Deammoniation of Sewage Sludges A. L. GENTER,Wyman Park Apartments, Baltimore, Md.
T
H E most difficult problem scissas, and t h e d e w a t e r i n g Ammonification accompanies the decomposiconnected with sewage rates in weight of dry solids tion of sewage sludges. The soluble decompositreatment becomes evid e w a t e r e d per t i m e unit as tion compounds accumulated in the sludge dent with the disposal of various ordinates, the successive curves liquors during decomposition chemically react sludges accumulated in sewage result. with coagulants intended for flocculating colloids d i s p o s a l p l a n t s . Sludge deThe numbers identifying the watering and disposal practice various curves show the parts present. About 90 per cent of the ferric chlohave r e m a i n e d uncoordinated per million of n i t r o g e n from ride added to well-decomposed raw and digested notwithstanding t h e v a r i o u s soluble compounds of ammonia sludges at Baltimore is chemically consumed. a n d ammonia derivatives chemical treatments, digestion, The use of chemical sludge deammoniating agents present in t h e particular and sand-bed drying methods such as formaldehyde together with deammoniasludge sample. T h e term evolved for solving this problem 11 a m i n o - a m m o n i a nitrogen” ( 5 ) . “The chief difficulty arises tion by elutriation is described, and typical rehas been chosen to designate from the fact that the sludge is sults are depicted. The results show that such this nitrogen, and its abbreviavery retentive of water, which deammoniation treatment produces remarkable tion U/h’is used on some of forms by far the greatest procoagulant savings on both raw and digested sewage the graphs. portion of the m i x t u r e , t h e sludges. Increased dewatering rates likewise Curve 100 r e p r e s e n t s t h e proportion being greater with flocculation graph for the fresh modern processes” (1). result. crude sludge which was pracThe writer’s investigations a t tically one day old when colB a l t i m o r e h a v e revealed the fact that the well-known resistance of sewage sludges to co- lected. This sludge contained 100 p. p. m. amino-ammonia agulation is practically entirely due to the soluble decompo- nitrogen. This fresh sludge required relatively small amounts sition products generated and accumulated in the various of ferric chloride for flocculation, as will become evident from sludges. Details of these investigations as well as the ex- the ferric chloride abscissa numbers. Maximum filtration haustive field tests conducted by Keefer and Kratz have rates were obtained a t about 2.2 per cent ferric chloride addition. On the second day the amino-ammonia nitrogen of been published elsewhere (9, 3). Aside from being rich in negatively charged colloids, all this sludge had increased to 180 p. p. m. as shown on the fresh domestic sewage sludges abound in highly decompos- amino-ammonia nitrogen graph. Flocculation graph 180 able organic solids. Because of the nature of a large portion has moved to the right and the ferric chloride consumption of these solids, their progressive decomposition involves the for equivalent dewatering rates shows an increase almost diproduction of ammonia derivatives and ammonia with a con- rectly proportional to the increase in soluble ammoniacal sequent increase in the concentration of soluble ammoniacal compounds. The same is true of the two succeeding flocculacompounds in the sludge. This is particularly true in stabiliz- tion graphs, 250 and 325. This is conclusive evidence that ing sludges by means of anaerobic digestion, where some of ferric chloride is being used mostly for chemical precipitation these solids and colloids are destroyed and simpler chemical purposes when added to such an aging sludge. To make this fact entirely obvious, the relatively wellcompounds result. The term “ammoniacal” for describing these simpler nitrogenous compounds is used in an extended decomposed and ammonified sludge of curve 325 was almost sense to include compounds of ammonia and ammonia de- entirely deammoniated by a method to be given in detail. rivatives resulting from deaminization and decarboxylation The concentration of the soluble ammoniacal products was diminished from 325 to 15 p. p. m. amino-ammonia nitrogen. of protein matter common to domestic sewage sludges. As a result of these disclosures fresh sludges should require Curve 15 to the extreme left resulted. This deammoniated less coagulant for effective flocculation than do older, well- sludge required surprisingly small amounts of ferric chloride, decomposed sludges. As a sludge progressively decomposes, mainly for colloidal flocculation. Gelatinous chemical preit should show a proportionate increase in coagulant demand cipitates no longer produced sluggish filtration rates. The necessary to produce equivalent dewatering rates. Mohlman effect of such precipitates becomes more evident the richer and Edwards (4) have shown that sludges from the Chicago such sludges become in certain decomposition products. FilSanitary District which have been decomposed by anaerobic tration becomes very sluggish with highly ammoniacal sludges digestion actually require more coagulant than do fresh and especially with sludges rich in both amino acids and amsludges. This was also found t o be true with Baltimore monium bicarbonate. A large variety of ammoniacal compounds can result in the sludges (I 3 ), where the scientific reasons for this phenomenon aerobic and anaerobic decomposition of raw and activated were disclosed. Figure 1 presents graphs resulting from following the de- sludges. Sludge digestion is necessarily a deaminization and composition of a fresh sludge in the Baltimore sewage plant ammonification process. When ammonium as bicarbonate, laboratories, The dotted diagonal graph belongs to the top succinate, propionate, etc., together with amino acids and reand right-hand coordinate system. The curves belonging to lated carbamino compounds, result in sewage sludges through the lower and left-hand coordinate system are flocculation progressive decomposition, and ferric coagulants are added, graphs. Details of the methods used for determining all both chemical precipitation and colloidal flocculation take points on such curves, both in the laboratory and on a place. Therefore an excess of coagulant is required to procommercial filter unit, have already been published ( 2 , 3). vide for these and similar reactions and to flocculate the colBy plotting the various percentages of anhydrous ferric loids present, These parallel reactions have heretofore 1ed:to chloride used on dry solids present in the sludges as ab- erroneous conclusions about the coagulation of colloids, in 218
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sewage sludges. In most cases the consequent consumption of coagulants has been so large that their use has been economically prohibitive. The disposal of sludges by coagulation and dewatering has therefore remained as difficult as stated in the introductory reference. SLIJDGE DEAMMONIATIOX
The term “deammoniation” is here intended to include broadly the partial or total removal of the decomposition products associated in solution with ammonia arid its derivatives. Deammoniation by sludge elutriation-i. e., reversal I ire*
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sample. As the ammonium compounds are diminished, the amounts of ferric chloride required for producing equivalent dewatering rates correspondingly decrease. That over 90 per cent of the ferric chloride added to the sludge of 1300 p. p. m. amino-ammonia nitrogen was required for chemical reactions becomes evident from comparison with the deammoniated sludge of curve 15. The original sludge contained 75,000 p. p. m. solids and 1300 p. p. m. amino-ammonia nitrogen and required 10.7 per cent or about 8000 p. p. m. ferric chloride to produce a maximum dewatering rate. The practically completely deammoniated sludge of curve 15 contained 77,000 p. p. m. solids of the same colloidal surface nature and produced the same dewatering rate a t 0.65 per cent or 500 p. p. m. ferric chloride addition. The ferric chloride addition necessary to obtain the same dewatering rate on the original untreated sludge was sixteen times the amount used on the sludge of curve 15. Curve 1000 was obtained by adding 0.1 per cent formalin solution (containing about 36 per cent formaldehyde) to the original sludge followed by 12 hours of standing. Otherwise each following curve was plotted from sludge samples taken from the original batch and treated with gradually increasing amounts of formaldehyde, accompanied or followed by elu-
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FIGURE1. EFFECTOF AGINGA RAWSLUDGE ON AMMONIFICATION AND COAGULANT CONSUMP~ON
of adsorption-has been detailed in the references to the published results obtained by the writer and Keefer and Kratz. It was pointed out that the solids present in sewage sludges are excellent adsorbents and hold a certain quantity of the soluble animoniacal compounds by simple adsorption. Furthermore, owing t o the shape of all adsorption isotherms it is difficult to wash out all of the adsorbed compounds. As the dissolved and adsorbed compounds, especially of ammonia and amino acids present in such sludges, can be effectively changed or destroyed by certain chemical means, it will be interesting to see the effect of removing more of such compounds than can be accomplished by simple elutriation. One of the most effective deaminizing and deammoniating agents known is formaldehyde. With amino and amine groups it forms methylene substitution products, suppresses the basic character of the amino acids, and leaves the acid character free. With ammonium compounds it produces hexamethylene tetramine and liberates the acid character of the salts. Investigations show that elutriation materially aids in such chemical deammoniakion. Because of the combination of both deammoniation steps, relatively small amounts of formalin and other deammoniating agents and water are required. The equilibrium liquid or elutriate, returned to the inflowing sewage froni this procedure, has a lower biochemical oxygen demand and contains agents that tend to arrest putrefaction. Figure 2 shows flocculation curves obtained in deammoniating a digested sludge in varying degrees by this method. The original sludge was well digested and rather rich in ammoniacal compounds. Tht: resulting precipitation curve is shown a t the right. This sludge contained 1300 p. p. m. aminoammonia nitrogen and required large amounts of ferric chloride for chemical precipitation. The maximum of precipitation and coagulation occurred a t 10.7 per cent ferric chloride addition on solids present.. The numbers identifying the remaining successive curves represent the amino-ammonia nitrogen left in each sludge
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FIGURE2. EFFECTOF DEAMMONIATING A DIGESTED SLUDGE ON COAGULANT CONSUMPTION
triation in tap water. In no case were more than two volumes of water to one of sludge used. Elutriation was found necessary to remove deammoniation products which tended to react with ferric chloride. Thoroughly mixing the sludge with formalin and allowing the mixture to stand several hours, followed by elutriation, further reduced the amounts of formaldehyde necessary for effective deammoniation. The application of heat was likewise found to accelerate the formaldehyde deammoniating-reaction speed. Heat was unnecessary for elutriation purposes. The sludge of curve 15 was obtained by adding 2 per cent of a formalin solution to 2500 cc. of digested sludge of curve 1300, allowing the formaldehyde to act over 17 hours, then elutriating the deammoniated sludge in two volumes of water followed by a 12-hour settling period. The curve obtained when completely deammoniating this sludge was almost identical in coordinate position with curve 15, which shows that the 15 p. p. m. nitrogen left in the sludge played a subordinate role in consuming coagulant in the presence of the 77,000 p. p. m. solids. Although the partial deammoniation of raw sludges by simple elutriation results in coagulant savings, especially when such sludges are a day or two old, the efficiency of the procedure is ordinarily not as high a4 when applied to digested
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sludges. This is not only due to the lower ammoniacal content of relatively fresh sludges, but also to their highly putrefactive nature. As digested sludges are relatively stabilized, they can be kept in contact with elutriating water or detained after elutriation over a m u c h l o n g e r period than can raw sludges. T h e amino-ammonia content of fresh sludges has been known to increase materially during the few hours required for c o m p l e t ing laboratory flocculation and filtration tests. Permitting the same sludge to stand 12 to 15 hours in contact with three times its volume of elutriating water, during warm weather, also showed a m a t e r i a l g a i n in 1 2 3 4 5 6 amino-ammonia nitroGmms FeCI, per \OOq.Dry Solids gen. This is not the FIGURE3. DEAMMOMATION OF A c a s e w h e n the use of ‘WELL-DECOMPOSED RAW SLUDGE formalin is combined with elutriation of such raw sludges, for then bacterial activity is arrested and further ammonification prevented. Chemical deammoniation, combined with elutriation, is therefore particularly applicable to crude sludges and mixtures of crude and activated sludges. This formalin treatment, followed by elutriation, was used in deammoniating the 4-day-old raw sludge of curve 325 (Figure 1) to produce the sludge of curve 15. By reversing the ammonification due to progressive bacterial decomposition of sludge 325 by deliberate deammoniation, the remarkable flocculation results of curve 15 were obtained.
FIGURE 4. EFFECT OF SLUDGE DEAMMONIATION ON COAGULANT CONSUVPTION AND TREATMENTS V ~ C U U FILTER V CAPACITIES
In Figure 3 a comparison is shown between deammoniation by simple elutriation and formalin treatment plus elutriation. Again a well-decomposed raw sludge was chosen for the experiment. As pointed out by the writer ( 2 ) and Keefer and Kratz (3), the raw sludges from present sludge accumulation methods in Baltimore are several days old when sent to digestion. They are therefore well decomposed, especially during the summer months, and rich in decomposition products which cause very sluggish filtration rates when treated with
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ferric chloride. Keefer and Kratz have also pointed out that this is not a t all true of raw sludges that are but an hour or so old when subjected to coagulation tests. Curve A of Figure 3 is a typical coagulation curve of a welldecomposed Baltimore raw sludge. This same sludge was first partially deammoniated by elutriation in three volumes of tap water. The middle curve shows the results. Then a large sample of the sludge of curve A was treated with formalin followed by elutriation with less water-i. e., but two volumes of water to one of sludge. The remarkable results shown by the left-hand curve were obtained. This sludge behaved quite like a similarly deammoniated digested sludge Here again about 90 per cent of the coagulant added to a welldecomposed raw sludge is chemically consumed. The unpublished data of Keefer and Kratz contain results obtained when applying this combined formalin-elutriation treatment to digested sludge in field tests. The same vacuum filter equipment and tank system detailed in their published article (3) were used. Although the sludge was not identical with the sludge investigated in this paper, and even though 1100-pound sludge lots were used to determine each point on every curve, Figure 4 shows that the field results confirm the laboratory results. Since less formalin was used for treatment of the digested sludge of Figure 4, deammoniation was not carried to the extreme limits obtained in the laboratory. However, the ferric chloride figures of Figures 2 and 4 show that the flocculation curve for the deammoniated sludge of Figure 4, containing 84 p. p. m. amino-ammonia nitrogen, lies slightly to the right of curve 60 of Figure 2, which is as it should be. The digested sludge of Figure 4, containing 1140 p. p. m. amino-ammonia nitrogen, should fall between curves 1000 and 1300 of Figure 2, which it does. The use of formaldehyde as a valuable adjunct to this process depends on obtaining a cheap commercial product. As the anaerobic digestion of seiyage sludge produces relatively large amounts of gas rich in methane, this gas offers a potentially cheap source of formaldehyde. The formaldehyde of the Baltimore tests was a relatively pure product s u p plied by the R & H Chemical Division of E. I. du Pont de Nemours & Company. Excellent deammoniation results were also obtained by properly combining the use of hypochlorites or chlorine with sludge elutriation. With digested sludge, chlorine and hypochlorites act to produce ammonium chloride which is readily removed from the sludge by elutriation. ACKNOWLEDGMEKT The Baltimore Sewage Works, where these investigations were conducted, are under the general supervision of B. L. Crozier, chief engineer of the Department of Public Works, G. E. Finck, sewerage engineer, and C. E. Keefer, principal assistant engineer, with G. K. Armeling, superintendent and E. C. Cromwell, principal sanitary chemist. Appreciation is expressed to C. E. Keefer and his associates for their encouraging cooperation and assistance in making possible this research. Acknowledgment is also due E. I. du Pont de Nemours & Company and the Pennsylvania Salt nhnufacturing Company for generous cooperation in supplying formaldehyde and ferric chloride for these investigations.
LITERATURE CITED Encyclopaedia Bntannica, 14th ed., 5’01. 20, p. 403. Genter, Sewage W o r k s J., 4, 689 (July, 1934). Keefer and Kratz, Ibid., 5 , 845-96 (Sept., 1934). Mohlman, F. W., and Edwards, G. P., IND.ENQ.CHEM.,26, 226 (1934). (5) Sewage W o r k s J., 5 , 701-2 (July, 1933).
(1) (2) (3) (4)
RECEIVED October 4, 1934 Presented before the Division of Water, Sewage, and Sanitation Chemistry at the 88th Meeting of the Amerlcan Chemical Society, Cleveland, Ohio, September 10 to 14. 1934.
