Distributor arms extend across the trickling filter inner diameter and have variable openings to provide a uniform application rate per unit area. The distributor arms are rotated by the force of the water exiting through their opening or by the use of electric drives In some cases, especially for square or rectangular filters, fixed flat-spray nozzles have been usedDistributor arms extend across the trickling filter inner diameter and have variable openings to provide a uniform application rate per unit area. The distributor arms are rotated by the force of the water exiting through their opening or by the use of electric drives. In some cases, especially for square or rectangular filters, fixed flat-spray nozzles have been used