October 15, 1931
INDUSTRIAL A N D ENGINEERING CHEMISTRY
359
Sludge Ripeness Studies',' E. L. Pearson and A. M. Buswell ILLINOIS STATE
I
WATER
N A study of the factors influencing the drying qualities
of digested sludge, various well-known tests denoting the degree of digestion were applied to the sludges in question. Lack of correlation between some of the tests prompted this study of the older tests and a search for a new one which might be more usable. Since the way in which the tests are conducted has considerable influence on the results obtained, the methods used in this experiment will be briefly outlined. VOLATILEACIDS-A 200-cc. sample was acidified with 2 cc. of concentrated sulfuric acid and distilled until 100 cc. had come over; the distillate was titrated with 0.1 N sodium
Figure 1-Results
SURVEY,
URBANA, ILL.
total present was plotted against time. The moisture remaining after 7 hours' draining was also determined. The weight of the sludge cake gave a check on the evaporation which was found to be negligible when the sieve is covered. I n all cases, of the total moisture draining in 24 hours more than 90 per cent had drained in 7 hours. The drainability data given here are for a 7-hour period. ASH-A 40- to 50-cc. sample was weighed in a platinum dish, evaporated on a steam bath, and dried for 11/2 hours at 105" C., weighed, and ignited in a muffle a t 850" to 900" C. for 1 hour. %-HOUR BIOCHEMICAL OXYGENDEMAND-A 10-cc. pipet (with tip broken off) graduated to tenths of a cubic centimeter was used to measure the sludge. Aerated distilled water with 500 p. p. m. of sodium bicarbonate was used as dilution water. A four- to fivefold increase in the dilution employed indicated a 40 to 70 per cent increase in B. 0. D. The dilution was selected, therefore, which would result in the consumption of about half of the available oxygen. A duplicate detennination using the same dilution checked reasonably well. The results are reported in parts per million per per cent volatile matter. 2 4 - H o u ~GASPRODUCTION (%)-Five hundred cubic centimeters of sludge (or more, if gasification was slow) were placed in a bottle and connections made whereby the gas evolved could be collected over saturated salt solution and measured. The samples were incubated a t 30" C. and rotated gently a t the end of 5 or 6 hours, and again just before
of Tests to Determine Ripeness of Sludge
hydroxide and the alkali used calculated to acetic acid. According to the Duclaux distillation curves, this procedure gives approximately 35 per cent of the acetic acid present when distilled from distilled water solution, and a slightly higher percentage when distilled from a solution of the character Qf sewage sludge. In this experiment the results are reported as titrated and are, therefore, relative. DRAINABILITY-one liter of sludge was weighed and poured onto a 3/4-inch (1.90-em.) layer of standard Ottawa sand in a 28-mesh sieve. The amount of liquid drained in per cent of 1 Received May 11, 1931. Presented before the Division of Water, Sewage, and Sanitation Chemistry at the 81st Meeting of the American Chemical Society, Indianapolis, Ind , March 30 to April 3, 1931. * This work was carried out with the assistance of funds furnished by the Chemical Foundation
Figure 2-Relation
of B. 0. D. a n d Gas Production to Ash C o n t e n t of Digested Sludge
ANAL Y TlCAL ED1TION
360
measuring the gas. The gas produced was reported in cubic centimeters per gram of volatile matter in the sludge. Before being connected to the gas collecting bottles, the samples were allowed to stand for 2 hours to avoid collecting the gas liberated by reason of a change in temperature or pressure, DIscussrox-Figure 1shows the results of these determinations as made on a series of sludge samples. The samples connected by lines were obtained from the experimental plant, those not connected were obtained from the UrbanaChampaign, Decatur, and Springfield sewage treatment plants. For purposes of comparison they are arranged in the order of their decreasing 24-hour gas production, considering t4heseries as a whole. The results of the various tests show a general correlation. On individual samples, however, wide discrepancies are very evident. As indicated in Figure 2, the best agreement is between the ash and the 24-hour gas production determinations.
Vol. 3, No. 4
Figure 3-Sludge-Drainability Curves
course of the volatile-acid content in conjunction with the gas production would give an indication of the completion of digestion. With continuous feeding, however, the volatile acids have been shown to follow a trend opposite to that of gas evolution (1). 24-Hou~BIOCHEMICAL OXYGENDEMAND-The biochemical oxygen demand of this series of sludges shows, in general, a trend parallel to the other tests. I t shows less correlation with the ash content, however, than does the 24-hour gas evolution. From these studies it does not seem that a definite B. 0. D. below which sludges are sufficiently ripe to draw can be set which will be applicable to sludges of all sources. Some sludges in this experiment having a B. 0. D. below 1000 p. p. m. per per cent volatile matter, although inoffensive and nonputrescible, would hardly be considered ripe for drawing because of their unsatisfactory drying qualities and the character of the cake produced. ASH CONTENT-The ash content of the digested sludge is probably the most used ripeness test. Its significance is dependent on a knowledge of the ash of the fresh solids fed, and the results from one plant are not comparable to those of another plant whose fresh solids do not have a similar ash content, or where the ash determination is not carried out at the same temperature. !%-HOURGAS PRODUCTION TEST-AS with the foregoing tests, the gas production of the series of sludges showed a trend similar to the results of the other tests, with wide variations, however, between the results on individual samples. At present it necessitates an ash determination, but it has advantages in its simplicity, in the large sample that may be used, and in the minimum apparatus and reagent requirements. If the gas evolution can be successfully correlated with the ripeness of a digested sludge, it should prove a useful test.
Results of Tests
Conclusions
DRAINABILITY-Figure 3 shows typical drainability curves of a slow, a medium, and a rapid draining sludge. In general, the more thoroughly ripened the sludge, the poorer its draining qualities. The moisture which may be removed by draining, therefore, appears to be, within certain limits, in inverse relation to the degree of ripeness of the sludge. The percentage of the moisture removed by draining is lower and the amount of moisture retained by the drained sludge higher for a well-ripened sludge than for a partially ripened sludge. I n view of this, the drainability is not strictly a function of the original solids cpntent. From these studies, the draining quality does not appear to be the important consideration in the drying of digested sludge. The ease with which the non-draining moisture is released and the character of the resulting cake appear to be of greater significance. A longer time is required to remove the smaller amount of remaining moisture in the better draining sludges, and a tough fibrous cake is produced instead of the porous granular material from well-ripened sludge. However, the better draining qualities, together with the fact that it is inoffensive and has been degasified, suggest the possibility that mechanical dewatering of this type of sludge might be more successful than has been the case with well-ripened sludge. VOLATILEAcms-The volatile-acid content of a wellripened sludge will be low, but a low volatile-acid content does not necessarily mean that the sludge is well ripened. The volatile-acid content is governed by the rate at which they are formed by the decomposition of more complex substances, and the rate at which they are decomposed to the gaseous end products. A rapidly gassing sludge may, therefore, be low in volatile acids. With batch feeding, following the
Within the limits of the degrees of ripeness shown by the series of sludges herein studied the following tentative conclusions may be drawn: The draining quality of a digested sludge is roughly inversely related to its ripeness and to the ease with which the non-draining moisture is removed. I n the continuous feed process of sludge digestion, volatile acids are a better index of the course of digestion than of its degree of completion. The ash and solids content of a digested sludge may be satisfactory locally as an indication of ripeness, but both are also dependent on other factors, and are, therefore, not of general applicability. Most of the sludges from the experimental plant had a biochemical oxygen demand below 16OQp. p. m. (2) but were not considered sufficiently ripened to permit ready disposal. Correlation of the rate of gas production under standard conditions with the ripeness of a digested sludge is being attempted. Literature Cited
T h e h Hu
(1) Buswell and Pearson, "Further Studies on Rapid Stage Sludge Digestion." (2) Rudolfs and Fischer, Public Works, 67, 5 , 171 (1926).
Expansion of Paint Industry in India-The paint industry of India is growing and each year is affording increasing competition to imported paint products, according to the Department of Commerce. There are now sixteen paint factories maintained by large British paint manufacturers. The other plants are relatively small and in most instances do not manufacture a complete line of paints. Varnishes and lacquers are not produced in India on a commercial scale a t present.
