Trickling filter at a sewage treatment works. Trickling filters are a type of aerobic wastewater treatment, where sewage or wastewater flows down into a bed of rocks. The flow of wastewater promotes a biofilm (microbial community forming an adherent layer) to grow on the bed. Pollutants are removed from the wastewater through adsorption and absorption of organic matter by the biofilm. Photographed in Llangyndeyrn, Wales, UK.