Aerobic and Anaerobic Decomposition of Sewage Solids 11. Effect of Seeding with Aerated Sludge H. HEUKELEKIAN, New Jersey Agricultural Experiment Station, Yew Brunswick, N. J. Proper seeding of sludge undergoing aerobic terial required 65 days of aeraEWAGE solids c o n t a i n decomposition reduces the tirne of decomposition tion or nearly twice as long as b o t h a n a e r o bi c a n d the seeded material. a e r o b i c flora* U n d e r to half. Such a seeded sludge decomposes in A similarly striking effect is p r o p e r environmental conditions one or the other type of the Same length Of lime as Properly seeded illustratedin connection with fat rediction, which increased with anaerobic sludge. Howecer, in the same length flora can be made to predomiof time, properly seeded sludge undergoing aerobic the seeded material rapidly and nate without resorting to exthorough decomporegularly up t o a total of 99.9 per decomposition git,es a ternal inoculation. The relative cent on the thirty-fifth day. I n rates Of decomposition Of the sition of organic matter than the anaerobic the unseeded material fat reducdifferent components of sewage s o l i d s , while the aerobic and process* tion was not as rapid, and only 40 per cent reduction of fat was the anaerobic flora were being established, were discussed in Part I of this series (1). The obtained in 35 days; the total reduction was not over 87 seeding value of a sludge in which the anaerobic flora has per cent even after 92 days of aeration. The reduction of total nitrogen was neither as regular nor been developed has been adequately established. It was therefore natural to expect a similar benefit to be derived in as high as the volatile matter and fat reduction. I n the early the aerobic decomposition from the use of a sludge in which period of aeration the reduction for unseeded material was the aerobic flora has been established. It was for the proper higher than the seeded, but a 30 per cent reduction was obevaluation of this point that the following investigation tained in 35 days with the seeded material while it required 70 days for the unseeded material t o give a similar reduction. was undertaken. The aerobic sludge developed by the continuous aeration The loss of nitrogen probably took place in the form of of fresh solids for 90 days was used for seeding fresh solids. ammonia as a result of aeration. The &day b. 0 . d. reductions increased rapidly. A total The relative volumes and compositions of each were as of 74 per cent b. 0 . d. reduction took place in 35 days in the f 0110~~~s : seeded material, while it required nearly 70 days for a similar dSa VOLATILE hT4TTER SOLIDS YOLUJlE SOLID8 CC . % % % Grams reduction in the unseeded material. Fresh Aerated
1800 600
3 1 1 6
23 6 40 5
2 36 0 95
42 5 5 7
DISCUSSIOS From the above results it is apparent that seeding of solids Thus the relative amounts of dry volatile matter used for aerobic decompositionwith suitwere 7.5 parts of raw to 1 part of able material accelerates the decomseeding material. position and diminishes the time reThe above mixture was placed quired for complete decomposition. i n a g l a s s cylinder fitted with a From the different analysis it seems porous plate in the bottom for the that, under the conditions of the exblowing of air. periment, the rapid decomposition RESULTSOF SEEDEDAERATIOX of the materials was completed in OF SLUDGE 70 days without seeding and 35 days with seeding; e. g., the same point The r e s u l t s of the continuous was reached in 35 days with seeding aeration of fresh solids seeded with as was reached without seeding in aerobic sludge are given in Figure 70 days. 1. For c o m p a r i s o n the figures Table I shows the effect of seeding from Part I (1) on the aeration of on aerobic and anaerobic decomunseeded fresh solids are also plotted p o s i t i o n . At 20' C. u n s e e d e d in the same figure. The volatile sewage solids require 150-200 days m a t t e r reduction of the seeded for digestion; with aerobic process, sludge proceeded regularly and complete decomposition is obtained rapidly up to the thirty-fifth day of 90 days without initial seeding. in aeration when the experiment was Seeding reduces the time of anaerobic terminated. A 50 per cent reducdigestion to 36-40 days and that of tion of volatile matter was obtained aerobic decomposition to nearly the in this period, while with an equal same length of time. The aerobic period of aeration the u n s e e d e d process is more efficient without material gave a b o u t 35 per cent seeding than the anaerobic. This DAYS reduction of volatile matter. To might be attributed to the lack of obtain a 50 per cent reduction of FIGURE 1. AERATIOS OF FRESHSOLIDSWITH accumulation of organic acids under AND WITHOUT SEEDMATERIAL volatile matter, the unseeded ma1162
October, 1933
1 ?J D U S T R 1.4 L A N D
E IC' G I N E E R I ? G i C H E MI ST R Y
1163
With longer periods of digestion it is natural to assume that the destruction of volatile matter under anaerobic process would go on slowly and probably be equal t o the aerobic. Experiments with prolonged anaerobic digestion show a very don- rate of destruction of organic matter after the initial rapid rate of decomposition. From a practical standpoint it has been shown that the aerobic decomposition of fresh solids is feasible in 35 days, provided the material is seeded a i t h sludge produced under aerobic conditions. At the present time there does not seem to be any decided advantage in decomposing sewage solids TABLE I. COMPARISON O F AEROBIC AND *4NAEROBIC DECOMPO-under aerobic conditions, however, since with proper seeding SITION .kND T H E EFFECT O F SEEDISGO O S T H E 'rOTAI2 DIGESTIONand temperature control sewage solids can be handled in TIME.4SD VOLATILE M A T T E R DESTRUCTION (20" c.) the same period of time under anaerobic conditions. The VOL.ATILE cost of continuous aeration would a t the present time seem DIGESTIOX MATTER to be a deciding factor. Hoxever, the seeding ratio for aerobic TIYE DEUTROYED Days % decomposition was 7 . 5 parts of fresh volatile matter to 1 .inaerobic, n o t seeded 150-200 25-30 part of volatile matter in the seed sludge. With such a wide .inaerobic. seeded ( 2 : l ) b 35-40 35-40 Aerobic, not seeded 90 60-65 ratio of seeding it is not possible to digest fresh solids anAerobic, seeded (7.5:l)b 35 55-60 aerobically in 35 days. It might be possible with further T h e destruction in the seed material itself is considered negligible. b T h e ratios refer t o t h e d r y volatile m a t t e r i n fresh sollds t o t h a t of seed work to show a considerable advantage in digestion time in sludge. favor of aerobic process under comparable conditions of The percentages of volatile matter destroyed under seeding. If such a n advantage is shoJvn, then the cost of aerobic and anaerobic conditions as given in Table I reveal aeration may be overbalanced by thtl advantage gained. some striking differences. Under anaerobic conditions the From this laboratory experiment a t least the possibility has destruction of volatile matter is less than under aerobic been opened of decomposing sewage solids aerobically, and conditions. Seeding does not change the relationship. Thus, further work should shoTT its practicability. although the decomposition in the seeded material under I n regard to the application of the findings to activated aerobic conditions is completed in the same length of time sludge process, it can be seen that with sludge recirculation a as under anaerobic conditions, the destruction of yolatile considerable stabilization and reduction of volatile matter matter is more thorough. Therefore t,he rate of destruction will take place. The extent of this reduction mill depend on per unit of time must be higher under aerobic conditions. the length of time the sludge is in recirculation. The higher volatile matter destruction is due, to a great The aboye findings substantiate Tenney and Waksman's extent, to the higher percentage destruction of fats and claim ( 2 ) that the rate of aerobic decomposition is greater nitrogenous materials under aerobic conditions. than the anaerobic. It was shown in Part I (1) that such Comparison of aerobic and anaerobic decomposition with was the case when the solids were not seeded. The present properly seeded materials leads to the conclusion that, work indicates that, when solids are seeded with aerobically although the total time required for decomposition is the produced sludge, the rate of decomposition is greater than same under both conditions, the end point reached in the when the solids are seeded with anaerobically produced same length of time is more complete under aerobic condi- sludge. tions. aerobic conditions, Tvhich enables the natural development of the flora responsible for the decomposition of theorganic matter more quickly. Under anaerobic conditions the accumulation of organic acids delays the natural development and activity of the flora. K h e n the solids are seeded, the accumulation of acids does not become an important factor, and therefore with proper seeding there are 110 appreciable differences in the digestion time between aerobic and anaerobic processes. I n order to make a fair coniparison of the rate of decomposition under aerobic and anaerobic conditions, the material should be amply provided with the respect,ive flora. ,
0
. . e * .
111. Effect of Reversing the Process after Initial Stabilization O n aerobic decomposition the sludge f r o m tanks was d i v i d e d into two P o r t i o n s . One Portion Was ticles of this series it apanaerobic digestion process undergoes a further aerated continuously as in the p e a r s that t h e aerobic material transformation as measured by tolatile prior e x p e r i m e n t s , a n d the and anaerobic processes of dematter, organic nitrogen, f a t , and b* 0. d. reducother was kept under anaerobic composing organic m a t t e r are similar from the s t a n d p o i n t lions. The reducfion in organic nitrogen was conditions. No seed material higher than in similarly aerated fresh solids. was used in either case. The of time r e q u i r e d to a c h i e v e After the complete aerobic stabilization, anperiod Of i n c u b a t i o n W a s 92 the major portion of decompodays; eight samples were taken sition. However, in the same aerobic digestion yields additional reduction of a t intervals but only the length of time a more thorough d e c o m p o s i t i o n of o r g a n i c datiLe analyses in the beginning and end of the experiment are given matter takes place under aerobic conditions, Therefore it might be expected that a sludge because the trends of the decomposition are similar to those subjected to initial anaerobic decomposition wiuld further reported before in connection with the a6robic decomposition decompose when put under aerobic conditions. If this is of fresh solids. true, the alternating conditions of aerobiosis and anaerobiosis A E R O B I C DECOMPOSITIOX OF ASAEROBICRIPESLUDGE as might take place in a sludge bank might yield a more The results are incorporated in Table 11. The pH and the thorough decomposition. This paper therefore deals with the aerobic decomposition of anaerobic ripe sludge and the ash figures of the initial sludge would indicate a well-staanaerobic decomposition of sludge previously subjected to bilized sludge from anaerobic digestion process. This sludge under further anaerobic conditions decomposed only slowly. aerobic decomposition. A quantity of ripe sludge from separate sludge digestion Volatile matter destruction did not exceed 17.0 per cent,
F
ROh4 the previous ar-
