March. 1924
INDUSTRIAL A N D ENGINEERING CHEMISTRY
225
The Filtration of Activated Sludge‘ By F. W. Mohlman THE SANITARY DISTRICT O F CHICAGO. CHICAGO, ILL.
The results showed considerable improvement over the use of acid, and as soon and testing stations of The Sanitary District of Chicago. Seoeral as possible alum was used I’PROXIMA T E L Y varieties of filters andfilter presses have been tested, mostly on afullfor pretr’eatment of all 3000 pounds of dry size, working scale. The work has been carried on at the Des sludge produced in the activated sludge are Plaines River Treatment Works, where operation began in August, plant. A marked improveproduced daily a t the Des 1922; at the Calumet Treatment Works, where operation of the actiment in filtration was Plaines Treatment Works. cated sludge units began in April, 1923; and at the Corn Products noted immediately. The As removed from the aeraTesting Station, which has been in operation since 1921. The procakes were dryer, the time tion tanks the moisture concedure in use at each plant is discussed separately. of filtration was shortened, tent averages 98 per cent, and the bags were cleaner. although it frequently is as low as 97 per cent. Sludge is removed from the Dorr thick- The use of alum was so satisfactory that it was adopted as eners daily and stored in rectangular tanks containing Filtros routine procedure in the pretreatment of sludge. A comparison of the relative efficiency of acid and alum for tile aerators for pretreatment. From these storage tanks pretreatment is shown in Table I. These results were obthe sludge is pumped directly to the filters. When the press house was put into operation, in September, tained by filtration of 250 cc. of treated sludge through a 1922, two types of presses were available. The first was a battery of Buchner funnels, using a sludge containing 99.06 12O-pla,te Simplex press, a typical recessed-plate filter press per cent water. with a special type of grooved recesses. A perforated sheetTABLE I-RELATIVE EFFICIENCY OF ACIDAND ALUMFOR PRETREATMENT metal plate was riveted over the grooves, on which the filter -SULFURIC F -L I--T --E --R ALUM---ACID66O Be.cloth rested. The plates were center-feed, the thickness of Time of FilTime of Filtration, Mintration, Minthe cakes 1 inch. The filtering area was 1815 square feet. Cc. per utes and Pounds per utes and Gallon Seconds DH 1000 Gallons Seconds DH Pressure could be built up to 125 pounds per square inch. 8:OO 7:30 7.1 0 7.1 The other press was a Worthington platen press, similar 2: 00 .3 6 .7 10: 15 6.2 1:50 6.3 6 6.4 9:30 to the one used a t Milwaukee in 1918.2 This press held 1:40 6:OO 9 6 .2 4.3 eighteen bags, each 5 x 8 feet, supported between perforated 1:30 6.0 5: 00 12 3.2 15 1: 15 2 . 5 4: 00 5.8 platens. The filtering area was 1440 square feet. The first few pressings were made with untreated sludge. These results have been verified a number of times, and The bags soon clogged and very slimy cakes were produced, show facts which indicate why alum has proved so much even with a long pressing of from 7 to 8 hours. It became inmore satisfactory than acid. An insufficient amount of acid creasingly difficult to handle the sludge produced. Experimental pressings in 1917 a t the Stockyards Testing or a drift back to a pH between 5 and 6 may actually cause Station3 of The Sanitary District of Chicago had shown that an increase in the time required for filtration. On the other acidification with sulfuric acid greatly improved filtration of hand, the addition of even small amounts of alum produces a the activated sludge produced from packing-house wastes. sharp decrease in time of filtration, with little tendency toward Subsequent work by Wilson4 and Copeland indicated that deflocculation and a t no time an actual increase in time of iilthe degree of acidification required could be controlled most tration. The curve shows no “humps” and no sharply deeasily by pH determinations, also that only a short interval fined isoelectric point. Similar results have been noted by van der Meulen and Smith5 in experiments on the use of should intervene between acidification and filtration. Acid was used for a number of pressings, and as far as alum for coagulating Imhoff sludge. A rather interesting fact has been demonstrated by a practicable the pH was adjusted to the optimum value, apnumber of laboratory tests and proved by months of practiproximately 3.5 as shown by laboratory tests with Buchner funnels. Results were very unsatisfactory, with wet, slimy cal operation-namely, that the sludge may be too concencakes and dirty bags. It was necessary to press 24 hours per trated for effective use of alum. For example, 97 per cent day, with a chemist present only during the day shift; con- sludge treated with the usual amount of alum (say 10 pounds sequently, perfect adjustment of the acidity could not be ob- per 1000 gallons) will not iilter so rapidly as the same sludge tained. The sludge produced in the plant exceeded the diluted to twice its volume-that is, to 98.5 per cent moisture output of presscake, and bags had to be washed frequently. and treated with the same amount of alum. The flocculaSmall-scale work a t Argo a t the Corn Products Testing tion is visibly not so good in the heavy as in the light sludge. These observations have resulted in the following procedure Station (Zoc. cit.) had indicated that filter alum was very effective in preparing activated sludge for filtration. Ac- for the pretreatment of the sludge: The sludge is drawn into the storage tank, where its moiscordingly, laboratory tests were made with the Des Plaines sludge, using varying amounts of filter alum, Alz(S0&.18HzO. ture content is determined approximately by means of a special hydrometer which has been calibrated by many tests 1 Presented before the Division of Water, Sewage, and Sanitation a t on sludges of various moisture contents. If the apparent the 66th Meeting of the American Chemical Society, Milwaukee, Wis., moisture content is below 98.5 per cent, effluent or city water September 10 t o 14, 1923. is added to bring the moisture up to that point. Alum is 2 Hatton and Copeland, Fourth Annual Report of the Milwaukee then added, never more than 10 pounds per 1OOOgallons. Sewerage Commission, p. 70 (January 15, 1918). * Pearse and Mohlman, Report on Industrial Wastes from the Stock- If possible, the sludge is used up within 6 hours; if it has to A practical, simple method of treatment of activated sludge before DESPLAINES RIVERTREATfiltration has been in use for the past year at the activated sludge plants MENT WORKS
A
yards and Packingtown in Chicago, p. 160 (January 20, 1921). 4 Wilson and Heisig, THISJOURNAL, 13, 406 (1921).
6
THISJOURNAL, 16, 281 (1923).
INDUSTRIAL A N D ENGINEERING CHEMISTRY
226
stand 24 hours, more alum is added. A small amount of air is used to keep the sludge uniform in composition and aerobic. The pH is usually decreased to 5.8 to 6.0, at which point practically All the alum is precipitated as Al2(0H)O. It will be noted that the function of the alum is thus as a coagulant as used in water filtration, and not for its salt effect, after the isoelectric point has been obtained by the use of acid. Much lower amounts are required if alum is used as outlined than if acid and alum are used together. The amount of alum left in the sludge as a diluent of the valuable nitrogenous material is absolutely negligible. A t the rate of 10 pounds per 1000 gallons of 98.5 per cent sludge, only about 30 pounds, or 1.5 per cent, of A1203will be added to a ton of dried sludge. The costs of alum and sulfuric acid are compared in Table 11, Sludge moisture is taken at 98.5 per cent, the cost of alum at 1.75 cents per pound, and of acid at 1cent per pound. These quotations are both high, due to the fact that the supplies were bought in small lots and had to be brought in by truck. T A B L E 11-COMPARATIVE
--
COSTS OR
ACID 66’ Be.--, Pounds per Cost per T o n Dry Ton Dry Sludge Sludge 0 $0.00
4 U L F U R I C
Cc. per Gallon 0
1 2 3 4
65 130 195 260 325 380
0.65 1.30 1.95 2.60 3.25 8.80
.
ALUM A N D
SULFURIC
FILTSR ALUM--
Pounds Founds per per 1000 Ton Dry Gallons Sludae 0 0
2 4
32 64 96 128 160 192
ACID Cost per
Ton Dry Sludge $0.00
0.56 1.12 1.68 2.24 2.80 3.36
Vol. 16, No. 3
CALUMET TREATMENT WORKS The Calumet Treatment Works include two activated sludge units, each with a capacity of 1.75 million gallons per 24 hours. Sludge was built up and ready for pressing in June, 1923. A n Oliver filter has been installed and has been filtering all sludge produced. This filter is 11 feet 6 inches in diameter and 14 feet long, covered with 495 square feet of filter cloth. It has not been pomible to run for more than 7 or 8 hours continuously with this filter, as the sewage is quite dilute and the sludge accumulates very slowly. As a t the Des Plaines Works, it was found that untreated sludge would not filter satisfactorily. Alum was used for preparation of the sludge, but it was found desirable to addmore alum, up to 14 pounds per 1000 gallons. A number of tests with Buchner funnels indicated much more rapid filtration with alum treatment than with acidification. The filter has worked very satisfactorily during the short time it has been operated. The average rate of filtration has been approximately 0.011 pound dry sludge per square foot filtering surface per minute. This high rate of filtration combined with the fact that filtration is continuous and cleanly has indicated that this type of filter is preferable to the pressure filters in use a t the Des Plaines Works. CORNPRODUCTB TESTII~G STATION
An unusual type of activated sludge has been produced at the Corn Products Testing Station from the wastes 5 from the Corn Products Refining Company. This sludge 6 is usually very light and voluminous, frequently containing over 99 per cent water. It contains over 75 per Since at least 4 cc. of sulfuric acid per gallon at a cost of $2.60 cent organic matter on the dry basis, and from 6.5 to 7.5 per ton of dry sludge would probably be required, it is appar- per cent nitrogen. The problem of pressing this sludge is ent that the cost of alum as it has been used here has been unusually difficult because of its watery nature and the at least as low as if acid had been used. rapid changes in its character due to biological conditions. Alum has been used as described above since October, 1922, Two filters have been used for pressing this sludge, a rewith uniform success. No so-called “winter sludge” was cessed-plate filter press and a small American continuous encountered, pressing and pretreatment were for practical filter. The latter is a rotating leaf suction filter containing purposes just as satisfactory in January and February as in 20 square feet of filtering surface. In principle it is similar July and August. On the other hand, one period of difficulty to the Oliver filter. Many substances have been used for was encountered in April, 1923. During this month an at- pretreatment of the sludge before filtration. It was found tempt was made to cut the air in the aeration tanks to the mini- in the winter of 1921-22 that frequently it was impossible to mum amount capable of giving an effluent that would not de- produce a cake in the filter press, no matter what pretreatcolorize methylene blue in less than 8 or 10 days. ‘The air ment the sludge received. It would be absolutely impossiwas gradually reduced to as low as 0.66 cubic foot per gallon, ble a t times to produce a cake in a reasonable length of time but as the air was cut the nitrates in the effluent were reduced with the recessed-plate type of press. from 7 or 8 parts per million down to 1 or 2 parts per million. The American filter has offered more hope. Sludge has The effluent was still clear, but stabilities began to drop off. been pressed both hot and cold many times, using as filter The main difficulty was noted, however, in filtration of the aids sulfuric acid, alum, dicalcium phosphate, spent bone sludge. Although the same pretreatment was continuedash, sulfur dioxide, and Filter-cel. General conclusions were : in fact, the alum dosage was increased-slimy cakes were ob1-By suitable pretreatment of the sludge i t should be possible tained and filter cloths clogged rapidly. The air was soon increased to 1.2 cubic feet per gallon and the nitrates were at all times to produce a cake with a type of suction filter similar American filter. built up to 5 to 10 parts per million. Coincidentally, the sludge to 2the -A very dilute sludge (99.5 per cent water) or one not well began to press more easily, and soon was back to its former activated cannot be pretreated with any of these substances, satisfactory condition. Confirmation of this phenomenon including heating to 150” F., so as to give a high rate of filtrahas been obtained a number of times at the Corn Products tion-that is, a rate greater than 0.080 pound per square foot per minute, A well-activated sludge with a moisture content of Testing Station. 98.5 or 99 per cent can he treated with either alum or acid, with The present trend of development of the activated sludge or without heating, and will give a satisfactory cake with a high process seems to be in the direction of decreased aeration and rate of filtration. The moisture content without heat is usually minimum nitrification. Numerous mechanical and biologi- about 85 per cent, with heat from 80 to 82 per cent. Apparently heating does not greatly increase the rate of filtration. Alum cal schemes are proposed which require less power than that treatment is generally more effective than acidification. required by aeration with compressed air, but generally no 3--Phosphates, bone ash, Filter-cel, and sulfur dioxide are of nitrification is obtained. The writer believes that thorough little value. activation, as evidenced by formation of 5 parts per million 4-The ratio of time of pick-up to time of drying is fixed. or more of nitrate nitrogen, is essential for the production of It would be desirable to be able to vary the ratio-that is, to the time of pick-up with a heavy sludge and to lengthen a sludge that will filter easily, and that this factor must be ishorten t with a dilute sludge. Control of the degree of vacuum and remembered in attempts to turn out a satisfactory effluent blanking off of one or more sectors are unsatisfactory methods without nitrification. for accomplishing such variations. 6
8 10 12
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
March, 1924
CONCLUSION
227
ACKNOWLEDGMENT
As a result of the uniformly successful use of alum for pretreatment of the activated sludges produced a t Des Plaines, Calumet, and Argo, the writer believes it to be preferable to acid as a practical aid in filtration. It has the advantages of being safe to handle, there is no possibility of increasing time of filtration, it is as cheap as acid, it gives a very clear filtrate, and it is not necessary to control the amount used within narrow limits. The only disadvantage is that it may very slightly dilute the nitrogenous material of the sludge.
The operation of the plants and testing stations of The Sanitary District of Chicago is under the supervision of E. J. Kelly, chief engineer, and Langdon Pearse, sanitary engineer. Edward Bartow represented the Corn Products Refining Company and supervised tests made a t the Corn Products Testing Station. Credit is due E. H. Morgan, principal assistant chemist, and S. L. Tolman and A. H. Goodman, assistant engineers, for progress in solving the problems encountered in the filtration of activated sludges produced in the plants and testing stations.
A Sectional System of Laboratory Desks’ By P. Borgstrom TULANE UNIVERSITY
I
T is an interesting fact that chemical laboratory desks have changed but little in construction since the time of Liebig and Bunsen. The idea of sectional furniture, now uriiversally adopted in modern office equipment, has apparently found no place in the workshop of the chemist. It frequently happens that in chemical laboratories rearrangement of desks becomes desirable, particularly in small rooms or instructors’ laboratories, and with the classical style of furniture such changes are impossible without considerable expense. With this idea in mind, what may be termed a sectional type of chemical laboratory desk has been designed, which has been installed in the new laboratory of physiological chemistry a t Tulane University. Two sizes of sections, 18 and 30 inches, were chosen because they suited the conditions here-that is, a major and a minor course. The method of setting up, as well as type of section used, is shown in the drawing. At the end of the desk, A is a “finished end,” stained and finished to match the desk fronts. This end is fastened to the desk by the same method as used to hold the section together-namely, three screws a t D. B is a device to lock the drawers. C shows the section removed to receive a lead-lined trough that runs through the student, desks. E is a shelf, a small one being used in the 18-inch and a larger one in the 30-inch section. The tops are 1.25 inches thick, built up of strips, glued and doweled. On the under side of the top are cleats to hold the sections in place, as well as to prevent warping of the top. Where a sink 1
LA.
OA LOUISIANA, NEWORLEANS,
was to be installed the section was made without bottom or back and with a drawer front only. As the water, gas, pressure, and vacuum lines all run above the desk, the sink can be installed independent of the desk and the desk can be removed without the aid of a plumber. In the instructors’ laboratories a few 60-inch sections were made (combining two 30-inch sections) with a removable wall so that long apparatus which would not go into the 30-inch section could be stored. The main advantage consists in its flexibility. When time of installation came it was found that some of the rooms had been altered in size due to changes in construction, and these changes caused no inconvenience or loss in installation. Moreover, ideas of equipment of the different rooms had changed in the two years that had elapsed since the plans for the building had been drawn. If the laboratory should be moved at any time in the future, the sections could be adjusted to the new building a t relatively small expense. The one objection advanced against this system is the cost. ActualIy, 629 linear feet of desks were installed and the expense would have been 5.00 per cent less if longer sections (8 and 12 foot) had been used. This is calculated on desk frontage only, independent of the tops, hoods, and plumbing. If these factors had been included in the calculation, the cost would have been 2.73 per cent less in the longer sections. It may also be said that these desks were made a t a local cabinet works, whose specialty is office and store fixtures.
Received November 28, 1923.
SECTIONAL LABORATORY DESK
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