Management of dissolved methane from anaerobic effluents

Anaerobic digesters in wastewater treatment plants discharge dissolved methane, which usually stripped off to the environment on subsequent handling leading to GHG emission and loss of an energy source. To prevent the emission of methane gas, we utilized a packed bed column as a post-treatment biological process. The packing in the column is enriched with methane-oxidizing bacteria which oxidizes methane in aerobic conditions to CO2. At 37˚C, the average methane oxidation rate was 0.38 Kg COD/m3/d at a residence time of 1d. Further, on decreasing the residence time to 2 h the average methane oxidation rate was 0.98 Kg COD/m3/d indicating maximum activity of methanotrophs occurs in the first 2 h. Two methods were employed to measure dissolved methane in the samples, and they were very well correlated with an R-value of 0.93. PCR results indicate that pmoA gene responsible for methane oxidation is present in the enriched bacteria.

Management of Dissolved Methane from Anaerobic Effluents Vedansh Gupta, Ramesh Goel Civil and Environmental Engineering Department University of Utah ABSTRACT • Anaerobic digesters in municipal wastewater treatment plants discharge dissolved methane, which is usually stripped off to the environment. • To prevent the emission of methane gas, we utilized a packed bed column as a post-treatment biological process. • The packing in the column is enriched with methane-oxidizing bacteria which oxidizes methane in aerobic conditions to CO2. • At 37˚C, the average methane oxidation rate was 0.38 Kg COD/m3/d at a residence time of 24 h. Further, on gradually decreasing the residence time to 0.5 h the average methane oxidation rate was 1.28 Kg COD/m3/d. • Two methods were employed to measure dissolved methane in the samples, and they were very well correlated with an R value of 0.94. • PCR results indicate that pmoA gene responsible for methane oxidation is present in the enriched bacteria. Figure 1: Natural Pathways for methane oxidation METHODS Experimental set-up: • 550 ml of column is used in this study. • Sponge media is used as packing. • Sponge media is enriched with methanotrophs. • Synthetic feed is purged with CH4. Temperature of the Earth is increasing every year due to global warming. 150 µl of 10%Sodium Azide Effluent Methane emissions from WWTP. [𝐶𝐻4 ]𝑑𝑖𝑠 = Biogas CH4 Influent 20 g of NaCl for 50 ml of effluent Effluent [%𝐶𝐻4 ]𝑔𝑎𝑠 /100 ∗ 𝑑 ∗ 𝑉𝑔𝑎𝑠 + 𝑃𝑇 − 𝑃𝑉 ∗ 𝐾𝐻 ∗ 𝑉𝐿 𝑉𝐿 Volume of CH4= (% CH4 /100)*Volume of headspace n= PV/RT; Concentration =n/Volume of liquid Salting-Out Method RESULTS & DISCUSSION Effluent An example: Total amount of CO2 produced= 343 g/m3 considering 100% COD removal CH4 produced= 1050 g equivalent CO2/m3. COD ~ 200 mg/l liquid. A Figure 7: Gel Electrophoresis results B Figure 8: Live and Dead analysis for viability of cells using salt (A) and using sodium azide (B) CONCLUSIONS Henry's law Method CH4 Figure 6: Dependence of the headspace concentration on the time of phase equilibrium between headspace and Figure 2: Schematic of Packed Bed Column Reactor (PBR) Operated in 7 phases: Phase 1: 25˚C, Residence time= 24 h; Phase 2: 37˚C, Residence time = 24h; Phase 3: 37˚C, Residence time = 22h; Phase 4: 37˚C, Residence time = 16h; Phase 5: 37˚C, Residence time = 4h; Phase 6: 37˚C, Residence time = 2h; Phase 7: 37˚C, Residence time = 0.5h INTRODUCTION CH4 Figure 5: Scatter plot of dissolved methane concentration determined with salting-out method and with Henry's law method • Biological Oxidation of Methane in synthetic effluent purged with methane using a packed bed column, which were enriched with Methylococcus capsulatus. • Methane removal rate increased from phase 1 to 7 with maximum removal rate of 1.28 Kg COD/ m3/d. • Methane removal rates were similar using Henry's law and salting-out method and they were very well correlated. • Time required to acquire equilibrium between headspace and liquid is 5 mins. • This packed bed column can be used for post-treatment of anaerobic effluent and could make wastewater treatment more eco-friendly by oxidizing the dissolved methane. REFERENCES Figure 3: CH4 removal performance measured using Henry's law method. Figure 4: CH4 removal performance measured using Salting-out method. Results from one-way ANOVA shows statistical difference between Phase 1 and Phase 2 for both the methods. • • • • • • Hatamoto, M., et al., 2010. Water Research, 1409-1418. Souza, C., et al., 2011. Water Science & Technology, 2259-2264. Daelman, M., et al., 2012. Water Research, 46, 3657-3670. Liu, Z., et al., 2013. Environmental Science & Technology, 48, 889-890. Bandara, W., et al., 2011. Water Research,45, 3533-3540. Cakir, F., et al., 2005. Water Research,39, 4197-4203.