Effectiveness of a Desorption Chamber for the Removal of Dissolved Gases from Anaerobic Effluents R. M. Glória 1 , C. L. Souza 1 , C. A. L. Chernicharo 1 , M. E. A. do Carmo 1 , P. V. O. Silva 1 Department of Sanitary and Environmental Engineering Federal University of Minas Gerais - Brazil
Brief Background UASB technology for domestic wastewater treatment Considered a established technology in some warm climate countries Mostly used treatment technology in Brazil: more than 600 full-scale reactors in operation Many advantages: low sludge production, low O&M costs, small foot print, good efficiency, possibility of resource recovery 13th SWWS – Athens – 2016
Brief Background: Acceptance of the Anaerobic Technology in Brazil Sample universe: 1439 STPs (9 states + Federal District) Treatment installed capacity (m 3 .s -1 ) STPs 10,000 – 100,000 inhab. STPs > 100,000 inhab. STPs < 10,000 inhab. 13th SWWS – Athens – 2016
Brief Background UASB technology: some limitations still exist Loss of dissolved methane o emission of GHG o loss of energy potential Emission of dissolved hydrogen sulfide o can cause bad odour and corrosion 13th SWWS – Athens – 2016
Brief Background Losses of dissolved methane 13th SWWS – Athens – 2016
Brief Background: Previous studies Methane losses in UASB reactors • Recovery inside the gas-collectors is only partial as the effluent stream is often supersaturated with dissolved CH 4 • Mass transfer from the open liquid surface to the atmosphere • Dissolution in the liquid phase and washing out with the final effluent • Release to the atmosphere through the hydraulic structures that produce turbulence Influent 13th SWWS – Athens – 2016
Brief Background: Previous studies Hydrogen Sulfide losses in UASB reactors Due to its high solubility in water, H 2 S tends to remain in solution when the liquid effluent exits the reactor However, turbulence produced by the free fall of the effluent (outlet structures of the reactor) can cause severe emissions of odorous compounds (Pagliuso et al. ,2002) Measured values inside splitting boxes can reach values as high as 500 ppm 13th SWWS – Athens – 2016
Brief Background – Previous studies Micro-aeration using biogas (Hartley and Lant, 2006) Micro-aeration using air (Bartacek et al., 2013) Degasifying membranes (Cookney et al., 2010; Bandara et al., 2011, 2012) Air injection in the upper part of the settler compartment (Gloria et al., 2015) Vacuum chamber None of them have yet proved to be fully viable or effective. 13th SWWS – Athens – 2016
Objective To evaluate the effectiveness of the desorption technique for the removal of methane and hydrogen sulfide dissolved in the effluent of a pilot-scale UASB reactor. 13th SWWS – Athens – 2016
The idea behind the proposal Biogas flare Biogas Waste gas from settlers treatment and pumping station Waste gas from preliminary Effluent saturated with CH 4 Combined biological oxidation Fan of sulfide and methane Fan Degasification unit Degasified effluent Possible alternative for combined management of methane and hydrogen sulfide in small size plants 13th SWWS – Athens – 2016
Material and Methods Schematic configuration of experimental apparatus. Experiments carried out in a 360-L pilot-scale UASB reactor and a Desorption Chamber (DC) The reactor was fed on real wastewater taken from a chamber upstream the primary clarifiers of a full-scale treatment plant, after being submitted to preliminary treatment UASB reactor: average HRT of 7 hours The DC was installed downstream the UASB reactor 13th SWWS – Athens – 2016
Material and Methods Positioning and view of the Desorption Chamber 13th SWWS – Athens – 2016
Material and Methods Characteristics and operating conditions of the Desorption Chamber Diameter: 20 cm Drop heights tested: 0.5 and 1.0 m Hydraulic loading rate: 0.132 m 3 .m -2 .min -1 Operational Exhaustion Exhaustion Number of air Free drop Chamber RQ ** Phases rate time renovations * height inside volume (times) (L.min -1 ) (min) (renews.h -1 ) DC (m) (L) 1 1.2 3.3 18 0.5 4 1,1 2 1.6 2.5 24 0.5 4 1,5 3 1.6 5 12 1.0 8 1,5 4 3.2 2.5 24 1.0 8 3,1 RQ: air to wastewater flow ratio 13th SWWS – Athens – 2016
Material and Methods Gas analyses Sulfide in the liquid samples: protocol adapted by Plas et al. (1992), Dissolved methane: Alberto et. al. (2000) and Hartley and Lant (2006). Waste gas (oxygen, nitrogen, CO 2 and H 2 S): portable analyser LANDTEC type GEM TM 5000. 3 4 Methane in the waste gas: gas chromatography 1 13th SWWS – Athens – 2016
Results Dissolved Methane Removal Efficiencies Concentration in the waste gas 100 2,0 Median Median 25%-75% 90 25%-75% 1,8 Min-Max Min-Max 80 1,6 Methane removal (%) Waste gas - CH 4 (%) 70 1,4 60 1,2 50 1,0 40 0,8 3 30 0,6 20 0,4 10 0,2 0 0,0 phase 1 phase 2 phase 3 phase 4 phase 1 phase 2 phase 3 phase 4 Experimental phases Experimental phases 2 1 13th SWWS – Athens – 2016
Results Hydrogen Sulfide Removal Efficiencies Concentration in the waste gas 1000 Median 100 25%-75% 900 Min-Max 90 800 Waste gas - H 2 S (ppm) Sulfide removal (%) 80 700 70 600 60 500 50 400 40 300 30 200 20 Median 25%-75% 100 10 Min-Max 0 0 phase 1 phase 2 phase 3 phase 4 phase 1 phase 2 phase 3 phase 4 Experimental phases Experimental phases 13th SWWS – Athens – 2016
Conclusions The use of the Desorption Chamber (DC) allowed good removal efficiencies of the dissolved gases contained in the effluent of the UASB reactor. For the best operating condition (free fall of 1.0 m, air to wastewater flow ratio of 3.1, and 24 renews per hour), the dissolved methane removal efficiency was close to 60%. As related to the removal of dissolved hydrogen sulfide, efficiencies as high as 80% were achieved for the same operating conditions. Overall, these results prove that simple devices such as the DC tested in this research can effectively contribute for the control of methane and hydrogen sulfide emissions in anaerobic-based sewage treatment plants. 13th SWWS – Athens – 2016
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