Trickling filters using rock media have long been a common form of biological treatment in the U.S. Used mostly in small and moderate size communities, they are capable of providing good removal of organic contaminants and are known for being stable, reliable, and economical. By themselves, however, rock trickling filters (preceded by primary treatment) cannot provide the high effluent quality usually required today. Discharge requirements often specify much lower levels of biochemical oxygen demand (BOD), suspended solids (ss), and coliform organisms than in the past; ammonia, nutrient, and toxicity removal may also be required.