The performance and dynamics of fixed-film pressurized tubular bioreactors

A Fixed-Film Pressurized Tubular Bioreactor (FFPTB), which used the inner surface of a tube as the support surface, was evaluated under laboratory conditions to treat synthetic wastewater where glucose was used as the carbon source. Dissolved oxygen (DO) at the entrance of the tube was supplied in proportion to the influent chemical oxygen demand (COD) or total organic carbon (TOC) through a pressurized dissolution tank where compressed air or oxygen from a liquid oxygen cylinder was applied. Key variables included tube length and diameter, liquid flow rate, initial organic strength and gas pressure.The bioreactor was a coiled tube, 15 to 30 m long and 6.4 or 8 mm in diameter. It was a closed system operated under pressure at constant temperature (20$ sp circ$C) and pH (7.0). The hydraulic retention time (HRT) in the gas dissolution tank was 13 minutes, and the pressure applied was 210 to 350 kPa. The tube HRTs ranged from 16 to 38 minutes, and organic loadings from 2.29 to 11.46 g COD/m$ sp2$-d (the area being the inside surface area of the tube), depending on the liquid flows and initial waste concentrations.The FFPTB performed well (80-85% organic removal) under COD loadings of up to 7.64 g/m$ sp2$-d at relatively short HRTs (40-45 minutes) using the compressed air supply. When pure oxygen was used, the raw wastewater with an initial COD varying between 80-120 mg/L could be treated with even higher efficiency (90-95% organic removal).It was observed that the COD, TOC and DO decreased dramatically over the initial portion of the tube, then decreased more slowly. Biofilm thickness along the tube varied in similar fashion, with the greatest thickness (350-750 $ mu$m when the compressed air was used, and 1,900 $ mu$m with the pure oxygen supply) registered at the beginning of the tube and decreasing to 90 to 345 $ mu$m at the end. The biofilm thickness at the beginning of the tube was found to be proportional to the initial strength of the wastewater used. Furthermore, as the influent substrate concentration increased, the pressure had to increase appropriately to maintain a sufficient DO level for aerobic respiration in the system. The application of elevated pressures appeared to reduce the thickness of the liquid diffusion layer, thereby increasing the mass transfer rate, and consequently increasing the substrate utilization rate in the biofilm.Besides the experimental investigations, a mathematical model based on Monod-type kinetics, molecular diffusion, liquid layer mass transport, bacterial growth and decay as well as shear stress under steady-state conditions was developed and verified for the FFPTB. Based upon numerical simulations, for a given bulk substrate concentration, the model would give a solution for substrate flux into the biofilm and predict the TOC concentration along the tube. The model was also able to predict the biofilm thickness along the tube with reasonable accuracy.The system appears to be particularly suitable for high rate treatment of wastewaters, especially at low initial concentrations and for small systems. The high level of efficiency of the system could be achieved with short detention times and confined gaseous emissions as well as a low production of sludge