Combining environmental isotopes with Contaminants of Emerging Concern (CECs) to characterise wastewater derived impacts on groundwater quality

Long-term impact of wastewater effluent discharge on groundwater: Identification of contaminant plume by geochemical, isotopic, and organic tracers' approach

Infiltration of effluents from wastewater treatment plants (WWTP) into groundwater can be a source of Contaminants of Emerging Concern (CECs), such as pharmaceutical compounds, that are not fully removed during the treatment processes. A multi-tracer approach, based on hydrogeochemical, isotopic, and organic tracers, is applied in the Vistrenque Aquifer (Gard, France) to assess the dispersion of such unintentional plumes and its potential implication on groundwater quality for CECs in a small catchment area. In this area, a point source of WWTP effluent causes contaminant infiltration and unintentional transfer to the aquifer. This strong impact of an urban effluent was revealed from the Br/Cl ratio, boron concentrations and δ11B isotopic signature of the groundwater in the direct vicinity of the infiltration point. With increasing distance from that point, dilution with groundwater rapidly attenuates the urban signal from these hydrogeochemical and isotopic tracers. Nevertheless, a gadolinium anomaly, resulting from discharges of urban wastewater containing the contrast agents used for magnetic resonance imaging (MRI), highlights the presence of a wastewater plume further along the flow line, that comes with a series of organic molecules, including pharmaceutical residues. Monitoring persistent or reactive molecules along the plume provides a more detailed understanding of the transfer of CECs into groundwater bodies. This highlights the relevance of pharmaceutical compounds as co-tracers for WWTP plume delineation. The present multi-tracer approach for groundwater resource vulnerability towards CECs allows a more in-depth understanding of contaminant transfer and their fate in groundwater.

Suitability of treated wastewater for irrigation and its impact on groundwater resources in arid coastal regions: Insights for water resources sustainability

Water scarcity threatens agriculture and food security in arid regions like Saudi Arabia. The nation produces significant quantities of municipal wastewater, which, with adequate treatment, could serve as an alternative water source for irrigation, thereby reducing reliance on fossil and non-renewable groundwater. This study assessed the appropriateness of using treated wastewater (TWW) for irrigation in a dry coastal agricultural region in Eastern Saudi Arabia and its impact on groundwater resources. Field investigations were conducted in Qatif to collect water samples and field measurements. A multi-criteria approach was applied to evaluate the TWW's suitability for irrigation, including complying with Saudi Standards, the Irrigation Water Quality Index (IWQI), the National Sanitation Foundation water quality index (NSFWQI), and the individual irrigation indices. In addition, the impact of TWW on groundwater was assessed through hydrogeological and isotope approaches. The results indicate that the use of TWW in the study area complied with the Saudi reuse guidelines except for nitrate, aluminum, and molybdenum. However, irrigation water quality indices classify TWW as having limitations that necessitate the use for salt-tolerant crops on permeable and well-drained soils. Stable isotopic analysis (δ2H, δ18O) revealed that long-term irrigation with TWW affected the shallow aquifer, while deep aquifers were minimally impacted due to the presence of aquitard layer. The application of TWW irrigation has successfully maintained groundwater sustainability in the study area, as evidenced by increased groundwater levels up to 2.3 m. Although TWW contributes to crop productivity, long term agricultural sustainability could be enhanced by improving effluent quality, regulating irrigation practices, implementing buffer zones, and monitoring shallow groundwater. An integrated approach that combines advanced wastewater treatment methods, community involvement, regulatory oversight, and targeted monitoring is recommended to be implemented.

Can we redevelop ammonia nitrogen contaminated sites without remediation? The key role of subsurface pH in human health risk assessment

Nitrogen fertilizer supports global food production, but its manufacturing results in substantial ammonia nitrogen (AN) contaminated sites which remain largely unexplored. In this study, ten representative AN contaminated sites were investigated, covering a wide range of subsurface pH, temperature, and AN concentration. A total of 7232 soil samples and 392 groundwater samples were collected to determine the concentration levels, migration patterns, and accurate health risks of AN. The results indicated that AN concentrations in soil and groundwater reached 12700 mg/kg and 12600 mg/L, respectively. AN concentrations were higher in production areas than in non-production areas, and tended to migrate downward from surface to deeper soil. Conventional risk assessment based on AN concentration identified seven out of the ten sites presenting unacceptable risks, with remediation costs and CO2 emissions amounting to $1.67 million and 17553.7 tons, respectively. A novel risk assessment model was developed, which calculated risks based on multiplying AN concentration by a coefficient fNH3 (the ratio of NH3 to AN concentration). The mean fNH3 values, primarily affected by subsurface pH, varied between 0.02 and 0.25 across the ten sites. This new model suggested all investigated sites posed acceptable health risks related to AN exposure, leading to their redevelopment without AN-specific remediation. This research offers a thorough insight into AN contaminated site, holds great realistic significance in alleviating global economic and climate pressures, and highlights the need for future research on refined health risk assessments for more contaminants.

Combination of multivariate data analysis and mixing modelling to assess tracer potential of contaminants of emerging concern in aquifers

Collected evidence has shown that contaminants of emerging concern (CECs) in conjunction with more conventional tracers (major ions, nutrients, isotopes etc.) can be used to trace pollution origin in aquatic systems. However, in highly mixed aquifer systems signals obtained from conventional tracers overlap diminishing their potential to be used as tracers. In this study, we present an approach that incorporates multivariate statistical analysis (principal component analysis (PCA) and Kohonen's Self-Organizing Map method (SOM)) and mixing modelling to identify the most suitable CECs to be employed as anthropogenic tracers. The study area is located in the Besòs River Delta (Barcelona, NE Spain) and represents the highly mixed aquifer system. A one-year monthly based monitoring campaign was performed to collect the information about the concentrations of 105 CECs as well as major and minor ions in the river and along the groundwater flow. The dimensionality of the obtained dataset was reduced to 25 CECs, based on their estimated health risk effects, for multivariate data analysis. The obtained results showed the overlap of conventional tracers' signals obtained from PCA. In case of CECs, PCA revealed differences in their distributions allowing the differentiation of the roles of natural attenuation processes, local and regional flows on their occurrence in different parts of the aquifer. This was not possible to do using solely CECs' distribution profiles. SOMs provided the lacking information about the modality of the distribution of each CECs, revealing their ability to represent factors controlling the groundwater hydrochemistry, which assist in defining their tracer potential. Based on the obtained results four identified persistent CECs, two with unimodal (lamotrigine and 5-Desamino-5-oxo-lamotrigine) and two with bimodal (carbamazepine and diazepam (higher modality was not revealed)) distributions, were selected to run a mixing model to compare their tracer performance.

Vulnerability of wells in unconfined and confined aquifers to modern contamination from flood events

Groundwater is a primary potable water supply for coastal North Carolina (NC), but the increased intensity of extreme rainfall events and floods may exacerbate surface and subsurface processes that contribute anthropogenic chemicals to wells in the major confined aquifers of this region. We evaluated groundwater for organic chemicals of emerging concern (CEC) and the presence of tritium using flooded and not-flooded wells in the NC Department of Environmental Quality well monitoring network across the NC Coastal Plain. Flooded wells experienced standing water around the well casing at least once during the study period. Tritium concentrations, which indicate modern water presence (water recharged after 1953), were significantly greater in groundwater from flooded wells than not-flooded wells. In confined aquifers, modern water was detected at greater depths in flooded wells (206 m) than not-flooded wells (100 m). Suspect-screening high resolution mass spectrometry (HRMS) analysis of 150 groundwater samples yielded a total of 382 unique organic chemicals. Each groundwater sample contained, on average, 19 tentatively identified chemicals from the NIST 20 mass spectral database (M1) and 9 USEPA ToxCast chemicals. The number of tentatively-identified chemicals per sample was not significantly different among aquifers demonstrating the pervasive presence of the detected CECs in unconfined and confined aquifers. The presence of modern water in groundwater from flooded wells coincided with higher detection frequencies of certain organic contaminant classes, particularly pharmaceuticals, food additives, and regulated aromatic hydrocarbons. These results indicate that wells in both unconfined and confined aquifers are susceptible to modern water contamination during flood events; this finding has critical public health implications for coastal communities.

Assessing groundwater recharge rates, water quality changes, and agricultural impacts of large-scale water recycling

The over-exploitation and insufficient replenishment of groundwater (GW) have resulted in a pressing need to conserve freshwater and reuse of treated wastewater. To address this issue, the Government of Karnataka launched a large-scale recycling (440 million liters/day) scheme to indirectly recharge GW using secondary treated municipal wastewater (STW) in drought-prone areas of Kolar district in southern India. This recycling employs soil aquifer treatment (SAT) technology, which involves filling surface run-off tanks with STW that intentionally infiltrate and recharge aquifers. This study quantifies the impact of STW recycling on GW recharge rates, levels, and quality in the crystalline aquifers of peninsular India. The study area is characterized by hard rock aquifers with fractured gneiss, granites, schists, and highly fractured weathered rocks. The agricultural impacts of the improved GW table are also quantified by comparing areas receiving STW to those not receiving it, and changes before and after STW recycling were measured. The AMBHAS_1D model was used to estimate the recharge rates and showed a tenfold increase in daily recharge rates, resulting in a significant increase in the GW levels. The results indicate that the surface water in the rejuvenated tanks meets the country's stringent water discharge standards for STW. The GW levels of the studied boreholes increased by 58-73 %, and the GW quality improved significantly, turning hard water into soft water. Land use land cover studies confirmed an increase in the number of water bodies, trees, and cultivated land. The availability of GW significantly improved agricultural productivity (11-42 %), milk productivity (33 %), and fish productivity (341 %). The study's outcomes are expected to serve as a role model for the rest of Indian metro cities and demonstrate the potential of reusing STW to achieve a circular economy and a water-resilient system.

Determination of groundwater origins and vulnerability based on multi-tracer investigations: New contributions from passive sampling and suspect screening approach

Knowledge about groundwater origins and their interactions with surface water is fundamental to assess their vulnerability. In this context, hydrochemical and isotopic tracers are useful tools to investigate water origins and mixing. More recent studies examined the relevance of contaminants of emerging concern (CECs) as co-tracers to distinguish sources contributing to groundwater bodies. However, these studies focused on known and targeted CECs a priori selected regarding their origin and/or concentrations. This study aimed to improve these multi-tracer approaches using passive sampling and qualitative suspect screening by exploring a larger variety of historical and emerging concern contaminants in combination with hydrochemistry and water molecule isotopes. With this objective, an in-situ study was conducted in a drinking water catchment area located in an alluvial aquifer recharged by several water sources (both surface and groundwater sources). CECs determined by passive sampling and suspect screening allowed to provide in-depth chemical fingerprints of groundwater bodies by enabling the investigation of >2500 compounds with an increased analytical sensitivity. Obtained cocktails of CECs were discriminating enough to be used as chemical tracer in combination with hydrochemical and isotopic tracers. In addition, the occurrence and type of CECs contributed to a better understanding of groundwater-surface water interactions and highlighted short-time hydrological processes. Furthermore, the use of passive sampling with suspect screening analysis of CECs lead to a more realistic assessment and mapping of groundwater vulnerability.

Deciphering natural and anthropogenic nitrate and recharge sources in arid region groundwater

Recently, the subsoils of ephemeral stream (arroyos) floodplains in the northern Chihuahuan Desert were discovered to contain large naturally occurring NO₃^- reservoirs (floodplain: ~38,000 kg NO₃-N/ha; background: ~60 kg NO₃-N/ha). These reservoirs may be mobilized through land use change or natural stream channel migration which makes differentiating between anthropogenic and natural groundwater NO₃^- sources challenging. In this study, the fate and sources of NO₃^- were investigated in an area with multiple NO₃^- sources such as accidental sewer line releases and sewage lagoons as well as natural reservoirs of subsoil NO₃^-. To differentiate sources, this study used a large suite of geochemical tools including δ¹⁵N[NO₃], δ¹⁸O[NO₃], δ¹⁵N[N₂], δ¹³C[DIC], ¹⁴C, tritium (³H), dissolved gas concentrations, major ion chemistry, and contaminants of emerging concern (CEC) including artificial sweeteners. NO₃^- at sites with the highest concentrations (25 to 229 mg/L NO₃-N) were determined to be largely sourced from naturally occurring subsoil NO₃^- based on δ¹⁵N[NO₃] (<8 ‰) and mass ratios of Cl^-/Br^- (〈100) and NO₃^-/Cl^- (>1.5). Anthropogenic NO₃^- was deciphered using mass ratios of Cl^-/Br^- (>120) and NO₃^-/Cl^- (<1), δ¹⁵N[NO₃] (>8 ‰), and CEC detections. Nitrogen isotope analyses indicated that denitrification is fairly limited in the field area. CEC were detected at 67 % of sites including ³H dead sites (<1 pCi/L) with low percent modern carbon-14 (PMC; <30 %). Local supply wells are ³H dead with low PMC; as ³H does not re-equilibrate and ¹⁴C is very slow to re-equilibrate during recirculation through infrastructure, sites with low PMC, ³H < 1 pCi/L, and CEC detections were interpreted as locations with substantial anthropogenic groundwater recharge. Neotame was used to identify locations of very recent (<15 years before present) or ongoing wastewater influxes to the aquifer. This work shows the important influence of naturally occurring subsoil NO₃^- reservoirs on groundwater in arid regions and the major contribution of artificial recharge.

Soil self-cleaning capacity: Removal of organic compounds during sub-surface irrigation with sewage effluent

Globally, the reuse of treated sewage effluent for irrigation purposes is increasingly encouraged as a practical solution against the mismatch between the demand for and availability of freshwater resources. The reuse of sewage effluent for sub-surface irrigation (SSI) in agriculture serves the dual purpose of supplying water to crops and diminishing emissions of contaminants of emerging concern (CoECs) into surface water. To investigate such reuse, in a real scale cropland with SSI using sewage effluent, from September 2017 to March 2019 including the extremely dry year 2018, residues were followed of 133 CoECs as related to their physicochemical properties and quantified by liquid chromatography coupled to high-resolution mass spectrometry. Of the 133 target CoECs, 89 were retrieved in the field, most non-detect CoECs have low persistency. During the growing season with sub-surface irrigation, CoECs spread to the shallow groundwater and rhizosphere. Significantly lower concentrations are found between infiltration pipes as compared to directly next to the pipes in shallow groundwater for all persistency-mobility classes. CoECs belonging to the class pm (low persistency and low mobility) or class PM (high persistency and high mobility) class show no change amongst their removal in the rhizosphere and groundwater in a dry versus normal year. CoECs belonging to the class pM (low persistency and high mobility) show high seasonal dynamics in the rhizosphere and shallow groundwater, indicating that these CoECs break down. CoECs of the class Pm (high persistency and low mobility) only significantly build up in the rhizosphere next to infiltration pipes. Climatic conditions with dry summers and precipitation surplus and drainage in winter strongly affect the fate of CoECs. During the dry summer of 2018 infiltrated effluent is hardly diluted, resulting in significantly higher concentrations for the CoECs belonging to the classes pM and Pm. After the extremely dry year of 2018, cumulative concentrations are still significantly higher, while after a normal year during winter precipitation surplus removes CoECs. For all persistency-mobility classes in the shallow groundwater between the pipes, we find significant removal efficiencies. For the rhizosphere between the pipes, we find the same except for Pm. Next to the pipes however we find no significant removal for all classes in both the rhizosphere and shallow groundwater and even significant accumulation for Pm. For this group of persistent moderately hydrophobic CoECs risk characterization ratio's were calculated for the period of time with the highest normalized concentration. None of the single-chemical RCRs are above one and the ΣRCR is also far below one, implying sufficiently safe ambient exposures. Overall the deeper groundwater (7.0-11.8 m below soil surface) has the lowest response to the sub-surface irrigation for all persistency-mobility. When adopting a SSI STP effluent reuse system care must be taken to monitor the CoECs that are (moderately) hydrophobic as these can build up in the SSI system. For the deeper groundwater and for the discharge to the surface water, we find significant removal for the pM and the PM class but not for other classes. In conclusion, relatively high removal efficiencies are shown benefiting the surface waters that would otherwise receive the STP effluent directly.

Semiconductors Application Forms and Doping Benefits to Wastewater Treatment: A Comparison of TiO2, WO3, and g-C3N4

Photocatalysis has been vastly applied for the removal of contaminants of emerging concern (CECs) and other micropollutants, with the aim of future water reclamation. As a process based upon photon irradiation, materials that may be activated through natural light sources are highly pursued, to facilitate their application and reduce costs. TiO2 is a reference material, and it has been greatly optimized. However, in its typical configuration, it is known to be mainly active under ultraviolet radiation. Thus, multiple alternative visible light driven (VLD) materials have been intensively studied recently. WO3 and g-C3N4 are currently attractive VLD catalysts, with WO3 possessing similarities with TiO2 as a metal oxide, allowing correlations between the knowledge regarding the reference catalyst, and g-C3N4 having an interesting and distinct non-metallic polymeric structure with the benefit of easy production. In this review, recent developments towards CECs degradation in TiO2 based photocatalysis are discussed, as reference catalyst, alongside the selected alternative materials, WO3 and g-C3N4. The aim here is to evaluate the different techniques more commonly explored to enhance catalyst photo-activity, specifically doping with multiple elements and the formation of composite materials. Moreover, the possible combination of photocatalysis and ozonation is also explored, as a promising route to potentialize their individual efficiencies and overcome typical drawbacks.

On inorganic tracers of wastewater treatment plant discharges along the Marque River (Northern France)

Increase of water quality in aquatic systems has become a hot button issue in recent decades. However, with the aim to implement an effective remediation strategy, the first step is to identify the sources of diffuse and point-source pollution using several tracers. In urban areas, B isotopes, Gd enrichment, Cl^- or carbamazepine concentrations can be used as wastewater treatment plant tracers. In this study, a focus was made on the quantification of a wide variety of inorganic compounds (elements, ions, isotopic ratios) all along the Marque River, a small stream located in Northern France receiving effluents coming from seven wastewater treatment plants (WWTPs). The objectives were (i) to determine the importance of the WWTPs discharge during low water events, (ii) to assess the efficiency of conventional tracers in quantifying the contribution of the WWTPs and (iii) to investigate new potential tracers less commonly used. The results have shown, through statistical analyses ANOVA (Analysis Of Variance) tests, PCA (Principal Component Analysis) and contribution calculations, that the WWTPs discharges strongly impact the water composition of all the watercourse and particularly during the first 6 km. However, due to high discharges of wastewaters not always well treated, some classical indicators (e.g. B, Rb/Sr) have shown limitations when used alone. The use of a set of relevant tracers including alkali metals could therefore be one solution for overcoming such a problem. Finally, other indicators like Rb/B or Gd/Pt ratios may also be a way to tackle this issue; they are indeed promising to discriminate the source of wastewaters.

Decoupling anthropogenic vs. natural impacts at a wastewater treatment plant situated on acid sulfate soils

Decoupling natural and anthropogenic impacts on the subsurface environment can be difficult, particularly when it has been subject to a wide range of influences over time and space. In this work we show how the use of hydrogeochemical plotting tools, time-series analysis of key contaminants of concern, and targeted isotopic analysis can be used to better understand the contamination sources/processes in a complex environment - a Wastewater Treatment Plant (WWTP) located on coastal acid sulfate soils (ASS). Analysis of soil profiles for potential oxidisable sulfur, acid neutralising capacity (ANC), and pH_fox along with groundwater chemistry, revealed that oxidation of pyritic sediments, initially deposited during the mid-Holocene, have led to significant pH declines and the secondary mobilisation of metals into the groundwater environment. This is further complicated by historic anthropogenic inputs associated with the WWTP (e.g., effluent leakages) and the surrounding agricultural land uses. There is distinct separation between spatial and temporal trends in the nutrient and heavy metals data in groundwater, suggesting these reflect different contaminant sources and/or processes. Isotopic data indicate nutrients are largely derived from the WWTP, whereas time-series analysis of key contaminants of concern and hydrogeochemical plotting tools indicate metals are largely derived from the secondary mobilisation of ASS due to acidity generated during sulfide oxidation. This work highlights the importance of understanding the hydrogeological environment and need for careful planning and ongoing management of WWTP sites, particularly those constructed on potential acid sulfate soils (PASS), which, if disturbed or exposed, can lead to impacts beyond the area of ASS via groundwater discharge to nearby surface water bodies (in this case the site is adjacent to a Ramsar-listed wetland). The outcomes of this work have significant global application in the identification, assessment, and control of ASS, the practice of contaminant source attribution, and the siting and design of future WWTPs, which will continue to be sited in coastal areas to meet population needs.

Transport-based source tracking of contaminants in a karst aquifer: Model implementation, proof of concept, and application to event-based field data

Identification and location of contamination sources is crucial for water resource protection - especially in karst aquifers which provide 25% of the world´s population with water but are highly vulnerable to contamination. Transport-based source tracking is proposed and verified here as a complementary approach to microbial and chemical source tracking in karst aquifers for identifying and locating such sources of contamination and for avoiding ambiguities that might arise from using one method alone. The transport distance is inversely modelled from contaminant breakthrough curves (BTC), based on analytical solutions of the 1D two-region non-equilibrium advection dispersion equation using GNU Octave. Besides the BTC, the model requires reliable estimates of transport velocity and input time. The model is shown to be robust, allows scripted based, automated 2D sensitivity analyses (interplay of two parameters), and can be favourable when distributed numerical models are inappropriate due to insufficient data. Sensitivity analyses illustrate that the model is highly sensitive to the input time, the flow velocity, and the fraction of the mobile fluid region. A conclusive verification approach was performed by applying the method to synthetic data, tracer tests, and event-based field data. Transport distances were correctly modelled for a set of artificial tracer tests using a discharge-velocity relationship that could be established for the respective karst catchment. For the first time such an approach was shown to be applicable to estimate the maximum distance to the contamination source for coliform bacteria in karst spring water combined with microbial source tracking. However, prediction intervals for the transport distance can be large even in well-studied karst catchments mainly related to uncertainties in the flow velocity and the input time. Using a maximum transport distance is proposed to account for less permeable, "slower" pathways. In general, transport-based source tracking might be used wherever transport can be described by the 1D two-region non-equilibrium model, e.g. rivers and fractured or porous aquifers.

The Variation in Groundwater Microbial Communities in an Unconfined Aquifer Contaminated by Multiple Nitrogen Contamination Sources

Aquifers provide integral freshwater resources and host ecosystems of largely uncharacterized, truncated endemic microorganisms. In recent history, many aquifers have become increasingly contaminated from various anthropogenic sources. To better understand the impacts of nitrogen contamination on native groundwater ecosystems, 16S rRNA sequencing of the groundwater microbial communities was carried out. Samples were taken from an aquifer known to be contaminated with nitrogen from multiple sources, including fertilizers and wastewater treatment plant effluents. In total, two primary contaminants were identified: NH4+ (<0.1–3.7–26 mg L−1 NH4+ min-median-max), and NO3− (<0.01–18–150 mg L−1 NO3− min-median-max). These contaminants were found to be associated with a decrease/increase in microbial species richness within affected groundwater for NH4+/NO3−, respectively. Important phyla were identified, including Proteobacteria, which had the highest abundance within samples unaffected by NH4+ (36–81% NH4+ unaffected, 4–33% NH4+ affected), and Planctomycetes (0.05–10% NH4+ unaffected, 43–72% NH4+ affected), which had the highest abundance within the NH4+ affected samples, likely due to its ability to perform anaerobic ammonia oxidation (ANAMMOX). Planctomycetes were identified as a potential indicator for the presence of NH4+ contamination. The analysis and characterization of sequencing data alongside physicochemical data showed potential to increase the depth of our understanding of contaminant behavior and fate within a contaminated aquifer using this type of data and analysis.

Groundwater CSI: Unravelling Pollution Sources in Complex Environments with Liquid Chromatography Triple Quadrupole Mass Spectrometry

Wastewater treatment plants (WWTP) are used worldwide to purify domestic and industrial wastewater before it is returned to the environment. Even after treatment, wastewater may still contain a variety of contaminants, including nutrients (usually nitrogen and phosphorus) and organic pollutants, such as endocrine disruptors and pesticides. These compounds can leak into groundwater via old and/or damaged infrastructure, leaching from biosolids storage areas and/or release of effluents. However, similar contaminant impacts can come from other sources, such as agriculture. It is often very difficult to distinguish the true source of such contamination, especially where wastewater treatment plants are located in agricultural areas. Robust and sensitive techniques are needed to characterize impacts where there are multiple potential sources. This study analyzed a variety of synthetic chemicals using liquid chromatography triple quadrupole mass spectrometry (LC–QQQ–MS) to generate unique fingerprints of pollution. These were used to distinguish the impact on the local groundwater of a WWTP in southeast Victoria (Australia) from that of local agriculture. The use of such novel tracers could become a valuable tool in environmental monitoring, management, and remediation in the future.

Using stable isotope probing and fluorescence spectroscopy to examine the roles of substrate and soluble microbial products in extracellular polymeric substance formation in activated sludge process

In this study, we used stable isotope-labeled soluble microbial products (SMP) and substrates to explore their assimilation into the formation of new biological products (i.e., extracellular polymeric substances and biomass) in two adjacent sequencing batch reactors. The isotope labeling approach along with fluorescence spectroscopy allowed us to distinguish between refractory and labile portions of SMP constituents as well as their roles in the formation of extracellular polymeric substances (EPS). Comparison of SMP fluorescence and the specific UV absorbance values between the two reactors revealed the presence of humic-like aromatic substances in the non-consumable part of SMP, which can be ultimately released as effluent organic matter. Parallel factor analysis modeling of fluorescence spectra showed that the hydrolysis of EPS contents mostly resulted in humic-like components in SMP rather than protein-like components, which were initially abundant in EPS (>80%). From variations in carbon and nitrogen isotopic contents in EPS and biomass, it was found that carbon-containing substrates were enriched faster than their nitrogenous counterparts. The contributions to new EPS formation reached 87.5% for carbon and 60.5% for nitrogen. Meanwhile, the isotopic tracking of the labeled SMP revealed that only 11.0% and 11.9% of carbon and 13.3% and 11.6% of nitrogen from the influent SMP were finally assimilated into EPS and biomass, respectively. In contrast, the isotopic enrichment in SMP was higher (~50%) than that of EPS and biomass, indicating the low bioavailability and refractory nature of the feed SMP. This study proposed a promising approach for estimating the relative contributions of different forms of labile substrate and SMP to the formation of EPS in activated sludge processes. This approach could be suggested as a versatile method for establishing the kinetics, substrate element flow, mass balance on organic substrates and nutrients, as well as for tracking the consumption and uptake pathways of hazardous materials.