Versus Activated . 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 of containing biodegradable 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 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, 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 •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 () 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. Factors such as: a) concentration of effluent BOD5, b) a nitrified or non- nitrified effluent, c) sludge disposal problems, d) inorganic nutrient removal requirements, and e) reaction to shock loads, i.e., process reliability, are examples of items to be considered. It has been shown in Table II that high and low-rate processes respond in a different manner with respect to these items. Therefore, the first step in determining whether to select an activated sludge or trickling filter process is to determine that process modification which will meet the treatment objectives. A realization that it may not be possible to achieve all of the treatment objectives with the selected process modification should be understood. Therefore, a list of treatment objective priorities must be made to make a sound decision. Included in a list of priorities should be economic considerations and operational problems. Economic considerations may become of less importance as the Federal Water Control Act Amendments of 1972 are enforced, i.e., effluent standards will be met regardless of cost. It is felt that at present, however, costs must be considered in process selection, and that costs are primarily a function of local conditions, i.e., no hard and fast rule can be established to indicate that one process is more economical than another. Economic factors that influence the decision making process can be divided between construction and maintenance (operating) costs. It is not possible to state positively that construction costs for one process will be less than for the other.This is true especially because of varying land prices, the increased use of plastic and redwood media for trickling filters, and innumerous local considerations. In general, however, operating costs for a trickling filter facility will be less than for an activated sludge facility. A major factor contributing to lower costs is manpower requirements. Based on the data reported in Table III one can conclude that, in general, 11 to 12 percent more manpower is needed at an activated sludge facility than at a trickling filter facility. Another factor contributing to the economic considerations is the power requirement to impart dissolved oxygen into the wastewater. Power requirements to aerate activated sludge with mechanical or diffused air aerators are, in general, larger than the power requirements to overcome the loss of head through a trickling filter.

TABLE III Operators Required per Plant as a Function of Flow Capacity (3) Average Capacity (MGD) Type of Plant 1 5 10 50 100 Trickling Filter 6-7 9.5-11.5 13-16 37-44 63.5-76.5 Activated Sludge 7-8 11.5-13 15-18 43-49 71.0-82.0

Factors related to operational problems that should be considered when choosing between the two processes are listed in Table IV. Based on those factors listed, it appears that operational problems are encountered more frequently with the activated sludge process than with the trickling filter process. It appears also that operating problems associated with the trickling filter can be rectified more easily than those associated with the activated sludge process. From this information one may surmise that problems of operation associated with the activated sludge process are in many instances related directly to the skill of the operator and that problems of operation associated with the trickling filter are related more closely to adequacy of the original design.

73 TABLE IV Common Operating Problems (3) Activated Sludge Trickling Filter 1. Sludge bulking 1. Ice buildup on media 2. Erratic Sludge Volume Indexes 2. Filter odors 3. Difficulty in maintaining balanced 3. Fly nuisance in vicinity of filter mixed liquor and dissolved oxy- . „. , „,. . • „ , J 4. Clogging and ponding orf filter gen in aeration tanks f. 6 r 6 • media 4. Excessive foam in aeration tanks c „, , .. . ~.- .,. . 5. Clogging oi distribution nozzles 5. Digestor supernatant and/or centrifuge centrate upsetting activated sludge process 6. Unable to maintain balanced F/ M ratio in aeration unit 7. Facilities inadequate for disposal of waste activated sludge 8. Uneven hydraulic and solids loading of aeration tanks

Although there are a number of disadvantages for using the activated sludge process, significant advantages exist when skilled operators are available. Most importantly is that solids levels can be controlled in the aeration basin to enable a given plant to operate at a constant F/ M or to change F/ M as conditions dictate. For an example, if the strength of the incoming waste increases the solids level in the aeration basin could be increased to maintain a constant F/ M. This virtually impossible in the trickling filter process. Thus with skilled operators it is easier to maintain operational control over the activated sludge process than the trickling filter and under certain conditions this could be an important factor when selecting the treatment process.

CONCLUSIONS A decision to select an activated sludge or trickling filter process for a particular wastewater first requires a determination of whether a high-rate or low-rate process will meet the desired treatment objectives. It has been shown that if activated sludge and trickling filter processes are compared on a equitable basis (i.e., high-rate or low-rate) that effluent quality and operational characteristics are similar. Therefore, a rational selection procedure should be based primarily on desired treatment objectives and then on economic and operational considerations. Both economic and operational considerations are related to the geographical location of the proposed treatment process. Factors such as: a) availability of skilled operators, b) power requirements, c) land costs, d) total manpower requirements and e) construction and maintenance expenses would be expected to largely influence the final decision.

74 REFERENCES 1. Metcalf and Eddy, Inc., Wastewater Engineering: Collection, Treatment and Disposal, McGraw-Hill Book Company, New York, (1972). 2. Process Design Manual for Upgrading Existing Plants, Environmental Protection Agency, October (1972). 3. Procedural Manual for Evaluating the Performance of Wastewater Treatment Plants, Environmental Protection Agency, October (1972). 4. Kornegay, B.H. and J.F. Andrews, "Kinetics of Fixed-Film Biological Reactors," Journal Water Pollution Control Federation, 40, R 460, November (1968). 5. Sherrard, J.H., E.D. Schroeder and Q.W. Lawrence, "Mathematical and Opera­ tional Relationships for the Completely Mixed Activated Sludge Process," Pro­ ceedings, 24th Industrial Wastewater Conference, Oklahoma State University, Stillwater, Oklahoma, April (1973). 6. Sherrard, J.H. and E.D. Schroeder, "Importance of Cell Growth Rate and Stoichiometry to the Removal of Phosphorus from the Activated Process," Water Research, 6 1051,(1972). 7. Sherrard, J.H. Unpublished data, February-August 1972. 8. Eckhoff, D.W. and D. Jenkins, "Activated Sludge Systems: Kinetics of the Steady and Transient States," SERL Report No. 67-12, University of California, Berkeley, December (1967). 9. Chipperfield, P.N.J., M.W. Askew, and J.H. Benton, "Multi-Stage Plastic Media Treatment Plants," Proceedings, 25th Annual Purdue Industrial Waste Conference, Purdue University, May (1970).

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