A method for treating landfill leachate by supercritical CO2 oxidation

Abstract

The application discloses a method for treating landfill leachate by supercritical CO2 oxidation, which comprises the following steps: firstly, carrying out flocculation and sedimentation pretreatment on the landfill leachate; introducing nitrogen into a gas-tight reaction kettle; discharging mixed gas; adding the landfill leachate after the flocculation and sedimentation pretreatment into the reaction kettle; simultaneously introducing O2 and CO2 gas; controlling the reaction temperature of the reaction kettle; and making the reaction system be in a supercritical state; and after the reaction, the pollutants in the landfill leachate are degraded into non-toxic substances such as CO2, N2 and H2O. The application makes full use of the characteristics that CO2 has strong dissolving capacity for most organic matters and O2 under supercritical conditions, and the pollutants such as COD, NH3-N and heavy metal ions in the landfill leachate can be efficiently removed at a lower temperature, the reaction is rapid, the effluent is colorless and transparent, there is no secondary pollution, and the application prospect is extremely wide. Furthermore, the application realizes the reduction and resource utilization of CO2.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sewage treatment, in particular to a method for treating landfill leachate by supercritical CO2 oxidation. BACKGROUND

[0002] With the continuous development of industry, the rapid improvement of national economy, the continuous acceleration of urbanization, the rapid increase of urban population, the total amount of municipal solid waste also increases at a rate of about 10% per year, and the types are also increasing. At present, sanitary landfill is still the main way of garbage disposal, and the treatment amount accounts for about 80% of the total garbage. During the landfill and stacking process, due to compaction, fermentation and other physical, biological and chemical effects, as well as the seepage effect of precipitation and other external water, a kind of high-concentration liquid containing organic and inorganic substances is produced, which is called landfill leachate. Landfill leachate contains a large amount of organic pollutants, humus, heavy metals, microbial metabolites, salt ions, etc. And has the following characteristics: poor biodegradability, low B / C, high organic matter concentration; high content of inorganic salt components; large variation of water quality and quantity with time; high color and high hardness; high concentration of heavy metals. If landfill leachate is directly discharged into rivers, reservoirs and other receiving water bodies without harmless treatment, it will seriously damage the ecological environment. For example, some receiving water bodies are used for irrigation, which seriously pollutes crops and aquatic organisms, and enters the human body through the food chain, causing various diseases; drinking surface water contaminated by leachate will seriously endanger human health. At the same time, landfill leachate contains a variety of heavy metal ions, and some heavy metal ions cannot be degraded in the natural environment and are continuously deposited and accumulated in the soil in surface water runoff, polluting the soil.

[0003] The harmless treatment technology of landfill leachate has been a difficult problem for landfill sites at home and abroad. If not properly treated, it will cause serious pollution to the surrounding environment, bottom soil and groundwater of the landfill site. At present, the main methods for treating landfill leachate include landfill leachate incineration, fly ash solidification and humidification, MVR evaporation method and advanced oxidation technology, etc. Among them, landfill leachate incineration has the disadvantages of reducing power generation, boiler corrosion and increasing fly ash; fly ash solidification and humidification will produce a large amount of fly ash, thereby increasing the difficulty of landfill leachate treatment; MVR evaporation technology has the disadvantages of serious scaling, evaporation residue lacking suitable disposal, high energy consumption and high investment cost; the combination process of advanced oxidation technology also has the problems of complex process and high operation cost. These methods cannot completely and efficiently remove pollutants, and there are still many problems in practical application. Therefore, the development of purification treatment technology for such high-concentration and difficult-biodegradable wastewater to make it meet the discharge standard is a hot and difficult point in the field of wastewater treatment technology research.

[0004] In recent years, the research of green chemistry has been generally valued, the green chemical process research is still in the level of theory and laboratory exploration, and still can not become a real sense of the productive forces. Among them, using supercritical CO2 to replace the toxic and harmful organic solvents and developing the research of chemical reaction in supercritical CO2 has become a front hot field. Supercritical CO2 has moderate critical constant (Tc=304.3K, Pc=7.39MPa), is not easy to react with solute, stable chemical property, colorless and odorless, non-toxic and non-polluting environment, non-corrosive, sufficient source and the like, and has very strong solubility to most organic compounds at low temperature and pressure, has very large solubility to oxygen, in addition to the relative low viscosity, high diffusion coefficient and high molecular energy of supercritical CO2, so that the reaction in supercritical CO2 can form a homogeneous system, so that the original process of heterogeneous reaction becomes homogeneous reaction, which is helpful to improve the reaction rate and reaction degree. Therefore, supercritical CO2 has a wide application prospect in the field of supercritical fluid technology, and strengthening the research and reasonably and efficiently using CO2 have great significance to solve the problems of energy, environment and chemical industry. The inventor team of the present application tries to use supercritical CO2 as the reaction medium of oxidized landfill leachate, so as to find an environment-friendly green process for treating landfill leachate. SUMMARY

[0005] In view of the deficiencies of the current landfill leachate treatment method, the present application provides a kind of efficient, clean supercritical CO2 oxidation method for treating landfill leachate.

[0006] The technical scheme of the present application is as follows:

[0007] A kind of supercritical CO2 oxidation method for treating landfill leachate, the steps are as follows:

[0008] Step 1, the landfill leachate original solution is subjected to flocculation and sedimentation pretreatment;

[0009] Further, 5% to 10% of polymeric ferric chloride is added to the landfill leachate original solution by mass percentage, and the mixture is stirred uniformly and settled for 10 to 25 minutes, and the precipitate is removed by filtration.

[0010] The flocculation and sedimentation pretreatment process can also remove particles and suspended solids in the water quality, prevent the blockage of the treatment system, and improve the treatment effect.

[0011] Step 2, supercritical CO2 oxidation reaction treatment

[0012] The reaction kettle is connected with nitrogen gas, mixed gas is discharged, the flocculation and precipitation pretreated landfill leachate is added into the reaction kettle, O2 and CO2 gas are introduced, the reaction temperature of the reaction kettle is controlled, the reaction system is in supercritical state, after a period of reaction, the pollutants in the landfill leachate are degraded into non-toxic substances, and the reaction gas phase and liquid phase products are collected and analyzed.

[0013] Further, before the reaction starts, the high-pressure nitrogen gas cylinder outlet valve is opened, high-pressure nitrogen gas enters the reaction kettle, and the airtightness of the reaction kettle and each valve is checked; in the case of good airtightness, the flocculation and precipitation pretreated landfill leachate is added into the reaction kettle; O2 and CO2 gas are introduced through the air valve; the electric heating device and the temperature control device are opened, and the temperature in the reaction kettle is controlled to keep at the set reaction temperature. At the same time, the water cooling device is opened to cool the reaction kettle; under the joint action of the electric heating device, the temperature control device and the water cooling device, when the temperature of the reaction kettle reaches the set reaction temperature and stabilizes, the timing starts, and after the set reaction time is reached, the reaction is completed, and the electric heating device is closed. After the temperature in the reaction kettle cools to below 50℃, the reaction gas phase and liquid phase products are collected and analyzed.

[0014] Further, in the supercritical CO2 oxidation reaction treatment process, the temperature in the reaction kettle is adjusted to 360-440℃; the oxygen required for complete oxidation is calculated according to the COD concentration in the leachate, the oxidation coefficient is controlled to be 3-7 according to the reading of the pressure gauge, the CO2 partial pressure is adjusted to 0.2-0.6MPa according to the reading of the pressure gauge, and the reaction time is controlled to be 10-50min. The oxidation coefficient is calculated according to the ratio of the actual oxygen added to the theoretical oxygen required for complete oxidation of the landfill leachate entering the reaction kettle.

[0015] Further, the temperature in the reaction kettle is controlled by the electric heating device and the temperature control device, so that the reaction system is in a supercritical state, and the high-concentration organic matter in the landfill leachate is rapidly decomposed.

[0016] Further, the landfill leachate treated by supercritical CO2 oxidation is sampled and analyzed, the gas is collected through the gas outlet valve, the COD is measured by potassium dichromate titration, the NH3-N is measured by Nash reagent spectrophotometry, and the mass concentration of metal ions in the liquid phase is measured by inductively coupled plasma emission spectrometry.

[0017] Compared with the prior art, the beneficial effects of the present application are:

[0018] The application utilizes the feature that CO2 has strong dissolving capacity for most organic matters and O2 under supercritical condition, and the refractory organic matters in landfill leachate can be removed efficiently at low temperature, the refractory organic matter pollutants in landfill leachate are degraded into non-toxic and non-hazardous substances such as CO2, N2, H2O and small molecular organic matters, the mass concentration of COD and NH3-N in the effluent reaches the secondary discharge standard in the Integrated Wastewater Discharge Standard GB8978-1996, the process is simple, the operability is good, the effluent quality is stable, there is no secondary pollution, and the application prospect is extremely wide. Moreover, the application realizes the reduction and resource utilization of CO2. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 It is a supercritical CO2 oxidation system flow chart;

[0020] Figure 2 It is the influence of temperature on the mass concentration and removal rate of COD and NH3-N in effluent;

[0021] Figure 3 It is the influence of CO2 input amount on the mass concentration and removal rate of COD and NH3-N in effluent;

[0022] Figure 4 It is the influence of oxidation coefficient on the mass concentration and removal rate of COD and NH3-N in effluent;

[0023] Figure 5 It is the influence of residence time on the mass concentration and removal rate of COD and NH3-N in effluent;

[0024] Figure 6 It is the mass concentration of heavy metal ions in landfill leachate before and after supercritical CO2 oxidation treatment. DETAILED DESCRIPTION

[0025] The method of the application is further illustrated by the following examples:

[0026] Example

[0027] The device used in the example includes a supercritical CO2 oxidation reactor, an oxygen cylinder, a carbon dioxide cylinder, a temperature controller, a cooler, a pipeline, a valve and the like, all of which are common equipment and instruments, and the supercritical CO2 oxidation system flow chart is shown in Figure 1 .

[0028] The landfill leachate is collected from a landfill site in Dalian, the original leachate is black-brown, has more suspended solids and heavy odor, the PH is 8.07, the COD is 9800 mg / L, and the NH3-N is 2655 mg / L.

[0029] The method for treating landfill leachate by supercritical CO2 oxidation is as follows.

[0030] 1. Flocculation and precipitation pretreatment of raw landfill leachate

[0031] Add 7% of polymeric ferric chloride to the raw landfill leachate by mass percentage, stir uniformly, and stand for 20 minutes. Remove the precipitate by filtration.

[0032] The flocculation and precipitation pretreatment process has a certain effect on the removal of particles and suspended solids in the influent, prevents the clogging of the treatment system, and improves the treatment effect.

[0033] After flocculation and precipitation, the COD and NH3-N concentrations of the landfill leachate are 8400 mg / L and 2465 mg / L, respectively.

[0034] 2. Supercritical CO2 oxidation reaction treatment

[0035] Before the reaction starts, open the high-pressure nitrogen cylinder outlet valve, and let the high-pressure nitrogen enter the reaction kettle. Check the airtightness of the reaction kettle and each valve. Under good airtightness, add the flocculation and precipitation pretreated landfill leachate to the reaction kettle. Pass O2 and CO2 gases through the aeration valve. Turn on the electric heating device and temperature control device to control the temperature in the reaction kettle to maintain the set reaction temperature. At the same time, turn on the water cooling device to cool the reaction kettle. Under the combined action of the electric heating device, temperature control device, and water cooling device, when the temperature of the reaction kettle reaches the set reaction temperature and stabilizes, start timing. After reaching the set reaction time, the reaction ends, and the electric heating device is turned off. After the temperature in the reaction kettle cools to below 50°C, collect the gas and liquid phase products and analyze them.

[0036] 3. Gas-liquid separation treatment

[0037] Sample and analyze the landfill leachate after supercritical CO2 oxidation treatment. Collect the gas from the gas outlet valve. Measure the COD by potassium dichromate titration, the NH3-N by Nash reagent spectrophotometry, and the mass concentration of metal ions in the liquid phase by inductively coupled plasma emission spectroscopy.

[0038] During the supercritical CO2 oxidation reaction treatment process, adjust the oxidation coefficient to 6, the CO2 input amount to 0.5 MPa, and control the residence time to 40 minutes. Keep these parameters unchanged, and adjust the temperature in the reaction kettle to 360, 380, 400, 420, and 440°C, respectively. The effluent mass concentration and removal rate of the landfill leachate under different temperatures are shown in Table 1. Figure 2

[0039] ​In the supercritical CO2 oxidation reaction treatment process, the temperature is adjusted to 440℃, the oxidation coefficient is 6, and the residence time is controlled to 40 min. Keeping these parameters unchanged, the CO2 input amount in the reaction kettle is adjusted to 0.2, 0.3, 0.4, 0.5, and 0.6 MPa, respectively. The effluent mass concentration and removal rate of landfill leachate under different CO2 input amounts are shown in Figure 3 .

[0040] In the supercritical CO2 oxidation reaction treatment process, the temperature is adjusted to 440℃, the CO2 input amount is 0.5 MPa, and the residence time is controlled to 40 min. Keeping these parameters unchanged, the oxidation coefficient in the reaction kettle is adjusted to 3, 4, 5, 6, and 7, respectively. The effluent mass concentration and removal rate of landfill leachate under different oxidation coefficients are shown in Figure 4 .

[0041] In the supercritical CO2 oxidation reaction treatment process, the temperature is adjusted to 440℃, the CO2 input amount is 0.5 MPa, and the oxidation coefficient is 6. Keeping these parameters unchanged, the residence time in the reaction kettle is adjusted to 10, 20, 30, 40, and 50 min, respectively. The effluent mass concentration and removal rate of landfill leachate under different residence times are shown in Figure 5 .

[0042] The above reaction conditions are as follows: the temperature in the reaction kettle is 440℃, the CO2 input amount is 0.5 MPa, the oxidation coefficient is 6, and the residence time is 40 min. Under these conditions, the removal rates of COD and NH3-N are 98.99% and 99.1%, respectively, the effluent mass concentrations of COD and NH3-N are 85 mg / L and 22 mg / L, respectively, which have reached the secondary discharge standard in the "Integrated Wastewater Discharge Standard" GB8978-1996, and the pollutants in the landfill leachate are degraded into non-toxic substances such as CO2, N2, and H2O.

[0043] At the same time, as shown in Figure 6 , under the above reaction conditions, other conditions remain unchanged, and the CO2 input amount is changed to 0, the content of heavy metal ions in the landfill leachate is greatly reduced. By comparing the removal rates of COD and NH3-N and heavy metal ions under the conditions of 0 MPa and 0.5 MPa CO2 input amount, it is proved that the supercritical CO2 condition is more conducive to the conversion of heavy metal ions into metal oxides and metal salts.

Claims

1. A method for treating landfill leachate by supercritical CO2 oxidation, characterized by, The steps are as follows: Step 1, flocculation and sedimentation pretreatment of raw landfill leachate; Step 2, supercritical CO2 oxidation reaction treatment; Nitrogen gas is introduced into a reaction kettle with good airtightness, and mixed gas is discharged. The flocculation and sedimentation pretreated landfill leachate is added to the reaction kettle, and O2 and CO2 gases are introduced at the same time. The reaction temperature of the reaction kettle is controlled to make the reaction system in a supercritical state. After a period of reaction, the pollutants in the landfill leachate are degraded into non-toxic substances. The reaction gas phase and liquid phase products are collected and analyzed. During the supercritical CO2 oxidation reaction treatment process, the temperature in the reaction kettle is adjusted to 360-440℃. The oxygen required for complete oxidation is calculated according to the COD concentration in the leachate. The oxidation coefficient is controlled to be 3-7 according to the pressure gauge reading. The CO2 partial pressure is adjusted to 0.2-0.6 MPa according to the pressure gauge reading. The reaction time is controlled to be 10-50 min. The oxidation coefficient is calculated according to the ratio of the actual oxygen added to the theoretical oxygen required for complete oxidation of the landfill leachate entering the reaction kettle.

2. The method of claim 1, wherein the supercritical CO2 oxidation method is characterized by, According to the mass percentage, 5-10% of polymeric ferric chloride is added to the raw landfill leachate, and stirred uniformly. After standing and settling for 10-25 min, the precipitate is removed by filtration.

3. The method of claim 1, wherein the supercritical CO2 oxidation method is characterized by, The flocculation and sedimentation pretreatment process can also remove particles and suspended solids in the influent water to prevent blockage of the treatment system.

4. The method of claim 1, wherein the supercritical CO2 oxidation method is characterized by, Before the reaction starts, open the outlet valve of the high-pressure nitrogen cylinder, and high-pressure nitrogen enters the reaction kettle to check the airtightness of the reaction kettle and valves. In the case of good airtightness, the flocculation and sedimentation pretreated landfill leachate is added to the reaction kettle. O2 and CO2 gases are introduced through the air valve. The electric heating device and temperature control device are turned on to control the temperature in the reaction kettle to maintain the set reaction temperature. At the same time, the water cooling device is turned on to cool the reaction kettle. Under the joint action of the electric heating device, temperature control device and water cooling device, when the temperature of the reaction kettle reaches the set reaction temperature and stabilizes, the timing starts. When the set reaction time is reached, the reaction is complete, and the electric heating device is turned off. After the temperature in the reaction kettle cools down to below 50℃, the reaction gas phase and liquid phase products are collected and analyzed.

5. The method of claim 1, wherein the supercritical CO2 oxidation method is characterized by, The temperature in the reaction kettle is controlled by the electric heating device and temperature control device to make the reaction system in a supercritical state. The high-concentration organic matter in the landfill leachate is rapidly decomposed.

6. The method of claim 1, wherein the supercritical CO2 oxidation method is characterized by, The landfill leachate treated by supercritical CO2 oxidation is sampled and analyzed. The gas is collected from the gas outlet valve. The COD is measured by potassium dichromate titration, the NH3-N is measured by Nash reagent spectrophotometry, and the mass concentration of metal ions in the liquid phase is measured by inductively coupled plasma emission spectrometry.

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