INDUSTRIAL A N D EXGINEERING CHEAfISTRY
Marsh, 1931
Special thanks are due Charles H. Cole, who assisted in obtaining much of the experimental data, and W.H. McAdams and A. P. Colburn, who kindly reviewed the manuscript. Table of N o m e n c l a t u r e c
=
d
=
D
=
h,
=
il,
=
k
=
I,
=
Q
=
r
=
s = ATs = u
=
1:
=
Z/
zm g
specific heat tube diameter inside diameter of tube, inches film coefficient of heat transfer Cipe to water, B. t. u per hour per square foot per F., based on arithmetic mean temperature drop across water film (see text) film coefficient of heat transfer, $earn to pipe, B. t. u. per hour per square foot per F., based on a temperature difference between the mean pipe surface temperature and the temperature corresponding to the steam pressure thermal conductivity, B. t. u. per hour per square foot per-’ F. per foot thickness, taken as constant a t 0.33 for water tube length, inches latent heat of condensation, B. t. u. per pound L I D = ratio of length to diameter specific gravity temperature drop across condensate film, taken as difference between mean pipe surface temperature and temperature corresponding to steam pressure average velocity, feet per second over-all coefficient of heat transfer, steam to water, B. t. u. per hour per square foot outer tube surface per O F., using arithmetic mean over-all temperature differtmce
p
309
= viscosity, centipoises, taken a t the arithmetic mean
film temperature-i. e., the arithmetic mean of inlet and outlet temperatures plus one-half mean temperature drop across fluid film = viscosity, centipoises, taken a t arithmetic mean of average inlet and average outlet temperatures = absolute viscosity = density, pounds per cubic foot L i t e r a t u r e Cited
(1) Burbach, “Stromungswiderstand und D‘armeiibergang in Rohren,” Akad. Verlag, Leipzig, 1930. (2) Cox, Trans. A m . SOL.diech. Eng.,60, Pet., 2 (1928). (3) Graetz, Ann. Physik, 18, 79 (1883); 26, 337 (1885). (4) Keevil and hlcAdams, Chem. diel. Eng., 36, 464 (1929). (5) McAdams, Cnpublished lecture notes on Heat Transmission, M. I. T . , 1925,. (6) McAdams and Frost, Refrigerating Eng., 10, No. 9 (1924). ( 7 ) McAdams, Sherwood, and Turner, Trans. .4m.SOC.Mech. Eng., 48, 1233 (1926). (8) Monrad and Badger, IXD,ENG.CHEY.,22, 1103 (1930). (9) Morris and Whitman, Ibid., 20, 234 (1928). (10) Nusselt, Mitt. iiber Forsch., Heft 89 (1910). (11) Susselt, 2 . Ver. deut. Ing., 54, 1154 (1910). (12) Nusselt, I b i d . , 67, 197 (1913). (13) Susselt, I b i d . , 60, 541 (1916). (14) Nusselt, Fest. d. tech. Hochschule Karlsruhe, 1925. (15) Rice, I X D . E N G . CHSn*., 16, 454 (1924). (16) Soennecken, Mitt. iiber Forsch., Heft 108-9 (1911). (17) Stanton, Trans. Roy. SOL. (London), 190A, 67 (1897); Tech. Rept. Adv. Comm. Aero. (Gt. Britain), 1916-17, p. 31. (18) Stender, U’iss. l’er6j$enentZick. Siemens-Konzern, 9, 88 (June, 1930). (19) Webster, Trans. Inst. Eng. Shipbuilders Scotland, 61, 58 (1913-14). (20) Wilson, Trans. 4 m . Soc. h’ech Eng.,37, 47 (1915).
The Role of Protozoa in Activated Sludge’ Robert C r a m e r 2 757 NORTH BROADWAY, MILWAUKEE, WIS.
HE process of sewage
Research work performed i n t h e laboratory of t h e ing to about 25 per cent of p u r i f i c a t i o n t8hat is Milwaukee Sewerage Commission i s described which the volume of the raw sewage practiced in hIil\yaUdiscloses i m p o r t a n t f a c t s d e m o n s t r a t i n g t h e essential G hours’ aeration sufficed, biochemical character of the activated-sludge process under the conditions of their keel the a c t i v a t e d - s ludge of sewage purification. These experiments indicate experiment, to produce clariprocess, is a development of the last eighteen years. The that clarification of sewage c a n be accomplished by fication. These experit h i s process if t h e following conditions exist simulmenters originated the term fundamental and ori gin81 work \vas performed in 1912 taneously : aerobic bacterial life, live protozoa, a n d “activated sludge.” at the Lawrence experiment oxygen in solution. The significance of these f a c t s Still later experiments were a n d t h e i r practical implication a r e briefly discussed. made with large plants that station of the Alassachusetts Microphotographs of s o m e organisms which a r e active operated on continuous flow State Board of Health by H a r r y W . C l a r k , Tyho is i n t h e purification process a r e given. instead of on the fill-and-draw still a t t h e head of t h i s principle. The design of the famous institution, called by Gilbert J. Fowler. of Manchester, Milwaukee plant was based on the experience gained in the operation of an experimental plant capable of treating 5 million England, “the Mecca of sanitary engineers.” Doctor Clark found that sewage could be clarified by mixing gallons of sewage daily. it with sludge obtained from sewage and agitating it by bubbling air through it, subsequently keeping the aerated liquor Theories quiescent, without agitation, to permit all the suspended solids to settle out. By this procedure, which Doctor Clark Although practical methods for applying activated sludge carried out in fill-and-draw fashion in large bottles and car- to seJwge purification have been developed, no adequate boys, he was able t o produce supernatant liquor of great theoretical explanation of the process has been worked out. purity, practically free from suspended solids and bacteria, In older methods of purification, such as contact beds and with 24 hours’ aeration. trickling or sprinkling filters, the presence of nitrates in the Later the quantitative relations of the process, such as effluent has been used as an indication of the degree of purithe proper proportion of sludge to raw sewage and necessary fication obtained. The terms “purification” and “oxidation” aeration and settling periods, were worked out by experiment- Tvere, in fact, often used interchangeably. ing on a larger scale, notably by I. E. Ardern and W.T. It is interesting, in this connection, to note the definition Lockett a t hlanchester, who showed that with sludge amount- of the activated-sludge process suggested by a committee appointed in 1916 by the American Society of Civil En1 Received January 12, 1931. gineers: ’ Consulting Engineer, Milwaukee, Wis.
T
310
f.vi>umrtI’.iI.
..i.vi) fiwi‘NEERING (:fffiMIIsTIZY
Bctiviltrd sludge trcatniciit may Lw defined as a biochrmicai process by which tlie purification of sewage is accomplished hy passing it through tsnks, in which sewage sludge is artificially agitated and intimately mixed with sewage and is supplied with the requisite onygeu for thr, optimum development of countless numbers of nitrifying organisms incorporated in and adhering t o the iludgr. the hnal scttlernent of which causes a distinct clarification of tlic oxidized sewage.
I’ract~iid cxperienoe with the process, in Milwaukee a i d other places, has shown tliat. nitrification is unnecessary, and en itndcsirable if the waste sludge is used as fcrtiliarr. 110 solid>le nit,rates pass out with the effluent, instead of rimaiiring in tho fertilizer, of which the nitrogcr c!om p o u n d s are thc valuaGlc part,. Clarificatioii--that is, ttrefreeingofthc sewage from suspended inat,ter and iiaotrria---is an ai‘tirm q u i t e indrI*.ndent of oxidnthin ani1 nitrific:ition. Oxygen, on tlic o t h e r h a n d , is imcess. This has always I m n reaogi i i a e d , aiid W E
dcfinitely proved in (xpcrinwnts coniluctril in Milwaiikee unilr*r the iiircctioii of I)oct,or Wilson. W h e n ripurL. I~~ ~ y ~ ~ and ~ ~ s, ~~ ~~ agitntion ~ t ~I ~ ~ was ,~ pro, Similar OrCsniam nrnone l‘aarficlra of Actidumd by bubbljng vnfed Sludee. 85 X nitrogen, carbon dioxide. or hydrogen through the sewage niixed with activated sliidge. t*he effectiveness of the latter waa rapidly destroyed, rtnii tlieri 110 clarification was produced. Air, or oxygcn, oidy w i e effective. It has often been yoiiitcd out that botb bacteria and protozoa are always present in ar.t,ivnted sludge. Bacteria were considered essential, but no such statement was made conrcrniiig protosoa. T h o nearest approach to a11 appreciation of bhe rnechanisin of tile process is fouiid in a tentative definition proposed by Buswell and I.ong in 1922 ( I ) : Activated sludzf Aocs are cumposcd of a syutlietic gelatinous matrix. similar to that of nostoc or merismopedia, in which iilamcntous and rinicellular bacteria are imhcdded and on wliich ~ a r i o m protorria and some metazoa crawl and ired. The uurification is accomnlished by inEcstion and assimilation of ille organic matter i n the sewa& by&ganisms, and its resynthusis by them into the living niaterial of tlie floc. This process c11snges organic matter from colloidal and dissolved states of dispersion to a state in which i t will settle out.
Xo proof of this ststenlent was given. ( h i t l w ot Iiw I h ~ l sorile , investigat.ors consider the presenre of protozoa liarrnful t,o the process. In 1922 Fairiimtlicr s r r d R i w l m w (#) srigg&ed that protozoa m k h t
killcd by tlrc use of certain dyes. Io commenting on this, A. J. Martin (4)says: “It is hy no moans certain that protozoa are as hamifill ns is sometimes supposed.” Ardern, Jepson, and Giiunt, in a paper read before the Manchester scetion of thc Society of Chemical Industry in 4larcb, 19M1, discussing a statement of Richards and Sawyer iic
Vol. 23,
50. 3
(i that ), the high tutrogen coiltent of activated sludge is due to the presence of large numbers of protozoa, do not refer to any role which the protozoa might play in the production of tho sludge, Martin (4) ijiiot,es Harris, Cockburn, and Auderaori i n connection with a close study they hare inade of the iiifiuenee of prot,isaoa:
Observations nadc by tis daily over a period of years lmve shown t h a t tlie protoma in activated sludge are subject to rapid and wide variations in numbers, and changes in type due in part t o seasoiial iduences. We have obtained experimental evidcnce tliat the presence or absence of protozoa neitlier iw creases nor detracts from tlic ctlicirncy of fully matured activated sludge. The significance of their presence and the value attached t o the identification of prevailing forms lie r a t l r r in the informetion thereby afforded as t o the condition of the sludge. arid the timely warning pivrri of thr mset of unhealthy and undesirahlc developmerits. Our experience has shown t h a t tlic predominating types of protozoa iiiliabiting t h e sludge particles differ with the system in opcration, and vary s i t h the SCL~SOOF, whilc a comparison of our records with tlic published results of other olmrvers suggests that the strength aiid character or the sewage treated arc also dctcrmining factors. Thougli the microscopical cxarninatioti ai activated sludge has, in OUT hands. proved a rapid and useful method of control, its utility and reliahility primarily rest upon a knowledge of the forms of protozoal lifc n?rmallv mcscnt under conditions existinr at individual works
About eight years ago John Arthur \Vileon poiirttd out to the writer that. his observations indicated t.hat tlie relatiun between the length of aeration time required to prorliice clarification and tlie amount of siiq*.nilcd matter in tlie nuxed liquor follows simple cliemical l a m , partiaularly tlie law of mass action. I n other words? the aeration time required t,o produce clarification is indirectly proportional to the yuantity of suspended matter. Later, under the writer’s direction as chief engineer of the Milwaukee Sewerage Commission, this statement w a fully confirmed by carefully controlled operation in the Tcsting and Control Station, where four complete miniature plant,s can be operated simultaneously and any operating condition varied at will. Doctor Wilson’s statement has often becn falsely interpreted as meaning that clarification by activated sludge is a purely chemical process, and that microscopic life is incidental, not essential. Research Work a t Milwaukee
Toward the end of 1929 the research lalmatory of tlrc Milwaukee Sewerage Commission took up the study of the fundamental character of the activated-sludge process. In the course of this investigation some facts of far-reaching importance were disclosed. It uas recogniieil that, the rnaiutcnance of aerobic conditions by means of bubbling air in laboratory experiments wa difficult, if at the same time &her conditions had to IJC strictly controlled. The use of mild oxidizing agents v-as therefore tried. In the experiment8 here described sodium chlorate was used. It was iound that sodium chlorate added to raw sewage i n a concentration as low as 0.05 per cent prevented the sewage from becoming septic. When the concentrntion was increased to 0.3 per cent the sewage clarified in from 2 to 4 days, the solids forming a typical sludge, which could in no way be disthgujshed from activrit,ed sludge, taken from the plant, when the sludge was in good condition. The term “clarification,” as here used, means that the supernatant liquor from which the sludge had settled was free from suspended solids and bacteria, comparable to the emuent of the plant when it was in the best working condition. The bacteria count of the samples of supernatant liquor was
311
0.3 0 3
0.3 0.8 0.3 R a r sewage
21 22
Raw Y
;x,%ex
sewage sterile s c w a ~ ris i i i ~ i i f i ~ i i ~itt i WAS , stciiiiied i n
0.3 0.06 15 pounds lor 20 minutes. the s t i l c x l a ~at~
from 100,ooO to 110,000 per cubic ci:iitimcter, that of tile raw sewage used from 3,500,000 t,o 4,000,000 per cubic centimeter. This clarification was :tccomplished only when the atmospheric air lrad free ai s to tlic surface of the liquid, and when the dept,li of the liquid was not too great relative to tlie exposed surface. Most of tlie experiments tabulated herein were maiic with 100-cc. siimples of 4.3 em. depth and having 23 sq. cm. of exposed suriace. If the sewage wils first stcrilized, either by lieirting in the autoclave a t 15 pounds for 28 minutes or with cliloroform. addition of sodium chlorate did not produce clarification, Addition of one drop of activated-sludge suspension to the heat-sterilized sewage caused it to become septic. Addition of both 0.3 per cent sodium clilorate and one drop of activated-sludge suspension caused clarification. The addition of raw sewage or plaiit effluent instead of the actirated sludge produced tire same result. Addition of supposed enfiymes, obtained by grinding activated sludge with sand and subsequently passing it through a Berkefcld filter or hy passing plant effluentthrough a Berkcfeld filter, produced no clarification in sewage to which chlorate had been added and to the surface of which air had access. When sewage bacteria and yeasts, obtained hy inoculating an agar slant, were introduced into lreahterilized sewage which contained 0.3 per cent of sodium chlorate, the sewage neither clarified nor did it becomo septic. Further addition of I cc. of raw sewage caused it to clarify. It was found that in activated sludge, when it was heated a t 60" C. for 30 minutes, all protozoa were killed hut many hackria remained alive. Inoculation with this sludge did not produce clarification in sterile sewage, which contained 0.3 per cent of sodium chlorate. Further addition of a drop of water coiitaining a single protozoan produced clarification in 1 week. If a small amount of sludge from this clarified sample, which now contained many iridividuals of one type of protozoa only, was added to sewage that had been first sterilized and then inoculated with bacteria and yeasts only, and to which 0.3 per cent of sodium chlorate hail heen added, clarification resulted in 48 hours or less. In all the experiments in which air was allowed access to the surface of the liquid, the neck of the bottle was closed with a sterile cotton plug to prevent contamination. When this plug was removed from a bottle containing sterile eew-
age and 0.3 per cent of sodiuin i:l:lurate, the sewage did 1101 clarify. Miarosc.opic cxaminatlim slrowcd that it contained bacteriir and ycasts but no proto. . The inRuenn: of tcmpernt.irre %as shown by keeping two sirmglos of raw scwagr, cscli dofled with 0.3 per cent, of sodiuni chlorabe, air having access through a cotton plug, one sample a t 10" C., the other at, 20" C. The first clarified in 10 days, tlie otlier in 2 days. 130th contained protozoa in abundance. I n ttre microscopic cxamination of tlie sewage and sludge used in t i m e eqxriments, tlie presence of algae was always noted. In one experiment tliese algae were killed by the addition of 50 p. p. m. of copper sulfate. Addition of 0.3 per cent of sodium chlorate to the sewage so treated and access uf air to the surface prodiired clarification. I n all the experiments so far d e scribed, except one, no a t t e m p t was made to work with special types of protozoa. It was found that one type, vorticella, can he produced in a culture which is pure with reference to protozoa but, of course, con t ai ns bacteria. The procedure was to heat s l u d g e a t 50" for Figure 2--Parameclum and a Smaller utes, which kills all Organism Attackinn a Particle of Sludge. 7u x protozoa except vorticella. In the one case to which reference was made, the organism was of a type wliich we have not identified. It was a ciliate ahout 0.020 mm. in length, shaped like an elongated pear, the neck of the pear acting like a snout or trunk. In experiments conducted by the writer in his own laboratory, a medium consisting of supernatant liquid from clarified sewage, to which a few grains of barley had been added, heated to 1M)" C . and kept at that temperatiire for an hour, waa inoculated with a few drops of a sludge suspension obtained from a clarifid sewage. I n this sample only two types of protozoa survived, paramecium and a similarly shaped, smaller form, from one-tenth to one-sixth the length
''
'lmin-
_.
Pipurr 3 ~ ~ A c l i rPsrsmrciurn e and Smatler Protoiiciracterinl life deeoinpose organic matter and leave it in a colloid state, in which it will not settle, hilt wiil remaiii dispersed. I tiat I,rotoooii are iieccssal'y ill t i i e p T O ( under t l i e coiiditions of tlic experimciits, seems fiilly demonstriited hy the results. 'She fact that clarificatiml does not m u l t when air has no acce~sto tlic w~rfaccof the liquid, together with the fact. that putrefaction is prevented by the addit,ion of sodiirrn clilorate a i d e bacterial life persists, demoirstrate tlrtit the bacteria ea11 utiiix! the cllemioally ixiiind oxvzeii &;reas bhe ixotozoa milst have oxygen in " ,, solution iri the water. One inicrosconio observation oftellmade in the course of this work is significant. When protozoa die, either by drying
/.
Pieurr 4 -Yorriccllit Attached f n Sludge.
400 X
eflwtiw clarifiixttiim Ijy settling is in indirect proprtio,) to the ainoiint US solid limiter i i i stispcnsi(m iii the mixed liquor, is i:onfinned by these esperirrients froin a new piiiiiL of view. :\ giveii quantity of disporsed orpaiiio Iriatter (iu the raw sewagi.) will be converted by a greater nunrber of organimrs iii D sliorter time, if in both cases they air equally x-igoroiis. This work, having produced definite results: also indicates t,lic iitxxl of further research. I'articoiarly, the relative effectivoocss of differmrt types of protozoa, the influerice of suhstatices found in industrial wastes, and nieaiis of keeping the life iii the sludge liealtlry and vigorous sbo~ildhi: irivcstigated iiiider rigidly cont.rollcd conditions. Acknowledgment
Slost, of the work described herein \vas periuriiied in the research laboratory of tlie Sewrage Cornmission of the City of Milwaukec, iiiider the writer's supervision and the direetioii of John Arthur Wilson. The laboratory work was perfiritled I,y Hiliiieyer Crhen, research eliernist. h small a r l i ~ n l n tof the work w a s rforie in the writer's uwii lahorrttorJ-. 'l'lianks are tqmially due to I h c t o r Wilson for putting at tho writer's disposal his elaimrate mieri~pliotographieal qnipmeiit,. The plwtograpliic work mas done by the writer's WII, ltoiwt J. Craincr. Literature Cited
