I N D U S T R I A L A N D ENGINEERING CHEiMISTRY
956
Vol. 15, No. 9
A Practical Solution of t h e Problem of Dewatering Activated Sludge' By J. A. Wilson, W. R. Copeland, and H. M. Heisig SEWERAGE TESTINQSTATION, MILWAUKEE, WIS.
H E present investisludge for filtering consists The activated sludge process of sewage disposal was adopted by gation is part of a of a battery of Buchner Milwaukee because the efluenf is run directly into Lake Michigan, larger work underfunnels set in filter flasks, the source of its drinking water, and must, therefore, be purified taken for the purpose of all connected to one pump to the highest degree attainable. The plant is designed to treat increasing the efficiency of furnishing a high vacuum. 85,000,000 gallons of sewage daily, from which 1,275,000 gallons operation of the activated When the sludge in the of sludge, containing 98 per cent of water, are obtained. Although sludge process by bringing plant was in such condition this process furnishes an efluent of the desired purity at all times about a clearer understandthat dewatering by means of the year, it possessed one weak point at the time of its adoptioning of its fundamental of a vacuum filter, under namely, the extreme dificulty of dewatering the waste sludge during mechanism. Some phases fixed conditions, was only the winter season. Chemical investigations carried out at the of this work have been rejust satisfactory, 500 cc, of Milwaukee station over a period of several years have led to a successported in earlier papers.2#s f u l solution of the problem of dewatering winter sludge. It was the sludge diluted to conI n this process the raw sewtain 1 per cent of solid found that relatively enormous increases in eficiency of jilter-pressing age, after being screened, matter would filter through the sludge can be obtained by properly adjusting the temperature is passed into aeration tankg a standard laboratory filter and pH value of the sludge and the amount of solid matter delivered containing activated sludge in just 20 minutes. This per unit of filter area, and by adding aluminium sulfate followed and sewage through which made it convenient to deby a corresponding adjustment of p H calue. air is forced in streams of fine the relative Jiltering e$tiny bubbles. This is acd e n c y of a given sample of complished by having the bottoms of the tanks fitted with sludge as 2000 divided by the number of minutes required Filtros plates, through which the air is forced. As the to filter 500 cc. of 1 per cent sludge through a standard particles of sludge are knocked about by the air, they occlude laboratory filter. Changing the size of the Buchner funnels bacteria and suspended matter from the incoming sewage. or the kind of filter paper employed merely changes the conThe mixture travels the length of the aeration tanks and stant in the equation of proportionality between the small finally enters a thickener, where the clear water, freed from tests and the large-scale plant operation. Two years of opermore than 95 per cent of the bacteria and suspended matter ation of large-scale filter-pressings show that the efficiency from the original sewage, is decanted from the sludge and obtained for any given type of sludge is directly proportional allowed to flow into the lake. The concentrated sludge, to the value defined as the relative filtering efficiency, p ~ o augmented by the bacteria and suspended matter from vided the press is operated under fixed conditions, including the sewage, is drawn out from the bottom and divided, the delivery of the same amount of sludge per unit of filter part being returned for mixing with the incoming raw area in each pressing. The vacuum filter, dksigned for consewage and the rest being sent to be dewatered ao that it tinuous operation, was found to be superior to all other types may be shipped away as fertilizer. The dewatering oper- of filter presses tried, because it permits the regulation of the ation is divided into two parts-filter-pressing to reduce the quantity of sludge applied to each unit of filter area to corwater content of the sludge from about 98 per cent to less respond to the condition of the sludge, which varies widely. than 80 per cent, and then reducing the water content further, ANXUAL CYCLE to about 10 per cent, by the application of heat in a drier. The waste sludge obtained in the summer time parts with The lowest curve in Fig. 1 indicates the seasonal changes its water easily and has never presented any serious difficulty in filter-pressing. I n the winter time the sludge ob- in the relative efficiency of filter-pressing the untreated waste tained is of a very different character, clinging tenaciously sludge from the Milwaukee plant during the year. The curves to its water and resisting filter-pressing. The condition of do not show the rather wide daily fluctuations, but were the sludge for dewatering by means of filtration passes through plotted from series of averages obtained from daily tests over a yearly cycle; in February it is most difficult to filter, but a period of more than two years. Only during a very short as the weather gets warmer it filters more and more easily period, late in August and early in September, can the ununtil September, when it is at its best. From September to treated sludge be filter-pressed in a reasonably satisfactory February the sludge filters with increasing difficulty. Under manner, The other curves show the seasonal changes in the fixed conditions, it takes about twenty times as long to sludge treated in ways before filtering that will be discussed filter a given quantity of sludge in February as it does in Sep- presently. An examination of sludge from the plant showed that the tember. The sludge produced in cold weather quickly becomes septic and its solid contents dispersed to such a degree average size of the individual particlesincreases from February to September and then decreases. The fact that the as to clog and blind the filter cloths. average temperature of the sewage is lowest in February and RELATIV~ FILTERINU EFFICIENCY highest early in September naturally led us to suspect that The laboratory equipment used to study the effect of change the condition of the sludge was determined by the temperature of each of the several variable factors upon the condition of in the aeration tanks. Since other causes had been suggested as well for the improvement of the sludge in September, it I Presented before the Division of Water, Sewage, and Sanitation Chemistry at the 65th Meeting of the American Chemical Society, New appeared advisable to settle this point definitely. At the Haven, Conn., April 2 to 7, 1923. suggestion of T. Chalkley Hatton,' the experimental plant
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*THISJOURNAL,18, 406 (1921); 14, 128 (1922). a J . A m . Water Works Assoc., 8 , 486 (1921).
4
Chief Engineer, Milwaukee Sewerage Commission.
September, 1923
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
was divided into two independent systems at the beginning of the year 1922. One of these systems was operated in the normal manner and the other was operated hexactly like
Time of
FIG.1 Seasonal changes in the relative efficiency of filter-pressing activated sludge from Milwaukee sewage after treating in various ways. Sulfuric acid was used for acidifying the sludge and the p H values noted are those of the filter eauent. Sludge was heated only after acidifying.
it except for the fact that all the raw sewage entering it was heated to a temperature of 70’ F. The sludge from the cold system, in which the temperature varied from 43 to 55 F., O
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was characteristic of winter sludge. I n the other system, however, the quality of the sludge at first became worse, but after about two weeks it began to improve and the average size of the particles began to increase until early in March the sludge was typical of the best ever obtained in September. The curves in Fig. 2 represent the relative filtering efficiency obtained when the p H value of the sludge was adjusted to 3.4 with sulfuric acid before filtering. The experiment shows clearly that the higher temperature assists in the building of large particles of sludge from the smaller ones. The decrease in quality of sludge during the first two weeks of January may be attributed to increased putrefaction in the thickener caused by the higher temperature, As the condition of the sludge entering the thickener improved, the tendency towards septic action decreased correspondingly. EFFECT OF p H VALUE It is well known in the field of colloid chemistry that the tendency for particles of a dispersion to coalesce, forming larger ones, is markedly affected by change of pH value, the greatest tendency towards coalescence occurring at the isoelectric point. This is also the point a t which organized jellies swell the least and most readily part with their water of imbibition. Although each amphoteric constituent of the sludge should be looked upon as having an isoelectric point of its own, not necessarily the same as that of the others, it is convenient t o assume an isoelectric point of the sludge as a whole, which may be taken as that pH value a t which the positive charges on the sludge just balance the negative ones. That this point will vary with the composition of the sludge is obvious. The continuous curve in Fig. 3 shows the effect of decreasing the pH value of February sludge with sulfuric acid.
957
Lowering the p H value to 3.4 causes an increase in relative filtering efficiency from 5 ta 25. Although this is a large increase, it is the greatest possible with change of pH value alone, and is insufficient to bring February sludge into condition suitable for filter-pressing on a practical scale. Considering the heterogeneous nature of the sludge, it is remarkable that the optimum filtering efficiency is obtained at pH = 3.4 the year round. The continuous curve in Fig. 1 shows the seasonal changes in rate of filtration of standard samples of sludge whose pH values have been adjusted to 3.4. Lowering the pH value from about 7.5 to 3.4, by adding sulfuric acid, multiplies the relative filtering efficiency by an average of about 5, regardless of the initial condition of the sludge-a fact of great practical importance since it enables us to filter-press the sludge satisfactorily from May to November simply by adjusting the pH value. As would be expected, change of pH value also has an important effect upon the action in the aeration tanks. Ordinarily, the p H value of the liquor in the aeration tanks is about the same as that of the blood of living animals-7.4. It is possible that there is some relation between the building up and breaking down of tissues in the animal body and the actions proceeding in the aeration tanks. In a recent lecture on “The Chemical Mechanism of Atrophy in the Animal Body,”6 H. C. Bradley pointed out that the stability of the body tissues is decreased when the oxygen supply is cut off and the pH value is allowed to fall by the introduction of acid. Immediately after the death of an animal there is a fall in the pH value of the blood from 7.4 to values below 7.0, and the tissues begin to atrophy. This action can be accelerated by the addition of acid, although an excess of strong acid retards the action. When the air supply is cut off from the aeration tanks, the large particles of sludge immediately begin to break down. As in the case of animal tissues, adding just enough acid to bring the pH value to 6.0 caused the sludge particles to atrophy with extreme rapidity. The action is retarded by adding an excess of strong acid. However, if the pH value
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Time of Year FIG. 2
Effect of heating raw sewage during the winter months upon the relative efficiency of filter-pressing the activated sludge produced, The aeration tanks were divided into two independent systems a t the beginning of the year and all raw sewage entering one of them was heated to 70° F. 6 Delivered before the Milwaukee Section of the American Chemical Society, March 23, 1923.
Vol. 15, No.9
INDUSTRIAL A N D ENGINEERING CHEMISTRY
958
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pH Volue o f F i l t e r Effluent (producedwith sulfuric a c i d ) FIG.3 Effect of change of p H value, produced with sulfuric acid, upon the relative efficiency of filter-pressing February sludge treated in various ways,
is again brought to 7.4 and the liquor aerated, the particles coalesce, reforming the large particles of the original liquor. In aerating sludge at different pH values, two points were found at which large particles were built from the smaller ones, 3.4 and about 7.4. As the pH value increased from 3.4, this action became less pronounced and then reversed. Between pH = 5 and pH = 6 the particles rapidly disintegrated in spite of the aeration. Between the pH values 7 and 8 the air was again very effective in building up large particles. Below 3 and above 8 the air was without effect. It was also noticed that the putrefaction of protein matter in contact with an aqueous solution tended to cause the p H value of that solution to shift towards the region 5 to 6, regardless of the initial value. At a pH value of 7.4, bubbling oxygen through the sludge causes a building of larger particles, while pure hydrogen causes a breaking down. At pH = 3.4, oxygen and hydrogen both show some tendency to cause a building of large particles, although oxygen is the more effective. When the initial sludge is already very septic, hydrogen may fail to show this building effect a t p H = 3.4. Between 5 and 6 the disintegration of the particles proceeds rapidly regardless of the presence of oxygen or hydrogen.6 Apparently, a continual struggle is going on within the sludge, certain bacteria or enzymes present, which act best between the pH values 5 and 6, tending to break down the sludge. Outside of this range their action appears to be retarded by oxygen. At pH = 3.4, the apparent average isoelectric point of the sludge, the building up of the sludge particles is probably brought about by the forces of cohesion. Whether these forces alone are responsible for the action a t 7.4 in the presence of oxygen may be questioned. As the sludge particles disintegrate, they become more susceptible to putrefaction, probably because of the greater surface exposed to the action of bacteria and enzymes. The e Compare
THISJOURNAL, 14, 128 (1922), Fig. 2.
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$0 8 0 90 100 110 120 130 140 150 160 170 180 190 200 Tcmperafure [%I t o Which Sludqr Was,Heafed offer A c i d i f y i n g w i t h S u l f u r i c Acid FIG.4
Effect of rise of temperature upon the relative efficiency of filter-pressing February sludge a t optimum p H value, with and without the addition of aluminium sulfate.
filter-pressing efficiency is also decreased because the smaller particles tend to clog the interstices of the filter cloth.
EFFECT OF TEMPERATURE While operating the vacuum filter in the plant, G. S. Backus tried heating sludge to a temperature of 160" F. after it had been acidified to make the p H value 3.4. A large and immediate increase in relative filtering efficiency was thereby obtained. But when untreated sludge was heated, its relative filtering efficiency rapidly decreased. The effect obtained is evidently a function of both temperature and pH value. One of the curves in Fig. 3 shows the effect of pH value upon sludge heated to a temperature of 160" F. The greatest beneficial effect is found at pH = 3.4. The heat is without effect at p H = 5.6, but above this value the heat is decidedly harmful. In each case the sludge was acidified before the application of heat. When the sludge is heated a t its normal p H value of about 7.5, its condition becomes much worse. Bringing the pH value t o 3.4 after heating improves its condition, but not nearly to the same extent as though it had been acidified before heating. The seasonal changes in sludge acidified to make pH = 3.4 and then heated to 160" I?. are indicated by one of the curves in Fig. 1. The combined effects of acid and heat cause the relative filtering efficiency to be multiplied by about 25 a t all times of the year, thus making it possible to filterpress even February sludge in a satisfactory manner. The cost of the acid and heat for the purpose is not at all unreasonable. If the effect of the high temperature were due to the coagulation of albuminous material, we should expect to find a break in the temperature curve. The continuous curve in Fig. 4 shows that no such break occurs; there is an increase in relative filtering efficiency for every degree rise of temperature frpm 60" to 190" F.
September, 1923
INDUSTRIAL A N D ENGINEERING CHEMISTRY
EFFECT OF ALUMINIUM SULFATE
the treated sludge a t its optimum pH value is shown in Fig. 4, the best result being obtained a t the boiling point of water. A still further increase in relative filtering efficiency can The seasonal changes in sludge treated with aluminium be obtained by adding 1 pound of aluminium sulfate to every 50 gallons of waste sludge and adjusting the pH value to 4.4. sulfate, acidified to make pH = 4.4 and heated to 160" The reason for the shift in optimum pH value to 4.4 may be F., are shown in the top curve in Fig. 1. This treatment raises explained by the necessity for the sludge to carry a negative the increase in relative filtering efficiency to a grand total electrical charge in order to co-precipitate the positively of about 4000 per cent. The problem of dewatering activated sludge at all times of charged alumina. Fig. 3 shows that the optimum efficiency is obtained at 4.4,both for cold and heated sludge contain- the year may, therefore, be considered as solved for the cliing the aluminium sulfate. The effect of temperature upon mate of Milwaukee.
Effect of Deaeration of Natural Waters on t h e Carbonate Equi li bri urn' By D. H. Jackson and J, R. McDermet ELLIOTT Co., JEANNETTE, PA.
N VIEW of the rather I n this paper it is shown that deaEration remoues all free carbon so that the condensate large volume of literadrains back into the vacuum dioxide and decomposes about 35 per cent of the bicarbonate present, chamber and falls with the ture on aggressive carmost of it being precipitated and a small amount remaining in bon dioxide and hydrogenwater from the spray heads solution as calcium carbonate. The percentage bicarbonate reion concentration of natural over a series of cascade pans moued increases slightly with increase in concentration of &carwaters, it was considered into the storage space in bonate. The p H calues are changed by deazration from slightly an appropriate and interthe bottom of the chamber. acid to a fairly high alkaline calue. the change aueraging p H 2.5. esting problem to study the The water level in the effect of deaeration of such chamber is regulated by waters on these factors. Experiments in this field should be a float-controlled valve. The vacuum is produced by an especially significant at this time owing to the growing com- ejector of the steam jet type or by a motor-driven air pump mercial interest in the application of apparatus for removing connected with the condenser. I n the former case an auxdissolved gases from water in systems where it is desired iliary condenser is used to recover the heat from the steam to prevent corrosion. The principal reason for removing dis- and the air extracted from the vacuum chamber passes solved gases is to eliminate the dissolved oxygen, which is through a vent to the atmosphere. the chief cause of corrosion of iron and steel by water. HowWater treated as above in practically all cases is delivered ever, the carbon dioxide content should by no means be with an oxygen content of from zero to a trace, a trace meanneglected, and the experiments recorded in this article were ing less than 0.02 cc. per liter as measured by the Winkler run with the idea of determining the change in hydrogen-ion iodimetric method. concentration and the extent of bicarbonate removal when CARBON DIOXIDE TESTS the free carbon dioxide in natural water is removed by complete deaeration. The water on which the carbon dioxide tests were run was the regular water supply of a power plant which takes water DEAERATION PROCESS from a small stream about one mile north of Jeannette, Pa., A complete description of this apparatus from both and is characteristic of the water of the surrounding section. theoretical and practical standpoints is given by McDermet.2 The water is used in the power plant without being filtered The deaerator t o which the writers had access for running or otherwise treated than by the deaerator. Tests were these tests is of the same type as is being extensively in- run at different seasons of the year, so that the conditions stalled at the present time to prevent corrosion in power could be varied as much as possible. A complete mineral plants and hot-water service systems. analysis of this water might be of interest in connection with Before entering the deaerator, water must be heated to the experimental results of this paper, and a representative a minimum temperature of 160' F. The apparatus may analysis is given in Table I. be designed to handle water at any temperature from 140"F. up to the boiling point. Water enters the deaerator through TABLE I-ANALYSIS OF WATER TESTED a spring-loaded valve and passes through spray heads into PARTSPER MILLION PARTS PER MILLION Calcium 29.6 Chlorides 4 ;97 a vacuum chamber. The spring-loaded valves tend to keep 5.16 Carbonates a s COa 71.4 Magnesium a constant pressure on the spray heads with change in water 21 66 Iron oxide 4.62 Sulfates R R1 Alumina 3.10 Silica rate. The hot water entering the vacuum chamber is subSodium 2.34 Organic matter 28.00 jected to a process of explosive boiling a t the expense of the Ammonia Trace Suspended matfer 16.7 Nitrites Trace Total solids 186.20 heat it contains. The vacuum is regulated so that enough Nitrates 1.1s pH = 6.8 maximum, 5.8 minimum water is boiled to reduce the temperature of the water about 22' F., as this gradieht has been found to produce perfect I n connection with the test of deaeration on carbon dioxide deaeration. The steam produced in the boiling process is drawn off into a small surface condenser which is cooled removal, determinations of free carbon dioxide, bicarbonate, by the water going to the heater. The condenser is located and carbonate were all carried out by t'itration, using the methods and precautions suggested by Johnston. The 1 Received April 12, 1923.
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2
Mech. Eng., 44. 273 (1920).
* J. A m . Chem. SOL.,88, 947 (1916).
