A. A. Kalinske
feature
Eimco Corp. Salt Lake City, Utah 84110
Economic evaluation of aerator systems Reliable data and design principles, together with knowledge of test method limitations, are key elements predicting performance
I
ndustries and municipalities which have the problem of reducing the biological oxygen demand (BOD)-or chemical oxygen demand (COD)-of their liquid organic wastes to the degree required by regulatory agencies generally must resort to some type of aerobic biological oxidation and stabilization of the soluble and suspended pollutants present in the wastes. The most economical processes-except for certain unusual industrial wastesare those using suspensions of aerobic organisms. The activated sludge process, developed some 50 years ago, is one which, so far as present knowledge is concerned, can reduce the organics in wastes to a very high degree. This process has been the subject of intensive and detailed study during the past decade, and can be used in many modifications. It can be adapted for treatment of a wide variety of organic wastes, including compounds which normally are bactericides such as phenolics. pesticides, and many others. There are many commercially available aerators and aeration systems for supplying oxygen to activated sludge systems and also to aerated lagoons. another method of using suspensions of aerobic organisms for waste treatment. Unfortunately, there is a great deal of misinformation and a lack of technically correct data and methods for evaluating the performance and economics of aeration systems. An engineer preparing specifications often is confused by the different claims. and cannot afford to objectively investigate the many variables that influence the performance of the various aerators and aeration systems for the particular waste treatment he is designing. Under these conditions, he is forced to incorporate unnecessary and expensive safety factors, and waste treatment is
not the place where anyone who pays the bill wants to have any such unnecessary luxuries. Therefore. in the following discussion. I try to set forth basic guides. substantiated design principles, and methods of economic and technical evaluation which may aid engineers in designing biologic aerobic treatment plants that often must be supplied with very large amounts of oxygen. T h e three principal oxygenation methods presently in use in liquid waste treatment are: Diffusion of compressed air. Diffused air with submerged turbine dispersers. Mechanical surface entrainment aerators. There are, in addition, two o r three miscellaneous methods which will be discussed briefly.
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Diffused air
For many years, the most widely used method for obtaining aerobic biological oxidation of organic matter was to diffuse compressed air into the liquids through porous diffusers. These were installed, usually in rectangular basins, in such a way that the rising air bubbles would create strong liquid movement and turbulence. Many types of diffusers have been developed in an attempt to reduce the problem of clogging of the small openings by chemical deposition and biological growths. The rate of oxygen transfer is basically influenced by the total air-water interfacial area produced, and the mixing and turbulence generated. To obtain quantitative information relating to this method of oxygenation, it is not necessary to discuss all the various types of spargers and diffusers that have been developed; it is sufficient merely to set d o h n the basic factors on which the power requirements
and installation costs depend. The basic criterion used to evaluate all oxygenation equipment is the amount of oxygen supplied to the liquid per unit of power in a unit time. The most widely used term is pounds of oxygen per horsepower-hour. This factor is expressed for standard conditions, which are clean water at 20" C. with no dissolved oxygen. at sea level barometric pressure of 29.92 inches of Hg. This is done so that there is a common basis for comparing the performance of various aeration systems and devices. The approximate power required to compress air can be calculated using the adiabatic equation. F o r about 7 5 3 0 % compressor and motor efficiency, which is average for larger compressors, we have:
P = HP = 0.30 Q,
[(E)
o.28 -
11
where Q, is the c.f.m. of air under standard conditions, pa is the absolute atmospheric pressure, and p2 is the absolute pressure of compression. The value of p2 will depend on the hydrostatic head above the diffusers. the losses in the lines and fittings between the compressor and the diffuser, and the head loss in the diffuser itself. The oxygen absorption efficiency can be measured using the unsteady state. clean water technique which will be discussed later. It has been customary to use as the value for oxygen saturation the value at mid-depth over the diffuser. For standard conditions, the oxygen absorption efficiency for various types of diffusers will vary from 3-10%,. Higher values can be found in the literature, but, invariably, I have discovered that either the test conditions used to obtain such values are entirely Volume 3, Number 3, March 1969 229
Testing. Oxygen tranrfer rates of aerators vary widely with the size of the unit and basin geometry, and proper scale up factors are important design criteria. These technicians are testing a 150 hp. surface aerator in a basin measuring 80 feet in diameter and 20 feet in depth
Multiple unit. Surface aerators are in widespread use f o r the treatment of both municipal and industrial wastes. This surface aeration system, installed at Kalamazoo, Mich., uses thirty 30 hp. and twelve 60 hp. units to treat the city's domestic sewage as well as effluent from several paper mills
230 Environmental Science & Technology
artificial and d o not duplicate, even approximately, actual field installations, o r else the test techniques were improper. Let us use an average efficiency of 6 % . Of course. the character of the waste and the barometric pressure will also affect the actual oxygen absorption efficiency. Using the above equation me can calculate:
where W,, is the pounds of 0, absorbed per hp.-hr., and E is the absorption efficiency expressed as a decimal. For diffusers in a basin with a liquid depth of, say, 15 ft.. taking a value for E of 0.06, and letting p2 = 7.0 p.s.i.g.. we have for W,, the value of 1.85 Ibs./ hp.-hr.. for standard conditions. F o r actual conditions, this will reduce to at least 60-7555 of this value, primarily due to the fact that we usually want to maintain about 2 m g . / l . of dissolved oxygen in the liquid to have an ample reservoir of oxygen available for the organisms which are fed with a waste of varying strength and quantity. Also, the character of the waste, particularly the presence of certain types of soluble surface-active agents, can reduce the actual oxygen transfer rate very significantly. This involves the determination of the alpha factor, which relates the transfer coefficient of oxygen from bubbles into the waste as compared to clean, or tap, water. This particular item will be discussed in more detail later, as it can have a profound influence on the power that must be supplied to dissolve the required oxygen into the waste liquid. To get the true cost of comparison with other aeration systems. it w’ould be necessary to obtain the installed cost of compressors. building. air piping. valves, and diffusers. and amortize these, over, let LIS say, a 20-year period. The maintenance costs for keeping the diffusers clean will, of course, depend on the type used. Dispersing compressed air
Various designs of radial-flow turbine mixers, supplied with compressed air, have been used and studied in considerable detail for supplying oxygen to aerobic fermentation broths. The normal design is to supply compressed air through a sparge ring below the turbine, and the rotating tur-
bine disperses the air and mixes it with the liquid that it pumps. T h e turbine pumps a relatively large quantity of Mater uith the entrained air bubbles. As this mixture passes through the turbine, the bubbles are sheared up, creating a large air-water interfacial area. The turbine also generates considerable turbulence and mixing to aid in the dissolution of the oxygen from the small air bubbles. I first adopted this oxygenation system for the activated sludge process in 1948, and. during the next 10-12 years, a great many of these systems were installed. Submerged turbines have certain distinct advantages over plain air diffusers. First. the openings in the sparge ring are sufficiently large so that no clogging occurs. Second. the basin depth can be reduced. since the intense mixing of the turbine retains the bubbles in the liquid and they do not rise directly upward. Another important feature is that when the BOD load is light. such as overnight or over weekends. the air can be reduced to a relatively small quantity. while the turbines keep the solids in suspension. Also, it is possible to supply a very large amount of oxygen as needed into a mass of liquid with this system, if power considerations are not important. Depending on the peripheral speed of the turbine, oxygen absorption efficiencies of up to 50% can be obtained if desired. However. in order to obtain such high oxygen absorption efficiencies, high turbine speeds are required. Tests have indicated that the absorption efficiency increases with about the 31’2 power of the peripheral speed of the turbine, while the power increases with almost the third power. I and others have shown that, from a practical standpoint, it would appear that the total horsepower (compressor plus turbine) remains about constant for oxygen absorption efficiencies between 10-25%. For efficiency values above and below this range, the total horsepower increases. A n average efficiency of about 15-20% is obtained with the power about equally divided between the compressors and the turbines. which gives the least total power per pound of oxygen dissolved. These efficiency values are for standard conditions and must be corrected for waste characteristics, temperature of liquid. desired dissolved oxygen in the liquid, and barometric pressure. Volume 3, Number 3, March 1969
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In order to obtain an approximate indication of the total power required for dissolving a required amount of oxygen. we can set up a relation for the total power: P,=HP=.30Q,
[
11 iQ,p,
(@)n''s-
where pr is the horsepower required per s.c.f.m. of air to attain an oxygen absorption efficiency of E. By substituting the previous expression for W, for Q, in the above equation. we can obtain the relation: ._.
W, w, =0.30
E
[(E)"is (E)"'is
- 1 1 f pr
The value of pr is set once a decision is made as to the desired value of E (expressed as decimal) and depends on the peripheral speed of the turbine and the air-loading of the turbine. Taking a value of E equal to 0.15, an experimentally determined value for pt of .025. and p2 equal to 7 p.s.i.g., then W,, = 2.6 Ibs./hp. 1-hr. In general, a good figure to keep in mind for this type of aeration system is 2.5 Ibs. of oxygen per hp.-hr. for standard conditions. Thus. we note that this type of aeration system will have an oxygen input per hp.-hr. about 35-50% greater than a plain diffused air system. Again, the true comparison must be made o n the basis of total installed cost, properly amortized, and with a proper figure for maintenance. Surface aerators
Though the mechanical surface entrainment aerator was used rather extensively some 30-40 years ago, especially for small activated sludge plants, its use waned until about eight years ago, when interest in the device increased. At present, this general type of aerator is used extensively for large activated sludge plants and aerated lagoons, and especially for industrial waste treatment plants. For example, the activated sludge treatment plant at Kalamazoo, Mich.. which treats domestic wastes and the wastes from several paper de-inking plants, has thirty 30-hp. and twelve 60-hp. units. Single units up to 150 hp. are being specified, both on fixed platforms and on floats, the latter generally for use in aerated lagoons. During the past five years, many designs of such surface aerators have been put forth by various equipment 232
Environmental Science & Technology
manufacturers. Though the designs are different, there are certain general common design features that any such aerator must have, if it is to dissolve oxygen from the atmospheric efficiently, and satisfy the necessary oxygen demand of the organisms in the entire basin in which the aerator or aerators is installed. These features are : The aerator must be basically a liquid pumping unit, either of axial or radial flow type, installed near o r at the liquid surface. The liquid pumped must create a zone of intense turbulence around the aerator, consisting of entrained atmospheric air and water. A relatively large quantity of liquid must be pumped to keep the driving force for dissolving oxygen as high as possible as the liquid flows through the oxygenation zone, and to distribute the oxygen enriched liquid throughout the basin. In addition, there must be sufficient liquid movement and turbulence to keep any solids properly dispersed and in uniform suspension in all parts of the basin. As with any type of equipment there are certain limitations that must be given consideration if these aerators are to perform properly. For example. proper basin baffling is necessary to provide hydraulic conditions and insure good mixing. Basin depth and basin width-depth ratio are also important. If these are not within the proper limits. it may be necessary to use a lower pumping turbine, with axial flow preferable, installed on the same shaft as the aerator. Multiple units must be spaced properly. with a suitable distance between obstructions, such as walls, columns, and floats, and the aerator periphery. Finally, the liquid pumpage per unit of basin volume must be such that the input oxygen is quickly dispersed, and the organisms are adequately mixed with the oxygenated liquid and kept in suspension. Various designs of these aerators have widely different pumping capacities per input hp., and the manufacturer should provide this information, in addition to the oxygen input per hp. per unit time. These aerators have been tested using various techniques, discussed briefly later, for oxygen input per hp. per unit time. Because of the extremely complex hydraulic-pneumatjc phenomena involved. it has been found that
tests on small units o r models cannot quantitatively predict the performance of large size units. In general, the smaller units have a higher oxygen input per hp.-hr. than the very large units. However. for any specific aerator the volume of liquid under aeration has a very significant influence on the oxygen input. The smaller the liquid volume per unit of aerator hp. or, more correctly. the aerator pumpage, the higher the oxygen input. Basin geometry also has an effect. Thus, the aerator supplier must be aware of the influence of all these and other variables on the performance of his unit. Generally, for activated sludge aeration basins, these aerators should have a sufficient pumping capacity to turn over the basin volume about every 4-5 minutes. In general, these surface aerators can supply oxygen at rates of about 2.8-4.5 Ibs./hp.-hr. under standard conditions. Thus, an approximate criterion is that these surface aerators can supply the same rate of oxygen input, for any specific treatment process, at about half the power cost of that for plain air diffusion. The installation cost usually is also less, since the need for compressors, building. and airpiping are eliminated. Miscellaneous aeration methods
Probably the next most common method of aeration, after the three already described, is the use of venturitype aspirators. or some modification of them. There are very little reliable data, obtained under properly controlled conditions, that indicate the efficiency of this method of aeration. Probably the best data were those reported by Jackson who tested a 34 -inch venturi aspirator that had a %-inch throat. and obtained a figure of 5.9 Ibs. of O,/hp.-hr. H e scaled this unit up by a factor of 5 ( 4 inch inlet and 1 ?Ainch throat), and the best performance of this unit was only 1.4 Ibs./hp.-hr. Obviously, there is a tremendous scale effect that influences the oxygen transfer ability of these units. Such an effect also exists in regard to surface aerators. since, for example, using a small laboratory size turbine of about 4 inch diameter, it was possible to obtain 8 Ibs. of O,/hp.-hr., a figure that cannot be even approached with 3-5 hp. units, to say nothing of 150 hp. units. Packed towers of various designs have been used for oxygenating liquid
wastes. However, a study of several references to such use indicates that the absorption efficiency is quite low, somewhere between 1-1.5 lbs./hp.-hr. This appears logical, from mass transfer considerations, for a slightly soluble gas such as oxygen. Performance evaluation of aerators
N o economic analysis of aeration equipment would be complete without a discussion of methods and their limitations for evaluating the performance of aerators, either under artificial test conditions, o r in the field under actual operating conditions. The three methods that have been used for determining the oxygen input rate of any aerator, or aeration system, during the past 10 years are: Steady-state, sodium sulfite oxidation, using a suitable catalyst. Non-steady-state oxidation, using clean water, in which oxygen is added after the water has been deoxygenated with sodium sulfite using a suitable catalyst. Laboratory measurement of the oxygen uptake rate of a sample of the mixed-liquor suspended solids from an activated sludge aeration basin, after equilibrium conditions are established with respect to waste inflow and dissolved oxygen. The steady-state method has been shown to give entirely erroneous results which can be 50% or more too high. Since the dissolved oxygen is zero in the liquid at all times and the oxygen transferred immediately reacts with the catalyzed sulfite, the physical conditions at the air-water interfaces are entirely different from the conditions that exist when supplying oxygen to a waste water or to activated sludge mixed-liquor. However, the method is simple, and can be used for qualitative comparisons of different designs of aerators. Non-steady-state oxidation has more or less been adopted as the standard testing technique. If done properly with an understanding of its limitations. it will give reproducible results and permits quantitative comparison of different designs of aerators. Though the theory on which it is based is strictly applicable only where the mixing and oxygenation conditions are uniform throughout the volume of water in which an aerator is being tested, the basic gas-liquid transfer relationship can be used within limits. A
transfer coefficient can be determined for a specific aerator o r aeration system, which then can be used to evaluate the oxygen input capacity of the aerator for any other condition o r waste. The test is made in clean. o r tap, water. Unfortunately, the theoretical diffusion relationship of gas into a liquid o n which this test technique is based is not strictly applicable for a localized surface aerator. As a result, through ignorance or other reasons. the technique has been grossly misused and. frequently, totally meaningless results have been obtained and presented as factual. Space does not permit me to go into all details relating to this test. However, I have found that in testing surface aerators (ranging from 1-150 hp.), compressed air diffusers, and submerged turbine dispersers, the following rules and limits must be observed if meaningful results are to be obtained :
If a localized type aerator is tested, such as the new surface aerators, the basin contents must be completely mixed. A quantitative statement of this is that the dissolved oxygen must be increased from 10-80% of saturation within a period of 10-30 minutes. There have been attempts to describe a basin as completely mixed by such means as specifying a minimum hp. per 1000 gal. This is entirely improper, since different aerators designs have widely varying liquid pumping capacities, which is what really mixes a basin. Since the oxygen input per hp.-hr. varies with the ratio W/Q,,, where W is the basin volume and Q,,. is the aerator pumpage, the manufacturer must provide data showing how the oxygen input varies with W/Q,, from previous research and test studies. If this data is not available, an evaluation test must be made in a basin where W/Q,, will be comparable to that in the actual installation. F o r example. one aerator design gave 4.5 Ib. of oxygen/hp.-hr. in a certain size basin, but, when this same aerator was installed in a basin 16 times as large, the oxygen input dropped to 3.0 lb./ hp.-hr. Another design of aerator, having a much lower pumpage per hp., gave an oxygen input of 3.5 lb./hp.-hr., and only 2.65 lb. in a basin 5 times larger in volume. This is a factor relating to surface aerators that, to date. has not received proper attention by
engineers using and specifying surface aerators. The samples of water for determining the dissolved oxygen at different times, as the dissolved oxygen (DO) is increasing, should be taken in at least three different points in the basin. The plotting of the oxygen deficit (C,-C,) , where C. is the oxygen saturation of the test water, and C , is the concentration of dissolved oxygen at various times ( t ) , when plotted against t on semi-log paper, must yield a straight line whose slope is proportional to the transfer coefficient, K , a . The amount of sodium sulfite to be added to deoxygenate the test water should be at least 25-50% greater than that theoretically required. It is very useful to have a fast-response DO electrical probe which will indicate when the DO starts to build up due to the aerator, at which time sampling should start. Since it is customary to determine the DO in the samples by the standard Winkler test, it is very important that the catalyst used does not cause a n interference with the Winkler test. The most effective catalyst is a cobalt salt, either the chloride o r sulphate. Unfortunately, however, these cobalt compounds undergo reactions due to the sulfite and oxygen and result in a residual compound that causes the DO value from the Winkler test to be o n the high side. The higher the cobalt concentration, the greater will be this error. For example, it has been common to specify 2 m g . / l . as Co. This will give high readings of C, which will result in the transfer coefficient being about 20% high after one sulfite reaction, and about 40% too high after 10 sulfite additions, even though the cobalt is added only before the first test. This, by the way, is the only time it should be added. Extensive tests, under precisely controlled laboratory conditions, have shown that the dosage of cobalt should not be greater than 0.05 m g . / l . as Co if the error in K,a is to be less than 3 % . The transfer coefficient measured is. of course, directly proportional to the oxygen input capacity of any aerator. Copper salts can also be used as a catalyst, but at much higher concentrations. Even so, they cause no interference with the Winkler test. Certain soluble compounds, especially those that are surface active, even in small concentrations, can influVolume 3, Number 3, March 1969
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A. A. Kalinske is consultant, sanitary engineering division, Eimco Corp. Previously (1965-68). he was director of R & D in sanitary engineering at Eimco, superviring work in process imd equipment development f o r treatnient o f wafer and liquid and solid wastes. He received his B.S. (1933) and M.S. (1935) f r o m the University of Wisconsin, and completed doctoral studies at the University of Iowa. During World War I I , Kalinske was consultant on fluid mechanics problems to the US. Navy, and carried on special research rtudies in atmospheric diflusion f o r the Chemical Warfare Service. The author o f some 50 technical papers in fluids mechanics and water and waste treatment processes, Kalinske is a fellow of American Society of Civil Engineers, and n member of American Water Works Association, Water Pollution Control Federation, American Chemical Society, Tnu Beta Pi, Chi Epsilon, and Sigma Xi.
ence the oxygen transfer coefficient. Usually these compounds tend to reduce the transfer coefficient, though some increase it. Therefore, it is necessary to determine for any waste a so-called alpha factor, which is the ratio of the transfer coefficient of the waste water to that of clean water. This is done in the laboratory using the same type of aeration device as will he used in the field installation, It has been noted that surface aerators will give a higher alpha factor than diffused air systems. There is an explanation for this, related to the migration of various molecules of materials in solution into the time-dependent air-water interface. It is my experience that carefully determined alpha values for clean tap waters are always unity, within the inherent experimental errors. The value of C , to be used in the determination of K,a in the clean water test should be the theoretical value, corrected for water temperature and barometric pressure. Certain waste waters may have a lower C , than that of clean water and this must be known before the oxygen input value for standard conditions can be applied in calculating the oxygenation hp. required to supply the necessary oxygen. The determination of the oxygen input capacity of any aerator under actual operating conditions also has many problems which can give misleading results. First of all, with localized aerators, it is physically impossible to have a uniform dissolved oxygen value throughout the basin, even if the oxygen uptake is small. Secondly, since uptake is influenced by turbulence, with localized aerators such turbulence varies from being extremely intense near the aerator to relatively quiescent at some distance from the aerator. Thus, the uptake rate for oxygen determined in the laboratory on a mixed liquor sample may not be indicative of the average uptake rate in the aeration basin. It is essential in determining such up-take rates in the laboratory that an electrical DO probe be used, and that the sample he agitated with a mixer at the bottom. The surface of the sample must, of course, he either covered or quiet so that no oxygen is absorbed from the atmosphere. There is a further complicating factor which may lead to a lower value of uptake rate (which will be used to
234 Eonvironmentai Science & Technology
evaluate the aerator performance), determined in the laboratory, than actually exists in the aeration basin. In the presence of air-water interfaces, organisms can absorb oxygen directly from the air without the necessity of dissolving it into the liquid. This eliminates the necessity for oxygen to diffuse through the air-water interface. Such a phenomenon can increase the up-take rate of oxygen in an aeration basin, as compared to what is determined in an unaerated sample in the laboratory. Perhaps a laboratory technique can be evolved that will take this factor into account. For this can have, apparently, a very significant affect on the oxygen up-take in concentrated suspensions of organisms, such as occur in activated sludge aeration basins. ADDITIONAL READING
Benncir. G.F., an0 Kernpe. L.L., "Oxygen Transfer 'I) B ologica Systems." Proceeoinosof7Orh Pdrd.ie Inddsiria Waste Conference (1965). Jackson, M.L.,andCollins,W.D.. "ScaleUp of a Venturi Aerator," Ind. Eng. Chem. Process Design Develop. 4, 386 (Oct. 1965). Kalinske, A.A., "Evaluation of Oxygenation Capacity of Localized Aerators." J. Water Pollution Control Federation 37, 1521 (1965). I(R
risr(e A A ,
"Oxygen
Absorption
Stdoles r~ ng Mechaiiical A i r D spcr wrs '' S e ~ a o eana Inoustrral Wasrrs 21.
572 (May 1955).
Kalinske, A.A.. "Pilot Plant Tests on High-Rate Biological Oxidation of Sewage." Water & Sewage Works, 175 (April 1950). Kalinske, A.A., Lash, L.D.. and Shell, G.L.. "Study of Cobalt Interference with W i n d er D ssolico 0 . Tesr When -sea to Cata yze Sod Lrn S d l l i t e n Aerator Tesls." Presented a t AnnLal Mcctinp. Control Federat'on. Nater Po 1.1'01, Chicago, 111. (Sept. 23, 1968) Morgan. P.F., and Bowtra. "A r Difldser Cfficcnc cs:' J. Water Po!lut!on Control Federarmn 32. 1047 (1160)
Oldshue. J.Y., "Aeration of Biological Systems Using Mixing Irnpellors," Pro. ceedings of Biological Waste Treatment Conference, Manhattan College, N.Y.. Reinhold Publishing Corp. (1956).
