Investigation of microbial population in hybrid biofilm nitrification systems

Abstract

Due to their persistence nature, potential toxicity and widespread presence of micropollutants, mostly pharmaceuticals residues, have gained lots of attention during the recent decades. Among these micropollutants, non-steroidal anti-inflammatory drugs (NSAIDs) are known as one of the most frequently detected therapeutic groups of pharmaceutical micropollutants in aquatic systems. Their widespread usage, both in human and animal medicine, together with their resistance to degradation, have resulted in the accumulation of these drugs in wastewater treatment effluents, surface waters, and even drinking water supplies. Conventional wastewater treatment plants are designed primarily to remove organic matter and nutrients, mostly having the deficiency to effectively eliminate such complex and recalcitrant compounds. Consequently, presence of NSAIDs in treated wastewater, posing ecological and public health risks. The aim of this study is to evaluate the efficiency and performance of hybrid treatment systems in removing a group of widely used NSAIDs—namely diclofenac, ibuprofen, indomethacin, ketoprofen, naproxen, and mefenamic acid—from wastewater. In addition to evaluation of removal efficiencies, this study aimed to investigate the impact of these NSAIDs on the microbial community structure and diversity within the treatment system. Detecting the impacts of these drugs on the microbial populations responsible for NSAIDs degradation is important in order to design and operation of future wastewater treatment systems. Laboratory-scale sequencing batch reactors (SBRs) were operated under controlled conditions. These reactors were set as moving bed biofilm reactor (MBBR) systems that combined the advantages of suspended biomass with attached biomass. A control hybrid reactor with no NSAID addition, and a micropollutant hybrid reactor with the addition of target NSAIDs at environmentally relevant concentrations were monitored. Over a period of 217 days, various parameters were monitored to assess system performance, including concentrations of chemical oxygen demand, ammonia, nitrite, and nitrate, as well as the influent and effluent levels of each NSAID. Based on the achieved removal efficiencies, it can be said that the hybrid system performed efficiently in degrading most of the NSAIDs studied. Ibuprofen and indomethacin were removed completely (with 100% removal efficiency), while ketoprofen, naproxen, and mefenamic acid showed removal efficiencies of 86%, 93%, and 84% respectively. Diclofenac, due to its low biodegradability, exhibited the lowest removal rate of 54%, which was higher than the performance of conventional activated sludge systems which was mostly reported less than 30% removal efficiency. These findings highlight the superior performance of hybrid systems in the biological treatment of NSAIDs. The hybrid system facilitates the growth of slow-growing nitrifiers, enhances the nitrification and also increases the resistance of the system to of parameters such as high loading rates and micropollutants. Using advanced molecular techniques, including high-throughput 16S rRNA gene sequencing, the microbial communities in the reactors were analyzed at phylum and species taxonomic levels. The results showed significant changes in the microbial community composition as a result of NSAID exposure. At the phylum level, dominant phyla were detected to be Proteobacteria, Bacteroidota, Acidobacteriota, Planctomycetota and Chloroflexi in the MP hybrid reactor. The results also indicated that the presence of NSAIDs caused a reduction in microbial diversity, as measured by different indices such as Shannon and Chao1 diversity indices. This reduction shows that NSAIDs cause selective pressure on microbial populations, potentially inhibiting sensitive species while improving the presence of resistant species. At the species level, Leadbetterella, Thauera, OLB8, uncultured Caldilineaceae, Fimbriiglobus and uncultured Planctomycetales species, showed the most relative abundance in the MP hybrid reactor, indicating their potential role in NSAID degradation. These findings are consistent with literature suggesting that certain microorganisms are capable of breaking down NSAIDs. Moreover, important treatment functions such as COD removal and nitrification were not inhibited showing that the microbial community was able to adapt to the presence of NSAIDs, maintaining its functions. Future researches should focus on applying the hybrid systems for full-scale cases with actual wastewater with complex matrix. Additionally, further exploration into the dynamics of NSAID-degrading microorganisms, may result in deeper insights into the mechanisms of NSAIDs biodegradation. In conclusion, this study highlights the potential of hybrid systems as an eco-friendly and sustainable solution to the presence of NSAIDs in the wastewater. Considering both high removal efficiency and microbial resistance, such systems represent a new insight for further systems operation and configuration.