Département Eaux de surface

Waterborne pathogens pose a serious threat to public health, emphasizing the need for reliable and efficient water treatment technologies. Wastewater treatment plants employ a range of processes to reduce microbial contamination, with membrane filtration emerging as a promising solution due to its ability to physically remove pathogens without the production of harmful chemical by-products. This study investigates the effectiveness of a gravity-driven membrane (GDM) filtration system for pathogen removal from wastewater and evaluates the associated public health risks with and without treatment. A quantitative microbial risk assessment model was employed to estimate infection probabilities for various waterborne pathogens. The results demonstrated a significant decrease in pathogen concentrations following treatment, with up to a 10⁴-fold reduction in norovirus infection risk. Three critical factors influencing membrane performance were identified: membrane integrity, pore size characteristics, and membrane fouling. Maintaining membrane integrity was found to be essential for ensuring consistent pathogen removal. While nominal pore size is commonly used to predict rejection efficiency, the overall pore size distribution was found to have a greater influence on virus retention. Additionally, although membrane fouling is often considered detrimental, it was shown to enhance virus removal by up to two orders of magnitude. These findings underscore the potential of GDM systems for effective virus removal and highlight the importance of proper membrane design, maintenance, and monitoring in ensuring long-term operational efficiency and maximizing public health protection in wastewater treatment applications.

Harmful algal blooms (HABs) caused by the dinoflagellate Alexandrium minutum can pose risks to human and ecosystem health. HABs of different species can coexist in coastal waters and accumulate near the shoreline, challenging their detection through Earth observation (EO). In this study, we use in situ geo-bio-optical and taxonomical data from the Rías Baixas (NW Spain) to develop a new method for identifying high-concentration blooms of A. minutum and its application to Sentinel-2 Multispectral Instrument (S2 MSI). Our approach named A. minutum index (AMI) was developed to capture the low absorption and high backscattering properties of A. minutum cells between 560 and 570 nm. We tested and validated the performance of three atmospheric correction algorithms (AC) (C2RCC, POLYMER and ACOLITE) using matchups between in situ and satellite-derived R_rs. Results show that C2RCC had the lowest error across most wavelengths. Applying AMI to S2 MSI indicates that our approach can accurately identify high-concentration blooms of A. minutum (F1 score: 70 %, Kappa: 68.3 %, balanced accuracy: 87.7 %, MCC: 68.3 %) and discriminate blooms of A. minutum from other phytoplankton species. We compared AMI with three existing indices for detecting HABs in coastal waters and found that our approach achieved a better performance, with the NDTI, RGCI and NDCI yielding F1 scores of 21.28, 21.74, and 0.0 % and MCC values of 15.0, 15.0 and 0.0 %, respectively. We also investigated the spatial resolution of S2 MSI to Sentinel-3 Ocean and Land Colour Instrument (S3 OLCI) for mapping fine-scale variations in A. minutum blooms. We found that the higher spatial resolution data from S2 MSI were highly useful for detecting small-scale variations in A. minutum in nearshore waters, enabling their detection in the mid-inner part of the Rías, where aquaculture activities are more prominent. This study also showcases the significance of accurate AC in near-shore waters, where high-concentration blooms can be more prevalent. Our findings show that greater errors in AC are observed in near-shore pixels, where the socio-economic and environmental impact of HABs are typically more severe.

Reconstructions of past changes in algal community composition provide important context for future alterations in biogeochemical cycling. However, many existing phytoplankton proxies are indicative of individual algal groups and are not fully representative of the whole community. Here, we evaluated hydrogen isotope ratios of algal lipids (δ²H_Lipid) as a potential proxy for phytoplankton community composition. We sampled the water column of Rotsee, a small eutrophic lake in Switzerland, every second week from January 2019 to February 2020 and analyzed distributions and the relative offsets between δ²H_Lipid values (δ²H_{Lipid1/Lipid2}) from short-chain fatty acids, phytosterols and phytol. Comparing these data with phytoplankton cell counts, we found that δ²H_{C16:0 Acid/Sterol} and δ²H_{Sterol/Phytol} values reflect shifts within the eukaryotic algal community. To assess whether the selected phytoplankton groups were the main sources of the selected lipids, we further modeled algal δ²H_{Lipid1/Lipid2} values based on δ²H_{C16:0 Acid}, δ²H_Sterol and δ²H_Phytol values from batch cultures of individual algal groups and their biovolume in Rotsee and evaluated the role of heterotrophy on δ²H_{Lipid1/Lipid2} values using a model incorporating δ²H_{C16:0 Acid} and δ²H_Sterol values from microzooplankton. Annually-integrated and amount-weighted δ²H_{Lipid1/Lipid2} values measured in Rotsee were within 2 to 20 ‰ of the mean of modeled algal δ²H_{Lipid1/Lipid2} values, demonstrating a strong link with the phytoplankton community composition, while δ²H_{Lipid1/Lipid2} values including microzooplankton lipids had a larger offset. Additionally, cyanobacterial biovolume was positively correlated with the ratio of phytol and phytosterols (phytol:sterol ratio) as well as the ratio of unsaturated C18 and C16:0 fatty acids (C18:C16 ratio). Our results support the application of sedimentary δ²H_{Lipid1/Lipid2} values in eutrophic lakes as a proxy for past phytoplankton community assemblages. Moreover, the calculation of sedimentary phytol:sterol and C18:C16 ratios provides an additional proxy for reconstructing cyanobacterial blooms.