A method for ecologically treating municipal sludge

Abstract

The present application belongs to the technical field of sludge treatment, and particularly relates to a municipal sludge ecological treatment method. The present application aims at the core problem of complex composition of municipal sludge and difficult treatment, and realizes deep purification of sludge, nitrogen and phosphorus resource recovery and safe resource utilization in a systematic, efficient and low-cost manner through composite adsorption, microwave catalytic oxidation, stripping-oxidation coupling denitrification, acid phosphorus recovery and hierarchical-mineral reinforced heavy metal solidification technologies. The present application breaks through the bottleneck of traditional technologies that are difficult to simultaneously solve the problems of deep removal of pollutants, resource recovery and safe utilization of products, and provides a new path of sludge treatment and disposal which is environmentally friendly and resource recycling.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of sludge treatment, and relates to a municipal sludge ecological treatment method. BACKGROUND

[0002] Municipal sludge has complex components, 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.

[0003] 1) Low dewatering efficiency and residual pollutants. Traditional mechanical dewatering (such as centrifugation and belt filter pressing) is difficult to reduce the water content to below 60%, and the dewatering process cannot remove dissolved heavy metals, organic pollutants, and nitrogen and phosphorus. Subsequent treatment is heavy, landfilling is prone to leachate pollution, and incineration is energy-intensive and prone to produce 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. A large amount of auxiliary materials are needed for aerobic composting, which is prone to produce odors and has no solidification effect on heavy metals. Incineration is complete but energy-intensive, and tail gas treatment is complex (dioxins, heavy metal fly ash). Existing oxidation technologies often require strong acidic conditions, produce a large amount of iron sludge, have high reagent costs, and have unstable oxidation efficiency for complex organic matter.

[0005] 3) Ammonia nitrogen removal problem. The concentration of ammonia nitrogen (NH4 + / NH3) in sludge is high, and traditional biological denitrification (nitrification and denitrification) has low efficiency and is inhibited by many factors in sludge treatment. Chemical precipitation (such as struvite method) mainly targets dissolved ammonia nitrogen, and has limited effect on solid organic nitrogen and complex ammonium. The blow-off method is energy-intensive, prone to fouling, and the tail gas ammonia needs additional treatment, which is prone to cause air pollution.

[0006] 4) Difficulty in phosphorus recovery and conflict with heavy metal stabilization. Phosphorus is a valuable resource, and traditional chemical stabilization / heavy metal stabilization methods (such as cement stabilization and lime stabilization) will also fix phosphorus, making it difficult to recover and utilize. Specialized phosphorus recovery technologies (such as struvite recovery) often ignore the phosphorus in the solid phase of sludge, and do not solve the problem of heavy metal pollution, resulting in the inability to achieve both phosphorus recovery and heavy metal stabilization.

[0007] 5) Heavy metal stabilization is not long-lasting and resource utilization is difficult. Simple chemical stabilization (such as adding lime to generate hydroxide precipitate) is easily affected by changes in environmental pH and re-dissolved, and sulfide precipitation has limited effect on some heavy metals. There is a lack of methods that can effectively fix multiple heavy metals and ensure long-term stability.

[0008] 6) The existing sludge treatment technology has a long treatment chain, low resource utilization degree, often requires multiple independent units in series, complex process, high cost, and the final product cannot be safely resourceized, resulting in resource waste and environmental risk.

[0009] Therefore, it is an urgent problem to provide a municipal sludge ecological treatment method which can simultaneously remove multiple pollutants and is easy to operate. SUMMARY

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

[0011] Step one, preliminary removal of pollutants

[0012] Filter the municipal sludge with a moisture content of ≥80%, remove insoluble impurities, dewater the sludge to a moisture content of 50-60% by a screw extruder, and add a composite adsorbent, stir and adsorb at a humidity of 40-50%, a temperature of 55-65℃, and a speed of 120-150rpm for 2-2.5h, and then filter to remove the composite adsorbent.

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

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

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

[0016] Step two, oxidation of COD

[0017] Mix the sludge treated in step one with calcium peroxide at a mass ratio of 100:(3-5), and microwave treat at 2400-2500MHz and 80-95℃ for 15-20min.

[0018] During this process, calcium peroxide is stepwise hydrolyzed in water to produce hydrogen peroxide, which will decompose to generate hydroxyl radicals under certain conditions to remove organic matter such as oxidized COD. The specific principle is as follows:

[0019] (1) Stepwise hydrolysis of calcium peroxide in water: CaO2+2H2O=Ca(OH)2+H2O2.

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

[0021] Process (2) homolytic cleavage need to overcome the O-O bond energy, conventional heating efficiency is low, microwave makes the high frequency oscillation of molecules, instantaneously improves the thermal motion of molecules, reduces the decomposition activation energy of H2O2. Microwave makes H2O2 molecule polarize into H + , -OOH - Dipole, weaken O-O bond. O-O bond vibration frequency matches with microwave frequency, molecular resonance occurs, and active ·OH is directly broken. In addition, Ca 2+ Can form a short [Ca(OH)] + Complex with ·OH, prolong the life of free radicals, and improve the oxidation efficiency.

[0022] Step three, ammonia nitrogen removal

[0023] The sludge treated in step two and the hypochlorite solution are mixed and sent into a stripping tower, 90-120℃ steam is introduced, the gas-liquid volume ratio is 10:1, after steam condensation, it is adsorbed by an adsorption tower and discharged.

[0024] Preferably, the mass ratio of the sludge and 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] In this process, ClO - On the one hand, it further oxidizes the COD not removed in step two, and on the other hand, it oxidizes ammonia nitrogen into N2 and is discharged with steam. The steam also further carries out the excess ammonia nitrogen in the sludge. After condensation and adsorption, the ammonia nitrogen content is significantly reduced and can be directly discharged.

[0027] Step four, phosphorus recovery

[0028] The sludge treated in step three is filtered to remove the filtrate, MgCl2·H2O and CaHPO4 are sequentially added to the sludge, 200-300rpm stirring is carried out for 30-40min, and then the insoluble impurities are removed by filtration.

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

[0030] Step three only removes free ammonia nitrogen (NH4 + And NH3) in the sludge, and has limited effect on organic nitrogen (such as proteins, amino acids, etc.) and complex ammonium in the solid phase of the sludge. In the process of step four, the hydrolysis of CaHPO4 provides an acidic environment, which promotes the release of NH 4+ , HPO4 - , Ca 2+ , Mg2+ It will release NH 4+ The formation of struvite allows for the recovery of phosphorus.

[0031] Step 5, heavy metal curing

[0032] Mix the sludge and mineral materials treated in step four, add an alkaline solution to adjust the pH to 10-11, then add Na2HPO4 solution and NaHCO3 powder in sequence, stir at 200-300 rpm for 30-40 min, filter, remove the filtrate and mineral materials, add an organic acid solution to the filtered sludge to adjust the pH to 4.5-6.0, then add Na2S solution, stir at 200-300 rpm for 20-30 min, filter, remove the filtrate, and retain the sludge.

[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 the following: citric acid solution with a mass fraction of 10-12%, acetic acid solution with a volume fraction of 15-20%, lactic acid solution with a mass fraction of 12-15%, tartaric acid solution with a mass fraction of 10-12%, and malic acid solution with a mass fraction of 10-15%.

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

[0038] During this process, the mineral material adsorbs some of the free heavy metal ions (such as Hg) in the solution. 2+ Ni 2+ Cd 2+ and Pb 2+ (etc.), PO4 under alkaline conditions 3- (including PO4 that was not completely removed in step four) 3- ) and Cd 2+ and Pb 2+ When insoluble phosphates are formed (such as Pb3(PO4)2), bioavailability is reduced, and CO3... 2- Will with Zn 2+ Cu 2+and S form basic carbonate precipitates, S 2- and As 3+ and Hg 2+ to form extremely stable sulfides.

[0039] Step six, the product is utilized

[0040] After step five, the sludge is dried at 30-40 DEG C to a water content of less than or equal to 10%, and then crushed to a particle size of less than or equal to 5 mm, and can be reused as a plant cultivation substrate or brick-making raw material.

[0041] The present application has the following advantages:

[0042] (1) Efficient dewatering and pollution source reduction, the present application dewatering sludge to a water content of 50-60%, greatly reducing the volume and energy consumption of subsequent treatment, the composite adsorbent has a large specific surface area, multi-level pores, multiple active sites and other characteristics, can simultaneously and efficiently adsorb free heavy metal ions, ammonia nitrogen and organic pollutants, achieving preliminary removal of pollutants, laying a foundation for subsequent deep treatment.

[0043] (2) The present application uses calcium peroxide and the synergistic effect of microwave and temperature to deeply oxidize COD, the microwave frequency matches the O-O bond vibration frequency in H2O2, causing molecular resonance, greatly reducing the activation energy of hydrogen peroxide decomposition to generate strong oxidizing hydroxyl radicals, and the oxidation efficiency is much higher than that of conventional heating, the reaction generates Ca 2+ which can form a short [Ca(OH)] + complex with hydroxyl radicals, prolonging the life of the radicals and improving the oxidation efficiency, and the energy consumption is relatively low, without secondary pollution, avoiding the strong acid conditions and iron sludge problems of the prior art.

[0044] (3) Stripping-chemical oxidation coupled deep denitrification, the present application innovatively combines hypochlorite oxidation with steam stripping, ClO-oxidizes residual COD and dissolved ammonia nitrogen into harmless substances such as N2, and high-temperature steam significantly improves the volatility of ammonia nitrogen, which is efficiently stripped out of the sludge solid phase, the ammonia-containing steam is treated by condensation+adsorption tower and then discharged, avoiding air pollution, and this process can efficiently remove dissolved ammonia nitrogen.

[0045] (4) Acid-driven struvite crystallization for phosphorus recovery is carried out after ammonia nitrogen removal, which ingeniously uses the hydrolysis of CaHPO4 to provide an acidic environment, the acidic conditions promote the dissolution of insoluble phosphate in the sludge solid phase, and release NH4 + from residual organic nitrogen and complex ammonium, and generate struvite crystals with Mg 2+ , PO4 3- . This process not only efficiently recovers phosphorus resources in the sludge, but also effectively removes organic nitrogen and complex ammonium in the solid phase, making denitrification more thorough, and avoiding the problem that the existing stripping method has limited effect on the solid phase nitrogen.

[0046] (5)Hierarchical stabilization of heavy metal ions, in the present application, under alkaline conditions of pH 10-11, the addition of mineral materials preliminarily adsorbs heavy metal ions, PO4 3- (From Na2HPO4 and residual phosphorus) and Cd 2+ , Pb 2+ , etc. to form insoluble phosphates, CO3 2- and Zn 2+ , Cu 2+ , etc. to form basic carbonates precipitate, the mineral material provides a large adsorption surface and enhances the stability of the precipitate. Then adjust the pH to weakly acidic, when the acidity is weak, S 2- and As 3+ , Hg 2+ form extremely stable and extremely low solubility sulfides (such as As2S3 (orpiment), HgS (cinnabar)).

[0047] This process is a hierarchical treatment of first alkaline and then acid, which preferentially treats heavy metals that are easy to form hydroxides / carbonates / phosphates under alkaline conditions, and then specifically stabilizes As 3+ , Hg 2+ , etc. under weakly acidic conditions, significantly reducing the bioavailability and leaching toxicity of heavy metals. Avoid the problem that existing stabilization methods are difficult to effectively immobilize multiple heavy metal ions and have insufficient long-term stability.

[0048] (6) The present application integrates dewatering, oxidation, denitrification, phosphorus recovery, and heavy metal stabilization in one, and the final product can be used as a plant cultivation substrate or brick making raw material after simple drying and crushing, truly realizing the ecological recycling goal of turning waste into treasure and treating and utilizing at the same time. Solving the problem that the prior art stops at reduction or preliminary stabilization, and it is difficult to produce high value-added and safe resource products. DETAILED DESCRIPTION

[0049] The technical solutions in the embodiments of the present application will be described below in a clear and complete manner. Obviously, the described embodiments are only some of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present application.

[0050] Embodiment 1

[0051] Step one, preliminary removal of pollutants

[0052] The municipal sludge with water content of 80% or more is filtered to remove insoluble impurities. The sludge is dewatered to a water content of 55% by a screw extruder and mixed with a composite adsorbent at a mass ratio of 50:1. The sludge is adsorbed at a humidity of 45% and a temperature of 60°C for 2 hours at a stirring speed of 120 rpm, and then filtered to remove the composite adsorbent. The composite adsorbent is activated carbon, diatomite, SiO2, Al2O3 and magnetic Fe3O4 particles at a mass ratio of 10:5:4:2:3.

[0053] Step two, COD oxidation

[0054] The sludge treated in step one is mixed with calcium peroxide at a mass ratio of 25:1, and then treated by microwave at 2450 MHz and 90°C for 20 min.

[0055] Step three, ammonia nitrogen removal

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

[0057] Step four, phosphorus recovery

[0058] The sludge treated in step three is filtered to remove the filtrate. MgCl2·H2O and CaHPO4 are sequentially added to the sludge, which is stirred at 250 rpm for 35 min, and then filtered to remove insoluble impurities. The mass ratio of the sludge, MgCl2·H2O and CaHPO4 is 100:0.5:1.

[0059] Step five, heavy metal solidification

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

[0061] Test example 1

[0062] 1. Experimental materials

[0063] Raw material: municipal sludge from a sewage treatment plant, water content 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 procedure and detection method

[0065] Table 1 Experimental procedure and detection method

[0066]

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

[0068] Table 2 Removal effect of main pollutants

[0069]

[0070] Table 3 Heavy metal solidification effect

[0071]

[0072] Table 4 Heavy metal form distribution change

[0073] Cd residue state proportion Pb residue state proportion Hg residue state proportion raw sludge 12％ 35％ 8％ final sludge 89％ 93％ 95％

[0074] From Tables 2-4, it can be seen that the microwave catalytic calcium peroxide makes the COD removal rate reach 98%, the stripping-hypochlorite coupling technology removes 96% of ammonia nitrogen, and the ammonia nitrogen in the condensed water is less than 10 mg / L, the acid environment drives the struvite crystallization to recover 82% of phosphorus, and the solid-phase organic nitrogen is removed simultaneously, the graded mineral solidification makes the residual heavy metal account for more than 89%, the TCLP leaching concentration is far lower than the national standard, and the garden substrate requirements are met.

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

Claims

1. A method for the ecological treatment of municipal sludge, characterized in that, The method comprises the following steps: Step one, municipal sludge is dewatered to a moisture content of 50-60%, and a composite adsorbent is added to the dewatered sludge, which is stirred and adsorbed, and then the composite adsorbent is removed by filtration; Step two, the sludge treated in step one is mixed with calcium peroxide, and then subjected to microwave treatment at 2400-2500 MHz and 80-95℃; Step three, the sludge treated in step two is mixed with a hypochlorite solution, and then fed into a stripping tower and subjected to steam treatment at 90-120℃; Step four, the sludge treated in step three is filtered to remove the filtrate, and then MgCl2·H2O and CaHPO4 are added to the sludge in sequence, which is stirred and then filtered to remove insoluble impurities; Step five, the sludge treated in step two is mixed with a mineral material, and then an alkali solution is added to adjust the pH to 10-11, and then Na2HPO4 solution and NaHCO3 are added in sequence, which is stirred, filtered, and then the filtrate and the mineral material are removed, and then 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, which is stirred, filtered, and then the filtrate is removed, and the filter residue is the treated sludge.

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

3. The method for ecological treatment of municipal sludge according to claim 1, characterized in that, The composite adsorbent in step one is active carbon, diatomite, SiO2, Al2O3, and magnetic Fe3O4 particles, and the mass ratio is 10:5:4:2:

3.

4. The method for ecological treatment of municipal sludge according to claim 1, characterized in that, The mass ratio of the sludge to calcium peroxide in step two 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 to the hypochlorite solution in step three is (10-20):

1.

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

7. A method for ecologically treating municipal sludge according to claim 1, characterized in that, The mass ratio of the sludge, MgCl2·H2O, and CaHPO4 in step four is 100:0.5:

1.

8. A method for ecologically treating municipal sludge according to claim 1, characterized in that, The mass ratio of the sludge, the 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 alkali solution in step five is NaOH solution with a mass fraction of 10-15% or KOH solution with a mass fraction of 10-15%.

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

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