Model assessment on the non-isothermal methane biofiltration at ambient conditions

Direct anaerobic sewage treatment is increasingly adopted in several intertropical developing countries. The reduced carbon footprint of such facilities is based on its low electricity requirements and potential energy (biogas) recovery; however, if dissolved CH4 in the effluent is not controlled, this advantage will be drastically reduced. A suitable option to overcome this situation is to adopt CH4 desorption by an air stream, followed by bio-oxidation in compost biofilters. In such systems, the methanotrophic activity will increase the temperature in the biofilter to inhibiting values (above 40 degrees C), reducing the active volume of the compost media and its removal efficiency. This work addresses this process limitation, aiming to assess the temperature variations in the filter media and their effects on methane oxidation performance under different operating conditions. A comprehensive mathematical 2-D porous-medium based model was developed and then calibrated and validated with experimental data. The model considers heat, momentum, and mass balances under non-steady-state conditions, including heat exchange at the biofilter container wall influenced by daily ambient temperature fluctuations and solar radiation. Results obtained from 6 model simulations at different operating conditions show that temperatures above 40 degrees C are reached at the center-upper zones of the compost biofilter due to the heat transported from the methanotrophic active zones located at the bottom (inlet) and container wall. At midday (12 - 14 h), solar radiation on the wall contributes 14% of the total heat gained in a non-covered biofilter. The model highlights the importance of facilitating heat transport from the compost media to the surroundings; some practical recommendations are presented for that purpose.

Automated summary

Direct anaerobic sewage treatment is becoming more popular in intertropical developing countries due to its low carbon footprint. However, if dissolved methane in the effluent is not controlled, this advantage will be significantly reduced. This study develops a mathematical model to assess the temperature variations in the biofilter media and their effects on methane oxidation performance under different operating conditions. Practical recommendations for facilitating heat transport from the compost media to the surroundings are presented.