Granular media filters

The filter application is typically applied to handling streams containing less than 100 to 200 mg/Liter suspended solids, depending on the required effluent level. Increased-suspended solids loading reduces the need for frequent backwashing. The suspended solids concentration of the filtered liquid depends on the particle size distribution, but typically, granular media filters are capable of producing a... 

Both media filters and cartridge filters can be used in a pretreatment process. Granular media filters involve the filtration of large particles through different layers of fine particles, usually coal, pumice, sand or garnet (Bonnelye et al. 2004). Cartridge filters act as the final filtration step before the water passes through the membranes, and remove fine particles as small as 1 pm. 

Similar to filter backwash, the concentrate from these membranes requires treatment before it can be disposed of with the membrane concentrate. However, the total amount of solids produced after the treatment of filter backwash can be 60-80% greater than MF and UF concentrate due to the addition of coagulants prior to the granular media filters (Bergman 2007). 

PSD measurements were also used in the selection of design criteria for granular-media filters. One design problem evaluated was the duration and intensity of flocculation required prior to filtration. Results of... 

As seen in Figure 7, the anticipated shift in the shape of the influent size distribution curve occurs because of flocculation. However, the effluent particle size distributions of both filters are not statistically different. Influent and effluent turbidity measurements for both filters were also identical, thus indicating that flocculation prior to filtration could be eliminated. These results also suggest that the efflciency of particulate removal by granular-media filters with identical physical design variables (media type, size and depth, and flow rate) may depend primarily on particulate destabilization (17), with particle size being less important in determining removal efflciency than previously considered (33). 

Activated carbon, in powdered (PAC) or granular (GAC) form, has many applications in drinking water treatment. It can be used for removing taste and odor (T O) compoimds, synthetic organic chemicals (SOCs), and dissolved natural ot] nic matter (DOM) from water. PAC typically has a diameter less than 0.15 mm, and can be applied at various locations in a treatment system (Fig. 1). GAC, with diameters ranging from 0.5 to 2.5 mm, is employed in fixed-bed adsorbers such as granular media filters or post filters. Despite difference in particle size, the adsorption properties of PAC and GAC are fundamentally the same because the characteristics of activated carbon (pore size distribution, internal surface area and smface chemistry) controlling the equilibrium aspects of adsorption are independent of particle size. However, particle size impacts adsorption kinetics. 

Figure 2.4 A granular media filter with multimedia layers.

Sometimes coagulants are injected in the feed water hne upstream of granular media filters to remove suspended matter of colloidal nature when the particulate matter is either too small or electrostatically repeUed from the media. Polymeric coagulants are added in low dosages (<10 mg/1) to remove particles down to 0.5 pm particle size as compared to 10 pm without the coagulant. Anionic coagulants have been shown to reduce the turbidity of raw water to less than 0.5 NTU at a nominal dosage of 1—2 mg/1. 

Table 4.2 Granular media filter valve operation Process cycle Open valves ...

Flow rate Granular media filters AP (bar) Turbidity (NTU)... 

Slow sand filters are operated at very low filtration rates without the use of coagulation. Slow sand filters are a simple, reliable and easy to operate system. The filtration rate for slow sand filters is typically 50-100 times slower than that of granular media filters. Therefore, a much larger area is needed for the filter bed to produce an equivalent amount of water. Contaminants are removed... 

Granular media filters are usually preceeded by sedimentation in order to reduce the suspended solids load on the filter. Granular media filtration can also be installed ahead of biological or activated carbon treatment units to reduce the suspended solids load and in the case of activated carbon to minimize plugging of the carbon columns. 

Granular-media filters are used in water treatment to remove relatively low concentrations of suspended matter. Typically the feed contains around 50 mg/L suspended sohds, or less, and 90... 

FIGURE 3.8 DynaSand continuous-flow granular-media filter. 

Gravity-fed or pressurized granular media filter systems are less energy-intensive than the three systems mentioned above but require highly qualified operators with sufficient experience in backflushing procedures. These systems are not applicable to the removal of dissolved chemical species. 

The granular media filters are backwashed using filtered water or, if acceptable, concentrate from the SWRO membrane system. The backwash rate is typically designed to provide 30-50% media bed expansion. The number of individual filter units is usually selected such that the filtration system can operate at fiiU production capacity with one filter out of service in backwash and one out of service for maintenance. 

The two types of granular media filters most widely used for pretieatment of SWRO plants are pressure and gravity filters. The major differenees between pressure and... 

The terms "granular media filter" and "sand filter" refer to a number of filter designs in which fluid passes through a bed of granular medium. Typically, these filters consist of a pressure vessel filled with the filter media, as shown in Figure 4.10. Media support screens prevent the media solids from leaving the filter vessel. 

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