Figure 8-5. - Cutaway view of gravity sand filter with rotary surface wash.

2. Quarterly, during a backwashing period, 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. When clogged areas appear because of cementation of sand grains with mud balls or because of carbonate deposits or if the sand (or anthrafilt) grains have increased in size because of incrustation (e.g., in softening plants or where lime and ferrous sulfate are used for coagulation), clean the sand by treating the idle filter with an inhibited muriatic acid or sulfurous acid. The advice of the public works officer should be obtained if the operator is unfamiliar with the use of these chemicals.

Add the inhibited muriatic acid at the surface and allow it to pass downward through the bed and out the filter drain or "rewash" the line or add it to an empty filter through a small tap on the bed side of the ash-water line.

Use sulfurous acid as follows: Allow the sulfur dioxide gas from a cylinder to discharge into the filter wash waterline while slowly filling the filter bed with wash water. Use one 150-pound cylinder to 6,000 gallons of water to produce a 0.3-percent solution. Allow it to stand for 6 hours.

Semiannually, ascertain any change in the rate of wash water rise, as determined during operating procedures, and check sand expansion. Inspect the sand and, if you do not see the condition of the medium, locate the elevation of the top of the bed to determine if the bed has "grown" in depth. Also, remove a sand sample and analyze it as follows:

Make a sampling tube 12 inches square by 36 inches deep. Force a tube to the gravel level and