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I N D U S T R I A L A N D ENGINEERING CHEMISTRY
Vol. 21, No. 3
Effect of Pressure upon Sludge Digestion’ Melville C. Whipple, Gordon M. Fair, and Louis Klein HARVARD ENGINEERING SCHOOL,CAMBRIDGE, MASS,
T IS a general belief that pressure exercises slight, if any, direct influence upon the metabolic activity of bacteria. If this is so, the digestion of sludge by these organisms in sewage treatment processes is not accelerated or retarded by reason of the direct physical change when pressure is varied above or below normal. There is a possibility, however, that departure from normal pressures may so alter physical and chemical conditions as to iduence indirectly the activity of the bacteria, and so the speed, if not the completeness, of the sludge digestion process. The solubility and, therefore, the release of the gaseous endproducts of digestion are affected by pressure changes. Further, solubility of certain gases affects reaction and ionic concentration of the medium. Also, entrained gases may erect a physical barrier between the bacteria and their food. Indirectly, then, the pressure factor may be of some importance. Subnormal pressures are not encountered in digestion tanks. Those above normal are incidental to the depth of the tanks and vary with the depth. I n practice the dimensions of sludge-digestion tanks have not been standardized, both deep and shallow ones being utilized. EddyZhas pointed out that there has been a decided advantage for some deep tanks in preventing excessive scum formation and in producing sludge of a higher solid content. He also states that “if tanks must be shallow, substantial additional capacity must be provided in the digestion compartments.” I n view of the indicated operating advantages with higher hydrostatic pressures and the uncertainties surrounding the influence of pressure upon digestion, there has been a need ifor investigation of the subject. Sierp,3 of Essen, reports a few laboratory studies made of digestion of sludge a t depths of 3, 1, and 0.15 meters, where there was a greater removal of fresh organic matter with the shallower depths although the gas production was about the same for all three. I n another experiment using both normal and diminished pressure there was a slightly greater removal of fresh organic matter under diminished pressure, but an 80 per cent greater volume of gas per gram of fresh organic matter. Further studies have been made during the past year a t the Laboratory of Sanitary Engineering, Harvard Engineering School, during which parallel experiments were made of the progress of digestion under normal pressure and under high and low pressures. This paper presents the results of these studies.
I
Preparation of Samples Mixtures were first prepared for digestion from fresh sewage solids and Imhoff sludge, both collected a t the Fitchburg, Mass., sewage plant. The ratio of organic solids in the mixtures was 2.1 of fresh organic solids to 1 of Imhoff organic solids. Water was added to bring the total solids to 6 p r cent, and the reaction adjusted to pH 6.7 by means of limewater. About 1.5 liters were used for each experiment, digestion being carried out in clear flint glass bottles. 1 Presented before the Division of Water, Sewage, and Sanitation Chemistry at the 76th Meeting of the American Chemical Society, Swampscott, Mass., September 10 t o 14, 1928. 1 Eddy, Proc. Am. Sod. CioiL E m . , 60, 616 (1924). , a Sierp, Tech. Gemcindcblalf, February 5, 1927.
One portion of the mixture was incubated a t 20” C. and 760 mm., and the gas collected over water in a eudiometer. A second portion was incubated a t 20” C. and a pressure of 625 mm. in excess of 760 mm. The third portion had the same temperature conditions and a pressure of 300 mm. less than 760 mm. Pressure Apparatus The details of the apparatus used for digestion under pressure greater than atmospheric are given in Figure 1. This consisted, first, of the sludge bottle, encased with wooden slats as a protection against breakage. The outlet was connected by means of rubber pressure tubing to a gas collector, or manometer, which was enclosed with fine screen wire. A tube led from the gas manometer to a brass pressure tank. Tapped off from this line was a connection to a mercury manometer. The pressure tank was connected with the tap water supply and also had a relief valve that functioned when the pressure exceeded 625 mm. above normal. The system was filled with water as far back as the gas manometer. There was special reikforcement of all stoppers and joints. Gas samples were taken by closing the pinchcock on the sludge bottle outlet, then placing a n inverted bottle of water over the delivery tube of the gas holder and slowly opening the pinchcock. Gas was discharged by the pressure and water filled the holder. Diminished Pressure Apparatus
A diagrammatic presentation of the apparatus for digestion under diminished pressure is given in Figure 2. The sludge bottle was connected to a safety flask to prevent water accidentally running back to the sludge if the suction failed. A bottle partially filled with water was interposed to provide a continual head of water on the gas manometer. The purpose of the valve bottle was to prevent backward leakage of air into the system from the exhaust pump. The valve was built in the laboratory and was made of a glass ball that had its seat on a piece of thin rubber tubing. The exhaust pump was operated with a 110-volt house current connecting with a relay that was excited by current from dry cells. The condenser reduced the spark and wear and tear on the mercurymanometer contact point. This point was the end of a c o p per wire inserted in the manometer jacket. Its position was fixed and determined the amount of vacuum that would be maintained in the apparatus. The experiment was conducted with a vacuum of 300 mm.-that is, an absolute pressure of 460 mm. Whenever leakage increased the pressure the mercury rose in the manometer jacket, made contact with the copper wire, ‘current flowed from the batteries, the relay was excited, and the exhaust pump operated until the vacuum lowered the mercury and broke the contact. Samples of gas were removed after opening the switch, closing the tube from the safety bottle, and opening the aircock on the valve bottle. The pressure on the gas in the manometer was then equal to the difference in the level of water in the water bottle and in the gas manometer. Gas was removed from the apparatus by opening the cock a t the top of the manometer and inserting the delivery tube beneath an inverted bottle filled with water.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
March, 1929
Discussion of Results
Bottles of the sludge mixture were digested for 7 weeksone a t normal, one a t increased, and one a t reduced pressure. Digestion was practically complete after that time, as shown by the decreased rate of gas production and the character of the sludge.
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On the basis of methane production digestion was 85 per cent complete after the following periods: Reduced pressure Atmospheric pressure Under pressure
Days 37 40 42
There was, therefore, a slight acceleration of digestion under the influence of suction and a slight retardation under pressure as compared with atmospheric pressure over a period of 5 to 6 weeks. Table I-Cumulative
Gas Production
1 GAS PRODUCTION - LITERS PER KILOGRAM OB FRESH ORGANIC
8 I
Total
Other CH4 than CHc
1
PRESSURE MM. HG)
(fans
Other CHI than CHc
Total
Other Total CHc than CHd
51
uu Figure 1-Apparatus
Table I gives data for the volume and nature of the gases evolved during the experiments. Initially and for a period of 5 or 6 weeks the total gas production per unit of fresh organic matter was considerably higher for the experiment a t reduced pressure than for that at atmospheric pressure. The pressure experiment produced somewhat less gas, a t first, than the one held a t atmospheric pressure, and the difference continued to increase up to the sixth week. Finally, a t the end of 7 weeks the total gas produced was fairly comparable for the three experiments, especially if the fact is kept in mind that the gases were collected over water and that there would be less loss by absorption in the diminished pressure apparatus than in that kept a t normal pressures, and that there would be a greater loss by absorption in the pressure apparatus. Solubility increases, of course, with the pressure. Digestion, then, as judged from total gas production was a t first more rapid under a pressure of 0.6 atmosphere than under 1.0 and 1.8 atmospheres. The total time required for digestion was approximately the same for all three conditions. These facts are in agreement with Sierp’s findings so far as pressures greater than atmospheric are concerned, but he noted a higher - gas Droduction when suction was applied. I
66 147 215 301 391 501 576
for Digestion under Pressure
Table 11-Composition ~~
SLUDGE SAMPLE
Fresh sludge Imhoff sludge Incubated mixture
I
40 92 140 207 279 369 432
26 55 75 94 112 132 144
of Sludge Used in Experiment8
% :: Pet cent 4.73
1
DRYSOLIDS Mineral matter
Organic matter
Per cent 30.7 62.0 47.5
Per cent 69.3 38.0 52.5
I
VALUE pH
Composition of Sludge
The percentage composition of the raw sludges used is given in Table 11. The differences are typical for the raw and digested sludges. The incubated mixture was a 2:l composite of fresh and raw sludge plus water that was added to prevent caking and to facilitate ebullition of gases. The mixture had at first a pH value of 5.5, and this was adjusted to 6.7 to assist in overcoming acid fermentation.
_
Methane and Other Gases
The production of methane under diminished pressure and a t atmospheric pressure was found to be fairly comparable for the total time of the experiment, although slightly greater during the first 5 weeks for the experiment under decreased pressure. Gases other than methane were produced in considerably greater volumes under diminished pressure. They were largely carbon dioxide, the most soluble of any, and therefore the most likely to have its concentration affected by the lowered pressure. Furthermore, there was a lower loss of gas in the reduced pressure experiment than in the others from replacement of water in the manometer when samples were taken. Figure 2-Apparatus for Digestion under Diminished Pressure There was a decidedly lower production of methane when a pressure of 1.8 atmospheres was used-that The composition of the digested sludges appears in Table is, up to the last week of the experiment, when i t was com- 111. The differences in total solids and in organic solids parable with the volume produced under the other conditions. vary within narrow limits. The latter show a slight, but Other gases were the same in volume as under atmospheric not a significant, advantage in favor of the experiment at pressure, but lower than under reduced pressure. As pointed reduced pressure. There is a similar indication for the presout, in the diminished pressure apparatus they were less sub- sure conditions over those of atmospheric pressure. ject to loss from the replacement of manometer water. Physical examination of all three digested sludges showed
I N D U S T R I A L A N D ENGINEERING CHEXISTRY
256
FINAL
TOTAL SOLIDS
ORGANIC MATTER IN SOLIDS
I !
Per cent Per cent Per cent Per cent Per cent Per cent
Diminished pressure Atmospheric pressure Pressure a
7.5 7.4 7.5
i
6.47
4.72
27.0
52.5
41.9
42.0
6.47 6.47
4.96 4.73
23.0 27.0
52.5 52.5
43.2 42.6
37.0 40.0
Reduction of organic solids is on basis of fresh organic solids.
Conclusions
1-The incubation of a 2: 1 mixture of fresh and Imhoff sludges under pressures of 0.6,1.0, and 1.8 atmospheres gave a completed period of digestion comparable for all three conditions as determined by methane production and the reduction of organic matter. This was 7 weeks. The period for 85 per cent completion of digestion, based on methane production, was 37 days for diminished pressure, 40 days for atmospheric pressure, and 42 days for a pressure of 1.8 atmospheres, an indicated slight advantage for diminished pressure and a disadvantage for pressure as compared with normal conditions.
Vol. 21, No. 3
2-Total gas production for some time prior to, and subsequent to, the middle point of digestion showed consistently different values for the different experiments, being greatest for decreased and least for increased pressure. If these differences could be adjusted for loss of gas due to the different pressure-solubility conditions, they would not be so great nor would they be significant of more than slight differences in the rates of digestion. 3-Methane production throughout was nearly parallel for the reduced and atmospheric pressure experiments. It was lower for the pressure experiment until the latter part of the digestion. This may have been due to an unfavorable influence upon the chemical environment by the entrained gases or to the inability of the bacteria to reach digestible solids by reason of the solids being surrounded by a barrier of gas bubbles. 4-As judged by the total period required for sludge digestion, there was no significant difference for any of the experiments. Therefore, devices aiming to establish pressures unusual in practice would presumably not shorten the digestion time. &The use of deep tanks for digestion of sludge may have certain advantages in operation. It probably has no disadvantage on the ground of the increased pressures a t which digestion must proceed.
Chemical and Biological Correlations in a Polluted Stream’,’ Willem Rudolfs NEW
JERSEY
AGRICULTURAL EXPERIMEXT S T A T l O N , NEW BRUNSWICK. N. 1.
The water of the Raritan River system in New Jersey and low mineral content place URING 1927 a study is in its natural condition admirably fit for potable and them definitely among the was made of the polluindustrial purposes. Unfortunately the lower section better waters of the United tion conditions in the of the river is heavily polluted. The studies conducted States for industrial use.” lower Raritan River and its show an apparent direct relation between the amounts The lower Raritan River is tributaries. The s t u d y exof ammonia present and the biochemical oxygen depolluted by the raw sewage of tended over one year and inmand of the river water. Oxygen depletion was far about 100,000 people and by cluded all seasons. greater in summer than in winter. The effect of two the industrial wastes of about The Raritan River system small dams upon the self-purification of the river 85,000 population equivalent. is the largest in New Jersey, is considerable. There was a direct relation between I n addition the effluent from except the Delaware. The B. 0. D., bacteria, and plankton. The highest number the Joint Plainfield Sewage tributaries do not combine of B. coli recorded was 11,000 per cubic centimeter Disposal Plant, treating the until they reach points within found opposite a bathing beach. During the summer waste of about 50,000 people, 7 miles of tide water. The months the average number of B . coli at this point was is discharged into a brook and whole Raritan drainage area 3240 per cubic centimeter. The relation between reaches the Raritan in about is 1105 square miles. The rainfall and oxygen saturation is pointed out. an hour. There is only one upper part of the drainage s e p t i c t a n k , treating the svstem lies in the hills and the -* -physiography of the territory changes until the lower part of sewage of a small borough, in the whole region. the Raritan runs through marshes into the Raritan Bay. Discharge Testing Procedure Collins and Howard3 state, in a paper on the surface waters in New Jersey, that “the river waters of New Jersey do not Discharge measurements for the years 1924-1927, inclusive, change in composition so much as many other waters in the United States.” Disregarding pollution and on the basis were supplied by the courtesy of 0. W. Hartwell, district of their chemical analyses, these authors conclude that the engineer, U. S. Geological Survey, Trenton, N. J. There were twelve sampling stations established, located waters of the Raritan basin with “their comparative softness as far as practicable in such a way that large sewer outfalls 1 Presented under the title “Some Results Obtained in Raritan River and the mouths of the larger tributaries had no disturbing Pollution Studies” before the Division of Water, Sewage, and Sanitation influence on the samples. Chemistry at the 76th Meeting of the American Chemical Society, SwampScott, Mass., September 10 to 14, 1928. The chemical analyses performed consisted of determina1 Paper 75, Department of Water Supplies and Sewage Disposal. tions of pH values, alkalinity, chlorides, ammonia, nitrites, 8 Collins and Howard, U. S. Geol. Survey, Water-Supply Paper 696-E nitrates, suspended solids, ash, dissolved oxygen, and 5-day (1927).
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