Multimedia filters

Deep Bed Filters. Deep bed filtration is fundamentally different from cake filtration both in principle and appHcation. The filter medium (Fig. 4) is a deep bed with pore size much greater than the particles it is meant to remove. No cake should form on the face of the medium. Particles penetrate into the medium where they separate due to gravity settling, diffusion, and inertial forces attachment to the medium is due to molecular and electrostatic forces. Sand is the most common medium and multimedia filters also use garnet and anthracite. The filtration process is cycHc, ie, when the bed is full of sohds and the pressure drop across the bed is excessive, the flow is intermpted and solids are backwashed from the bed, sometimes aided by air scouring or wash jets. 

The BAT level of treatment consists of all components of BPT except segregation and recirculation of quench wastewater. The combined wastewater after sedimentation is treated in multimedia filters and then discharged. 

Quench waste recirculation requires installation of a cooling tower to lower the temperature of the quench wastewater stream. The multimedia filter system for the final polishing of effluent includes a backwash mechanism, pumps, control media, and the filter structure. 

The filtration system consists of a granular bed multimedia filter unit, a GAC filter unit, and/or a membrane filtration unit.10 12... 

Multimedia filters, which consist of a top layer of coarse and low density anthracite, layers of silica, and then dense finest medium vitreous silicate, remove about 98% of particulates >20 tm. These filters are regularly back-washed to avoid buildup of particulates. Finer filters (S-lO tm) are used to remove suspended matter and colloidal materials. To prevent scaling due to water hardness, sodium ions generated from brine are exchanged with calcium and magnesium ions in the water. Activated carbon or metabisulfite is used to remove chlorine. 

Paticlc sizes of 7-10 p can be removed without a filter aid Dual flow filter. A dual flow filter is a modi tied veision ot a multimedia filter. The media are arranged by particle size and specific gravitv for simultaneous up and downtlovv nitration Filtered water leaves through an outlet in the middle of the bed. Coarse-to-fine filtration occurs in both directions. 

The rating of this filter is 20-40 gpnv sq rt essentially twice mat ot tire downflow multimedia filter because of the dual flow. Backwash water enters at the bottom and expands the bed in the same fashion as the graded sand bed and multimedia filters In some cases, raw water may be used, it would be prudent to follow backwashing with a short rinse before returning to service. 

Conley, W. R. and Hsiung, Kou-ying Design and Application of Multimedia Filters, J. Am. Water Works Assn. (Feb. 1969) 61. 

Triple media (multimedia) filters use anthracite, sand, and garnet, with the garnet providing the final and finest filtering quality, such that water with a turbidity less than 5 nephelometric turbidity units (NTU) and 20 p maximum particle size can be expected. Media specific gravity, grain size, and bed depth are given in Table 3.1. 

Figure 3.7 shows SDI filter pads taken before and after a multimedia filter treating RO influent water. The pads in this figure illustrate 2 important issues. First, the filter pads provide a visual confirmation about the efficacy of the multimedia filter to reduce the concentration of suspended solids in the RO influent water. Second, the filters pads can be analyzed to determine the nature of the deposit on them. Visually, the following colors are indicative of specific potential foulants ... 

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 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... 

Table 8.2 shows a typical cooling water particle size distribution. As seen in the table, nearly 98% of all particles are smaller than 0.5 microns, and greater than 99% of all particles are smaller than 5 microns. Recall that multimedia filters were only about 50% efficient at 10 -15 microns. By comparison, HEFs can remove 50% of particles a small as 0.25 microns in size. 

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. 

High-efficiency filters are gaining in acceptance for RO pretreatment.3 These filters offer some advantages over conventional multimedia filters, the most important of which may be the ability to remove particles down to 0.25 microns in size for the top-over-bottom filters, and 0.45 microns for the vortex filters. 

Channeling through multimedia filters leading to fouling of the membranes... 

The most common process used for this is depth filtration through a bed of sand or similar material charged in a vertical vessel. The incoming water flows from top to bottom. To improve the efficiency of such filters, two or more layers of media with various particle sizes are used. Coarse and less dense material such as anthracite is located at the top of the bed, whereas finer and denser particles of sand are placed at the bottom. Such multimedia filters can remove most particles larger than 10-20 pm. Periodically the filter bed is back-washed by reversing the flow direction (from bottom to top) and by increasing the flow rate. During backwash, the captured particles are removed and sent to drain, whereas heavier particles of the filter bed remain in the vessel and settle back at the end of the cycle. 

For some industries such as pharmaceuticals, electronics, and toiletries, ultra-pure water is always demanded. Pathogens, organic substances, and inorganic substances must be effectively removed to a very low level (e.g., less than 1 ppb TOC in semiconductor fabrication manufacturing). The source water is first filtered by multimedia filters and disinfected by UV light. The water is then treated by membrane units (usually reverse osmosis) and stored. Later on, UV photolysis, ion exchange resin and micro-filters are used alternatively to produce the high pure process water. 

Multimedia filters and canister filter media from the SCWO. The SCWO effluent is passed through multimedia Alters and canister Alters before entering the reverse osmosis (RO) unit. The Alter media will constitute secondary waste, but at present there... 

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