Anaerobic digestion has great potential for application to the treatment of organic solid waste. However, process efficiency is still limited by hydrolysis of polymeric material. This thesis investigates the use of the rumen as a model to enhance rates of anaerobic degradation of cellulosic material and describes this system in terms of process kinetics, fermentation stoichiometry and microbial community composition.
A novel reactor system combining favourable aspects of previous designs was developed and applied for solid waste treatment. The reactor was operated as a sequencing batch reactor (SBR) and continuously stirred tank reactor (CSTR) to allow comparison of two operating modes. The SBR system was found to achieve high rates of hydrolysis with a shorter residence time (7 days) than other documented solid waste anaerobic digestion systems. SBR performance also exceeded that of the CSTR, using VFA production rate as an estimate of hydrolysis rate. At a cellulose loading rate of 10gL-1d-1, the SBR reactor had a VFA production rate of 230 mgCOD L-1 h-1. The reactor system performance was comparable to other efficient rumen reactor systems, hence demonstrating excellent potential for scale-up to treat solid waste.
Mass balances over the reactor resulted in a fully defined system allowing reaction stoichiometry to be estimated. Overall yields for acid and biomass were consistent across all trials indicating that varied fermentation products were likely due to differences in microbial community structure. Biomass yields were higher than those previously found in anaerobic systems but approximated yields obtained from pure cultures of rumen cellulolytic microorganisms. This may be due to a fundamental difference in the systems.
Significant variations in performance were found in the start-up phase of the reactor which resulted from changing microbial community composition. This was attributed largely to the change from a heterogeneous substrate in the rumen to pure cellulose in the reactor.
This study represented the first application of fluorescence in situ hybridisation (FISH) in a rumen reactor system. Probes designed to target rumen cellulolytic bacteria revealed that Fibrobacter and Ruminococcus were key microorganisms present in the system. The presence of these bacteria in biofilms on cellulose particles also confirmed cellulolytic function in a mixed culture system. The proliferation of Fibrobacter and Ruminococcus from low numbers detected in the rumen to significantly higher abundances in the reactor was noted during start-up. This indicated a selection process had occurred, which was in agreement with similar process data found in both reactor modes, despite different operating parameters. High propionate production was correlated with a high proportion of Fibrobacter, therefore demonstrating a link between microbial community structure and reactor performance.
Contrary to expected problems with autofluorescence from cellulosic material, actual autofluorescence levels were low precluding the use of this occurrence for imaging of feed particles. An innovative method combining the cellulose specific dye Congo Red with FISH using fluorescent microscopy was developed to overcome this issue. It is expected that this technique will be extremely useful in future studies for imaging cellulolytic microorganisms, which was demonstrated by the creation of 3-D reconstructions using image analysis.
The adaptation of the titrimetric and off-gas analysis (TOGA) sensor for use in anaerobic systems was a major achievement in this thesis. The sensor provided a powerful technique for real-time analysis of anaerobic processes and was found to be capable of accurate quantification of the inorganic carbon equilibrium system. The TOGA sensor was validated using acetatefed upflow anaerobic sludge blanket (UASB) granules to accurately determine reaction stoichiometry. The stoichiometry of formate uptake by hydrogen utilising methanogens was also established including an estimate of biomass yield. The ability of the sensor to monitor complex degradation processes was proven by qualitative monitoring of the multi-step process of glucose consumption by UASB granules. Analysis of soluble carbohydrate utilisation by rumen microorganisms was also successfully examined in the presence of background cellulose degradation. Cellulose was digested at a higher rate than the soluble substrates of glucose and cellobiose, indicating that attached microorganisms did not uptake these soluble substrates.
Application of the advanced methods developed in this thesis has allowed new insight into anaerobic digestion of cellulose by rumen microorganisms. These new techniques will be critical to facilitate the scale-up of an efficient rumen anaerobic system for solid waste treatment. Such a system has the potential to considerably reduce treatment costs and provide a feasible alternative to current unsustainable practices.