Multimedia pressure filters

Multimedia pressure filters are designed to reduce turbidity and colloids (measured as SDI) in water. These filters can remove particles down to about 10 microns in size. If a coagulant is added to the filter influent stream, reduction of particles down to 1-2 microns can sometimes be accomplished. Typical removal efficiency for multimedia pressure filters is about 50% of particles in the 10-15 micron size range. Influent turbidity for RO pretreatment is limited to about 10 NTU. At turbidity greater than 10 NTU, these filters may backwash too frequently to provide consistent effluent quality at reasonable run lengths. 

Service flow rates for RO pretreatment should be about 5 gpm/ ft2 of media. Throughput can be estimated using a filter about 0.45 lb of suspended solids per square foot of filter loading of area. Backwash rates should be 15 gpm/ft2 at 60°F. Lower water temperatures require higher flow rates to adequately raise the bed for a complete backwash. A 30 - 50% bed expansion is necessary to achieve good backwashing of the media. Raw, unfiltered water can be used for backwash. Alternatively, a filtered product tank can be provided that also acts as a reservoir for backwash water. Some operators choose to recovery RO reject by using this water to backwash filters. However, for best results, use of RO reject water to backwash a multimedia filter is not recommended. In some applications where 

Multimedia pressure filters can be used as stand alone treatment, when the feed sources is relatively clean water, such as a potable municipal supply, or in series with clarification, for river and other surface waters. 

An air scour involves air that is bubbled up through the bed to help release some stubborn solids from the media. 

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Figure 8.7 Multimedia pressure filter showing coarse, medium and fine media, typically anthracite, sand, and garnet, respectively.

Multimedia pressure filters can be vertically or horizontally oriented. Figure 8.7 shows a vertical filter while Figure 8.8 shows a horizontal multimedia pressure filter. Horizontal multimedia filters are separated internally into "tanks" or compartments. Each tank acts as an individual filter. When it is time to backwash one of the tanks, the effluent from the other tanks provides the backwash water. The key in selecting horizontal filters is that the filter should have enough internal tanks so that productivity (required effluent flow rate plus the... 

Figure 8.8 Horizontal multimedia pressure filter with 4 internal tanks.

Table 8.3 compares performance parameters or multimedia pressure filters and HEFs. The higher throughput of HEFs reduces the footprint of the system required when compared to multimedia filters. Also, the lower backwash flow requirements for HEFs leads to less waste water to dispose of and smaller backwash components, on these filters. 

Multimedia pressure filters contain graduated layers of anthracite on top of sand on top of garnet. Figure 8.7 shows a cross section of a multimedia... 

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