Study On Optimization Of Synthesis Conditions For PHAs From Undomesticated Surplus Sludge

Polyhydroxyalkanoates(PHAs)is an biomass energy storage substance in which microorganisms transform external carbon sources into internal carbon sources and store them in the cell to maintain their normal growth under adverse environment.PHAs is similar to traditional plastics in material properties and it is biodegradable.So PHAs is a good substitute for traditional plastics.Using surplus sludge as mixed microbial culture and waste fermentation liquid as carbon source to synthesize PHAs can reduce the cost of PHAs synthesis and realize the resource utilization of waste,which has important research significance for the sustainable development of society.At present,most studies on the synthesis of PHAs from surplus sludge are "three-stage" synthesis of PHAs,which takes a lot of time in the cultivation and domestication of sludge.In addition,the instantaneous aerobic feeding method was adopted during the enrichment of the bacterial community and the synthesis of PHAs.The concentration of dissolved oxygen in the reactor was kept above 2-3mg/L,which is not conducive to energy saving.The purpose of this study was to study the optimization of conditions for the direct synthesis of PHAs without domestication from surplus sludge.Two different types of residual sludge were selected,which were the residual sludge(R1)from the simultaneous nitrifying/denitrifying phosphorus removal system and the residual sludge(R2)from the A2 O process.Under the same conditions,the synthesis quantity,rate and intracellular substance of PHAs were measured and the ability of PHAs synthesis from two kinds of surplus sludge was compared and analyzed through testing various indexes.The content of bacteria with PHAs synthesis ability in two kinds of residual sludge was analyzed by molecular microbiology.Then,in order to improve the production of PHAs from the two kinds of surplus sludge,the regulation methods of micro-oxygen and pre-aeration were studied,and the optimal control parameters were obtained.The yield effect of PHAs under high VFAs load was also investigated.On this basis,R1 surplus sludge with higher PHAs production was selected,and two different types of fermentation liquid were added as carbon sources,and each of them ran for several long periods,to investigate the stability of the synthesis system and the influence of different types of fermentation broth on the total amount of PHAs synthesis and the proportion of each component.The experimental results showed that :(1)under the same conditions,the synthesis amount and rate of PHAs of residual sludge R1 were greater than R2.In R1,the necessary content of intracellular glycogen and intracellular phosphorous to synthesize PHAs was also higher than R2.Two strains capable of PHAs synthesis in the excess sludge are mainly Thauera,Dechloromanas,Candidatus accumulibacter,Caldilinea and Zoogloea.Among them,Thauera?Dechloromanas and Candidatus accumulibacter accounted for the largest proportion,in which R1 is more than R2.(2)the synthesis effect of the two kinds of residual sludge in microoxygen environment is the best when the air flux is 10L/H.Under the combined process of pre-aeration + micro-oxygen,the yield of PHAs can be further increased.When the pre-aeration time is controlled to be 20 min and the pre-aeration air flux is 60L/H,PHAs synthesis effect is the best.Moreover,the biomass of R1 has not decreased under the pre-aeration condition.Excessive VFAs load will reduce the synthesis rate.The optimal VFAs concentration is 460-520 mg /L,and the COD concentration is 650-750mg/L In addition,ORP can be used as a guiding parameter in the process of synthesis of PHAs in the micro-oxygen segment.The moment when ORP reaches the valley point is the moment when PHAs accumulates to the maximum value in the surplus sludge.(3)the synthesis of PHAs with undomesticated surplus sludge under the combined process of pre-aeration and micro-oxygen has a high stability.The total amount and synthesis rate of PHAs were higher in the fermentation liquid with even VFAs as the main carbon source than in the fermentation broth with odd acid as the main carbon source.Moreover,the ratio of PHB and PHV to PHAs can be regulated by using different types of fermentation broth.Thus,R1 surplus sludge has a higher synthesis capacity of PHAs.The regulation mode of previa aeration + micro-oxygen can increase the PHAs yield of undomesticated surplus sludge.The fermentation broth mainly with even carbon VFAs as the carbon source has higher PHAs yield and synthesis rate,and PHB accounts for a higher proportion.