Impact of polystyrene nanoplastics on primary sludge fermentation under acidic and alkaline conditions: Significance of antibiotic resistance genes.

As a part of industrial or commercial discharge, the influx of nanoplastics (NPs) to the wastewater treatment plants is inevitable. Consequently, it has become a must to understand the effects of these NPs on different unit processes. This study aimed to investigate the impact of three different concentrations of polystyrene nanoplastics (PsNPs) on the fermentation of primary sludge (PrS), implemented in batch anaerobic bioreactors, at pH 5 and 10, considering the pH-dependent nature of the fermentation process. The results showed that PsNPs stimulated hydrogen gas production at a lower dose (50 µg/L), while a significant gas suppression was denoted at higher concentrations (150 µg/L, 250 µg/L). In both acidic and alkaline conditions, propionic and acetic acid predominated, respectively, followed by n-butyric acid. Under both acidic and alkaline conditions, exposure to PsNPs boosted the propagation of various antibiotic resistance genes (ARGs), including tetracycline, macrolide, ß-lactam and sulfonamide resistance genes, and integrons. Notably, under alkaline condition, the abundance of sul2 gene in the 250 µg PsNPs/L batch exhibited a 2.4-fold decrease compared to the control batch. The response of the microbial community to PsNPs exposure exhibited variations at different pH values. Bacteroidetes prevailed at both pH conditions, with their relative abundance increasing after PsNPs exposure, indicating a positive impact of PsNPs on PrS solubilization. Adverse impacts, however, were detected in Firmicutes, Chloroflexi and Actinobacteria. The observed variations in the survival rates of various microbes stipulate that they do not have the same tolerance levels under different pH conditions.

Impact of aging of primary and secondary polystyrene nanoplastics on the transmission of antibiotic resistance genes in anaerobic digestion.

Sewage sludge is a significant reservoir of nano/microplastics (NPs/MPs) and antibiotic resistance genes (ARGs). Research has revealed that NPs/MPs may exert an inhibitory effect on anaerobic digestion (AD) of sludge. Moreover, NPs/MPs can influence microbial community diversity and composition, potentially increasing ARGs dissemination. The morphological changes to NPs/MPs surface due to aging contribute to modifying hydrophobic properties. To date, there is limited comprehension regarding how various surface properties of NPs influence ARGs dissemination during AD. This study investigated the impact of primary aged/non-aged and secondary aged/non-aged polystyrene nanoplastics (PSNPs) on ARGs and mobile genetic elements (MGEs) propagation during AD. The findings indicated that the UV-aging process resulted in surface oxidation and distinct morphological characteristics in both primary and secondary PSNPs, while the surface oxidation effect was more pronounced in the secondary aged PSNPs. High concentrations (150 µg/L) of primary and secondary PSNPs inhibited methane production, with secondary PSNPs causing greater inhibition by 16 to 20 % compared to control. In contrast, low concentration (25 µg/L) had negligible or slightly positive effects on methane production. PSNPs at 150 µg/L reduced total VFA concentration, indicating an inhibitory effect on the fermentation step in the AD process. Primary and secondary PSNPs exhibited changes in EPS characteristics. ARGs abundance was enriched in reactors amended with PSNPs, with the highest abundance of 8.54 × 105 copies/g sludge observed in the secondary aged PSNPs (150 µg/L) reactor. Reactors exposed to aged PSNPs exhibited a relatively higher abundance of ARGs compared to reactors exposed to non-aged PSNPs. Exposure to PSNPs increased the microbial community diversity within the digesters and triggered the enrichment of Comamonadaceae and Syntrophaceae, belonging to Proteobacteria phylum. On the other hand, archaeal communities tended to shift towards hydrogenotrophic methanogens in PSNPs reactors. The correlation analysis showed that Comamonadaceae were positively correlated with the majority of ARGs and intl1. A positive correlation was observed between MGEs and most ARGs, suggesting that the increased proliferation of ARGs under PSNPs exposure may be linked to the abundance of MGEs, which in turn promotes the growth of hosts carrying ARGs. These findings suggest that aged and non-aged NPs could substantially impact the spread of ARGs and MGEs, which also led to notable alterations in the composition of the microbial community. Overall, this study provides valuable insights into the multifaceted impacts of PSNPs with various characteristics on AD processes, microbial communities, and ARGs proliferation, highlighting the urgent need for comprehensive assessments of NPs pollutants in the environment.

Comprehensive hydrothermal pretreatment of municipal sewage sludge: A systematic approach.

This study provides a comprehensive analysis of the potential impact of hydrothermal pretreatment (HTP) on municipal thickened waste-activated sludge (TWAS) and its integration with anaerobic digestion (AD). The research demonstrates that HTP conditions (170 °C, 3 bars for 30 min) can increase the solubilization of macromolecular organic compounds by 41%, which enhances biodegradability in semicontinuous bioreactors. This treatment also results in a 50% reduction in chemical oxygen demand (COD) and a 63% increase in the destruction of volatile solids (VS). The combination of HTP with AD significantly boosts methane yields by 51%, reaching 176 ml/g COD, and improves the digestate dewaterability, doubling the solid content in the dewatered cake. However, a higher polymer dose is required compared to conventional AD. Microbial community analysis correlates the observed performance and alterations; it indicates that HTP enhances resilience to stress conditions such as ammonia toxicity. This comprehensive study provides valuable insights into the transition from wastewater treatment plants (WWTPs) to resource recovery facilities (RRF) in line with circular economy principles.

Coupling hydrothermal liquefaction and anaerobic digestion for waste biomass valorization: A review in context of circular economy.

In light of the substantial global production of biomass waste, effective waste management and energy recovery solutions are of paramount importance. Hydrothermal liquefaction (HTL) and anaerobic digestion (AD) have emerged as innovative techniques for converting biomass waste into valuable resources. Their integration creates a synergistic framework that mitigates inherent limitations, leading to improved efficiency, enhanced product quality, and the comprehensive utilization of biomass. This review paper investigates the integration of HTL and AD, highlighting its significance and potential benefits as well as the optimal sequencing (HTL followed by AD and AD followed by HTL). The review encompasses experimental procedures, factors influencing both sequencing options, energy recovery characterizations, final product outcomes, as well as toxicological assessments and discussions on reduction. Additionally, it delves into the transition towards a circular bioeconomy and discusses the challenges and opportunities intrinsic to these processes. The findings presented in this review offer valuable insights to shape future research in this evolving field.

Utilizing waste eggshells as a calcium precursor for contact precipitation of phosphorus from digested sludge centrate.

Phosphorus (P) recovery from wastewater is an essential component of the global P cycle. A contact precipitation process using chicken eggshells as a calcium (Ca) precursor was used to recover P from synthetic wastewater and real digested sludge centrate. Up to 96.4 % of P could be recovered from the digested sludge centrate after three repeated cycles of the contact precipitation process. In addition, 36.1 % of total chemical oxygen demand and 37.6 % of total ammonia nitrogen were removed from the centrate. Finally, most of the precipitates could be collected by a simple washing step. Scanning electron microscopy-energy dispersive spectroscopy and x-ray diffraction results indicated that the eggshells played three roles in this process: Ca source, precipitation substrate, and filter medium. Precipitates were mainly brushite. This research provides a new perspective on P recovery from wastewater using waste eggshells, and if further optimized, has a potential for practical future applications.

Can low-temperature thermal hydrolysis mitigate the oxidative stress of polystyrene nanoplastics on anaerobic digestion?

The presence of micro/nanoplastics (MPs/NPs) in sewage sludge has sparked considerable apprehensions over their potential negative effects on anaerobic digestion (AD) performance. The occurrence of MPs/NPs can trigger oxidative stress on the anaerobic microbiome, leading to potential inhibition of the AD process. While the thermal hydrolysis process (THP) is an extensively utilized sludge pretreatment method for AD, its impact on stress induced by MPs/NPs during AD remains poorly understood. In this study, we assessed the impacts of low-temperature THP (90 °C, 90 min) on AD of sewage sludge in the presence of 150 µg/L of polystyrene nanoplastics (PsNPs) under different solid retention times (SRTs) of 20, 15, and 10 d. The presence of PsNPs resulted in a higher reactive oxygen species (ROS) production and a higher abundance of antibiotic resistance genes (ARGs). Additionally, their presence caused a significant inhibition of methane production by 28.2%, 29.3%, and 38.8% for SRTs of 20, 15, and 10 d, respectively. Introducing low-temperature THP prior to the AD could partially recover methane production by mitigating ROS-induced stress and curbing the propagation of ARGs during the AD process. These results shed light on the potential benefits of THP and further optimization opportunities in alleviating the adverse effects of MPs/NPs-induced stress during sewage sludge AD.

Low-temperature thermal hydrolysis for enhancing sludge anaerobic digestion and antibiotic resistance management: Significance of digester solids retention time.

Recently, there has been a growing inclination towards utilizing primary sludge (PS) fermentation prior to anaerobic digestion (AD) in water resource recovery facilities (WRRFs), where sludge liquor containing volatile fatty acids is used for biological nutrient removal. Nevertheless, using a low-temperature thermal hydrolysis process (THP) to improve AD in WRRFs adopting PS fermentation remains an area that has received limited research attention. Here, we studied the impact of THP (90 °C, 90 min) on anaerobic co-digestion of thickened waste activated sludge (TWAS) and fermented primary sludge (FPS) under varying solids retention times (SRTs) in semi-continuous mode. The study involved two THP schemes: scheme 1, where THP was done for both TWAS and FPS, and scheme 2, where THP was applied to TWAS only. The results demonstrated that reducing SRT from 20 to 15 and 10 d leads to decreased methane yield in both schemes. However, THP significantly enhances methane production, showing improvements of up to 37.9 % (scheme 1) and 31.2 % (scheme 2) under a 15-d SRT. Furthermore, while decreasing SRT increased the proliferation of antibiotic resistance genes (ARGs), thermal hydrolysis could effectively reduce most ARGs, indicating its potential to mitigate antibiotic resistance in the AD process. Overall, these results provide useful perceptions regarding the potential adoption of low-temperature THP in WRRFs with PS fermentation.

Fate of disposable face masks in high-solids anaerobic digestion: Experimental observations and review of potential environmental implications.

Face masks became a part of our daily life amid the global COVID-19 (SARS-CoV-2) pandemic. Most of the face masks are made for single-use and primarily disposed of in garbage bins with other non-recyclable wastes. To date, little is known about how disposable face masks in municipal solid waste (MSW) would interfere with high-solids anaerobic digestion (HSAD) in waste management facilities. Here, we first report preliminary results from a lab experiment conducted with the organic fraction of municipal solid waste (OFMSW) amended with used disposable face masks. The lab-scale HSAD systems were operated with percolate recirculation comparable to commercial HSAD systems typically used for full-scale processing of OFMSW. The results suggested that the presence of face masks in OFMSW could negatively affect methane productivity and kinetics. In the digesters amended with face masks, total cumulative methane production decreased by up to 18%, along with a 12-29% decrease in maximum methane production rates than the control digester (without face masks). Moreover, lag phases increased by 7-14%. The results also suggested that the type of polymeric materials used in face masks would be more critical than their total number/loading in the digester, which warrants further investigation. The visual inspection of digestate showed that the face masks were mostly undegraded after 40 days of operation. Much remains unknown about how the undegraded face masks will affect the digestate management practices, such as composting, land application, and landfilling. However, the review of existing literature suggested that they can be a potential source of plastic and microplastic pollution and amplify transmission of antibiotic resistance genes to the ecosystem. In summary, this study underscores the importance of developing safe and reliable disposal guidelines and management plans for single-use face masks.