Extraction Of Short Chain Fatty Acid From Food Waste Fermentation Liquor By In-Situ Synthesis Of LDHs

At present,almost all of China's urban sewage treatment plants are using biological denitrification for sewage denitrification treatment.However,the shortage of carbon source is widespread,especially in southern China with low C/N ratio,and a large number of organic matters are needed to be used as carbon source for achieving successful biological denitrification to meet the discharge standards of sewage.Food waste?FW?anaerobic fermentation liquor is rich in high concentrations of short chain fatty acids?SCFAs?.It has a good bioavailability and can be used as a biological denitrification carbon source.However,high ammonia nitrogen in the FW anaerobic fermentation liquor will increase the overall denitrification load and inhibit the denitrifying bacteria activity,thus,causing the poor effect of denitrification process.At the same time,excessive dosing of the FW anaerobic fermentation liquor will be added in order to ensure high nitrogen removal efficiency,nevertheless,easily resulting the secondary pollution and low utilization rate of the carbon source.In layered double hydroxides?LDHs?,the species and the amount of anions in the interlayer are adjustable.In this paper,we try to extract the SCFAs from the FW anaerobic fermentation liquor and use it as an interlayer anion of LDHs via in-situ synthesis of SCFAs-LDH,then it can be further appiled as a solid slow-release carbon source for denitrification.The aim of this study was to extract the SCFAs efficiently from the FW anaerobic fermentation liquor.In this paper,the extraction ability of LDHs towards each component of SCFA was first studied in the simulation system based on the experimental data in our previous work.And the effects of different ratio of SCFAs with metal salt concentration on the loading carbon content of SCFAs-LDH and the extraction ability of SCFAs were then investigated.At the same time,the intercalation mechanism of SCFAs and the structural changes of LDHs were further discussed by using X-ray diffraction,elemental analysis and infrared spectroscopy.On the other hand,the intercalation characteristics of SCFAs anions and CO₃^2-coexisting were studied in the actual FW anaerobic fermentation liquor system.As regards the interference of CO₃^2-on the formation of SCFAs-LDH,pretreatment of the actual FW anaerobic fermentation liquor was carried out to solve this problem.And the secondary assembly of CO₃^2--LDH has been performed to form SCFAs-LDH.The main results and conclusions of this paper were as follows:1)In the simulated fermentation liquor system,the characteristic peaks of?003?were shifted to the low 2?direction when the single short-chain fatty acid was extracted to produce LDHs,with the charge density was 2,SCFA/Al³⁺=3,pH=9.And the interlayer spacing increased with the growth of the carbon chain,indicating that the anions of acetic acid,propionic acid and butyric acid were successfully intercalated to the LDHs interlayer.2)In single SCFA intercalated LDHs,the loading carbon content of CH₃COO-LDH,C₂H₅COO-LDH and C₃H₇COO-LDH could reach to 115.8 mg COD/g LDH,187.9 mg COD/g LDH and 288.6 mg COD/g LDH,respectively;and butyric acid accounts for the majority of the SCFAs-LDH with percentage of 46%,indicating that the butyric acid is more likely to be the interlayer anions in LDHs than acetic acid and propionic acid.3)In the simulated mixed SCFAs system,the loading carbon content of SCFAs-LDH increased obviously with the increase of SCFAs/Al³⁺ratio,as well as the increasing trend of acetic acid,propionic acid and butyric acid in SCFAs-LDH.While the extraction ability of SCFAs increased first and then decreased with the increase of SCFAs/Al³⁺ratio.The highest extraction rate reached to 29.8%when the molar ratio of Zn:Al:SCFAs=2:1:3,the loading carbon content reached to 186.5 mg COD/g LDH.4)In the actual FW anaerobic fermentation liquor,the carbonate has a significant effect on the extraction of SCFAs by in situ synthesis of LDHs because the carbonate takes precedence over various SCFAs acid ion intercalation into the LDHs.5)Compared with the SCFAs-LDH_FW obtained by in-situ synthesis,the loading carbon content of Ca-SCFAs-LDH_FW and HNO₃-SCFAs-LDH_FW were 1.68 and 2.3times of the original SCFAs-LDH_FW.It was suggested that the carbon content of the final product can be increased by using chemical method to remove the carbonate in the fermentation liquor.6)As for the reuse of the generated CO₃^2--LDH,it was found that the loading carbon content of SCFAs-LDH_FW-IX and SCFAs-LDH_FW-RC obtained by ion exchange and re-coprecipitation were 2.3 times and 3.84 times of the original SCFAs-LDH_FW,meanwhile the contents of acetic acid,propionic acid and butyric acid in LDHs also increased.