If severe algae growths exist on media or walls, remove the
filter from service and treat it with a strong hypochlorite solution. Add enough
hypochlorite to produce 2 to 4 mg/L of free residual chlorine in a volume of water 6
inches deep above the filter surface. Draw down the filter until the water level is just
above the bed surface. Allow the filter to stand 6 to 8 hours, then backwash the
surface. Follow this procedure with a complete backwashing. Repeat if necessary.
On a quarterly basis, probe the filter for hard spots and uneven
gravel. Examine the sand below the surface by digging to gravel with the water drawn
down to the gravel level. Clogs may appear because sand grains have cemented with
mud balls or because grains have increased in size due to calcium carbonate deposit
encrustation (for example, in softening plants or where lime and ferrous sulfate are
used for coagulation). If so, clean the sand by treating the idle filter with inhibited
muriatic acid (hydrochloric acid to which a chemical has been added to reduce
corrosion of metal) or sulfurous acid. It is good practice to notify the utility managers
before these chemicals are used.
Add the inhibited muriatic acid at the surface and allow it to
pass downward through the bed and out the filter drain or "rewash" line. Alternatively,
add it to an empty filter through a small tap on the bed side of the wash water supply
Use sulfurous acid as follows. Allow the sulfur dioxide gas
from a cylinder to discharge into the filter wash water supply line while slowly filling the
filter bed with wash water. Use one 150-pound cylinder with 6,000 gallons of water to
produce a 0.3 percent solution. Allow solution to stand for 6 hours.
Twice a year, usually when seasonal water temperature changes
occur, determine any change in the rate of wash water rinse and check sand expansion
The flow rate of backwash water should be sufficient for
cleaning the media but should not provide so much pressure that loss of media results.
In general, the backwash flow rate should be at least 15 gallons per minute per square
foot (10 liters per second per square meter [Lps/sq m]), which is equivalent to a rise
rate of 2 feet per minute (600 mm/min) as measured by a hook gage. Higher rates may
be required for some types of filter media, but rapid sand filters typically backwash at a
rise rate of about 2.0 to 2.5 feet per minute (600 to 750 mm/min). The highest rate for