Rapid 'wet' sand filter

The rapid 'wet' sand filter is a container with sand. The water level is controlled in a way that all the sand is underwater. This is the reason that his type of filter is called a wet filter. The down-side of a wet filter is that the available amount of oxygen in the water is limited. The removal of ammonia limited because this process uses oxygen. The up-side of a wet filter is that the contact time is much longer.
The filter nozzles let the water through but not the sand. If the filter is full of "dirt" the filter is stopped. With water and air the filter is back-washed and the dirt is deposited in a settling pond.