Dual-media filter

A filter with a coarse upper layer over the sand layer — a dual-media filter — was introduced to overcome this shortcoming. Capped" filters — that is, filters with 4 to 6 in. of coal over sand — have been used for many years, operating at 2 to 3 gal/min per square foot of filter area. In the 1940 s, Conley and Pittman at the Atomic Energy Works in Hanford, Wash., refined the use of dual-media filters for opera-... 

Fig. 2 illustrates a typical dual-media filter. Essentially, this is a two-layer filter with a roughing layer oa top of the sand. Because the coal has a lighter density, it remains on top of the sand after backwash. Dual-media filters are operated in the range of 3 to 5 gal/min per square foot of filter area with applied suspended solids of 3 to 10 ppm and with steady flow-rate operation. 

Removal of the fine sand has reduced the stopping power or breakthrough efficiency. Filter efficiency is also reduced. Filter efficiency has been directly related to the media particle surface area. The number of particles in a given volume of a dual-media filter has been reduced. The average size of the particles has also increased. As the filtered material is relatively loosely held in the coal layer, the materials are dislodged and will likely penetrate the sand layer and appear in the filter effluent if a flow change or applied solids surge occurs. 

The susceptibility of the dual-media filter to breakthrough, particularly under changing flow conditions, indicated the need for further development in the design of filters. 

The advantages of UF pretreatment in comparison with traditional ones, has been also examined for an Eastern Mediterranean feed water [7]. In this study it has been shown that the UF/MF can be cheaper than conventional pretreatment (dual-media filters followed by cartridges) by 0.7 cents/m3 for total water cost [7]. 

FIGURE 7.1 (a) Dual-media filter (b) triple-media filter. 

The results obtained for 18 PNAs found in the refinery dual-media filter and final filter are presented in Table II. Only traces of PNAs were found in the final filters from the refinery. These data illustrate that the refinery-activated sludge unit removed 95% of the 5-ring PNAs and 99% of the 4-ring PNAs. 

Figure 5. UV spectrum of BaA peak in dual-media filter effluent...

The largest seawater desalination plant in the World operates in Jeddah, Saudi Arabia. It has a capacity of 56,800,000 liters per day, and the TDS content of the seawater is approximately 44,000ppm. Pretreatment modules for this plant include chlorine treatment, a dual media filter, and a cartridge filter for the coagulation and filtration of dead cells of microorganisms (Figure 8). 

Mechanical filtration is first performed in dual media components featuring a first layer of anthracite to capture the final oil traces, followed by a bed of quartz sand of proper granulometry to prevent silica entrainment by water. Dual media filters can retain particles larger than few microns and are followed by cartridge filters. They represent the last filtration stage before the membrane and are suited to remove fine particles having size larger than about 1 pm. 

Brody MA, Lumkins RJ. Performance of dual-media filters—1. Chem Eng Prog 73(4), 83-86, 1977. 

In water filtration, the dual media filters are usually designed using coarse anthracite coal on the top of fine silica sand. The coarse anthracite layer serves to prevent the formation of surface deposits on the sand bed, resulting in the formation of a compressible cake along the bed depth. Thus the depth removal of particulates throughout the bed would be the key feature. 

Raw seawater is taken from the Red Sea and then disinfected by sodium hypochlorite produced by a chlorine generation plant using filtered seawater as feed. Then ferric chloride as coagulant is added to the seawater feed ahead of dual-media filter (DMF) to help in reducing SDI values to suit the recommended SDI required by the membrane manufacmrer. The filtered feed water is then collected in a clear well, and particles greater than 10 p,m will not pass a cartridge filter in order to avoid membrane plugging. Sulfuric acid is added to the feed water to adjust pH to about 6.5. Sodium bisulfite (SBS) is injected for 7 h every... 

In 2000, rehabilitation work was conducted to obtain improved performance of the pretreatment by means of ultrafiltration (UF) membranes that proved most effective among the tests conducted. The work included modification of existing dual-media filter to singlemedia filter and additional installation of spiral-wound-type UF having a capacity of... 

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