Effluent Dissolved Carbon Discharge from Two Municipal Wastewater Treatment Plants to the Mississippi River

Nutrient and carbon transport from the Mississippi River to the Gulf of Mexico have been investigated intensively. However, little is known about the direct human contribution of carbon from wastewater treatment plants (WWTPs) to this large river, a source that can be termed as Cultural Carbon. This study analyzed dissolved carbon in effluents from two municipal WWTPs on the bank of the Mississippi River in Baton Rouge, South Louisiana, USA. One of the WWTPs (WWTP North) is a conventional wastewater treatment facility with a treatment capacity of 40 million gallons per day (MGD), while the other (WWTP South) is a recently upgraded facility with a treatment capacity of 200 MGD. From September 2022 to November 2024, river water and effluent samples were collected monthly to analyze dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) concentrations and their mass transport. The study found significantly higher monthly average DIC (56.80 ± 16.51 mg/L) and DOC (29.52 ± 8.68 mg/L) concentrations in the effluent of WWTP North than in the effluent of WWTP South (DIC: 42.64 ± 10.50 mg/L and DOC: 12.93 ± 3.68 mg/L). Effluents from both WWTPs had substantially greater DOC and DIC levels than the Mississippi River water (DIC: 28.92 ± 4.91 mg/L and DOC: 5.47 ± 2.35 mg/L). WWTP North discharged, on average, 3.80 MT of DIC and 1.95 MT of DOC per day, whereas WWTP South discharged 6.27 MT of DIC and 1.92 MT of DOC per day, resulting in a total annual load of 3808 MT of DIC and 1459 MT of DOC entering the Mississippi River. Considering the large number of WWTPs within the Mississippi River Basin, these findings highlight a significant contribution of effluents to riverine carbon, suggesting that basin-wide carbon budgets and regional climate assessments must take them into account. The findings from this study can be useful for federal and state policymakers, as well as researchers and engineers involved in carbon science, climate change, and water quality assessment of the Mississippi River Basin and beyond.