Emerging investigators series: revisiting greenhouse gas mitigation from conventional activated sludge and anaerobic-based wastewater treatment systems

Abstract

Elucidation of N2O formation mechanisms in aerobic-based wastewater treatment is essential for effective greenhouse gas mitigation, whereas mainstream anaerobic treatment requires improved methane recoverability.

Reducing Cost and Environmental Impact of Wastewater Treatment with Denitrifying Methanotrophs, Anammox, and Mainstream Anaerobic Treatment

“…N 2 O remains a major factor of uncertainty in any quantification of GHG emissions from wastewater treatment due to both global warming potential (298 times more potent than CO 2 ) and variability in emission rates . In conventional nitrification/denitrification, N 2 O emissions can vary between 0 to 14.6% of influent nitrogen and can be affected by factors as diverse as COD/N ratio, pH, nitrite concentration, dissolved oxygen concentration, sudden increase in ammonium concentrations, and even size and scale of the nitrogen removal system . In a conventional system, N 2 O is produced through three mechanisms: nitrifier denitrification and hydroxylamine oxidation, both performed by autotrophic ammonium oxidizing organisms (AOB, ammonium oxidizing archaea, and complete ammonium oxidizing NOB) in the nitrification reactor, and incomplete denitrification, performed by heterotrophic denitrifying organisms. ,,, N 2 O emissions from denitrification at minimum COD/N ratios (∼5) have been measured between 8 and 4% of influent nitrogen and decreases as COD/N ratios increase .…”

“…Two primary GHGs of concern in WRRF operation are methane (CH 4 ) and carbon dioxide (CO 2 ) . Although nitrous oxide (N 2 O) is also a contributor to GHG emissions from WRRF (with a global warming potential 298 times that of CO 2 ), it is difficult to include N 2 O into stoichiometric modeling as the microbial origin, amounts emitted, and conditions triggering its emission are controversial. − CH 4 has a global warming potential of approximately 34 times that of CO 2 . Therefore, even small quantities can greatly impact atmospheric quality .…”

Hydrophobic Gas Transfer Membranes for Wastewater Treatment and Resource Recovery

“…The membrane may be coated with catalysts for in situ production (e.g., H 2 and NH 3 ) and extraction. (B) Conversion rate (mmol m –2 h –1 ) and (C) energy reduction by membrane gas extraction of CH 4 , ,,− H 2 , ,,, NH 3 , ,, and butanol. , (D) Hydrophobic membranes for the recovery of methane . (E) Integrated hydrophobic membrane + electrode for ammonia recovery …”

“…Recovery modes (i.e., sweep-gas or vacuum) also affected CH 4 recovery. Studies found sweep-gas (e.g., N 2 ) mode was energetically favorable with good removal (up to 97% methane), but the dilution (CH 4 /N 2 = 2.32 × 10 –1 to 4.01 × 10 –8 ) reduced the product value and economic viability . In contrast, vacuum harvesting generated a high purity product with high output-to-input ratio (up to 32.4) in terms of energy .…”

“…Studies found sweep-gas (e.g., N 2 ) mode was energetically favorable with good removal (up to 97% methane), but the dilution (CH 4 /N 2 = 2.32 × 10 –1 to 4.01 × 10 –8 ) reduced the product value and economic viability . In contrast, vacuum harvesting generated a high purity product with high output-to-input ratio (up to 32.4) in terms of energy . Notably, membrane gas extraction via vacuum reduced energy need by >85% over the conventional heating processes, but it promoted pore wetting and clogging in porous membranes that leads to considerate water carryover. , To overcome such issues, Li et al replaced vacuum with an organic solvent (i.e., dodecane) for CH 4 adsorption and achieved >90% methane recovery and net energy production .…”