Distribution of Polycyclic Aromatic Hydrocarbons (PAHs) in sludge organic matter pools as a driving force of their fate during anaerobic digestion

Retention time and organic loading rate as anaerobic co-digestion key-factors for better digestate valorization practices: C and N dynamics in soils

The growing use of anaerobic co-digestion (AcoD) in processing organic waste has led to a significant digestate production. To effectively recycle digestate back into soils, it is crucial to understand how operational variables in the AcoD process influence the conversion of organic matter (OM). To address this, a combination of biochemical fractionation and various soil incubation tests were employed to assess the stability of OM in digestates generated from anaerobic continuous reactors fed with a food waste-hay mixture and operating at different hydraulic retention times (HRT) and organic loading rates (OLR). This study revealed that digester performance and operating parameters impacted carbon dynamics in soils. A decrease in the carbon mineralization in soils when increasing the HRT was reported (48 ± 4 % for 70 days compared to 59 ± 1 % for 42 days). Specific HRT and OLR values were found to be linked to carbon accessibility and complexity, confirming that longer HRT lead to higher OM removal and increased complexity in soluble OM, despite minor discrepancies in relative carbon distribution. Furthermore, comparable rates of nitrogen mineralization in soils were observed for all digestates, consistent with the accessibility of nitrogen from the particulate OM. Nevertheless, AcoD converted substrates with the potential to immobilize nitrogen in soils into fast-acting fertilizers. In summary, this study underscores the importance of controlling the AcoD performances to evaluate the suitability of digestates for sustainable agricultural practices.

The occurrence characteristics, removal efficiency, and risk assessment of polycyclic aromatic hydrocarbons in sewage sludges from across China

Sewage sludge is considered to be an important sink for polycyclic aromatic hydrocarbons (PAHs) in wastewater treatment plants and the potential risks from sludge contaminated with PAHs during land application has attracted attention. To identify the priority PAHs for control and enhance their removal from sludge, the occurrence characteristics, removal efficiency, and risk assessment of PAHs in sewage sludges from across China were analyzed. Data collection was from 2001 to 2023. Results showed that 16 PAHs were widely detected in Chinese sewage sludge with total amounts (∑16PAHs) between 0.06 and 34.93 mg kg dw-1. Fossil fuel, coal, and biomass combustion are main anthropogenic sources of PAHs in China. In general, phenanthrene (PHE), anthracene (ANT), fluorescein (FL), chrysene (CHR), pyrene (PYR), and benzo[b]fluoranthene (BbF) are regarded as the main components and PAHs with 3-5 rings dominate (84.01%-91.53%) sewage sludge in China. Although aerobic composting and anaerobic treatment significantly improve ∑16PAHs removal, sludge stabilization treatment only reduced the risk by a small amount, especially for high-molecular-weight (HMW) PAHs. The benzo[a]anthracene (BaA), benzo[a]pyrene (BaP), and dibenzo[a,h]anthracene (DahA) are proposed as the priority control contaminants for sewage sludge in China because they have consistently high-risk quotient (RQ) values of 2.42-7.47, 1.28-3.16, 1.06-1.83 before and after sludge stabilization, respectively. More attention should be paid to BaA, BbF, benzo[k]fluoranthene (BkF), BaP, DahA, and indeno[1,2,3-cd]pyrene (IcdP) in Beijing; ANT, BaA, and BaP in Shanghai; and BaA and BaP in Guanghzou. Although the toxic equivalent quotient (TEQ) for PAHs met the limit concentration requirements of the national standard, the potential health risks due to long-term exposure to HMW PAHs cannot be ignored because the incremental lifetime cancer risk (ILCR) was consistently in the risk threshold range (>1 × 10-6). Some suggestions on enhanced treatment approaches and land use standards are proposed to further alleviate the risk from HMW PAHs.

Concentration of the main persistent organic pollutants in sewage sludge in relation to wastewater treatment plant parameters and sludge stabilisation

Concentration of 16 polycyclic aromatic hydrocarbons (PAHs), 7 polychlorinated biphenyls (PCBs), and 11 organochlorine pesticides (OCPs) in sewage sludge from 40 wastewater treatment plants (WWTPs) was investigated. Relationship between pollutant sludge contents, main WWTP parameters and type of sludge stabilisation was carefully evaluated. Average load of PAHs, PCBs, and OCPs in different sludges from Czech Republic was 3096, 95.7 and 76.1 μg/kg dry weight, respectively. There were moderate/strong correlations among the individual tested pollutants in sludge (r = 0.40-0.76). Relationship between total pollutant contents in sludge, common WWTP parameters and sludge stabilisation was not evident. Only individual pollutants such anthracene and PCB 52 correlated significantly (P < 0.05) with biochemical oxygen demand (r = -0.35) and chemical oxygen demand removal efficiencies (r = -0.35), suggesting recalcitrance to degradation during wastewater treatment. When sorted according to the design capacity, a linear correlation between WWTP size and pollutant contents in sludge was evident with growing WWTP capacity. Our study indicated that WWTPs with anaerobic digestion are prone to accumulate a statistically higher content of PAHs and PCBs (P < 0.05) in digested sludges compared to aerobically digested ones. The influence of anaerobic digestion temperature of treated sludge on tested pollutants was not evident.

Deciphering the contribution of microbial biomass to the properties of dissolved and particulate organic matter in anaerobic digestates

The recalcitrant structures either from substrate or microbial biomass contained in digestates after anaerobic digestion (AD) highly influence digestate valorization. To properly assess the microbial biomass contribution to the digested organic matter (OM), a combination of characterization methods and the use of various substrate types in anaerobic continuous reactors was required. The use of totally biodegradable substrates allowed detecting soluble microbial products via fluorescence spectroscopy at emission wavelengths of 420 and 460 nm while the protein-like signature was enhanced by the whey protein. During reactors' operation, a transfer of complex compounds to the dissolved OM from the particulate OM was observed through fluorescence applied on biochemical fractionation. Consequently, the fluorescence complexity index of the dissolved OM increased from 0.59-0.60 to 1.06-1.07, whereas it decreased inversely for the extractable soluble from the particulate OM from 1.16-1.19 to 0.42-0.54. Accordingly, fluorescence regional integration showed differences among reactors based on visual inspection and orthogonal partial latent structures (OPLS) analysis. Similarly, the impact of the substrate type and operation time on the particulate OM was revealed by ¹³C nuclear magnetic resonance using OPLS, providing a good model (R²X = 0.93 and Q² = 0.8) with a clear time-trend. A high signal resonated at ∼30 ppm attributed to CH₂-groups in the aliphatic chain of lipid-like structure besides carbohydrates intensities at 60-110 ppm distinguished the reactor fed with whey protein from the other, which was mostly biomass related. Indeed, this latter displayed a higher presence of peptidoglycan (δH/C: 1.6-2.0/20-25 ppm) derived from microbial biomass by ¹H-¹³C heteronuclear single-quantum coherence (HSQC) nuclear magnetic resonance. Interestingly, the sample distribution obtained by non-metric multidimensional scaling of bacterial communities resembled the attained using ¹³C NMR properties, opening new research perspectives. Overall, this study discloses the microbial biomass contribution to digestates composition to improve the OM transformation mechanism knowledge.

A strategy to reduce the effect of organic matter on fluorescence intensity for improving the detection accuracy of polycyclic aromatic hydrocarbons in soil

Fluorescence spectroscopy has been used for rapid detection of PAHs in soil, but soil organic matter (SOM) produces strong interference to the fluorescence intensity of PAHs, which restricts the application of fluorescence spectroscopy for rapid detection of PAHs in soil. A correction method of reducing the interference of SOM on PAHs fluorescence intensity was proposed combining fluorescence and near-infrared (NIR) spectroscopy. Six soil samples with different concentrations of humic acid (HA) at a given phenanthrene concentration (5 mg/g) were prepared and scanned for obtaining the fluorescence and NIR diffuse reflectance spectra. The spectral data showed that the fluorescence intensity and NIR diffuse reflectance had an approximate trend with the change of HA concentration. It was found that the NIR diffuse reflection at 4672 cm^-1 as a calibration factor could effectively reduce the interference of HA on the fluorescence intensity of phenanthrene. Subsequently, a standard curve for the quantitative analysis of phenanthrene in soil was established based on the fluorescence intensity before and after calibration. For the unknown samples, the predicted average relative errors of the standard curves before and after calibration were 27.46 % and 9.00 %, respectively. The results showed that the proposed correction method could reduce the interference of HA on the quantitative analysis of PAHs, and provide a reference for eliminating the interference constraint of fluorescence spectroscopy technique for rapid real-time detection of PAHs in soil.

Predicting the Stability of Organic Matter Originating from Different Waste Treatment Procedures

Recycling organic wastes into farmland faces a double challenge: increasing the carbon storage of soil while mitigating CO₂ emission from soil. Predicting the stability of organic matter (OM) in wastes and treatment products can be helpful in dealing with this contradiction. This work proposed a modeling approach integrating an OM characterization protocol into partial least squares (PLS) regression. A total of 31 organic wastes, and their products issued from anaerobic digestion, composting, and digestion-composting treatment were characterized using sequential extraction and three-dimension (3D) fluorescence spectroscopy. The apportionment of carbon in different fractions and fluorescence spectra revealed that the OM became less accessible and biodegradable after treatments, especially the composting. This was proven by the decrease in CO₂ emission from soil incubation. The PLS model successfully predicted the stability of solid digestate, compost, and compost of solid digestate in the soil by using only the characterized variables of non-treated wastes. The results suggested that it would be possible to predict the stability of OM from organic wastes after different treatment procedures. It is helpful to choose the most suitable and economic treatment procedure to stabilize labile organic carbon in wastes and hence minimize CO₂ emission after the application of treatment products to the soil.

Combining sequential extraction and 3D fluorescence to investigate the behavior of antibiotic and polycyclic aromatic hydrocarbons during solar drying of sewage sludge

Solar drying and liming are commonly used for sludge treatment, but little is known about their efficiency on antibiotics and Polycyclic Aromatic Hydrocarbons (PAHs) removal. This study aimed to investigate the removal of antibiotics and PAHs during solar drying of Limed Sludge (LS) and Non-Limed Sludge (NLS). Thus, organic matter fractionation and 3D fluorescence were used to assess the accessibility and the complexity of organic matter. 2 experiments have been conducted using LS and NLS for 45 days of drying in a pilot scale tunnel. Physicochemical results indicated significant decrease of water content (90%) for both sludge samples within 15 days of drying. For both treatments, the removal of total organic carbon and total nitrogen was low and similar for both treatments. Through this study, it has been confirmed that liming and drying contributed to a strong modification of the organic matter quality with an increase of its accessibility. On the other hand, drying alone increased the less accessible compartments, while the presence of lime affected the interconnexion between the organic matter pools. 3D fluorescence confirmed the obtained results and indicated that LS leads to obtaining more simple molecules in the most accessible compartments, while NLS leads to obtaining more complex molecules in the less accessible compartments. In addition, solar radiations and leaching may contribute to the significant removal (p < 0.01) of roxithromycin, benzo(a)anthracene, chrysene, benzo[k]fluoranthene, benzo[a]pyrene, and benzo(g, h, i) perylene in the presence of lime. Furthermore, the evolution of organic matter pools in terms of accessibility and complexity may drive the bioavailability of these pollutants, leading to their significant removal.

Prediction of organic matter accessibility and complexity in anaerobic digestates

Further characterization to properly assess the fate of organic matter quality during anaerobic digestion and organic carbon mineralization in soils is required. Organic matter quality based on its accessibility and complexity was employed to successfully classify 28 substrate/digestate pairs through principal components and hierarchical clustering analysis. The two first components explained 58.02% of the variability and four main groups were separated according to the feedstock type. A decrease in the accessibility (16-66%) and an increase in the complexity (34-98%) of the most accessible fractions was noticed. Besides, an increase of non-biodegradable compounds (17-66%) was globally observed after anaerobic digestion. The observed trends in the conversion of organic matter during anaerobic digestion have allowed to fill the gap in the modeling of the anaerobic digestion process chain. Indeed, partial least squares regressions have accurately predicted the organic matter quality of digestates from their inputs (R² = 0.831, Q² = 0.593) although the digester operational conditions (temperature and hydraulic retention time) were non-explicative enough. As a novel approach, the predicted digestate quality was used to feed a partial least squares regression model previously developed to predict organic carbon mineralization in soil. The combined models have predicted experimental organic carbon mineralization in soil (R² = 0.697) with a model quality similar to the model for organic carbon mineralization in soil (R² = 0.894). This is the first study that has successfully conceived an additional step in the prediction of organic matter fate from raw substrate before anaerobic digestion to soil carbon mineralization.

Bioelectrochemical systems for environmental remediation of estrogens: A review and way forward

Globally estrogenic pollutants are a cause of concern in wastewaters and water bodies because of their high endocrine disrupting activity leading to extremely negative impacts on humans and other organisms even at very low environmental concentrations. Bioremediation of estrogens has been studied extensively and one technology that has emerged with its promising capabilities is Bioelectrochemical Systems (BESs). Several studies in the past have investigated BESs applications for treatment of wastewaters containing toxic recalcitrant pollutants with a primary focus on improvement of performance of these systems for their deployment in real field applications. But the information is scattered and further the improvements are difficult to achieve for standalone BESs. This review critically examines the various existing treatment technologies for the effective estrogen degradation. The major focus of this paper is on the technological advancements for scaling up of these BESs for the real field applications along with their integration with the existing and conventional wastewater treatment systems. A detailed discussion on few selected microbial species having the unusual properties of heterotrophic nitrification and extraordinary stress response ability to toxic compounds and their degradation has been highlighted. Based on the in-depth study and analysis of BESs, microbes and possible benefits of various treatment methods for estrogen removal, we have proposed a sustainable Hybrid BES-centered treatment system for this purpose as a choice for wastewater treatment. We have also identified three pipeline tasks that reflect the vital parts of the life cycle of drugs and integrated treatment unit, as a way forward to foster bioeconomy along with an approach for sustainable wastewater treatment.

Process type is the key driver of the fate of organic micropollutants during industrial scale treatment of organic wastes

Organic micropollutants (OMPs) such as polycyclic aromatic hydrocarbons, nonylphenols and pharmaceutical products are ubiquitous in organic wastes generated by most human activities. Those wastes are mainly recycled by land spreading, most often after treatments, such as liming, dewatering, composting or anaerobic digestion. It has been shown essentially at lab scales that biological treatments have an effect on the removal of some OMPs. However, less is known on the role of each step of industrial treatment lines combining physico-chemical and biological treatments on the OMP fate and removal. The present study focuses on the impact of waste treatment on the fate of 53 OMPs along 10 industrial treatment lines treating urban, agricultural wastes or mixtures. The combination of studying a diversity of organic wastes and of OMPs with different characteristics (solubility, ionic charges, hydrophobicity etc.), sampling in situ industrial sites, quantifying native OMP concentrations and looking at each step of complete treatment lines allows for a global and representative view of the OMP fate in the French organic waste treatment sector. Less studied wastes, i.e. territorial mixtures, revealed intermediate OMP contents and compositions, between urban and agricultural wastes. Dewatering and liming, usually dismissed, had a noticeable effect on concentrations. Anaerobic digestion and composting had significant effects on the removal of all pollutant families. Combination of processes enhanced most OMP dissipation. Here we showed for the first time that the process type rather than the waste origin affects dissipation of organic micropollutants. Such data could be used to build and validate dynamic models for the fate of OMPs on solid waste treatment plants.

Traceability of organic contaminants in the sludge line of wastewater treatment plants: A comparison study among schemes incorporating thermal hydrolysis treatment and the conventional anaerobic digestion

The traceability of conventional pollutants and 10 organic microcontaminants in the sludge line of a wastewater treatment plant (WWTP) was evaluated. The application of thermal hydrolysis (TH) as pre-treatment to anaerobic digestion (AD) or as inter-treatment (between two AD stages) was considered and compared with the conventional digestion scheme. TH scenarios reduced the mass flow rate of biosolids (40-60%) as well as the ratio of solids (50-100%), organic matter (5-26%) and nitrogen (8-13%) destined to biosolids. Micropollutants showed a strong tendency to accumulate in the solid phase (more than 90% were sorbed) in spite of thermal and dewatering processes, but TH scenarios exhibited greater removal efficiency (80%) in comparison to conventional AD (50%), reducing the ratio of micropollutants destined to biosolids from a conventional 48% to 7-8%. These findings reveal that TH could increase the value of biosolids from sewage sludge treatment because of greater removal of pollutants and dewaterability.

Effects of cellulose on polycyclic aromatic hydrocarbons removal and microbial community structure variation during anaerobic digestion of sewage sludge

Waste-activated sludge (WAS) with trace organic pollutants, e.g., polycyclic aromatic hydrocarbons (PAHs), has become an environmental concern. Anaerobic technology is a feasible option for WAS treatment due to its advantages of low-energy consumption and high-energy recovery compared to aerobic technology, but it still has many shortcomings, such as low biogas production and a low organic pollutant removal efficiency. Thus, this study investigated the effects of cellulose on PAHs degradation and microbial community structure variation during anaerobic digestion of sewage sludge. Four semicontinuous experiments were set by adding cellulose to sewage sludge based on the volatile solids (VS) concentration. The proportions of sludge VS to cellulose VS were 1:0 (CK), 1:0.2, 1:0.5 and 1:1. The results showed the following: (1) The biodegradation of 2-ring, 3-ring and 4-ring PAHs was enhanced by cellulose addition, with total PAHs removal efficiencies of 14.82%, 20.75% and 19.35%, respectively. (2) The abundance of bacteria that could degrade PAHs, such as Chloroflexi, Bacteroidetes, Aminicenantes, Planctomycetes and Spirochaeta, was obviously increased by cellulose addition. (3) The abundance of Methanosaeta during sludge anaerobic digestion was apparently increased by cellulose addition. Methanobacterium and Methanolinea appeared after cellulose addition, while they were not observed in the blank experiment.

Acidogenesis is a key step in the anaerobic biotransformation of organic micropollutants

Understanding the role of the different anaerobic digestion stages on the removal of organic micropollutants (OMPs) is essential to mitigate their release from wastewater treatment plants. This study assessed the fate of 21 OMPs during hydrolysis and acidogenesis to elucidate the contribution of these stages to the overall anaerobic removal. Moreover, the removal mechanisms and factors influencing them were investigated. To this purpose, a fermentation reactor was operated and fed with two different substrates: starch (to jointly evaluate hydrolysis and acidogenesis) and glucose (to isolate acidogenesis). Results indicate that sulfamethoxazole was highly biotransformed (>80 %), while galaxolide, celestolide, tonalide, erythromycin, roxithromycin, trimethoprim, octylphenol and nonylphenol achieved a 50-80 % biotransformation. Since no significant differences in the biotransformation efficiencies were found between starch and glucose fermentation, it is stated that the enzymatic activities involved in starch hydrolysis do not significantly contribute to the cometabolic biotransformation of OMPs, while acidogenesis appears as the major player. Moreover, a higher biotransformation (≥15 percentage points and p ≤ 0.05) was found for galaxolide, celestolide, tonalide, erythromycin and roxithromycin during acidogenesis in comparison with the efficiencies reported for the acetogenic/methanogenic step. The biotransformation of some OMPs was explained considering their chemical structure and the enzymatic activities.

Extraction of nonylphenol, pyrene and phenanthrene from sewage sludge and composted biosolids by cyclodextrins and rhamnolipids

Sewage sludge generated by Waste Water Treatment Plants (WWTPs) are frequently used as organic amendments in agriculture, but they contain pollutants such as Potentially Toxic Elements (PTEs) and organic contaminants which contaminate the agricultural soils. The study presented here is part of a larger study based on the application of environmentally friendly chemical and biological techniques to decrease the content of organic pollutants in sewage sludge before agricultural application. The aim of this study was to evaluate the performance of biodegradable extractants, such as some cyclodextrins (CDs), β-cyclodextrin (BCD), hydroxypropyl-β-cyclodextrin (HPBCD) and randomly methylated-β-cyclodextrin (RAMEB), and a biosurfactant (rhamnolipid, RL) on the removal and availability of pyrene (PYR), phenanthrene (PHE) and nonylphenol (NP) from several biosolids samples in order to improve their subsequent biodegradation. The influence of pollutants retention time on biosolids was studied, as well as the effect of each extractant on PTEs solubilization. Results obtained were pollutant and extractant-dependent. BCD extracted similar amounts of pollutants compared to water, whereas HPBCD and RAMEB actually increased the availability of the three pollutants in most of the samples and aging times. RL seems to be the best election for Polycyclic Aromatic Hydrocarbons (PAHs) extraction from biosolids, with percentages of extraction multiplied by more than 80 and 40 times for PHE and PYR, respectively, relative to water extraction. The extraction enhancement was the highest for NP, the most hydrophobic pollutant, reaching more than 500-fold higher with HPBCD and RAMEB. PTEs extractability was not affected by the different CDs used, but RL caused an increment in their soluble content what could endanger a subsequent biodegradation of the organic pollutants.

Effect of soil organic matter (SOM) on the degradation of polycyclic aromatic hydrocarbons using Pleurotus dryinus IBB 903-A microcosm study

Bioremediation of polycyclic aromatic hydrocarbons (PAHs) in soil matrix is often arbitrated by the presence of soil organic matter (SOM). The present study focuses on determining the effect of the soil organic content on the mycoremediation of two model PAHs, phenanthrene (Phe) and benzo(a) pyrene (BaP) through microcosm studies. On comparing various microcosm strategies, a maximum degradation of Phe (99%) and BaP (48.5%) in soil bioaugmented with Pleurotus dryinus IBB 903, followed by, biostimulation with the degradation of Phe (89.9%) and BaP (24.8%) were noted. On relating the degradation pattern with lignolytic enzyme cocktail production, a laccase activity of 108 U/Kg at day 21, aryl alcohol oxidase (411 U/Kg) and manganese peroxidase (52.2 U/Kg) at day 14 along with lignin peroxidase (481 U/Kg) at day 21 were noted in fungal augmented soils, which were comparatively higher than levels observed in the bio-stimulation. Investigating the impact of different concentration of SOM (3-12%), a maximum remediation of Phe by 100% at 9% SOM in days 28 and 58.19% for BaP at 12% SOM, respectively was exhibited. Further, the biosorption effect of PAHs in abiotic condition showed a positive correlation with the increase in SOM, with a maximum adsorption of 3.78% Phe, and 6.93% BaP. The results support that the nominal adsorption ability of SOM, and helps in enhancing the microbial growth, thereby improving their degradation potentials, when less than 6% of SOM was utilized. Overall, this work establishes the critical role of organic matter in the soil with reference by simultaneous stimulation and degradation capability in complete PAHs remediation.

Organic micropollutants in sewage sludge: influence of thermal and ultrasound hydrolysis processes prior to anaerobic stabilization

Organic micropollutants (OMP) in the household and industrial wastewater are not efficiently removed by conventional treatment processes and a significant fraction ends in sludge. Proper valorization technologies become fundamental to attain sustainable sewage sludge management, with anaerobic digestion (AD) as one of the preferred strategies. However, it exhibits some limitations that can be overcome with pre-treatment processes. In this study, the influence of different pre-treatment configurations over OMP concentration and removal during AD was assessed. The incorporation of a sequential US - TT-PT resulted in decreased concentrations of 7 of the 9 detected compounds in biosolids compared to conventional AD digestate, with bisphenol-A and ter-octylphenol showing the opposite effect. The results suggest that the assessed PT could improve the removal of sequestered or highly hydrophobic compounds through their solubilization and increased bioavailability.

Thermal hydrolysis of sewage sludge partially removes organic micropollutants but does not enhance their anaerobic biotransformation

Pretreatment technologies prior to anaerobic digestion (AD) have been developed with the aim of enhancing biogas productivity and reducing the presence of pathogens in digested sludge. Among them, thermal hydrolysis (TH) appears as the most promising one. In wastewater treatment plants (WWTPs) sludge is the end point of many organic micropollutants (OMPs), which was proved to lead to important environmental and human risks since sludge is commonly used in agriculture. The objective of this work is to determine the fate OMPs in TH and subsequent AD. Sewage sludge was pretreated in a TH pilot plant at 170 °C for 20 min. Afterwards, two anaerobic digesters with a working volume of 14 L fed with fresh and pretreated sludge were operated in parallel in mesophilic conditions. TH proved to be an effective technology to partially or totally remove the dissolved fraction of OMPs as well as the fraction sorbed into those suspended solids that are solubilised after this pretreatment. However, it did not affect the OMPs sorbed concentration into solids that are not solubilised. Globally, the OMPs removal efficiency during TH appears to be linked to the solids solubilisation during this process. Afterwards, the OMPs biotransformation efficiency in AD of fresh and pretreated sludge was determined. Noticeable differences between the microbiome of both reactors was determined, but the anaerobic biotransformation was not substantially different for most of the OMPs. However, it affected musk fragrances, which presented considerably lower biotransformation efficiency in the reactor fed with pretreated sludge. Therefore, TH was proved effective in partially removing OMPs but not in enhancing their bioavailability and subsequent anaerobic biotransformation.

Post-aerobic treatment to enhance the removal of conventional and emerging micropollutants in the digestion of waste sludge

Low content of micropollutants in sewage sludge, essential to allow its safe re-use in agriculture, requires effective removals during the digestion phase. To this purpose, we investigated the performance of the anaerobic-aerobic sequential digestion process applied to real waste sludge in the removal of several classes of standard pollutants, i.e. extractable organic halogens (EOXs), polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), di(2-ethylhexyl)phthalate and alkylphenolethoxylates. In addition, emerging pollutants were also investigated based on their widespread occurrence and on their physicochemical characteristics and eco-toxicological relevance: quaternary ammonium compounds, a number of pharmaceuticals and selected biocides. The anaerobic step was conducted at mesophilic conditions, while two operating temperatures were tested for the post-aerobic treatment, i.e. 20 and 37 °C, respectively. Results showed that the post-aerobic digestion step enhanced the removal of all investigated standard and emerging micropollutants, even in presence of high accumulation in the anaerobic digestate (occurred for some PAHs and PCB congeners). Increased removals (up to 30%) have been generally observed at 37 °C aerobic temperature in comparison with tests at T = 20 °C for all investigated organic micropollutants, with the only exception of halogenated compounds (i.e. EOXs and PCBs). Low biodegradability and high bioaccumulation of the investigated pollutants were successfully faced by the sequential process, which has been demonstrated as an effective alternative solution to produce digested sludge for safe agricultural re-use.

Environmental availability of sulfamethoxazole and its acetylated metabolite added to soils via sludge compost or bovine manure

The fate of antibiotics and their metabolites in soils after application of organic waste depends on their environmental availability, which depends on the quality and biodegradability of the added exogenous organic matter (EOM). This study aimed at better understanding the fate of sulfamethoxazole (SMX) and N-acetyl-sulfamethoxazole (AcSMX) metabolite added to soils via sludge compost or cow manure application, during a 28-day incubation. Experimental results obtained for mineralized, extractable, and non-extractable fractions as well as EOM mineralization were used to couple SMX and AcSMX dynamics to the EOM evolution using the COP-Soil model. According to various mechanisms of extraction, CaCl₂, EDTA and cyclodextrin solutions extracted contrasted available fractions (31-96% on day 0), resulting in different sets of parameter values in the model. CaCl₂ extraction was the best method to assess the sulfonamide availability, leading to low relative root mean squared errors and best simulations of SMX and AcSMX dynamics. The decrease of SMX and AcSMX availability over time went with the formation of non-extractable residues, mostly of physicochemical origin. Using the COP-Soil model, the co-metabolism was assumed to be responsible for the formation of biogenic non-extractable residues and the low mineralization of SMX and AcSMX.

Organic micropollutants' distribution within sludge organic matter fractions explains their dynamic during sewage sludge anaerobic digestion followed by composting

The simultaneous fate of organic matter and 4 endocrine disruptors (3 polycyclic aromatic hydrocarbons (PAHs) (fluoranthene, benzo(b)fluoranthene, and benzo(a)pyrene) and nonylphenols (NP)) was studied during the anaerobic digestion followed by composting of sludge at lab-scale. Sludge organic matter was characterized, thanks to chemical fractionation and 3D fluorescence deciphering its accessibility and biodegradability. Total chemical oxygen demand (COD) removal was 41% and 56% during anaerobic digestion and composting, respectively. 3D fluorescence highlighted the quality changes of organic matter. During continuous anaerobic digestion, organic micropollutants' removal was 22 ± 14%, 6 ± 5%, 18 ± 9%, and 0% for fluoranthene, benzo(b)fluoranthene, benzo(a)pyrene, and nonylphenols, respectively. Discontinuous composting allowed to go further on the organic micropollutants' removal as 34 ± 8%, 31 ± 20%, 38 ± 10%, and 52 ± 6% of fluoranthene, benzo(b)fluoranthene, benzo(a)pyrene, and nonylphenols were dissipated, respectively. Moreover, the accessibility of PAH and NP expressed by their presence in the various sludge organic matter fractions and its evolution during both treatments was linked to both the quality evolution of the organic matter and the physicochemical properties of the PAH and NP; the presence in most accessible fractions explained the amount of PAH and NP dissipated.

Impact of sludge conditioning treatment on the bioavailability of pyrene in sewage sludge

Conditioning is an indispensable step to improve mechanical dewatering of municipal sewage sludge. However, it is still unclear how sludge conditioning treatments impact the bioavailability of polycyclic aromatic hydrocarbons (PAHs) in sewage sludge that potentially influences the biodegradation of PAHs during the composting of dewatered sludge cake. In the present study, five sludge conditioning treatments, including chemical acidification, bioleaching driven by Acidithiobacillus ferrooxidans, chemical conditioning with Fe[III] and CaO, and chemical conditioning with either aluminum polychloride (PACl) or polyacrylamide (PAM), were investigated to reveal their respective impacts on the bioavailability of pyrene in sewage sludge. The bioavailability of pyrene in conditioned sludge was evaluated by using the n-butanol extraction method. The results showed that the bioavailable fraction of pyrene increased from 59.1% in raw sludge to 68.7% in chemically acidified sludge and 79.3% in bioleached sludge, while the other three conditioning approaches did not significantly change the bioavailability of pyrene. During chemical acidification or bioleaching of sludge, cellular membrane damage of sludge microbial cells induced changes in sludge chemical and physical properties. Ridge regression analysis revealed that during these two conditioning processes the contribution rates of the changes in sludge chemical properties and physical properties on the enhancement of pyrene bioavailability were 33.0% and 67.1%, respectively. Therefore, chemical acidification and bioleaching conditioning treatments can enhance the bioavailability of pyrene in sewage sludge, mainly through changing the relative hydrophobicity and particle size of sludge flocs.

Why are organic micropollutants not fully biotransformed? A mechanistic modelling approach to anaerobic systems

Biotransformation of most organic micropollutants (OMPs) during wastewater treatment is not complete and an unexplained steady decrease of the biotransformation rate with time is reported for many OMPs in different biological processes. To minimize and accurately predict the emission of OMPs into the environment, the mechanisms and limitations behind their biotransformations should be clarified. Aiming to achieve this objective, the present study follows a mechanistic modelling approach, based on the formulation of four models according to different biotransformation hypotheses: Michaelis-Menten kinetics, chemical equilibrium between the parent compound and the transformation product (TP), enzymatic inhibition by the TP, and a limited compound bioavailability due to its sequestration in the solid phase. These models were calibrated and validated with kinetic experiments performed in two different anaerobic systems: continuous reactors enriched with methanogenic biomass and batch assays with anaerobic sludge. Model selection was conducted according to model suitability criteria (goodness of fitting the experimental data, confidence of the estimated parameters, and model parsimony) but also considering mechanistic evidences. The findings suggest that reversibility of the biological reactions and/or sequestration of compounds are likely the causes preventing the complete biotransformation of OMPs, and biotransformation is probably limited by thermodynamics rather than by kinetics. Taking into account its simplicity and broader applicability spectrum, the reversible biotransformation is the proposed model to explain the incomplete biotransformation of OMPs.

The fate of trace organic contaminants during anaerobic digestion of primary sludge: A pilot scale study

A pilot-scale study was conducted to investigate the fate of trace organic contaminants (TrOCs) during anaerobic digestion of primary sludge. Of the 44 TrOCs monitored, 24 were detected in all primary sludge samples. Phase distribution of TrOCs was correlated well with their hydrophobicity (>67% mass in the solid phase when LogD > 1.5). The pilot-scale anaerobic digester achieved a steady performance with a specific methane yield of 0.39-0.92 L/gVS_removed and methane composition of 63-65% despite considerable variation in the primary sludge. The fate of TrOCs in the aqueous and solid phases was governed by their physicochemical properties. Biotransformation was significant (>83%) for five TrOCs with logD < 1.5 and electron donating functional groups in molecular structure. The remaining TrOCs with logD < 1.5 were persistent and thus accumulated in the aqueous phase. Most TrOCs with logD > 1.5 were poorly removed under anaerobic conditions. Sorption onto the solid phase appears to impede the biodegradation of these TrOCs.

Comparison of pre- and inter-stage aerobic treatment of wastewater sludge: Effects on biogas production and COD removal

The aim of this study was to investigate thermophilic (55°C) aerobic digestion (TAD) as pre- and inter-stage treatment of sludge anaerobic digestion and to analyse the change in organic matter accessibility and complexity. Pre-treatment decreased methane yield (up to -70%), due to oxidation losses whereas inter-stage treatment slightly improved overall methane yield (+2.6%) and total COD removal (+5%) compared to control. Anaerobic degradability and COD removal in the second anaerobic stage significantly increased, by 13-40%. Organic matter fractionation showed that TAD led to an increase in sludge organic matter accessibility in all cases. Organic matter complexity, measured by fluorimetry, increased after TAD pre-treatment whereas it remained constant after inter-stage treatment. TAD was shown to be more efficient if applied to a more recalcitrant substrate and should thus be used as inter-stage treatment to avoid decreasing methane production.

Development of a soft extraction method for sulfamethoxazole and transformation products from agricultural soils: Effects of organic matter co-extraction on the environmental availability assessment

The recycling of biosolids and livestock manure in agriculture may lead to the introduction of antibiotic residues, i.e., parent molecule and transformation products, into amended soils. Their fate in soils can be approached through the assessment of their environmental availability. In this work, the environmental availability of sulfamethoxazole (SMX) and three transformation products (N⁴-acetyl-SMX, 3-amino-5-methylisoxazole, aniline) was assessed in soils amended with sludge compost or cow manure throughout a three-month incubation, using soft extractions with CaCl₂, EDTA or cyclodextrin solutions. First, the freeze-storage of soil samples was shown to decrease the SMX extractability. The SMX extractability depended on the initial concentration, the amendment type and the extracting solution at day 0. From 1.9% up to 63% of the SMX total content was initially extractable. The lowest fractions were quantified in EDTA extracts in which the dissolved organic matter was the most complex and responsible for high matrix effects in mass spectrometry compared to CaCl₂ extracts. The purification of cyclodextrin extracts highly reduced the matrix effects, but CaCl₂ was considered as the most suitable extractant. SMX extractability strongly decreased after the first 8days of incubation to finally reach 0.4-0.8% after 84days, whatever the initial conditions. This high decrease could be related to humification observed through the increasing complexity of extracted dissolved organic matter. Very low levels of transformation products were quantified throughout the incubation period. The low environmental availability of SMX was mainly due to its sorption on soil organic matter and resulted in its low biotransformation in these amended soils.

Removal of phenanthrene in aqueous solution containing photon competitors by TiO2-C-Ag film supported on fiberglass

Surface interactions with pollutants and photons are key factors that affect the applications of TiO₂ in environmental remediation. In this study, the solubilizing agents dimethylsulfoxide and polyoxyethylene sorbitan monooleate, which act as photon competitors, had no effect on the photocatalytic activity of TiO₂-C-Ag film in phenanthrene (PHE) removal. Fiberglass with TiO₂-C-Ag coating removed 91.1 ± 5.2 and 99.7 ± 0.4% of PHE in treatments using UVA (365-465 nm) and UVC (254 nm) irradiation, respectively. The use of fiberglass as a support increased the superficial area, thus allowing PHE sorption. C and Ag, which are electrically active impurities in TiO₂, enhanced its photocatalytic activity and thus the attraction of the pollutant to its surface. The use of high-frequency UV light (UVC) decreased the amount of carbon species deposited on the TiO₂CAg film surface. X-ray photoelectron spectroscopy of the TiO₂-C-Ag film revealed extensive oxidation of the carbon deposited on the film under UVC light and loss of electrons from Ag clusters by conversion of Ag⁰ to Ag³⁺.