Study On Cassava Stillage Convert Into Medium Chain Fatty Acids By Anaerobic Microbials

Cassava stillage is a kand of wastewater with high concentration of organic,strong acidity and complex composition,and which contains a large amount of suspended cellulose.If not properly treated,cassava stillage may cause serious environmental pollution.It is an effective measure to treat cassava stillage by anaerobic digestion.Firstly,cassava stillage was converted to lactic acid and n-butyric acid with high selectivity,then converted to high value medium chain fatty acids,which is very important recycling treament.It not noly decreases envirmental pollution,but also creates considerable economic benefits.The pH value was an important factor for efficient acids production,and appropriate pH was required to produce acid efficiently from cassava stillage.A series of experiments of cassava stillage anaerobic digestion was carried out with different initial pH.While ensuring efficient and stable acid production,suitable pH value were selected to produce lactic acid and n-butyric acid with high selectivity.At the same time,the ability of lactic acid and n-butyric acid,which were main components of cassava stillage fermented effluent,converted into medium chain fatty acids was studied.By studying the pH and the ratio of lactic acid to n-butyric acid on carbon chain elongation,constructed an efficient medium chain fatty acids production system.And revealed the mechanism of changing the lactic acid conversion pathway during the carbon chain elongation under different pH adjustments.Finally,in order to improve the production of medium chain fatty acids and inhibit the production of propionic acid,the effect of pH on the microbial community and COG functional genes was clarified at the microbial level.Producing medium chain fatty acids and propionic acid of key microorganisms were elucidated during anaerobic digestion.Three pathways of lactic acid degraded were successfully constructed during the carbon chain elongation process,and the feasibility of those pathways were proved by the presence of key enzyme genes.A new way was providesd for resource recovery high-efficiency of cassava stillage.The main conclusion from this study are as follows:(1)The effect of initial pH on the acids production and selectivity of cassava stillage was investigated.The results indicated that:at acidic and alkaline initial pH,the hydrolysis effect of organic matter is better,and at around partial neutral initial pH,the acidification effect of solution organic matter is better.When initial pH was at 5.0,6.0 and 7.0,lactic acid to n-butyric acid production were 11.3±0.30 g COD/L,11.1 ± 0.32 g COD/L and 11.0 ± 0.28 g COD/L,respectively.They were more suitable for carbon chain elongatin to produce medium chain fatty acids than others.The dominant species in the microbial community were almost thermophilic anaerobe:g_Thermoanaerobacterium?g_Ruminiclostridium?g_Caloribacterium and g_Caldanaerobius.(2)The effects of pH and ratio of lactic acid to n-butyric acid on the prolongation of medium chain fatty acids were investigated.The results indicated that pH and ratio of lactic acid to n-butyric acid had an important effect on the carbon chain elongation.Lower pH could increase the conversion of lactic acid to medium chain fatty acid and inhibit to propionic acid;lower ratio of lactic acid to n-butyric acid could reduce the conversion of lactic acid to propionic acid.At pH 6.0 and the ratio of lactic acid to n-butyric acid was 1.2,carbon chain elongation had the highest efficiency in producing medium chain fatty acids(32.9 ± 1.00 g COD/L).Based on the results of cassava stillage acid production and lactic acid/n-butyric acid production of medium chain fatty acids.It was found that when phase I at initial pH 6.0 and phase II at pH 6.0,Cassava stillage were efficiently converted to medium chain fatty acids.(3)The 16S rRNA high-throughput sequencing was used to analyze the composition of microbial communities of carbon chains elongation anaerobic digestion.Microbes was reveals which play a key role in the conversion of lactic acid to medium chain fatty acids(g_Caproiciproducens and g_Bacteroide).Lactic acid as an electron donor was converted medium-chain fatty acids by g_Caproiciproducens,but when g_Bacteroides missing,the efficiency of g_Caproiciproducens converted lactic acid into medium-chain fatty acids was decreased significantly.The COG and KRGG gene function analysis revealed that that pH had an important effect on COG function:carbohydrate transport and metabolism,coenzyme transport and metabolism.There were three pathways in anaerobic digestion of lactic acid:acrylic acid pathway:reverse beta oxidation pathway(RBO)and fatty acid biosynthesis pathway(FAB).And the carbon chain elongation potential of FAB was much greater than RBO.