What is the difference between a hydraulic driven distributor and a speed controlled distributor?

Rotary distributors have historically used the reaction thrust of water flowing over spreader plates on the trailing edge of distributor arms to rotate the distribution mechanism forward. In order to slow the distributor down, a small number of spreader plates are placed on the leading edge of the distributor arms to provide a backward thrust. The spreader orifice openings are set based on what is required to achieve even distribution of the wastewaterover the filter media. Once the orifices are set, these are seldom adjusted unless major changes in flows occur. Most hydraulically driven or Reaction driven distributors rotate at 1 to 3 revolutions per minute (rpm) with wastewater dosed at 0.2 to 0.6 inches per pass of each distributor arm. In the early 90’s many Engineers recommended slowing down the speed of rotary distributors so that water would penetrate deeper into the filter tank with each pass. Speed controlled or Motor driven distributors allow for a controlled rate of distribution which is independent of the flow or orifice settings. Speed controlled distributors can be operated 10 to 20 times slower than hydraulic driven distributors while operating at the same flow.