture is filtered, the filtrate is removed, and the sludge is retained. The mass ratio of the sludge, mineral material, Na2HPO4 solution, NaHCO3, and Na2S solution is 100:9:1:2:1. The mineral material includes zeolite and vermiculite in a mass ratio of 2:1.

[0061] Test Example 1

[0062] 1. Experimental Materials

[0063] Raw materials: Municipal sludge from a sewage treatment plant, with a moisture content of 82%, initial COD = 15200 mg / L, ammonia nitrogen = 1850 mg / L, total phosphorus = 3200 mg / kg, heavy metal content: Cd = 28 mg / kg, Pb = 420 mg / kg, Hg = 15 mg / kg, As = 75 mg / kg.

[0064] 2. Experimental process and detection methods

[0065] Table 1 Experimental process and detection method

[0066]

[0067] 3. Experimental results data table Experimental results data table

[0068] Table 2 Removal effects of main pollutants

[0069]

[0070] Table 3 Heavy metal curing effect

[0071]

[0072] Table 4 Changes in distribution of heavy metal forms

[0073] The proportion of Cd residual state Pb residual state ratio The proportion of Hg in residual state Raw sludge 12％ 35％ 8％ Final sludge 89％ 93％ 95％

[0074] As shown in Tables 2-4, the microwave-catalyzed calcium peroxide of the present invention achieves a COD removal rate of 98%, the stripping-hypochlorite coupling technology removes 96% of ammonia nitrogen, and the ammonia nitrogen in the condensed water is <10 mg / L. The acidic environment drives the crystallization of struvite to recover 82% of phosphorus, and solid-phase organic nitrogen is simultaneously removed. The graded mineral solidification reduces the proportion of residual heavy metals to over 89%, and the TCLP leaching concentration is far below the national standard, meeting the requirements of garden substrates.

[0075] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for ecological treatment of municipal sludge, characterized in that: The following steps are involved: Step 1: dewatering the municipal sludge to a moisture content of 50-60% to obtain dewatered sludge, adding a composite adsorbent to the dewatered sludge, stirring and adsorbing the sludge, and then filtering to remove the composite adsorbent; Step 2: Mix the sludge treated in step 1 with calcium peroxide and microwave at 2400-2500 MHz and 80-95°C; Step 3: Mix the sludge treated in step 2 with the hypochlorite solution and send it into a stripping tower, where steam at 90-120°C is introduced. Step 4: Filter the sludge treated in step 3, remove the filtrate, add MgCl2·H2O and CaHPO4 to the sludge in sequence, stir, and then filter to remove insoluble impurities; Step 5: Mix the sludge treated in step 2 with the mineral material, add an alkaline solution, adjust the pH to 10-11, then add Na2HPO4 solution and NaHCO3 in sequence, stir, filter, remove the filtrate and mineral material, add an organic acid solution to the filtered sludge to adjust the pH to 4.5-6.0, then add Na2S solution, stir, filter, remove the filtrate, and the filter residue is the treated sludge.

2. A municipal sludge ecological treatment method according to claim 1, characterized in that: The mass ratio of the dewatered sludge and the composite adsorbent in step 1 is 100:(1-2).

3. A municipal sludge ecological treatment method according to claim 1, characterized in that: The composite adsorbent in step 1 is activated carbon, diatomaceous earth, SiO2, Al2O3 and magnetic Fe3O4 particles, with a mass ratio of 10:5:4:2:

3.

4. A municipal sludge ecological treatment method according to claim 1, characterized in that: The mass ratio of the sludge to calcium peroxide in step 2 is 100:(3-5).

5. The method for ecological treatment of municipal sludge according to claim 1, characterized in that: The mass ratio of the sludge and the hypochlorite solution in step 3 is (10-20):

1.

6. A municipal sludge ecological treatment method according to claim 1, characterized in that: The hypochlorite solution in step 3 is a NaClO solution with a mass fraction of 10-12% or a Ca(ClO)2 solution with a mass fraction of 15-20%.

7. The method for ecological treatment of municipal sludge according to claim 1, characterized in that: The mass ratio of sludge, MgCl2·H2O and CaHPO4 in step 4 is 100:0.5:

1.

8. The method for ecological treatment of municipal sludge according to claim 1, characterized in that: The mass ratio of the sludge, mineral material, Na2HPO4 solution, NaHCO3 and Na2S solution in step five is 100:(8-10):(3-4):(2-3):(0.5-1).

9. The method for ecological treatment of municipal sludge according to claim 1, characterized in that: The alkaline solution in step five is a NaOH solution with a mass fraction of 10-15% or a KOH solution with a mass fraction of 10-15%.

10. The method for ecological treatment of municipal sludge according to claim 1, characterized in that: The organic acid solution in step five is one of a citric acid solution with a mass fraction of 10-12%, an acetic acid solution with a volume fraction of 15-20%, a lactic acid solution with a mass fraction of 12-15%, a tartaric acid solution with a mass fraction of 10-12%, and a malic acid solution with a mass fraction of 10-15%.

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