Design of Physical Facilities Factors that must be considered in the design of trickling filters include (1) type and physical characteristics of filter packing to be used;(2) dosing rate;(3) type and dosing characteristics of the distribution system;(4)configuration of the underdrain system; (5) provision for adequate airflow (ie ventilation), either natural or forced air; and(6)sealing tank design Filter Packing. The ideal Tab.8-2 physical properties of trickling filter packing materials filter packing is a material that has a high surface area per unit of volume. is low in cost. has a high durability, and has a high Packing material enough porosity so that 800-1000 50 CNN clogging is minimized and good tich 6595 CN air circulation can occur Typical trickling filter packing slie random pocking 3060 materials are shown on Fig 8-3 Plastic random pocking hig 50-B0 N The physical characteristics of commonly used filter packings, including those shown on Fig. 8-3, are reported in Table 8-2. Until the mid-1960s, the material used was either high-quality granite or blast-furnace slag. Since the 1960s, plastic packing material, either cross-flow or vertical-flow, has become the packing of choice in the United States of low cost. the most suitable material is rounded fiver rock or a uniform size so that 95 e ithin th ange of 75 to 100 mm. uniformity is a way of ensuring adequate pore flow and air circulation. Other Important characteristics of filter typical packing material for tricking filters: (al rock, ( b) and (c) plastic verticol-Row, Id) plastic cross Rlow. (e) redwood stre horinontal, and ( f) random pock. / Figs, fe) and (d) fom American Surfpoc Corp, (e) from Neptune Microfloc, ond ff) from Products, Ine. Note: the random pock material is often used in air stripping towers Durability may be determined by odium sulfate test, which is used to test the soundness of concrete aggregates. Because of the weight of the packing, the depth of rock filters is usually on the order of 2 m. The low void volume of rock limits the space available for airflow and increases the potential for plugging and flow short circuiting Because of plugging, the organic loadings to rock filters are more commonly in the range of 0.3 to 1.0 kg BOD/m.d Various forms of plastic packings are shown on Fig 8-3. Molded plastic packing materials have the appearance of a honeycomb. Flat and corrugated sheets of polyvinyl chloride are bonded together in rectangular modules. The sheets usually have a corrugated surface for enhancing slime growth and retention time. Each layer of modules is turned at right angles to the previous layer to further improve wastewater distribution. The two basic types of corrugated plastic sheet packing are vertical and cross flow(see Fig. 8-3b, c, and d). Both types of packing are reported to be effective in BOD and TSS removal over a wide range of loadings. Biotowers as deep as 12 m have been constructed using plastic packing with depths in the range of 6 m being more common In biotowers with vertical plastic packing, cross-flow packing can be used for the uppermost layers to enhance the distribution across the top of the filter. The high hydraulic capacity, high void ratio, and resistance to plugging offered by these types of packing can best be used in a high-rate-type filter. Redwood or other wood packings have been used in the past, but with the limited availability of redwood, wood packing is seldom used currently. Plastic packing has the advantage of requiring less land area for the filter structure than rock due to the ability to use higher loading rates and taller trickling filters. Grady et al. (1999)noted that when loaded8-6 Design of Physical Facilities Factors that must be considered in the design of trickling filters include (1) type and physical characteristics of filter packing to be used; (2) dosing rate; (3) type and dosing characteristics of the distribution system; (4) configuration of the underdrain system; (5) provision for adequate airflow (i.e., ventilation), either natural or forced air; and (6) sealing tank design. Filter Packing. The ideal filter packing is a material that has a high surface area per unit of volume, is low in cost, has a high durability, and has a high enough porosity so that clogging is minimized and good air circulation can occur. Typical trickling filter packing materials are shown on Fig. 8-3. The physical characteristics of commonly used filter packings, including those shown on Fig. 8-3, are reported in Table 8-2. Until the mid-1960s, the material used was either high-quality granite or blast-furnace slag. Since the 1960s, plastic packing material, either cross-flow or vertical- flow, has become the packing of choice in the United States. Where locally available, rock has the advantage of low cost. The most suitable material is rounded fiver rock or crashed stone, graded to a uniform size so that 95 percent is within the range of 75 to 100 mm. The specification of size uniformity is a way of ensuring adequate pore space for wastewater flow and air circulation. Other important characteristics of filter packing materials are strength and durability. Durability may be determined by the sodium sulfate test, which is used to test the soundness of concrete aggregates. Because of the weight of the packing, the depth of rock filters is usually on the order of 2 m. The low void volume of rock limits the space available for airflow and increases the potential for plugging and flow short circuiting. Because of plugging, the organic loadings to rock filters are more commonly in the range of 0.3 to 1.0 kg BOD/m3·d. Various forms of plastic packings are shown on Fig. 8-3. Molded plastic packing materials have the appearance of a honeycomb. Flat and corrugated sheets of polyvinyl chloride are bonded together in rectangular modules. The sheets usually have a corrugated surface for enhancing slime growth and retention time. Each layer of modules is turned at right angles to the previous layer to further improve wastewater distribution. The two basic types of corrugated plastic sheet packing are vertical and cross flow (see Fig. 8-3b, c, and d). Both types of packing are reported to be effective in BOD and TSS removal over a wide range of loadings. Biotowers as deep as 12 m have been constructed using plastic packing, with depths in the range of 6 m being more common. In biotowers with vertical plastic packing, cross-flow packing can be used for the uppermost layers to enhance the distribution across the top of the filter. The high hydraulic capacity, high void ratio, and resistance to plugging offered by these types of packing can best be used in a high-rate-type filter. Redwood or other wood packings have been used in the past, but with the limited availability of redwood, wood packing is seldom used currently. Plastic packing has the advantage of requiring less land area for the filter structure than rock due to the ability to use higher loading rates and taller trickling filters. Grady et al. (1999) noted that when loaded at Fig. 8-3 Tab. 8-2