Extending ASM1 to Model a Tubular Biofilm Reactor

The world’s largest tubular biofilm reactor for biological oxidation of hydrogen sulphide (H2S) in geothermal power station cooling water was successfully commissioned in NZ in 2012. The original design of the biofilm reactor was based on pilot studies conducted over several years and an empirical model was developed to predict the biofilm reactor performance. The biofilm reactor has operated for six years and continues to perform as predicted by this model.

To enable a more rigorous evaluation of the fundamental biokinetics from first principles, and to consider the application of the tubular biofilm reactor to alternative configurations and operation, a more mechanistic design approach was required. This paper describes the development of a mechanistic biofilm reactor model based on an extension of activated sludge model 1 (ASM1) to simulate the biological oxidation of H2S by sulphur oxidising bacteria (SOB) and the incorporation into a tubular biofilm reactor simulation using commercial software.

A review of literature identified strategies for the incorporation of SOB and sulphur reducing bacteria (SRB) kinetics into ASM1. The extension of ASM1 was carried out by adding processes for H2S biological oxidation to elemental sulphur or sulphate endpoints with kinetic rate equations and component stoichiometry drawn from literature values.

The extended ASM1 model was incorporated into a biofilm reactor simulation using commercial software (GPS-X Hydromantis). The paper describes how the tubular biofilm reactor was simulated in GPS-X as a series of reactors that incorporate both fixed film and bulk fluid biokinetics. A total of ten reactors in series were modelled to represent individual 20m x 100mm diameter pipe sections for the total full-scale tubular length of 200m. Sulphide removal data from the pilot studies collected at 20m intervals was used to modify the model SOB kinetic and stoichiometric parameters to achieve a simulation performance fit. Model values were compared with published literature values.

The biofilm reactor’s performance was simulated for different flow velocities, temperature and dissolved oxygen conditions. The evaluation of the extended model for simulation of the full-scale biofilm reactor, provided a better understanding of the biofilm reactor’s performance and presents the opportunity to optimise the operating conditions to improve energy consumption and sulphide removal.

The process for incorporating further processes into the ASM models extends beyond the scope of the current biofilm reactor and could be applied within the wastewater industry to simulate the fate of contaminants in alternative biofilm reactor configurations.