8 Attached Growth Biological Treatment Processes 8-1 Background Evolution of Attached growth Processes Attached growth processes can be grouped into three general classes: (1)nonsubmerged attached growth processes, (2)suspended growth processes with fixed-film packing, and (3)submerged attached growth aerobic processes Nonsubmerged Attached Growth Processes. Trickling filters with rock packing have been a common simple, and low-energy process used for secondary treatment since the early 1900s. A trickling filter is a nonsubmerged fixed-film biological reactor using rock or plastic packing over which wastewater is distributed continously. Treatment occurs as the liquid flows over the attached biofilm. The concept of a trickling filter grew from the use of contact filters in England in the late 1890s. Originally they were watertight basins filled with broken stones and were operated in a cyclic mode. The bed was filled with wastewater from the top, and the wastewater was allowed to contact the packing for a short time. The bed was then drained and allowed to rest before the cycle was repeated a typical cycle required 12 h(6 h for operation and 6h of resting). The limitations of the contact filter included a relatively high incidence of clogging, the long rest period required, headloss, and the relatively low loading that could be used Because of the clogging problems, larger packing was used until a rock size of 50 to 100 mm was reached In the 1950s, plastic packing began to replace rock in the United States. The use of plastic packing allowed the use of higher loading rates and taller filters(also known as biotowers) with less land area, improved process efficiency, and reduced clogging In the 1960s, practical designs were developed for otating biological contactors(RBCs), which provided an alternative attached growth process where the packing is rotated in the wastewater treatment tank, versus pumping and applying the wastewater over a static packing. Both trickling filters and RBCs have been used as aerobic attached growth processes for BUD removal only, combined BOD removal and nitrification, and for tertiary nitrification after secondary treatment by suspended growth or attached growth processes. The principal advantages claimed for these aerobic attached growth processes over the activated-sludge process are as follows ed Simpler operation with no issues of mixed liquor inventory control and sludge wasting No problems of bulking sludge in secondary clarifiers Better sludge thickening properties Less equipment maintenance needs Better recovery from shock toxic loads In comparison to the activated-sludge process, disadvantages encountered for trickling filters are a poorer effluent quality in terms of BOD and TSS concentrations, greater sensitivity to lower temperatures, odor production, and uncontrolled solids sloughing events. In general, the actual limitations of the processes(1 make it difficult to accomplish biological nitrogen and phosphorus removal compared to single-sludge biological nutrient removal suspended growth designs, and(2)result in an effluent with a higher turbidity than activated-sludge treatment. Trickling filters and RBCs have also been used in combined processes with activated sludge to utilize the benefits of both processes, in terms of energy savings and effluent alit Suspended Growth Processes with Fixed-Film Packing. The placement of packing material in the aeration tank of the activated-sludge process dates back to the 1940s with the Hays and Griffith proce Present-day designs use more engineered packings and include the use of packing materials that are spended in the aeration tank with the mixed liquor fixed packing material placed in portions of the aeration tank, as well as submerged RBCs. The advantages claimed for these activated-sludge process enhancements are as follows Increased treatment capacit Greater process stability Reduced sludge production Enhanced sludge settleability Reduced solids loadings on the secondary clarifier No increase in operation and maintenance costs Submerged Attached Growth Processes. Beginning in the 1970s and extending into the 1980s, a nev class of aerobic attached growth processes became established alternatives for biological wastewater treatment. These are upflow and downflow packed-bed reactors and fluidized-bed reactors that do not use8-1 8 Attached Growth Biological Treatment Processes 8-1 Background Evolution of Attached Growth Processes Attached growth processes can be grouped into three general classes: (1) nonsubmerged attached growth processes, (2) suspended growth processes with fixed-film packing, and (3) submerged attached growth aerobic processes. Nonsubmerged Attached Growth Processes. Trickling filters with rock packing have been a common, simple, and low-energy process used for secondary treatment since the early 1900s. A trickling filter is a nonsubmerged fixed-film biological reactor using rock or plastic packing over which wastewater is distributed continously. Treatment occurs as the liquid flows over the attached biofilm. The concept of a trickling filter grew from the use of contact filters in England in the late 1890s. Originally they were watertight basins filled with broken stones and were operated in a cyclic mode. The bed was filled with wastewater from the top, and the wastewater was allowed to contact the packing for a short time. The bed was then drained and allowed to rest before the cycle was repeated. A typical cycle required 12 h (6 h for operation and 6h of resting). The limitations of the contact filter included a relatively high incidence of clogging, the long rest period required, headloss, and the relatively low loading that could be used. Because of the clogging problems, larger packing was used until a rock size of 50 to 100 mm was reached. In the 1950s, plastic packing began to replace rock in the United States. The use of plastic packing allowed the use of higher loading rates and taller filters (also known as biotowers) with less land area, improved process efficiency, and reduced clogging. In the 1960s, practical designs were developed for rotating biological contactors (RBCs), which provided an alternative attached growth process where the packing is rotated in the wastewater treatment tank, versus pumping and applying the wastewater over a static packing. Both trickling filters and RBCs have been used as aerobic attached growth processes for BUD removal only, combined BOD removal and nitrification, and for tertiary nitrification after secondary treatment by suspended growth or attached growth processes. The principal advantages claimed for these aerobic attached growth processes over the activated-sludge process are as follows: . Less energy required . Simpler operation with no issues of mixed liquor inventory control and sludge wasting . No problems of bulking sludge in secondary clarifiers . Better sludge thickening properties . Less equipment maintenance needs . Better recovery from shock toxic loads In comparison to the activated-sludge process, disadvantages encountered for trickling filters are a poorer effluent quality in terms of BOD and TSS concentrations, greater sensitivity to lower temperatures, odor production, and uncontrolled solids sloughing events. In general, the actual limitations of the processes (1) make it difficult to accomplish biological nitrogen and phosphorus removal compared to single-sludge biological nutrient removal suspended growth designs, and (2) result in an effluent with a higher turbidity than activated-sludge treatment. Trickling filters and RBCs have also been used in combined processes with activated sludge to utilize the benefits of both processes, in terms of energy savings and effluent quality. Suspended Growth Processes with Fixed-Film Packing. The placement of packing material in the aeration tank of the activated-sludge process dates back to the 1940s with the Hays and Griffith processes. Present-day designs use more engineered packings and include the use of packing materials that are suspended in the aeration tank with the mixed liquor, fixed packing material placed in portions of the aeration tank, as well as submerged RBCs. The advantages claimed for these activated-sludge process enhancements are as follows: . Increased treatment capacity . Greater process stability . Reduced sludge production . Enhanced sludge settleability . Reduced solids loadings on the secondary clarifier . No increase in operation and maintenance costs Submerged Attached Growth Processes. Beginning in the 1970s and extending into the 1980s, a new class of aerobic attached growth processes became established alternatives for biological wastewater treatment. These are upflow and downflow packed-bed reactors and fluidized-bed reactors that do not use