Microplastic (MP) pollution is a global environmental issue, and traditionally treated wastewater has been identified as a source of land-based microplastics into the aquatic environment. This study evaluated the performance of a pilot-scale biofilter to polish wastewater treatment plant (WWTP) effluent before it enters the environment. The filter was divided into four zones, allowing the concentration of microplastics to be followed through the filter. It was fed with secondary effluent from a conventional WWTP in Denmark. The raw effluent from the WWTP contained 917 items m⁻³ which corresponded to a mass concentration of 24.8 µg m⁻³. After the top layer of the biofilter, the concentration had decreased to a median value of 197 item m⁻³ and 2.8 µg m⁻³, indicating an overall removal efficiency of 79% in terms of particle number and 89% in terms of particle mass. We also observed a tendency that MP of larger size and higher particle mass were more likely to be retained. After the last filtration zone, all MP larger than 100 µm had been removed. The results of this study demonstrate that biofilters are able to lower the MP abundance in treated wastewater significantly, but a complete removal is not ensured, hence some MP, particularly small-sized ones, can still be discharged into the receiving environment.

Summary: Chlorophyll (Chl) f and d are the most recently discovered chlorophylls, enabling cyanobacteria to harvest near‐infrared radiation (NIR) at 700–780 nm for oxygenic photosynthesis. Little is known about the occurrence of these pigments in terrestrial habitats. Here, we provide first details on spectral photon irradiance within the photic zones of four terrestrial cave systems in concert with a detailed investigation of photopigmentation, light reflectance and microbial community composition. We frequently found Chl f and d along the photic zones of caves characterized by low light enriched in NIR and inhabited by cyanobacteria producing NIR‐absorbing pigments. Surprisingly, deeper parts of caves still contained NIR, an effect likely attributable to the reflectance of specific wavelengths by the surface materials of cave walls. We argue that the stratification of microbial communities across the photic zones of cave entrances resembles the light‐driven species distributions in forests and aquatic environments.

At a former Danish polluted landfill, a field experiment using biofilters as an ex-situ remediation strategy for leachate water was carried out. The leachate water was polluted with phenoxy acids, mecoprop, dichlorprop, and their impurities originated from previous years of disposal of production wastes. Three individual biofilters were set up and each was filled with different a support material, e.g., sand, stonewool, and peat amended sand. The sand biofilter was spiked with ammonium acetate in pulses lasting a week to stimulate biomass growth and thereby enhance the removal of the phenoxy acids. The effects on removal and enantioselectivity were studied during a 69-day sampling campaign. Results showed that stimulation of the microbial community with ammonium acetate provided a boost, hence removal in the sand biofilter increased after the dosing whereas the stonewool and peat biofilters showed generally low removal. The highest removal was observed after stimulation in the sand biofilter for both herbicides. After a starting period, the removal was compound-specific but ranged from 60–100%. The final concentrations exceeded the drinking water limits slightly (0.25 µg L⁻¹) (mecoprop and 2-(2/4-chlorophenoxy)propanoic acid), while it was considerably below the limit for all other compounds (2-(2-methylphenoxy)propanoic acid and dichlorprop). Enantioselective fractions were already 0.41, and 0.75 for mecoprop and dichlorprop, respectively, in the inlet, probably due to in-situ degradation in the landfill—Mecoprop showed some enrichment of the (R)-enantiomer in the sand biofilter whereas no real trends were seen in the stonewool and peat biofilter. Only minor alterations in enantiomeric fractions were observed for dichlorprop in all three biofilters. This experiment shows that it is feasible to remove micropollutants from landfill leachates and it is possible to stimulate biomass and thereby initiate and obtain increased removal faster.