CN115094095B - A method for promoting anaerobic fermentation of excess sludge to produce medium-chain fatty acids and recover phosphorus - Google Patents

Abstract

The invention provides a method for promoting recovery of medium-chain fatty acid and phosphorus in anaerobic fermentation production of excess sludge, belonging to the field of pollution control and recycling of solid waste. The invention provides a method for promoting recovery of medium-chain fatty acid and phosphorus in sludge anaerobic fermentation production, which comprises the following steps: (1) naturally settling the sludge to obtain settled sludge; (2) Mixing the settled sludge with ferrate, and regulating pH to be acidic after pretreatment to obtain tempered sludge; (3) Mixing the tempering sludge with an electron donor and inoculating sludge, and carrying out anaerobic fermentation to obtain a fermentation product rich in medium-chain fatty acid and blue iron stone. The invention breaks the sludge structure and carries out anaerobic fermentation on the sludge, so that the sludge hydrolysis, acidification and carbon chain extension microorganisms are in the same reactor, the sludge anaerobic fermentation is strengthened to synchronously realize medium chain acid generation and phosphorus recovery, the process cost is reduced, the sludge treatment efficiency is improved, and the operation is simplified.

Description

A method for promoting anaerobic fermentation of excess sludge to produce medium-chain fatty acids and recover phosphorus

Technical Field

The invention belongs to the technical field of solid waste pollution control and resource utilization, and specifically relates to a method for promoting the production of medium-chain fatty acids and phosphorus recovery by anaerobic fermentation of excess sludge.

Background Art

With the further acceleration of urbanization, the construction of sewage treatment plants has gradually increased, accompanied by a large amount of residual sludge generated during the biological treatment of sewage. The annual output of dry sludge is 11.2 million tons, and the cost of sludge treatment and disposal can be as high as 55% of the total operating cost of the sewage treatment plant. How to reasonably treat and dispose of sludge to avoid secondary environmental pollution has become one of the major challenges facing sewage treatment plants.

Excess sludge is rich in organic compounds, such as proteins and polysaccharides, accounting for about 40% to 80% of the dry weight of sludge. It is a potential resource to be utilized and can be used as a substrate for anaerobic fermentation to recover energy or resources to alleviate the growing energy consumption of sewage treatment plants. During the anaerobic fermentation process, by adding electron donors, the metabolic intermediates short-chain fatty acids (such as ethanol, propionic acid and butyric acid) or hydrogen/carbon dioxide generated by the anaerobic decomposition of sludge organic matter can be converted into medium-chain fatty acids through the carbon chain extension process. Medium-chain fatty acids refer to straight-chain monocarboxylic acids containing 6 to 12 carbons, including hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, undecanoic acid and dodecanoic acid. Medium-chain fatty acids have many advantages, such as strong hydrophobicity, easy separation and purification from liquid media, high energy density, and can be used in many industrial fields, such as energy (predecessors of renewable diesel and aviation fuel), medicine (antibacterial agents), and food (spices and food additives). On the other hand, excess sludge is also an important phosphorus sink, and its total phosphorus content can be as high as 500 mg P/L. Under anaerobic conditions, ferric iron is converted into divalent iron by the action of dissimilatory iron-reducing bacteria, and the generated divalent iron further reacts with phosphate released from sludge to form blue iron. Blue iron has a certain paramagnetic property and can be recovered from sludge by magnetic separation technology. Therefore, it is of great economic value to realize the simultaneous recovery of carbon and phosphorus resources in sludge by anaerobic fermentation to produce medium-chain acids and blue iron.

Sludge organic matter is composed of extracellular polymeric substances (EPS) and microbial cells. EPS has a highly polymerized complex structure, and the hard cell wall of microorganisms hinders the dissolution and hydrolysis of organic matter in the sludge, which will affect the production of medium-chain acids. At the same time, during the carbon chain extension process, methanogens in the sludge will compete with medium-chain microorganisms for electron acceptors, making it difficult for carbon chain extension to be carried out effectively. In addition, the sludge's own dissolution capacity is limited, and the phosphorus released into the liquid phase from the sludge is very limited, making it difficult to recover phosphorus from the sludge. Therefore, some pretreatment technologies are usually used to achieve sludge wall breaking and EPS cracking, such as ultrasound, acid/alkali, and heat treatment. Although these technologies can effectively break sludge flocs, they usually require a lot of energy consumption and only have a single function (i.e., breaking the sludge structure), which weakens the economy of sludge fermentation. Therefore, there is an urgent need to find more economical and effective pretreatment technologies.

Summary of the invention

The object of the present invention is to provide a method for promoting the production of medium-chain fatty acids and phosphorus recovery by anaerobic fermentation of sludge, by breaking the sludge structure and performing anaerobic fermentation on the sludge, so that the sludge hydrolysis, acidification and carbon chain extension microorganisms are in the same reactor, the anaerobic fermentation of the sludge is strengthened to synchronously realize the production of medium-chain acids and phosphorus recovery, thereby reducing the process cost, improving the sludge treatment efficiency and simplifying the operation.

In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

The present invention provides a method for promoting anaerobic fermentation of sludge to produce medium-chain fatty acids and phosphorus recovery, comprising the following steps:

(1) After the sludge settles naturally, the settled sludge is obtained;

(2) the settled sludge is mixed with ferrate, pretreated, and the pH is adjusted to acidic to obtain tempered sludge;

(3) The conditioned sludge is mixed with an electron donor and inoculated sludge for anaerobic fermentation to obtain a fermentation product rich in medium-chain fatty acids and blue iron stone.

Preferably, the sludge is sludge that has been biologically treated in a sewage treatment plant; the volatile suspended solids content of the settled sludge is 11 to 16 g/L.

Preferably, the temperature of the natural sedimentation is 22 to 28° C., and the time is 16 to 32 hours.

Preferably, the ferrate includes potassium ferrate and/or sodium ferrate; the dosage of the ferrate is 34-85 mg/g total suspended solids calculated as hexavalent iron.

Preferably, the mixing speed in step (2) is 220-380 rpm, the time is 3-6 min, and the temperature is 22-28° C.; the pretreatment speed in step (2) is 100-150 rpm, the time is 18-36 h, and the temperature is 20-40° C.

Preferably, the pH in step (2) is adjusted to 5.5-6.5.

Preferably, the electron donor in step (3) is ethanol or lactic acid; the final concentration of the electron donor is 5-10 g/L; and the volume ratio of the sludge to the inoculated sludge is 6-9:1.

Preferably, the fermentation speed in step (3) is 100-150 rpm, the fermentation time is 12-18 days, and the temperature is 30-40°C.

Preferably, the inoculated sludge is prepared according to the following steps:

(1) adding ethanol or lactic acid to the sludge to a final concentration of 5 to 10 g/L, then adding 5 to 10 g/L of sodium 2-bromoethane sulfonate in a ratio of 0.8 to 0.9:1 of the mass ratio of sodium 2-bromoethane sulfonate to the volatile suspended solids of the settled sludge, and performing anaerobically fermentation at 30 to 40° C. for 12 to 18 days to obtain acclimated sludge;

(2) heat-treating the acclimated sludge to obtain inoculated sludge.

Preferably, the heat treatment time in step (2) is 20 to 40 minutes and the temperature is 85 to 105°C.

The present invention provides a method for promoting the production of medium-chain fatty acids and phosphorus recovery by anaerobic fermentation of sludge, using potassium ferrate with strong oxidizing properties to release a certain amount of alkalinity during the reaction process to generate trivalent iron as a byproduct. The strong oxidizing property quickly decomposes sludge EPS and microbial cell walls, and the increase in pH is conducive to the dissolution of sludge protein, accelerating the hydrolysis and acidification of sludge organic matter, thereby providing a large amount of short-chain fatty acids for microorganisms producing medium-chain acids; after the ferrate reaction is completed, the trivalent iron generated in situ in the system can generate divalent iron through dissimilatory iron reduction, and this process can promote the hydrolysis and acidification of macromolecular organic matter dissolved in the sludge; at the same time, the generated divalent iron combines with the phosphate dissolved in the sludge to generate blue iron, thereby realizing phosphorus recovery.

The method of the present invention breaks the sludge structure and performs anaerobic fermentation on the sludge, so that the sludge hydrolysis, acidification and carbon chain extension microorganisms are in the same reactor, and the anaerobic fermentation of the sludge is strengthened to simultaneously realize the generation of medium-chain acids and phosphorus recovery, thereby reducing the process cost, improving the sludge treatment efficiency and simplifying the operation. The content of n-hexanoic acid in the fermentation product obtained by the present invention can reach 2826.7-8106 mg COD/L, the content of n-octanoic acid can reach 302.2-2113 mg COD/L, and the XRD spectrum shows that blue iron stone can be synthesized to realize phosphorus recovery.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a process flow chart of the present invention;

FIG2 is a comparison of the XRD diffraction pattern of the fermentation product obtained from the total suspended solids in Example 1 when the addition amount of potassium ferrate is 85 mg/g and the blue iron standard diffraction pattern; wherein the off-white columnar graph is the blue iron standard pattern; all arrows point to the blue iron peak;

Figure 3 is a comparison of the XRD diffraction pattern of the fermentation product obtained in Comparative Example 1 and the standard diffraction pattern of blue iron stone; wherein the off-white columnar pattern is the standard pattern of blue iron stone.

DETAILED DESCRIPTION

The technical solutions provided by the present invention are described in detail below in conjunction with the embodiments, but they should not be construed as limiting the protection scope of the present invention.

Example 1

The invention provides a method for promoting anaerobic fermentation of sludge to produce medium-chain fatty acids and recover phosphorus, using ethanol as an electron donor, and the specific process is as follows: selecting residual sludge from a municipal sewage treatment plant after biological treatment, naturally settling it at 25°C for 24 hours, removing the supernatant, and obtaining settled sludge, wherein the volatile suspended solid content of the settled sludge is 11.38 g/L. Potassium ferrate of 34, 51, and 85 mg/g of total suspended solids calculated as hexavalent iron is added to the settled sludge, respectively, and rapidly stirred at 25°C for 5 minutes at a speed of 300 rpm to fully mix the potassium ferrate with the sludge; and then pre-treating it at 35°C for 24 hours at a speed of 120 rpm. After the pre-treatment, the pH is adjusted to 5.5 to obtain tempered sludge. The conditioned sludge was transferred to an anaerobic fermentation tank as a fermentation substrate, nitrogen was charged into the sludge, and ethanol was added to the substrate until the ethanol concentration was 7.83 g/L as an electron donor for carbon chain extension. Then, the inoculated sludge was added at a volume ratio of 9:1 between the residual sludge and the inoculated sludge, and then the headspace of the fermentation tank was purged with nitrogen to ensure an anaerobic environment and sealed. The temperature of the fermentation tank was controlled at 35 ° C, the stirring intensity was controlled at 120 rpm, and the fermentation was carried out for 16 days. After the fermentation, the yield of medium-chain fatty acids in the three groups of fermentation products was detected by gas chromatography, and the results are shown in Table 1.

The preparation method of inoculated sludge is as follows: nitrogen is charged into another portion of settled sludge to remove oxygen from the sludge, ethanol is added to the substrate to a concentration of 7.83 g/L as an electron donor for carbon chain extension, and 10 g/L of sodium 2-bromoethane sulfonate is added as a methanogen inhibitor at a mass ratio of 0.88 to the volatile suspended solids of the settled sludge, and then nitrogen is charged into the headspace of the fermentation tank and sealed, and then placed in a constant temperature incubator at 35°C for anaerobic fermentation for 16 days. The fermented sludge is heat treated at 95°C for 30 minutes to remove potential methanogens to obtain inoculated sludge.

Table 1 Medium chain fatty acid production at different potassium ferrate addition levels

It can be seen from Table 1 that when the addition amount of potassium ferrate is 34 mg/g total suspended solids, the content of medium-chain fatty acids produced by the fermentation of residual sludge is the lowest, and n-hexanoic acid and n-octanoic acid can be detected at the same time; when the addition amount of potassium ferrate is 51 mg/g total suspended solids, the content of medium-chain fatty acids produced by the fermentation of residual sludge is relatively high, and n-hexanoic acid and n-octanoic acid can be detected at the same time; when the addition amount of potassium ferrate is 85 mg/g total suspended solids, the total content of medium-chain fatty acids produced by anaerobic fermentation of residual sludge is the highest, but n-octanoic acid is not detected. The fermentation product obtained when the potassium ferrate addition amount was 85 mg/g total suspended solids was subjected to XRD scanning and compared with the standard diffraction pattern of violet iron ore (PDF#30-0662Vivianite), as shown in Figure 2. It can be seen that the gray-white column is the peak position of the violet iron ore standard, and the fermentation product obtained has obvious peaks at 11.1°, 13.1°, and 18.1°, indicating that violet iron ore was generated when the potassium ferrate addition amount was 85 mg/g total suspended solids. It shows that the anaerobic fermentation of excess sludge according to the above steps can recover phosphorus while producing acid.

Example 2

The invention provides a method for promoting the production of medium-chain fatty acids and phosphorus recovery by anaerobic fermentation of sludge, using lactic acid as an electron donor, and the specific process is as follows: selecting residual sludge from a municipal sewage treatment plant after biological treatment, naturally settling it at 25°C for 24 hours, removing the supernatant, and obtaining settled sludge, wherein the volatile suspended solid content of the settled sludge is 11.38 g/L. Potassium ferrate is added to the settled sludge at 51 mg/g of total suspended solids calculated as hexavalent iron, and the potassium ferrate is rapidly stirred at 25°C for 5 minutes at a speed of 300 rpm to fully mix the potassium ferrate with the sludge; and then pre-treated at 35°C for 24 hours at a speed of 120 rpm. After the pre-treatment, the pH is adjusted to 5.5 to obtain tempered sludge. The conditioned sludge was transferred to an anaerobic fermentation tank as a fermentation substrate, nitrogen was charged into the sludge, and lactic acid was added to the substrate until the lactic acid concentration was 9g/L as an electron donor for carbon chain extension. Then, the inoculated sludge was added at a volume ratio of 9:1 between the residual sludge and the inoculated sludge, and then the headspace of the fermentation tank was purged with nitrogen to ensure an anaerobic environment and sealed. The temperature of the fermentation tank was controlled at 35°C, the stirring intensity was controlled at 120rpm, and the content of medium-chain fatty acids was detected by gas chromatography every 2 days for 16 days of fermentation. After the fermentation was completed, the yield of medium-chain fatty acids in the fermentation product was detected by gas chromatography.

The preparation method of inoculated sludge is as follows: nitrogen is charged into another portion of settled sludge to remove oxygen from the sludge, lactic acid is added to the substrate to a lactic acid concentration of 9 g/L as an electron donor for carbon chain extension, and 10 g/L of sodium 2-bromoethane sulfonate is added as a methanogen inhibitor at a mass ratio of 0.88 to the volatile suspended solids of the settled sludge, and then nitrogen is charged into the headspace of the fermenter and sealed, and then placed in a constant temperature incubator at 35°C for anaerobic fermentation for 16 days. The acclimated sludge is heat treated at 95°C for 30 minutes to remove potential methanogens to obtain inoculated sludge.

The fermentation product was prepared according to the above steps, and the yield of medium-chain fatty acids in the fermentation product was detected by gas chromatography. The final yield of hexanoic acid was 3600 mg COD/L, and the yield of octanoic acid was 1800 mg COD/L.

Comparative Example 1

(1) Select the residual sludge after biological treatment from the municipal sewage treatment plant, let it settle naturally at 25°C for 24h, remove the supernatant, and obtain the settled sludge, the volatile suspended solid content of which is 11.38g/L. Adjust the pH of the settled sludge to 5.5 to obtain the tempered sludge. Transfer the tempered sludge to an anaerobic fermentation tank as a fermentation substrate, then fill the sludge with nitrogen, and then add ethanol to the substrate until the ethanol concentration is 7.83g/L as an electron donor for carbon chain extension, then add the inoculated sludge at a volume ratio of 9:1 between the residual sludge and the inoculated sludge, and then purge the headspace of the fermentation tank with nitrogen to ensure an anaerobic environment and seal it. The temperature of the fermentation tank was controlled at 35°C, the stirring intensity was controlled at 120rpm, and the content of medium-chain fatty acids was detected by gas chromatography every 2 days for 16 days. After the fermentation, the yield of medium-chain fatty acids in the fermentation product was detected by gas chromatography.

The preparation method of inoculated sludge is as follows: nitrogen is charged into another portion of settled sludge to remove oxygen from the sludge, ethanol is added to the substrate to a concentration of 7.83 g/L as an electron donor for carbon chain extension, and 10 g/L of sodium 2-bromoethane sulfonate is added as an inhibitor of methanogens at a mass ratio of 0.88 to the volatile suspended solids of the settled sludge, and then nitrogen is charged into the headspace of the fermenter and sealed, and then placed in a constant temperature incubator at 35°C for anaerobic fermentation for 16 days. The acclimated sludge is heat treated at 95°C for 30 minutes to remove potential methanogens to obtain inoculated sludge.

The fermentation product was prepared according to the above steps, and the yield of medium-chain fatty acids in the fermentation product was detected by gas chromatography, and the final yield of n-hexanoic acid was 138.3 mg COD/L; the fermentation product was subjected to XRD scanning and compared with the standard diffraction pattern of violet iron ore (PDF#30-0662Vivianite), as shown in Figure 3, it can be seen that the gray-white column chart is the peak position of the violet iron ore standard, and the obtained fermentation product has no obvious peaks at 11.1°, 13.1°, and 18.1°, indicating that the anaerobic fermentation of the residual sludge according to the steps of Comparative Example 1 did not produce violet iron ore, phosphorus recovery could not be performed, and acid production and phosphorus recovery could not be performed simultaneously.

The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims (6)

1. A method for promoting anaerobic fermentation of sludge to produce medium-chain fatty acids and recover phosphorus, characterized in that it comprises the following steps: (1) After the sludge settles naturally, the settled sludge is obtained; (2) the settled sludge is mixed with ferrate, pretreated, and the pH is adjusted to acidic to obtain tempered sludge; The ferrate includes potassium ferrate and/or sodium ferrate; the dosage of the ferrate is 34-85 mg/g total suspended solids calculated as hexavalent iron; the pH is adjusted to 5.5; (3) mixing the conditioned sludge with an electron donor and inoculated sludge, and performing anaerobic fermentation to obtain a fermentation product rich in medium-chain fatty acids and blue iron stone; The electron donor is ethanol or lactic acid; the final concentration of the electron donor is 5-10 g/L; the volume ratio of the sludge to the inoculated sludge is 6-9:1; The inoculated sludge is prepared according to the following steps: (i) adding ethanol or lactic acid to the sludge to a final concentration of 5 to 10 g/L, then adding 5 to 10 g/L of sodium 2-bromoethane sulfonate in a ratio of 0.8 to 0.9:1 of the mass ratio of sodium 2-bromoethane sulfonate to the volatile suspended solids of the settled sludge, and performing anaerobically fermentation at 30 to 40° C. for 12 to 18 days to obtain acclimated sludge; (ii) heat-treating the acclimated sludge to obtain inoculum sludge. 2. The method according to claim 1, characterized in that the sludge is sludge after biological treatment in a sewage treatment plant; and the volatile suspended solids content of the settled sludge is 11 to 16 g/L. 3. The method according to claim 2, characterized in that the temperature of the natural sedimentation is 22 to 28°C and the time is 16 to 32 hours. 4. The method according to claim 3, characterized in that the mixing speed in step (2) is 220-380 rpm, the time is 3-6 min, and the temperature is 22-28°C; the pretreatment speed in step (2) is 100-150 rpm, the time is 18-36 h, and the temperature is 20-40°C. 5. The method according to claim 4, characterized in that the fermentation speed in step (3) is 100-150 rpm, the fermentation time is 12-18 days, and the temperature is 30-40°C. 6. The method according to claim 1, characterized in that the heat treatment in step (ii) is carried out for 20 to 40 minutes at a temperature of 85 to 105°C.