Methane is the most important organic greenhouse gas emitted to the atmosphere for its contribution to the global warming. The gas has a strong infrared absorbance (i.e., 25 times more efficiently than carbon dioxide) which makes it a more effective greenhouse gas than carbon dioxide although having a shorter lifetime in the atmosphere (~9 years). Driven by the anthropogenic emission due to the increase of global population and energy demand, methane emission is set to increase in the future. Several anthropogenic methane emission mitigation has been applied in various sectors (agriculture, energy, and waste). The use of a biochemical reactor can be an alternative to remediate methane emission at low concentration (< 1 % (v/v)) as it environmentally friendly and economically more beneficial. In the biochemical oxidizer, Methane Oxidizing Bacteria (MOB) are used as the biocatalyst. MOB are part of methylotrophic bacteria, a group of bacteria capable of utilizing one carbon compounds as their carbon and energy sources. For gaseous waste having low solubility like methane, the typical bioreactor used to treat methane gas waste is biofilter where MOB are immobilized on a carrier material.
When designing a biofilter, carrier material selection is arguably the most crucial step. The preferred carrier materials possess a high porosity and surface area to provide space for the bacteria to grow and to increase the contact area between the bacteria and methane, respectively. Based on these criteria, building materials have the potential to be a good carrier material for a methane biofilter. Using this concept of “housing” MOB on building material, another biotechnological application of the bacteria was explored. Microbiologically Induced Carbonate Precipitation (MICP) is the production of carbonate mineral driven by environmental condition (e.g., pH) alteration as a result of microbial activity. In the construction industry, the typically applied urea hydrolysis based MICP poses several disadvantages such as ammonia release to the air and nitric acid production. In this thesis, the capacity of MOB to induce calcium carbonate precipitation as the basis for a more environmentally friendly biogenic building material surface protection was also explored. Therefore the thesis is divided into two parts: Part 1 deals with the exploration of building material utilization as the carrier material for methane biofiltration (Chapter 2 to 4) whereas Part 2 deals with the exploration of MOB application on building materials as an alternative biocatalyst for the material surface treatment (Chapter 5 and 6).
In Chapter 2 a screening of different building material and MOB culture was done to select the combination of both which allow the bacteria to exhibit the highest methane removal capacity. Experiments were performed with different MOB inoculated on building materials at high (~20 % (v/v)) and low (~100 ppmv) methane concentrations. Methylocystis parvus in Autoclaved Aerated Concrete (AAC) exhibited the highest methane removal rate at high (28.5 ± 3.8 μg CH4 g-1 building material h-1) and low (1.7 ± 0.4 μg CH4 g-1 building material h-1) methane concentration. Due to the higher volume of pores with diameter > 5 μm compared to other materials tested, AAC was able to adsorb more bacteria which might explain for the higher methane removal observed. The total methane and carbon dioxide-carbon in the headspace was decreased for 65.2 ± 10.9 % when M. parvus in AAC was incubated for 100 hours. AAC was therefore selected for the carrier material for the subsequent methane bioremediation studies (Chapter 3 and 4) and M. parvus was selected as the MOB strains for MICP studies (Chapter 5 and 6).
In Chapter 3, the methane removal capacity of mixed MOB culture in a biofilter setup using AAC as a highly porous carrier material was tested. Although it was found that M. parvus exhibited the highest methane removal capacity on AAC (Chapter 2), mixed MOB culture was the selected culture for the biofilter inoculation in this study. This was based on the fact that non-aseptic practice was preferred to keep the operating cost lower if the biofilter was to be applied to remove methane in places with high methane emission (atmospheric concentration < 1 % (v/v)). Batch experiment was performed to optimize MOB immobilization on the AAC specimens where optimum methane removal was obtained when calcium chloride was not added during bacterial inoculation step and 10 mm thick AAC specimens were used. The immobilized MOB could remove methane at low methane concentration (~1000 ppmv) in a biofilter setup for 127 days at an average Removal Efficiency (RE) of 28.7%. MOB also exhibited a higher abundance at the bottom of the filter, in proximity with the methane gas inlet where a high methane concentration was found. It was concluded here that a reasonably efficient and a more environmentally friendly methane biofilter performance can be obtained using AAC as the carrier material. Hence, the setup was used in a field test application (Chapter 4).
The performance of MOB immobilized on AAC to remove methane from ruminants effluent gas was investigated in Chapter 4. A biofilter employed in Chapter 3 was used as the biofilter in this study. Two dairy cows were housed in respiration chambers for two days where the exhaust gas from the chambers was used as the biofilter feed. MOB consumed methane at an average RE of 17.52 % and elimination capacity (EC) of 67.3 g m-3 d-1. Several factors that might cause the lower RE and EC compared to the lab scale study (RE = 28.7 %) in Chapter 3 are: (a) the lower methane concentration and (b) the presence of ammonia in the livestock effluent gas, (c) the higher gas flow rate into the biofilter, and (d) the lowering humidity level in the biofilter. By using AAC as carrier material, carbon dioxide in the effluent gas as well as the one from the methane oxidation by MOB were removed by the carbonation reaction with AAC. Thus, complete carbon sequestration from methane was obtained. Overall, in part 1 of this thesis (Chapter 2 to 4) it was concluded that a more environmentally friendly methane biofilter than the ones previously tested could be achieved when using ACC as the carrier material.
An alternative MICP from calcium formate by Methylocystis parvus OBBP is presented in Chapter 5. To induce calcium carbonate precipitation, M. parvus was incubated at different calcium formate concentrations and starting culture densities. Up to 91.4 % ± 1.6 % of the initial calcium was precipitated in the methane amended cultures compared to 35.1 % ± 11.9 % when methane was not added. Because the bacteria could only utilize methane for growth, higher culture densities and therefore calcium removals were exhibited in the cultures when methane was added. A higher calcium carbonate precipitate yield was obtained when higher culture densities were used but not necessarily when more calcium formate was added. This was mainly due to salt inhibition of the bacterial activity at a high calcium formate concentration. A maximum of 0.67 ± 0.03 CaCO3 Ca(CHOOH)2-1 (g/g) calcium carbonate precipitate yield was obtained when 109 cells mL-1 and 5 g L-1 of calcium formate were used. Compared to the current strategy employing biogenic urea degradation as the basis for MICP, the approach in this study presents significant improvements in terms of pollutant emission reduction if applied in the construction industry. The process was subsequently applied on building material as an alternative surface treatment (Chapter 6).
The effectiveness of MICP from the formate oxidation by Methylocystis parvus as an alternative concrete surface treatment was investigated in Chapter 6. MICP was induced on AAC by immersing the material in 109 M. parvus cells mL-1 containing 5 g L-1 of calcium formate. A 2 days immersion of the material gave the highest weight increase of the specimen due to the calcium carbonate deposition. The deposition mainly occurred on the wall of the pores on the surface of the specimen. Due to this surface deposition, a significantly lower water absorption was observed in the bacterially treated specimens compared to the non-treated ones (i.e., up to 2.92 ± 0.91 kg m-2). A concomitant atmospheric methane removal (152.2 ± 40.1 μg of CH4 m-2 h-1) was also observed in the bacterially treated specimens. Overall, in part 2 of this thesis (Chapter 5 and 6) it was concluded that compared to the currently employed biogenic processes, the formate-based MICP by M. parvus offers a more environmentally friendly approach for the biotechnological application to protect concrete surface.
The results obtained from part 1 and 2 in this thesis were subsequently positioned in their related biotechnology field and the outlook for the respective researches was presented in Chapter 7. The AAC-based methane biofilter had lower methane removal efficiency compared to the previously reported biofilters, although the other biofilters operated with higher Empty Bed Residence Time (EBRT) which might increase the overall methane conversion in the biofilter. The AAC-based methane biofilter, however, offers an advantage of carbon dioxide sequestration and this advantage is not found in the other biofilters. For the formate-oxidation based MICP by M. parvus, it was found that, on per cell basis, the optimum biomineralization rate obtained in Chapter 5 (i.e., at 5 g L-1 calcium formate and 109 cells ml-1) was still approximately three times lower than the maximum urea based biomineralization rate by B. pasteurii ATCC 6453. Moreover, unlike the urea-based MICP where a high urea / calcium source concentration could be employed, the influence of the formate-oxidation based MICP on the building material characteristics was smaller. Nevertheless, the resulting calcium carbonate deposition could effectively lowered water intrusion into the material. Based on the results obtained, several suggestions were made. To construct the biofilter like the “green façade” concept, MOB should be applied by brushing / spraying on existing building material with consecutive applications of nutrient applications to sustain the MOB growth on the building structure. For the MOB-based MICP, the application on natural stones to test the effectiveness of the process as the surface treatment for this type of material should be performed. Future studies should also look into the use of mixed culture MOB as it may lower the cost of this type of application.