Fine Solids Removal
Technical Profile
The objective of this process is removal of fine particles from water that may affect downstream processes such as steam generation or membrane separation. Fine solids filtration is used to remove solids present from lime softening or other sources prior to processing on ion exchange equipment to remove remaining hardness. Filtration of water removes colloidal and larger particles from water by passage through a porous filter usually sand or granular coal. Suspended solids from less than 1 micron to 50 microns and larger may be removed.
Contaminants are captured on the filtration media in downflow operation. Contaminants are removed using backwash and in some instances gas scouring. Filters may contain sand or a combination of filter media. Sand, the most common filter media, should be clean silica sand with a specific gravity not less than 2.5. Its effective size should fall in the range of 0.35 to 0.5 millimeters. Anthracite is an alternative media of hard anthracite coal particles. When used alone its effective size should fall in the range of 0.45 to 0.6 millimeters.
Often multimedia filters are used for this application. Multimedia filters generally utilize separate layers of anthracite, sand and garnet to progressively clean water. Anthracite helps boost oil removal properties of sand alone. Media size distribution is as follows:
- Anthracite 0.8 to 2.0 millimeters
- Sand 0.4 to 1.0 millimeters
- Garnet 0.2 to 0.4 millimeters
Multimedia filters are generally designed with flux rates of 3 to 5 gallons per minute per square foot of surface area. The three filter media on top of each other provide the possibility to filter a larger distribution of solid sizes while minimizing cake filtration. The filters need to be backwashed when they are loaded with a typical amount of solids. Despite the attempt to minimize cake filtration, backwash is generally based on a solids load per unit of area between 2 to 3.7 kg/m2 (0.5 to 0.75 lbs./ft2). A fixed number is hard to give as it depends on susceptibility to clog the filter, which may be higher when iron like particles are suspected.
The rate at which the filter is operated is always a hot topic. 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 when Reverse Osmosis is present downstream. Experience indicates that a velocity of 8-9 m/h (0.44 to 0.49 ft/s) gives acceptable values. When no RO is present downstream 15 m/h (0.82 ft/s) is a conservative value. Higher rates may be possible, but small particle removal becomes significantly more difficult as rate increases.
When a coagulant such as FeCl3 is dosed at proper rate turbidity in multimedia filter effluent may be reduced by a factor ten with respect to influent and SDI<3 should be possible to obtain. Addition of an anionic polymer will enhance the removal. Cationic polymers should be avoided if ion exchange softening is used downstream of the filters.
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.
