Upflow sand filters

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. 

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. 

Stratification of the particles making up the bed, caused by the fluidization (fines on top), is not desirable. The soflds holding capacity of the bed is best utilized if the filtration flow encounters progressively finer sand particles. This is achieved in upflow filters where the fluidization due to backwash produces the correct stratification in the bed. Unfortunately, the filtration flow and the backwash take place in the same direction the disadvantage is that the washwater goes to the clean side of the filter. 

Water Treatment. The source of water for this experiment was a pilot plant located on the Seine River upstream from Paris, France (Figure 1). The pilot plant uses an upflow solids contact clarifier (Pulsator, Degremont, Rueil Malmaison, France) followed by rapid sand filtration (RSF). The filtered water is then distributed over four treatment lines to evaluate the efficiency of various ozone-GAC combinations (ozonation rates of 1 or 5 ppm O3 and 10-30 min of contact time). The GAC used in this study was Calgon F-400 (Calgon Corp.). Disinfection by chlorine or chlorine dioxide completed the process. In this chapter, line 3 treatment was not considered a complete treatment for the water supply. This line was studied to evaluate the efficiency of a high ozonation rate. 

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. 

However, in upflow filtration on a noncompact granular medium, the aim is to associate the cyclic filtration of SS retained at the bottom of the filter bed with the limited and deliberate migration of oil. This corresponds in fact to the enlargement and breakaway of the thick oil film which has fed on the coalescence of HC droplets on the sand walls. 

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