Trickling Filter Versus Activated Sludge. When to Select Each Process DON F. KINCANNON, Associate Professor JOSEPH H. SHERRARD, Assistant Professor School of Civil Engineering Oklahoma State University Stillwater, Oklahoma 74074 INTRODUCTION Secondary treatment of wastewaters containing biodegradable organic matter usually requires a choice between the trickling filter process or the activated sludge process. A rational procedure to follow to select either of these processes for a proposed wastewater treatment project appears to be needed because standard textbooks either ignore proposing a selection method or only list advantages and disadvantages of each process in vague terms. Typical historical comparisons between these two processes yield such concepts as trickling filters: a) require more land area, b) are more costly to construct, c) may cause fly nuisance and odor problems, and d) are less sensitive to shock loads. Activated sludge processes, however, are credited with: a) the ability to produce a higher quality effluent, b) a quicker response to control measures, c) being difficult to operate and therefore unstable and unreliable, and d) a production of sludge in excess to that found from trickling filters. Advantages and disadvantages, such as those listed, may or may not describe adequately either of the two processes because of the wide assortment of modifications found within a single process, i.e., high-rate versus low-rate activated sludge and high-rate versus low-rate trickling filters. At best, therefore, previous comparisons have given the design engineer a poor basis for comparing the two processes. The purpose of this paper will be an attempt to offer an alternate and more reasonable basis for comparison of these two biological processes. To accomplish this objective a comparison of process similarities and differences will be presented, a discussion of process modifications will be given, a comparison of modifications will be made, and a criteria for process selection will be established. PROCESS SIMILARITIES AND DIFFERENCES To establish a criteria for choice between the two treatment processes requires a knowledge of the manner in which wastewater is treated in each process and how the process can be operated to optimize desired results. Basically, both processes are aerobic in nature and removal of organic compounds from solution occurs as a result of microbial metabolic activities. Carbon compounds are both incorporated into cellular tissue and oxidized to metabolic end products.While the trickling filter process relies on microbial growth on media surfaces, the activated sludge process required the formation of flocculated microorganisms held in suspension in an aeration basin. Differences between these two processes result due to physical parameters. Besides the difference between microorganisms held in suspension or attached to media surface, oxygen is supplied by mechanical or diffused air aeration to the activated sludge process and is supplied by natural air convection currents to the trickling filter. An additional difference is the operation of recycling. While microorganisms are recycled back to the aeration basin in the activated sludge process, clarified effluent is returned to the trickling filters. A summary of differences and similarities are listed in Table I. Typical process flow sheets for each process are shown in Figure 1. As shown, the flow diagram for the trickling filter process is composed of towers of plastic media (as opposed to rocks) because of the growing trend toward use of plastic media. 69 TABLE I Basic Similarities and Differences Between the Trickling Filter and Activated Sludge Process Similarities Differences 1. Wastewater stabilized by conver­ 1. Sludge recycled to activated sion of organic matter to cellular sludge process while clarified material and end products of effluent recycled to trickling metabolism filters 2. Stabilization of wastewater 2. Microbial growth in suspension organics is an aerobic reaction in activated sludge process and adhered to surface in trickling filter 3. Level of treatment that can be 3. Oxygen supplied by mechanical achieved is similar or diffused air aeration to activa­ ted sludge process, but supplied by convection currents in trick­ ling filter 4. Operational problems ACTIVATED SLUDGE AIR HO RETURN SLUDGE WASTE SLUDGE TRICKLING FILTER EFFLUENT RECYCLE 3-STAGE TOWERS INFLUENT | . 1, x-—. fr 1 \r J L_ J FINAL\ EFFLUENT INTERMEDIATE " Figure 1— Treatment process flow CLARIFIERS •WASTE SLUDGE (OPTIONAL) diagrams. PROCESS MODIFICATIONS Within the past few years many modifications of both the trickling filter and activated sludge process have been advanced. Principally, these modifications can be classified as either high-rate or low-rate. High-rate processes require a higher mass loading rate of organic material per mass of organisms than do low-rate processes. For example, a high- rate activated sludge process may have a ratio between lbs BOD5 applied per lbs MLVSS under aeration per day (often called the food to microorganism ratio, F/ M) of 2 while a 70 low-rate (extended aeration) process may have a ratio of 0.1 (1). The terms high-rate and low-rate as applied to the trickling filter have, in the past, required a modified definition. In this case, the mass loading rate of organic material per 1000 cubic feet media has been used. An example of ratios obtained for high-rate and low-rate trickling filters are 100 and 10 lbs BOD5 applied per 1000 cubic feet of filter media. In principle, a relationship between the ratio used for the activated sludge process exists with that used for the trickling filter process. This occurs because of the mass of organisms adhered to the surface area of the trickling filter media. Hence, if the mass of organisms adhered to the surface per unit volume of media were known, the two ratios could be related. In the following development an attempt to relate these ratios is made. Development of a food to microorganism ratio for a trickling filter may be made by drawing on information reported by previous researchers and by making reasonable engineering judgements. To obtain the weight of microorganisms contained in 1000 cubic feet of filter media requires a knowledge of the surface area per cubic foot of filter media, the active film thickness of the microorganisms attached to the surface, and the dry weight of microorganisms per unit volume. Kornegay and Andrews (4) have reported that the active film thickness on a trickling filter is 70jU and that the dry weight of microorganisms per unit volume is 95 mg/cc. Using a surface area of 27 sq. ft. per cubic foot of media (3) yields a value of 36.5 lbs dry weight of biological solids per 1000 cu. ft. of media. To obtain F/ M values for high-rate and low-rate filters now requires only a knowledge of the lbs of BOD5 applied to 1000 cu. ft. of filter per day. Application of 100 and 10 lbs BOD5/day/1000 cu. ft. gives F/ M values of 2.75 and 0.275 for high and low-rate processes, respectively. Comparison of these values with values commonly reported for high-rate and low-rate activated sludge processes are remarkedly similar. Thus, it would appear that an equivalent basis for comparing the two processes exists. An additional method of comparison is found by using the concept of sludge age, 0C, as applied to activated sludge for use with trickling filters. This comparison would appear to be valid as the F/ M ratio has been shown to be related to 0C, i.e., a low F/ M is equivalent to a high 0C and vice versa, (5). Calculations of the "sludge age" on high and low-rate trickling filters give 0C values of 2.3 and 19.2 days, respectively. These values also are nearly similar to those reported for high and low-rate activated sludge processes. Therefore, it is felt that because the values of F/ M and 0C are similar between both types of processes, that F/ M and 0C can be used as meaningful parameters upon which a basis for process comparison can be built. COMPARISONS OF PROCESS MODIFICATIONS A comparison of operational characteristics that are found for high and low-rate trickling filter and activated sludge units are shown in Table II. As shown, both processes display like characteristics when compared on an equivalent basis. Hence, a discussion of process modifications involves only a comparison between high-rate and low-rate units. High-rate processes have been reported to: a) give larger amounts of sludge (5), b) remove larger amounts of inorganic nutrients (6), c) produce a non-nitrified effluent (1,2,3), d) operate at low 0C and high F/ M (5), and e) be more unstable to shock loading conditions (7,8,9). Low-rate systems have been found to operate, however, in a manner almost exactly opposite to high-rate systems. Characteristics of low-rate systems are: a) low sludge production (5), b) a nitrified effluent (1,2,3), c) a lower than normal removal of inorganic nutrients (6), d) operation at a low F/ M and high 0C(5), and e) stability when shock loaded (7,8,9). Based on the above analysis relating high and low-rate activated sludge units to high and low-rate trickling filters, respectively, and a knowledge of the different response to be tound by employing either type of process modification, a reasonable criteria for process selection can be established. 71 TABLE II Comparison Between Processes High - Rate Low - Rate Activated Sludge Trickling Filter Activated Sludge Trickling Filter High Sludge Production High Sludge Production Low Sludge Production Low Sludge Production Effluent not nitrified Effluent not nitrified Nitrified effluent nitrified effluent Higher than normal Higher than normal Lower than normal removal Lower than normal removal removal efficiencies of removal efficiencies of efficiencies of inorganic efficiencies of inorganic inorganic nutrients inorganic nutrients nutrients nutrients Low»c, High F/M Low 0C, High F/M Low 0C, low F/M High 0C, low F/M Highly unstable when Unstable when shock Stable when shock Stable when shock shock loaded loaded loaded loaded CRITERIA FOR PROCESS SELECTION Before a particular process is selected, the desired treatment objectives should be determined.