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The Potential of Ozonation to Reduce Impact of Waste Sludge-Entrapped Microplastics to Biogas Production
Summary
Wastewater treatment plants concentrate microplastics from sewage into the resulting sludge, and this study tested whether ozonation could reduce the harm those microplastics cause during anaerobic digestion used to produce biogas. The findings showed that PET and polypropylene microplastics alter methane yields from sludge digestion in concentration-dependent ways, and that ozone pretreatment partially mitigates the inhibition caused by polypropylene — though the interactions are complex and require further optimization before widespread use.
Due to low degradability of plastic materials, high usage, and low cost, microplastics (MPs) is becoming ubiquitous environmental pollutant. Wastewater treatment plants (WWTP) are one of main sources, where most of the MPs from wastewaters ends in waste sludge. Anaerobic digestion is one of most promised techniques of waste sludge management. The aim of this paper was to evaluate the impact of polyethylene terephthalate (PET) and polypropylene (PP) microplastics in the waste sludge with and without ozonation as possible pretreatment, on biogas and methane production with OxiTop® method. In the first set of experiments, PET inhibited (up to 6% at 0.1 g L−1) or promoted (up to 3% at 1 g L−1) methane production but increased cumulative biogas production (up to 38% at 0.5 g L−1). PP inhibited methane production (up to 5% at 0.5 g L−1) but its impact on biogas production was dependent upon concentration of MPs added. In the second set of experiments, pretreatment of non-contaminated waste sludge by ozonation (3.54 gozone h−1) inhibited methane yield. Longer time of ozonation of PP-contaminated waste sludge reduced inhibition of methane yield, while in the case of PET-contaminated waste sludge, it was increased.