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Hydrothermal versus photo-aged polyethylene microplastics: Aging mode-dependent oxidative stress and enzyme inhibition in waste activated sludge fermentation
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When plastic waste breaks down through weathering (like heat and water exposure) or sunlight, it releases chemicals and additives that can throw off the microbes used in wastewater treatment plants to process sewage sludge. This study found that these leached chemicals cause oxidative stress and block key enzymes microbes need to break down waste, making treatment less efficient, raising concerns that aging plastic pollution could quietly undermine wastewater systems that protect our water supply and, ultimately, human health.
The aging of microplastics (MPs) inevitably occurs during sludge treatment, leading to the leaching of dissolved organic matter (DOM) and associated chemical additives. However, the impact and mechanism of aged leachate on anaerobic fermentation of waste activated sludge (WAS), particularly its ecological risks and implications for sludge resource recovery, remain poorly understood. This study investigated the effects of leachates prepared from photochemically (PC) and hydrothermally (HT) aged polyethylene (PE) MPs at an initial PE-MPs concentration of 5 g/L on short-chain fatty acids (SCFAs) production during WAS anaerobic fermentation. The results showed that PC and HT leachates reduced SCFAs production by 21% and 30.6%, respectively, and shifted the product composition from acetate-dominant to a mixed-acid profile. Biochemical analyses revealed that the aged leachates induced oxidative stress and inhibited protease and α-glucosidase activities, resulting in only 2.3%-6.0% protein degradation and a 23.1%-23.8% decrease in carbohydrate hydrolysis efficiency. Microbial community analysis further demonstrated that both PC and HT leachates reduced microbial diversity and the abundance of key hydrolytic genera, thereby compromising the functions of acid-producing and nitrogen-cycling bacteria. Analysis of leachate composition indicated that HT leachates released more oxygen-containing depolymerization products and additives, leading to stronger inhibition of microbial activity and hydrolysis processes. ECOSAR predictions suggested that several phenolic antioxidants and phthalate plasticizers may pose relatively high aquatic hazards. Their presence may partly contribute to the stronger biological inhibition observed in the aged-leachate treatments. These findings provide critical insights into the toxicity mechanisms of aged MP leachates in sludge fermentation systems.
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Researchers investigated how HDPE and LDPE microplastics affect aerobic and anaerobic digestion in wastewater treatment, while also characterizing whether the plastics themselves degrade. HDPE reduced methane production in anaerobic digestion by 26% but LDPE had no negative impact on either process, and only additive-free LDPE showed signs of bio-aging.
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Researchers examined how the degree of weathering of polyethylene and polypropylene microplastics affects their tendency to promote antibiotic resistance gene spread during sewage sludge digestion, and found a counterintuitive U-shaped relationship: moderately aged plastics actually suppressed resistance gene proliferation by up to 50% compared to fresh plastics, while more heavily aged plastics saw the effect bounce back. The mechanism involves weathering altering how microplastics affect microbial stress responses and gene transfer pathways. The findings highlight that the environmental history of microplastics matters when assessing their biological risks.
Revealing the Mechanisms of Polyethylene Microplastics Affecting Anaerobic Digestion of Waste Activated Sludge
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Researchers studied how polyethylene microplastics affect the anaerobic digestion of sewage sludge, a common wastewater treatment process. They found that higher concentrations of microplastics significantly reduced methane production by disrupting microbial communities and enzyme activities essential for digestion. The study reveals that microplastic contamination in wastewater systems can undermine the efficiency of sludge treatment and biogas generation.
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