| In recent years, the amount of excess sludge produced from municipal and industrial wastewater plants increased very rapidly with the sustainable and fast development of economy in China. The treatment and disposal of sludge have become one of the most important and complex problems. In fact, the main part of sludge is organic biomass, and it can become a source for valuable biochemicals such as volatile fatty acids (VFAs). The bioprocess strategy that results in the production of VFAs not only achieves the objective of controlling pollution and reducing sludge volumes but also efficiently explores the resource of organic substance in sludge.It is known that volatile fatty acids are important intermediate compounds in the metabolic pathway of anaerobic digestion. In this dissertation, using excess sludgs from three different sources, the tests were firstly performed to analyze the relationship between VFAs and consumed organic matter, and to investigate the effects of initial carbon-nitrogen-ratio (C/N) on the acidification efficiency of sludges. And then, hydrolysis and acidogenesis of sludge pre-treated with different techniques was analyzed. Based on the results of the study on those pretreatment techniques, control strategies of pH and initial C/N were investigated in order to achieve the maximum and stable production of total VFAs or different types of anaerobic acidogenesis. The microbial community structure and the dominant microbial population was discovered through the analysis of Terminal-Restriction Fragment Length Polymorphism (T-RFLP). Combining the means of fluorescence in situ hybridization analysis with the analysis of T-RFLP, the reason for accumulating acetic acid was further explored. Furthermore, the main metabolic pathway of different individual VFAs was investigated by analyzing the relative abilities of some key enzymes. Main results of this dissertation were shown as follows:(1) The main component of sludge including proteins and carbohydrates is enclosed in microbial cells. The autolysis bioprocess of microbial cells was slow, which resulted in the relatively low conversion ratio from proteins and carbohydrates into VFAs. Moreover, the results also showed that acetic acid was the main product in total VFAs during the anaerobic digestion. (2) Effects of various pretreatment methods including thermo-acid, thermo-alkaline, ultrasonic-alkaline and ultrasonic-acid on the solubilization of excess sludge were investigated. The results showed that both thermo or ultrasonic -alkaline significantly improved the solubilization of sludge at a high concentration (7.4% of total solid). Solubilization of volatile solids (VS) and total proteins was 60.2%~61.6% and 66.8%~67.5%, respectively. Moreover, STOC and STN concentration increased more than before pretreatment. STOC increased 7.62 times, while STN increased 4.97 times. The ultrasonic- or thermo- alkaline pretreatment significantly decreased the particle size of WAS and account of less than 10μm particles was for 50% and many minute cavities appeared on the surface of particles. They could not only hydrolyze extracellular biopolymer but also destroy some microbial cells. But the thermo- or ultrasonic- acid pretreatment just disintegrated some sludge flocs. On the contrary, the effect of the thermo- or ultrasonic- acid method on the solubilization of WAS was not obvious. They just disintegrated some sludge flocs.(3) Effects of these four pretreatment techniques on the subsequent acidification efficiency of excess sludge were further analyzed. The results showed that the acidification efficiency of sludge was improved in the case of the thermo- or ultrasonic- alkaline pretreatment. Compared with the untreated sludge, the concentration of total VFAs increased by 59.1% for the ultrasonic-alkaline pretreatment, and increased by 68.2% for the thermo-alkaline pretreatment at the final fermentation. Soluble protein was the main source for the total VFAs production in the pretreated WAS slurry with the thermo- or ultrasonic- alkaline methods. On the contrary, the yield of total VFAs from the thermo- or ultrasonic- acid pretreated sludge was lower than that from the untreated sludge. Combining the cost of pretreatment with the acidogenic efficiency of sludge, thermo-alkaline was established the optimal pretreatment technique.(4) Effects of pH on the sedimentation of soluble proteins from the thermo-alkaline pretreated sludge were investigated. The results showed that the concentration of soluble protein decreased slowly with the decrease of pH from 12.0 to 7.0, but it became fast when pH was adjusted from 7.0 to 3.0. The data strongly implied that the most part of insoluble protein deposited after the pH adjustment was not consumed as the substrate for producing VFAs during the fermentation.(5) Bacterial community structure was monitored through T-RFLP at different pHs. The results revealed that the impact of pH on the distribution pattern of VFAs resulted from the varieties of bacterial community structure. The dominant bacterial population varied with the change of pH in the anaerobic process. Granulicatella was the dominant bacterial population at pH 12.0, while Peptostreptococcus dominated at pH 10.0. With the adjustment of pH from 7.0-5.0 to 3.0, the dominant bacterial population evolved from Clostridium to Bacillus.(6) Base on the analysis of fluorescence in situ hybridization, the amount of syntrophic acetogenic bacteria was very small and its relative abundance was only 0.01% in total microorganisms when pH was controlled at 10.0. Peptostreptococcus played an main role on the accumulation of acetic acid. The main metabolic pathway for producing acetic acid was the Stickland reaction between different amino acids. Controlling the pH of fermentation process at 10.0, not only the activity of methanogenic archaea but also the growth of sulfate-reducing bacteria was inhibited. The yield of VFAs could be significantly improved and maintained stable by controlling fermentation pH at 10.0.(7) Control strategy of initial carbon-nitrogen-ratio (C/N) could implement different types of acid-forming fermentation during the anaerobic acidogenesis. Controlling initial C/N in the range of 12~44, the main product was acetic acids, while propionic and acetic acids became the main products under the conditions of initial C/N varied between 56 and 69. With the increase of initial C/N from 156 to 256, butyric acid-type of fermentation appeared in the fermentation process. The change in the different types of acid-forming fermentation was caused by the changes in the dominant microbial populations, from acetic acid-producing bacterial (Peptostreptococcus) to propionic acid-producing bacterial (Propionibacterium), and then to butyric acid-producing bacterial (Clostridium). At the low initial C/N, the increase of H2 and CO2 was slow, but with the increase of initial C/N from 156 to 256, they increased rapidly. Moreover, the main metabolic pathway resulted in the accumulation of predominant VFAs was changed from the Stickland reaction at low initial C/N to the metabolic pathways of pyruvic acid from glycolysis.
|
PDF Full Text Request