SR0033424_ENG DESIGNED PLAN - Laserfiche WebLink

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11-2 REMOVAL MECHANISMS IN PACKED-BED FILTERS 711 waste products, and other debris retained in the filter from the previous dose. The material flushed from the filter tends to accumulate in the bottom of the recirculation tank. In highly loaded multipass filters, a solids trap should be included to remove the material flushed from the filter before the effluent is discharged. Failure of Single- and Multipass Filters I Both single- and m- ultipas"s filters can fail when the.hydraulic; solids, and/or organic 'application rates exceed certain limits specific'to"each type`bf filter. The typical failure mode is manifested by surface ponding.between liquid applications. In some cases, the applied liquid overflows the enclosure. Failure usually occurs as the result... of the accumulation of untreated organic and inorganic solid materials and oil and grease from the influent, and residual cell tissue that has not undergone endogenous respiration to such an extent that the pore space becomes clogged. Other causes of failure include clogged underdrains and the presence of too many fines in the filter medium. When the amount of dissolved and particulate organic material applied exceeds the amount of food needed to maintain the microorganisms in the slime.layer in'a low growth rate, an increased growth rate will occur. For the purpose of illustration, assume only soluble organic constituents are applied to the filter. Further, assume the microorganisms on the surface of the filter medium are capable of absorbing an excess amount of organic matter. During the time until the next dose, the organisms will process the stored organic matter,-converting some of it to cell tissue. If there is not sufficient time for the organisms to reduce their mass through endogenous respiration between liquid applications (doses), the mass of organisms and unpro- cessed organic matter in the slime layer will gradually increase with time. As the film thickness increases, the rate of oxygen transfer and, in turn, biological activity is decreased, ultimately leading to the failure of the filter. ` in a similar manner, if particulate organic matter and oil and grease are applied and trapped in the slime layer, the organisms in the slime layer must be able to solubilize and process the applied constituents before the next dose.If they are unable to do so, a gradual accumulation of solids will occur within the filter, ultimately leading to failure as evidenced by ponding: A solids balance for a filter is presented in Example 11-1 to illustrate that,if the filter is loaded lightly,it will take a very long h time to clog. EXAMPLE 11-1. SOLIDS BALANCE FOR AN INTERMUTENT SAND FILTER. Prepare a solids balance for an intermittent sand filter to assess how-long it would take to fill half of the void space in the upper 6 in (150 mm)of the filter bed, assuming that 60 percent of the material retained in the filter is accumulated in the upper portion of the filter. Assume the following conditions also apply.The values for the influent BOD and TSS are based on the use of an effluent filter as described in Chap. 5.- 1. :1. Filter depth = 0.6 m(2 ft) 2. Porosity for sand = 40% 3. Average hydraulic loading rate = 40L/m2•d (1.0 gal/ft'-•d) 4. influent BOD = 130 mg/L I i f