| It is of great significance to synthesize polyhydroxyalkanoates (PHA) bybacterial consortium from the excess sludge fermentation liquid which will greatlylower the cost of PHA production and result in the sludge reduction and recycling.However, the high initial carbon concentration without limit of nitrogen in sludgefermentation liquid and its complexity and instability of components limits thehydrolysate application in actual PHA production. Therefore, it is important to studythe laws and kinetics of PHA production by bacterial consortium from sludgefermentation liquid in order to provide theoretical guidance and technical support forthe technology development.Firstly, the laws of PHA accumulation by bacterial consortium under high initialcarbon source concentration and nitrogen-unlimited conditions were studied insubstrate of mixed violatile fatty acids (VFAs), which were the main components offermented liquid. It shows that under feast and famine regime, the highest PHAcontent (62.43%, of cell dry weight) and PHA yield (0.71g/L) were obtained whencarbon was exhausted. The surplus nitrogen and phosphorus as the essential elementsinfluenced the active cell growth. Kinetics analysis indicated that PHA accumulationwas the predominant in the process comparing with the cell growth(YP/S≈3YX/S). Theconsortium was prior to utilize even number carbon acids to synthesize HB and thenthe odd number carbon acids for HV. The thiourea, one kind of nitrification inhibitors,could facilitate the PHA synthesis.Secondly, the composition of VFAs played an important role in the PHAsynthesis as well as the HV content in the PHA. When acetic acid, propionic acid,butyric acid and iso-valeric acid was adopted as the sole carbon source separately,both of the substrate uptake rate and PHAs%were acetic acid> butyric acid>propionic acid> iso-valeric acid, indicating that the bacteria consortium was prior touse short-chain and even-number carbon sources for PHA synthesis. When themixture of acetate/propionic acids and butyric/iso-valeric acids were adopted ascarbon source, the HV monomer ratio increased with the ratio of odd-number carbonacids(1:3,1:1,3:1) in substrates, indicating that odd-number carbon acids favored HVsynthesis. The group using acetic acid and propionic acid as carbon source presented ahigher yield of PHA (YP/S) than that of butyric acid and pentanoic acid, indicating that short chain VFAs were favorable for the transformation from substrates to PHA.Finally, the non-VFAs in sludge fermentation liquid (such as soluble proteinsand carbohydrates, etc) played an important role in the allocation of carbon sourcebetween cell growth and PHA accumulation. Taking the bovine serum albumin (BSA)and glucose as model substrates, it showed that an increase of non-VFAs content(0~50%) facilitated the cell growth while restrained the PHA accumulation. With thesame non-VFAs content, the sludge fermentation liquid with carbohydrates as themain non-VFAs will be better for PHA synthesis than that with the protein as themain non-VFAs. When using the glucose, BSA and yeast extract as the sole carbonsubstrate separately, none of the consortium accumulated PHA well, indicating noneof these carbon sources was ideal substrate for the bacteria consortium to accumulatePHA.In conclusion, sludge fermentation liquid with higher content of VFAs andodd-number carbon acids, will probably obtain a relatively high content as well as abetter quality of the PHA product. Besides, a balance should be found between cellgrowth and PHA accumulation in order to achieve a relative high PHA yield. |
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