Effects and fate of microfibres on wastewater treatment: Insights into treatment performance, microbiome selection and synergetic impacts on functional organisms in granular sludge reactors

The potential impacts of anthropogenic fibres, particularly denim and polyester, on wastewater treatment plants (WWTPs) remains poorly understood. In this research, the effects of commonly encountered denim and polyester fibres on the performance of aerobic granular sludge (AGS) reactors were investigated. Various concentrations of fibres (ranging from 10 to 8000 microfibres per litre) were explored, with a focus on nutrient removal, system performance, and efficacy. Firstly, the mechanism and formation of aerobic granules were explored in bubble reactors, the treatability of microfibres using aerobic granular sludge was determined, and it was discovered that aerobic granular sludge could remove microfibres up to 95-99%. The study also revealed that denim and polyester fibres had a noticeable impact on the reactor's performance and nutrient removal at very high concentrations. At concentrations corresponding to the median (70 microfibres per litre), three times the median (210 microfibres per litre), the 95th percentile (1500 microfibres per litre) and the maximum (8000 microfibres per litre), there was a statistically significant inhibition in the removal of chemical oxygen demand and total nitrogen. However, at the minimum concentration (10 microfibres per litre), inhibition was minimal or absent. iii Notably, the reactor with fibres exhibited a remarkable enhancement in the granulation process, as observed by a slower granulation rate in the control reactor relative to the reactor with fibres. These findings suggest a beneficial effect of fibres on granulation. Modelling simulations indicated that most fibres would aggregate with organic matter in the marine environment and potentially accumulate in sediments, highlighting river sediments as a sink for microfibres. The microbial community analysis conducted on the reactors revealed a range of 400-650 observed species, indicating a high alpha-diversity. Interestingly, the reactors with high fibre concentrations exhibited a more diverse microbial community compared to the control reactor. At the genus level, the most abundant genera included Paracoccus, Thauera, Azoarcus, Neomegalonema, Comamonas, and Pseudomonas, which is a potentially known fibre-degrading bacteria. This finding emphasizes the influence of fibres in enhancing microbial diversity in this specific study. The findings from this study provide valuable insights into the effectiveness of aerobic granular sludge in removing microfibres from wastewater treatment plants, as well as the inhibitory impacts of microfibres on nutrient concentrations, chemical oxygen demand, and total suspended solids in the final effluents of wastewater treatment plants. This information carries significant implications for both regulatory bodies and treatment plant operators in terms of optimizing wastewater treatment processes. Keywords: Aerobic granular sludge, Aerobic granules, Biofilm, Conventional activated sludge, Wastewater, Wastewater treatment plants, Total suspended solids, Microbial diversity, Mixed liquor suspended solids, Removal efficiency