CN113462727A - Method for improving AD (AD) methane production efficiency by pretreating protein wastewater based on pH adjustment - Google Patents
Method for improving AD (AD) methane production efficiency by pretreating protein wastewater based on pH adjustment Download PDFInfo
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- C12P5/02—Preparation of hydrocarbons or halogenated hydrocarbons acyclic
- C12P5/023—Methane
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- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F9/00—Multistage treatment of water, waste water or sewage
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Abstract
The invention discloses a method for improving AD methanogenesis efficiency by pretreating protein wastewater based on pH adjustment, which comprises the following steps: step 1: centrifuging the anaerobic granular sludge, and cleaning with distilled water to obtain centrifugal granular sludge for later use; step 2: the protein wastewater is pretreated in an acidic or alkaline way, the pH value of the protein wastewater is adjusted, and the protein wastewater after the pH pretreatment is obtained by shaking in an air bath shaker; and step 3: readjusting the protein wastewater after the pH pretreatment to a neutral initial value to obtain neutral protein wastewater; then the neutral protein wastewater, the microelement stock solution and Na are added2HPO4、NaHCO3And placing the centrifugal granular sludge into a reaction tank for mixing, and then shaking on a shaking table for anaerobic digestion and collecting the produced methane. The method of the invention adjusts the pH valueThe protein wastewater is pretreated to expand the conformation and the structure of the protein, thereby eliminating the limit of low methanation rate of the protein wastewater in the AD process.
Description
Technical Field
The invention relates to the technical field of wastewater treatment, in particular to a method for improving AD (AD) methane production efficiency by pretreating protein wastewater based on pH adjustment
Background
High concentration organic waste water from slaughterhouses, fish, whey, casein, cheese and certain vegetable processing processes often contain large amounts of protein, which has a high energy density and a non negligible biological value. For example, more than 40% of the total chemical oxygen demand in dairy waste water is derived from protein, and how to efficiently treat and utilize such high protein waste water is a current challenge facing sewage treatment plants. Considering that high protein wastewater is rich in biodegradable organic nutrients, a great deal of research has been conducted on the biotransformation of such wastewater by biological methods. Conventionally, the aerobic biological treatment process is difficult to cope with high concentration of organic matters in wastewater and suffers from high load impact, so that the operational stability of the reactor is deteriorated. At the same time, the aerobic biological treatment unit needs to be additionally provided with oxygen, which leads to further increase of operation cost. In contrast, the anaerobic biological treatment method not only circumvents the disadvantages of the aerobic biological treatment method, but also effectively recovers energy, making it an ideal technology for treating high-concentration organic wastewater. Proteins with higher energy density possess significant biogas production potential as measured by Chemical Oxygen Demand (COD) quantification of the amount of organic matter in the waste stream compared to carbohydrates such as glucose, crop residues and livestock manure. According to Buswell's equation, the theoretical value of methane production per unit COD at standard temperature and pressure is 0.35L, the protein to COD conversion factor is 1.5, and the carbohydrate is only 1.07. Therefore, it is of great interest to study the process and efficiency of protein biotransformation to methane in Anaerobic Digestion (AD) systems.
AD is a metabolic microenvironment formed by the mutual cooperation of a plurality of functional bacteria under the anaerobic or anoxic condition, and the complex high-concentration organic wastewater is biologically convertedIs a process capable of replacing clean energy. AD is generally summarized as the following three steps: hydrolytic acidification, acetoxylation and methanogenesis. In particular, proteins are natural polymers formed from amino acid units joined to each other by peptide bonds (or amides). Proteins are first hydrolyzed by extracellular enzymes (called proteases) into oligomeric polypeptides and monomeric amino acids before being utilized by functional bacteria for conversion to methane. Which way the amino acid is then routed to the fermentation stage depends on the nature and concentration of the monomeric amino acid. However, the actual rate of proteolysis is rather slow and the methane production of AD is much lower (less than 0.5L CH)4Per gram of protein) due to system disturbances caused by direct impact of large amounts of highly loaded protein wastewater and increased risk of anaerobic microbial ammonia inhibition. Recent work has found that the stability and complexity of protein structure is another important factor that hinders methanation because proteins have a three-dimensional structure, with a multi-layered conformation that makes them less susceptible to cleavage by proteases in native folding, such as a stable hydrogen bonding network. Therefore, the adoption of proper pretreatment means is crucial to relieving the hydrolysis rate limitation of the protein wastewater in the AD methanation process.
In recent years, physical and chemical methods such as acid, alkali, heat, ultrasonic waves and ultraviolet rays are applied to a pretreatment process of protein-rich biomass such as sewage or sludge, dissolution of extracellular polymers in the sludge and cell wall breakage are promoted, a large amount of intracellular organic substances are released, and methane production is further promoted. However, the complexity of the biomass composition and the diversity of the sources of organic monomers after hydrolysis make it quite difficult to establish a direct link between the methanogenic properties of AD and the biomass composition, which can only be easily deduced using theoretical contributions. Based on this consideration, it is logical to explore the conversion relationship between hydrolysate and methanogenesis efficiency by pre-treating a single organic material, such as protein. Numerous studies have shown that proteolysis can be accelerated by protein denaturation, but these reports mostly focus on emphasizing the conversion of protein waste water to hydrogen or VFAs in AD systems. Thus, there remains a gap in the intrinsic relationship and mechanism between the unfolding of the native conformation and structure of proteins and the intensification of the hydrolysis and methanogenesis processes.
Disclosure of Invention
In order to solve the technical problems, the invention provides a method for improving AD methanogenesis efficiency by pretreating protein wastewater based on pH adjustment.
The technical scheme of the invention is as follows: a method for pretreating protein wastewater based on pH adjustment to improve the efficiency of AD methanogenesis comprises the following steps:
step 1: microbial preparation
Centrifuging the anaerobic granular sludge at 12000rpm for 10min to remove soluble organic matters, and washing with distilled water for 2 times to obtain centrifuged granular sludge for later use;
step 2: pH pretreatment
Changing the pH value of the protein wastewater through pH adjustment treatment, and shaking the protein wastewater in an air bath shaker at room temperature for 6-24 h to obtain protein wastewater subjected to pH pretreatment;
and step 3: anaerobic digestion
1) Readjusting the protein wastewater after the pH pretreatment to a neutral initial value of pH to obtain neutral protein wastewater;
2) then according to the proportion of 150mL of neutral protein wastewater, 1mL of trace element stock solution and 0.3g of Na2HPO4、1g NaHCO3And 15g of the centrifugal granular sludge is placed in a reaction tank for mixing, and meanwhile, helium is introduced to evacuate oxygen in the reaction tank, so that an oxygen-free environment in the reaction tank is ensured;
3) the temperature in the reaction tank was then controlled to 35. + -.1 ℃ and anaerobic digestion was carried out on a shaker at a shaking speed of 120rpm and the produced methane was collected.
According to the invention, the pH is adopted to pretreat the wastewater, so that the conformation and the structure of the protein can be developed, and the limit of low methanation rate of the protein wastewater in the AD process is eliminated.
Further, the pH adjustment processing in step S2 specifically includes: and (3) adopting acidic pretreatment, namely adjusting the pH value of the protein wastewater to 2-6 by 4M hydrochloric acid. The protein wastewater is pretreated by acidic pretreatment, so that the conformation and the structure of the protein are expanded, and the limit of low methanation rate of the protein wastewater in the AD process is eliminated.
Further, the pH adjustment processing in step S2 specifically includes: and (3) alkaline pretreatment is adopted, namely the pH value of the protein wastewater is adjusted to 8-12 by 4M sodium hydroxide. The alkaline pretreatment is carried out, particularly when the pH value is 12, the methane generation rate is obviously increased by 35.7 percent compared with the method without the pH pretreatment, and the methane generation rate reaches 149.6 +/-16.1 ml/g protein; this is due to the collapse of the hydrogen bond network caused by the C ═ O stretching vibration of the amide I band, which causes the secondary structure of the protein to undergo a transition from ordered to disordered; however, time economy evaluations indicate that the optimal protocol for alkaline pretreatment (pH 12) of protein wastewater requires a trade-off between time benefit and methane production rate benefit.
Further, the air bath shaking table in the step S2 specifically comprises: shaking was carried out at room temperature, 25 ℃ and a shaking speed of 100 rpm. The protein wastewater can be fully subjected to pH pretreatment through the temperature and the shaking speed, so that the treatment effect of the protein wastewater subjected to pH pretreatment is ensured.
Further, in the step S2, before the protein wastewater is subjected to pH pretreatment, the protein wastewater is subjected to variable power microwave pretreatment at a temperature range of 20 to 30 ℃, wherein the microwave frequency is 2 to 3 GHz. The protein can be hydrolyzed into amino acid rapidly by microwave, and simultaneously, the microwave can promote the second-order conformational change of the protein to different degrees, so as to promote the hydrolysis of the protein.
Furthermore, the variable parameter variation of the microwave power satisfies formula (1), specifically as follows:
P＝4C·f2 (1)
wherein C represents the temperature of the protein wastewater, P represents the microwave power of the microwave pretreatment, and f represents the microwave frequency of the microwave pretreatment. Researches show that different temperatures, microwave powers and microwave frequencies have different influences on the secondary conformational change of proteins in protein wastewater, and the microwave pretreatment effect keeps higher treatment effect by adjusting and changing along with the adaptation of temperature environment, so that the subsequent AD methanogenesis efficiency is improved.
Further, the mixing manner of step S3 is specifically: putting 150mL of neutral protein wastewater into mixed slurry of gelatin-coated high-carbon steel balls and 15g of centrifugal granular sludge, wherein the gelatin-coated high-carbon steel balls account for 20-30% of the total volume of the neutral protein wastewater, and 1mL of trace element stock solution and 1g of NaHCO are added into a mixed gelatin layer adopting the gelatin-coated high-carbon steel balls3And 0.3g of Na2HPO4. By using gelatin containing microelement stock solution and Na2HPO4Can be slowly released into neutral protein water body, thereby maintaining the long-acting anaerobic fermentation treatment efficiency.
Furthermore, before the mixed slurry is put into the neutral protein wastewater, the gelatin-coated high-carbon steel balls and the centrifugal granular sludge are required to be mixed and placed under the condition that the magnetic field intensity is 0.1-0.2T, and the mixture is stirred for 15 min. Through adding the magnetic field intensity of establishing this scope and handling, carry out the magnetic activation to centrifugal particle mud in order to improve microbial activity to can magnetize gelatin cladding type high carbon steel ball, utilize the high carbon steel characteristic of gelatin cladding type high carbon steel ball to store magnetic for a long time, not only can long-term promotion microbial activity when it drops into central protein waste water, can magnetize moreover and influence neutral protein waste water, promote the anaerobic digestion treatment to neutral protein waste water.
Furthermore, the preparation method of the gelatin-coated high-carbon steel ball specifically comprises the following steps: selecting high carbon steel ball with particle diameter of 1 + -0.1 cm, specifically preparing microelement stock solution and NaHCO3And Na2HPO4Mixing with gelatin to obtain mixed gelatin, and coating the mixed gelatin on the surface of high carbon steel bead to make the thickness of the high carbon steel coated with the mixed gelatin be 0.6 + -0.05 cm. The high-carbon steel beads in the range can avoid the problems of small particle size, small adhesive surface of the mixed gelatin layer and the like, and the magnetization efficiency is low due to the overlarge particle size, the quality is overlarge, and the rapid sedimentation is easy to realize; the thickness range of the mixed gelatin can meet the requirement of anaerobic digestionWhen the whole of the structure is long, the influence of the over-thick mixed gelatin layer on the magnetization efficiency is avoided.
The invention has the beneficial effects that:
(1) the method of the invention carries out pretreatment on the protein wastewater by adjusting the pH value, so that the conformation and the structure of the protein are expanded, thereby eliminating the limitation of low methanation rate of the protein wastewater in the AD process.
(2) The invention provides a scheme for adjusting protein wastewater by various pH values, and the optimal scheme for adjusting the pH value of the protein wastewater can be selected according to the balance between time benefit and methane yield benefit.
(3) The invention provides a microwave pretreatment method, and the microwave power and the microwave frequency are adjusted according to the change temperature, so that the effect of promoting the second-stage conformational change of the protein in the protein wastewater is improved, and the subsequent efficiency of producing methane by AD is improved.
(4) The invention provides a mode for adding each component in anaerobic digestion, which adopts gelatin containing trace element stock solution and NaHCO3And Na2HPO4The neutral protein wastewater can be slowly released into a neutral protein water body, so that the long-acting anaerobic fermentation treatment efficiency is kept, and meanwhile, the high-carbon steel magnetization is utilized to promote the microbial activity for a long time, influence the neutral protein wastewater water body through magnetization and improve the anaerobic digestion treatment effect of the neutral protein wastewater.
Drawings
FIG. 1 is a graph of methane production after 24h pretreatment at various pH values, (A) acidic conditions (B) basic conditions, (error bars represent standard deviations of triplicate samples);
figure 2 is the methane yield at different pretreatment times (a) pH 3, (B) pH 12, (error bars represent standard deviation of triplicate samples);
FIG. 3 shows the results of simultaneous fluorescence spectroscopy analysis of pretreated protein wastewater: (A) pretreatment time based on pH 3; (B) pretreatment time at pH 12 (Ex 292 nm);
FIG. 4 is a transition of secondary structure of protein, (A-D) far ultraviolet-CD spectra of protein wastewater pretreated with different pH conditions, (a-D) secondary structure composition change based on CD spectrum decoding;
fig. 5 is an infrared spectrum of the protein after acidic and basic pretreatments at optimal pH (a) pH 3 for 24h and (B) pH 12 for 24 h.
Detailed Description
The present invention will be described in further detail with reference to specific embodiments thereof for better understanding the advantages of the invention.
Example 1
A method for pretreating protein wastewater based on pH adjustment to improve the efficiency of AD methanogenesis comprises the following steps:
step 1: microbial preparation
Centrifuging the anaerobic granular sludge at 12000rpm for 10min to remove soluble organic matters, and washing with distilled water for 2 times to obtain centrifuged granular sludge for later use;
step 2: pH pretreatment
Changing the pH value of the protein wastewater through pH adjustment treatment, and shaking the protein wastewater in an air bath shaker at room temperature for 18h to obtain protein wastewater subjected to pH pretreatment; the pH adjustment processing in step S2 specifically includes: the pH of the protein wastewater was adjusted to 3 by acidic pretreatment, i.e., by 4M hydrochloric acid. The protein wastewater is pretreated by acidic pretreatment, so that the conformation and the structure of the protein are expanded, and the limit of low methanation rate of the protein wastewater in the AD process is eliminated;
the air bath shaking table in the step S2 specifically comprises the following steps: shaking was carried out at room temperature, 25 ℃ and a shaking speed of 100 rpm. The protein wastewater can be fully subjected to pH pretreatment through the temperature and the shaking speed, so that the treatment effect of the protein wastewater subjected to pH pretreatment is ensured.
And step 3: anaerobic digestion
1) Readjusting the protein wastewater after the pH pretreatment to a neutral initial value of pH to obtain neutral protein wastewater;
2) then according to the proportion of 150mL of neutral protein wastewater, 1mL of trace element stock solution and 0.3g of Na2HPO4、1g NaHCO3And 15g of centrifugal granular sludge are placed in a reaction tank for mixing and simultaneously communicatedHelium is added to evacuate oxygen in the reaction tank, so that an oxygen-free environment in the reaction tank is ensured;
3) the temperature in the reaction tank was then controlled at 35 ℃ and anaerobic digestion was carried out on a shaker at a shaking speed of 120rpm, and the produced methane was collected.
According to the invention, the pH is adopted to pretreat the wastewater, so that the conformation and the structure of the protein can be developed, and the limit of low methanation rate of the protein wastewater in the AD process is eliminated.
Example 2
This example is substantially the same as example 1, except that the pH adjustment process in step S2 specifically includes: the pH of the protein wastewater was adjusted to 2 by acidic pretreatment, i.e., by 4M hydrochloric acid.
Example 3
This example is substantially the same as example 1, except that the pH adjustment process in step S2 specifically includes: the pH of the protein wastewater was adjusted to 6 by acidic pretreatment, i.e., by 4M hydrochloric acid.
Example 4
This example is substantially the same as example 1, except that the pH adjustment process in step S2 specifically includes: the pH of the protein wastewater was adjusted to 12 by alkaline pretreatment, i.e., by 4M sodium hydroxide. The alkaline pretreatment, especially at pH 12, significantly increased the methane production rate by 35.7% to 149.6 ± 16.1ml/g protein relative to the method without pH pretreatment; this is due to the collapse of the hydrogen bond network caused by the C ═ O stretching vibration of the amide I band, which causes the secondary structure of the protein to undergo a transition from ordered to disordered; however, time economy evaluations indicate that the optimal protocol for alkaline pretreatment (pH 12) of protein wastewater requires a trade-off between time benefit and methane production rate benefit.
Example 5
This embodiment is substantially the same as embodiment 4, except that the pH adjustment process in step S2 specifically includes: the pH of the protein wastewater was adjusted to 8 by alkaline pretreatment, i.e., by 4M sodium hydroxide.
Example 6
This embodiment is substantially the same as embodiment 4, except that the pH adjustment process in step S2 specifically includes: the pH of the protein wastewater was adjusted to 10 by alkaline pretreatment, i.e., by 4M sodium hydroxide.
Example 7
This example is essentially the same as example 1, except that the pH pretreatment of step S2 was shaken in an air bath shaker for 6 h.
Example 8
This example is essentially the same as example 1, except that the pH pretreatment of step S2 was shaken in an air bath shaker for 24 h.
Example 9
This example is substantially the same as example 1, except that the protein wastewater is subjected to variable power microwave pretreatment at a frequency of 2.8GHz while varying the temperature range of 25 ℃ before being subjected to pH pretreatment in step S2. The protein can be hydrolyzed into amino acid rapidly by microwave, and simultaneously, the microwave can promote the second-order conformational change of the protein to different degrees, so as to promote the hydrolysis of the protein.
The variable parameter variation of the microwave power satisfies formula (1), which is specifically as follows:
P＝4C·f2 (1)
wherein C represents the temperature of the protein wastewater, P represents the microwave power of the microwave pretreatment, and f represents the microwave frequency of the microwave pretreatment. Researches show that different temperatures, microwave powers and microwave frequencies have different promotion influences on the secondary conformational change of proteins in protein wastewater, and the microwave pretreatment effect keeps higher treatment effect by adjusting and changing along with the adaptation of temperature environment, so that the subsequent AD methane production efficiency is improved;
c is 25, f is 2.8, and the result of the belt expression (1) is P784W.
Example 10
The present embodiment is substantially the same as embodiment 9, and the difference is that the parameters of the microwave pretreatment are different, specifically:
c is 20, f is 2, and the result is P ═ 320W with formula (1).
Example 11
The present embodiment is substantially the same as embodiment 9, and the difference is that the parameters of the microwave pretreatment are different, specifically:
c is 30, f is 3, and the result is P ═ 1080W, with formula (1).
Example 12
The present embodiment is substantially the same as embodiment 1, except that the mixing manner of step S3 is specifically as follows: putting 150mL of neutral protein wastewater into mixed slurry of gelatin-coated high-carbon steel balls and 15g of centrifugal granular sludge, wherein the gelatin-coated high-carbon steel balls account for 27% of the total volume of the neutral protein wastewater, and 1mL of trace element stock solution and 1g of NaHCO are added into a mixed gelatin layer adopting the gelatin-coated high-carbon steel balls3And 0.3g of Na2HPO4. By using gelatin containing microelement stock solution and Na2HPO4Can be slowly released into neutral protein water body, thereby maintaining the long-acting anaerobic fermentation treatment efficiency.
Wherein, before the mixed slurry is put into the neutral protein wastewater, gelatin-coated high-carbon steel balls and centrifugal granular sludge are required to be mixed and placed under the condition that the magnetic field intensity is 0.15T for stirring for 15 min. Through adding the magnetic field intensity of establishing this scope and handling, carry out the magnetic activation to centrifugal particle mud in order to improve microbial activity to can magnetize gelatin cladding type high carbon steel ball, utilize the high carbon steel characteristic of gelatin cladding type high carbon steel ball to store magnetic for a long time, not only can long-term promotion microbial activity when it drops into central protein waste water, can magnetize moreover and influence neutral protein waste water, promote the anaerobic digestion treatment to neutral protein waste water.
The preparation method of the gelatin-coated high-carbon steel ball comprises the following specific steps: selecting high carbon steel balls with the grain diameter of 1cm, and concretely selecting high carbon steel balls with the grain diameter of 1cmStock solution of trace elements, NaHCO3And Na2HPO4Mixing with gelatin to obtain mixed gelatin, and coating the mixed gelatin on the surface of high carbon steel bead to make the thickness of the high carbon steel coated with the mixed gelatin be 0.6 cm. The high-carbon steel beads in the range can avoid the problems of small particle size, small adhesive surface of the mixed gelatin layer and the like, and the magnetization efficiency is low due to the overlarge particle size, the quality is overlarge, and the rapid sedimentation is easy to realize; the thickness range of the mixed gelatin can meet the requirement of the whole anaerobic digestion treatment duration, and meanwhile, the phenomenon that the magnetization efficiency is influenced by the excessively thick mixed gelatin layer is avoided.
Example 13
This example is substantially the same as example 12 except that the gelatin-coated high-carbon steel beads account for 20% of the total volume of the neutral protein wastewater.
Example 14
This example is substantially the same as example 12 except that the gelatin-coated high-carbon steel beads account for 30% of the total volume of the neutral protein wastewater.
Example 15
This example is substantially the same as example 12, except that the mixed slurry was mixed with gelatin-coated high-carbon steel beads and centrifuged granular sludge and stirred at a magnetic field strength of 0.1T for 15min before being fed into neutral protein wastewater.
Example 16
This example is substantially the same as example 12, except that the mixed slurry was mixed with gelatin-coated high-carbon steel beads and centrifuged granular sludge and stirred at a magnetic field strength of 0.2T for 15min before being fed into neutral protein wastewater.
Experiment of AD methanogenesis efficiency of protein wastewater
BSA (bovine serum albumin, purchased from Equitech-Bio, USA) is used as a carbon source to prepare synthetic protein wastewater, and the COD set value is kept at 5000 mg/L; meanwhile, anaerobic granular sludge is obtained from a UASB reactor of a sewage treatment plant of a certain city of Jiangsu in China; the microelement stock solution is Hunter microelement solution;
in biogasThe methane content is determined by gas chromatography (Scientific)TMTRACE 1310), equipped with a thermal conductivity detector with nitrogen as carrier gas; circular dichroism spectroscopy (CD), fluorescence spectroscopy, and FTIR spectroscopy were used to describe changes in protein secondary structure; briefly, a CD spectrum was obtained using a JASCO J-715 autoscription spectrophotometer (tokyo, japan), controlled by JASCO software, using a 0.1cm quartz cell at room temperature; the pretreated sample was diluted to a concentration of about 45 mg BSA/L and then transferred to a quartz cell having an optical path length of 1 cm; measuring the molecular ellipticity within the range of 190-250nm, keeping the bandwidth at 1nm and the scanning speed at 50 nm/min; during the scanning, each spectrum was automatically corrected according to the choice of distilled water as a blank.
Calculating the alpha-helix content according to the following formulas (2) and (3):
wherein MRE is the mean residue ovality (deg cm)2 dmol-1) CP is the molar concentration of the protein, n is the number of amino acid residues (BSA 583), and L is the path length of the cell (mm);
wherein MRE208Is the MRE observed at 208nm, 4000 is the MRE at 208nm where the beta form crosses the random coil conformation, 33000 is the MRE value of the pure helix at 208 nm.
Three-dimensional excitation-emission matrix fluorescence spectroscopy and simultaneous fluorescence spectroscopy were used for luminescence spectroscopy to characterize protein samples (Fluoromax-4 Spectrofluorometer, HORIBA Scientific, france); the sample was first filtered through a 0.45mm hydrophilic filter membrane and then diluted to an approximate concentration prior to testing to avoid exceeding the measurement range of the instrument; in order to obtain a simultaneous fluorescence spectrum, the excitation wavelength is from 250 to 450nm, the step size is 5nm, and the offset (delta lambda) is constant at 60 nm; fourier infrared spectroscopy forDetecting the characteristic absorption peak of the functional group in the protein, wherein the scanning range is 500-4000cm-1。
The following experimental investigations are now made:
firstly, the influence of different pH values on the efficiency of producing methane by the protein wastewater AD in the acidic pretreatment is explored
The AD methanogenesis performance of protein wastewater pretreated with different pH gradients is shown in FIG. 1A, compared with the experiments using examples 1-3 as acidic pretreatment at different pH. No significant improvement in methane production and no difference between groups occurred during the first 12 h. This is because anaerobic granular sludge needs to undergo a short adaptation period, which does not show differences due to different pretreatment conditions. As shown in FIG. 1A, the methane production rate of 5g of untreated COD/L BSA synthetic wastewater at 120h was 110.2. + -. 5.1mL/g protein. After the acidic pretreatment, the synthetic wastewater has the lowest and highest methane generation rates of 125.5 +/-2.6 mL/g protein (pH 5) and 142.6 +/-4.0 mL/g protein (pH 3) at 120 h. The methane production rate increased by 13.9% (pH 5) and 29.4% (pH 3), respectively, compared to the control group, which was not pH pretreated. This may be based on the fact that at low pH, the protein molecules will be in an unstable intermediate molten globule state, the conformation of the protein will change, and the active fraction susceptible to hydrolytic enzymes will be exposed, which will significantly increase the hydrolysis and acidification efficiency of anaerobic fermentation, resulting in better methane production rates.
Secondly, the influence of different pH values on the efficiency of producing methane by the protein wastewater AD in the alkaline pretreatment is explored
Using examples 4-6 as experimental comparisons with alkaline pretreatment at different pH, after pretreatment under alkaline conditions of pH 8-12, as shown in fig. 1B, the results showed that the lowest and highest methane production rates at 120h were 138.4 ± 3.8mL/g protein (pH 8) and 149.6 ± 16.1mL/g protein (pH 12), respectively, with 25.6% and 35.7% increases compared to the control group (110.2 ± 5.1mL/g protein) without pH pretreatment. In addition, the adaptability of the anaerobic fermentation functional bacteria to the protein wastewater subjected to alkaline pretreatment seems to be higher than that of the protein wastewater subjected to acidic pretreatment. Particularly, after the anaerobic fermentation was carried out for 24 hours, a significant difference in methane accumulation was observed, wherein the methane production rates of the pretreated protein wastewater at pH 12 and pH 3 were 47.4 ± 1.4mL/g protein and 16.9 ± 4.2mL/g protein, respectively, while the control group was 13.0 ± 0.9mL/g protein. The rapid increase in methane production rate of alkaline pretreated protein wastewater continued until 60 hours, and then the increasing trends of both pretreatments tended to be consistent.
Thirdly, the influence of the shaking time of the air bath shaking table in the pretreatment of different pH values on the efficiency of producing methane from the protein wastewater AD is researched
The methane productivity of AD was obtained by subjecting the synthetic protein wastewater to different pretreatment times (6h, 12h, 18h and 24h) using examples 1, 7 and 8 as experimental comparisons of shaking times of the shaker for different air baths. As shown in fig. 2A, after pretreatment at pH 3 for 6h, the methane production rate could reach 134.3 ± 1.3mL/g protein through the methanation process, and the methane gain effect was 74.4% compared to the experimental group (142.6 ± 4.0mL/g protein) pretreated for 24 h. Therefore, it is economically desirable to pretreat the synthetic protein wastewater for 6 hours under acidic pretreatment (pH 3). Similarly, the synthetic protein wastewater was subjected to alkaline pretreatment at pH 12 (fig. 2B), which indicated that the methane production rate could reach 142.6 ± 17.3mL/g protein through 120h of methane production at a pretreatment time of 6 h. Compared with the methane production rate (149.6 +/-16.1 mL/g protein) of 24h of alkaline pretreatment, the methane gain effect reaches 82.2 percent. Therefore, whether acidic pretreatment or alkaline pretreatment, it is scientifically reasonable to select a pretreatment time of 6h based on economic considerations. In addition, compared with the synthetic wastewater treated by acid, the anaerobic fermentation functional bacteria have better adaptability to the wastewater treated by alkali no matter how long the pretreatment time is.
Fourthly, the influence of microwave pretreatment on the efficiency of producing methane by the protein wastewater AD is explored
Examples 1 and 9 were methods without microwave pretreatment and with microwave pretreatment, and the methane yield after 120h was determined, wherein the methane yield after 120h in example 1 was about 142.6 + -4.0 mL/g protein, and the methane yield after 120h in example 9 was about 151.3 + -8.9 mL/g protein; by comparison, example 9 after microwave pretreatment had higher methanogenesis efficiency than example 1.
Fifthly, the influence of different microwave pretreatment parameters on the efficiency of producing methane from the protein wastewater AD under the microwave pretreatment is explored
Examples 9-11 are methods under different microwave pretreatment parameters, and the methane yield after 120h was determined, wherein the methane yield after 120h was about 151.3 + -8.9 mL/g protein in example 9, about 149.1 + -7.3 mL/g protein in example 10, and about 152.6 + -9.5 mL/g protein in example 11; it can be seen that the methane production rates of examples 9-11 are not significantly different, and the microwave pretreatment parameters are dynamically adjusted according to the temperature change through the formula (1) to maintain a relatively stable methane production efficiency promoting effect;
meanwhile, the results of pretreatment using 784W microwave power of example 9 were recorded as comparative examples 1 and 2, based on the temperature of 20 ℃ and 30 ℃ of examples 10 and 11, and the methane yield of comparative example 1 was about 141.3. + -. 9.1mL/g protein after 120 hours, and the methane yield of comparative example 2 was about 143.7. + -. 8.8mL/g protein after 120 hours, and it can be seen from comparison that the methane production efficiency of comparative example 1 was somewhat lower than that of example 10 and that of comparative example 2 was lower than that of example 11.
Sixthly, the influence of different mixing modes in anaerobic digestion on the efficiency of producing methane by the protein wastewater AD is researched
Examples 1 and 12 were conducted by direct mixing and using gelatin-coated high-carbon steel beads, and the methane yield after 120 hours was measured, wherein the methane yield after 120 hours was about 142.6. + -. 4.0mL/g protein in example 1, and about 156.5. + -. 3.7mL/g protein in example 12, and it can be seen by comparison that example 12 using gelatin-coated high-carbon steel beads had higher methane production efficiency than example 1;
meanwhile, considering that gelatin is introduced into example 12, the same content of gelatin as that of example 12 is added based on example 1, which is referred to as comparative example 3, and the methane yield of comparative example 3 after 120 hours is about 145.2 ± 5.7mL/g protein, and it can be seen by comparison that example 12 is still superior to comparative example 3 in which gelatin is introduced at the same content.
Seventhly, the influence of the adding amount of different gelatin-coated high-carbon steel beads on the efficiency of producing methane from the protein wastewater AD is researched
Examples 12 to 14 are methods for different amounts of gelatin-coated high-carbon steel beads, and the methane yield after 120 hours is respectively determined, wherein the methane yield after 120 hours in example 12 is about 156.5 ± 3.7mL/g protein, the methane yield after 120 hours in example 13 is about 150.5 ± 3.3mL/g protein, and the methane yield after 120 hours in example 14 is about 157.5 ± 3.9mL/g protein, and it can be seen that the increase of the methane yield in the interval of 20 to 27% of the amount of gelatin-coated high-carbon steel beads is obvious, and the increase of the methane yield in the interval of 27 to 30% is slow, so that the amount of gelatin-coated high-carbon steel beads can be selected according to actual requirements from the perspective of actual cost and the like.
Eighthly, the influence of treating the mixed slurry with different magnetic field strengths on the AD methanogenesis efficiency of the protein wastewater is explored
Examples 12, 15, and 16 are methods under different magnetic field strengths, and the methane yield after 120 hours is respectively measured, wherein the methane yield after 120 hours in example 12 is about 156.5 ± 3.7mL/g protein, the methane yield after 120 hours in example 13 is about 152.7 ± 3.5mL/g protein, and the methane yield after 120 hours in example 14 is about 157.3 ± 4.0mL/g protein, as can be seen by comparison, the increase of the methane yield in the interval of 0.1 to 0.15T is more obvious, and the increase of the methane yield in the interval of 0.15 to 0.2T is slower, so the magnetic field strength of example 12 is more suitable from the economical point of view.
Meanwhile, in order to further explore the influence of pH pretreatment on the efficiency of producing methane by the protein wastewater AD and explore the endogenous fluorescence characteristics of the protein wastewater, the secondary structure characteristics of the protein and the Fourier transform infrared spectrum of BSA after acid-base pretreatment, the method comprises the following steps:
endogenous fluorescence characteristics of protein wastewater
The endogenous fluorescence of proteins is mainly derived from tryptophan (Trp), tyrosine (Tyr) and phenylalanine (Phe) residues in the molecule. As a protein macromolecule with strong fluorescence, BSA has 2 Trp residues, 21 Tyr residues, and 26 Phe residues per molecule, and all three chromophores have their own characteristic fluorescence peak. Trp residues include Trp 134 and Trp 212, which are located in domain I and domain II, respectively, while fluorescence is primarily emitted by the Trp residue at position 212. Fluorescence due to tryptophan residues alone can be selectively measured by excitation near 295nm, since tyrosine does not absorb at this wavelength. In addition, Trp residues exhibit higher environmental polarity sensitivity and fluorescence quantum yield than Tyr and Phe residues, and thus exert long-term and productive optical spectroscopic effects in unwrapping the secrecy of protein conformation and structure.
The maximum emission peak of Trp residues in untreated BSA samples was determined to be 345nm at an excitation wavelength of 292nm, while the maximum emission peak of free L-Trp was located at 352nm, resulting in a blue-shift of the maximum emission wavelength of Trp residues by 7 nm. This indicates that most Trp residues in BSA are encapsulated inside the molecule, in a hydrophobic microenvironment of low polarity, and free contact with water is limited. After 24h acidic pretreatment at pH 3, the position of the fluorescence peak of the Trp residue in BSA changed from 345nm to 350nm, resulting in a red shift and a slight increase in stokes shift (Δ λ). At the same time, the fluorescence intensity at the peak was also significantly reduced, indicating a change of the microenvironment to a more polar environment, since the Trp residues were exposed from the hydrophobic cavity in which the molecule was originally located. However, under the same conditions, the emission wavelength of the fluorescence peak is slightly less than that of free L-Trp, and the peak position still has a certain degree of blue shift, which indicates that the Trp residue in the BSA molecule is not completely exposed to the water environment after 24h pretreatment at pH 3, and still has a part of the hydrophobic cavity in the molecule.
In contrast, the fluorescence peak position of the Trp residue in BSA treated at pH 12 for 24h changed from 345nm to 340nm, which was blue-shifted in emission wavelength, and the intensity of the fluorescence peak decreased by 43.8% (from 122670a.u. to 689989 a.u.). The decrease in the fluorescence intensity of a protein is more often caused by two factors, one is the quenching of the fluorescence of an aromatic residue and the shift of the position of the fluorescence peak, and the other is the oxidation of the aromatic residue. As can be seen from the data, the decrease in fluorescence intensity appears to be a result of the synergistic effect of the two quenching pathways. Furthermore, the increase in hydrophobicity indicates that the Trp residue is located in a hydrophobic region inside the protein molecule, which also indicates that the conformation of the protein may be shifted. Under other pH conditions, whether acidic or basic, the decrease in fluorescence intensity values may be caused only by oxidation of aromatic amino acids, and not by fluorescence quenching, since there is no blue shift in fluorescence maximum emission.
As shown in FIG. 3 and Table 1, the quenching of fluorescence under acidic pretreatment conditions was attributable to the oxidation of aromatic amino acids except for the pretreatment time of 24h, since no significant peak position change was found. Unlike the acidic pretreatment, the fluorescence intensity of the alkaline pretreated protein wastewater is significantly reduced, which may be due to the blue shift of the fluorescence maximum peak in the emission wave at different pretreatment times, resulting in fluorescence quenching. Furthermore, regardless of the pretreatment time, alkaline pretreatment tends to significantly alter the conformation and structure of the protein, resulting from the increase in hydrophobicity due to blue-shift. Thus, denaturation of the protein was achieved with alkaline pretreatment (pH 12) in only 6 h.
Second, secondary structure characteristics of protein
Proteolysis is the rate-limiting step in AD because of the complex conformation and structure of the protein itself, which needs to be degraded into peptides and amino acids by the action of hydrolytic enzymes in order to be utilized by methanogenic functional bacteria. The secondary structure of primary BSA is mainly an α -helix and β -sheet structure formed by hydrogen bond-mediated folding of the peptide backbone, and its changes have a significant impact on the bioavailability of proteins. Further investigation of the far uv-CD spectrum showed the conformational and structural patterns of the pretreated protein under different pH working conditions, as shown in figure 4. The results show that in the spectra of fig. 4(a-D), there are two distinct negative peaks at about 208 and 222nm, which are generally characteristic of the alpha-helical structure. And as shown in fig. 4(a-d), the content changes of four monomers in the secondary conformation and structure of the BSA molecule under different pH working conditions were obtained by correlation calculation. In detail, the contents of α -helix, β -sheet, β -turn and disordered structure in the blank group were 49.0%, 8.2%, 14.5% and 22.0%, respectively. The alpha-helix content of all samples pretreated under acidic conditions decreased, with the alpha-helix content of the samples pretreated at pH 3 decreasing to 47.5%, indicating that acidic pretreatment stimulated a decrease in the peptide chain ordered helix of BSA molecules, resulting in an increase in protein folding and disordered structure. Furthermore, as shown in fig. 4C and 4C, a time-economic investigation of acidic pretreatment (pH 3) revealed that there was no significant difference in secondary conformation and structure of BSA molecules with increasing pretreatment time, and thus 6h could be determined as the first choice for acid treatment.
In contrast, the second structure of the pre-treated BSA at pH 12 showed a clear transition from ordered to disordered, folded to disordered structure, as shown in fig. 4B and fig. 4B. It is emphasized that the contents of alpha-helix, beta-sheet, beta-turn and disordered structure are 26.3%, 29.5%, 18.9% and 32.2%, respectively. Compared to acidic pretreatment of BSA, alkaline pretreatment (pH 12) was more effective in disrupting the secondary structure of the protein, resulting in a transition of the protein from ordered to disordered, especially β -sheet and disordered structures, which is positively correlated with degradation and utilization of the protein. Furthermore, the alpha-helix content in the methanogenesis of AD was reduced by 46.3% before pH adjustment to neutrality, indicating that under alkaline conditions at pH 12, the hydrogen bonding network of the protein may be irreversibly broken. Furthermore, as shown in fig. 4D and 4D, time-economy studies of alkaline pretreatment (pH 12) showed that the longer the time, the more pronounced the changes in secondary conformation and structure of the protein. Thus, changes in protein conformation and structure upon alkaline pretreatment (pH 12) are time dependent.
Third, Fourier transform infrared spectrum of BSA after acid-base pretreatment
In general, in the FTIR spectrum, the amide I and amide II bands are associated with the BSA secondary structure, which occurs at 1600-1700cm-1The absorption of the region associated with C ═ O tensile vibration was recorded as amide I and measured at about 1550cm-1The absorption associated with the N-H bending vibration and the C-N tensile vibration as the center was designated as amide II. As shown in fig. 5, the vibration frequency and vibration intensity of BSA varied compared to the blank group regardless of pretreatment with acidic or basic conditions. With respect to the change in vibration frequency, protein molecules pretreated with acidic or basic conditions have red-shifts in both amide I and amide II bands, indicating that hydrogen bonds in the molecules are broken. In particular, the amide I band has a spectral range of 1645cm-1Left-right centered absorption can be attributed to exposure of the alpha-helical structure. While the shift in the exposed alpha-helix peak indicates that the BSA protostructure is distorted due to the unfolding of the helix structure, which is consistent with the observation of CD results. As for the vibration intensity, the alkaline pretreatment showed stronger vibration amplitude, indicating that the degree of breakage of hydrogen bonds was greater during the alkaline pretreatment than the acidic pretreatment, which may explain that the methane production performance of the alkaline pretreatment was superior. Thus, the greater sensitivity of the amide I band to changes in the BSA secondary structure relative to the amide II band suggests that C ═ O stretching vibration may be the primary trigger for the unfolding of the α -helix structure, leading to hydrogen bond cleavage.
Table 1 fluorescence spectra parameters at different pretreatment times of pH 3 and pH 12
F*Representing the fluorescence intensity.
Claims (8)
1. A method for pretreating protein wastewater based on pH adjustment to improve AD methanogenesis efficiency is characterized by comprising the following steps:
step 1: microbial preparation
Centrifuging the anaerobic granular sludge at 12000rpm for 10min to remove soluble organic matters, and washing with distilled water for 2 times to obtain centrifuged granular sludge for later use;
step 2: pH pretreatment
Changing the pH value of the protein wastewater through pH adjustment treatment, and shaking the protein wastewater in an air bath shaker at room temperature for 6-24 h to obtain protein wastewater subjected to pH pretreatment;
and step 3: anaerobic digestion
1) Readjusting the protein wastewater after the pH pretreatment to a neutral initial value of pH to obtain neutral protein wastewater;
2) then according to the proportion of 150mL of neutral protein wastewater, 1mL of trace element stock solution and 0.3g of Na2HPO4、1g NaHCO3And 15g of the centrifugal granular sludge is placed in a reaction tank for mixing, and meanwhile, helium is introduced to evacuate oxygen in the reaction tank, so that an oxygen-free environment in the reaction tank is ensured;
3) the temperature in the reaction tank was then controlled to 35. + -.1 ℃ and anaerobic digestion was carried out on a shaker at a shaking speed of 120rpm and the produced methane was collected.
2. The method for pretreating protein wastewater based on pH adjustment to improve the efficiency of AD methanogenesis according to claim 1, wherein the pH adjustment treatment in the step S2 is specifically: and (3) adopting acidic pretreatment, namely adjusting the pH value of the protein wastewater to 2-6 by 4M hydrochloric acid.
3. The method for pretreating protein wastewater based on pH adjustment to improve the efficiency of AD methanogenesis according to claim 1, wherein the pH adjustment treatment in the step S2 is specifically: and (3) alkaline pretreatment is adopted, namely the pH value of the protein wastewater is adjusted to 8-12 by 4M sodium hydroxide.
4. The method for pretreating protein wastewater based on pH adjustment to improve the efficiency of producing methane by AD according to claim 1, wherein the air bath shaking table in the step S2 is specifically: shaking was carried out at room temperature, 25 ℃ and a shaking speed of 100 rpm.
5. The method for pretreating protein wastewater based on pH adjustment to improve the efficiency of producing methane by AD according to claim 1, wherein the air bath shaking table in the step S2 is specifically: shaking at room temperature of 25 ℃ and a shaking speed of 100rpm for 6-24 h.
6. The method for pretreating protein wastewater based on pH adjustment to improve the efficiency of AD methanogenesis according to claim 1, wherein the mixing manner of the step S3 is specifically as follows: putting 150mL of neutral protein wastewater into mixed slurry of gelatin-coated high-carbon steel balls and 15g of centrifugal granular sludge, wherein the gelatin-coated high-carbon steel balls account for 20-30% of the total volume of the neutral protein wastewater, and 1mL of trace element stock solution and 1g of NaHCO are added into a mixed gelatin layer adopting the gelatin-coated high-carbon steel balls3And 0.3g of Na2HPO4。
7. The method for pretreating protein wastewater based on pH adjustment to improve AD methanogenesis efficiency according to claim 6, wherein the mixed slurry is prepared by mixing gelatin-coated high-carbon steel balls with centrifugal granular sludge and stirring at a magnetic field strength of 0.1-0.2T for 15min before adding neutral protein wastewater.
8. The method for pretreating protein wastewater based on pH adjustment to improve AD methanogenesis efficiency according to claim 6, wherein the method for preparing the gelatin-coated high-carbon steel beads comprises the following steps: selecting high carbon steel ball with particle diameter of 1 + -0.1 cm, adding microelement stock solution and NaHCO3And Na2HPO4Mixing with gelatin to obtain mixed gelatin, and coating the mixed gelatin on the surface of high carbon steel bead to make the thickness of the high carbon steel coated with the mixed gelatin be 0.6 + -0.05 cm.
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