Wastewater inputs reduce the CO2 uptake by coastal oceans

Sea surface carbon dioxide during early summer at the Tuandao nearshore time series site

To investigate air-sea CO2 flux at the Qingdao nearshore site and its temporal variations, a high-resolution continuous observation of surface carbon dioxide partial pressure (pCO2) was carried out at Zhongyuan Pier near Tuandao from May 25 to July 8, 2019. It was observed that during this period, surface pCO2 varied between ∼490 and ∼690 μatm, mainly associated with sea surface temperature. Surface pCO2 also displayed substantial diurnal variations, with an average amplitude of 64 ± 21 μatm, largely dominated by biological activities. During the observational period, this site acted as a source of atmospheric CO2, releasing 361 mmol CO2 m-2. The notable diurnal variations in air-sea CO2 flux, such as the observed average amplitude of 10.9 mmol m-2 d-1 in this study, pose a challenge for accurately estimating the air-sea CO2 flux in coastal regions without high-resolution observations.

Integrating bicarbonate-based microalgal production with alkaline sewage for ocean negative carbon emissions

Using sewage (wastewater) for ocean alkalinity enhancement (OAE) has been considered as one promising ocean negative carbon emissions (ONCE) approach due to its high carbon sequestration efficiency and low environmental risk. To make this process more profitable and sustainable, this perspective proposes to integrate bicarbonate-based microalgal production and sewage alkalinity enhancement for ONCE. In this concept, the spent aqueous alkaline bicarbonate-based microalgal medium is cheap or even free for OAE, while the produced microalgae with high value-added compositions make this process more profitable. To make the proposed idea more efficient and sustainable, the prospects for its future development are also discussed in this opinion article. This perspective provides a novel and practical idea for achieving efficient carbon neutralization and high economic value simultaneously.

Dissolved carbon in effluent of wastewater treatment plants and its potential impacts in the receiving karst river

The dissolved carbon cycling in river system fueled by wastewater treatment plant effluent have been a research hotspot. However, the composition of dissolved carbon (DC) in wastewater effluents from karst regions remains poorly understood, resulting in a lack of clarity regarding its impact on the dynamics of dissolved carbon in karst rivers. To address this knowledge gap, this study investigated variations of dissolved inorganic (DIC) and organic C (DOC) components in effluent in karst regions and preliminarily discussed their influence on the DC cycling in karst rivers. The results showed that bicarbonate (HCO3-) in WWTP effluents makes more than 90% of the total dissolved inorganic carbon (DIC). The partial pressure of aqueous CO2 (pCO2) of the effluent reached 14450 ± 10084μtam, and pCO2 level declined with increasing river distance from the effluent discharge, effluent acted as a strong CO2 emitter to the atmosphere. Stable carbon isotope and water chemistry evidence revealed that organic matter degradation made important contributions to the high CO2 concentrations in effluent. PHREEQC mixing simulation together with filed samples data indicated that the DIC species can be changed, and pCO2 increased in receiving karst river water after mixed with effluent. The dissolved organic carbon (DOC) of effluent contained humic-like and protein-tryptophan-like, both of them appeared important and recent autochthonous, which could interfere the distinguish the sources of DOC in receiving karst river water. Thus, these findings highlight that the effluent can be an essential factor for the changes of the karst riverine DC pool, which advance our understanding on karst riverine DC evolution under anthropogenic activities. As more than 30% of the earth surface in China, northern America, and Europe are covered by carbonate rocks, this study has relevant implications for other karst regions as it underscores the influence of WWTP effluents on the carbon cycle in karst rivers. Such information and knowledge are valuable for monitoring and managing effluent-receiving river in other karst regions in the world.

Significant CO2 emission in the shallow inshore waters of the southeastern Yellow Sea in 2020

This study investigated the carbonate system and air-sea CO2 exchange in the inshore waters along South Korea's western coastline in 2020. Overlooking these waters might introduce significant errors in estimating air-sea CO2 fluxes of the southeastern Yellow Sea, given their interaction with land, offshore regions, and sediments. During periods other than summer, seasonal variations in seawater CO2 partial pressure (pCO2) could be generally explained by thermal effects. Tidal mixing and shallow depths resulted in weaker stratification-induced carbon export compared to offshore regions. However, during summer, inshore waters exhibited high spatial variability in pCO2, ranging from approximately 185 to 1000 μatm. In contrast to offshore waters that modestly absorbed CO2, inshore waters shallower than 20 m emitted ∼100 Gg C yr-1 to the atmosphere. However, considering the high heterogeneity of the study area, additional observations with high spatial and temporal resolution are required to refine estimates of air-sea CO2 exchange.