| The anaerobic fermentation of excess sludge to produce volatile fatty acids(VFAs)is an efficient way of sludge disposal.Recently,the production of bioplastic polyhydroxyalkanoates(PHA)using VFAs-rich sludge hydrolysate is becoming one of the research focuses in sludge reclamation.This study compared the microbial responses to thermophilic and mesophilic conditions to illustrate the effects of fermentation temperature.Besides,in order to reveal the effects of sludge organic concentration and composition on VFAs production,the variations of biotic and abiotic factors during VFAs production under different organic concentrations were investigated,and the fermentation systems with different protein to carbohydrate ratios were artificially set up.Specially,this study focused on the application of the valerate-dominant sludge hydrolysate.First,the simulative valerate-dominant hydrolysate was utilized to enrich a PHA-producing mixed culture,and its characteristics and functions were studied.Second,the real valerate-dominant sludge hydrolysate was utilized in the overall mixed-culture PHA production process to evaluate the efficiency and feasibility.Results showed that compared with mesophilic condition,thermophilic condition facilitated the accumulation of VFAs and extended the acid-forming phase.This was because that the concentrations of soluble protein and soluble carbohydrate were much higher under thermophilic condition,and the activities of extracellular hydrolases were enhanced by more than twice.Thermophilic condition also increased the relative abundances of bacteria which could accelerate sludge hydrolysis and acidification,for example,the homoacetogens.Lower initial organic concentration was beneficial to the dissociation of sludge matrix,while higher initial organic concentration corresponded to the higher abundance and density of total bacteria(yet less located in the outer layer of sludge matrix).The protein in sludge was solubilized and converted better under lower organic concentration,while the carbohydrate was solubilized and converted better under higher organic concentration.Correlatively,the higher organic concentration increased the activity of ?-glucosidase and decreased that of protease.The biotic and abiotic factors together decided that the lower organic concentration(VSS 5~20 g/L)and higher organic concentration(VSS 30~40 g/L)exerted different influencs on the VFAs yield,percentage and composition.The sludge with more protein and less carbohydrate was adverse to the hydrolysis of particulate organics in the sludge,but beneficial to the production of VFAs.The increase of the fraction of protein in sludge could promote the secretion and activity of extracellular hydrolases.The PHA-producing mixed culture enriched by simulative valerate-dominant hydrolysate showed preference for odd-carbon VFAs,and had high ability to synthesize 3HV and 3H2 MV monomers.The PHA biosynthetic pathway of this enriched culture was different from that of the PHA culture enriched by simulative acetate-dominant hydrolysate.With mixed iso-valerate and n-valerate as substrate,its PHA production performed better than with single iso-valerate or single n-valerate,and the addition of moderate propionate could promote the production of 3HV and 3H2 MV.Using the real complex valerate-dominant sludge hydrolysate with abundant non-VFAs and nutrients,the overall two-stage mixed-culture PHA production process was established.Under feast-famine mode,the maximal PHA production reached 1.30 g/L,and the maximal PHA content reached 42.31%.In famine phase,the culture preferentially decomposed the stored PHA,other than using the extracellular non-VFAs,for maintenance and further growth.When N/COD and P/COD respectively exceeded the limitation of 150 and 15 mg/g,the PHA content would be apparently inhibited.The total yield of this overall PHA production process was estimated as 1 kg dewatered sludge resulting in 5 g PHA,and the COD conversion ratio was about 7.73%. |
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