Currently, approximately 1000 km³ of wastewater is generated around the world each year, making sewage treatment plants a crucial terrestrial source of DOC. However, the potential of terrestrial DOC, particularly from sewage effluents, to act as a carbon sink needs further investigation. Key areas of inquiry include understanding the transformation processes of DOC into recalcitrant forms in sewage treatment and assessing its carbon sequestration potential in oceanic environments.

We used Fourier transform ion cyclotron resonance mass spectrometry to analyze the molecular composition of organic carbon. We focused on carboxyl-rich alicyclic molecules (CRAMs, a kind of biologically refractory molecules) from sewage treatment plant effluent (eSTP), a river discharge outlet (RDO), and an estuary. We found that the O/C, I_DEG, MLBR, AI_mod, and the number of CRAMs also increased as the distance from the estuary increased, indicating an increase in molecular inertia. It indicates that estuaries, as biogeochemical reactors, facilitate the transformation and aggregation of organic matter, leading to elevated concentrations of refractory compounds, such as CRAMs. Furthermore, dark culture experiments (120 days) were performed on the eSTP and RDO samples to explore microbial carbon utilization. The CRAMs in eSTP increased significantly, the number of common molecules at different sites increased significantly, the molecular composition of eSTP changed little in dark culture. However, the composition of microbial communities changed significantly. It supported that after the secondary biological treatment of wastewater plant removes easily degradable organic matter, stable and inert tail water is produced, meanwhile, significant changes in microbial community structure indicate the importance of microbial carbon pump (MCP) in the degradation and subsequent transformation of organic matter.

To enhance comprehension, the RDOC, which holds potential for carbon sequestration in wastewater treatment, as“gray carbon”. The gray carbon, as an emerging field and concept, it is necessary to conduct adequate research. Strengthening the understanding of gray carbon can better evaluate the effect of realizing negative Marine discharge through the “enhance sewage alkalinity to promote carbon sinks".
 

Refractory dissolved organic carbon,Alkaline minerals,Wastewater alkalinity addition