Calculating expected effects of treatment effectivity and river flow rates on the contribution of WWTP effluent to the ARG load of a receiving river

The discharge of chlorinated effluent from wastewater treatment plants enhances dissolved oxygen in the receiving river: From laboratory study to practical application

Dissolved oxygen (DO) is essential for the health of aquatic ecosystems, supporting biogeochemical cycles and the decomposition of organic matter. However, continuous untreated external inputs from illicit discharges or sewer overflows, coupled with inadequate ecological base flow, have led to widespread river deoxygenation and serious ecological crises. This study demonstrates that chlorinated wastewater treatment plant (WWTP) effluent can significantly enhance DO levels in downstream rivers, particularly in areas with high pollution loads or poor ecological base flow. Notably, DO increases in receiving waters were positively correlated with initial chorine doses. Residual chlorine in WWTP effluent reduced inorganic nitrogen and dissolved organic matter (DOM). Analysis of DOM and molecular properties showed that residual chlorine preferentially reacts with low-molecular-weight organics like amino acids, increasing their hydrophobicity and electrophilicity. These molecular changes inhibit enzyme interactions, reducing the bioavailability of these compounds for oxygen-consuming processes. Field studies demonstrated that through on-site optimization of the full-scale WWTP disinfection process, specifically by controlling residual chlorine levels in effluents, DO levels downstream increased by an average of 15 %, with a maximum of 48 % compared to upstream levels, while typical disinfection byproducts (i.e., trihalomethanes, haloacetic acids and haloacetonitriles) remained below regulatory thresholds. This work provides new insights into the positive effects of chlorinated WWTP effluent on DO levels in receiving waters.

Occurrence of emerging contaminants in three river basins impacted by wastewater treatment plant effluents: Spatio-seasonal patterns and environmental risk assessment

The concern on the fate and distribution of contaminants of emerging concern (CECs) is a burning topic due to their widespread occurrence and potential harmful effects. Particularly, antibiotics have received great attention due to their implications in antimicrobial resistance occurrence. The impact of wastewater treatment plants (WWTP) is remarkable, being one of the main pathways for the introduction of CECs into aquatic systems. The combination of novel analytical methodologies and risk assessment strategies is a promising tool to find out environmentally relevant compounds posing major concerns in freshwater ecosystems impacted by those wastewater effluents. Within this context, a multi-target approach was applied in three Spanish river basins affected by different WWTP treated effluents for spatio-temporal monitoring of their chemical status. Solid phase extraction followed by ultra-high-performance liquid chromatography were used for the quantification of a large panel of compounds (n = 270), including pharmaceuticals and other consumer products, pesticides and industrial chemicals. To this end, water samples were collected in four sampling campaigns at three locations in each basin: (i) upstream from the WWTPs; (ii) WWTP effluent discharge points (effluent outfall); and (iii) downstream from the WWTPs (500 m downriver from the effluent outfall). Likewise, 24-h composite effluent samples from each of the WWTPs were provided in all sampling periods. First the occurrence and distribution of these compounds were assessed. Diverse seasonal trends were observed depending on the group of emerging compounds, though COVID-19 outbreak affected variations of certain pharmaceuticals. Detection frequencies and concentrations in effluents generally exceeded those in river samples and concentrations measured upstream WWTPs were generally low or non-quantifiable. Finally, risks associated with maximum contamination levels were evaluated using two different approaches to account for antibiotic resistance selection as well. From all studied compounds, 89 evidenced environmental risk on at least one occasion in this study.

Investigating the antibiotic resistance genes and their potential risks in the megacity water environment: A case study of Shenzhen Bay Basin, China

Antibiotic resistance genes (ARGs) constitute emerging pollutants and pose serious risks to public health. Anthropogenic activities are recognized as the main driver of ARG dissemination in coastal regions. However, the distribution and dissemination of ARGs in Shenzhen Bay Basin, a typical megacity water environment, have been poorly investigated. Here, we comprehensively profiled ARGs in Shenzhen Bay Basin using metagenomic approaches, and estimated their associated health risks. ARG profiles varied greatly among different sampling locations with total abundance ranging from 2.79 × 10-2 (Shenzhen Bay sediment) to 1.04 (hospital sewage) copies per 16S rRNA gene copy, and 45.4% of them were located on plasmid-like sequences. Sewage treatment plants effluent and the corresponding tributary rivers were identified as the main sources of ARG contamination in Shenzhen Bay. Mobilizable plasmids and complete integrons carrying various ARGs probably participated in the dissemination of ARGs in Shenzhen Bay Basin. Additionally, 19 subtypes were assigned as high-risk ARGs (Rank I), and numerous ARGs were identified in potential human-associated pathogens, such as Burkholderiaceae, Rhodocyclaceae, Vibrionaceae, Pseudomonadaceae, and Aeromonadaceae. Overall, Shenzhen Bay represented a higher level of ARG risk than the ocean environment based on quantitative risk assessment. This study deepened our understanding of the ARGs and the associated risks in the megacity water environment.

Mass-immigration shapes the antibiotic resistome of wastewater treatment plants

Wastewater treatment plants (WWTPs) are the hotspots for the spread of antibiotic resistance genes (ARGs) into the environment. Nevertheless, a comprehensive assessment of the city-level and short-term daily (inter-day) variations of ARG profiles in the whole process (influent-INF, activated sludge-AS and effluent-EF) of WWTPs is still lacking. Here, 285 ARGs and ten mobile gene elements were monitored in seven WWTPs in Xiamen for seven days via high-throughput qPCR. The average daily load of ARGs to WWTPs was about 1.32 × 1020 copies/d, and a total of 1.56 × 1018 copies/d was discharged to the environment across the entire city. Stochastic processes were the main force determining the assembly of ARG communities during sampling campaign, with their relative importance ranked in the order of INF > EFF > AS. There're little daily variations in ARG richness, abundance, β-diversity composition as well as assembly mechanisms. The results of SourceTracker, variation partitioning analysis, and hierarchical partitioning analysis indicated that bacteria and ARGs from upstream treatment processes played an increasingly dominant role in shaping ARG communities in AS and EFF, respectively, suggesting the importance of mass-immigration of bacteria and ARGs from the source on ARG transport in wastewater treatment processes. This emphasizes the need to revise the way we mitigate ARG contamination but focus on the source of ARGs in urban wastewater.

Microcosm experiments revealed resistome coalescence of sewage treatment plant effluents in river environment

Sewage treatment plant (STP) effluents are important contributors of antibiotic resistance (AR) pollution in rivers. Effluent discharging into rivers causes resistome coalescence. However, their mechanisms and dynamic processes are poorly understood, especially for the effects of dilution, diffusion, and sunlight-induced attenuation on coalescence. In this study, we have constructed microcosmic experiments based on in-situ investigation to explore these issues. The first batch experiment revealed the effects of dilution and diffusion. The coverage of water coalesced resistomes ranged 66.26∼152.18 × /Gb and was positively correlated with effluent volume (Mann-Kendall test, p < 0.01). Principal coordinate analysis (PCoA) and source tracking analysis demonstrated that dilution and diffusion stepwise reduced AR pollution. The second batch experiment explored the temporal dynamics and sunlight attenuation on coalesced resistomes. Under natural light, the coverage and diversity of water resistomes posed decreasing trends, primarily attributed to drastic erasure of effluent traces. The proportion of effluent-specific ARGs in coalesced resistomes significantly declined over time (Spearman's r = -0.83 and -0.94 in coverage and richness). While under dark condition, the coverage and diversity increased. Sunlight radiation intensified the interactions between water and sediment resistomes, as evidenced by more shared ARGs and less dissimilarities across niches. Network analysis, metagenome-assembled genome (MAG) analysis and variation partitioning analysis (VPA) showed that microbiome controlled resistome coalescence, explaining 56.5% and 58.4% of resistomes in water and sediment, respectively. Biotic and abiotic factors synergistically explained 40% of water resistomes. This study offers a comprehensive understanding of AR transmission and provides theoretical bases for grasping AR pollution and developing effective suppression strategies.

Prevalence of antibiotic resistance genes and virulence factors in the sediment of WWTP effluent-dominated rivers

In the context of increasing aridity due to climate changes, effluent from wastewater treatment plants (WWTPs) became dominant in some rivers. However, the prevalence of antibiotic resistance genes (ARGs) and virulence factors (VFs) in effluent-dominated rivers was rarely investigated. In this study, the profiles of ARGs and VFs in the sediment of two effluent-dominated rivers were revealed through the metagenomic sequencing technique. In each river, samples from the effluent discharge point (P site) and approximately 500 m downstream (D site) were collected. Results showed that the abundances of ARGs and VFs were both higher in D sites than those in P sites, indicating higher risks in the downstream areas. The compositions of ARGs were similar in the P sites of two rivers while being distinct in the D sites. The same was true for changes in the VFs compositions. Microbial community structure variations were the main driver for the changes in ARGs and VFs. Network analysis revealed that the interaction of ARGs and VF genes (VFGs) in sediment was intense. Two VFGs and eleven ARGs were identified to play important roles in the network. Metagenome-assembled genomes (MAGs) were generated to evaluate the coexistence of ARGs and VFGs at the single genome level. It was found that 38.4 % of the MAGs contained both ARGs and VFGs, and two MAGs were from pathogenic genera. These results suggested that high microbiological risks existed in effluent-dominated rivers, and necessary measures should be taken to prevent the potential threat to public health.

Spatio-seasonal patterns of the impact of wastewater treatment plant effluents on antibiotic resistance in river sediments

There is a growing concern about the risk of antibiotic resistance emergence and dissemination in the environment. Here, we evaluated the spatio-seasonal patterns of the impact of wastewater treatment plant (WWTP) effluents on antibiotic resistance in river sediments. To this purpose, sediment samples were collected in three river basins affected by WWTP effluents in wet (high-water period) and dry (low-water period) hydrological conditions at three locations: (i) upstream the WWTPs; (ii) WWTP effluent discharge points (effluent outfall); and (iii) downstream the WWTPs (500 m downriver from the effluent outfall). The absolute and relative abundances of 9 antibiotic resistance genes (ARGs), 3 mobile genetic element (MGE) genes, and 4 metal resistance genes (MRGs) were quantified in sediment samples, as well as a variety of physicochemical parameters, metal contents, and antibiotic concentrations in both sediment and water samples. In sediments, significantly higher relative abundances of most genes were observed in downstream vs. upstream sampling points. Seasonal changes (higher values in low-water vs. high-water period) were observed for both ARG absolute and relative abundances in sediment samples. Chemical data revealed the contribution of effluents from WWTPs as a source of antibiotic and metal contamination in river ecosystems. The observed positive correlations between ARG and MGE genes relative abundances point out to the role of horizontal gene transfer in antibiotic resistance dissemination. Monitoring plans that take into consideration spatio-temporal patterns must be implemented to properly assess the environmental fate of WWTP-related emerging contaminants in river ecosystems.

Effect of wastewater treatment plant discharge on the bacterial community in a receiving river

The effluent of wastewater treatment plants (WWTPs) is an important water resource for some rivers in regions with relatively low precipitation, which may pose ecological risks. Various pollutants and microorganisms are discharged into rivers, along with the WWTP effluent, but this process has not been thoroughly studied. The objective of this study was to evaluate the effect of WWTP effluent on the bacterial community in the sediment and water column of an urban river and to identify the relationship between the total and active bacterial communities. Five sites were sampled in the river, including the most upstream site of the river (Up-most), 200 m upstream of the WWTP (Up-200), at the point of effluent discharge of the WWTP (Eff-pl) and 50 m (Down-50) and 1000 m (Down-1000) downstream of the WWTP. Compared with the two upstream sites (Up-most and Up-200), the bacterial species composition of Eff-pl was significantly different (p < 0.05) in both the sediment and water columns, while the bacterial species composition at Down-1000 was significantly different (p < 0.05) in the sediment but not in the water. The relative abundance of Proteobacteria, Actinobacteriota and Verrucomicrobiota was significantly different (p < 0.05) at Eff-pl in both the sediment and water columns compared with that at the upstream sites. The shared bacterial species between the DNA and RNA 16 S rRNA analyses were only 45.5-62.2% and 43.2-52.3% for the sediment and water, respectively. Accordingly, WWTP effluent drainage significantly alters (p < 0.05) the bacterial composition in the receiving river but can be recovered in water within a short distance. However, in sediment, a longer recovery space is probably needed. Analyses of the combination of total and active bacterial compositions are recommended to evaluate the ecological consequences of WWTP effluent drainage on the bacterial composition.

Biogeochemical impacts of sewage effluents in predominantly rural river catchments: Are point source inputs distinct to background diffuse pollution?

Discharge of treated sewage effluent to rivers can degrade aquatic ecosystem quality, interacting with multiple stressors in the wider catchment. In predominantly rural catchments, the river reach influence of point source effluents is unknown relative to complex background pressures. We examined water column, sediment and biofilm biogeochemical water quality parameters along river transects (200 m upstream to 1 km downstream) during summer at five wastewater treatment works (WWTW) in Scotland. Treated sewage effluent (subset, n = 3) pollutant concentrations varied between sites. Downstream concentration profiles of water and sediment biogeochemical parameters showed complex spatial changes. A hypothesised point source signature of elevated concentrations of pollution immediately downstream of WWTW then a decaying pollution 'plume' did not commonly occur. Instead, elevated soluble reactive phosphorus (SRP), ammonium and coliforms (maximum 0.23 mgP/l, 0.33 mgN/l and >2 × 106 MPN/100 ml) occurred immediately downstream of two WWTW, whereas some downstream pollutant concentrations decreased. Microbial substrate respiration responses only differed 1 km downstream. Significantly greater concentrations of sediment metal occurred >500 m downstream, likely due to the redeposition of historic contaminated sediments. Significantly lowered chlorophyll-a downstream of one WWTW coincided with elevated metals, despite water SRP and sediment P increases. Overall, stress caused to microbes and algae by effluent contaminants outweighed the subsidy effect of WWTW nutrients. We observed variable effluent flows to the rivers limited localised pollution downstream of WWTW and overall influence of arable land cover on river water quality. Together, this challenges views of consistently discharging point sources impacting low dilution sensitive rivers in summer contrasting with 'diffuse' sources. Thus, river water column and benthic compartments are altered at varying scales by point source effluents in combination with rural catchment pollution sources, both discrete (e.g. farmyards and septic tanks) and diffuse.