Anaerobic Digestion: II. Nitrogen Changes and Losses during Anaerobic Digestion

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<ul><li><p>Anaerobic Digestion: II. Nitrogen Changes and Losses during Anaerobic DigestionAuthor(s): J. R. SnellSource: Sewage Works Journal, Vol. 15, No. 1 (Jan., 1943), pp. 56-70Published by: Water Environment FederationStable URL: .Accessed: 12/06/2014 16:58</p><p>Your use of the JSTOR archive indicates your acceptance of the Terms &amp; Conditions of Use, available at .</p><p> .JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact</p><p> .</p><p>Water Environment Federation is collaborating with JSTOR to digitize, preserve and extend access to SewageWorks Journal.</p><p> </p><p>This content downloaded from on Thu, 12 Jun 2014 16:58:40 PMAll use subject to JSTOR Terms and Conditions</p><p></p></li><li><p>ANAEROBIC DIGESTION * </p><p>II. NITROGEN CHANGES AND LOSSES DURING ANAEROBIC DIGESTION </p><p>By J. R. Snell </p><p>Sanitary Engineer, V. S. Engineers, Repairs and Utilities Division, Boston, Mass. </p><p>The purpose of this article is to present the results of the author's </p><p>experiments on the nitrogen losses and changes during anaerobic diges tion, and to review the literature on the same subject, explaining some </p><p>of the many inconsistencies which appear. It attempts to answer the </p><p>question: "Are nitrogenous gases evolved during anaerobic digestion and if so, under what conditions?" </p><p>The theory proposed by Waksman (1) seems to answer the question very definitely. For the limited condition in which no oxygen is pres ent, the theory states briefly that nitrogen cannot be evolved as a gas unless it is present in a nitrate or a nitrite. When in this form, how ever, a large percentage of the nitrogen may break down into N2, N20, and NO gases and the remainder may be reduced to ammonia. None of these nitrogenous gases can be evolved unless nitrates or nitrites are </p><p>present and then only in a total amount equivalent to the weight of nitrites or nitrates present. </p><p>The various articles appearing in the literature seem to disagree as to whether these nitrogenous gases are evolved, some supporting the </p><p>Waksman theory and others opposing it. The latter find the loss of </p><p>nitrogen as a gas from the anaerobic digestion of sewage sludge, which contains for all practical purposes no nitrates or nitrites. </p><p>The presentation of the author's experimental results and the con </p><p>clusions drawn from them will be followed by a review of articles re </p><p>porting the finding of nitrogenous gases evolved during anaerobic di gestion. Probable errors will be pointed out and explanations given for their contradictory results. Following this a review will be given of the articles reporting that no nitrogen gas is evolved, and at the end </p><p>will be given a general summary of results. </p><p>Description of Experiments </p><p>A series of four experiments was especially designed to prove or </p><p>disprove the Waksman theory as stated above. Each experiment was conducted in a four-liter Pyrex bottle (see Part I for description of apparatus) and each bottle contained 500 grams of digested sewage sludge for seeding purposes, and was diluted to 3,000 grams in the final </p><p>mix. The following nitrogenous substances (expressed as nitrogen) were added: in Experiment 65, 3,060 p.p.m. of proteose peptone as a </p><p>supply of organic nitrogen ; in Experiment 66, 1,000 p.p.m. of proteose * </p><p>This is the second of a series of papers based on two years of research at the Harvard Graduate School of Engineering. For the first paper, see This Journal, 14, 1304 (Nov., 1942). </p><p>56 </p><p>This content downloaded from on Thu, 12 Jun 2014 16:58:40 PMAll use subject to JSTOR Terms and Conditions</p><p></p></li><li><p>Vol. 15, No. 1 ANAEROBIC DIGESTION. II 57 </p><p>peptone and 2,000 p.p.m. of NaN03 ; in Experiments 67 and 68, no pro teose peptone, but 3,000 p.p.m. of NaN03 to each. In Experiment 67, 1,330 p.p.m. of sucrose was added after 408 hours and an equal amount </p><p>was added at the end of 792 hours to supply the energy for the denitri </p><p>fying bacteria. In Experiment 68, 1,670 p.p.m. of sucrose had been </p><p>added at the beginning and then after 94 hours 500 p.p.m. of proteose peptone was introduced. As was stated above, each of these experi ments had a total weight of digesting material of 3,000 grams. </p><p>The solids and nitrogen content of the digested sludge, proteose peptone, and sodium nitrate are given in Table I. </p><p>Table I.?Solids and Nitrogen Content of Proteose Peptone, Digested Sludge, and Sodium Nitrate </p><p>Digested Sludge </p><p>Proteose Peptone </p><p>Sodium Nitrate </p><p>% Total Solids % Volatile Solids Nitrate Nitrogen* Nitrite Nitrogen* Ammonia Nitrogen* </p><p>4.03 </p><p>2.08 </p><p>0.2 </p><p>0.05 </p><p>350.0 </p><p>95.66 </p><p>0.7 </p><p>0.00 </p></li><li><p>58 SEWAGE WORKS JOUENAL January, 1943 </p><p>trites, or nitrogenous gases are formed. The total nitrogen content </p><p>recorded in Table II shows that there is a gain of less than 1 per cent </p><p>during the experiment. This is well within the possible experimental error. The gas results have been corrected for the air present in the </p><p>bottle at the beginning; that is, the oxygen absorbed and remaining from this air. It was shown in this experiment that no nitrogen gas </p><p>was contained in the evolved gas. </p><p>O 200 ?400 600 800 1000 1200 WOO 1600 1800 2000 Time in H?ur* </p><p>Fig. 1.?Nitrogen balance during the digestion of 3000 P.P.M. of proteose peptone. Experiment No. 65. </p><p>Figure 2 (Experiment 66) shows the decomposition of 1,000 p.p.m. of proteose peptone and 2,000 p.p.m. of sodium nitrate. The ordinates between the ploted lines represent the concentrations of the various forms of nitrogen, as indicated. It is seen that most of the nitrogen breaks down into N20 and N2 gases, but that a small amount of ammonia nitrogen is also formed. About 75 per cent of the proteose peptone nitrogen was decomposed into ammonia nitrogen instead of over 90 per cent as in Experiment 65, when nitrate nitrogen was absent. Only about 15 per cent as much carbon dioxide and about 2 per cent as much </p><p>methane was produced as in Experiment 65. The nitrogen balance in Table II shows that considerable nitrogen was unaccounted for in this experiment. This was due to a loss incident to rapid evolution of gas </p><p>during one night. It was known that some gas was lost but the amount was uncertain. Over 2,000 ml. was measured as evolved during that </p><p>night. An additional 875 ml. would have accounted for all of the miss </p><p>This content downloaded from on Thu, 12 Jun 2014 16:58:40 PMAll use subject to JSTOR Terms and Conditions</p><p></p></li><li><p>Vol. 15, No. 1 ANAEROBIC DIGESTION. II 59 </p><p>Table II.?Nitrogen Balance for Experiments 65-68* </p><p>No. 65 No. 66 No. 67 No. 68 </p><p>At Start: Proteose Peptone Sodium Nitrate </p><p>Digested Sludge Total </p><p>3,060 0 </p><p>260 </p><p>3,320 </p><p>1,000 2,000 </p><p>260 </p><p>3,260 </p><p>0 </p><p>3,000 260 </p><p>3,260 </p><p>500 </p><p>3,000 260 </p><p>3,760 </p><p>At End: N20 Gas N2Gas </p><p>N02 Nitrogen NO* Nitrogen NH* Nitrogen Organic Nitrogen </p><p>Total Nitrogen </p><p>0 0 0 0 </p><p>2,940 410 </p><p>3,350 </p><p>l,060t 430t </p><p>0 0 </p><p>960 445 </p><p>2,895 </p><p>495 140 </p><p>280=fc30 </p><p>2,500*250 40 </p><p>180 </p><p>3,635*280 </p><p>1,325 1,010 </p><p>3 500?50 </p><p>400 465 </p><p>3,703?50 </p><p>C02 Producedt CH4 Produced? </p><p>137.5 266.0 </p><p>21.0 </p><p>5.7 </p><p>52.0 </p><p>1.4 </p><p>41.5 </p><p>5.4 </p><p>* AU nitrogen values are given as p.p.m. of nitrogen for the whole weight of 3,000 grams. </p><p>t Because of rapid evolution and escape of gas overnight these two values are low. When </p><p>they are corrected so that the total nitrogen at the end balances with that at the start they become </p><p>1,390 p.p.m. N20 gas and 465 p.p.m. N2 gas. </p><p>X Liters per kilogram of volatile solids of proteose peptone and sucrose. </p><p>i. </p><p>%~T Organic Nitrogen </p><p>Ammonia Hitregen. </p><p>0 p.-p.m. nitrite as nitrogen O trate as nitrogen . </p><p>HO} </p><p>Me0 Gas </p><p>kO? 600 1600 l?oo </p><p>F?o. 2.?Nitrogen balance during the digestion of 2000 P.P.M. of sodium nitrate and </p><p>1000 P.P.M. of proteose peptone. Experiment No. 66. </p><p>This content downloaded from on Thu, 12 Jun 2014 16:58:40 PMAll use subject to JSTOR Terms and Conditions</p><p></p></li><li><p>60 SEWAGE WOEKS JOUENAL January, 1943 </p><p>ing nitrogen. The results plotted in Fig. 2 are corrected for this loss of gas. </p><p>The apparent error in the changes of organic nitrogen (Fig. 2) is due to the method of calculating the quantities of N20 and N2 gases pro duced. Actually all this gas is produced before the end of 220 hours when all but 6 p.p.m. of the nitrate has been decomposed. The only </p><p>gases produced after that are C02 and CH4. Both N20 and N2 gas contained in the space above the digesting mixture, however, were not </p><p>measured and recorded until later. Experiment 66 shows that nitrogen present as nitrate may be lost during anaerobic digestion as N20 or N2 gas. NO gas was not found in any of the samples analyzed. </p><p>Organic nitrogen in Digested. Sludge Ammonia Nitrogen </p><p>Add 1670 p.p.?. of Socreee </p><p>S 2 Mitrate Httrogen </p><p>t 1 </p><p>?2G*? </p><p>N20 Ga* </p><p>NOO 60O 800 1000 1200 moo I6OO 1800 2000 Time In Hour? </p><p>Fig. 3.?Nitrogen balance during the digestion of 3000 P.P.M. sodium nitrate and 3300 P.P.M. sucrose. Experiment No. 67. </p><p>Figure 3 (Experiment 67) shows the digestion of sodium nitrate to be almost negligible when seeded with digested sludge alone. Sucrose added after 410 hours and again after 790 hours aided in the denitrifica tion of the sodium nitrate, but insufficient nitrogenous food was evi </p><p>dently present, for the bacteria did not complete the denitrification. </p><p>However, nitrous oxide (N20) and nitrogen gas were given off. The </p><p>nitrogen balance in Table II shows more nitrogen accounted for at the end than at the beginning of the experiment, but most of this error oc curred in the nitrate determination, which is accurate only within about </p><p>10 per cent. </p><p>This content downloaded from on Thu, 12 Jun 2014 16:58:40 PMAll use subject to JSTOR Terms and Conditions</p><p></p></li><li><p>Vol. 15, No. 1 ANAEEOBIC DIGESTION. II 61 </p><p>Figure 4 (Experiment 68) shows the digestion of sodium nitrate and sucrose when seeded with digested sludge. The break-down of the su </p><p>crose necessitated an adjustment of the pH with sodium hydroxide. Little nitrous oxide or nitrogen gas was evolved, but considerable ni </p><p>trite nitrogen (300 p.p.m.) was formed. After 94 hours 500 p.p.m. of </p><p>proteose peptone was added. It supplied the necessary food for the </p><p>bacteria, and digestion proceeded rapidly. Adjustment of the pH with HC1 was necessary because of the decomposition of the sodium nitrate. </p><p>At the end of the experiment denitrification had not reached completion, but a trend toward that end was exhibited. It should be noted that the </p><p>0 200 MOO 600 800 1000 1200 Time in Hours </p><p>Pig. 4.?Nitrogen balance during the digestion of 3000 P.P.M. of sodium nitrate, 3330 P.P.M. sucrose and 500 P.P.M. of proteose peptone. Experiment No. 68. </p><p>rate of denitrification was lower in this experiment than in Experiment 66, where the proteose peptone was added at the beginning and twice as </p><p>much of it was used. It would seem that at least 500 p.p.m. of organic </p><p>nitrogen is needed to supply nutrition for the bacteria to carry on com </p><p>plete denitrification of 3,000 p.p.m. of sodium nitrate. To generalize from these few experiments, one would say that to obtain complete denitrification of nitrates the ratio of available organic nitrogen to ni </p><p>trate nitrogen should be greater than one to six. The nitrogen balance </p><p>in Table II shows that most of the nitrogen is accounted for in Experi ment 68. The error again lies in the nitrate determination. </p><p>This content downloaded from on Thu, 12 Jun 2014 16:58:40 PMAll use subject to JSTOR Terms and Conditions</p><p></p></li><li><p>62 SEWAGE W0EKS JOURNAL January, 1943 </p><p>In other experiments, primarily designed to study the digestion of human excreta, nitrogen observations were made which further confirm </p><p>the above theory. Attention is first called to an early experiment (No. 24) in which seeded feces were digested with 4,280 p.p.m. of ammonium nitrate. Analysis of the evolved gas showed 4 per cent C02, 1 per cent </p><p>CH4, and 1.5 per cent 02. No analysis was made for N20 or NO, but </p><p>the expansion during combustion proved their presence. (See Article </p><p>I.) The remaining 93.5 per cent may, therefore, reasonably be as </p><p>sumed to be a mixture of N20 and N2 gases. Making this assumption, </p><p>3,480 p.p.m. of the original 4,280 p.p.m. of ammonium nitrate was </p><p>evolved as nitrogenous gases. Determinations of total nitrogen were run before and after digestion </p><p>on several of the feces and excreta digestion experiments. Direct nes </p><p>slerization with the Kjeldahl method was employed for total nitrogen determinations so that the results summarized in Table III are accurate </p><p>only to within 3 to 5 per cent. These results add further proof of the </p><p>theory that nitrogen is not lost during anaerobic digestion unless it is </p><p>present as nitrate or nitrite. </p><p>Table III.?Nitrogen Balance in Experiments on Feces and Excreta Digestion </p><p>Total Nitrogen in p.p.m. </p><p>Experiment Beginning End </p><p>No. 17?Unseeded Feces 1,970 1,940 </p><p>No. 41?Seeded Feces 1,440 1,450 No. 40?Seeded Feces -f 1/3 Urine 2,615 2,780 </p><p>Summary of Conclusions </p><p>1. Organic nitrogen is not lost as a gas during anaerobic digestion. About 75 to 90 per cent of it is broken down into ammonia nitrogen. </p><p>2. Nitrate nitrogen breaks down into nitrous oxide (N20), nitrogen gas (N2), and ammonia nitrogen when sufficient food is present for the bacteria. </p><p>3. About 90 to 95 per cent of the nitrate nitrogen is denitrified to N20 or N2 and the remainder is reduced to ammonia nitrogen when the </p><p>digestion is complete. 4. The ratio of the quantity of N2 gas to that of N20 gas produced </p><p>was found to vary from 1: 2.8 to 1:1.3. </p><p>5. The experiments indicate that, to obtain complete denitrification of the nitrates, a ratio of available organic nitrogen to nitrate nitrogen </p><p>greater than 1:6 should be established. This ratio is thought neces </p><p>sary to supply food for the bacteria. </p><p>Loss of Nitrogen During Digestion </p><p>Many authorities claim that organic nitrogen is lost during anaero </p><p>bic digestion. They usually state that it is reduced and nitrogen gas </p><p>This content downloaded from on Thu, 12 Jun 2014 16:58:40 PMAll use subject to JSTOR Terms and Conditions</p><p></p></li><li><p>Vol. 15, No. 1 ANAEROBIC DIGESTION. II 63 </p><p>is formed. Other authorities claim that it is impossible for nitrogen gas to form in anaerobic digestion and that no nitrogen is lost. An </p><p>attempt will be made h...</p></li></ul>