Flocculation is required for filter efficiency

Alum Flocculation Using Aluminum Sulphate

The process of Flocculation is defined as "forming woolly cloudlike aggregations". Flocculate means:" To cause (soil) to form lumps or masses". Flocculation takes place in the sand filter and allows the filter to function at its most effective. Here's how it works.

Rapid Gravity and Pressure Sand Filters must be coated on the surface media with a gelatinous floc of aluminum hydroxide in order to filter out the very fine or colloidal soil which would otherwise block the interstices between the sand particles or else become trapped and form mud balls in the filter bed. Besides removing very finely suspended matter the floc will also remove algae, colouring matter and a portion of the bacteria in the water.

Alum (aluminum sulphate Al2(SO4)3 17H20) is the common coagulant used. As this material is acidic it requires a reserve alkalinity in the water for the reaction to form the aluminum hydroxide floc.

For every pound of alum added to the water it will require 1/2 lb. of soda ash or slightly more than 3/4 lb. of sodium bicarbonate to complete the reaction.

Therefore before alum is added, the water should have a pH between 7.5 - 8.00 and a reserve alkalinity between 150 - 200 ppm.

If the pH of the water is below 5.5, or above 8.5, the alum will not form a satisfactory floc and some if not all of it will remain in solution.

Flocculation after Backwashing

It is only necessary to add the alum after backwashing. The amount to use to obtain efficient filtering will depend on the design of the filters and experience. As a general guide - 2 to 4 oz of alum per sq. ft. of filter area should be added. It should be added as a solution, 10% or less over a period of 1 -2 hours by drip feed to the inlet side of the filters, preferably at the skimmer boxes.

If the alum is added too quickly the pH of the water may be depressed below 5.5 at which level it will not form a floc but will pass through the filters. However, when this water mixes with the pool water the pH will rise causing the alum to floc in the pool causing cloudiness.

In the normal operation of a pool, backwashing will be required on an average once per week. This depends on loading, size of filters and turnover rates. If the loss of head gauges do not indicate that backwashing is required after a week's operation, this may indicate that a weak spot has developed in the sand and that most of the water is passing through this area of least resistance and is therefore only partially filtered. This can usually be indicated by the condition or clarity of the water. It is good practice therefore to backwash at least once per week. In doing so, the high velocity of backwashing expands the sand bed and resettles the sand in a more uniform layer, thereby eliminating the tendency for weak spots or holes to develop in the filter bed.