INDUSTRIAL AND ENGINEERING CHEMISTRY
226
cooking for 12 hours at 170' C. or for 2 hours a t 186" C. yielded equal quantities of cellulose a t the same concentration of sodium hydroxide. The percentage losses of Cross and Bevan cellulose are given in Table V and Figure 3. These data show that, a t the higher caustic soda concentrations, the losses of cellulose in these residual woods were equal or nearly equal to those sustained by cooking with water only. The destruction of Cross and Bevan cellulose depended to a great extent upon the pH of the cooking liquors, being greater with larger hydroxyl-ion concentrations. The residual woods of the trisodium phosphate cooks retained more cellulose than was found in the residues of the corresponding cooks made with water only or with any of the salts used in this series of investigations, except those made with sodium sulfite. The cellulose yields of the trisodium phosphate cooks were approximately equal to those obtained by cooking with sodium sulfite. ALPHA-CELLULOSE. The quantities of alpha-cellulose found in the residual woods or pulps of the sodium hydroxide cooks are given in Table IV and Figure 2. With 15.1 per cent caustic soda used in cooking, the yields of alpha-cellulose were greater than those found for the corresponding water cooks, but the reverse was true at the higher sodium hydroxide concentrations, except in the case of cook 91 (12 hours, 186" C.). The increases in time and temperature of cooking resulted in decreasing the quantities of alpha-cellulose in the residual woods of the caustic soda cooks. The alpha-cellulose contents of these pulps were generally higher than those of the sodium sulfite residual woods. They were higher than
Vol. 26, No. 2
the quantities of alpha-cellulose found in the sodium sulfide cooks at the lower concentrations of cooking chemicals, but lower when the amounts of chemicals used in cooking were doubled. The amounts of alpha-cellulose destroyed, or rendered soluble in 17.5 per cent caustic soda, in cooking with sodium hydroxide are given in Table V and Figure 3. It is apparent from these data that, while a certain hydroxyl-ion concentration of the cooking liquor is necessary to protect the alphacellulose against hydrolysis or transformation into betaand gamma-celluloses, an increase in the pH of the cooking liquors tends toward increased degradation of this constituent. The destruction of alpha-cellulose was greater when trisodium phosphate was used as the cooking agent than when water alone was used. Increasing the amount of salt used in cooking caused an increased destruction or conversion of the alpha-cellulose in the residual wood. LITERATURE CITED (1) Aronovsky and Gortner, IND.ENG.CHEM., 22, 264 (19301 (2) Ibid., 22, 941 (1930). (3) Ibid., 25, 305 (1933). (4) Ibid., 25, 1260 (1933). (5) Ibid., 25, 1349 (1933) (6) Ibid., 26, 6 1 (1934).
RECEIVED August 12, 1933. Presented before the Divisioa of Cellulose Chemistry at the 84th Meeting of the American Chemical Society, Denver, Colo., August 22 to 26, 1932. Published with the approval of the director ae Paper No. 1202, Journal Series, Minnesota Agricultural Experimeat Station. 9.I. Aronovaky is the Cloquet Wood Produota Fellow, Univeraity of Minnesota; the fellowship waa established by the Northwest Paper Company of Cloquet, Minn
Prefiltration Treatment of Sewage Sludge F. W. MOHLMAN AND G. P. E D W A R D S , The Sanitary District of Chicago, Chicago, 111.
A
TREMESDOUS amount of research has been carried on in various sewage works laboratories in the past several years concerning the Drefiltration treatment of sewage sludge. Practically none of the results of these investiga-
A
Parious chemicals and filter aids w e used to condition sludge prior 1; fifiltration. A large number of such filter aids are tested with cornparatine data on rates Of filtration through Biichner $filters. T h e materials used are cornpared as to rates of filtration, economy, andpracticability used with various types of sludges, bofh undigested and digested.
been but they are in the files of various municipalities or concerns interested in the mechanical dewatering of sludge. Here are described briefly the results of some preliminary laboratory investigations made during the winter and spring of 1931-32 in the Main Laboratory of the Sanitary District of Chicago in which various chemicals and conditioning agents were added t o several types of sewage sludge followed by vacuum filtration on Buchner funnels. This work was augmented a t the time by tests on small Oliver vacuum filters, and later by operating results with a large &foot Oliver filter. This paper, hoxever, deals only with the Biichner tests, since a large variety of materials were studied and their value as conditioning agents determined. The outstanding value of ferric chloride as a conditioner for sludge was discovered by John Palmer, in the Calumet Laboratory of the Sanitary District of Chicago, in 1925, and reported by Mohlman and Palmer.' Since then ferric chloride has been used in practically all sewage works where sludge has been filtered on vacuum filters. In the past few 1
Enp News-Record, 100, 147-50 (1925).
years 'increasing efforts have been m a d e to find some augmenting or cheapening agent to l,Se with f e r r i c c h l o r i d e . Consequently many m a t e r i a 1s have been tried in conjunction with ferric chloride. Most of the tests were made on mixed activated and settled s o l i d s , a b o u t o n e t o o n e on the dry basis. Tests were also made with settled and activated sludges separately, Imhoff sludges, and sludges from heated separate digestion tanks. Although the differences in characteristics of these various sludges with regard to conditioning were studied during the Biichner tests, the main purpose was to test the use of various conditioners on undigested activated plus settled solids rather than to determine the ferric chloride requirements of various types of sludges. ~
TECHNIC OF TESTS Samples of sludge were treated with various amounts of conditioning materials, mixed, and poured on a Biichner funnel 9 cm in diameter. Whatman No. 1 filter paper was used, and the vacuum was held at approximately 20 inches (50.8 cm.) of mercury. The filtrate was received in a 250-cc. graduated cylinder, whereby the volume of filtrate could be read at frequent intervals. The end point, when the cake cracked from the sides and the vacuum broke, was noted. The tests in most cases included determinations of moisture contents of feed and cake, volatile matter, pH, and iron content. Altogether about two hundred tests were made, of which only a few representative data sheets are included here.
AND ENGINEERING CHEMISTRY
INDUSTRIAL
February, 1934
TYPESOF CONDITIONERS There are two broad divisions in the types of conditioners or filter aids that have been used in this and other laboratories-namely, chemical coagulants and physical filter aids. The former consist of chemicals which flocculate the colloidal solids by formation of a chemical precipitate or by adsorption; the latter consist of more or less inert solids, which by surface or matting effect may improve filtration. The coagulants and filter aids tested were selected with due consideration for cost, although in some instances relatively expensive coagulants, such as aluminum chloride or titanium tetrachloride, were tested in order to demonstrate the coagulating power of tri- or tetravalent metal salts. The following chemical coagulants were studied, either alone or in combination with ferric chloride: ferric chloride, ferrous chloride, ferric sulfate, chlorinated copperas, sulfuric acid, nitric acid, hydrochloric acid, lime, sodium dichromate, chromic chloride, aluminum chloride, zinc chloride, and titanium chloride. The following filter aids were tested: coke breeze, petroleum coke, paper pulp, and ground slag. The amount of conditioners used is expres5ed in per cent of the weight of dry solids in the sludge. Chemicals are considered on the 100 per cent basis, with no water of crystallization. Solid filter aids used are on the air-dry basis. CHEMICAL COAGULANTS FERRICCHLORIDE.The differences in ferric chloride requirements between fresh and digested sludges are shown in Table I. A mixed fresh sludge (settled and activated) from the West Side Treatment Works required 5.0 per cent ferric chloride for optimum coagulation, another required 8.4 per cent (or more) with a pH of 3.6. Later operation indicated that 5 to 6 per cent of ferric chloride was the optimum amount. TABLEI. EFFECT OF FERRIC CHLORIDE ON VARIOUS
SLUDGES
ASH
(DRY M O ~ B T U RBASIS) E
5%
%
TIME
FILTRATE OBTAINEDWITH FeCls (AB % or DRY S L U D Q B ) A 0 FOLLOWB:
Min.
Blank
1.67
3.33
6.0
6.67
cc.
cc.
cc.
Ce.
Cc.
WEST BIDE BLUDQE ( M I X E D BBTTLED A N D .4CTIVATED)
96.4
31 2
(Oct. 5,
1931)
1.0 2.0 2.5 8.0 17 BO
36 46
..
.. ..
66 90
..
100 138 17QI;
119 156
.. ..
.. .. .. ..
136 1.7
15s;
, , , ,
Blank
3.36
5.04
6.72
8.40
cc.
cc.
cc.
cc.
Cc.
cc.
..
158"
rte:
114 164 172=
142 ' 6.7
l68h 5:9
150a
.. ..
5.2
..
3'8
~.
..
.. ..
i:2
316
..
..
..
W E ~ T S I D E SEPARATE DIGESTED BLWDQE (MIXED BETTLED A N D ACTIVATED)
7.74 Cc.
96.12
1.0 48 50 2.0 56 60 8.0 .. .. 12 .. .. 30 35+ 136' 14i' pH of Eltrate: 7.4 7.2 Cake curled from edge, vacuum broke.
(Feb. 11, 1932)
.
a
..
9.29
cc.
10.8
12.4
cc.
cc. 64
56 70
..
146; 6.9
82
134" 6.7
13.9 Cc.
78 98 140a ..
15.5
cc. 74 94 138= ,.
..
..
6:7
6:s
Digested sludge required much inore ferric chloride, even though the moisture content was approximately the same as that of the fresh sludge. The optimum amount was 13.9 per cent of the dry solids at a pH of 6.7. Many more tests on the same type of digested sludge indicated that from 8 to 14 per cent of ferric chloride would be required for optimum conditioning.
221
Differences in the ferric chloride requirements for the same type of sludge from two sewage treatment plants are shown in Table 11. Mixed fresh settled plus activated sludge from the Calumet Treatment Works required 9 to 10 per cent ferric chloride while similar sludge from the West Side Treatment Works required only 6.3 to 8.4 per cent, with much shorter time of filtration. The activated sludge in the West Side sludge was pumped from the North Side Treatment Works for disposal. Various tests indicated that the North Side activated ,sludge had slightly lower ferric chloride requirement than Calumet. At the present time frequent tests are made a t these plants to determine the optimum ferric chloride requirements from season to season. TABLE11. COMPARISON OF WEST SIDEAND CALUMET MIXEDFRESHSOLIDS (200-cc. sample used; moisture of Calumet feed, 96.02 per cent: moisture of West Side feed, 97.16 per cent; December 18, 1931) FROM WBBTSIDE FILTRATE FROM CALUMET FILTRATB SOLIDB SOLIDS FeCla as FeCla aa FeCla as FeClt as 9.05%,of 10.5% of 6.33% ,of 8.45% pf TIUE Blank dry solids dry solids Blank dry sollds dry s o l I d a
Min. 1.0 1.25 2.0 3.0 3.75 4.0 4.25 60.0 pH of Eltrate: 6
cc.
c c.
30
92
...
38 44
...
49
...
142 6.7
...
120 140
...
156 l6OU
...
4.6
c c.
100
...
132 155
167'
... ... ... 4.2
Cc.
26 ,
,
35 40 ,. ,.
.. ..
6.5
cc.
cc.
168 17Sa
176 184"
.. .. .. ... ,.. ...
...
4.8
... ..* ... ... ... ...
3.4
Cake curled from edge. vacuum broke
d s mould be expected, ferrous chloFERRous CHLOHIDE;. ride has practically no value as a coagulant. Analyses of inany filtrates from ferric chloride conditioning tests indicated that a large proportion of the ferric chloride is found in the effluent as ferrous chloride. A test made November 30, 1931, indicated that the ferrous chloride in the effluent could be reoxidized to ferric chloride by means of chlorine, and the ferric chloride solution re-used, provided the volume of liquid did not dilute the sludge too greatly. FERRICCHLORIDE PLUS CHLORINE.Several tests were made to determine whether chlorine might be of value in conjunction with ferric chloride. A test was made November 27, 1931, on West Side fresh solids (Table 111)in which 0.75 and 1.3 per cent chlorine, respectively, were added to a sample treated with 3.18 per cent ferric chloride. The larger amount of chlorine produced a slight improvement in filtration time, but the results were much inferior to those obtained with 6.37 per cent ferric chloride. I n a later test on Calumet sludge (Table 111) 3.6 per cent ferric chloride plus 1.0 per cent clilorine gave considerably better filtration than 3.6 per cent ferric chloride alone, but not nearly as good as 6.05 per cent ferric chloride alone; in both cases the pH was 4.7. These results indicate that little benefit was obtained by the use of chlorine plus ferric chloride. CHLORINE.I n the November 27 test on the West Side sludge, 1.3 per cent chlorine alone gave even poorer filtration than the blank. Several years ago, tests by Palmer' likewise indicated that chlorine alone was of little value as a coagulant. FERRICCHLORIDE PLUSSoDIuiv DICHROMATE. Interesting results have been obtained using a combination of ferric chloride and sodium dichromate. The latter salt is a powerful oxidizing agent in acid solution. Apparently i t prevents reduction and loss of iron in the effluent, and likewise acts as a good coagulant when reduced to chromic chloride. The value of sodium dichromate is shown by tests made January 13, 1932 (Tables I V and V). Results in Table I V indicate that as much as 12.7 per cent ferric chloride was
INDUSTRIAL AND ENGINEERING CHEMISTRY
22%
Vol. 26, No. 2
required to obtain most rapid filtration. From 31.6 to 43.5 per cent of the applied iron was found in the filtrate.
sufficient lime was added to bring the pH up to 9.0 or more, a great increase in rate of filtration was noted. Tests shown in Table V I were made in 1933 by G. G. PoinTABLE111. EFFECT OF FERRIC CHLORIDE AND CHLORINE dexter on West Side Imhoff sludge. The improvement shown FILTRATE OBTAINED WITH FeC13 (AS yo OF DRY SLUDGE) AS by use of lime is remarkable. With ferric chloride alone the FOLLOWS: most rapid filtration occurred a t a pH of 4.4, with 9.07 per 3.18 + 3.18 + 290 500 cent ferric chloride. The time for collection of 150 cc. of p. p. m. p. p. m. 500 filtrate was 282 seconds. The lowest amount of ferric chloride (0.75%) (1.3%) P. P. m. TIMB Blank 3.18 6.37 C~P ch clz used, 4.18 per cent, required 480 seconds. However, using Min. cc. cc. cc. cc. cc. cc. this lowest amount of ferric chloride and lime, increasing from WEST BIDE MIXED FRESH SOLIDS (86.13% MOISTURE) b 3.2 to 19.3 per cent, the time of filtration was rapidly reduced, 1 26 100 144 110 122 32 1.33 ... ... 174a ... ... .. with a minimum time of only 120 seconds when 16.1 per cent ... ... ... ... 1 4 P .. 1.87 lime had been applied. Even with as little as 6.45 per cent 2.75 . . . 154a ... 152O ... 46 10 70 ..... . .. .. .. ... ... 70 lime (following ferric chloride) the time of collection of 150 60 140 ... . , . 122 cc. was only 210 seconds, well below the minimum time with 6.9 5.3 3.4 4.3 4.5 6.1 pH of filtrate: CALUMET MIXED FRESH BOLIDB (36.6% XOIBTURE) the optimum amount of ferric chloride alone. The differ3.6 + 3.6 + 3.6 + ences are even more striking when based on the time that 0.75% 1.25% 1.0% 6.05 Clz 3.6 7.15 8.35 Clz CIZ elapsed before the cake broke away from the funnel. CC. cc. cc. Cr. cc. Cc . Cc. 50 70 84 94 104
82 110 134 150 163 176"
74 102 120 136 148
72 96 118 134 146
...
6.25 ... ... 6.50 . . . 166" ... 7 .I) ... 168" 9.0 ... 10 ... ... ... ... 14 156a oHof filtrate: 5 . 6 4.7 4.4 4.1 0 Cake curled from edge, vacuum broke. b November 27, 1931. e January 5, 1932.
... ...
62 62 98 112 122
... ... ... ... ...
72 94 114 126 140
... ... ...
52 72 66 100 110
... ...
CHLORIDE AND SODIUM DITABLE v. EFFECTOF FERRIC CHROMATE ON FILTRATION OF CALUMET MIXEDFRESH SOLIDS
(ZOO-cc. sample: moisture of feed, 96.7 per cent) FILTRATE OBTAINED WITH 1.5% NarCrzOi FOLLOWING 166" % FeCla IN D R YSOLIDS: 150" TIME 3.64 5.45 7.27 9.10 10.90 12.70 ... Min. cc. cc. Cc. cc. cc. cc. 5.1 4.7 4.7 1.0 70 78 92 114 106 106 2.0 94 110 130 158 146 146 ... 176O 176" ... 2.75 ... 3.0 114 136' 158 ... ... 174 3.5 176a ... ... 1844 4.0 l2i' 156 ... ... ON FILTRATIOX TABLEIV. EFFECTOF FERRICCHLORIDE 4.5 ... 164a ... ... ... OF CALUMET MIXEDFRESH SOLIDS 5.0 140 ... ... 6.0 155 ... ... ... ... ... (200-00. sample; moisture of feed, 96.7 per cent) 7.0 168 ... ... ... ... ... 1 0 . 5 1925 . . . . . . ... FILTRATE OBTAINED WITH FeCla (AS % O F DRYSOLIDS) A B FOLLOWS: 6 . 0 5 . 7 5 . 2 4 . 6 4 . 2 3.6 pH TIME 3.64 5.45 7.27 9.10 10.90 12.70 82.5 124 165 206 248 289 Fe added, mg. Fe in filtrate mg. 1.6 1.6 1.6 3.2 12.8 22.4 Mdn. cc. cc. cc. cc. cc. cc. AddedFeinhtrate, % ' 1.9 1.3 1.0 1.6 6.2 7.8 60 72 86 80 96 66 1.0 102 118 130 96 92 80 2.0 Cake curled from edge, vacuum broke 140 154 110 94 120 126 3.0 174 140 162 148 108 127 4.0 176 186 140 156 120 166 5.0 TABLEVI. FILTR.4TIOX OF IMHOFF TANKSLUDGE USING 190= ... ... ... ... ... 5.5 ... 184a ... ... ... 5.75 FERRIC CHLORIDE^ AND LIME AS COAGULANTS 130 152 ' 166 180 6.0 AT \!-EST SIDE 'TREATYEST \TORRS ... 196a ... ... 9.0 ... 1S6a ... ... 9.5 (Sludge moisture, 93 6 per cent: pH, 6 6 ; March 22, 1933) ... 154 175 10.0 TINE FOR 192a ... ... ... ... ... 14.0 TI>f\IE F O R C A K E TO ... ... ... 106" ... ... 22 0 FeCh AS COLLECTING BRE.AK FROY FILTRATB 4.7 4.2 5.4 4.6 6.0 5.6 DH % OF LI>fIE ON 150 CC. SIDESOF W H E N C l K E P H OF DRYSOLIDS DRY BASIS FILTRaTE FUNAEL BROKE FILTRATE 206 348 289 165 Fe added, mg. 82.5 124 100.4 108.8 70.4 41.6 60.8 F e i n f i l t r a t e mg. 25.6 70 % See. See Cc 34.4 43.5 37.6 36.9 33.5 AddedFeinhltrate, % 31.6 0 480 1020 186 6 4 4 16 a Cake curled from edge, vacuum broke. 5.4 0 405 Not recorded 5.58 312 600 185 5 2 0 6.96 4 8 8.36 0 285 500 184 0 282 520 185 4 4 9 06 Table V shows, however, that equally as good results could 9 0 310 575 187 3.7 . . .7. 5 be obtained with 7.27 per cent ferric chloride plus 1.5 per 7.3 930 3.22 478 4.16 8.9 270 6.45 210 4.16 cent sodium dichromate as with 12.7 per cent ferric chloride 9.4 210 167 9.68 4.18 alone. The iron lost in the effluent from the dichromate 9.6+ 165 130 12.6 4.18 9.6+ 140 120 16.1 4.18 test was only 1.0 per cent as compared with 37.6 per cent 9.6+ 140 123 19.3 4.16
...
... ...
...
...
.... ..
...
...
+
... ...
...
...
... ...
...
...
... .
.
.. .. ..
I
(1
...
...
...
...
.. .. .. ...
.
...
loss when using 12.7 per cent ferric chloride alone. Many other tests have demonstrated the value of sodium dichromate in reducing the quantity of ferric chloride required. The cost of dichromate is now considerably more than that of ferric chloride but the cost will probably be greatly reduced, just as was that of ferric chloride, if the material is used on a large scale in the treatment of sludge. Of all the chemicals used, sodium dichromate appeared to have most promise as an improvement over the use of ferric chloride alone on undigested sludge. FERRIC CHLORIDEPLUSLIME. A number of tests made early in 1932 indicated that, for digested sludge, a great saving in ferric chloride could be obtained by use of lime, following treatment with relatively small amounts of ferric chloride. The results indicated that fmall amounts of lime (5 per cent or less), added after addition of ferric chloride, did not cause a marked improvement in filtration, but, when
a The ferric chloride solution was added 10 seconds before the lime was introduced and was mixed in a 600-cc. beaker with a rotary motion. After addition of lime, the sludge was mixed with a spatula for one minute be'ore i t was poured on the filter, Hence, the contact period for the ferric chloride was 70 seconds.
Lime is much cheaper than ferric chloride; consequently, substitution of lime for ferric chloride will result in a substantial saving. The remarkable results obtained by use of ferric chloride and lime on digested sludge, with filtration a t a pH of 9.0 or more, have not been duplicated on fresh sludge. Apparently the fresh sludge yields best to treatment on the acid side, particularly with addition of an oxidizing agent such as dichromate, while digested sludge filters best on the alkaline side, with addition of ferric chloride followed by lime. Whether or not this difference in behavior is due to differences in charges or the sludge particles has not been investigated. The presence of soluble bicarbonates in the
February, 1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
sludge liquor of digested sludge may account, in part a t least, for the larger amount of ferric chloride required and may also explain the advantage in adding lime to coagulate a colloidal iron sol which otherwise will not flocculate. SULFURIC,HYDROCHLORIC, AND NITRIC ACIDS. Various acids have been used with ferric chloride with t.he thought that the pH might be lowered more cheaply with acid than with ferric chloride, thus permitting the use of smaller amounts of ferric chloride. Tests on West Side mixed fresh solids with ferric chloride and sulfuric acid were made September 25, 1931. The acid decreased the efficiency of ferric chloride as a coagulant. Hydrochloric acid was used with ferric chloride in a test ~iiadeNovember 17, 1931, on West Side mixed fresh solids. The use of 2.73 per cent ferric chloride gave a pH of 6.1; this was reduced to 4.4 by addition of hydrochloric acid, but there was no appreciable improvement in filtration. A sample treated with 5.46 per cent ferric chloride alone filtered in about half the time. Kitric acid was used with ferric chloride in a test made December 30, 1931, on Calumet raw solids. Addition of 2 per cent nitric acid to 4.2 per cent ferric chloride resulted in no improvement. It was thought that nitric acid might help to prevent reduction to ferrous iron, in a manner similar to the action of dichromate, but this did not seem to be the case, even with aeration. These tests indicated that practically no savings in the use of ferric chloride could be made, on fresh solids, by lowering the pH by means of sulfuric, hydrochloric, or nitric acid. CHLORINATED COPPERAS.When ferric chloride was quoted at 5 and 6 cents per pound in 1925, its use for sludge-conditioning was prohibitively expensive. As a substitute, chlorinated copperas was made a t the Calumet Treatment Works and used for some time. With the cost of ferric: chloride now wound 2 cents per pound, Ohere is no advantage in using chlorinated copperas, since ferric chloride is a much more effective coagulant. I n order to check the earlier work, tests were made December 8, 1931, with chlorinat'ed copperas on mixed West Side fresh solids. Various amounts of chlorinated copperas were added, ranging from 2 to 16 per cent of the dry sludge. Six per cent seemed to be the optimum amount, hut the rate of filtration was litt81ebetter than half of that obtained with 6 per cent ferric chloride. FERRICSULFATE. A commercial product containing approximately 75 per cent' ferric sulfate was used October 5, 1931, on West Side mixed fresh solids. The results are shown in Table VII. TABLE VII. COMPARISON O F FERRIC SULF.4TE WITH FERRIC CHLORIDE IN FILTRATION OF WEST SIDHFRESH
SOLIDS
(200-cc.sample, nioisture of feed, 96.4per cent) FILTR.4TI.E o B T . 4 I N E D WITH F I L T R A T E OBTAINEDW I T H FeC13 (AS 7 OF D R Y Fea(SO4)a ( A S % OF D R Y SOLIDS) A S 8 o L L o w s : SOLIDS)A S FOLLOWS: TIME Blank 3.34 6.64 2.50 3.75 5.00 Min. Cc. cc. cc. cc. cc. Cc. 1.0 32 97 148 72 88 97 1.75 ... ... 172n ... ... ... 2.00 40 140 ... 92 112 126 3.0 47 154 ... 108 130 144 4.5 ... 167" ... ... ... 5.0 68 ... .,. 130 152 164' 10.0 76 ... ... 156 170 168 15.0 88 , . . ... 164 176 18Z5 18.5 ... , . . ... ... 178" 20.0 96 ... ... 16SR ... ... 30.0 110 ... ... ... ... 45.0 134 ... ... ... ... ...
...
...
60.0 147 ... lloistureincake, 70 84.7 82.8 Ash in cake, % 31.6 32.2 a Cake curled from the edge,
...
...
...
...
83.7 31.4
81.0 31.3
79.7 31.8
79.0 31.7
From 2 to 5 per cent ferric sulfate [Fe2(S04)3]was used. The larger amount gave slightly poorer filtration t'han 3.3
229
per cent ferric chloride. Tests made by Palmer in 1925 also indicated that ferric sulfate was inferior to ferric chloride, pound for pound. On the iron basis the commercial ferric sulfate contains 21 per cent iron, while ferric chloride contains 34.5 per cent iron. Therefore, if equally efficient as coagulants on the basis of iron content, the commercial 75 per cent sulfate would have to be delivered a t about 60 per cent of the cost of the chloride (100 per cent FeCl3) to be equally economical, neglecting the cost of handling and preparation of solution. Further tests on ferric sulfate are to be made in order to obtain more detailed information concerning its efficiency. Cmohxrc CHLORIDE.Chromic chloride was used on Calumet mixed fresh solids in tests made January 4, 1932. While the salt proved to be a good coagulant, it was a p parently no more effective than ferric chloride and would cost considerably more. ALUMI~WM CHLORIDE.Aluminum chloride was used on West Side mixed fresh solids (Table VIII) in tests made November 10, 1931. It was not as good as an equal amount of ferric chloride and would cost more. CHLORIDE ON TABLE VIII. EFFECTO F ALUMINUM FILTRliTION O F WEST SIDE MIXEDFRESHSOLIDS (200-cc. sample: moisture of feed, 96.21 per cent) F I L T R A T E O B T A I N E D V I T H A1Ch (AS OF D R YSOLIDS) AS FOLLOWS:
7,
TIME .Win. 1.0 2.0 3.0 4.0 5.0 6.0 6.25 9.0 10.0 15.0 22.0
FILTRATE OBTAINED W I T H FeCk
A S 6.37, OF DRYSOLIDS
1.6
3.16
4.9
6.3
cc.
cc.
cc.
cc.
cc.
44 52 64 70 78 84
54 $2 87 99
64 84 100
94 120 140
110 116
134
64 86 102 114 126 136
148;
152R
100 116
135 144
..
...
30.0 138 60.0 150 Moistureincake,70 83 6 Curled from the edge.
... ...
112 116
...
...
...
...
...
15Za
...
...
...
... ...
86 2
85.2
...
83.7
...
...
152
160 164 166" . I .
... ... .
I
.
,..
.. 85.1
ZINC CHLORIDE. Zinc chloride was used March 25, 1932, on West Side mixed fresh solids. It had no value whatever as a coagulant. T I T a N I u h s TETRACHLORIDE. Titanium tetrachloride was used in a test made March 16, 1932, on West Side mixed fresh solids. It was realized that this salt would undoubtedly be far too expensive for practical use, but the test was made to determine whether a tetravalent salt would be more effective than a trivalent, as indicated by the Schulze-Hardy theorem. The results are shown in Table IX. Titanium chloride and ferric chloride were added in equal amounts, varying from 3.85 to 13.4 per cent of the weight of dry sludge solids, to two series of sludge samples. I n all cases ferric chloride was better than titanium tetrachloride. Apparently the tetravalent titanium id no improvement over the trivalent aluminum, chromium, or iron. OTHERCHEMICALS.Other substances tried were sodium chloride, potassium permanganate, cupric chloride, soap, lime, and ferrous sulfate plus aeration, and alum. None except the last was found to be of much value as a coagulant. PHYSICAL
FILTER AIDS
COKE BREEZEAND PETROLEUM COKE. Screened coke breeze and petroleum coke were used with mixed West Side fresh solids, both with and without ferric chloride. It was hoped that the porous structure of the coke particles might aid filtration, but the results indicated that no saving in ferric chloride could be made. The cake moistures were
INDUSTRIAL AND ENGINEERING
230
CHEMISTRY
Vol. 26, No. 2
TABLEIX. COMPARISON OF FERRIC CHLORIDB AND TITANIUM TETRACHLORIDE IN THE FILTRATION OF WEST SIDEMIDD FRESHSOLIDS (200-00.sample; moisture of feed, 96.88per cent) FILTRATE OBTAINED WITH FeCls (AS % OB D R YSOLIDS) FILTRATE OBTAINED WITH TiClr ( A S AS FOLLOWB: AB FOLLOWS: TIME 3.85 5.77 7.70 9.63 11.52 13.48 3.85 5.77 7.70 9.63 dfin. cc cc. cc cc. Cc. cc. cc . cc Cc. c c. 1.0 48 70 108 80 120 136 ... 1.5 ... ... 162a ... ... ... ... 2.0 58 90 136 130 166 ... 38 72 118 120 3.0 ... 88 140 66 102 152 154 178 44 70 4.0 110 160 164 182" ... 48 96 152 164O 150 5.0 74 120 166 170 ... 52 106 162 9.0 178O ... ... ... 10.0 92 146 17Sa ... ... ... ... ... 12.0 ... ... ... ... ... ... 1820 ... 15.0 ,.. ... ... 92' 170 ... uH 5.7 5 1 4.5 3.8 3.2 3.0 5.5 4.8 4.1 3.7 Curled froin the edge
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lower, owing to the added solid matter, and the B. t. u. value of the dry cake was increased, but no appreciable improvement in dewatering of the sewage solids was noted. PAPERPULP. Four series of tests were made with paper pulp; the results of one series are presented in Table X. Comparatively large amounts of paper pulp were required to approach the rate of filtration obtained with 5.6 per cent ferric chloride. Addition of 47 per cent paper pulp was required for satisfactory filtration. TABLEX. EFFECTOF PAPERPULPON FILTRATION OF WEST SIDEMIXEDFRESHSOLIDS (75-co. sample; moisture of feed, 95.75 per cent: moisture of paper pulp,
TIXE Min 0.16 1.0 1.5 2.0 2.5 3.0 8.0 10.0 15.0 35.0 45.0 60.0 Moisture in
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0
84.80 per cent) FILTRATE OBTAINED WITH PAPER OBTAINED FILTRATE PULP(AB% OF D R YSOLIDS) WITH FeCla AS 5.65% OF AB FOLLOWB: BLANK DRY SOLIDS 11.9 23.9 47.6 95.2
cc . ..
60°
40 48
..
50 60 62
.. .. ..
..
..
55
54 ..
58 64"
.. ..
..
64"
.. ..
64
67O
67.7 35.1 Curled from the edge.
.. 46 .. 50
S5,T
80.4
36.1
32.8
50
..
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79.5 29.4
.. 58 62O
.. ..
56
..
62
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.. .. ..
78.2 '26.4
64a
79.0 24.7
I n other tests, made October 27, 1931, 5.7 per cent ferric chloride plus 19.3 per cent paper pulp gave considerably better filtration than 5.7 per cent ferric chloride alone. The filter cakes from the sludge treated with ferric chloride plus paper pulp were drier than those from ferric chloride alone, but only in proportion to the added dry solids. The porous nature of the paper pulp seemed to help filtration if a relatively large amount, up to 50 per cent or more of the dry solids, was used. SLAG. Samples of water-quenched blast-furnace slag were obtained from the Illinois Steel Company and were used March 8, 1932, with West Side mixed fresh solids. Approximately 25 per cent slag, when used with 4.4 per cent ferric chloride, resulted in somewhat better filtration, and increase of slag to 37 per cent showed considerable improvement, but just about the same as was obtained by 5.8 per cent ferric chloride alone. A larger amount of slag, 48 per cent, gave no improvement. Slag apparently has very little if any value as a filter aid. EFFECTOF HEAT. Several series of test)s were made to determine the effect of moderate heating of sludge before addition of ferric chloride. Samples a t 72" F. (22.2" C.) were heated to 90" and 122' (32.2" and 48.9" C.). With digested sludge there was no improvement either a t 90" or 122", but with West Side mixed fresh solids there was considerable improvement a t 90" and deterioration a t 122". Further study is required to determine whether 90" is a critical point
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70 OB D R YSOLIDS\ 11.52 Cc .
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134 160 172 1800
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2.8
13.48 cc .
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96 132 156
170"
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in the filtrsbility of the fresh sludges the authors have worked with, although several series of tests confirmed this observation. SUMMARY 1. Digested sludge required more ferric chloride than fresh sludge, expressed in percentage of the dry weight of sludge solids, for optimum conditioning. 2. The amount of ferric chloride required by fresh sludge was not reduced appreciably by use of sulfuric, nitric, or hydrochloric acid, and only slightly by use of chlorine. Substantial reduction was obtained in the use of ferric chloride, however, by addition of a small amount, 1 per cent or less, of sodium dichromate. This oxidizing agent also prevented loss of iron, as ferrous salt, in the filtrate. Sodium dichromate would be of practical benefit in conjunction with ferric chloride if the cost could be reduced. 3. d large saving in ferric chloride on digested sludge was indicated by the use of lime, following ferric chloride treatment, in amounts sufficient to raise the pH to 9.0 or more. 4. Chlorine alone was not of much value as a coagulant 011 the fresh sludge tested. Ferrous chloride, zinc chloride, sodium chloride, and ferrous sulfate plus lime were of little value as coagulants. Aluminum chloride, chromic chloride, and titanium tetrachloride were good coagulants, but no better than ferric chloride. Alum, chlorinated copperas, and ferric sulfate were inferior to ferric chloride as coagulants for fresh. solids. 5. Inert materials tested, including coke breeze, petroleuni coke, and ground slag, gave no real improvement in filtration of the sewage solids. Paper pulp seemed to have some value as a filter aid if used in large amounts, equal to 50 per cent or more of the weight of dry sludge solids. 6. Moderate heating, from 72" to 90" F., before addition of ferric chloride improved filtration of fresh solids noticeably. Further heating to 122" F. hindered filtration. ACKNOWLEDGMENT The authors wish to express their appreciation to Langdoii Pearse for helpful suggestions and advice during the course of this work, which was carried out under the supervision of Philip Harrington, Chief Engineer of The Sanitary District of Chicago. RECEIVEDOotober 23, 1933. Presented befure the Divislon of Water, Sewage, and Sanitation Chemistry at the 86th Meeting of the American Chemical Society, Chicago, Ill., September 10 to 15, 1933
PIG-IROSPRODUCTION in 1933, according to final figures compiled by The Iron Age, shows a gain of 52 per cent over 1932, compared with an estimated increase of 71 per cent in steel ingot output. The pig-iron total for the year was 13,208,190tons, compared with 8,686,443 for 1932. December output was 1,177,484tons against 1,085,239in November. The daily rate, 37,983 tons, showed a gain of 5 per cent over the Wovemher average.
