Assessment of Microplastics Variation in a Sequence Batch Reactor

MPs are an emerging pollutant that can be found worldwide, raising concerns especially in aquatic ecosystems. WWTPs are the ultimate barrier preventing MPs from being discharged into natural waters. For this reason, the MPFs behavior in secondary treatment was evaluated. The analysis of the MPFs concentration in the SBR sludge showed a clear decrease in the first 40 days of the experiment, after which remained quite stable. The analysis of the MPFs concentration in the SBR effluent showed a similar trend. The decreasing trends were expected, as before the start of the experiment the sludge was treating influents with a much higher concentration of MPs compared to the simulated wastewater. The two series of data approached a horizontal at the later stage of the experiment, showing that an equilibrium between the MPFs entering in the SBR and exiting from the SBR was reached. The relatively slow decrease in MPFs concentration in the SBR sludge suggested that during the settling phase microplastic fibers have a strong tendency to settle with the sludge. This consideration is supported by the persistence of the difference in MPFs concentration at the later stages of the experiment and by previous studies. The results also showed that the WWTPs sludge can release MPs even when the influent has a low MPs concentration, which confirms the concerns over the land usage of the WWTPs sludge. To quantify the distribution of the entering MPFs into the AS and the effluent, a model of the concentration of MPFs in the SBR sludge was developed. The modeled concentration showed a decreasing trend, but slower than the decrease actually observed. To improve the fit of the model, further refinements would be needed: the quantification of the exact recovery rate of the effluent collection phase, the quantification of the recovery rate of the sludge collection phase, the monitoring of eventual variations of MPFs concentration in the tap water. The second aim was to assess the proportion between natural and synthetic fibers in the SBR. 88% of the suspected MPFs recovered from the initial sludge sample were natural fibers, confirming the findings of previous studies. The high proportion of natural fibers amongst suspected MPFs underlined the urge for the definition of a more reliable procedure to visually distinguish natural and plastic microfibers. The final adaptation of the H2SO4 digestion protocol was successful in eliminating natural fibers from the sample. Even though the results of the test were very promising, further studies are needed to better understand the effect of the adapted digestion protocol to the MPFs.