DynaSand filter

Several advanced filtration systems are finding applications in treating refinery wastewaters. Examples include the HydroClear filter (Zimpro, Rothschild, WI) and the Dynasand filter (Parkson Corporation, Fort Lauderdale, FL). The HydroClear filter employs a single sand medium (0.35-0.45 mm) with an air mix (pulsation) for solids suspension and regeneration of the filter surface. Filter operation enables periodic regeneration of the medium... 

The sand bed inside the DynaSand filter continually moves downwards, through the dirty sand hopper, rmtil it readies the bottom of an air lift pun, see Figure 6.8. Some previously deposited solids on the sand grains are removed in the turbulent air hft punq>, additional sand cleaning takes place in the washbox, where some filtered water is admitted to wash the sand in coimter-current flow. The dean sand r utns to the top of the bed. The washwater and fine solids flow over a weir into the washwater outlet. Process water is fed into the filter through the inlet distributor, and flows counter-currently to the sand bed. Cleaned water emerges fiom the bed, flows over an outlet wdr and leaves via the filtrate outlet. 

All of the types of sand filter so far discnssed are discontinuous in their operation, even if the cycle is quite long in the case of the slow gravity filter. The most recent development of the rapid sand filter has been to make it continuously operating, by having the bed of sand or other materials move downwards through the filter (as in the DynaSand filter). The dirty solids normally fall into a conical hopper at the base of the filter, whence they are carried by a jet of air to a wash zone above the filter (as seen in Figure 3.87). Here they are washed clean of trapped solids and then returned to the top of the bed in the filter. There can now be no stratification of the solids by size or density, but the flow of water, counter-current to the movement of the solids, ensures good solids removal. 

Although continuous operation can be achieved using conventional sand filters in parallel, truly continuous depth filters exist where filtration proceeds with virtually no head loss. These units, which are typified by the Tenten inclined bed and Dynasand devices, operate in the normal manner except a portion of the filter media is intermittently removed (see Figure 1.44). This portion is washed, and then automatically returned to the top of the filter bed in a cleaned condition. While being more expensive than conventional sand filters, continuous depth filters find use when frequent backwashing would otherwise be required. 

FIGURE 3.8 DynaSand continuous-flow granular-media filter. 

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