The authors have declared that no competing interests exist.
Conceived and designed the experiments: JL XT GS JG. Performed the experiments: XT RF. Analyzed the data: XT JL. Contributed reagents/materials/analysis tools: GS XT. Wrote the paper: XT JL.
Although biotrickling filters (BTFs) applied under acidic condition to remove H2S from waste gases have been reported, the removal behavior of the acidic BTF under transient condition which was normal in most industry processes, and corresponding bacterial community have not been thoroughly studied. In the present study, two BTFs were run under neutral (BTFn) and acidic (BTFa) conditions, respectively. The results revealed that the removal performance of BTFa under transient condition was superior to that of BTFn; the maximum H2S eliminating capacities (ECs) achieved by BTFa and BTFn were 489.9 g/m3 h and 443.6 g/m3 h, respectively. High-throughput sequencing suggested that pH was the critical factor and several other factors including nutrient and the inlet loadings also had roles in shaping bacterial community structure.
Hydrogen sulfide is the most prevalent odor compound generated from various industrial facilities such as municipal wastewater treatment plants, landfills, livestock farms and biogas plants. Under anaerobic conditions, the use of sulfate as the terminal electron acceptor by sulfate-reducing bacteria results in H2S production [
Many research works have been conducted to optimize the operating parameters, or to test the efficiency of filter materials and functional bacterial strains [
Based on the above considerations, in the present work, two BTFs were initially acclimated under steady condition at pH 7.0 and pH 4.0, respectively. The capabilities of both BTFs tolerant to the loading rates shock in short times were firstly evaluated. The composition and the structure of the bacterial communities within the two BTFs were also analyzed by a high throughput sequencing technique in attempt to find the main factors affecting the formation of the bacterial communities, and to reveal the interactive relationship between the bacterial composition and the environmental factors including pH, nutrient or inlet loading rates.
Two identical bench scale BTFs (
(1) H2S cylinder, (2) Mixing chamber, (3,4) Gas flowmeter, (5) Air compressor, (6) NaOH dosing pump, (7,8) Nutrient tank, (9,10) pH probe, (11,12) Peristaltic pump, (13–15) BTFa-u/m/b port for sampling packing materials, (16–18) BTFn-u/m/b port for sampling packing materials.
During the acclimation period, two BTFs operated under steady condition for a month with a constant inlet concentration of H2S. After that to assess the effect of loading-rates shock on the performances of the two BTFs, batch experiments were performed in a transient condition at the empty bed retention times (EBRT) of 60 s, 30 s and 15 s, respectively. At each EBRT, the inlet concentrations of H2S gradually increased within 14 h. The ranges for H2S concentration at 60 s, 30 s and 15 s were set at 175–5858 mg/m3, 169–5028 mg/m3 and 69–1029 mg/m3, respectively.
To obtain microorganisms that can adapt to various pH environments, both BTFs were simultaneously inoculated with an acclimated microbial consortium and activated sludge. The acclimated microbial consortium originated from a biofilter treating waste gases released from a landfill leachate treatment plant. H2S was found as a predominant pollutant in the waste gas with an average concentration of 224 mg/m3. Enrichment of the consortium was performed by series of transfer at one-week intervals with thiosulfate as the sole sulfur source. The activated sludge was collected from Liede municipal wastewater treatment plant in Guangzhou, China. The collection of the activated sludge was permitted for research purposes. After settling for 24 h, the concentrated sludge (5 mL) was mixed with the acclimated consortium of 35mL, and added to the nutrient solution (1 L).
The pH of the nutrient solution was measured on-line using a pH-meter. To determine the pH of the different layers of the BTFs bed, before each transient experiment, triplicate samples of the packing were taken from the each sampling site using a sterile tweezers, mixed with 3 mL of sterile water, and centrifuged at 7000 rpm for 10 min; the supernatants were analyzed with the pH-meter, and the pellets were used to measure the biomass. The biomass concentration was quantified based on the total protein measurement using the Bradford method. Sulfate, thiosulfate, nitrate and nitrite were analyzed using an ion chromatograph (Dionex ICS-1500, USA) with an AS19/AG19 column (Dionex, USA).
To analyze the H2S concentration, gas samples were collected in 2 L Tedlar bags, and immediately analyzed with a gas chromatograph (Shimadzu GC-2010, Japan) equipped with an FPD detector. The air was separated by a GS-GasPro capillary column (30 m × 0.32 mm × 1.0 μm, Agilent Technologies, USA) with nitrogen as the carrier gas at a flow rate of 1.5 mL/min. The injector and detector temperature were 70°C and 250°C, respectively. The GC oven temperature was programmed as follows: initial temperature of 80°C for 2 min, increase to 250°C at 10°C min−1 and maintain for 5 min.
After the acclimation phase, triplicate packing materials of 10 g wet weight were sterilely collected from each layer of both BTFs. The materials were mixed with 100 mL of phosphate buffer (137 mM NaCl, 2.7 mM KCl, 4.3 mM Na2HPO4, 1.4 mM KH2PO4; pH 7.3) and vortexed for 30 min. After detachment, the packing materials were discarded, and the biofilm-containing liquid phase was centrifuged at 8000 rpm for 10min. The resulting pellet was used to extract genomic DNA via the TIANamp Bacteria DNA Kit (Tiangen Biotech, Beijing) according to the manfacturer’s instructions. The DNA density and quality were checked using a NanoDrop Spectrophotometer. The extracted DNA was diluted to 10 ng/μL and stored at -40°C for downstream use. The universal primers 515F(5'-
The sequence data were processed using QIIME Pipeline–Version 1.7.0 (
At the beginning of this study, both BTFs were acclimated under steady state for a month with empty bed retention times (EBRT) of 60 s at an inlet concentration of 607 mg/m3. A short time was required to finish the acclimation for both BTFs. Their H2S removal efficiencies (REs) rapidly increased to nearly 100% within a week, and maintained this level throughout the rest of the acclimation period. The microbial consortium obtained from a previous biofilter used to treat waste gases from a landfill could be partly responsible for this successful and rapid acclimation.
The removal performances of the two BTFs under transient condition were provided in
(a): EBRT = 60s BTFa; (b): EBRT = 60s BTFn; (c): EBRT = 30s BTFa; (d): EBRT = 30s BTFn; (e): EBRT = 15s BTFa; (f): EBRT = 15s BTFn.
Similar removal behavior exhibited by BTFa was observed at 30s of EBRT with one exception occurred at the highest inlet concentration, but the H2S REs only declined by 2.5%. However, when the EBRT was set to 15 s, the H2S REs of BTFa dramatically decreased from near 100% to 55.1% while the inlet concentration increased from 69 to 1029 mg/m3. These findings were consistent with those of Chaiprapat et al. who reported that H2S RE increased 1.44 times in a single stage BTF treating a 5522-ppm H2S when the EBRT increased from 100 to 180 s [
The removal capacity of BTFn was decreased as compared with BTFa. The H2S REs of BTFn declined while the inlet concentration increased at every EBRT. The extent of the decrease in the REs was more serious than that observed in BTFa, and was also found to depend on the EBRTs employed. Therefore, it can be concluded that BTFa was more robust than BTFn under transient conditions. The maximum EC achieved by BTFn at 60 s, 30 s and 15 s were 265.0, 443.6 and 59.6 g/m3.h with inlet loading rates of 292.9, 502.3 and 124.2 g/m3.h, respectively. These EC values were significantly lower than those of BTFa. Interestingly, BTFn performed better at 30s of EBRT than at 60s. A previous study suggested that when the EBRT decreases, the H2S-removal rates of BTFs were usually limited by the diffusion rate of H2S from the gas into the liquid phase rather than by the rate of bacterial consumption of H2S [
The H2S mineralization rates were calculated based on the inlet loading rates of H2S and the amount of sulfate that accumulated in the leachate.
In this study, the bacterial community structures of the three layers (upper, middle and bottom) of both BTFs were analyzed. High-throughput sequencing of 18 samples yielded 33,557–102,326 sequence reads with an average length of 200 bps. The
Principal co-ordinates analysis (PCoA) demonstrated that the data can be reduced to two principal components (PC1 and PC2) with combined Eigenvalues explaining 86.7% of the variation.
An average of 98.7% of the sequence reads of the individual samples belong to Bacteria and only 1.0% to Archaea by comparing sequence reads to known 16S rRNA genes. Among the Archaea sequence reads, 13.7% and 85.7% were affiliated with the phyla Crenarchaeota and Euryarchaeota, respectively. No significant differences in the relative abundance of the two phyla were found between the BTFs. An average of 92.5% of the Bacteria sequence reads could be classified into 71 different phyla. However, the vertical distributions of the bacterial phyla between the two BTFs were very different.
The relative abundances of the main bacterial phylum were summarized in
Average relative abundance (%) | ||||||
---|---|---|---|---|---|---|
BTFa | BTFn | |||||
Bacterial phylum | BTFa-u | BTFa-m | BTFa-b | BTFn-u | BTFn-m | BTFn-b |
63.9 | 72.6 | 77.7 | 56.0 | 66.1 | 66.8 | |
|
7.9 | 4.8 | 3.0 | 13.3 | 14.4 | 10.8 |
|
20.0 | 32.9 | 23.6 | 25.2 | 29.9 | 19.1 |
|
1.2 | 1.0 | 1.2 | 3.0 | 2.4 | 2.9 |
|
0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
|
31.6 | 29.4 | 46.7 | 13.1 | 18.1 | 32.7 |
Other | 3.2 | 4.6 | 3.1 | 1.4 | 1.3 | 1.3 |
8.2 | 5.2 | 2.7 | 8.0 | 3.2 | 3.4 | |
6.3 | 6.2 | 6.2 | 5.0 | 5.6 | 6.2 | |
4.6 | 1.6 | 1.0 | 0.9 | 0.8 | 0.8 | |
2.9 | 2.9 | 1.2 | 1.4 | 1.3 | 1.4 | |
1.0 | 1.4 | 0.8 | 0.2 | 0.1 | 0.1 | |
1.1 | 1.1 | 1.3 | 10.6 | 9.5 | 8.1 | |
1.0 | 0.8 | 0.9 | 1.0 | 0.9 | 1.1 | |
0.4 | 0.4 | 0.5 | 1.8 | 1.1 | 1.1 | |
0.2 | 0.2 | 0.2 | 2.9 | 1.3 | 1.6 | |
0.1 | 0.1 | 0.1 | 3.5 | 2.3 | 2.0 | |
Other | 6.4 | 5.0 | 4.7 | 4.2 | 3.6 | 3.5 |
The Proteobacteria phylum mainly contained Beta proteobacteria and Gamma proteobacteria classes with average relative abundances of 25.5% and 35.9% in BTFa and 24.7% and 21.3% in BTFn, respectively. Notably, the percentage of Gamma proteobacteria in the upper layer was significantly higher than that in the bottom layer of the filter bed in both BTFs. Alpha proteobacteria was another predominant class in the Proteobacteria phylum. In contrast to the Gamma proteobacteria, the average relative abundance of Alpha proteobacteria in BTFa was significantly lower than that in BTFn. Other predominant phyla included Planctomycetes, Firmicutes, Acidobacteria, Cyanobacteria, Bacteroidetes and Chloroflexi. The spatial distributions of Planctomycetes, Firmicutes and Chloroflexi phyla were similar in both BTFs. However, the spatial distributions of the remaining bacterial phyla were significantly different between BTFa and BTFn.
When comparing the bacterial community structure at the genus level, differences in relative abundance between BTFa and BTFn increased in significance. As shown by the heat map in
Although the broader pH gradient formed in BTFn provided diverse environments that facilitated the development of more types of microorganisms in the bioreactor, the sulfur-oxidizing activities of those neutral microbes were easily inhabited under acidic conditions, causing the H2S removal to decline. In contrast, the
The acidic BTF performed better and robustly than the neutral BTF with regard to H2S removal under transient conditions. Considering the different solubility of H2S and O2, it could be inferred that the microbial activities relating to H2S degradation in the BTFa were greater than those in BTFn. H2S removal mainly occurred at the down layer of filter bed.
The acclimation under different pH conditions resulted apparently different bacterial communities. Besides pH, the nutrients and the inlet loading also influenced the formation of the bacterial community. The circulation of the nutrient solutions along the filter bed facilitated the transportation of the elemental sulfur and the microorganisms from the bottom layer to upper layer, hence resulted a similar distribution of microorganisms within same BTF.
This work was supported by the Scientific and Technology project of Guangdong Province, China (No.2013B030600002; No.2014A020216022), the Science and Technology project of Guangzhou, China (No.201504010014) and the Natural Science Foundation of China (No. 31270169).