Retention Time
To reiterate, HRT refers to the average time a certain amount of wastewater stays in a holding tank before exiting the tank. Where much of the confusion between retention and detention comes from is that these terms can describe different types of ponds that store water before the water moves on to the next phase of its journey. Knowing the definitions of these terms as they relate to ponds can help operators understand them in a greater context.
A retention pond can also go by the name “wet pond” because it is a permanent pool of water and holds water regardless of rainfall or snowmelt. There may be a nearby stream feeding the retention pond. The purpose of a retention pond is to hold water for a period of time — the retention time — before it moves on. The retention time allows sediment and other materials to separate from the water and sink to the bottom, reducing resuspension and sediment overflow.
In a sense, retention time in a pond is similar to hydraulic retention time in a wastewater treatment facility. In both instances, there is a body of water present that acts as the temporary home for water on its journey. While in an aeration tank or the wet pond, materials can separate from the water and sink to the bottom. The treated water can then continue to its next destination at a higher quality than before it entered the body of water.
Detention Time
Detention time is more closely associated with detention ponds, which are also known as dry ponds. The term “dry pond” should give away what these ponds are like — they have no water in them until water flows in from the outside. These ponds are common in areas with dry climates and provide flood protection in the event of a storm or extreme snowmelt.
Detention ponds will hold water for a certain period of time, depending on their size, slope and location. The time the water spends in the dry pond is its detention time. Thanks to a dry pond’s detention time, communities and cities can experience less flooding and less particle runoff. During the pond’s detention time, sediments will settle to the bottom, like in retention ponds. However, since the pond starts off empty, it can avert flooding in areas where excess water is unwanted.