Mathematical modelling of the anaerobic digestion process.

Summary

Anaerobic digestion is a complex sequential process, performed by symbiotic microorganisms with differing kinetics and growth rates in which liquid-gas equilibrium plays a critical role. Complex interactions between the physico-chemical and the biochemical system, both in dynamic and steady state conditions, in anaerobic digestion processes, make them quite difficult to intuitively comprehend. Insights into the process requires extensive laboratory testing, however, because of inherent slow growth rate of some anaerobes experimentation is rather time consuming, thus dynamic mathematical models are very useful in evaluation of digester control strategies and reactor arrangements.

Mathematical modelling and computer simulation of aerobic biological processes is well documented. Computer simulations of biological processes can aid in the design and control of full scale systems predicting biomass growth, oxygen consumption etc. Less information is available concerning the simulation of anaerobic systems as biogas production can affect process performance and it is more difficult to estimate the correct parameters for degradation kinetics.

A Windows® based Visual-Basic® simulator has been developed using the IWA Anaerobic Digestion Model No.1. Numerical integration of the models differential equations is achieved using a corrective fourth-order Runge-Kutta method. The simulator model is to be calibrated against data obtained using an iconic model under controlled conditions, and subsequently to be used to formulate and evaluate digester control strategies which are in-turn to be validated by performance data of the iconic model. The anaerobic technology modelled is a expanded granular sludge bed reactor

Research Team

Dr. Billy Fitzgerald. Principal Researcher Department of Science, IT, Sligo Mr. Brian Mulligan Joint Researcher Department of Engineering, IT, Sligo Mr. Anthony Lang Research Student Department of Engineering, IT, Sligo

Research Objectives

Provide by literature searches an expansive database on the kinetic formulae used to model the anaerobic sludge process.

Produce a Windows® based computer simulation based on the most recent kinetic formula as reported in literature for a latest generation anaerobic wastewater treatment technology.

Compare the results predicted by the computer model (simulator model) with the physical results found in the laboratory using an iconic model (physical model) of the same design and configuration as those specified for the computer model.

Improve the correlation between the computer model results and those of the pilot plant by modifying the kinetic equations and trying those produced by different researchers.

Investigate the effects of the various parameters on both the computer model and the pilot.

Summary Research Team Research Objectives Research Contribution Research Details

Anaerobic Technology Modelled The technology modelled is the expanded granular sludge bed (EGSB) reactor.

The expanded granular sludge bed (EGSB) is a modification of the traditional upflow anaerobic sludge blanket (UASB) reactor technology. Both use granular sludge, but the EGSB reactor operates at high superficial velocities, which are obtained by high effluent recirculation rates and an elevated height/diameter ratio. This causes high hydraulic mixing which improves the wastewater-sludge contact thereby providing more efficient treatment..

Degradation of substrate in EGSB reactors is limited by reaction kinetics more so than by mass transfer. A UASB reactor can suffer mass transfer limitations caused by short-circuiting through the sludge bed this is not the case for EGSB reactors. This is due to their high liquid and gas upflow velocities which allow for complete expansion of the sludge bed, assuring good mass transfer from the liquid to the sludge granules.

EGSB type reactors may be regarded as an optimised sludge bed system.

EGSB technologies have recently attracted commercial interest as they appear to be out-competing more conventional systems from both an economic and a performance standpoint.