Filtration Plant

Prefiltration involves the use of a filter designed to remove particulate matter (dirt, sediment, etc.) from water prior to further treatment. Prefiltration is important for two reasons. First, larger particulate matter has the potential to clog or prematurely exhaust the filter(s) which follow it  in the system. Second, the effectiveness of further treatments (e.g. chemical or mechanical) can be significantly reduced in the presence of particulate matter. 

A variety of prefilter types are available. The most important way in which these filter types differ is in their “fineness”, or the size of the smallest particle they are able to filter out. Water filter fineness is typically expressed in micron size. “Micron” refers to the filter pore diameter in micrometres (µm). 

Ultimately, the fineness of the prefilter required for a particular application will vary, and will depend upon the quality of the source water involved (which will help identify the size of particles present in the water that need to be removed). Multiple levels of filtration are often required – prefiltration before fine filtration, for example – as much larger particles must be filtered out before water reaches the filter intended for finer particles. Without prefiltration, these large particles may damage the finer filter(s). 

It is also important to choose the correct size of filter. The “size” of the filter refers to the maximum flow rate that it is able to accommodate. There are often multiple filter sizes available for each level of fineness. The water pressure of the system must also be taken into consideration. Each filter model will have a limit to the maximum pressure at which it can operate. Also, the filter’s minimum operating pressure should be noted to ensure this pressure will be maintained to optimize system performance.

Deciding whether to add another level of filtration is a cost/benefit decision. While any form of prefiltration requires extra cost, the operator must take into consideration the costs or losses that would be incurred without prefiltration. Prolonging the life of more expensive downstream fine filters, for example, may be well worth the investment.

Pressure filters are typically used with hot process softeners to permit high-temperature operation and to prevent heat loss. The use of pressure filters eliminates the need for repumping of filtered water. Pressure filters are similar to gravity filters in that they include filter media, supporting bed, underdrain system, and control device; however, the filter shell has no wash water troughs.

Pressure filters, designed vertically or horizon-tally, have cylindrical steel shells and dished heads. Vertical pressure filters (see Figure 6-2) range in diameter from 1 to 10 ft with capacities as great as 300 gpm at filtration rates of 3 gpm/ft². Horizontal pressure filters, usually 8 ft in diameter, are 10-25 ft long with capacities from 200 to 600 gpm. These filters are separated into compartments to allow individual backwashing. Backwash water may be returned to the clarifier or softener for recovery.

Pressure filters are usually operated at a service flow rate of 3 gpm/ft². Dual or multimedia filters are designed for 6-8 gpm/ft². At ambient temperature, the recommended filter backwash rate is 6-8 gpm/ft² for anthracite and 13-15 gpm/ft² for sand. Anthracite filters associated with hot process softeners require a backwash rate of 12-15 gpm/ft² because the water is less dense at elevated operating temperatures. Cold water should not be used to backwash a hot process filter. This would cause expansion and contraction of the system metallurgy, which would lead to metal fatigue. Also, the oxygen-laden cold water would accelerate corrosion.

UPFLOW FILTERS

Upflow units contain a single filter medium–usually graded sand. The finest sand is at the top of the bed with the coarsest sand below. Gravel is retained by grids in a fixed position at the bottom of the unit. The function of the gravel is to ensure proper water distribution during the service cycle. Another grid above the graded sand prevents fluidization of the media. Air injection during cleaning (not considered backwash because the direction of flow is the same as when in-service) assists in the removal of solids and the reclassification of the filter media. During operation, the larger, coarse solids are removed at the bottom of the bed, while smaller solids particles are allowed to penetrate further into the media