Integrated approach towards quantifying carbon dioxide and methane release from waste stabilization ponds

Accurate estimations of gaseous emissions and carbon sequestration in wastewater processing are essential for the design, operation and planning of treatment infrastructure, particularly considering greenhouse gas reduction targets. In this study, we look at the interplay between biological productivity, hydrodynamics and evasion of carbon-based greenhouse gases (GHG) through diffusion and ebullition in order to provide direction for more accurate assessments of their emissions from waste stabilization ponds (WSPs). The ponds stratified in the day and mixed at night. Buoyancy flux contributed between 40 and 75% to turbulence in the water column during nocturnal cooling events, and the associated mixing lead to increasing carbon dioxide (CO₂) and methane (CH₄) concentrations by up to an order of magnitude in the surface. The onset of stratification and phytoplankton surface blooms, associated with high pH as well as low and variable CO₂ partial pressure resulted in an overall reduction of CO₂ efflux. Ebullition represented between 40 and 99% of the total CH₄ efflux, and up to 95% of the integrated GHG release during wastewater treatment (in CO₂ equivalents). Hydrodynamic conditions, diurnal variability and ebullition need to be accounted for reliable assessments of GHG emissions from WSPs. Our study is an important step towards gaining a deeper understanding in the functioning of these hot spots of carbon processing. The contribution of WSPs to atmospheric GHG budget is likely to increase with population growth unless their performance is improved in this regard.