Modelling, optimisation, and control of high-rate anaerobic reactors

Abstract

Treatment costs for biodegradable organic wastes have increased dramatically in recent years making anaerobic treatment an economic alternative to the disposal of high-strength wastes into the environment or into a municipal sewage system. Previously, research has concentrated on the design aspects of high-rate anaerobic treatment systems rather than addressing many of the problems associated with the stable and reliable operation of these processes. This dissertation investigates the economic optimisation and the application of a model-based control strategy to high-rate anaerobic reactors. This is achieved through the development and verification of a structured mathematical model of anaerobic degradation incorporated into the dynamic equations for single-stage and two-stage reactor systems.Verification of the dynamic behaviour of the anaerobic degradation model was undertaken through the comparison of a single-stage reactor model to experiments conducted at the University of Queensland and two independent sets of experimental data obtained from the literature. A two-stage process model was verified against steady-state data obtained from a full-scale, high-rate, anaerobic treatment process. Three major developments in the modelling of anaerobic degradation processes arose from the model verification experiments. It was found that a recently developed model for hydrogen inhibition in anaerobic degradation was able to simulate the regulation of the acid-forming bacteria for partial pressures of hydrogen less than 1000 parts per million. Furthermore, lactic acid was verified as an important parameter in the anaerobic degradation model, especially under severe organic shock loadings. Finally, the physicochemical model developed in conjunction with the anaerobic process model was able to calculate the reactor pH without the assumptions or iterative techniques used in previous modelling studies.Using the verified model of a single-stage anaerobic reactor, the operating pH was manipulated to obtain the minimum operating cost at different influent organic loadings. An overall saving of 13 percent (corresponding to a 4 percent reduction in caustic consumption) was obtained by reducing the pH from 7.3 to 7.2. Further reductions in the pH (into the region of pH inhibition) reduced the stability of the process, and provided only marginal cost benefits. This highlights both the benefits of operating a single-stage reactor as close to the region of pH inhibition as possible, and the need for a high performance control strategy to achieve this aim.The minimum operating cost of the two-stage model was obtained by manipulating both the acidification tank pH and the recycle flowrate from the methanogenic reactor. The optimum pH was found to occur close to the point where the methanogenic bacteria in the second stage were completely inhibited. However, significant savings in the consumption of caustic soda (10 percent), and increases in the generation of viable methane (9 percent) were obtained, without significantly compromising the stability of the process, by the reduction in the acidification tank pH from 6.0 to 5.9. The optimum recycle flowrate was found to occur at a flowrate too high for the actual process modelled, but significant realisable savings were obtained by increasing the recycle flowrate from 72.7 to 100 kilolitres per hour. The major savings came from a 40 percent reduction in the estimated effluent discharge cost and a 20 percent decrease in the cost of caustic soda consumption.Maintaining a particular pH specified by a process optimiser requires a reliable control strategy. The performance and reliability of an advanced model-based control strategy (Generic Model Control) was compared with a conventional control strategy, using the single-stage and two-stage anaerobic reactor simulation models. Generic Model Control was able to outperform the conventional control strategy, and maintain stable operating conditions for both the single-stage and two-stage reactor simulations. Further improvements in the performance of the two-stage model were obtained by the dual control of the pH of the acidification tank and the methanogenic reactor. Normally only the acidification tank pH is controlled. By using the dual control technique on the two-stage model the maximum shock loading able to be handled without failure of the methanogenic reactor increased by 50 percent.In summary, the application of high-rate anaerobic systems has been hampered by their reputation as expensive unreliable processes. This has led to very conservative design strategies. The results of this dissertation have contributed to the realisation that these systems are more reliable than first thought. This dissertation has shown that the high-performance robust control of anaerobic treatment plants is feasible, and that significant cost savings are possible by making realisable changes in the operating conditions of reactors. In the long term, as confidence in the controllability of anaerobic treatment plants increase, savings may also be made through a reduction in the size of reactors built to treat high-strength organic wastes.