[Highlights]
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All three surfactants can improve the extraction of alginate like substances (ALE) from activated sludge (12.4% → 22.3%~28.2% VSS)
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Surfactants can also improve the purity of the extract alginate (50% → 54%~70%)
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Triton X-100 has the best promotion effect, followed by CTAB and SDS
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Surfactants promote the release of sludge flocs and extracellular polymers through micellar action
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Similar phase solubility and functional group adsorption help to improve the dissolution of ALE substances
[Graphic summary]
[Introduction]
Alginate like exopolysaccharides (ALE), as a high value-added product that can be recovered from the extracellular polymers (EPS) of excess sludge, has attracted much attention. Previous experiments have proved that the extraction amount of ALE from ordinary activated sludge (CAS) is between 90 and 190 mg/g VSS; Although its content is acceptable, it is still only the lower limit of its content compared with the amount of ALE extracted from aerobic granular sludge (AGS) (16%~31% SS) (see the public number article Research Express | Recovery of Alginate like Substances from Activated Sludge: Potential, Properties and Limiting Factors on October 1, 2021 (click to enter)). It can be seen from the analysis of reasons that the low ALE content or extraction amount in CAS is mainly limited by the low background value of flocs or the low efficiency of extraction methods.
Preliminary experiments have proved that different influent water quality (different carbon source, sludge load, C/N ratio) and operating parameters (temperature and SRT) have a certain impact on the background value of ALE; Except for the obvious effect of starch carbon source and low temperature environment cultivation sludge ALE on production increase, other factors have no obvious impact (see the public number article "Original Research | Control Factors of Alginate like Substance Formation in Activated Sludge EPS" on March 18, 2022 (click to enter)). Consider actual municipal administration Sewage treatment plant Although there are many types of influent carbon sources, the composition is basically stable, and the low temperature environment of sewage is not normal, so it is obviously difficult to improve ALE output by controlling the operating conditions. It can be seen that we can only try to optimize the extraction method. Based on the premise that the background content of ALE is basically fixed, we can increase the amount of ALE extraction as much as possible by improving the extraction efficiency.
Typical sludge EPS extraction methods include physical method, chemical method and a combination of the two methods. Among them, the physical method is relatively mild, but the extraction efficiency is low; Although the efficiency of chemical method is high, it may change the performance of EPS components, leading to a decrease in the purity of the extract. The research found that although these optimization methods can improve the EPS extraction yield to a certain extent, the high power, long time consumption, high speed and other means used in the optimization process mean high energy consumption and high cost. In early research, ALE was successfully extracted and purified from AGS by using the method of alginate extraction, high-temperature sodium carbonate, acid precipitation and organic extraction. However, this method is effective for granular sludge, but it has the disadvantage of low extraction efficiency when extracting ALE from CAS. Therefore, it is necessary to find effective means to optimize it.
The literature review found that surfactants seemed to play a significant role in the extraction of ALE, the high value product of EPS from CAS excess sludge. Therefore, this study takes surfactants as the research goal, and explores the enhanced extraction effect of different surfactants, such as anionic sodium dodecyl sulfate (SDS), cationic cetyltrimethylammonium bromide (CTAB) and non-ionic octylphenyl polyoxyethylene ether (Triton X-100), on ALE in CAS sludge EPS, Determine the appropriate surfactant type and dosage concentration. At the same time, the components of the extract were characterized, and the mechanism of the increase of ALE extraction and purification process was analyzed, which laid an experimental and theoretical foundation for the efficient extraction and recovery of the high-value product of CAS sludge ALE. This achievement has been published in Science of The Total Environment (SCI of Zone 1, IF=7.936).
[Main results]
When SDS=3 g/L, the yield of ALE extract increased by 132% to 287.7 mg/g VSS, corresponding to a purity of 70%; When CTAB=0.8 g/L, the maximum extraction amount is 281.9 mg/g VSS, and the incremental effect is 127%, corresponding to the purity of 54%; When Triton X-100=1.25 mL/L, the extraction amount is 250.6 mg/g VSS, the incremental effect is 102%, and the corresponding purity is 63%.
In conclusion, the extraction amount of ALE can be increased from 12% to 25%~28% after optimization of surfactant extraction, and the optimal sequence of adding effects is: 0.04 g/g TS (Triton X-100)<0.06 g/g TS (CTAB)<0.19 g/g TS (SDS). In addition, the experiment also found that using SDS and Triton X-100 as additives can also improve the purity of ALE extract, from about 50% of the initial purity of alginate (500 mg/g ALE) to nearly 70%.
[Mechanism analysis]
Solubilization and dispersion
The results showed that the solubility of hydrophobic substances could be improved when the critical micelle concentration (CMC) of surfactant was exceeded or below. ALE mostly exists in the EPS structure as insoluble calcium alginate. The addition of surfactant may cause it to fall off from the sludge floc and dissolve into the liquid phase, increasing Na + (from adding Na2CO three ）And Ca 2+ Exchange probability, causing more ALE to be separated and extracted. On the other hand, for polysaccharide and protein, the main components of ALE, the presence of surfactant will also promote the dissolution of cellular flocs, extracellular structural proteins and functional substances, thus increasing the amount of ALE extract.
Group binding
Surfactants are usually composed of hydrophilic groups that can be completely dissolved in water and hydrophobic groups that are not easily soluble in water. They will combine with various groups or sludge floc groups in ALE through electrostatic attraction, hydrogen bonding and hydrophobic interaction, occupy the adsorption site, make ALE fall off from the cell flocs into the solution system, and increase the chance of combining with Na+in the solution, So as to improve the conversion efficiency of insoluble calcium alginate to dissolved sodium alginate. This binding change is proved by the functional group change of ALE extract.
The group binding angle is also the reason for the difference of enhanced extraction effects of different surfactants. The biggest difference among the three surfactants is that the tail connecting groups are different; The sulfate group attached to the tail of SDS can destroy the non covalent bond between EPS and combine with substances with special functional groups, so that it can separate from the cell floc and enter the liquid phase; The tail of CTAB is connected with quaternary ammonium salt surfactants, which can form complexes with polysaccharides other than proteins and most acid polysaccharides (the composition is completely different from alginate like substances or amyloid proteins, mainly polymers composed of O-methyl glucose and galactose as monomers).
Triton X-100 is a non-ionic surfactant with weak adsorption and binding capacity, but it can destroy the lipid bilayer, dissolve and destroy most of the proteins adsorbed and bound between molecules; The destruction of protein can lead to the disintegration of sludge flocs, complete free release of extracellular polymers, and increase the extraction amount.
[Important conclusion]
1) SDS (3.0 g/L), CTAB (0.8 g/L), and Triton X-100 (1.25 mL/L) were the optimal addition concentrations respectively, and the extraction amount increased from 124.1 mg/g VSS to 222.8, 281.9, and 250.6 mg/g VSS, respectively; The promotion efficiency is 79.5%, 127.2% and 102.0%, and the corresponding purity of alginate is increased from 50% to 70%, 54% and 63%;
2) Based on the dry weight of sludge, the same extraction amount was achieved (ALE=25%~28% VSS), and the amount of surfactant was different: 0.04 g/g TS (Triton X-100)<0.06 g/g TS (CTAB)<0.19 g/g TS (SDS);
3) Surfactants can promote the sludge floc material to enter the liquid phase through the formation of micellar solubilization to peel off from the surface of the binding cells, so as to be extracted and recovered. Among them, Triton X-100 micelle has the lowest concentration and the best incremental effect;
4) SDS, CTAB and Triton X-100 can further increase the extraction amount of ALE by dissolving in alginate with similar structure, among which Triton X-100 can also achieve the purification of substances through similar dissolving;
5) The group combination can also increase the extraction amount of ALE and purify it, and this effect is significantly different due to different surfactant group structures.
