In systems where severe dust or suspended solids loading pose a serious problem, side stream filtration may be necessary to reduce the severity of fouling or underdeposit corrosion caused by the fouling. For the most part, polymeric antifoulants do not interfere with the filter operation because the particle size maintained in suspension by the antifoulant is usually smaller than that removed by the filter.

Several types of media are used in side stream filtration, but sand is the most common. For greater efficiency, anthracite or mixed media can be substituted. The filter or filters normally filter between 1 to 5% of the circulating water directly off the circulating pump and discharge it into the tower sump. Consequently, this reduces the amount of circulating water available for cooling so there must be an excess circulation rate or the installation of filters will reduce cooling capacity and limit production.

If suspended solids are in the range of 10 to 30 ppm, 50-75% removal can be achieved, and in highly turbid waters, 90% removal is possible . In general, a side stream filter allows cooling water turbidity to approach the turbidity of the filter effluent. With oil contamination, side stream filters are impractical because of rapid fouling of the filter medium.

For example, if a given water has 100 ppm of suspended sol ids and you wish to reduce that by 70% to 30 ppm, the required filter rate in gpm would be:

F = (100/30 -1) (B.D.)

F =(2.33)x(B.D.)