Browsing by Subject "Biotrickling filter"

An integrated biotrickling filter (BTF)-Anammox bioreactor system was established for the complete treatment of ammonia. Shortcut nitrification process was successfully achieved in the biotrickling filter through free ammonia and free nitrous acid inhibition of nitrite oxidizing bacteria. During transients, while increasing nitrogen loading, free ammonia was the main factor that inhibited the activity of ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB). During steady state operation, free nitrous acid was mainly responsible for inhibition of NOB due to the accumulation of nitrite at relatively low pH. Ammonia removal by the BTF reached up to 50 gN m-3 h-1 with 100% removal at an inlet concentration of 403 ppm and a gas residence time of 20.8 s. Average removal of ammonia during stable operation was 95%. The anammox bioreactor could remove 75% of total nitrogen discharged by the BTF when the two reactors were connected. The possibility of operating in complete closed loop mode for the liquid was investigated. However, due to the limited activity of the Anammox bioreactor or the fact that this reactor was undersized, recycling the Anammox effluent back to BTF caused accumulation of nitrite in the system which further inhibited activity of Anammox and progressively caused failure of the system.

A conceptual model of both bioreactors was also developed to optimize the integrated system. The model was developed by including mass balances of nitrogen in the system and inhibition factors in microbial kinetics. Parameters such as hydraulic residence time (HRT), empty bed residence time (EBRT) and pH had significant impact on the partial nitritation process in the BTF. Model simulations also indicated that implementing a recycle for the Anammox bioreactor was needed to reduce the inhibitory effect of nitrite on the performance of the system.

Ammonia is an odorous gaseous compound emitted by a variety of industrial facilities. This study aimed to address the feasibility of ammonia gas removal using a biotrickling filter (BTF) coupled with an anammox bioreactor. In the BTF, the influent ammonia gas partitioned into the trickling water and was converted to nitrite via partial nitrification. The effluent liquid from the BTF, containing nitrite and ammonium concentrations, was fed into the anammox reactor where autotrophic denitrifying bacteria converted the ammonium and nitrite to dinitrogen gas. For the anammox reactor to operate efficiently, the influent ammonium and nitrite concentrations must be in a 1 to 1 molar ratio. To evaluate the feasibility of this system, a lab scale BTF and anammox reactor were constructed and operated and a conceptual model for this system was developed. To obtain a nitrite to ammonium ratio close to 1, it was found that the effluent pH from the BTF must be maintained below 7, and the loading rate could not exceed 8.7 g N/m3h. At this loading rate, complete ammonia gas removal occurred. A recycle rate of 1.4 times that of the influent was implemented in the BTF to increase performance and improve the nitrite to ammonium ratio. The addition of the recycle line achieved a nitrite of ammonium ratio of 0.97 at a pH value of 7.67. The anammox reactor achieved 88% removal of ammonium and nitrite at a loading rate of 10.5 g N /m3h. The fact that the BTF was able to achieve a 1 to 1 nitrite to ammonium ratio indicated that coupling of a BTF with the anammox reactor should be feasible. The mathematical model underpredicted effluent ammonium and nitrite concentrations in the BTF and greatly overpredicted the effluent concentrations from the anammox reactor. To improve the BTF model inhibition factors and oxygen supply need to be accounted for. Further development of the growth kinetics in the annamox model are necessary as well.

Upgrading raw biogas (~60% CH4, 40% CO2, 1000-5000 ppmv H2S) to renewable natural gas (RNG) (> 97% CH4, < 2% CO2, < 4 ppmv H2S) for injection into the grid is a desirable endeavor. RNG would allow for a clean alternative to natural gas derived from fossil origin, and it also have a versatile use as a transportation fuel and source of heating energy. Current physical-chemical technologies, such as pressure swing absorption and organic chemical scrubbing, can successfully upgrade raw biogas to meet RNG standards (1,2). However, they are energy intensive, costly, and can remove fractions of methane gas along with the impurities. Recently, biological biogas upgrading technologies have emerged as a promising solution for converting raw biogas to RNG. The method relies on hydrogenotrophic methanogens to reduce the CO2 fraction of raw biogas to CH4 using H2 as the electron donor. This method is advantageous compared to traditional biogas upgrading methods because is sequesters carbon emissions while increasing the volumetric production of methane. While early studies on biological biogas upgrading in continuously stirred tank reactors were conceptually validating, hydrogen mass transfer resistance from the gas-to-liquid phase prevented fast upgrading capacities from being realized. Slow biogas upgrading rates hinder the economic feasibility of the process. Furthermore, these studies only focused on CO2 removal when in reality, other impurities, such as corrosive H2S, must also be removed before RNG injection into the natural gas pipeline.

The overall objective of this thesis research is to develop a biological trickling filter reactor that can upgrade biogas to RNG standards at fast upgrading capacities while biologically co-removing H2S. A biological trickling filter was chosen for this investigation because they are characterized by a high specific surface area for biofilm growth, high biomass density, and are known for their high overall mass transfer coefficients; all factors that contribute to high conversion rates. A proof-of-concept study validated that this approach could achieve upgrading rates that were 5 – 30 times faster than other bioreactor configurations. This finding supported further studies that aimed to investigate hydrogen mass transfer resistance specifically in a biological trickling filter reactor. This was accomplished using a highly sensitive dissolved hydrogen sensor, which collected concentrations in real-time. Using this sensor, experiments were conducted to assess mass transfer resistance in the gas and liquid films. It was discovered that there was no external resistance in the gas-film. Furthermore, the liquid phase was a main barrier for mass transfer and reducing the liquid film thickness can significantly improve biogas upgrading capacities by 20%.

In addition to laboratory experiments, a robust and conceptually correct mathematical model was developed for a biogas upgrading biological trickling filter. The model was used to provide deeper insight into process fundamental and identify biological versus mass transfer limitations in the bioreactor. The model successfully replicated complex experimental findings and confirmed that liquid transport through the bioreactor bed was faster than the rates of mass transfer and biological conversion. A sensitivity analysis revealed that the model was most sensitive to the empty bed contact time and the maximum rate of reaction. Interestingly, the mass transfer coefficient for the liquid film (kLa) did not significantly improve the biogas upgrading rate for the bioreactor. This is because the model predicts that the bulk of hydrogen mass transfer occurs from the gas to non-wetted biofilm phase.

Concluding mass transfer resistance testing and process optimization, it was demonstrated that the engineered bioreactor could successfully upgrade various biogas compositions to RNG standards. The rates achieved for these experiments (10 – 20 m3CH4 m-3 d-1) were 1.5 – 25 times faster than other comparable research studies. To determine the economic feasibility of this technology, a paper scale-up cost analysis was conducted to estimate the investment and operation costs of a biological trickling filter upgrading raw biogas (60% CH4, 40% CO2) to RNG (> 97% CH4 < 2% CO2). This was accomplished by using experimental findings to scale the dimensions and determine heating and cooling requirements based on seasonal temperatures. Cost estimates for parts were acquired through vendor quotes. The cost analysis showed that the bioreactor is economically feasible however, the H2 acquisition cost was ~ 650% of the bioreactor investment cost. This is because H2 was acquired from the electrolysis of excess wind and solar energy and the cost of the hydrolyzer was ~ $1,000,000. Despite this significant cost, the total amortized cost of the biological biogas upgrading system was comparable to current physical-chemical upgrading technologies.

The final study of this thesis investigated the potential to biologically co-treat CO2 and H2S using nitrate as the terminal electron donor. Since the addition of nitrate favored undesired oxidation-reduction reaction pathways with hydrogen, a method was developed to map electron transfers. The effect of nitrate on methanogensis was tested with and without sulfur oxidizing bacteria. Under both conditions, nitrate had a negative impact on methanogenesis and ultimately, prevented co-treatment from being achieved. While attempting to co-treat H2S and CO2, it was discovered that dissimilatory nitrate reduction to ammonium was favored over denitrification. The electron balance confirmed that a competition for electrons from hydrogen did exist. This competition required N:S feeding ratios upwards of 16:1, which far exceeded the theoretical ratios of (4:1) for denitrifying bacteria. While the high nitrate loading rates allowed for high H2S removal efficiencies (98%), they inhibited methanogenesis so that carbon dioxide removal efficiencies did not meet RNG standards. Thus, future work should focus on alternative electron donors for sulfur oxidation and quantifying methanogenesis inhibition caused by sulfur-oxidation/denitrification pathways.

Pre-treatment of hydrogen sulfide is required before the utilization of biogas to eliminate the detrimental effects of corrosive hydrogen sulfide to the following combustion engines and pipelines. Biotrickling filters as one of the biotechnological methods have been investigated in desulfurizing biogas in recent years. Although high removal efficiency has been achieved by conventional biotrickling filters, clogging of the biotrickling filter bed due to the accumulation of excess biomass and elemental sulfur, has been widely reported (Janssen et al. 1997, Fortuny et al. 2008). In this context, a novel biotrickling filter using a monolith as its filter bed has been proposed and studied in this work to investigate its performance in removing H2S and solving the bed-clogging problem through pigging, a common method used for pipeline and tubular reactor cleaning. The inlet H2S concentration was controlled around 1000 ppmv, corresponding to a loading rate of 122 g S–H2S m−3 h−1, and the empty bed gas residence time (EBRT) was 41 s. The influence of different H2S/O2 ratios on the removal performance was investigated at these conditions and results indicated that at H2S/O2 molar ratio of 1:2, an average removal efficiency of 95% was obtained. Under all conditions investigated, elemental sulfur and sulfate were measured to be the two dominant products and covered up to 93% of total end products. The monolith bed design also served to demonstrate that the risk of clogging was greatly reduced under this kind of design and bed-clogging problems could be resolved when bed pigging was implemented to remove excess biomass and elemental sulfur accumulated inside the bed. Based on the results reported here, the monolith filter bed can be an effective alternative to the conventional packing material with a high specific surface area and a comparable performance could also be achieved by this novel bioreactor.