Microplastic interference influences Pseudomonas fluorescens in denitrification efficiency of wastewater treatment

Wastewater treatment plants (WWTP) play an essential role in pathogen and contaminant removal in wastewater. While developed countries treat approximately 70% of industrial wastewater prior to discharge, only about 8% is treated in developing countries. WWTP solutions can reduce the solids load, including microplastics, by up to 98.4%. Still, it is estimated that about 65 million microplastics are released into aquatic environments every day. Microplastics that are 20-300 µm in size are additionally considered extremely difficult to remediate and may be problematic for the denitrifying gram-negative bacteria Pseudomonas fluorescens, which is capable of polyethylene degradation. In order to explore this interaction, four bioreactors were established that contained 0% (control), 0.1%, 0.25%, and 0.5% plastic waste by weight, and inoculated with a Pseudomonas fluorescens culture. In the next five days, nitrate, nitrite, and polyethylene degradation were monitored through two trials. The 0.5% group saw the slowest nitrate reduction, indicating that the higher concentration of microplastics were inhibiting denitrification. The nitrate concentrations observed at the 0.1% group were most similar to the control. All groups had decreasing amounts of microplastics present, thus indicating some degree of biodegradation. This study suggests that microplastics impede Pseudomonas mediated denitrification while simultaneously being biodegraded. Future studies should look at different types of microplastics, lengths of time, and using different bacterial strains for the same purpose.