Control Of Lactic And Propionic Acid Production During The Hydrolysis And Acidogenesis Of Food Waste

In the present study the effects of temperature and feeding loads on hydrolysis and acidogenesis of food waste were investigated in leaching-bed reactor (LBR) under different feeding modes. The objective of the present project was to reduce lactic acid and propionic acid production during anaerobic hydrolysis and acidogenesis of food waste for establishment of two-phase anaerobic digestion system.Firstly, the effect of temperature on hydrolysis and acidogenesis of food waste was investigated in LBRs in batch feeding mode. Results showed that hydrolysis and acidogenesis efficiency of food waste was enhanced when the temperature was raised from 20℃ to 30℃ or 40℃. In LBR operated at 20℃, forty-two days were required for anaerobic decomposition of food waste as indicated by an increase in the leachate pH to≥ 8.0 and decrease in soluble COD concentration to≤1000 mg/L.While the operation time required for anaerobic decomposition of food waste could be reduced to 21 days when the temperature was raised to 30℃ or 40℃.The contents of various VFAs in the leachate were also analyzed. The results showed that the hydrolysis and acidogenesis of food waste in all LBRs underwent two consecutive stages, namely lactic acid fermentation and mixed acid fermentation. In the initial lactic acid fermentation stage, lactic acid constituted the major organic acid which accounted for 60.96～83.37% of the total organic acids in the leachate. It could be drawn that LBR in batch feeding mode is impossible to be applied in two-phase anaerobic digestion system due to high concentration of lactic acid generated in its leachate which is toxic to methanogenic bacteria.Secondly,the effects of different temperature and different feeding loads on the hydrolysis and acidification of food waste were investigated in LBRs set at 20～40℃ in semi-continuous feeding mode. The results showed that the range of feeding loads suitable for semi-continuous operation for LBRs was different when they were set at different temperatures.Optimal feeding loads for the LBRs set at 20℃.30℃ and 40℃ were 25～50 g/d.50-150 g/d and 50～100 g/d. respectively. Regardless of the temperature set for LBRs. once the feeding loads exceeded above optimal range,sustainable operation would be impossible because of gradual accumulation of feeding materials in the reactors.Chemical analysis showed that the VFA composition in the leachate was also influenced by the feeding loads regardless of the temperature set for the LBR. When the LBRs were operated at above mentioned optimal feeding loads, acetic and butyric acids were always the main VFA species in the leachate. which accounted for as high as 76.82～94.71% of the TOA content. Meanwhile the percentages of lactic and propionic acids were only 5.29～23.18% and this is beneficial to methanogenesis. However, the percentages of lactic and propionic acids showed an increasing trend when the LBRs were operated at higher feeding loads and this may lead to adverse effects on subsequent methanogenesis during anaerobic digestion. Therefore semi-continuous operation of LBRs for anaerobic hydrolysis and acidogenesis is possible as long as the feeding loads were properly controlled regardless of the temperature.In order to disclose the mechanisms on the generation of VFAs during the anaerobic hydrolysis and acidogenesis of food waste, dominant fermentative bacteria were isolated using Hungate technique from LBRs which were operated at a feeding load of 100 g/d at different operating temperature. A total of 8 strains of fermentative bacteria were isolated and identified based on their physiological and biochemical properties and 16S rDNA sequence similarity analysis. Among them four strains (20B、20C、20D、20E) were isolated from LBR at 20℃ and were preliminarily identified as Selenomonas sp.20B、Megasphaera sp.20C、Bifidobacterium sp.20D and Bifidobacterium sp.20E, respectively; two strains (30A、30B) isolated from LBR at 30℃ showed similar physiological and biochemical properties and were identified as Bifidobacterium thermacidophilum 30A/3B;another two strains (40A、40B) isolated from LBR at 40℃ were identified as Staphylococcus sp.40A and Bifidobacterium thermacidophilum 40B,respectively. Strain 20C showed a quite relatively low similarity (95%) in its 16S rDNA sequence to any other strains in the genera Megasphaera was 95% and thus considered to be a new species in this genus.It should be noted that Bifidobacterium bacterium is always the dominant fermentative bacteria during the anaerobic decomposition in the LBRs, which were operated at a feeding load of 100 g/d at 20～40℃. However, concentrations of lactic acid in the leachate from all the three LBRs were extremely low. This presented a contradiction to VFA components in the leachate, which was probably due to Megasphaera 20C’s unique role in the transformation of lactic acid to acetic or butyric acid. It has been well established that lactic acid has a strong toxicity to methanogens during anaerobic digestion of organic matters. Obviously removing lactic acid from the leachate helps to improve the process stability of anaerobic digestion system. Therefore the mechanisms involved in the transformation of lactic acid during the anaerobic hydrolysis and acidogenesis of food waste deserved further study.