...
Industrial Wastes
R. E. GREENFIELD AND G. N. CORNELL A. E. Staley Manufacturing Company, Decatur, Ill.
W. D. HATFIELD Sanitary District of Decatur, Ill.
b b In the manufacture of starch from oorn by the
’
Recovery system were developed that proved of economic adwet milling prooess, sewer losses were extreme in the vantage to the plants using them and also gave relief from the early days of t h e industry. T h e increasing gravity of stream pollution standpoint. Several patents covering details t h e stream pollution problems in this country caused of these dcvelopmentswere granted, and much litigation resulted a great deal of activity in this industry aiming tofollowing the issuance of these patents. It is not possible to menward t h e reduotion of such losses. The outstanding tion and give credit to the many individuals who contributed to development in the reduction of these wastes was these de&apments, although the names of Widmer, Bartow, the development of processes which re-used waste Jeffries, Sjostrom, and others come to the mind of anyonefamilwater in place of fresh water. This resulted in the iar with the period. recovery of most of the soluble loss.- in t h e form With the settlmg of patent difficultiesin the early 1930’a, the of valuable feedstuffs. The small residual wastes muse of certain waste waters in the plants following what came left after t h e development of this bottled-up process to becalled the “bottled-up”systemwasrathergenerallyadopted; are adequately disposed of by standard sewage losses were then reduced to less than 0.5% of the dry substsnce methods when mixed in about equal proportions of the corn. In most cases this so decreased the sewage pollution with ordinary domestic sewage. This paper sumload as to bnng it mthin the range of ordinary sewage disposal marizes the results of these early developments and a 1 practices. illustrates t h e m hy t h e actual conditions prevailing ,?”?. This ~. a.p e rreviews recovery and subsequent treat. practices . in one wet milling process plant and in one sewage merrt of rrmxininp.plaut wm1Ps as thpy exist today. .bmiicli as disposal plant receiving t h e wastes from this plant. $??,‘possibleIhi4 review will represent a fair cross section of rnduatry,~,&widc prortices; but sincc there arc individiral differences and variation in d e t R i l s o i t h P ~ ~ o C ~ f i o m o n e p l tomother,itnlll xnt represent particular conditions in the one-plmt and sew& disHE problem of recovery or prevention of wastes from facp a l works with which the authors are direotly connected. tones producing starch from corn is an old one. Formerly THE PROCESS tbe only material produced was starch, and all the rest of the Figure 1presents a flow sheet of a typical plant illustrating an corn kernel became waste. Since the rest of the kernel consisted application of the bottled-up system and the recovery of valuable ofalmost 25% of the dry substance content of the corn and that componentsfrom the corn. This process consists in steeping airportion containing all the nitrogenous portions of the corn subcleaned shelled com in a dilute sulfurousmid solution in prepars,, stance, this was a formidable amount of material even for the tion for milling, the multistage wet milling itself, and the subsesmaller factories then in existence. An early commentator assures quent separation from one another of the principal eonstitu+ of us, “You could always fmd a starch factory by using your nose.” the milled com, partly by flotation and partly by wet screening. The recovery of solid feedstuffs came early and reduced the The milling and separation processee are performed within the loases to 7-8% of the dry content of the corn. Later evaporamillhouse and produce streams of a mixture of starch and gluten, tion and recovery of the steep water solubles reduced the lossea called mill starch, germ, coarse fiber or bran, and fme fiber or grit. tofrom24%of thedrysubstanceofthecorn (1). Commercially ALSO within the millhouse the last three components named are the r e s u l t i i yield of 9648% seemed fairly good, but the stream subjected to countercurrent washing processes which strip from pollution effectfrom even a small factory could still assume the them as much starch as possible for recovery with the millstarch. magnitude of that from a city of a quarter million to a half million Subsequent stages in the operations are indicated on Figure 1. people. Early in this century the focus of community attention The four principal points at which water enters the process are on these problems imposed pressure on the industry to reduce at the steeps for steeping the corn, at the milling process to serve stream pollution. While earliest work was directed toward as a carrying medium for the corn in the m i l l s and the subsequent chemical and physical treatment of the starcb plant wastes, separations, in the millhouse washing processes for stripping Wagner (6) concluded‘in 1911 that this attack alone would not starcb from feed components, and in the starch washing procesa solve the problem but that, rather, further process modifications for removing solubles from the starcb. In the early days all of to prevent the production of such wastes would %the only satisthis water entered as fresh water and was discarded to the &ewer factory solution. when it had served its purpose. The waste prevention systems which were eventually worked Evaporation of the steep water recovered a major portion of out and are in we today fully confirm the validity of this early the &7% of the soluble matter of the corn. Most of the rest opinion,.although it was not until the early 1920’sthat real progof the soluble matter enters the millhouse processes and evenress WBS schieved in some plants, such as those at Cedar Rapids, tually appeam dissolved in the waters discarded from those procIowa, Argo, Ill., and, to some extent, at other p h .
..
T.
!?a:I
H E M IS T P Y
Vol. 39, No. 5
water, and this once-used starch wash water serves to Supply the make-up necessary as a result of evaporation of Steep water. Since this water is of low soluble concentration, it is generally used in millhouse processes +hioh are most critical. This further recovery of solubles has added to the nutritive value of the feeds, since they are known to contain'valuable constituents. More recently these same steep water solubles have been shown to be the most successful nutrient for production of penicillin mold. The figures shown on the flow sheet indicate the water quantities involved in the typical wet milling plant and reveal the extent to whiob is achieved a balance of water input as fresh water and water output by evaporation, drying, and in product. Possibly the key figure is 5.5 gallons ofsteep water drawn per bushel; although the figure varies slightly from plant to plant, this is the amount which must be replaced with the dilute starch washing filtrate to ensure best operating mults. Draw muchin excess of 'this quantity probably has questionable benefit an operating results and requires the addition of a corresponding volume of fdtrite so dilute that recovery of its soluble content by evaporation is not economie. LOSSES FROM BOTTLED-UP PROCESS
'
Losses from the wet process m a of this plant result normally from the failure to achieve a balance between water input and output and as entrainment from the steep wateievaporato?, and abnormally as s p i h resulting generally from breakdowns. The first ntmed is t e d "process water lorn"; in thk plant it can be made to &wage 2.0 gaUons per bushel and can usually be con6ned to least concentrated starch washing filtrates by p.mper use of a 200 Figure 1. Flow Sheet of Corn Wet-Milliap Pkcess gallon water surge storage center. .Inactual practice variation from this amage lass may.be considerable became of factors esses, the major portion being the water separated from the wlnch mult in nonideal operatiou. 6ince this paper illustrates gluten which carried from 1 to 2% of the dry substance of the approaches employed by the wet milling industry to a general problem, some of these factors aze discussed briefly. cor9 to the sewer. . , , . , , MOISTW CONTENT OF CORN. The moisture content of corn ground in these plants ma vary from 13to 14% to as high as 24 AWLICsLTXON OF 5OTT.LED-UP PROCESS . , or 25%. An increase of &at magnitude am0un~sto almost ode gallon of water per bushel of corn hnd is a large factor, compared The appealing prospect of recovering the rest of the soluble to an awra loss of 2 gallons per bushel. matter by the re-use of the millihg process waters in a closed STEEP% & E R EVAPOEAnON RIF~+ICULTIES. &OSW Of the circuit led ultimately to the development of the bottled-np procdiversity of corn types rocessed, S h p water varies considerably ea. That this did,notoccur earlier in the growth of the industry and m y introduce di&culties which change the heat transfer characteristics of the evapratms and thereby cause reduction may have been paitialiy due to the lack of m economic and effiin capacity. This mu increase not only the direct loss of starch cient meam for Washing starch free bf solnblea. The abundant wash waters to the sewer hut, by disturbance of soluble levels in use of fresh water produced starch of a soluble content sufficiently the processes, may also increase the wash water requirement for low =''that it could be dried and used directly, and only' o c c e stsrch washing. POORGLUTENFILTRXTION. This same diversity of corn sionallf were efforts made to wash it in plati pkses.' types CaU8eS poor gluten fdtration which incresses the load on The oyclie use of water ,in the bottled-up process inereased the the feed dryers, sgsjn disturbing the water and soluble balances. soluble impurities in the finished starch 'And made essential the v4RlATION IN SPECIAL PRODUCTS. c&h Specid products we of a starch Washing process. Rotary vacuum filters which may take more or less wash water, so that the water balance ia again disturhed. we& utiliaed-in the mining industry filled this need,. Continuous These vanations in the dewatering process and in wash water withdrswal and evaporation of steep witer in the existing prbceSs were found 66 provide adequate me&for preventirqhuild-up of r e q m m e n t may result in dislocations of as much as 3 or 4 gallons solubles to levels which would impair the efficiency of m i l h g ?nd per bushel in the water balance and illustrate the variety of facseparation processes. .Of the foufpiincipal points of Use qf water tors which cam deet the loss problem. In the corn wet milling in the process, therefore, three-millhaw+ separation, millhouse mdustry, each Case of dislocation of the water balance usually washing, m d steeping-are supplied by re-use of glnten settler presents an individual Droblem, and effective control of 10lied wit,h frmh water. The starc
May 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
Table I. C a l c u l a t i o n of R a t i o of A c t u a l Sewage S t r e n g t h to C a l c u l a t e d S e w a g e S t r e n g t h f o r Tj-pica1P e r i o d ( O c t o b e r 1916) \\-t.,
7.b
Ilel!l
wiiter Feed l?(>ii.e ex s v ~ ~ r a r r ~ r Bz,lie nn.h I i i i L i ,.oi::,red fi,r
P T t i ..P
24 Hr. 1400
Factor. P.C 1.b.
1100 2800
3800
4
10 8 4
C:ilhd. i e a - 2 g e 3 r rengt h , 1' I: 2 4 H r . .5.600 :31,000 8.400
'fal,le 11.
Capacitiep of Treatrnent Plant t - n i t s
Secondarv 5ettline tank c a n a c i t r . c u . f t . Area, sq. it. D.P. before 1546, h r . D . P . d u r i n g 1946, hr.
42 9-12 12-16
ventional problem and is similar to the steep m-ater evapot,ation. CONTROL O F P L A N T LOSSES
In the particular plant with nhich the authors are f:tn:ili:ir,
CODE
18-10 0 i-1 2 5 1a1,500 I) 4-1 73 41,000 45,900 169,000 10-19 4-8 118,600 12.000 3 acres Y 6 I t deev. 1-1 3 I:: .rune :30,700 4430
0 2-5
0 4-1.0
1.azonn ~- - - ~ ~
Area. acres Depth, i t . Drying bed a r e a , -q f t .
:ire then diverted t o the sewer. Control of these bone xvasli losses is ,simply a matter of close operator attention t,o the cutoff point. Loss by entrainment from the corn sirup evaporators is a con-
control of plant losses really means their reduction. The, first requi-ite of loss control iq a knowledge of v-hat the l o s e ? actunlly are, and where and n-hen they are incurred. To achieve this knon.ledge, we h a r e set up a rather elaborate proceduw for metering sampling and controlling losses 1vhic:h is under the tiirect full-time supervision of a chemical engineer. The following meterit!g :\lid control points are indicated by letter on Figure 1:
13.200
80,200 67.000 1 11
11:~rcepiing s e a e r s , n~ilh,,:. EJ:. d:>y rota1 capacity llaily dry weather flou T>aily a e t weather f l o \ ~ Daily storm w-ater f l o ~ Iiiihoff t a n k s Detention period ( D . P . , ilr. Digestion capacity, cu. f t . Storm-water-tank D.P., hr. Primary heated digestion capacitL-, cu. i t . Secondary unheated digestion capacity, c u . i t . A c i i r a t e d sludge aeration paparity (A.C.), cu. i t . aeration of sludge ( D . P . ) ,hr. .A.C for aera:iun of mixed liquor ( D . P . ) ,hr. rling capacity, 1 Dorr tanks, r u i t . face area, 2 Dorr ranks. s q . fr.
585
8-10 53.000
The other normal source of loss from thg wet process is in the entrainment from the steep water evaporators. Tliia loss coniponent is affected by the conventional factors of evaporator design, vacuum, liquid level, maintenance, etc., and in addition by a variable tendency of steep water t o foam. With a well designed evaporator station, t h e control of these losses rvithin reasonable limits is up t o t h e operator and is a matter of providing him with such aid as he needs t o maintain optimum conditions. In addition t o t h e loss b y entrainment a substantial source of lo.-. taken from a number of key points in the sewer system. Cnusual results are reported immediately t o t he responsible people. To complete the picture, the loss engineer has available the volume of water entering the process a t the various points and the -tee11 water draw, and d a t a which can Le employed t o calculate the amount of evaporation from the various dryers. H e is thus able t o prepare a water balance similar t o t2-A shoivn in Figure 1 for any given period.
OTHER LOSSES
T\vo other principal sources of loss from a corn wet niilliiig plant are in bone char filters and evaporators in tlie glucose refinery. In the sirup decolorizing process employed in most refinery procesceB, t h e sirup is filtered through bone char. .ifter the filtration portion oi tlie cycle it is necessary t o backn-n.;h t o bone chiit to recover the s i r u p adhering t o it before later regeneratloll of the char. This is done Tvith water, and the major portion of the sirup is recovered by suhseqnerit concentration. B:~ckn.nsli waters rencli a point u-here recovery of t h e sirup ia not economical, in tlii. plant a h u t 30 pornids per thousand gallon., nnd the \v:tters
1943
F i g u r e 2.
1944
I945 PERIOD R a t i o of i c t u a l to C a l c u l a t e d Population EquiTaIeiit, 1913 through 1915
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Figure 3.
Flow Sheet of Sewage Treatment Plant
Armed with this knowledge, the logs engjneer is able to spot rather rapidly the particular factor responsible for a poor result. For example, this plant recently had unusually high losses of
Vol. 39, No. 5
process water. Examination of the water balance indicated an excessive usage of fresh water a t the starch washhg filters. Investigation revealed that the prescribed filter medium for this operation was not available and that a substitute material did not permit operation of the 6lters to produce a normally thick cake. This increased the ratio of spray water to starch washed and dislocated the water balance. The immediate remedy was to increase the steep water .@w; the permanentremedywastohda b&tm6ltermediuma search which, incidentally, resulted in the prescription of a medium better than the one originally prescribed. One other technique employed by the logs engineer is the comparison of a calculated plant effluent sewage strength in terms of population equivalent (P.E.) per day with the actual sewage strength of the effluent. Factors have been developed which relate weight of the loas wmponents to P.E. per day. This P.E. is totaled to give the calculated P.E. for the effluent. The actual P.E. is then divided by the calculated to provide a ratio for wmparison. A sample of this calculation iS shorn in Table I. This ratio is maintained on a current chart and normally varies only mcderately a b v e or below 1.10. Any significant variation in this value indicates either a hasic change in the procem, an important error in the loss accounting, or the presence of unusual wmponents in the unacwunted for losses. Figure 2 illustrates an actual case where a variation in this ratio showed up. Its canse waa run down and eorrective action taken. I n brief, the change in ratio occurringin the Summer of 1943 coincided with a basic change in steeping process
Table 111. Treatment Plant Loading Data 1
Date 1928 1929 1830 1831 1932 1933 1934 1935 1836 1937 1938 1939 1940 1841 1942 1043 1944 1845
1940 Sa". Feb. Mar. Apr. May June SUlY Aug. sept. oct. NO". DW.
A"
2 3 ImhoB Thnks Lb. settleable Diger solids/ tion CU. f t . temp.. mo. * F. 1.78 79 1.35 73 1.76 75 1.87 74 1.60 71 1.91 2.19 2.13 77 2.39 .. 2.38 .. 2.46 2.52 ii 2.61 75 2.59 08 2.93 75 3.04 75 3.50 73 3.25 75
e
3.90
3.34 3.74 3.34 4.04 4.31 4.01 4.12 3.87 3.48 4.07 6.09 4.07
04 08
67 67 67 80 88
88 83 85 74 74 73
4
5
Amation Plant InEuent Lb. B.O.D./ B.O.D.. 1OOOou.ft. p.p.m. A.C. 236 89 152 75 111 56 124 54 151 80 183 145 (138) (37)
tEY
7
6
Effluent p.p.m.
138 110 15 81 123
$%I (110) ... ...
p.p.m,
0 0
0 0 0
0
0
0
ci6)
(28:4) (71.5) 24.8 26.9
21 14
2.0 2.0
20.2 29.8 31.4 29.6
23 20 22 22
2.3 0.7 1.0 2.2
86.2 8S.l 84.8 84.3
25.4 30.5 34.5 30.8 34.7 34.4 33.8 31.5
is
3.3 0.8 1.4 0.6 0.4 0.2 1.2 1.3
&:e
cig) 115 155 117 144 140 101 119 125 139 152 168 203 226 153
...
...
... ...
... ... ...
18 20 10 11 25 24 20
...
8
10
11
12
... ...
... ...
... ... ... ...
'
TrioHiing Filter and Settling Lb. B.O.D. Influent B.O.D./ Effluent ~omovd. B.O.D.. 1MX)au. B.O.D., NOI. % p.p.m. ft. stone p.p.m. p.p.m. 138 14.5 21 4.9 43.6 110 13.9 29 2.3 27.7 15 15 3.8 7.3 23.6 1.8 34.5 81 7.4 15 3.2 125 11.1 17 18.3 3.2 122 12.1 18 20.0 110 11.6 21 1.5 24.2 135 13.9 3.9 23 20.3 158 18.3 30 1.8 124 15.3 28 ' 2 . 4 2.3 iaz is.6 18 1.1 148 16.1 26 2.6 179 19.4 27 21 1.1 102 20.6 1.6 156 17.1 26 Ti:& 1.1s 486 88 20.8 56.2 2.9 27 161 15.7 87.0 1.1 38 156 13.9 91.0
... ... ... ... ...
... ...
...
8
... ... ...
05.6 85.6 89.5 93.0 87.1 89.4 86.9
156 117 144 140 101 118 125 139 152 158 203 220 133
14.4 16.8 13.1 11.1 10.0 11.6 11.8 13.0 14.8 12.4 18.0 20.8 14.2
35 39 36 27 23 17 15 22 22 18 20 31 20
2.4 1.8 8.1 4.0 4.4 1.9 3.7 3.8 3.2 4.4 2.6 4.5 3.6
13
14
B.O.D. 2%
mod.
% 84.9 73.6
80.0
80.2 88.4 84.4 80.9 83.2 81.0 77.4 84.3
82.4 84.9
87.0
83.3 46.0b 83 2 74.8
TYPeOf operation sarias Series series
Series series
saris
W O S
Baiaa No m a t i o n No aemt!on Noneratlon No aeration
Noasration Noaenrtlon Parallel Seriea
Parallel Parallel
00.6 73.0 80.7
Pmaklel Parallel Parallel Psrallel
85.6
Paralld Parallel P*r& Parallel Parallel Parallel Pa*allsl
77.1 11.3 05.7 88.0 84.2
83.6 87.2 88.5 83.0
Parsllel
Data 'n paranthasas are averages of lass than 12 months o eration due to 5-day weeks breakdonna floods. eto. 6 In 1 8 b the plant wss Eooded. the mters were clogged wit{ silt, and the filter eSiuent'effiaienoiasw'm greatly reduoed. This oondition not aompletely Dveraome until Maroh 1946. Aeration plant shut dawn became of b r o a a o u t during May 1946.
May 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
587
Reference t o work by Pulfrey et nl. ( 4 ) led t o the conclusion t h a t this change had resulted in a substantial increase in the volatile constituents of the water from the steeps, and that these were responsible for about 30,000 P.E. per day. I t did not show in our regular accounting procedure because the principal analysis used is a solidi determination by evaporation t o dryness. These findings led quickly to the development of n stripping toxyer by means of which the volatiles from the condensate were sent to the atmosphere: this resulted in a reduction of sewage strength by about 20,000 P.E. The ratio also dropped back near its normal value of 1.1: this indicated that, the ratio was not 1.0 because about the same amount of volatiles was present in the steep water before the process change as x-as left in the stripping tower effluent. STARCH WASTE-SEB-.kGE 1 I I X T U R E S
The seFage from Decatur, 111. (65,000 populntion), is treat,ed by the Sanit,ary District of Decatur. a municipal taxing body with authority t o build and operate intercepting sewers and sevage treatment facilities. This sexage contains the normal domestic wastes from the city and industrial wastes from various metal works, foundries, plating works, and other smaller industries which do not adversely affect the sewage concentration. Plating \Tastes can be quite detrimental, but a t Decatur the control by the industry has made losses of chromium, copper, and cyanide of minor importance. In addition to these minor wastes are the w from the large corn processing factory described. The volume of t'his waste equals that of the city sewage largely because of the condenser water from the steep Ti-ater evaporators. The soluble organic matter which escapes the recovery processes, when expressed as population equivalent, equals or exceeds that of the city itself. The original selvage treatment plant a t Decatur (1) wis placed in operation in the summer of 1921and consisted of grit chambeis, Imhoff tanks, three acres of trickling filters, and secondary sedimentation. The capacity was liberally designed for a population of 60,000, since the city a t that time had only 45,000 inhabitants. During the design and construction period of the intercepting sell-ere, the starch Tyorks grew a t an unexpected rate, and the sea-age received a t the plant in 1924 had a population equivalent exceeding 300,000. In 1925-26 the Sanitary District operated a serl-age testing station (1) to find out if the sewage-vaste mixture could be treated by a short-period activated sludge process followed by trickling filters, and to study the possible loadings such a process could take. -.It the same time the Staley IIanufacturing Company began the st,udy of waste recovery processes. The result of these cooperative efforts culminated in the recoveries by the industry described in the first part of this paper, and the construction by the Sanitary District of a pre-aeration or short-period activated sludge plant which increased the treatment plant capacity to 150,000 population equivalent. This plant was placed in operation in 1928 ( 2 ) . During 1932 through 1934 the plant was operated four t o five days a week, depending on the grinding operation a t the Staley plant. In 1935 the aeration plant was discontinued and the entire load taken by the original Imhoff trick-
LOAD IN LBS. B.O.R/lOOO
CU. FT. C A R C I T Y
Figure i. .ictivated S l u d g e and Filter Loadings
ling filter plant. In 19.12 industrial activity nude it necessary to resume the operation of t h e aeration plant. These years of operation have proved that when residual corn product wastes are mixed with about an equal volume of domwtic sewage, they can be satisfactorily treated by the activated sludge procew, on trickling filters, or on a combinaxion of the t x o in aeries. During parts of 1947 the production 3.t the industry has been larger than the evaporation capacity, arid higher losses t h a n normal have been received. This affords an interesting study of capacity loading of the sewage purification units. TREATMENT P L A S T
Figure 3 is the flow sheet of the sewage treatment plant, Inid Table I1 shows the unit capacities. The normal sewage flow of 9 to 18 million gallons per day passes tlirough the grit chamber, then through the Imhoff tank for primary sedimentation and sludge digestion. The settled sewage may Tiass directly to the filters, directly t o the aeration tank, and thence to the filter.; in series, or the floa- may be split betneen the aeration plant and filters, each operating independently in pnrdlel. The activated sludge may be returned to the aeration tanks or to sludge reaeration tanks, and waste activated sludge may \>I? returned t o the influent of the Imhoff tanks or to the primrlry digestion tank. When desirable, digested Imhoff sludge or separate digestion
588
I N D U S T R I A L A N Il E N G IN EX R I N G C H E M I S T R Y
tank sludge may be pumped t u the activated sludge reaeration tank. All supernatant liquor from the primary or secbndary digester is returned to the sludge reaeration tank. Digested sludge of any type may be p u m p d t o theJsgoon or to the drying beds. Humus sludge from t h e trickling filter plant is pumped to the sludee reaeration tank or mav " be Dumned to the influent of the Imhiff tanks. For the Grst ei&t - vears _ and wain in 1943 the nlant was OD-ated in series, except for numerous experimental rum described by Hatfield (8). In 1942 and from 1944 through 1946 the a c r e tion plant was operated as an activated sludge plant run parallel with the trickling filters. By returning digested sludge and supernatant liquor from the sludgedigestion tankto the activated sludge reaeration tank, the sludge index of the activated sludge has been rather easily controlled, a nitrifying sludge has been developed, and B.O.D. loadings have been considerably increwd. Some of the loading data on the plant are s d a e d in Table 111. The figures are not complete in that sewage flows, detention periods, sludge returns, sludge conoentrations, sludge indices, and numerous operating data are omitted. Such date should be included in a paper gn p l h t operation, but for the purposes of tbis joint paper only the biochemical oxygen demand unit loadings are given with influeut and effluent analyass, which show the efficiencies of B.O.D. removal and the quality of the h a 1 effluenta produced.
. .
.
~
,
LOADINGS ON DIGESTION
~
j
.
In Table 111,column 2,the loadinga of sludge solids on the digestion compsrtmcnt of the Imhoff tanka indicate an increase from 1.35 to 4.07 pounds of dry settleable solids per cubic foot per month, the annual average digestion temperature being about 75' F. At these loadings and temperatures a well digested sludge is obtained which drys satisfactorily on the sludge drying beds. The analyses of the digested sludge vary during the year RE follows: dry solids &13%, pH 6.4-7.0, volatile solids 3545%. The relatively hi&.digeation tempexat-, are due to a raw sewage temperature of 80' to 100' F. The lower.average digeation temperatures from January through May are due to cold storm water temperatures and melting snow. Theseloadings on the Imhoff tank digestion compartment include dl the daily settleable solids, practically all the storm water settleable solids, and about 45% of the wastssctivated sludge.' The other 55% of the activated sludge has been wasted to the heated primary digestion tank at an average loading of 1.9 pounds of solids per month per cubic foot of heated digestion tank capacity at 86' F. Recent changes in piping have made two-stage digestion possible; this cute the loading to 0.95pouhd per cubic foot per month, including both heated and .unheated digestion. It is hoped to incresse the efficiency in operation of this digestion and thus reduce the waste-activated sludge load to the Imhoff tanks. WADING ON ACTIYATED SLUDGE PLANT
The loadings and efficienciesof the aeration plant are shown in Table I11 (columna 4 through 8) and Figure 4. 'During 1928 through 1935 and a&~ in 1943 the &fition plant was operated on the total flow of the settled sewage, the effluent from the aeration settling tanks flowing in series to the trickling filters. This method of operation produced loadinga in the order of 75 to 140 pounds of B.O.D. per loo0 cubic f&tof aeration capacity with correspondingly high effluent B.O.D. values. The 6nd effluent from the trickling filter settling tank was satisfactory; it contained 20 to 30 parts per million of B.O.D. and 2 to 4 p.p.m. of.nitrate. However, the sludge produced by thie short-period eeration had a pigpen odor, and the general odor conditionsabout the aeration plant were not so satisfactory as those about a conventional activated sludge plant. when the aeration plant i8 operated as a conventionala&vsted
Vol. 39, No. 5
sludge plant pardld with the trickling filters, the odor hmard is reduced, but the ease with which the process is upset by shock loads makes its operation difficult to control. Shock loads cause sludge bulking, the control of which is the key to the suecessful operation of activated sludge planta, particularly if they are loaded near or above the loading limita. The data on parallel operation in 1942, 1944, 1945,and 1946 show an increasing trend in loading from the usually accepted 25 pounds of B.O.D. per 1000 cubic feet of aeration capacity of 35 pounds. These data arc average ' m u d and monthly ligures. Many d d y loadings of 40 to 45 pounds of B.O.D. are handled satisfactorily, hut a week of such loadings will show a deterioration iooperation and 6nal effluent. These higher loadings are made pomible by reaeration of the ret- activated sludge for 10to 19 hours, feeding this sludgewith the high nitrogen containing supernatant liquor (250 p.p.91. of ammo-) from the sludge digestion tanks, and increasing the specific gravity of the acti,vated sludge with well mineralized digested sludge. By these last two procedures it is possible to control the sludge density and almost entirely prevent bulking of the activated sludge, and its attendant difficulties in plant operation. A similar use of supernantant liquor and digested sludge was reported by Kraus (3). His reaeration of the activated sludge with digested sludge is accomplished intermittently, whereas $t Decatur we have preferred continuous flow. It is expected that further improvements in the technique of this reaeration will all?, some further increase in plant loadin@. TRICKLING FILTER WADINGS
The loadings and efficiencies of the trickling filters are shown by the data in Table 111 (columns 9 through 13) and in Fi@e 4. With B.O.D. loadings up to 20 pounds per loo0 cubic feet of filter stone (875 pounds per acre-foot) satisfadory effluents were obtained. Individual days at loading of 27 pounds have been handled satisfactorily; however, continuous average loadings at 20 p o d s during December 1946 and January 1947 caused some poudingof the surface of the filter8 because of too heavy biological p w t h on the surface stone. These loadings are higher than the u s u d y recommended 11.5 to 14 pounds B.O.D. per loo0 cubic feet and are undoubtdy'attained because of the greater biological activity at the higher sewage temperatures which prevail throughout tbe year. "FIE RIVER
The paper would not be complete without a few words about the Sangamon River, which receives the effluent from the sewage treatment plant. From 1914 until 1928 the river was grossly polluted an& during periods of low water was septic for 25 miles downsham. Since the waste recovery system and the activated sludge addition were placed in operation in 1928, the river has been in good condition except for a relatively few times due to mechanical fdures at the industry or at the sewage treatment plant. These results have been due to a true cooperative spirit IV and the Smi-
(1) Greeley, 6 . A,, and H a t f i e 1 ~ : B . . Tmns. 94.54-99 (1980). (2) Hatfield. W. D., S e w e Woiks 3..--3 (3) Kraus. L.6..ZW.,17, 1177-
(5) Wagner, T.B.,Ibid.
3 h
.
E.,Im. Em?.
'
