Kinetics of autothermal thermophilic aerobic digestion - application and extension of Activated Sludge Model No 1 at thermophilic temperatures.

The application of an ASM1-based mathematical model for the modeling of autothermal thermophilic aerobic digestion is demonstrated. Based on former experimental results the original ASM1 was extended by the activation of facultative thermophiles from the feed sludge and a new component, the thermophilic biomass was introduced. The resulting model was calibrated in the temperature range of 20-60 degrees C. The temperature dependence of the growth and decay rates in the model is given in terms of the slightly modified Arrhenius and Topiwala-Sinclair equations. The capabilities of the calibrated model in realistic ATAD scenarios are demonstrated with a focus on autothermal properties of ATAD systems at different conditions.

Nitrification of high-strength ammonium wastewater by a fluidized-bed reactor.

A laboratory-scale fluidized-bed reactor with an external aeration loop was used for nitrification of high-strength ammonium wastewater (up to 500 mg NH4-N/L). The results demonstrated that the system is capable of handling ammonium removal rates of up to 2.5 kg NH4-N/m3 x d, while removal efficiencies were as high as 98% and independent of the applied ammonium loading rates. Ammonium loading rates higher than 2.5 kg NH4-N/m3 x d resulted in decreased ammonium removal efficiency. The data show that near complete ammonium removal occurred at DO concentrations as low as 0.3-0.5 mg/L. However, the nitrite-nitrogen fraction in the effluent increased from 3.5% to 23.2% when the DO dropped from 1.0 mg/L to approximately 0.4 mg/L, respectively. The high specific removal rates in this system are one order of magnitude higher than that of suspended-growth systems. This can reduce the supplementary reactor volumes required for nitrification to less than 10% of that needed in conventional activated sludge systems. These results clearly indicate the potential economic gains that could be achieved through implementation of this technology.

Identification of factors contributing to degradation in autothermal thermophilic sludge digestion.

A laboratory scale ATAD reactor (6 days SRT, 24 h feed cycle) was operated at quasi steady-state conditions with real waste activated sludge feed. Hydrolysis tests of waste activated sludge demonstrated that 10% of feed suspended matter is hydrolysed practically instantaneously and 20-30% of all fed suspended matter is dissolved after 60 minutes in tap water and in cell free reactor liqueur equally. Respirometric, VSS and COD concentration data served as basis for calibration of a simple VSS based kinetic model. The calibrated model provided good fit to two separate sets of measured data. This model was used to evaluate different operation strategies. Modification of cycle length does not affect overall VSS emoval rate, while shorter cycle length or continuous operation helps avoid oxygen limited conditions. Further advantages of shorter feed cycles (reduced cooling effect, greater realizable load) support choosing continuous operation of the ATAD system if the main goal is VSS reduction. While reactor cascades increase efficiency, this advantage diminishes with increasing load. At high load rates increased construction costs are not justified by the expected improvement in efficiency.

The evaluation of dioxin emissions from pilot scale incineration of organochlorine compounds.

Formation of the dioxins was investigated in case of two fuel types (kerosene-dichlorobenzene and kerosene-dichloroethane) on a pilot scale liquid waste incinerator. It was determined the effect of the molecule structure (aromatic, aliphatic) and Cl content (1, 3 and 6%) of the liquids and the combustion parameters (O2 content, residence time and temperature). In some experiments particulate matters (fly ash and soot) were injected. The measured dioxin concentrations ranged up to 160 ng/Nm3 or till 10 ng/Nm3 TEQ, except at addition of soot where extremely high dioxin concentrations (up to 750 ng/Nm3, or 30 ng/Nm3 TEQ) were measured. Main conclusions of the experiment are: The incineration of aromatic chlorinated products produce almost 10 times higher dioxin concentrations than the aliphatic ones. A relatively low dioxin concentration is caused at low (1 to 3%) feed Cl contents while over a threshold value of more than 3% Cl intensifies sharply the dioxin formation. The dioxin generation can be characterized with concentrations measured at 600-650 degrees C. Dioxin toxicity reduction occurs when cooling down to 200-250 degrees C. Changes between these sampling temperatures can be explained mostly by sorption phenomena occurring between dioxin content of flue gases and solid deposits.

Mathematical modeling of ethanol production by immobilized Zymomonas mobilis in a packed bed fermenter.

A mathematical model which describes ethanol production in a packed bed fermenter containing. Zymomonas mobilis entrapped in small spheres of calcium alginate within a packed bed fermenter has been developed. The equations combine simultaneous diffusion and reaction as well as a complex flow pattern to calculate glucose and ethanol profiles in the column type reactor. As part of the study, diffusivity values for glucose and ethanol in cell-loaded calcium alginate were determined. Also a freecell kinetic expression for Z. mobilis at 33 degrees C and ph 6.0 was developed. Comparison of the model with actual experimental results were made showing average deviations of ca. 30-40%.