Efficient sulfide and methane removal in anaerobic secondary effluent using a pilot-scale membrane-aerated biofilm reactor

Anaerobic secondary treatment can enable energy-efficient removal of organic matter but may produce effluent containing dissolved methane and sulfide that must be managed before discharge or reuse. In this study, we operated a Membrane-aerated Biofilm Reactor (MABR) to achieve reliable removal of dissolved sulfide and methane from anaerobic secondary effluent at pilot-scale. The pilot-scale MABR was equipped with gas permeable polymethyl pentene (PMP) membranes, promoting surface growth of aerobic biofilm via diffusion-based aeration (lumen-to-surface diffusion). The system treated anaerobic secondary effluent from a demonstration-scale anaerobic membrane bioreactor (AnMBR) processing 90 m3/d primary effluent. MABR influent flow rate was increased from 8.2 to 32.7 m3/d to elevate substrate loading rates to the biofilm. The MABR consistently achieved >99 % removal of sulfide and dissolved methane, even at the maximum substrate loading rate: 2.3 g-S/m2/d for sulfide and 2.5 g-CH4/m2/d for dissolved methane. Despite effective sulfide and methane removal, incomplete nitrification (<25 % ammonia removal) occurred, with a portion of ammonia converted into nitrous oxide (N2O), a greenhouse gas 298 times more potent than CO2. Operating the MABR incurred low energy costs: 0.01 to 0.05 kWh/m3 for the compressor supplying air to the membrane lumen and 0.01 kWh/m3 for the influent pump. An in-depth mass balance of N2O emissions from the MABR revealed that N2O was subject to counter-diffusion (surface-to-lumen diffusion) in which >99 % of the N2O produced within the biofilm was recovered in off-gas from the gas permeable membranes (hollow fibers).