Trickling filters that use plastic packing have been built in round, square, and other shapes with depths varying from 4 to 12 m In addition to the packing, other components of the trickling filter include a wastewater dosing or application system,an underdrain. and a structure to contain the packing. The underdrain system is important both for collecting the trickling filter effluent liquid and as a porous structure through which air can circulateTrickling filters that use plastic packing have been built in round, square, and other shapes with depths varying from 4 to 12 m. In addition to the packing, other components of the trickling filter include a wastewater dosing or application system, an underdrain, and a structure to contain the packing. The underdrain system is important both for collecting the trickling filter effluent liquid and as a porous structure through which air can circulate