To backwash, the custodian
adjusts a number of valves to redirect the water flow. He or she
closes the return pipe leading to the pool and opens the drainage
pipe, which lead to the sewer system.
she adjusts a valve at the filter to connect the pipe from the pump
to the outlet pipe and connect the drainage pipe to the inlet pipe.
With this arrangement, water from the pump pushes up through the
sand, dislodging the dirt and debris. At the top of the filter tank,
the dirty water flows out through the inlet pipe and into the sewer.
To redirect the water flow for backwash, the custodian turns large
handles to adjust plumbing valves.
In place of a sand filter, some pool systems use a diatomaceous
earth filter or a cartridge filter. In a diatomaceous earth filter,
water from the pool passes through filter grids coated with
diatomaceous earth, a fine powder made from the chemically inert,
fossilized remains of sea organisms called diatoms. In a cartridge
filter, dirty water passes through a filter made out of polyester
cloth or corrugated paper.
Instead of backwashing, you simply remove
the filter and hose it off. After a few years (or as many as eight
years), it's time to discard the old filter and put in a new one.
In most regions, the law dictates that all the water in the pool (or
more accurately, the equivalent volume) must pass through the filter
in a certain amount of time -- typically between 30 minutes and six
hours. For the apartment-complex pool pictured above, that means
pumping 167,000 gallons (630,000 liters) of water through the
filtering system every six hours!
The pump and filter system is also connected to a well or municipal
water line so fresh water can be added to the pool.
necessary to replace water lost to evaporation, backwashing and
"splash-out" (water that splashes on the deck or is carried out on
people's bodies and swim suits). When it's pretty hot out and
there's heavy swimmer activity, this 167,000-gallon pool could lose
300 gallons (1,100 liters) or more in one day.