Downflow sand filters

The conventional, and still the principal, type of filter used for cleaning bulk water is the sand bed, with backwashing carried out by a backfiow of water, or preferably water backwash combined with an air scour. The latter results in better fluidization of the bed and more effective cleansing. Normally, such sand filters are downflow, gravity types, but upflow filters are also used where higher flow rates are required, or where high turbidities make conventional downflow sand filters impractical. 

Conventional downflow sand filters are effective for liquid-solid separation at flow rates up to about 15m /h.m of filter area, although higher rate downflow filters are available. With proper selection of filter media, gelatinous as well as granular suspended matter can be filtered out, without a rapid differential pressure build-up. 

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. 

Unfortunately this is exactly the opposite of what is needed for a downflow depth filter, which should have the raw water meeting the coarsest particles first and the finest last. Nevertheless, the rapid sand filter was used in this manner for many years, because to have an upflow filter risked the expansion of the bed in the direction of the flow, and the consequent release of trapped solids into the filtrate. It was not until the development of the Immedium filter in the 1940s that upflow became possible, by virtue of an open grid of parallel bars just below the surface of the bed. 

In the operation of a downflow-type saturator, water is introduced at the top of the saturator tank (there is an air gap to avoid the possibility of a cross-connection) and the level is regulated with a float-operated controller valve on the water supply line. The water then trickles down through the bed of sodium fluoride solids the solution is clarified in the sand and gravel filter bed and ends up as a clear, saturated solution at the bottom of the tank where it is withdrawn by the feeder. The fluoride-water solution pump withdraws the solution and delivers it into the water system at a desired dosage rate. Operator attention is required only to ensure that an adequate quantity of sodium fluoride solids is kept in the saturator and that the saturator is kept in a reasonably clean condition. 

The filter medium is usually graded by size. Three or four layers of different screen cuts, with the finest material on top, are common. Sand is often placed above one or two layers of gravel or pebbles. The top layer is the deepest as well as the finest. An upflow filter then has a solids capacity of20-30kg m , compared with the 5-15 kg obtained in downflow. The brine flow rate must be kept below about 0.3 m min . The fluidization limit, which will depend on the size and density of the filter medium and the properties of the brine, is an absolute hmit which must not be exceeded in an attempt to overcome upsets or to compensate for filter downtime. Some form of assurance that excessive flow will not occur should be part of the design. 

Most deep bed filters are gravity fed but there are also some pressure types in use. The most common arrangement is the downflow filter, with backwash in the upward direction in order to fluidize the bed. The stratification of the particles that make up the bed, caused by the fluidization (fines on top), is not desirable, however. The solids holding capacity of the bed is best utihzed if the filtration flow encounters progressively finer sand particles. This is achieved in the upflow filters, where the fluidization due to backwash produces the correct stratification in the bed. Both the filtration flow and the backwash take place in the same direction and the disadvantage is that the wash goes to the clean side of the filter. 

Typical filter media for the downflow filter consist of selected silica sands, and coal or anthracite, which are tough inert solids, available in a range of particle sizes. One solution to the problem of matching the pore sizes in the bed is to use... 

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