Backwash has several meanings depending on the context, but the most common one people wonder about is the saliva and bacteria that flow back into a drink after someone takes a sip. The term also applies to water filtration systems (where it describes a cleaning process) and to ocean waves (where it refers to water retreating down a beach). Here’s what each version of backwash actually involves.
Backwash From Drinking: What Goes Back In
When you drink from a bottle, cup, or can, a small amount of saliva transfers from your mouth back into the liquid. This happens because your lips create a seal around the container’s opening, and as you tilt the drink back or release suction, fluid mixed with saliva flows in reverse. It’s nearly impossible to avoid completely, though drinking through a straw or pouring liquid into your mouth without contact reduces it.
A study analyzing plastic tea bottles found that immediately after someone drank from them, the mouth of the bottle carried an average of about 18,000 bacterial cells per milliliter. The dominant bacteria were Streptococcus species, making up roughly 60% of what was detected, which tracks with the fact that Streptococcus is one of the most abundant groups in the human mouth. Researchers also found Neisseria, Haemophilus, Rothia, and several other common oral bacteria on the bottle openings and in the remaining liquid.
For most healthy people sharing a drink with another healthy person, this bacterial transfer is harmless. Your mouth already hosts hundreds of bacterial species, and swapping small amounts with someone else rarely causes illness on its own. The risk increases when the person who drank first has an active infection, particularly something transmitted through saliva like a cold, flu, or mononucleosis. Bacteria also multiply over time, so a half-finished bottle left at room temperature for hours will have a significantly higher bacterial load than one finished quickly.
Backwash in Water Filtration
In water treatment, backwash refers to reversing the flow of water through a filter to clean it. During normal operation, water passes downward through a bed of granular material like sand, trapping particles and contaminants as it goes. Over time, those trapped particles clog the filter and reduce flow. Backwashing sends water upward from the bottom of the filter tank with enough force to lift and toss the filter media, dislodging the captured debris and flushing it out through the top to a drain.
The process is surprisingly physical. The upward water flow needs to be strong enough to expand the filter bed by 15 to 30%, essentially suspending the grains so they tumble against each other and release trapped particles. According to EPA testing, typical backwash rates in U.S. water treatment run between 24 and 30 inches per minute for 5 to 10 minutes. The exact speed depends on the size and weight of the filter media. Heavier materials like garnet sand require less water velocity to start moving (around 3 to 3.5 gallons per minute per square foot), while lighter silica sand needs roughly 7.5 gallons per minute per square foot to reach the point where the grains begin to float.
Pool and Home Filter Backwashing
If you own a pool with a sand filter, backwashing is part of routine maintenance. The rule of thumb is to backwash when your filter’s pressure gauge reads 10 PSI above its normal running pressure. You might also notice cloudy water or weak flow from the return jets, both signs the filter is clogged and needs cleaning. The process takes just a few minutes: you turn off the pump, set the multiport valve to “backwash,” run the pump until the sight glass runs clear, then switch back to normal filtration.
Home water filtration systems that use granular media work the same way. Most units have a control valve that triggers a backwash cycle automatically at a preset time, typically recommended every 7 to 10 days depending on water quality and usage. The valve reverses the water direction, flushes the trapped contaminants to drain, then resettles the filter bed to eliminate channels where water might have been bypassing the media.
Membrane Systems Use a Different Approach
Reverse osmosis and ultrafiltration membranes can also be backwashed, but the process works differently than with sand or granular filters. A backward flush pushes clean filtered water through the membrane in reverse at roughly twice the normal operating pressure. This forces out particles absorbed into the membrane’s pores, something a simple forward rinse can’t accomplish. If backflushing alone doesn’t restore performance, chemical cleaning with solutions like chlorine bleach or acid may be needed, where the membrane soaks in the solution before being flushed again. Some newer systems also inject air into the water stream during cleaning, creating turbulence that scrubs the membrane surface more effectively with less water.
Backwash in Ocean Waves
In coastal science, backwash is the return flow of water down a beach after a wave breaks and rushes up the shore. The upward rush is called swash, and the gravity-driven return is the backwash. This cycle shapes beaches over time because both movements carry sediment.
How much sand the backwash pulls back toward the sea depends largely on the beach material. On coarse-grained beaches made of gravel or large sand particles, water from the swash quickly soaks into the ground before it can flow back. Less return water means less sediment carried seaward, which is why coarse beaches tend to be steep. Fine-grained beaches saturated with water can’t absorb much, so the backwash runs freely across the surface, dragging more sediment with it. This produces a gentler, flatter beach slope. The balance between swash depositing sediment and backwash removing it determines whether a beach builds up or erodes over time.