Microplastics in sewage sludge destined to anaerobic digestion: The potential role of thermal pretreatment

Mesophilic anaerobic digestion of mixed sludge in CSTR and AnMBR systems: A perspective on microplastics fate

Most microplastics (MPs) end up in the biosolids produced in wastewater treatment plants (WWTPs) and can pose contamination risks when the biosolids are applied to agriculture. This study evaluated the impact of mesophilic anaerobic digestion on the fate of MPs in WWTP sludge. For this, two laboratory-scale anaerobic digesters were operated in parallel, consisting of a continuous stirred tank reactor (CSTR) and a membrane bioreactor (AnMBR) equipped with an ultrafiltration membrane to decouple the hydraulic and sludge retention times. Both digesters were continuously fed with mixed sludge from a municipal WWTP. The results showed a significant reduction in the MP concentration, with the AnMBR having the higher MP removal efficiency (88.6% vs. 62.1%) and obtaining a higher percentage of biomethanisation (58.3% vs. 43.7%). Polypropylene (PP) and polyacrylonitrile were the main polymers in the mixed sludge, while PP and polyethylene were the dominant polymers in the digested samples. The MP particles in all the samples were predominantly in the 500-104 μm size range. Microbiological analysis indicates a greater species diversity in the microbial community of the AnMBR, the results also revealed a symbiotic relationship between the Firmicutes and Patescibacteria phyla in this digester.

Dissecting the effects of co-exposure to microplastics and sulfamethoxazole on anaerobic digestion

Microplastics (MPs) and antibiotics are frequently and simultaneously detected in sewage and sludge, raising global concerns in recent years. However, their combined effects on anaerobic digestion (AD) remain unclear. Herein, we evaluated the effects of the combinations of different MPs (i.e., polyethylene, polystyrene, polyvinyl chloride and polyethylene terephthalate) with sulfamethoxazole (SMX) on AD performance and microbial communities. The combined stress slightly decreased the chemical oxygen demand removal rate and total gas/methane production. Furthermore, co-exposure to MPs and SMX visibly changed the anaerobic sludge morphology during AD, reduced the methanogen activity, and increased the residual propionic acid concentration versus a control. The decreased relative abundances of Euryarchaeota ranged from 1.88% to 4.63% in the experimental groups compared with CK, suggesting that the microbial communities were inevitably affected by exposure to SMX alone or combined MPs/SMX. Interestingly, among the top 50 genera, only two were negatively related to a few antibiotic resistance genes, implying that sludge exhibited widespread multiple resistances. The correlation analysis between the MPs and microbial communities suggested that the MP properties, such as the aperture-desorption of MPs, may impact the microbial variations. This study will contribute to a deeper understanding of the impact of coexisting MPs/SMX on AD.

Nationwide meta-analysis of microplastic distribution and risk assessment in China's aquatic ecosystems, soils, and sediments

Microplastic (MP) accumulation has recently become a pressing global environmental challenge. As a major producer and consumer of plastic products, China's MP pollution has garnered significant attention from researchers. However, accurate and comprehensive investigations of national-level MP pollution are still lacking. In this study, we systematically collated a national MP pollution dataset consisting of 7766 water, soil, and sediment sampling sites from 544 publicly published studies, revealing the spatiotemporal distribution and potential risks of MP pollution in China. The results indicate that MP distribution is influenced by various regional factors, including economic development level, population distribution, and geographical environment, exhibiting considerable range and complexity. MP concentrations are generally higher in economically prosperous areas, but the degree of pollution varies significantly across different environmental media. Given the uncertainty and lack of standardized data in traditional microplastic risk assessment methods, this article highlights the urgency of developing a comprehensive big data and artificial intelligence (AI)-based regulatory framework. This work provides a substantial amount of accurate MP pollution data and offers a fresh perspective on leveraging AI for microplastic pollution regulation.

Aging and mitigation of microplastics during sewage sludge treatments: An overview

Wastewater treatment plants (WWTPs) receive large quantities of microplastics (MPs) from raw wastewater, but many MPs are trapped in the sludge. Land application of sludge is a significant source of MP pollution. Existing reviews have summarized the analysis methods of MPs in sludge and the effect of MPs on sludge treatments. However, MP aging and mitigation during sludge treatment processes are not fully reviewed. Treatment processes used to remove water, pathogenic microorganisms, and other pollutants in sewage sludge also cause surface changes and degradation in the sludge MPs, affecting the potential risk of MPs. This study integrates MP abundance and distribution in sludge and their aging and mitigation characteristics during sludge treatment processes. The abundance, composition, and distribution of sludge MPs vary significantly with WWTPs. Furthermore, MPs exhibit variable degrees of aging, including rough surfaces, enhanced adsorption potentials for pollutants, and increased leaching behavior. Various sludge treatment processes further intensify these aging characteristics. Some sludge treatments, such as hydrothermal treatment, have efficiently removed MPs from sewage sludge. It is crucial to understand the potential risk of MP aging in sludge and the degradation properties of the MP-derived products from MP degradation in-depth and develop novel MP mitigation strategies in sludge, such as combining hydrothermal treatment and biological processes.

Differential effects of petroleum-based and bio-based microplastics on anaerobic digestion: A review

The number of plastics is increasing owing to the rapid development of the plastics industry. Microplastics (MPs) are formed during the use of both petroleum-based plastics and newly developed bio-based plastics. These MPs are inevitably released into the environment and are enriched in wastewater treatment plant sludge. Anaerobic digestion is a popular sludge stabilization method for wastewater treatment plants. Understanding the potential impacts of different MPs on anaerobic digestion is critical. This paper provides a comprehensive review of the mechanisms of petroleum-based MPs and bio-based MPs in anaerobic digestion methane production and compares their potential effects on biochemical pathways, key enzyme activities, and microbial communities. Finally, it identifies problems that must be solved in the future, proposes the focus of future research, and predicts the future development direction of the plastics industry.

Effect of thermal hydrolyzed sludge filtrate as an external carbon source on biological nutrient removal performance of A2/O system

Thermal hydrolyzed sludge filtrate (THSF) rich in biodegradable organics could be a promising external carbon source for biological nutrient removal (BNR). The use of THSF can effectively reduce wastewater treatment plants operating costs and recover bioresources and bioenergy from the waste activated sludge. In this study, the effect of THSF on the BNR process was investigated using a lab-scale anaerobic/anoxic/oxic (A²/O) system. Total nitrogen (TN) and total phosphorus (TP) removal efficiencies of 74.26 ± 3.36% and 92.20 ± 3.13% at a 0.3% dosing ratio were achieved, respectively. Moreover, 20.42% of the chemical oxygen demand (COD) contained in THSF contributed to denitrification, enhancing nitrogen removal efficiency from 55.30 to 74.26%. However, the effluent COD increased by approximately 36.80%, due to 18.39% of the COD contained in THSF discharged with effluent. In addition, the maximum denitrification rate was approximately 16.01 mg N g VSS^-1 h^-1, while the nitrification rate was not significantly affected by THSF. Nitrosomonas, a common chemoautotrophic nitrifier, was not detected after the introduction of THSF. The aerobic denitrifier Rubellimicrobium was stimulated, and its relative abundance increased from 0.16 to 3.03%. Moreover, the relative abundance of Dechloromonas was 3.93%, indicating that the denitrifying phosphorus removal process was enhanced. This study proposes an engineering application route of THSF, and the chemical phosphate removal pretreatment might be a means to suppress the phosphate recirculation.

Microplastics in agricultural soils in China: Sources, impacts and solutions

The detection of microplastics (MPs) in agricultural soils has raised alarms on their potential impacts on agricultural production, particularly in China where agriculture has great importance for domestic consumption and export. This review aims to present the abundance, sources and impacts of MPs in the agricultural soils of China. It has the novelty of synthesizing sustainable agronomic practices to reduce MPs pollution of agricultural soils based on the sources identified. According to the extant study, the abundance of MPs in the agricultural soils in China ranged from 4.94 items/kg in the lower reaches of Yangtze River to 40,800 items/kg in Yunnan Province. The MPs were predominantly ≤1 mm and were mainly composed of fragments, films and fibers. Polyethylene and polypropylene MPs were most reported. Plastic mulching films were the most significant source of MPs in agricultural soils, followed by abandoned greenhouses and the use of organic fertilizers containing fugitive MPs or whose sources were often MPs-polluted. MPs were found to alter soil physicochemical properties for instance, water flow, water-stable aggregates, soil aggregation, soil pH, bulk density and nutrient contents. MPs also affect soil biota through changing the richness and diversity of soil microbial community while retarding growth and disrupting physiological functions of soil macrofauna. The effects of MPs on crops vary and range from alteration of biomass, metabolism and nutrient demands to impacted photosynthesis. Sustainable solutions include the use of grass clippings - straw mix as organic mulches, the use of compost as soil amendment in conjunction with grass-straw mix and incorporation of weed-suppressing biomass into compost, the use of jute and biodegradable plastics for greenhouses, proper decommissioning of abandoned greenhouses as well as setting standards for allowable MPs contents in organic fertilizers and irrigation water.

Potential of Advanced Oxidation as Pretreatment for Microplastics Biodegradation

In the last two decades, microplastics (MP) have been identified as an emerging environmental pollutant. Due to their small size, MP particles may easily enter the food chain, where they can have adverse effects on organisms and the environment in general. The common methods for the removal of pollutants from the environment are not fully effective in the elimination of MP; thus, it is necessary to find a more suitable treatment method(s). Among the various approaches tested, biodegradation is by far the most environmentally friendly and economically acceptable remediation approach. However, it has serious drawbacks, generally related to the rather low removal rate and often insufficient efficiency. Therefore, it would be beneficial to use some of the less economical but more efficient methods as pretreatment prior to biodegradation. Such pretreatment would primarily serve to increase the roughness and hydrophilicity of the surface of MP, making it more susceptible to bioassimilation. This review focuses on advanced oxidation processes (AOPs) as treatment methods that can enhance the biodegradation of MP particles. It considers MP particles of the six most commonly used plastic polymers, namely: polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyethylene terephthalate and polyurethane. The review highlights organisms with a high potential for biodegradation of selected MP particles and presents the potential benefits that AOP pretreatment can provide for MP biodegradation.