When you think of percolation, making coffee might immediately come to mind, but the term is used to describe several types of water phenomena.
Percolation is a natural phenomenon scientists and water managers can calculate and use to ensure surface and groundwater are free from contaminants and pollutants and also ensure water intended for human consumption is potable.
The image of liquid dripping through coffee grounds at gravity’s bidding is not so far removed from percolation, which generally refers to the movement of a liquid through a medium.
Within the context of the water cycle, it describes how water moves underground. As the National Ocean and Atmospheric Adminstration’s Northwest River Forecast Center explained, “percolation is the movement of water though the soil and its layers, by gravity and capillary forces.”
There are several factors that determine how that water moves through the soil — the filtering medium — including the porosity of the soil and how saturated with water the earth may already be. The water that moves through the soil and silt strata typically flows underground until it reaches an impermeable rock, when it stops, becoming stored underground in an aquifer.
If a soil has more sand content than clay, this affects how rapidly water passes through it. Shale and clay are porous, but they typically restrict the flow of water.
Percolation can be modeled mathematically, which can help determine the siting of facilities such as septic or sewage treatment facilities. The evaluation method, known as a percolation or perc test, determines the rate at which water moves through the soil based on factors including the soil composition and compaction.
Percolation of Reclaimed Water
In 2015, the Clay County (Florida) Utility Authority constructed a series of percolation ponds that would allow as much as 2.2 million gallons a day of reclaimed water to gradually percolate through the ground into the shallow, local aquifer. The system eliminates potential problems associated with directly discharging treated water to surface streams.
Halting the rate of water percolation is the key to Subsurface Water Retention Technology. Alvin Smucker, a Michigan State University professor, has been working on the system, which not only helps improve agricultural production, but also helps protect groundwater supplies by installing a membrane that acts as a barrier on cropland.
Percolation can also be used to predict water transport factors such as the rate of leaching, the flow of materials into water. This is most often used in agriculture to determine the movement of fertilizers or the salt content of soil. Leaching also refers to the movement of water through substances such as chemicals disturbed during mining or waste present in landfills that may affect groundwater supplies.
The term is also used in water treatment to describe a technology known as a percolating filter or trickling filter. Trickling filters are a biological wastewater treatment process — water is oxidized and purified by the process of trickling through a filtration media. This form of aerobic wastewater treatment may be used in lieu of aeration basins in packaged wastewater treatment plants or larger aeration plants.
The RWL Water blog and website contain a wealth of information on various water treatment and quality topics, all of which is updated frequently.
Image courtesy Clay County (Florida) Utility Authority.