Figure 8-6. - Pressure-type rapid sand filter.

For iron oxide removal, treat the elements with a 0.5-percent solution of oxalic acid. Information is available from manufacturers on the amount of oxalic acid to use on different size units. The procedures used to remove iron oxide are as follows:

1. Start with an empty filter after a regular washing.

2. Close the drain valve and the main outlet valve; open the recirculation valve.

3. Fill the tank to a level covering the top of the elements.

4. Add the proper quantity of oxalic acid, and recirculate for 1 hour.

5. Drain and hose down the elements and the tank interior.

6. Close the drain valve; refill, circulate a few minutes, and then drain again. When the cleaning is not completely effective, repeat the procedure.

The procedure for manganese dioxide removal is the same as for the removal of iron oxide, except that anhydrous sodium bisulfite must be added to the solution. (See the manufacturer's instructions for the correct amount.)

To remove algae growths, add a 12 percent hypochlorite solution to the tank volume after filling the tank to the proper level. (See the manufacturer's instructions for the proper amounts to use for different size units.)

Semiannually, check the piping and valves and other equipment, including the body feed equipment. Make whatever adjustments the manufacturer's instruction says.

Clean and paint all exterior surfaces annually if necessary.