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 CIVIL ENGINEERING STUDIES 
 
 SANITARY ENGINEERING SERIES NO. 2 
 
 AERATION WITH A HIGH-OXYGEN ATMOS- 
 PHERE IN THE ACTIVATED SLUDGE PROCESS 
 
 By 
 
 HAROLD A BABBITT 
 . Professor of Solitary Engineering 
 
 )LU 
 
 These Studies deal with current activities in the 
 department of Civil Engineering. They serve as 
 progress reports of major investigations, and in 
 general as a means for disseminating information 
 not readily adaptable for presentation in formal 
 technical papers or bulletins. 
 
 November 15, 1951 
 
 DEPARTMENT OF CIVIL ENGINEERING 
 
 UNIVERSITY OF ILLINOIS 
 
 URBANA, ILLINOIS 
 
Return this book on or before the 
 Latest Pjlte. f&SlES&m below- 
 
 ENSInEErllL. 
 
 University of Illinois Library 
 
Sanitary Engineering Series No. 2 
 
 AERATION WITH A HIGH-OXYGEN ATMOSPHERE IN THE 
 ACTIVATED SLUDGE PROCESS 
 
 by 
 
 Harold E. Babbitt 
 Professor of Sanitary Engineering 
 
 November_15.,,_1951. 
 
 Hypotheses concerning the function of aeration and of 
 agitation in the activated-sludge process have varied from the 
 assumption that agitation alone is all that is necessary to the 
 assumption that it is the oxygen in the air that is solely 
 responsible for the functioning of the process. Intermediate 
 between these extremes are the following various hypotheses 
 which have attempted to attribute the functioning of the 
 process to: (1) electrostatically charged colloids, (2) sub- 
 surface attraction or adsorption, (3) bioprecipitation, 
 (4) enzymatic action, (5) clarification by protozoans, (6) base 
 exchange, and (7) biological metabolism under aerobic conditions. 
 One method of testing the truth of hypothesis that the effect 
 is due entirely to agitation, and that oxygen has nothing to do 
 with the process, might be to agitate the mixed liquor with an 
 atmosphere devoid of oxygen, such as an atmosphere of nitrogen 
 only. In view of experience in the operation of activated 
 sludge plants such a test would seem to be unnecessary since 
 it is known that the effect of inadequate aeration will be to 
 produce undesirable conditions in the plant and that septic 
 conditions develop rapidly when the air supply is cut off, 
 
:. ■. . 
 
-2- 
 
 These facts lead to the conclusion that some oxygen is 
 essential to the functioning of the process. 
 
 It has been shown in the operation of activated 
 sludge plants that, within certain limits, the greater the 
 amount of aeration per unit volume of sewage treated, other 
 things being equal, the better the quality of the effluent 
 and the better the quality of the sludge produced. A limit 
 may be reached where the application of additional air becomes 
 economically prohibitive or the agitation may break up the 
 floe so that it will not settle properly. 
 
 If, therefore, oxygen is the essential element in the 
 efficacy of aeration it occurs that the reduction of the required 
 volume of air, the reduction of agitation, an accelerated 
 oxidation, and other beneficial effects might result from the 
 aeration of the mixed liquor in an enclosed aeration tank using 
 a synthetic atmosphere containing a high proportion of oxygen. 
 The high-oxygen atmosphere, after passing through the mixed 
 liquor in the aeration tank would be caught and sent to the 
 compressor, where oxygen consumed in the aeration tank might 
 be replenished and the atmosphere recompressed and reused in 
 aeration. An apparatus to permit such a procedure was devised 
 and tested in the Sanitary Engineering Laboratory. A flowing- 
 through diagram of the apparatus is shown in Fig. 1. 
 
 In the operation of the equipment the aeration tank 
 was tightly closed at the top to enclose the synthetic 
 
-3- 
 
 atmosphere. Raw sewage and return sludge were mixed at A 
 to form the mixed liquor. The mixed liquor entered the bottom 
 of the aeration tank at B and arose with the rising gas bubbles 
 to leave the aeration tank at C at the top. The aerated liquor 
 was conveyed to a settling tank from which return sludge was 
 obtained; the clarified liquor and the excess sludge being wasted. 
 
 The synthetic atmosphere or aerating gas was stored in 
 a gasometer as at D, needed oxygen being added from the oxygen 
 cylinder marked C^. The oxygen was washed in water and in a 
 solution of caustic soda before being admitted to the aeration 
 lines. The aerating gas was circulated in a completely 
 enclosed system by means of a compressor. Samples of the 
 aerating atmosphere were taken frequently during each run and 
 oxygen was added, as needed, to maintain the desired percentage 
 of oxygen in the atmosphere. 
 
 Conditions that it was expected to control during a 
 run or test were: 
 
 1, Rate of flow of sewage, 
 
 2, Period of aeration. 
 
 3, Period of sedimentation. 
 
 4, Rate of return sludge, 
 
 5, Proportion of oxygen in the aerating atmosphere, 
 
 6, Ratio of volume of aerating atmosphere to volume of sewage 
 treated. 
 
* 
 
-4- 
 
 Among the observations to be made during a test were: 
 
 1, Dissolved oxygen in the mixed liquor in the aeration tank, 
 
 2, Dissolved oxygen in the effluent from the aeration tank. 
 
 3, Dissolved oxygen in the sedimentation tank, 
 
 4, Dissolved oxygen in the return sludge, 
 
 5, Solids in the influent, in the mixed liquor, in the return 
 sludge, and in the final effluent, 
 
 6, B,0,D, in the sewage, in the return sludge, and in the 
 final effluent, 
 
 7, Amount of oxygen required for replenishment of the aerating 
 atmosphere, 
 
 8, Sludge index, 
 
 9, Turbidity of the final effluent, 
 
 10, pH of incoming sewage, mixed liquor, return sludge, and of 
 final effluent. 
 
 In tests made after the apparatus was in good working 
 
 condition no serious difficulties were encountered in controlling 
 
 any of the desired conditions with the exception of the oxygen 
 
 in the synthetic atmosphere. Tests were made normally with: 
 
 a 3-hour aeration period; about 30 per cent, by volume, of 
 
 return sludge in the mixed liquor; and 95 per cent, by volume, 
 
 of oxygen in the synthetic atmosphere. The return sludge con* 
 
 tained between one and V6 per cent of solids. Various rates 
 
 of aeration were attempted from about 0,3 to 2,0 cu, ft, of gas 
 
 per gallon of sewage treated. 
 

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 In so far as the effect of an increased proportion of 
 oxygen in the aerating atmosphere was concerned observations 
 were inconclusive and no reliable quantitative results were 
 obtained. It was found that, in general, a satisfactory effluent 
 could be obtained, with relatively high dissolved oxygen and 
 low 3.O.D. with a low ratio of aerating gas to volume of sewage 
 treated. This ratio increased somewhat as the concentration 
 of oxygen fell below 95 per cent but no relationship between 
 the amount of gas required and the proportion of oxygen in 
 the atmosphere was established. Whatever the reduction of the 
 amount of gas required might have been, it was not commensurate 
 with an insurmountable difficulty encountered which rendered 
 the procedure impracticable and necessitated the termination 
 
 of the studies. 
 
 It became impossible to maintain the oxygen content 
 of the atmosphere constant at any figure without frequent 
 flushing of other accumulated gases from the lines. The 
 principal gas accumulating in the lines was nitrogen. The loss 
 of oxygen during the flushing process was appreciable. The 
 proportion of oxygen in the atmosphere tended to fall steadily 
 as the proportion of the nitrogen increased, and the lines were 
 flushed. The required amount of flushing and the resulting loss 
 of oxygen had not been foreseen when the test apparatus was 
 designed and the tests planned. 
 
-6- 
 
 The difficulties due to the accumulation of nitrogen 
 in the synthetic atmosphere were probably due to the fact that the 
 mixed liquor entering the apparatus, having been exposed to the 
 air had, presumably absorbed or dissolved some air. The oxygen 
 absorbed was consumed in the activated sludge process, the 
 nitrogen remaining in solution. As additional oxygen was blown 
 through the mixed liquor in the aeration tank nitrogen was washed 
 out of solution and oxygen was dissolved. The nitrogen removed 
 from the mixed liquor accumulated in the synthetic atmosphere 
 making it impossible to maintain a desired proportion of oxygen 
 in the aerating atmosphere without first disposing of the excess 
 nitrogen. The need for the frequent flushing of the gas lines, 
 accompanied by an inevitable loss of oxygen, rendered the process 
 impracticable however promising it might be biologically or 
 economically, 
 
 A study was made of the possible economy of the process 
 as compared with the conventional activated sludge process. For 
 the purpose of comparison alone it was assumed that the annual 
 cost of the conventional activated sludge process consists of the 
 cost of: (l) power for the compression or air and the circulation 
 of sludge; (2) oxygen; and (3) interest on the construction cost 
 of the aeration tank and its appurtenances. Other features of 
 the two processes may be considered as similar and costing the 
 same, and need not, therefore, be considered in cost comparison. 
 
-7- 
 
 The cost of the conventional process of a plant 
 treating one million gallons of sewage daily may be estimated 
 as follows: 
 
 1, Cost of compressing one cubic foot pf^srir per gallon of 
 sewage to a pressure of one atmosphere will be taken as 
 $5,00 per day for power alone, 
 
 2, Cost of pumping 300,000 gal, of return sludge per day 
 against a total head of 15 ft, will be taken at $0,21 
 for power alone, with power costing 1,5 cents per kwh. 
 
 3, Cost of aeration tank. In the conventional process the 
 cost of a diff used-air aeration tank can be estimated, 
 roughly, as about $100,000 per million gallons per day 
 treated. If interest may be taken at 4 per cent, the 
 annual cost will be $4,000 if depreciation and other factors 
 are neglected, 
 
 The total annual cost of the conventional activated sludge 
 process will be, therefore, for comparative purposes only, 
 about $5,900 per mgd. 
 
 The annual comparative cost of the synthetic- 
 atmosphere process can be estimated as follows: 
 
 1, Cost of compressing atmosphere, Although there are no 
 convincing observations to justify the assumption, it will 
 be assumed that the synthetic atmosphere, containing 
 approximately 5 times as much oxygen as air, will be 5 times 
 as effective in oxidation and that only one-fifth as much 
 atmosphere will be required. Hence, the cost of compression 
 of the atmosphere will be 1/5 x $5 o 00, or $1,00 per day, 
 
 2, Cost of pumping return sludge will be considered to be one- 
 half that in the conventional process, since only one-half 
 as much sludge will be required, Kence, the daily cost 
 will be 1/2 x 0,21 or $0.11 per day, 
 
 3, Cost of additional oxygen required. No reliable figures 
 are available concerning the amount of oxygen required. 
 However, it will be assumed that the B,O.D. of the sewage 
 may be reduced by 100 ppm. This is equivalent to an 
 
-8- 
 
 3. (cont.) 
 
 oxygen requirement of about 850 pounds per million gallons 
 of sewage. At a cost of 5 cents per lb, the daily cost 
 will be $42. This figure is based on the availability 
 of 100 per cent' of the oxygen added to the atmosphere. 
 Losses of oxygen will occur from many causes such as 
 solution in the mixed liquor, oxidation of inorganic matter 
 not involved in the B.O.D. test, and in the flushing of 
 the line. If the added oxygen may be considered as 67 
 per cent effective in reducing the B, 0, D,, the daily cost 
 for oxygen will be $63 , 
 
 4, Cost of the aeration tank* It will be assumed that the 
 aeration period in the new process will be one-half that in 
 the conventional process due to the more rapid action of the 
 higher concentration of oxygen in the aeration tank* Hence 
 the cost of the aeration tank without a cover, will be one- 
 half of that of the conventional tank, or $2,000 per year. 
 However, the new tank must be covered with an air-tight 
 cover and there will be additional appurtenances needed for 
 the collection of the gas from beneath the cover and to 
 return it to the compressor, It may be estimated, conserva- 
 tively, that the additional cost of the cover and its 
 appurtenances will amount to about $500 per year. 
 
 The total annual cost of the process using a synthetic 
 
 atmosphere for comparative purposes only, will be, therefore: 
 
 Power for compressing atmosphere. ,,.,.,,,,, ,$ 365,00 
 Pumping return sludge C 11 x 365, , c 9 , , , a . , , „$ 40,00 
 
 Aeration tank and appurtenances, . ., e ... , B , „ $ 2,50 0, -00 
 
 The approximate cost of $26 } 000 per year for the 
 synthetic-atmosphere process as compared with approximately 
 $6,000 per year for the conventional process is discouraging 
 to further investigations to seek refinements of the proposed 
 process in the hope of reducing its cost. It is concluded, 
 therefore, that any attenpt to use an oxygen-enriched atmosphere 
 in the activated sludge process is economically unjustifiable 
 however effective it may otherwise be. 
 
-9- 
 
 Acknowledgement is made of the work done on these 
 tests by Air, D. E. Drier who performed most of the tests in 
 the preparation of his thesis for the master's degree in 
 sanitary engineering, and to Mr. Lee Ming, who qs a Research 
 Graduate Assistant did much of the work in the cost comparison 
 of the two processes. 
 
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