CN112939384A

CN112939384A - Method for degrading heavy metals in sludge by combination of alkaline hydrolysis and electrolysis

Info

Publication number: CN112939384A
Application number: CN202110109031.0A
Authority: CN
Inventors: 周集体; 柳懿函
Original assignee: Dalian University of Technology
Current assignee: Dalian University of Technology
Priority date: 2021-01-27
Filing date: 2021-01-27
Publication date: 2021-06-11

Abstract

The invention provides a method for degrading heavy metals in sludge by combination of alkaline hydrolysis and electrolysis, belonging to the technical field of sludge resource utilization. Sieving and settling sludge in a secondary sedimentation tank of a sewage treatment plant, removing supernatant, breaking cells of microorganisms in the sludge by adopting a thermokalite combined method, releasing inner dissolved matters in the microorganisms, and preparing sludge lysate. Adding electrolyte into the sludge lysate which releases the intracellular heavy metals of the microorganisms, and adding an electric field and a circulating device. According to the invention, microbial cells are firstly broken through a thermokalite combination method to release intracellular heavy metals, and then the concentration of the heavy metals in the sludge hydrolysate is effectively degraded through electrolysis. The electrochemical method has the advantages of short degradation period, simple equipment, low energy consumption, no secondary pollution, contribution to resource recycling of subsequent sludge, controllable cost by controlling electrolysis operation parameters, economy, stability and reliability.

Description

Method for degrading heavy metals in sludge by combination of alkaline hydrolysis and electrolysis

Technical Field

The invention relates to a method for degrading heavy metals in sludge by combination of alkaline hydrolysis and electrolysis, belonging to the technical field of sludge resource utilization.

Background

In recent years, the total amount of wastewater discharged in China is increased year by year, and a large number of sewage treatment plants are built, so that the pollution of sewage to the environment is effectively relieved, but a large amount of sludge is generated in the process of treating sewage by applying methods such as activated sludge and the like.

The sludge contains rich nutrients necessary for the growth of various crops such as nitrogen, phosphorus, potassium, amino acid and the like, so the resource utilization prospect is very wide. However, after the sewage is treated, the heavy metals are absorbed and precipitated for a half, and are transferred from the sewage to the sludge, and although the concentration of the heavy metals is not high, the sludge is large in amount, and the sludge is accumulated for a long time, so that a large amount of heavy metals are enriched in the sludge.

Heavy metals have the characteristics of difficult migration, easy enrichment, long-term property, latency and the like in the environment, can be absorbed by plants, are enriched through a food chain, finally damage the ecological environment and threaten the health of human bodies. Therefore, the removal of heavy metal substances in the sludge has important practical significance for sludge recycling.

Sludge farming is now becoming the primary disposal of most european countries and the united states. China is a big agricultural country, and the agricultural application of the excess sludge is an ideal choice.

However, in many municipal sludges, the heavy metal content exceeds the agricultural standard of the sludge and cannot be directly used for agriculture. Therefore, it is imperative to remove heavy metals from sludge.

Because the sludge cells also contain a certain amount of heavy metals, the sludge cells need to be broken to completely release the heavy metals, and then the heavy metals are removed. Because other components in the sludge can be recycled subsequently, the condition for breaking the sludge cells needs to be controlled, so that the nutrient substances in the sludge cells keep activity, and the sludge cell breaking effect can be judged by using the content change of indexes such as COD (chemical oxygen demand), protein, polysaccharide and the like.

The sludge cell disruption method comprises the following steps: pyrolysis, physical, chemical, biological. Wherein the hot alkali combination has good promotion effect on sludge disintegration. In high concentration of OH^-When the sludge hydrolysis reaction kettle exists, extracellular polymers of the sludge are subjected to the action of alkali to accumulate a large amount of negative charges, so that electrostatic repulsion is generated, and an original stable colloid structure is destroyed, so that the cell wall is easier to generate hydrolysis reaction. Meanwhile, peptidoglycan, cellulose, lipid organic matters and other substances of the sludge cell wall are decomposed or lose activity under the action of alkali, so that the sludge cells cannot maintain normal osmotic pressure, and a large amount of intracellular substances are dissolved out. In addition, these reactions are endothermic reactions, and when the temperature is raised, the reaction equilibrium is shifted in the positive direction, and the effect of dissolving out organic substances is improved, so thatThe alkaline method and the thermal method are combined for use, so that the sludge cracking efficiency can be greatly improved, and a better cracking effect can be obtained.

The sludge cells are broken, so that after the heavy metals in the sludge cells are released, the heavy metals in the whole sludge system need to be degraded. At present, the treatment method of heavy metals in the sludge mainly comprises a chemical method, an electric restoration technology, a plant restoration technology, an adsorption method and the like. The electric repairing technology mainly utilizes the action of an external electric field to lead heavy metals to be gathered at the cathode through the actions of electrochemical dissolution and ion migration. The electric restoration technology can effectively avoid the defects of other treatment technologies, such as temporary solution and permanent solution of the stabilization technology, large acid consumption and treatment cost of a chemical treatment method, reduction of fertilizer value of sludge, bacterial survival limitation of a biological removal technology, sulfate pollution and the like.

The electric restoration technology can effectively remove heavy metals in the sludge, does not damage fertilizer components of the sludge, has no secondary pollution, can recover the heavy metals, has obvious advantages, and is a novel high-efficiency in-situ green restoration technology. The application of the electrochemical technology in sludge treatment is just started, and the technology is a novel technology for efficiently removing heavy metals, at present, the technology mainly focuses on the researches on repairing the soil polluted by the heavy metals and removing the heavy metals in sewage, and the adoption of thermokalite decomposition-electrochemical degradation of the content of the heavy metals in the sludge is a brand-new topic.

Disclosure of Invention

The invention aims to provide a method for degrading heavy metals in sludge by alkaline hydrolysis-electrolysis combination, which effectively solves the problems that the content of heavy metals is increased after deposition and the heavy metals cannot be directly recycled in agriculture due to large sludge amount and long standing time, realizes reduction and resource utilization of excessive residual sludge, and provides an electrochemical treatment method for treating the heavy metals in the sludge.

The technical scheme of the invention is as follows:

a method for degrading heavy metals in sludge by using alkaline hydrolysis-electrolysis combination comprises the following steps:

(1) passing the sludge in the secondary sedimentation tank through a 40-mesh screen, naturally settling, and removing supernatantPreparing 1mol/L NaOH solution, adding excessive alkali to adjust the pH of the sludge to 13-14, and then placing the sludge in a 30 ℃ shaking table to shake and mix for at least 12 hours. By OH in lye^-To hydrolyze or saponify cell walls, cell membrane proteins and lipopolysaccharides, destroy the floc structure of the sludge and the cell structure of microorganisms, and dissolve the intracellular heavy metals out of the cells to prepare sludge hydrolysate.

(2) Adding the sludge hydrolysate into an electrolytic tank, adding electrolyte, ensuring that the solution can fully flow in the whole electrolytic process by adding a circulating pump outside the electrolytic tank, controlling the electrolyte concentration to be not less than 0.1mol/L, controlling the rotating speed of the circulating pump to be 30-50 rpm, adjusting the voltage to be 20-40V, and ensuring the electrolytic time to be more than 12 h.

The content of heavy metals before and after the electrolysis of the experiment is determined by digesting the heavy metals and then applying an inductively coupled plasma spectrum generator. The specific digestion method comprises the following steps: putting 0.1g of sludge powder which is evaporated to dryness and ground into a digestion tank, adding a little deionized water for wetting, adding 10mL of newly prepared aqua regia, covering a digestion cap, setting a temperature rise program of a graphite digestion instrument, firstly heating for 1h at 120 ℃, cooling, then adding 10mL of aqua regia again, heating for 1h at 150 ℃, and then heating to 180 ℃ until the digestion reaction is finished (basically no solid residue exists in the digestion tank, and the solution is light yellow). Cooling, adding a little high-purity water, and adding 1mLH for each time₂O₂Until the reaction is no longer vigorous (note H)₂O₂Add no more than 10mL total), continue heating until 5mL solution remains, cool, add 10mL HCl, heat to 180 ℃ until 5mL solution remains.

The invention has the beneficial effects that: according to the invention, microbial cells are firstly broken through a thermokalite combination method to release intracellular heavy metals, and then the concentration of the heavy metals in the sludge hydrolysate is effectively degraded through electrolysis. The electrochemical method has the advantages of short degradation period, simple equipment, low energy consumption, no secondary pollution, contribution to resource recycling of subsequent sludge, controllable cost by controlling electrolysis operation parameters, economy, stability and reliability.

Drawings

FIG. 1 is a schematic diagram of a system used in the process of the present invention.

Detailed Description

The following further describes the specific embodiments of the present invention in combination with the technical solutions.

Example 1

(1) Hot alkali hydrolysis: screening the residual sludge at the secondary sedimentation tank of the sewage treatment plant to remove substances blocking pipelines, such as leaves, and the like, standing overnight, pouring out the supernatant, and measuring the sludge parameters, wherein the TCOD content of the sludge is 13392.02 mg/L. Preparing 1mol/L NaOH solution, slowly adjusting the pH value of the sludge solution to 13, putting the sludge solution in a shaking table with the temperature of 30 ℃ and the rpm of 150 for full reaction to ensure that sludge cells are fully broken, and after 12 hours, increasing the TCOD of the sludge by 55.1 percent.

(2) Electrolysis: adopting potassium dihydrogen phosphate as electrolyte, controlling the concentration of the electrolyte potassium dihydrogen phosphate contained in the sludge solution to be 0.1mol/L, adding the sludge hydrolysate after adding alkali and oscillating for 12 hours into an electrolytic cell, adding a circulating pump to ensure that the solution can fully flow in the whole electrolytic process, and adjusting the rotating speed of the circulating pump to be 30 rpm. Ruthenium-iridium noble metal is selected as an electrode, the electrode spacing is 18cm, the set voltage is 30V, and the current change condition in the electrolysis process is recorded. After electrolysis for 12h, samples were taken at 7 different positions 0, 3cm, 6cm, 9cm, 12cm, 15cm and 18cm away from the anode, dried at 105 deg.C for 10h, ground, and sieved with 100 mesh sieve.

(3) And (3) determining the content change of the heavy metal: after digestion is completed, liquid in the digestion tank (deionized water repeatedly washes the digestion tank and the digestion cap for 2-3 times) passes through a 0.22um membrane and is transferred to a 50mL volumetric flask, a nitric acid solution with a certain concentration is prepared for constant volume, so that the concentration of nitric acid in the liquid in the final volumetric flask is 5%, and the liquid in the 10mL volumetric flask is taken to be detected in a 10mL centrifugal tube. The ICP method is adopted to measure the contents of 12 heavy metals such as Cu, Pb, Cr, Ni, Zn, Hg, Cd and the like in the sludge before and after electrolysis, and the degradation rates of the heavy metals are 47.06%, 18.31%, 17.20%, 22.79%, 14.06%, 12.01%, 1.51% and the like respectively.

Example 2

(1) Hot alkali hydrolysis: and (3) taking the residual sludge at the secondary sedimentation tank of the sewage treatment plant, sieving, settling, pouring out supernatant, and measuring the TCOD content of the sludge to be 13392.02 mg/L. Preparing 1mol/L NaOH solution, slowly adjusting the pH value of the sludge solution to 14, putting the sludge solution in a shaking table with the temperature of 30 ℃ and the rpm of 150 for full reaction to ensure that sludge cells are fully broken, and increasing the TCOD of the sludge by 57.2 percent after 12 hours.

(2) Electrolysis: adopting potassium dihydrogen phosphate as electrolyte, controlling the concentration of the electrolyte potassium dihydrogen phosphate contained in the sludge solution to be 0.1mol/L, adding the sludge hydrolysate after adding alkali and oscillating for 12 hours into an electrolytic cell, adding a circulating pump to ensure that the solution can fully flow in the whole electrolytic process, and adjusting the rotating speed of the circulating pump to be 40 rpm. Ruthenium-iridium noble metal is selected as an electrode, the electrode spacing is 18cm, the set voltage is 30V, and the current change condition in the electrolysis process is recorded. After electrolysis for 24h, samples were taken at 7 different positions 0, 3cm, 6cm, 9cm, 12cm, 15cm, 18cm away from the anode, dried at 105 deg.C for 10h, ground, and sieved with 100 mesh sieve.

(3) And (3) determining the content change of the heavy metal: after digestion, the liquid in the digestion tank is subjected to membrane filtration, and the contents of 12 heavy metals such as Cu, Pb, Cr, Ni, Zn, Hg, Cd and the like in the sludge before and after electrolysis are measured, wherein the degradation rates of the heavy metals are 49.81%, 22.18%, 19.36%, 21.15%, 17.28%, 11.92%, 2.27% and the like respectively.

Claims

1. A method for degrading heavy metals in sludge by using alkaline hydrolysis-electrolysis combined technology is characterized by comprising the following steps:

(1) sieving sludge in a secondary sedimentation tank of a sewage treatment plant, naturally settling, removing supernatant, adding 1mol/L NaOH solution to adjust the pH of the sludge to 13-14, placing the sludge in a shaking table at 30 ℃ for shaking and mixing, carrying out cell breaking on microorganisms in the sludge by a physical and chemical combination method, releasing inner solutes of the microorganisms, and preparing sludge lysate;

(2) adding electrolyte into the sludge lysate, and ensuring the sufficient flow of the solution in the whole electrolysis process under the action of an external electric field and a circulating device; the concentration of the electrolyte is controlled to be not less than 0.1mol/L, the rotating speed of a circulating pump is controlled to be 30-50 rpm, the reaction temperature is 25-35 ℃, the voltage is adjusted to be 20-40V, and the electrolysis time is not less than 12 h.

2. The method of claim 1, wherein the electrolyte is selected from the group consisting of potassium dihydrogen phosphate, sodium sulfate, potassium chloride, and potassium nitrate.