Fine Solids Removal
Technical profile
The objective of this process is removal of particles from water that might clog or foul reverse osmosis (RO) membranes used to prepare seawater for potable water use. Multimedia filtration is common technology for this process. It is described under Secondary Solids Removal.
Multimedia filters remove colloidal and larger particles from water by passage through a porous filter media of sand, anthracite and fine garnet. Suspended solids from less than 1 micron to 50 microns and larger may be removed. Multimedia filters are generally designed with flux rates of 3 to 5 gallons per minute per square foot of surface area. In general, a higher filter rate produces a higher silt density index (SDI) in the output. This SDI is of great importance in predicting filterability through membrane systems. Experience indicates that a velocity of 8-9 m/h (25 to 30 ft/h) gives acceptable SDI values below 3.
Filter backwash facilities should be capable of expanding the filter bed by 50%. Rates of between 10 to 20 gallons per minute per square foot filter area are often used. Filter downtime during backwash typically averages 10 to 20 minutes including a wash period.
