The increasing concentration of carbon dioxide (CO 2 )-the most dominant component of greenhouse gases-in the atmosphere has been of growing concern for many years. Many methods focus on CO 2 capture and storage and there is always the risk of CO 2 release to the environment. In this study, a new method to convert CO 2 to biogas with a high content of methane (CH 4 ) in an anaerobic system with a lab-scale upflow anaerobic sludge blanket reactor at 35°C was developed. In a series of experiments, the reactor was run with and without CO 2 -saturated solutions including volatile fatty acids (VFAs) as sources of hydrogen. The concentration of dissolved CO 2 in the influent solutions was 2.2-6.1 g/L, with corresponding chemical oxygen demand (COD) values of 2.6 -8.4 g/L for the solutions. Overall CO 2 removal values of 2.7-20 g/day (49 -88% conversion) were obtained for the organic loading rates (OLR) and CO 2 loading rates of 8 -36 gCOD/L ⅐ day and 6 -26 gCO 2 /L ⅐ day, respectively with CH 4 purity of above 70%. Also, VFA and COD removal were in the range of 79 -95% and 75-90%, respectively. Methanogenic activities of the cultures with the concentrations measured as volatile suspended solids (VSSs) were 0.12-0.40 L CH 4 /gVSS ⅐ d with the highest value for the system containing acetic acid. This anaerobic method can be applied to reduce CO 2 emitted to the atmosphere from a wide variety of industrial point sources with a valueadded product, CH 4 .

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A new wastewater treatment technology, called BioCAST, has been designed and developed for high rate and simultaneous removal of organic carbonaceous compounds as well as nitrogen and phosphorus, along with reduced sludge generation. The treatment system has two interlinked reactors containing four independent zones with different environmental conditions of aerobic, microaerophilic, anoxic and anaerobic for the biological treatment of wastewater, as well as two clarification zones and a filtration unit for solid-liquid separation. The treatment system contains suspended as well as fixed-film microorganisms. The performance evaluation of the BioCAST system was carried out at organic loading rates of 0.95 to 1.86 kg/m(3) d, and nitrogen and phosphorus loading rates of 0.02 to 0.08 kg/m(3) d and 0.014 to 0.02 kg/m(3) d, respectively. The results demonstrated high removal efficiencies of carbon and nitrogen throughout the operation period, reaching 98.9 and 98.3%, respectively. Phosphorus removal efficiency was lower than 50% during the first 160 days of operation but it increased with the increase of nitrogen loading rate above 0.05 kg/m(3) day and concomitant reduction of C/N ratio below 15. Phosphorus removal efficiency reached 94.1%, producing an effluent concentration of 1.4 mg/L after 225 days of operation. The overall biomass yield based on the consumed COD was 3.7%.

The theoretical and experimental aspects of the hydrodynamics and mixing in a new multi-environment bioreactor that uses the air-lift design were investigated. This study focused on the mixing characteristics, residence time distribution, liquid circulation between three zones of aerobic, microaerophilic and anoxic, and liquid displacement in the bioreactor at influent flow rates of 720-1,450 L/day and air flow rates of 15-45 L/min. The theoretical analysis of liquid displacement led to the estimation of the specific rate of liquid discharge from the bioreactor at any given influent flow rate, and the number of liquid circulations between various bioreactor zones before the discharge of a given quantity of wastewater. The ratio of mean residence time to the overall hydraulic retention time (t m/HRT) decreased with the increase of air flow rate at any given influent flow rate, and approached unity at higher air flow rates. Mixing was characterized in terms of the axial dispersion coefficient and Bodenstein number, demonstrating a linear relationship with the superficial gas velocity. A correlation was developed between the Bodenstein number and the Froude number. The study of liquid circulation between the zones showed that less than 1.5 % of the circulating liquid escapes circulation at each cycle and flows towards the outer clarifier, while the percentage of escaped liquid decreases with increasing air flow rate at a given influent flow rate. The specific rate of liquid discharge from the bioreactor increased from 0.19 to 0.69 h⁻¹ with the increase of air and influent flow rates from 15 to 45 L/min and 500 to 1,450 L/day, respectively. Under the examined operating conditions, mixed liquor circulates between 364 and 1,698 times between the aerobic, microaerophilic and anoxic zones before 99 % of its original volume is replaced by the influent wastewater.

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