Combining submerged membrane technology with anaerobic and ...

Chapter 6where subindex i correspond to each component (nitrogen anions, oxygen andmethane) of the mass balance, Q IN,i is the input flow [L·d -1 ], C 0i is the input concentration[mg·L -1 ], Q OUT, is the output flow from the first (anoxic) chamber [L·d -1 ], C i is the outputconcentration [mg·L -1 ], V is the volume of the first (anoxic) chamber [L], r i is the volumetricreaction rate [mg·L -1·d -1 ], C i*is the concentration of either methane or oxygen in equilibriumwith air [mg·L -1 ] and k La i is the mass transfer coefficient for either methane or oxygen. Thelast term of equation 6 was not taken into account for nitrogen ions mass balance.Operational data was grouped depending on the recirculation ratio. Average valuesof dissolved methane concentration (input and output), dissolved oxygen concentration(input and output) and nitrogen anions concentration (input and output) were calculatedfrom experimental data for each one of the recirculation scenarios. Dissolved methaneinput was due to the UASB effluent whereas nitrogen anions and dissolved oxygenentered the first chamber through the recirculation from the aerobic membrane filtrationchamber. Desorbed methane from the first chamber and oxygen input due to oxygentransferred from the surface air to this chamber were calculated according to section 2.4.6.4. Results and discussion6.4.1. General resultsThe system was operated at ambient temperature, and wastewater temperatureschanged with seasons (21.5 – 19.0 °C). Despite operating in psychrophilic conditionsvolatile fatty acids (VFA) concentration in the UASB effluent was below minimum detectionlimit of the method used (20 mg·L -1 ) during the six experimental periods. Biogas productionin the UASB reactor was detected during the six experimental periods, with an averageproduction rate of 50.9±10.8 L·d -1 , depending on OLR applied. Biogas production yieldwas around 0.15 m³ methane·kgCOD eliminated-1. Methane reached more than 70% of the biogascomposition during the whole operation. Methane in the biogas correspondedapproximately to the 75% of the total methane produced. Therefore, up to 25 % of themethane produced in the anaerobic reactor would be dissolved in the effluent, whichconfirmed the values reported by previous studies (Noyola et al., 2006; Souza et al. 2011).The system treated an average of 280 L·d -1 of wastewater with a total CODconcentration in the feeding varying between 800 and 1300 mg·L -1 . The concentration oftotal COD in the permeate of the MBR was normally lower than 20 mg·L -1 . Therefore CODremovals achieved in the system were above 97.5%. The average organic loading rate(OLR) applied to the UASB reactor was between 1.7 and 2.8 kgCOD·m -3·d -1 . With respect158